Solution: Consider a two-stage cascade refrigeration system

Problem 1P Why is the reversed Carnot cycle executed within the saturation dome not a realistic model for refrigeration cycles?

Problem 2P Why do we study the reversed Carnot cycle even though it is not a realistic model for refrigeration cycles?

Problem 5P Why is the throttling valve not replaced by an isen-tropic turbine in the ideal vapor-compression refrigeration cycle?

Problem 4P Refrigerant-134a enters the condenser of a steady-flow Carnot refrigerator as a saturated vapor at 90 psia, and it leaves with a quality of 0.05. The heat absorption from the refrigerated space takes place at a pressure of 30 psia. Show the cycle on a T-s diagram relative to saturation lines, and determine (a) the coefficient of performance, (b) the quality at the beginning of the heat-absorption process, and (c) the net work input.

Problem 3P A steady-flow Camot refrigeration cycle uses refriger-ant- 134a as the working fluid. The refrigerant changes from saturated vapor to saturated liquid at 60°C in the condenser as it rejects heat. The evaporator pressure is 140 kPa. Show the cycle on a T-s diagram relative to saturation lines, and determine (a) the coefficient of performance, (b) the amount of heat absorbed from the refrigerated space, and (c) the net work input.

Problem 133P An ideal vapor compression refrigeration cycle with R-134a as the working fluid operates between the pressure limits of 120 kPa and 700 kPa. The mass fraction of the refrigerant that is in the liquid phase at the inlet of the evaporator is (a) 0.69 ________________ (b) 0.63 ________________ (c) 0.58 ________________ (d) 0.43 ________________ (e) 0.35

Problem 6P It is proposed to use water instead of refrigerant-134a as the working fluid in air-conditioning applications where the minimum temperature never falls below the freezing point. Would you support this proposal? Explain.

Problem 7P In a refrigeration system, would you recommend condensing the refrigerant-134a at a pressure of 0.7 or 1.0 MPa if heat is to be rejected to a cooling medium at 15°C?Why?

Problem 8P Does the area enclosed by the cycle on a T-s diagram represent the net work input for the reversed Carnot cycle? How about for the ideal vapor-compression refrigeration cycle?

Problem 9P Consider two vapor-compression refrigeration cycles. The refrigerant enters the throttling valve as a saturated liquid at 30°C in one cycle and as subcooled liquid at 30°C in the other one. The evaporator pressure for both cycles is the same. Which cycle do you think will have a higher COP?

Problem 10P The COP of vapor-compression refrigeration cycles improves when the refrigerant is subcooled before it enters the throttling valve. Can the refrigerant be subcooled indefinitely to maximize this effect, or is there a lower limit? Explain.

Problem 11P An ice-making machine operates on the ideal vapor-compression cycle, using refrigerant-134a. The refrigerant enters the compressor as saturated vapor at 20 psia and leaves the condenser as saturated liquid at 80 psia. Water enters the ice machine at 55°F and leaves as ice at 25°F. For an ice production rate of 15 lbm/h, determine the power input to the ice machine (169 Btu of heat needs to be removed from each l bm of water at 55°F to turn it into ice at 25°F).

Problem 12P A refrigerator operates on the ideal vapor-compression refrigeration cycle and uses refrigerant-134a as the working fluid. The condenser operates at 300 psia and the evaporator at 20°F. If an adiabatic, reversible expansion device were available and used to expand the liquid leaving the condenser, how much would the COP improve by using this device instead of the throttle device?

Problem 14P Consider a 300 kJ/min refrigeration system that operates on an ideal vapor-compression refrigeration cycle with refrigerant-134a as the working fluid. The refrigerant enters the compressor as saturated vapor at 140 kPa and is compressed to 800 kPa. Show the cycle on a T-s diagram with respect to saturation lines, and determine (a) the quality of the refrigerant at the end of the throttling process, (b) the coefficient of performance, and (c) the power input to the compressor.

Problem 16P Repeat Prob. 11–14 assuming an isentropic efficiency of 85 percent for the compressor. Also, determine the rate of exergy destruction associated with the compression process in this case. Take T0 = 298 K. Problem 11–14 Consider a 300 kJ/min refrigeration system that operates on an ideal vapor-compression refrigeration cycle with refrigerant-134a as the working fluid. The refrigerant enters the compressor as saturated vapor at 140 kPa and is compressed to 800 kPa. Show the cycle on a T-s diagram with respect to saturation lines, and determine (a) the quality of the refrigerant at the end of the throttling process, (b) the coefficient of performance, and (c) the power input to the compressor.

11-13 An ideal vapor-compression refrigeration cycle that uses refrigerant-134a as its working fluid maintains a condenser at and the evaporator at \(-12^{\circ} \mathrm{C}\). Determine this system's COP and the amount of power required to service a cooling load. Answers: Equation Transcription: Text Transcription: -12°C

Problem 19P Refrigerant-134a enters the compressor of a refrigerator at 100 kPa and ?20°C at a rate of 0.5 m3/min and leaves at 0.8 MPa. The isentropic efficiency of the compressor is 78 percent. The refrigerant enters the throttling valve at 0.75 MPa and 26°C and leaves the evaporator as saturated vapor at ?26°C. Show the cycle on a T-s diagram with respect to saturation lines, and determine (a) the power input to the compressor, (b) the rate of heat removal from the refrigerated space, and (c) the pressure drop and rate of heat gain in the line between the evaporator and the compressor.

11-21 A refrigerator uses refrigerant-134a as the working fluid and operates on the ideal vapor-compression refrigeration cycle except for the compression process. The refrigerant enters the evaporator at with a quality of 34 percent and leaves the compressor at \(70^{\circ} \mathrm{C}\). If the compressor consumes of power, determine the mass flow rate of the refrigerant, the condenser pressure, and the COP of the refrigerator. Equation Transcription: Text Transcription: 70°C

Reconsider Prob. 11–19. Using EES (or other) software, investigate the effects of varying the compressor isentropic efficiency over the range 60 to 100 percent and the compressor inlet volume flow rate from \(0.1 \text { to } 1.0 \mathrm{~m}^{3} / \mathrm{min}\) on the power input and the rate of refrigeration. Plot the rate of refrigeration and the power input to the compressor as functions of compressor efficiency for compressor inlet volume flow rates of , and \(1.0 \mathrm{~m}^{3} / \mathrm{min}\), and discuss the results. Equation Transcription: Text Transcription: 0.1 to 1.0 m3/min 1.0 m3/min

Problem 23P An actual refrigerator operates on the vapor-compression refrigeration cycle with refrigerant-22 as the working fluid. The refrigerant evaporates at ?15°C and condenses at 40°C. The isentropic efficiency of the compressor is 83 percent. The refrigerant is superheated by 5°C at the compressor inlet and subcooled by 5°C at the exit of the condenser. Determine (a) the heat removed from the cooled space and the work input, in kJ/kg and the COP of the cycle. Determine (b) the same parameters if the cycle operated on the ideal vapor-compression refrigeration cycle between the same evaporating and condensing temperatures. The properties of R-22 in the case of actual operation are: h1, = 402.49 kJ/kg, h2 =454.00 kJ/kg, h3 = 243.19 kJ/kg The properties of R-22 in the case of ideal operation are: h1 = 399.04 kJ/kg, h2 = 440.71 kJ/kg, h3 = 249.80 kJ/kg Note: state 1: compressor inlet, state 2: compressor exit, state 3: condenser exit, state 4: evaporator inlet.

A commercial refrigerator with refrigerant-134a as the working fluid is used to keep the refrigerated space at \(-30^{\circ} \mathrm{C}\) by rejecting its waste heat to cooling water that enters the condenser at \(18^{\circ} \mathrm{C}\) at a rate of \(0.25 \mathrm{~kg} / \mathrm{s}\) and leaves at \(26^{\circ} \mathrm{C}\). The refrigerant enters the condenser at and \(65^{\circ} \mathrm{C}\) and leaves at \(42^{\circ} \mathrm{C}\). The inlet state of the compressor is and \(-34^{\circ} \mathrm{C}\) and the compressor is estimated to gain a net heat of from the surroundings. Determine the quality of the refrigerant at the evaporator inlet, (b) the refrigeration load, (c) the COP of the refrigerator, and the theoretical maximum refrigeration load for the same power input to the compressor. Equation Transcription: Text Transcription: -30°C 18°C 0.25 kg/s 26°C 65°C 42°C -34°C

Problem 17P Refrigerant-134a enters the compressor of a refrigerator as superheated vapor at 0.20 MPa and ?5°C at a rate of 0.07 kg/s, and it leaves at 1.2 MPa and 70°C. The refrigerant is cooled in the condenser to 44°C and 1.15 MPa, and it is throttled to 0.21 MPa. Disregarding any heat transfer and pressure drops in the connecting lines between the components, show the cycle on a T-sdiagram with respect to saturation lines, and determine (a) the rate of heat removal from the refrigerated space and the power input to the compressor, (b) the isentropic efficiency of the compressor, and (c) the COP of the refrigerator.

Problem 24P How is the energy efficiency of a refrigerator operating on the vapor-compression refrigeration cycle defined? Provide two alternative definitions and explain each term.

Problem 25P How is the energy efficiency of a heat pump operating on the vapor-compression refrigeration cycle defined? Provide two alternative definitions and show that one can be derived from the other.

Problem 27P A space is kept at –15°C by a vapor-compression refrigeration system in an ambient at 25°C. The space gains heat steadily at a rate of 3500 kJ/h and the rate of heat rejection in the condenser is 5500 kJ/h. Determine the power input, in kW, the COP of the cycle and the second-law efficiency of the system.

Problem 29P A vapor-compression refrigeration system absorbs heat from a space at 0°C at a rate of 24,000 Btu/h and rejects heat to water in the condenser. The water experiences a temperature rise of 12°C in the condenser. The COP of the system is estimated to be 2.05. Determine (a) the power input to the system, in kW, (b) the mass flow rate of water through the condenser, and (c) the second-law efficiency and the energy destruction for the refrigerator. Take T0 = 20°C and cp,water = 4.18 kJ/kg•°C.

Problem 22P The manufacturer of an air conditioner claims a seasonal energy efficiency ratio (SEER) of 16 (Btu/h)/W for one of its units. This unit operates on the normal vapor compression refrigeration cycle and uses refrigerant-22 as the working fluid. This SEER is for the operating conditions when the evaporator saturation temperature is ?5°C and the condenser saturation temperature is 45°C. Selected data for refrigerant-22 are provided in the table below. T, °C Psat, kPa hf, kJ/kg hg, kJ/kg Sg, kJ/kg•K ?5 421.2 38.76 248.1 0.9344 45 1728 101 261.9 0.8682 (a) Sketch the hardware and the T-s diagram for this air conditioner. ________________ (b) Determine the heat absorbed by the refrigerant in the evaporator per unit mass of refrigerant-22, in kJ/kg. ________________ (c) Determine the work input to the compressor and the heat rejected in the condenser per unit mass of refrigerant-22, in kJ/kg.

Problem 26P Consider isentropic compressor of a vapor-compression refrigeration cycle. What are the isentropic efficiency and energy efficiency of this compressor? Justify your answers. Is the energy efficiency of a compressor necessarily equal to its isentropic efficiency? Explain.

Problem 30P A refrigerator operating on the vapor-compression refrigeration cycle using refrigerant-134a as the refrigerant is considered. The temperature of the cooled space and the ambient air are at 10°F and 80°F, respectively. R-134a enters the compressor at 20 psia as a saturated vapor and leaves at 140 psia and 160°F. The refrigerant leaves the condenser as a saturated liquid. The rate of cooling provided by the system is 45,000 Btu/h. Determine (a) the mass flow rate of R-134a and the COP, (b) the energy destruction in each component of the cycle and the energy efficiency of the compressor, and (c) the second-law efficiency of the cycle and the total energy destruction in the cycle.

Problem 32P A refrigerator operates on the ideal vapor-compression refrigeration cycle with refrigerant-134a as the working fluid. The refrigerant evaporates at ?10°C and condenses at 57.9°C. The refrigerant absorbs heat from a space at 5°C and rejects heat to ambient air at 25°C. Determine (a) the cooling load, in kJ/kg, and the COP, (b) the energy destruction in each component of the cycle and the total energy destruction in the cycle, and (c) the second-law efficiency of the compressor, evaporator, and the cycle.

11-31 A room is kept at \(-5^{\circ} \mathrm{C}\) by a vapor-compression refrigeration cycle with R-134a as the refrigerant. Heat is rejected to cooling water that enters the condenser at \(20^{\circ} \mathrm{C}\) at a rate of \(0.13 \mathrm{~kg} / \mathrm{s}\) and leaves at \(28^{\circ} \mathrm{C}\). The refrigerant enters the condenser at \(1.2 \mathrm{MPa} \text { and } 50^{\circ} \mathrm{C}\) and leave as a saturated liquid. If the compressor consumes of power, determine the refrigeration load, in and the the second-law efficiency of the refrigerator and the total exergy destruction in the cycle, and the exergy destruction in the condenser. Take \(T_{0}=20^{\circ} \mathrm{C} \text { and } \mathrm{c}, \text { water }=4.18 \mathrm{~kJ} / \mathrm{kg} \cdot{ }^{\circ} \mathrm{C}\). Equation Transcription: Text Transcription: -5°C 20°C 0.13 kg/s 28°C 1.2MPa and 50°C T0=20°C and c, water=4.18 kJ/kg°C

Problem 36P Consider a refrigeration system using refrigerant-134a as the working fluid. If this refrigerator is to operate in an environment at 30°C, what is the minimum pressure to which the refrigerant should be compressed? Why?

Problem 37P A refrigerant-134a refrigerator is to maintain the refrigerated space at ?10°C. Would you recommend an evaporator pressure of 0.12 or 0.14 MPa for this system? Why?

Problem 33P A refrigeration system operates on the ideal vapor-compression refrigeration cycle with ammonia as the refrigerant. The evaporator and condenser pressures are 200 kPa and 2000 kPa, respectively. The temperatures of the low-temperature and high-temperature mediums are ?9°C and 27°C, respectively. If the rate of heat rejected in the condenser is 18.0 kW, determine (a) the volume flow rate of ammonia at the compressor inlet, hi L/s, (b) the power input and the COP, and (c) the second-law efficiency of the cycle and the total energy destruction in the cycle. The properties of ammonia at various states are given as follows: h1 = 1439.3 kJ/kg, s1 = 5.8865 kJ/kg•K, v1 = 0.5946 m3/kg, h2 = 1798.3 kJ/kg, h3 = 437.4 kJ/kg, s3 = 1.7892 kJ/kg•K, s4 = 1.9469 kJ/kg•K. Note: state 1: compressor inlet, state 2: compressor exit, state 3: condenser exit, state 4: evaporator inlet.

Problem 35P When selecting a refrigerant for a certain application, what qualities would you look for in the refrigerant?

Problem 38P A refrigerator that operates on the ideal vapor-compression cycle with refrigerant-134a is to maintain the refrigerated space at ?10°C while rejecting heat to the environment at 25°C. Select reasonable pressures for the evaporator and the condenser, and explain why you chose those values.

Problem 39P A heat pump that operates on the ideal vapor-compression cycle with refrigerant-134a is used to heat a house and maintain it at 26°C by using underground water at 14°C as the heat source. Select reasonable pressures for the evaporator and the condenser, and explain why you chose those values.

Problem 28P Bananas are to be cooled from 28°C to 12°C at a rate of 1140 kg/h by a refrigerator that operates on a vapor-compression refrigeration cycle. The power input to the refrigerator is 8.6 kW. Determine (a) the rate of heat absorbed from the bananas, in kJ/h, and the COP, (b) the minimum power input to the refrigerator, and (c) the second-law efficiency and the energy destruction for the cycle. The specific heat of bananas above freezing is 3.35 kJ/kg•°C.

Problem 42P A heat pump that operates on the ideal vapor- compression cycle with refrigerant-134a is used to heat water from 15 to 45°C at a rate of 0.12 kg/s. The condenser and evaporator pressures are 1.4 and 0.32 MPa, respectively. Determine the power input to the heat pump.

Problem 45P A heat pump that operates on the ideal vapor- compression cycle with refrigerant-134a is used to heat a house and maintain it at 75°F by using underground water at 50°F as the heat source. The house is losing heat at a rate of 60,000 Btu/h. The evaporator and condenser pressures are 50 and 120 psia, respectively. Determine the power input to the heat pump and the electric power saved by using a heat pump instead of a resistance heater.

Problem 41P What is a water-source heat pump? How does the COP of a water-source Heat pump system compare to that of an air-source system?

Problem 40P Do you think a heat pump system will be more cost-effective in New York or in Miami? Why?

11-43 A heat pump with refrigerant-134a as the working fluid is used to keep a space at \(25^{\circ} \mathrm{C}\) by absorbing heat from geothermal water that enters the evaporator at \(50^{\circ} \mathrm{C}\) at a rate of \(0.065 \mathrm{~kg} / \mathrm{s}\) and leaves at \(40^{\circ} \mathrm{C}\). The refrigerant enters the evaporator at \(20^{\circ} \mathrm{C}\) with a quality of 23 percent and leaves at the inlet pressure as saturated vapor. The refrigerant loses of heat to the surroundings as it flows through the compressor and the refrigerant leaves the compressor at at the same entropy as the inlet. Determine the degrees of subcooling of the refrigerant in the condenser, (b) the mass flow rate of the refrigerant, the heating load and the COP of the heat pump, and the theoretical minimum power input to the compressor for the same heating load. Equation Transcription: Text Transcription: 25°C 50°C 0.065 kg/s 40°C 20°C

Problem 46P A heat pump using refrigerant-134a heats a house by using underground water at 8°C as the heat source. The house is losing heat at a rate of 60,000 kJ/h. The refrigerant enters the compressor at 280 kPa and 0°C, and it leaves at 1 MPa and 60°C. The refrigerant exits the condenser at 30°C. Determine (a) the power input to the heat pump, (b) the rate of heat absorption from the water, and (c) the increase in electric power input if an electric resistance heater is used instead of a heat pump.

11-44 Refrigerant- 134 a enters the condenser of a residential heat pump at and \(50^{\circ} \mathrm{C}\) at a rate of \(0.022 \mathrm{~kg} / \mathrm{s}\) and leaves at subcooled by \(3^{\circ} \mathrm{C}\). The refrigerant enters the compressor at superheated by \(4^{\circ} \mathrm{C}\). Determine (a) the isentropic efficiency of the compressor, the rate of heat supplied to the heated room, and the COP of the heat pump. Also, determine the and the rate of heat supplied to the heated room if this heat pump operated on the ideal vapor-compression cycle between the pressure limits of 200 and . Equation Transcription: Text Transcription: 50°C 0.022 kg/s 3°C 4°C

Problem 49P A certain application requires maintaining the refrigerated space at ?32°C. Would you recommend a simple refrigeration cycle with refrigerant-134a or a two-stage cascade refrigeration cycle with a different refrigerant at the bottoming cycle? Why?

Problem 48P How does the COP of a cascade refrigeration system compare to the COP of a simple vapor-compression cycle operating between the same pressure limits?

Problem 51P Can a vapor-compression refrigeration system with a single compressor handle several evaporators operating at different pressures? How?

Problem 52P In the liquefaction process, why are gases compressed to very high pressures?

A two-stage compression refrigeration system operates with refrigerant-134a between the pressure limits of 1.4 and 0.10 MPa. The refrigerant leaves the condenser as a saturated liquid and is throttled to a flash chamber operating at 0.4 MPa. The refrigerant leaving the low-pressure compressor at 0.4 MPa is also routed to the flash chamber. The vapor in the flash chamber is then compressed to the condenser pressure by the high-pressure compressor, and the liquid is throttled to the evaporator pressure. Assuming the refrigerant leaves the evaporator as saturated vapor and both compressors are isentropic, determine (a) the fraction of the refrigerant that evaporates as it is throttled to the flash chamber, (b) the rate of heat removed from the refrigerated space for a mass flow rate of \(0.25 \mathrm{~kg} / \mathrm{s}\) through the condenser, and (c) the coefficient of performance. Equation Transcription: Text Transcription: 0.25 kg/s

Problem 54P Repeat Prob. 11–57 for a flash chamber pressure of 0.6 MPa.

Problem 50P Consider a two-stage cascade refrigeration cycle and a two-stage compression refrigeration cycle with a flash chamber. Both cycles operate between the same pressure limits and use the same refrigerant. Which system would you favor? Why?

Problem 57P Repeat Prob. 11–56 for a heat exchanger pressure of 0.55 MPa. Problem 11–56 Consider a two-stage cascade refrigeration system operating between the pressure limits of 0.8 and 0.14 MPa. Each stage operates on the ideal vapor-compression refrigeration cycle with refrigerant-134a as the working fluid. Heat rejection from the lower cycle to the upper cycle takes place in an adiabatic counterflow heat exchanger where both streams enter at about 0.4 MPa. If the mass flow rate of the refrigerant through the upper cycle is 0.24 kg/s, determine (a) the mass flow rate of the refrigerant through the lower cycle, (b) the rate of heat removal from the refrigerated space and the power input to the compressor, and (c) the coefficient of performance of this cascade refrigerator.

Consider a two-stage cascade refrigeration system operating between the pressure limits of 1.4 MPa and 160 kPa with refrigerant-134a as the working fluid. Heat rejection from the lower cycle to the upper cycle takes place in an adiabatic counterflow heat exchanger where the pressure in the upper and lower cycles are 0.4 and 0.5 MPa, respectively. In both cycles, the refrigerant is a saturated liquid at the condenser exit and a saturated vapor at the compressor inlet, and the isentropic efficiency of the compressor is 80 percent. If the mass flow rate of the refrigerant through the lower cycle is \(0.11 \mathrm{~kg} / \mathrm{s}\), determine (a) the mass flow rate of the refrigerant through the upper cycle, (b) the rate of heat removal from the refrigerated space, and (c) the COP of this refrigerator. Equation Transcription: Text Transcription: 0.11 kg/s

Problem 56P Consider a two-stage cascade refrigeration system operating between the pressure limits of 0.8 and 0.14 MPa. Each stage operates on the ideal vapor-compression refrigeration cycle with refrigerant-134a as the working fluid. Heat rejection from the lower cycle to the upper cycle takes place in an adiabatic counterflow heat exchanger where both streams enter at about 0.4 MPa. If the mass flow rate of the refrigerant through the upper cycle is 0.24 kg/s, determine (a) the mass flow rate of the refrigerant through the lower cycle, (b) the rate of heat removal from the refrigerated space and the power input to the compressor, and (c) the coefficient of performance of this cascade refrigerator.

Consider a two-stage cascade refrigeration system operating between the pressure limits of 1.2 MPa and 200 kPa with refrigerant-134a as the working fluid. The refrigerant leaves the condenser as a saturated liquid and is throttled to a flash chamber operating at 0.45 MPa. Part of the refrigerant evaporates during this flashing process, and this vapor is mixed with the refrigerant leaving the low-pressure compressor. The mixture is then compressed to the condenser pressure by the high-pressure compressor. The liquid in the flash chamber is throttled to the evaporator pressure and cools the refrigerated space as it vaporizes in the evaporator. The mass flow rate of the refrigerant through the low pressure compressor is \(0.15 \mathrm{~kg} / \mathrm{s}\). Assuming the refrigerant leaves the evaporator as a saturated vapor and the isentropic efficiency is 80 percent for both compressors, determine (a) the mass flow rate of the refrigerant through the high-pressure compressor, (b) the rate of heat removal from the refrigerated space, and (c) the COP of this refrigerator. Also, determine (d) the rate of heat removal and the COP if this refrigerator operated on a single-stage cycle between the same pressure limits with the same compressor efficiency and the same flow rate as in part (a). Equation Transcription: Text Transcription: 0.15 kg/s

Problem 61P A two-evaporator compression refrigeration system like that in Fig. P11–61 uses refrigerant-134a as the working fluid. The system operates evaporator 1 at 30 psia, evaporator 2 at 10 psia, and the condenser at 180 psia. The cooling load for evaporator 1 is 9000 Btu/h and that for evaporator 2 is 24,000 Btu/h. Determine the power required to operate the compressor and the COP of this system. The refrigerant is saturated liquid at the exit of the condenser and saturated vapor at the exit of each evaporator, and the compressor is isentropic.

11-60 A two-evaporator compression refrigeration system as shown in Fig. P11-60 uses refrigerant-134a as the working fluid. The system operates evaporator 1 at \(0^{\circ} \mathrm{C}\), evaporator 2 at \(-26.4^{\circ} \mathrm{C}\), and the condenser at . The refrigerant is circulated through the compressor at a rate of \(0.1 \mathrm{~kg} / \mathrm{s}\) and the low-temperature evaporator serves a cooling load of . Determine the cooling rate of the high-temperature evaporator, the power required by the compressor, and the COP of the system. The refrigerant is saturated liquid at the exit of the condenser and saturated vapor at the exit of each evaporator, and the compressor is isentropic. Equation Transcription: Text Transcription: 0°C -26.4°C 0.1 kg/s

Problem 63P How does the ideal-gas refrigeration cycle differ from the Brayton cycle?

Problem 64P Devise a refrigeration cycle that works on the reversed Stirling cycle. Also, determine the COP for this cycle.

Repeat Prob. 11–61E if the 30 psia evaporator is to be replaced with a 60 psia evaporator to serve a \(15,000 \mathrm{Btu} / \mathrm{h}\) cooling load. Equation Transcription: Text Transcription: 15, 000Btu/h

Problem 65P How is the ideal-gas refrigeration cycle, modified for aircraft cooling?

Problem 66P In gas refrigeration cycles, can we replace the turbine by an expansion valve as we did in vapor-compression refrigeration cycles? Why?

Problem 68P Air enters the compressor of an ideal gas refrig-eration cycle at 40°F and 10 psia and the turbine at 120°F and 30 psia. The mass flow rate of air through the cycle is 0.5 lbm/s. Determine (a) the rate of refrigeration, (b) the net power input, and (c) the coefficient of performance.

Problem 67P How do we achieve very low temperatures with gas refrigeration cycles?

Problem 69P An ideal gas refrigeration cycle using air as the working fluid is to maintain a refrigerated space at –23°C while rejecting heat to the surrounding medium at 27°C. If the pressure ratio of the compressor is 3, determine (a) the maximum and minimum temperatures in the cycle, (b) the coefficient of performance, and (c) the rate of refrigeration for a mass flow rate of 0.08 kg/s.

Problem 71P Repeat Prob. 11–73 for a compressor isentropic efficiency of 80 percent and a turbine isentropic efficiency of 85 percent.

Air enters the compressor of an ideal gas refrig-eration cycle at \(7^{\circ} \mathrm{C} and 35 \mathrm{kPa}\) and the turbine at \(37^{\circ} \mathrm{C} and 160 \mathrm{kPa}\). The mass flow rate of air through the cycle is \(0.2 \mathrm{~kg} / \mathrm{s}\). Assuming variable specific heats for air, determine the rate of refrigeration, the net power input, and the coefficient of performance. Equation Transcription: Text Transcription: 7°C and 35kPa 37°C and 160kPa 0.2 kg/s

Problem 74P A gas refrigeration system using air as the working fluid has a pressure ratio of 4. Air enters the compressor at –7°C. The high-pressure air is cooled to 27°C by rejecting heat to the surroundings. It is further cooled to –15°C by regenerative cooling before it enters the turbine. Assuming both the turbine and the compressor to be isentropic and using constant specific heats at room temperature, determine (a) the lowest temperature that can be obtained by this cycle, (b) the coefficient of performance of the cycle, and (c) the mass

Problem 73P A gas refrigeration cycle with a pressure ratio of 4 uses helium as the working fluid. The temperature of the helium is –6°C at the compressor inlet and 50°C at the turbine inlet. Assuming isentropic efficiencies of 85 percent for both the turbine and the compressor, determine (a) the minimum temperature in the cycle, (b) the coefficient of performance, and (c) the mass flow rate of the helium for a refrigeration rate of 25 kW.

Problem 75P Repeat Prob. 11–74 assuming isentropic efficiencies of 75 percent for the compressor and 80 percent for the turbine. Problem 11–74 A gas refrigeration system using air as the working fluid has a pressure ratio of 4. Air enters the compressor at –7°C. The high-pressure air is cooled to 27°C by rejecting heat to the surroundings. It is further cooled to –15°C by regenerative cooling before it enters the turbine. Assuming both the turbine and the compressor to be isentropic and using constant specific heats at room temperature, determine (a) the lowest temperature that can be obtained by this cycle, (b) the coefficient of performance of the cycle, and (c) the mass

An ideal gas refrigeration system with two stages of compression with intercooling as shown in Fig. P11-77 operates with air entering the first compressor at and \(-24^{\circ} \mathrm{C}\) Each compression stage has a pressure ratio of 3 and the two intercoolers can cool the air to \(5^{\circ} \mathrm{C}\). Calculate the coefficient of performance of this system and the rate at which air must be circulated through this system to service a \(45,000 \mathrm{~kJ} / \mathrm{h}\) cooling load. Use constant specific heats at room temperature. Equation Transcription: Text Transcription: -24°C 5°C 45, 000kJ/h

Problem 78P How will the answers of Prob. 11–81 change when the isentropic efficiency of each compressor is 85 percent and the isentropic efficiency of the turbine is 95 percent?

Problem 79P What is absorption refrigeration? How does an absorption refrigeration system differ from a vapor-compression refrigeration system?

A gas refrigeration system using air as the working fluid has a pressure ratio of 5 . Air enters the compressor at \(0^{\circ} \mathrm{C}\). The high-pressure air is cooled to \(35^{\circ} \mathrm{C}\) by rejecting heat to the surroundings. The refrigerant leaves the turbine at \(-80^{\circ} \mathrm{C}\) and then it absorbs heat from the refrigerated space before entering the regenerator. The mass flow rate of air is \(0.4 \mathrm{~kg} / \mathrm{s}\). Assuming isentropic efficiencies of 80 percent for the compressor and 85 percent for the turbine and using constant specific heats at room temperature, determine the effectiveness of the regenerator, the rate of heat removal from the refrigerated space, and the COP of the cycle. Also, determine the refrigeration load and the if this system operated on the simple gas refrigeration cycle. Use the same compressor inlet temperature as given, the same turbine inlet temperature as calculated, and the same compressor and turbine efficiencies Equation Transcription: Text Transcription: 0°C 35°C -80°C 0.4 kg/s

Problem 81P In absorption refrigeration cycles, why is the fluid in the absorber cooled and the fluid in the generator heated?

Problem 80P What are the advantages and disadvantages of absorption refrigeration?

Problem 82P How is the coefficient of performance of an absorption refrigeration system defined?

Problem 83P An absorption refrigeration system that receives heat from a source at 95 °C and maintains the refrigerated space at 0°C is claimed to have a COP of 3.1. If the environmental temperature is 19°C, can this claim be valid? Justify your answer.

Problem 84P An absorption refrigeration system receives heat from a source at 120°C and maintains the, refrigerated space at 0°C. If the temperature of the environment is 25°C, what is the maximum COP this absorption refrigeration system can have?

Problem 85P Heat is supplied to an absorption refrigeration system from a geothermal well at 110°C at a rate of 5 × 105 kJ/h. The environment is at 25°C, and the refrigerated space is maintained at –18°C. Determine the maximum rate at which this system can remove heat from the refrigerated space.

Problem 87P An ammonia-water absorption refrigeration cycle is used to keep a space at 25°F when the ambient temperature is 70°F. Pure ammonia enters the condenser at 300 psia and 140°F at a rate of 0.04 lbm/s. Ammonia leaves the condenser as a saturated liquid and is expanded to 30 psia. Ammonia leaves the evaporator as a saturated vapor. Heat is supplied to the generator by geothermal liquid water that enters at 240°F at a rate of 0.55 lbm/s and leaves at 200°F. Determine (a) the rate of cooling provided by the system, in Btu/h, the COP, and (b) the second-law efficiency of the system. The enthalpies of ammonia at various states of the system are: condenser inlet h2 = 665.7 Btu/lbm, evaporator inlet h4 = 190.9 Btu/lbm, evaporator exit h1 = 619.2 Btu/lbm. Also, take the specific heat of geothermal water to be 1.0 Btu/lbm•°F.

Problem 89P Describe the Seebeck and the Peltier effects.

Problem 91P An iron and a constantan wire are formed into a closed circuit by connecting the ends. Now both junctions are heated and are maintained at the same temperature. Do you expect any electric current to flow through this circuit?

reversible absorption refrigerator consists of a reversible heat engine and a reversible refrigerator. The system removes heat from a cooled space at \(-15^{\circ} \mathrm{C}\) at a rate of . The refrigerator operates in an environment at \(25^{\circ} \mathrm{C}\). If the heat is supplied to the cycle by condensing saturated steam at \(150^{\circ} \mathrm{C}\), determine the rate at which the steam condenses, and (b) the power input to the reversible refrigerator. If the COP of an actual absorption chiller at the same temperature limits has a COP of , determine the second-law efficiency of this chiller Equation Transcription: Text Transcription: -15°C 25°C 150°C

Problem 88P What is a thermoelectric circuit?

Problem 90P Consider a circular copper wire formed by connecting the two ends of a copper wire. The connection point is now heated by a burning candle. Do you expect any current to flow through the wire?

Problem 92P A copper and a constantan wire are formed into a closed circuit by connecting the ends. Now one junction is heated by a burning candle while the other is maintained at room temperature. Do you expect any electric current to flow through this circuit?

Problem 93P How does a thermocouple work as a temperature measurement device?

Problem 94P Why are semiconductor materials preferable to metals in thermoelectric refrigerators?

Problem 97P A thermoelectric refrigerator removes heat from a refrigerated space at ?5°C at a rate of 130 W and rejects it to an environment at 20°C. Determine the maximum coefficient of performance this thermoelectric refrigerator can have and the minimum required power input.

Problem 95P Is the efficiency of a thermoelectric generator limited by the Carnot efficiency? Why?

Problem 96P A thermoelectric generator receives heat from a source at 340°F and rejects the waste heat to the environment at 90°F. What is the maximum thermal efficiency this thermoelectric generator can have?

Problem 98P A thermoelectric cooler has a COP of 0.15 and removes heat from a refrigerated space at a rate of 180 W. Determine the required power input to the thermoelectric cooler, in W.

Problem 99P A thermoelectric cooler has a COP of 0.18 and the power input to the cooler is 1.8 hp. Determine the rate of heat removed from the refrigerated space, in Btu/min.

11-100 A thermoelectric refrigerator is powered by a 12-V car battery that draws of current when running. The refrigerator resembles a small ice chest and is claimed to cool nine canned drinks, \(0.350-L\) each, from \(25 \text { to } 3^{\circ} \mathrm{C}\) in . Determine the average COP of this refrigerator. Equation Transcription: Text Transcription: 0.350 - L 25 to 3°C

Problem 102P It is proposed to run a thermoelectric generator in conjunction with a solar pond that can supply heat at a rate of 7 × 106 kJ/h at 90°C. The waste heat is to be rejected to the environment at 22°C. What is the maximum power this thermoelectric generator can produce?

Problem 103P A typical 200-m2 house can be cooled adequately by a 3.5-ton air conditioner whose COP is 4.0. Determine the rate of heat gain of the house when the air conditioner is running continuously to maintain a constant temperature in the house.

Problem 104P Consider a steady-flow Carnot refrigeration cycle that uses refrigerant-134a as the working fluid. The maximum and minimum temperatures in the cycle are 30 and ?20°C, respectively. The quality of the refrigerant is 0.15 at the beginning of the heat absorption process and 0.80 at the end. Show the cycle on a T-s diagram relative to saturation lines, and determine (a) the coefficient of performance, (b) the condenser and evaporator pressures, and (c) the net work input.

11–105 A heat pump water heater (HPWH) heats water by absorbing heat from the ambient air and transferring it to water. The heat pump has a COP of 3.4 and consumes 6 kW of electricity when running. Determine if this heat pump can be used to meet the cooling needs of a room most of the time for “free” by absorbing heat from the air in the room. The rate of heat gain of a room is usually less than \(45,000 \mathrm{~kJ} / \mathrm{h}\) Equation Transcription: Text Transcription: 45,000 kJ/h

Problem 101P Thermoelectric coolers that plug into the cigarette lighter of a car are commonly available. One such cooler is claimed to cool a 12-oz (0.771-lbm) drink from 78 to 38°F or to heat a cup of coffee from 75 to 130°F in about 15 min in a well-insulated cup holder. Assuming an average COP of 0.2 in the cooling mode, determine (a) the average rate of heat removal from the drink, (b) the average rate of heat supply to the coffee, and (c) the electric power drawn from the battery of the car, all in W.

Problem 106P A heat pump that operates on the ideal vapor- compression cycle with refrigerant-134a is used to heat a house. The mass flow rate of the refrigerant is 0.25 kg/s. The condenser and evaporator pressures are 1400 and 320 kPa, respectively. Show the cycle on aT-s diagram with respect to saturation lines, and determine (a) the rate of heat supply to the house, (b) the volume flow rate of the refrigerant at the compressor inlet, and (c) the COP of this heat pump.

Problem 107P A large refrigeration plant is to be maintained at –15°C, and it requires refrigeration at a rate of 100 kW. The condenser of the plant is to be cooled by liquid water, which experiences a temperature rise of 8°C as it flows over the coils of the condenser. Assuming the plant operates on the ideal vapor-compression cycle using refriger- ant-134a between the pressure limits of 120 and 700 kPa, determine (a) the mass flow rate of the refrigerant, (b) the power input to the compressor, and (c) the mass flow rate of the cooling water.

Problem 109P Repeat Prob. 11–107 assuming the compressor has an isentropic efficiency of 75 percent. Also, determine the rate of exergy destruction associated with the compression process in this case. Take T0 = 25°C. Problem 11–107 A large refrigeration plant is to be maintained at –15°C, and it requires refrigeration at a rate of 100 kW. The condenser of the plant is to be cooled by liquid water, which experiences a temperature rise of 8°C as it flows over the coils of the condenser. Assuming the plant operates on the ideal vapor-compression cycle using refriger- ant-134a between the pressure limits of 120 and 700 kPa, determine (a) the mass flow rate of the refrigerant, (b) the power input to the compressor, and (c) the mass flow rate of the cooling water.

Problem 113P Consider a two-stage compression refrigeration system operating between the pressure limits of 1.4 and 0.12 MPa. The working fluid is refrigerant-134a. The refrigerant leaves the condenser as a saturated liquid and is throttled to a flash chamber operating at 0.5 MPa. Part of the refrigerant evaporates during this flashing process, and this vapor is mixed with the refrigerant leaving the low-pressure compressor. The mixture is then compressed to the condenser pressure by the high-pressure compressor. The liquid in the flash chamber is throttled to the evaporator pressure, and it cools the refrigerated space as it vaporizes in the evaporator. Assuming the refrigerant leaves the evaporator as satu-rated vapor and both compressors are isentropic, determine (a) the fraction of the refrigerant that evaporates as it is throttled to the flash chamber, (b) the amount of heat removed from the refrigerated space and the compressor work per unit mass of refrigerant flowing through the condenser, and (c) the coefficient of performance.

11-111 An air conditioner with refrigerant-134a as the working fluid is used to keep a room at \(26^{\circ} \mathrm{C}\) by rejecting the waste heat to the outside air at \(34^{\circ} \mathrm{C}\). The room is gaining heat through the walls and the windows at a rate of \(250 \mathrm{~kJ} / \mathrm{min}\) while the heat generated by the computer, TV, and lights amounts to . An unknown amount of heat is also generated by the people in the room. The condenser and evaporator pressures are 1200 and , respectively. The refrigerant is saturated liquid at the condenser exit and saturated vapor at the compressor inlet. If the refrigerant enters the compressor at a rate of and the isentropic efficiency of the compressor is 75 percent, determine (a) the temperature of the refrigerant at the compressor exit, (b) the rate of heat generation by the people in the room. (c) the COP of the air conditioner, and the minimum volume flow rate of the refrigerant at the compressor inlet for the same compressor inlet and exit conditions. Equation Transcription: kJ/min Text Transcription: 26°C 34°C 250kJ/min

11-110 A refrigeration unit operates on the ideal vapor compression refrigeration cycle and uses refrigerant-22 as the working fluid. The operating conditions for this unit are evaporator saturation temperature of \(-5^{\circ} \mathrm{C}\) and the condenser saturation temperature of \(45^{\circ} \mathrm{C}\). Selected data for refrigerant- 22 are provided in the table below. 45 1728 101 For R-22 at and and Also, take \(c_{p, \text { air }}=1.005 \mathrm{~kJ} / \mathrm{kg} \cdot K\) (a) Sketch the hardware and the \(T-s\) diagram for this heat pump application. (b) Determine the COP for this refrigeration unit. (c) The evaporator of this unit is located inside the air handler of the building. The air flowing through the air handler enters the air handler at \(27^{\circ} \mathrm{C}\) and is limited to a \(20^{\circ} \mathrm{C}\) temperature drop. Determine the ratio of volume flow rate of air entering the air handler \(\left(m_{\text {air }}^{3} / m i n\right)\) to mass flow rate of \(R-22\left(k g_{R-22} / s\right)\) through the air handler, in \(\left(m_{\text {air }}^{3} / m i n\right) /\left(k g_{R-22} / s\right)\). Assume the air pressure is . Equation Transcription: Text Transcription: -5°C 45°C cp, air = 1.005 kJ/kgK T-s 27°C 20°C (m air 3/min) R-22(kgR-22/s) (m air 3/min)/(kgR-22/s)

Problem 112P An air-conditioner operates on the vapor-compression refrigeration cycle with refrigerant-134a as the refrigerant. The air conditioner is used to keep a space at 21°C while rejecting the waste heat to the ambient air at 37°C. The refrigerant enters the compressor at 180 kPa superheated by 2.7°C at a rate of 0.06 kg/s and leaves the compressor at 1200 kPa and 60°C. R-134a is sub- cooled by 6.3°C at the exit of the condenser. Determine (a) the rate of cooling provided to the space, in Btu/h, and the COP, (b) the isentropic efficiency and the exergy efficiency of the compressor, (c) the exergy destruction in each component of the cycle and the total exergy destruction in the cycle, and (d) the minimum power input and the second-law efficiency of the cycle.

11-114E A two-evaporator compression refrigeration system as shown in Fig. P11-114E uses refrigerant-134a as the working fluid. The system operates evaporator 1 at \(30^{\circ} F\) evaporator 2 at \(-29.5^{\circ} F\), and the condenser at 160 psia. The cooling load of evaporator 1 is double that of evaporator 2 . Determine the cooling load of both evaporators per unit of flow through the compressor, as well as the COP of this system. The refrigerant is saturated liquid at the exit of the condenser and saturated vapor at the exit of each evaporator, and the compressor is isentropic. Equation Transcription: Text Transcription: 30°F -29.5°F

Problem 115P Reconsider Prob. 11–121E. The refrigeration system of that problem cools one reservoir at ?15°F and one at 40°F while rejecting heat to a reservoir at 80°F. Which process has the highest energy destruction?

11-116 A two-stage compression refrigeration system with an adiabatic liquid-vapor separation unit as shown in Fig. P11-116 uses refrigerant-134a as the working fluid. The system operates the evaporator at \(-32^{\circ} \mathrm{C}\), the condenser at , and the separator at \(8.9^{\circ} \mathrm{C}\). The refrigerant is circulated through the condenser at a rate of \(2 \mathrm{~kg} / \mathrm{s}\). Determine the rate of cooling and power requirement for this system. The refrigerant is saturated liquid at the inlet of each expansion valve and saturated vapor at the inlet of each compressor, and the compressors are isentropic. Equation Transcription: Text Transcription: -32°C 8.9°C 2 kg/s

Problem 118P An aircraft on the ground is to be cooled by a gas refrigeration cycle operating with air on an open cycle. Air enters the compressor at 30°C and 100 kPa and is compressed to 250 kPa. Air is cooled to 70°C before it enters the turbine. Assuming both the turbine and the compressor to be isentro- pic, determine the temperature of the air leaving the turbine and entering the cabin.

11-117 Which process of the cycle in Prob. 11-116 has the greatest rate of exergy destruction when the low-temperature reservoir is at \(-22^{\circ} \mathrm{C}\) and the high-temperature reservoir is at \(20^{\circ} \mathrm{C}\)? Equation Transcription: Text Transcription: -22°C 20°C

Problem 120P An absorption refrigeration system is to remove heat from the refrigerated space at 2°C at a rate of 28 kW while operating in an environment at 25°C. Heat is to be supplied from a solar pond at 95°C. What is the minimum rate of heat supply required?

Reconsider Prob. 11–120. Using EES (or other) software, investigate the effect of the source temperature on the minimum rate of heat supply. Let the source temperature vary from 50 t \(250^{\circ} C\). Plot the minimum rate of heat supply as a function of source temperature, and discuss the results. Equation Transcription: Text Transcription: 250°C

Problem 119P Consider a regenerative gas refrigeration cycle using helium as the working fluid. Helium enters the compressor at 100 kPa and ?10°C and is compressed to 300 kPa. Helium is then cooled to 20°C by water. It then enters the regenerator where it is cooled further before it enters the turbine. Helium leaves the refrigerated space at ?25°C and enters the regenerator. Assuming both the turbine and the compressor to be isentropic, determine (a) the temperature of the helium at the turbine inlet, (b) the coefficient of performance of the cycle, and (c) the net power input required for a mass flow rate of 0.45 kg/s.

11-122 A gas refrigeration system using air as the working fluid has a pressure ratio of 5 . Air enters the compressor at \(0^{\circ} \mathrm{C}\). The high-pressure air is cooled to \(35^{\circ} \mathrm{C}\) by rejecting heat to the surroundings. The refrigerant leaves the turbine at \(-80^{\circ} \mathrm{C}\) and enters the refrigerated space where it absorbs heat before entering the regenerator. The mass flow rate of air is . Assuming isentropic efficiencies of 80 percent for the compressor and 85 percent for the turbine and using variable specific heats, determine the effectiveness of the regenerator, (b) the rate of heat removal from the refrigerated space, and the of the cycle. Also, determine the refrigeration load and the COP if this system operated on the simple gas refrigeration cycle. Use the same compressor inlet temperature as given, the same turbine inlet temperature as calculated, and the same compressor and turbine efficiencies. Equation Transcription: Text Transcription: 0°C 35°C -80°C

11-123 The refrigeration system of Fig. P11-123 is another variation of the basic vapor-compression refrigeration system which attempts to reduce the compression work. In this system, a heat exchanger is used to superheat the vapor entering the compressor while subcooling the liquid exiting from the condenser. Consider a system of this type that uses refrigerant-134a as its refrigerant and operates the evaporator at \(-10.09{ }^{\circ} \mathrm{C}\), and the condenser at . Determine the system COP when the heat exchanger provides \(5.51{ }^{\circ} \mathrm{C}\) of subcooling at the throttle valve entrance. Assume the refrigerant leaves the evaporator as a saturated vapor and the compressor is isentropic. Equation Transcription: Text Transcription: -10.09°C 5.51°C

Repeat Prob. 11–123 if the heat exchanger provides \(9.51^{\circ} \mathrm{C}\) of subcooling. Equation Transcription: Text Transcription: 9.51°C

11–125 An ideal gas refrigeration system with three stages of compression with intercooling operates with air entering the first compressor at 50 kPa and \(230^{\circ} C \). Each compressor in this system has a pressure ratio of 7, and the air temperature at the outlet of all intercoolers is \(15^{\circ} C \). Calculate the COP of this system. Use constant specific heats at room temperature. Equation Transcription: Text Transcription: 230°C 15°C

Using EES (or other) software, investigate the effect of the evaporator pressure on the COP of an ideal vapor-compression refrigeration cycle with R-134a as the working fluid. Assume the condenser pressure is kept constant at 1.4 MPa while the evaporator pressure is varied from 100 kPa to 500 kPa. Plot the COP of the refrigeration cycle against the evaporator pressure, and discuss the results. Equation Transcription: Text Transcription:

Using EES (or other) software, investigate the effect of the condenser pressure on the COP of an ideal vapor-compression refrigeration cycle with R-134a as the working fluid. Assume the evaporator pressure is kept constant at \(150 \mathrm{kPa}\) while the condenser pressure is varied from \(400 to 1400 \mathrm{kPa}\). Plot the COP of the refrigeration cycle against the condenser pressure, and discuss the results. Equation Transcription: Text Transcription: 150 kPa 400 to 1400 kPa

Problem 128P Derive a relation for the COP of the two-stage refrigeration system with a flash chamber as shown in Fig. 11–14 in terms of the enthalpies and the quality at state 6. Consider a unit mass in the condenser.

Problem 129P Consider a heat pump that operates on the reversed Carnot cycle with R-134a as the working fluid executed under the saturation dome between the pressure limits of 140 and 800 kPa. R-134a changes from saturated vapor to saturated liquid during the heat rejection process. The net work input for this cycle is (a) 28 kJ/kg ________________ (b) 34 kJ/kg ________________ (c) 49 kJ/kg ________________ (d) 144 kJ/kg ________________ (e) 275 kJ/kg

Problem 130P A refrigerator removes heat from a refrigerated space at 0°C at a rate of 2.2 kJ/s and rejects it to an environment at 20°C. The minimum required power input is (a) 89 W ________________ (b) 150 W ________________ (c) 161W ________________ (d) 557 W ________________ (e) 2200 W

Problem 132P A heat pump operates on the ideal vapor compression refrigeration cycle with R-134a as the working fluid between the pressure limits of 0.32 and 1.2 MPa. If the mass flow rate of the refrigerant is 0.193 kg/s, the rate of heat supply by the heat pump to the heated space is (a) 3.3 kW ________________ (b) 23 kW ________________ (c) 26kW ________________ (d) 31 kW ________________ (e) 45 kW

Problem 131P A refrigerator operates on the ideal vapor compression refrigeration cycle with R-134a as the working fluid between the pressure limits of 120 and 800 kPa. If the rate of heat removal from the refrigerated space is 32 kJ/s, the mass flow rate of the refrigerant is (a) 0.19 kg/s ________________ (b) 0.15 kg/s ________________ (c) 0.23 kg/s ________________ (d) 0.28 kg/s ________________ (e) 0.81 kg/s

Problem 137P An absorption air-conditioning system is to remove heat from the conditioned space at 20°C at a rate of 150 kJ/s while operating in an environment at 35°C. Heat is to be supplied from a geothermal source at 140°C. The minimum rate of heat supply is (a) 86 kJ/s ________________ (b) 21 kJ/s ________________ (c) 30 kJ/s ________________ (d) 61 kJ/s ________________ (e) 150 kJ/s

Problem 138P Consider a refrigerator that operates on the vapor compression refrigeration cycle with R-134a as the working fluid. The refrigerant enters the compressor as saturated vapor at 160 kPa, and exits at 800 kPa and 50°C, and leaves the condenser as saturated liquid at 800 kPa. The coefficient of performance of this refrigerator is (a) 2.6 ________________ (b) 1.0 ________________ (c) 4.2 ________________ (d) 3.2 ________________ (e) 4.4

Problem 136P Consider an ideal gas refrigeration cycle using helium as the working fluid. Helium enters the compressor at 100 kPa and 17°C and compressed to 400 kPa. Helium is then cooled to 20°C before it enters the turbine. For a mass flow rate of 0.2 kg/s, the net power input required is (a) 28.3 kW ________________ (b) 40.5 kW ________________ (c) 64.7 kW ________________ (d) 93.7kW ________________ (e) 113kW

Problem 144P A refrigerator using R-12 as the working fluid keeps the refrigerated space at –15°C in an environment at 30°C. You are asked to redesign this refrigerator by replacing R-12 with the ozone-friendly R-134a. What changes in the pressure levels would you suggest in the new system? How do you think the COP of the new system will compare to the COP of the old system?

Problem 140P Design a vapor-compression refrigeration system that will maintain the refrigerated space at –15°C while operating in an environment at 20°C using refrigerant-134a as the working fluid.

Problem 135P Consider a heat pump that operates on the ideal vapor compression refrigeration cycle with R-134a as the working fluid between the pressure limits of 0.32 and 1.2 MPa. The coefficient of performance of this heat pump is (a) 0.17 ________________ (b) 1.2 ________________ (c) 3.1 ________________ (d) 4.9 ________________ (e) 5.9

Problem 79P What is absorption refrigeration? How does an absorption refrigeration system differ from a vapor-compression refrigeration system?

Problem 11.111C An air conditioner with refrigerant-134a as the working fluid is used to keep a room at by rejecting the waste heat to the outside air at . The room is gaining heat through the walls and the windows at a rate of while the heat generated by the computer, TV, and lights amounts to . An unknown amount of heat is also generated by the people in the room. The condenser and evaporator pressures are 1200 and 500 kPa, respectively . The refrigerant is saturated liquid at the condenser exit and saturated vapor at the compressor inlet. If the refrigerant enters the compressor at a rate of and the isentropic efficiency of the compressor is 75 percent, determine A(a) the temperature of the refrigerant at the compressor exit, (b) the rate of heat generation by the people in the room, (c ) the COP of the air conditioner, and (d) the minimum volume flow rate of the refrigerant at the compressor inlet for the same compressor inlet and exit conditions.

Why do we study the reversed Carnot cycle even though it is not a realistic model for refrigeration cycles?

Consider a two-stage compression refrigeration system operating between the pressure limits of and . The working fluid is refrigerant-134a. The refrigerant leaves the condenser as a saturated liquid and is throttled to a flash chamber operating at . Part of the refrigerant evaporates during this flashing process, and this vapor is mixed with the refrigerant leaving the low-pressure compressor. The mixture is then compressed to the condenser pressure by the high-pressure compressor. The liquid in the flash chamber is throttled to the evaporator pressure, and it cools the refrigerated space as it vaporizes in the evaporator. Assuming the refrigerant leaves the evaporator as saturated vapor and both compressors are isentropic, determine (a) the fraction of the refrigerant that evaporates as it is throttled to the flash chamber, (b) the amount of heat removed from the refrigerated space and the compressor work per unit mass of refrigerant flowing through the condenser, and (c) the coefficient of performance.

A two-evaporator compression refrigeration system as shown in Fig. P11–114E uses refrigerant-134a as the working fluid. The system operates evaporator 1 at , evaporator 2 at , and the condenser at . The cooling load of evaporator 1 is double that of evaporator 2. Determine the cooling load of both evaporators per unit of flow through the compressor, as well as the COP of this system. The refrigerant is saturated liquid at the exit of the condenser and saturated vapor at the exit of each evaporator, and the compressor is isentropic.

Reconsider Prob. 11–114E. The refrigeration system of that problem cools one reservoir at and one at while rejecting heat to a reservoir at . Which process has the highest energy destruction?

A two-stage compression refrigeration system with an adiabatic liquid-vapor separation unit as shown in Fig. P11–116 uses refrigerant-134a as the working fluid. The system operates the evaporator at , the condenser at , and the separator at . The refrigerant is circulated through the condenser at a rate of . Determine the rate of cooling and power requirement for this system. The refrigerant is saturated liquid at the inlet of each expansion valve and saturated vapor at the inlet of each compressor, and the compressors are isentropic.

Which process of the cycle in Prob. 11–116 has the greatest rate of exergy destruction when the low-temperature reservoir is at and the high-temperature reservoir is at ?

An aircraft on the ground is to be cooled by a gas refrigeration cycle operating with air on an open cycle. Air enters the compressor at and and is compressed to . Air is cooled to before it enters the turbine. Assuming both the turbine and the compressor to be isentropic, determine the temperature of the air leaving the turbine and entering the cabin.

Consider a regenerative gas refrigeration cycle using helium as the working fluid. Helium enters the compressor at and and is compressed to . Helium is then cooled to by water. It then enters the regenerator where it is cooled further before it enters the turbine. Helium leaves the refrigerated space at and enters the regenerator. Assuming both the turbine and the compressor to be isentropic, determine (a) the temperature of the helium at the turbine inlet, (b) the coefficient of performance of the cycle, and (c) the net power input required for a mass flow rate of .

The COP of vapor-compression refrigeration cycles improves when the refrigerant is subcooled before it enters the throttling valve. Can the refrigerant be subcooled indefinitely to maximize this effect, or is there a lower limit? Explain

An ice-making machine operates on the ideal vapor-compression cycle, using refrigerant-134a. The refrigerant enters the compressor as saturated vapor at and leaves the condenser as saturated liquid at . Water enters the ice machine at and leaves as ice at . For an ice production rate of , determine the power input to the ice machine (169 Btu of heat needs to be removed from each 1 bm of water at to turn it into ice at ).

A refrigerator operates on the ideal vapor-compression refrigeration cycle and uses refrigerant-134a as the working fluid. The condenser operates at and the evaporator at . If an adiabatic, reversible expansion device were available and used to expand the liquid leaving the condenser, how much would the COP improve by using this device instead of the throttle device?

An ideal vapor-compression refrigeration cycle that uses refrigerant-134a as its working fluid maintains a condenser at and the evaporator at . Determine this system's COP and the amount of power required to service a cooling load.

Consider a refrigeration system that operates on an ideal vapor-compression refrigeration cycle with refrigerant-134a as the working fluid. The refrigerant enters the compressor as saturated vapor at and is compressed to . Show the cycle on a T-s diagram with respect to saturation lines, and determine (a) the quality of the refrigerant at the end of the throttling process, (b) the coefficient of performance, and (c) the power input to the compressor.

Reconsider Prob. 1114. Using EES (or other) software, investigate the effect of evaporator pressure on the COP and the power input. Let the evaporator pressure vary from 100 to 400 kPa. Plot the COP and the power input as functions of evaporator pressure, and discuss the results. 1116 Repeat Prob. 1114 assuming an isentropic efficiency of 85 percent for the compressor. Also, determine the rate of exergy destruction associated with the compression process in this case. Take T0 5 298 K.

Refrigerant-134a enters the compressor of a refrigerator as superheated vapor at and at a rate of , and it leaves at and . The refrigerant is cooled in the condenser to and , and it is throttled to . Disregarding any heat transfer and pressure drops in the connecting lines between the components, show the cycle on a T-s diagram with respect to saturation lines, and determine (a) the rate of heat removal from the refrigerated space and the power input to the compressor, (b) the isentropic efficiency of the compressor, and (c) the COP of the refrigerator.

A commercial refrigerator with refrigerant-134a as the working fluid is used to keep the refrigerated space at by rejecting its waste heat to cooling water that enters the condenser at at a rate of and leaves at . The refrigerant enters the condenser at and and leaves at . The inlet state of the compressor is and and the compressor is estimated to gain a net heat of 450 W from the surroundings. Determine (a) the quality of the refrigerant at the evaporator inlet, (b) the refrigeration load, (c) the COP of the refrigerator, and (d) the theoretical maximum refrigeration load for the same power input to the compressor.

Refrigerant-134a enters the compressor of a refrigerator at and at a rate of and leaves at . The isentropic efficiency of the compressor is 78 percent. The refrigerant enters the throttling valve at and and leaves the evaporator as saturated vapor at . Show the cycle on a T-s diagram with respect to saturation lines, and determine (a) the power input to the compressor, (b) the rate of heat removal from the refrigerated space, and (c) the pressure drop and rate of heat gain in the line between the evaporator and the compressor.

Reconsider Prob. 1119. Using EES (or other) software, investigate the effects of varying the compressor isentropic efficiency over the range 60 to 100 percent and the compressor inlet volume flow rate from 0.1 to 1.0 m3 / min on the power input and the rate of refrigeration. Plot the rate of refrigeration and the power input to the compressor as functions of compressor efficiency for compressor inlet volume flow rates of 0.1, 0.5, and 1.0 m3 /min, and discuss the results.

A refrigerator uses refrigerant-134a as the working fluid and operates on the ideal vapor-compression refrigeration cycle except for the compression process. The refrigerant enters the evaporator at with a quality of 34 percent and leaves the compressor at . If the compressor consumes of power, determine (a) the mass flow rate of the refrigerant, (b) the condenser pressure, and (c) the COP of the refrigerator.

The manufacturer of an air conditioner claims a seasonal energy efficiency ratio (SEER) of for one of its units. This unit operates on the normal vapor compression refrigeration cycle and uses refrigerant-22 as the working fluid. This SEER is for the operating conditions when the evaporator saturation temperature is and the condenser saturation temperature is . Selected data for refrigerant-22 are provided in the table below. -5 421.2 38.76 248.1 0.9344 45 1728 101 261.9 0.8682 (a) Sketch the hardware and the T-s diagram for this air conditioner. (b) Determine the heat absorbed by the refrigerant in the evaporator per unit mass of refrigerant-22, in kJ/kg. (c) Determine the work input to the compressor and the heat rejected in the condenser per unit mass of refrigerant-22, in kJ/kg.

An actual refrigerator operates on the vapor-compression refrigeration cycle with refrigerant-22 as the working fluid. The refrigerant evaporates at and condenses at . The isentropic efficiency of the compressor is 83 percent. The refrigerant is superheated by at the compressor inlet and subcooled by at the exit of the condenser. Determine (a) the heat removed from the cooled space and the work input, in kJ/kg and the COP of the cycle. Determine (b) the same parameters if the cycle operated on the ideal vapor-compression refrigeration cycle between the same evaporating and condensing temperatures. The properties of R-22 in the case of actual operation are: , , . The properties of R-22 in the case of ideal operation are: , , Note: state 1: compressor inlet, state 2: compressor exit, state 3: condenser exit, state 4: evaporator inlet.

How is the second-law efficiency of a refrigerator operating on the vapor-compression refrigeration cycle defined? Provide two alternative definitions and explain each term.

How is the second-law efficiency of a heat pump operating on the vapor-compression refrigeration cycle defined? Provide two alternative definitions and show that one can be derived from the other.

Consider isentropic compressor of a vaporcompression refrigeration cycle. What are the isentropic efficiency and second-law efficiency of this compressor? Justify your answers. Is the second-law efficiency of a compressor necessarily equal to its isentropic efficiency? Explain.

A space is kept at 2158C by a vapor-compression refrigeration system in an ambient at 258C. The space gains heat steadily at a rate of 3500 kJ/h and the rate of heat rejection in the condenser is 5500 kJ/h. Determine the power input, in kW, the COP of the cycle and the second-law efficiency of the system.

Bananas are to be cooled from 288C to 128C at a rate of 1140 kg/h by a refrigerator that operates on a vaporcompression refrigeration cycle. The power input to the refrigerator is 8.6 kW. Determine (a) the rate of heat absorbed from the bananas, in kJ/h, and the COP, (b) the minimum power input to the refrigerator, and (c) the second-law efficiency and the exergy destruction for the cycle. The specific heat of bananas above freezing is 3.35 kJ/kg8C.

A vapor-compression refrigeration system absorbs heat from a space at 08C at a rate of 24,000 Btu/h and rejects heat to water in the condenser. The water experiences a temperature rise of 128C in the condenser. The COP of the system is estimated to be 2.05. Determine (a) the power input to the system, in kW, (b) the mass flow rate of water through the condenser, and (c) the second-law efficiency and the exergy destruction for the refrigerator. Take T0 5 208C and cp,water 5 4.18 kJ/kg8C.

A refrigerator operating on the vapor-compression refrigeration cycle using refrigerant-134a as the refrigerant is considered. The temperature of the cooled space and the ambient air are at 108F and 808F, respectively. R-134a enters the compressor at 20 psia as a saturated vapor and leaves at 140 psia and 1608F. The refrigerant leaves the condenser as a saturated liquid. The rate of cooling provided by the system is 45,000 Btu/h. Determine (a) the mass flow rate of R-134a and the COP, (b) the exergy destruction in each component of the cycle and the secondlaw efficiency of the compressor, and (c) the second-law efficiency of the cycle and the total exergy destruction in the cycle.

A room is kept at 258C by a vapor-compression refrigeration cycle with R-134a as the refrigerant. Heat is rejected to cooling water that enters the condenser at 208C at a rate of 0.13 kg/s and leaves at 288C. The refrigerant enters the condenser at 1.2 MPa and 508C and leave as a saturated liquid. If the compressor consumes 1.9 kW of power, determine (a) the refrigeration load, in Btu/h and the COP, (b) the second-law efficiency of the refrigerator and the total exergy destruction in the cycle, and (c) the exergy destruction in the condenser. Take T0 5 208C and cp,water 5 4.18 kJ/kg8C.

A refrigerator operates on the ideal vapor-compression refrigeration cycle with refrigerant-134a as the working fluid. The refrigerant evaporates at 2108C and condenses at 57.98C. The refrigerant absorbs heat from a space at 58C and rejects heat to ambient air at 258C. Determine (a) the cooling load, in kJ/kg, and the COP, (b) the exergy destruction in each component of the cycle and the total exergy destruction in the cycle, and (c) the second-law efficiency of the compressor, evaporator, and the cycle.

A refrigeration system operates on the ideal vaporcompression refrigeration cycle with ammonia as the refrigerant. The evaporator and condenser pressures are 200 kPa and 2000 kPa, respectively. The temperatures of the lowtemperature and high-temperature mediums are 298C and 278C, respectively. If the rate of heat rejected in the condenser is 18.0 kW, determine (a) the volume flow rate of ammonia at the compressor inlet, in L/s, (b) the power input and the COP, and (c) the second-law efficiency of the cycle and the total exergy destruction in the cycle. The properties of ammonia at various states are given as follows: h1 5 1439.3 kJ/kg, s15 5.8865 kJ/kgK, v1 5 0.5946 m3 /kg, h2 5 1798.3 kJ/kg, h3 5 437.4 kJ/kg, s3 5 1.7892 kJ/kgK, s4 5 1.9469 kJ/kgK. Note: state 1: compressor inlet, state 2: compressor exit, state 3: condenser exit, state 4: evaporator inlet

Using EES (or other) software, repeat Prob. 1133 if ammonia, R-134a, and R-22 is used as the refrigerant. Also, for the case of ammonia, investigate the effects of evaporator and condenser pressures on the COP, the second-law efficiency, and the total exergy destruction. Vary the evaporator pressure between 100 and 400 kPa and the condenser pressure between 1000 and 2000 kPa.

When selecting a refrigerant for a certain application, what qualities would you look for in the refrigerant?

Consider a refrigeration system using refrigerant- 134a as the working fluid. If this refrigerator is to operate in an environment at 308C, what is the minimum pressure to which the refrigerant should be compressed? Why?

A refrigerant-134a refrigerator is to maintain the refrigerated space at 2108C. Would you recommend an evaporator pressure of 0.12 or 0.14 MPa for this system? Why?

A refrigerator that operates on the ideal vaporcompression cycle with refrigerant-134a is to maintain the refrigerated space at 2108C while rejecting heat to the environment at 258C. Select reasonable pressures for the evaporator and the condenser, and explain why you chose those values.

A heat pump that operates on the ideal vaporcompression cycle with refrigerant-134a is used to heat a house and maintain it at 268C by using underground water at 148C as the heat source. Select reasonable pressures for the evaporator and the condenser, and explain why you chose those values.

Do you think a heat pump system will be more cost-effective in New York or in Miami? Why?

What is a water-source heat pump? How does the COP of a water-source heat pump system compare to that of an air-source system?

A heat pump that operates on the ideal vaporcompression cycle with refrigerant-134a is used to heat water from 15 to 458C at a rate of 0.12 kg/s. The condenser and evaporator pressures are 1.4 and 0.32 MPa, respectively. Determine the power input to the heat pump.

used to keep a space at 258C by absorbing heat from geothermal water that enters the evaporator at 508C at a rate of 0.065 kg/s and leaves at 408C. The refrigerant enters the evaporator at 208C with a quality of 23 percent and leaves at the inlet pressure as saturated vapor. The refrigerant loses 300 W of heat to the surroundings as it flows through the compressor and the refrigerant leaves the compressor at 1.4 MPa at the same entropy as the inlet. Determine (a) the degrees of subcooling of the refrigerant in the condenser, (b) the mass flow rate of the refrigerant, (c) the heating load and the COP of the heat pump, and (d) the theoretical minimum power input to the compressor for the same heating load.

Refrigerant-134a enters the condenser of a residential heat pump at 800 kPa and 508C at a rate of 0.022 kg/s and leaves at 750 kPa subcooled by 38C. The refrigerant enters the compressor at 200 kPa superheated by 48C. Determine (a) the isentropic efficiency of the compressor, (b) the rate of heat supplied to the heated room, and (c) the COP of the heat pump. Also, determine (d ) the COP and the rate of heat supplied to the heated room if this heat pump operated on the ideal vapor- compression cycle between the pressure limits of 200 and 800 kPa.

A heat pump that operates on the ideal vaporcompression cycle with refrigerant-134a is used to heat a house and maintain it at 758F by using underground water at 508F as the heat source. The house is losing heat at a rate of 60,000 Btu/h. The evaporator and condenser pressures are 50 and 120 psia, respectively. Determine the power input to the heat pump and the electric power saved by using a heat pump instead of a resistance heater.

A heat pump using refrigerant-134a heats a house by using underground water at 8C as the heat source. The house is losing heat at a rate of 60,000 kJ/h. The refrigerant enters the compressor at 280 kPa and 08C, and it leaves at 1 MPa and 608C. The refrigerant exits the condenser at 308C. Determine (a) the power input to the heat pump, (b) the rate of heat absorption from the water, and (c) the increase in electric power input if an electric resistance heater is used instead of a heat pump.

Reconsider Prob. 1146. Using EES (or other) software, investigate the effect of varying the compressor isentropic efficiency over the range 60 to 100 percent. Plot the power input to the compressor and the electric power saved by using a heat pump rather than electric resistance heating as functions of compressor efficiency, and discuss the results.

How does the COP of a cascade refrigeration system compare to the COP of a simple vapor-compression cycle operating between the same pressure limits?

A certain application requires maintaining the refrigerated space at 2328C. Would you recommend a simple refrigeration cycle with refrigerant-134a or a two-stage cascade refrigeration cycle with a different refrigerant at the bottoming cycle? Why?

Consider a two-stage cascade refrigeration cycle and a two-stage compression refrigeration cycle with a flash chamber. Both cycles operate between the same pressure limits and use the same refrigerant. Which system would you favor? Why?

Can a vapor-compression refrigeration system with a single compressor handle several evaporators operating at different pressures? How?

In the liquefaction process, why are gases compressed to very high pressures?

A two-stage compression refrigeration system operates with refrigerant-134a between the pressure limits of 1.4 and 0.10 MPa. The refrigerant leaves the condenser as a saturated liquid and is throttled to a flash chamber operating at 0.4 MPa. The refrigerant leaving the low-pressure compressor at 0.4 MPa is also routed to the flash chamber. The vapor in the flash chamber is then compressed to the condenser pressure by the high-pressure compressor, and the liquid is throttled to the evaporator pressure. Assuming the refrigerant leaves the evaporator as saturated vapor and both compressors are isentropic, determine (a) the fraction of the refrigerant that evaporates as it is throttled to the flash chamber, (b) the rate of heat removed from the refrigerated space for a mass flow rate of 0.25 kg/s through the condenser, and (c) the coefficient of performance.

Repeat Prob. 1153 for a flash chamber pressure of 0.6 MPa.

Reconsider Prob. 1153. Using EES (or other) software, investigate the effect of the various re frig erants for compressor efficiencies of 80, 90, and 100 percent. Compare the performance of the refrigeration system with different refrigerants.

Consider a two-stage cascade refrigeration system operating between the pressure limits of 0.8 and 0.14 MPa. Each stage operates on the ideal vapor-compression refrigeration cycle with refrigerant-134a as the working fluid. Heat rejection from the lower cycle to the upper cycle takes place in an adiabatic counterflow heat exchanger where both streams enter at about 0.4 MPa. If the mass flow rate of the refrigerant through the upper cycle is 0.24 kg/s, determine (a) the mass flow rate of the refrigerant through the lower cycle, (b) the rate of heat removal from the refrigerated space and the power input to the compressor, and (c) the coefficient of performance of this cascade refrigerator

Repeat Prob. 1156 for a heat exchanger pressure of 0.55 MPa.

Consider a two-stage cascade refrigeration system operating between the pressure limits of 1.4 MPa and 160 kPa with refrigerant-134a as the working fluid. Heat rejection from the lower cycle to the upper cycle takes place in an adiabatic counterflow heat exchanger where the pressure in the upper and lower cycles are 0.4 and 0.5 MPa, respectively. In both cycles, the refrigerant is a saturated liquid at the condenser exit and a saturated vapor at the compressor inlet, and the isentropic efficiency of the compressor is 80 percent. If the mass flow rate of the refrigerant through the lower cycle is 0.11 kg/s, determine (a) the mass flow rate of the refrigerant through the upper cycle, (b) the rate of heat removal from the refrigerated space, and (c) the COP of this refrigerator

Consider a two-stage cascade refrigeration system operating between the pressure limits of 1.2 MPa and 200 kPa with refrigerant-134a as the working fluid. The refrigerant leaves the condenser as a saturated liquid and is throttled to a flash chamber operating at 0.45 MPa. Part of the refrigerant evaporates during this flashing process, and this vapor is mixed with the refrigerant leaving the low-pressure compressor. The mixture is then compressed to the condenser pressure by the high-pressure compressor. The liquid in the flash chamber is throttled to the evaporator pressure and cools the refrigerated space as it vaporizes in the evaporator. The mass flow rate of the refrigerant through the lowpressure compressor is 0.15 kg/s. Assuming the refrigerant leaves the evaporator as a saturated vapor and the isentropic efficiency is 80 percent for both compressors, determine (a) the mass flow rate of the refrigerant through the high-pressure compressor, (b) the rate of heat removal from the refrigerated space, and (c) the COP of this refrigerator. Also, determine (d) the rate of heat removal and the COP if this refrigerator operated on a single-stage cycle between the same pressure limits with the same compressor efficiency and the same flow rate as in part (a).

A two-evaporator compression refrigeration system as shown in Fig. P11-60 uses refrigerant-134a as the working fluid. The system operates evaporator 1 at 08C, evaporator 2 at 226.48C, and the condenser at 800 kPa. The refrigerant is circulated through the compressor at a rate of 0.1 kg/s and the low-temperature evaporator serves a cooling load of 8 kW. Determine the cooling rate of the high-temperature evaporator, the power required by the compressor, and the COP of the system. The refrigerant is saturated liquid at the exit of the condenser and saturated vapor at the exit of each evaporator, and the compressor is isentropic.

A two-evaporator compression refrigeration system like that in Fig. P1160 uses refrigerant-134a as the working fluid. The system operates evaporator 1 at 30 psia, evaporator 2 at 10 psia, and the condenser at 180 psia. The cooling load for evaporator 1 is 9000 Btu/h and that for evaporator 2 is 24,000 Btu/h. Determine the power required to operate the compressor and the COP of this system. The refrigerant is saturated liquid at the exit of the condenser and saturated vapor at the exit of each evaporator, and the compressor is isentropic.

Repeat Prob. 1161E if the 30 psia evaporator is to be replaced with a 60 psia evaporator to serve a 15,000 Btu/h cooling load.

How does the ideal-gas refrigeration cycle differ from the Carnot refrigeration cycle?

Devise a refrigeration cycle that works on the reversed Stirling cycle. Also, determine the COP for this cycle.

How is the ideal-gas refrigeration cycle modified for aircraft cooling?

In gas refrigeration cycles, can we replace the turbine by an expansion valve as we did in vapor-compression refrigeration cycles? Why?

How do we achieve very low temperatures with gas refrigeration cycles?

Air enters the compressor of an ideal gas refrigeration cycle at 408F and 10 psia and the turbine at 1208F and 30 psia. The mass flow rate of air through the cycle is 0.5 lbm/s. Determine (a) the rate of refrigeration, (b) the net power input, and (c) the coefficient of performance.

An ideal gas refrigeration cycle using air as the working fluid is to maintain a refrigerated space at 2238C while rejecting heat to the surrounding medium at 278C. If the pressure ratio of the compressor is 3, determine (a) the maximum and minimum temperatures in the cycle, (b) the coefficient of performance, and (c) the rate of refrigeration for a mass flow rate of 0.08 kg/s.

Air enters the compressor of an ideal gas refrigeration cycle at 78C and 35 kPa and the turbine at 378C and 160 kPa. The mass flow rate of air through the cycle is 0.2 kg/s. Assuming variable specific heats for air, determine (a) the rate of refrigeration, (b) the net power input, and (c) the coefficient of performance.

Repeat Prob. 1170 for a compressor isentropic efficiency of 80 percent and a turbine isentropic efficiency of 85 percent.

Reconsider Prob. 1171. Using EES (or other) software, study the effects of compressor and turbine isentropic efficiencies as they are varied from 70 to 100 percent on the rate of refrigeration, the net power input, and the COP. Plot the T-s diagram of the cycle for the isentropic case.

A gas refrigeration cycle with a pressure ratio of 4 uses helium as the working fluid. The temperature of the helium is 268C at the compressor inlet and 508C at the turbine inlet. Assuming isentropic efficiencies of 85 percent for both the turbine and the compressor, determine (a) the minimum temperature in the cycle, (b) the coefficient of performance, and (c) the mass flow rate of the helium for a refrigeration rate of 25 kW.

A gas refrigeration system using air as the working fluid has a pressure ratio of 4. Air enters the compressor at 278C. The high-pressure air is cooled to 278C by rejecting heat to the surroundings. It is further cooled to 2158C by regenerative cooling before it enters the turbine. Assuming both the turbine and the compressor to be isentropic and using constant specific heats at room temperature, determine (a) the lowest temperature that can be obtained by this cycle, (b) the coefficient of performance of the cycle, and (c) the mass flow rate of air for a refrigeration rate of 12 kW.

Repeat Prob. 1174 assuming isentropic efficiencies of 75 percent for the compressor and 80 percent for the turbine.

A gas refrigeration system using air as the working fluid has a pressure ratio of 5. Air enters the compressor at 08C. The high-pressure air is cooled to 358C by rejecting heat to the surroundings. The refrigerant leaves the turbine at 2808C and then it absorbs heat from the refrigerated space before entering the regenerator. The mass flow rate of air is 0.4 kg/s. Assuming isentropic efficiencies of 80 percent for the compressor and 85 percent for the turbine and using constant specific heats at room temperature, determine (a) the effectiveness of the regenerator, (b) the rate of heat removal from the refrigerated space, and (c) the COP of the cycle. Also, determine (d ) the refrigeration load and the COP if this system operated on the simple gas refrigeration cycle. Use the same compressor inlet temperature as given, the same turbine inlet temperature as calculated, and the same compressor and turbine efficiencies.

An ideal gas refrigeration system with two stages of compression with intercooling as shown in Fig. P1177 operates with air entering the first compressor at 90 kPa and 2248C. Each compression stage has a pressure ratio of 3 and the two intercoolers can cool the air to 58C. Calculate the coefficient of performance of this system and the rate at which air must be circulated through this system to service a 45,000 kJ/h cooling load. Use constant specific heats at room temperature.

How will the answers of Prob. 1177 change when the isentropic efficiency of each compressor is 85 percent and the isentropic efficiency of the turbine is 95 percent?

What is absorption refrigeration? How does an ab sorption refrigeration system differ from a vapor-compression refrigeration system?

What are the advantages and disadvantages of absorption refrigeration?

In absorption refrigeration cycles, why is the fluid in the absorber cooled and the fluid in the generator heated?

How is the coefficient of performance of an absorption refrigeration system defined?

An absorption refrigeration system that receives heat from a source at 958C and maintains the refrigerated space at 08C is claimed to have a COP of 3.1. If the environmental temperature is 198C, can this claim be valid? Justify your answer.

An absorption refrigeration system receives heat from a source at 1208C and maintains the refrigerated space at 08C. If the temperature of the environment is 258C, what is the maximum COP this absorption refrigeration system can have?

Heat is supplied to an absorption refrigeration system from a geothermal well at 1108C at a rate of 5 3 105 kJ/h. The environment is at 258C, and the refrigerated space is maintained at 2188C. Determine the maximum rate at which this system can remove heat from the refrigerated space.

A reversible absorption refrigerator consists of a reversible heat engine and a reversible refrigerator. The system removes heat from a cooled space at 2158C at a rate of 70 kW. The refrigerator operates in an environment at 258C. If the heat is supplied to the cycle by condensing saturated steam at 1508C, determine (a) the rate at which the steam condenses, and (b) the power input to the reversible refrigerator. (c) If the COP of an actual absorption chiller at the same temperature limits has a COP of 0.8, determine the second-law efficiency of this chiller.

An ammonia-water absorption refrigeration cycle is used to keep a space at 258F when the ambient temperature is 708F. Pure ammonia enters the condenser at 300 psia and 1408F at a rate of 0.04 lbm/s. Ammonia leaves the condenser as a saturated liquid and is expanded to 30 psia. Ammonia leaves the evaporator as a saturated vapor. Heat is supplied to the generator by geothermal liquid water that enters at 2408F at a rate of 0.55 lbm/s and leaves at 2008F. Determine (a) the rate of cooling provided by the system, in Btu/h, the COP, and (b) the second-law efficiency of the system. The enthalpies of ammonia at various states of the system are: condenser inlet h2 5 665.7 Btu/lbm, evaporator inlet h4 5 190.9 Btu/lbm, evaporator exit h1 5 619.2 Btu/lbm. Also, take the specific heat of geothermal water to be 1.0 Btu/lbm8F.

What is a thermoelectric circuit?

Describe the Seebeck and the Peltier effects.

Consider a circular copper wire formed by connecting the two ends of a copper wire. The connection point is now heated by a burning candle. Do you expect any current to flow through the wire?

An iron and a constantan wire are formed into a closed circuit by connecting the ends. Now both junctions are heated and are maintained at the same temperature. Do you expect any electric current to flow through this circuit?

A copper and a constantan wire are formed into a closed circuit by connecting the ends. Now one junction is heated by a burning candle while the other is maintained at room temperature. Do you expect any electric current to flow through this circuit?

How does a thermocouple work as a temperature measurement device?

Why are semiconductor materials preferable to metals in thermoelectric refrigerators?

Is the efficiency of a thermoelectric generator limited by the Carnot efficiency? Why?

A thermoelectric generator receives heat from a source at 3408F and rejects the waste heat to the environment at 908F. What is the maximum thermal efficiency this thermoelectric generator can have?

A thermoelectric refrigerator removes heat from a refrigerated space at 258C at a rate of 130 W and rejects it to an environment at 208C. Determine the maximum coefficient of performance this thermoelectric refrigerator can have and the minimum required power input.

A thermoelectric cooler has a COP of 0.15 and removes heat from a refrigerated space at a rate of 180 W. Determine the required power input to the thermoelectric cooler, in W.

A thermoelectric cooler has a COP of 0.18 and the power input to the cooler is 1.8 hp. Determine the rate of heat removed from the refrigerated space, in Btu/min.

A thermoelectric refrigerator is powered by a 12-V car battery that draws 3 A of current when running. The refrigerator resembles a small ice chest and is claimed to cool nine canned drinks, 0.350-L each, from 25 to 38C in 12 h. Determine the average COP of this refrigerator.

Thermoelectric coolers that plug into the cigarette lighter of a car are commonly available. One such cooler is claimed to cool a 12-oz (0.771-lbm) drink from 78 to 388F or to heat a cup of coffee from 75 to 1308F in about 15 min in a well-insulated cup holder. Assuming an average COP of 0.2 in the cooling mode, determine (a) the average rate of heat removal from the drink, (b) the average rate of heat supply to the coffee, and (c) the electric power drawn from the battery of the car, all in W.

It is proposed to run a thermoelectric generator in conjunction with a solar pond that can supply heat at a rate of 7 3 106 kJ/h at 908C. The waste heat is to be rejected to the environment at 228C. What is the maximum power this thermoelectric generator can produce?

A typical 200-m2 house can be cooled adequately by a 3.5-ton air conditioner whose COP is 4.0. Determine the rate of heat gain of the house when the air conditioner is running continuously to maintain a constant temperature in the house.

Consider a steady-flow Carnot refrigeration cycle that uses refrigerant-134a as the working fluid. The maximum and minimum temperatures in the cycle are 30 and 2208C, respectively. The quality of the refrigerant is 0.15 at the beginning of the heat absorption process and 0.80 at the end. Show the cycle on a T-s diagram relative to saturation lines, and determine (a) the coefficient of performance, (b) the condenser and evaporator pressures, and (c) the net work input.

A heat pump water heater (HPWH) heats water by absorbing heat from the ambient air and transferring it to water. The heat pump has a COP of 3.4 and consumes 6 kW of electricity when running. Determine if this heat pump can be used to meet the cooling needs of a room most of the time for free by absorbing heat from the air in the room. The rate of heat gain of a room is usually less than 45,000 kJ/h.

A heat pump that operates on the ideal vaporcompression cycle with refrigerant-134a is used to heat a house. The mass flow rate of the refrigerant is 0.25 kg/s. The condenser and evaporator pressures are 1400 and 320 kPa, respectively. Show the cycle on a T-s diagram with respect to saturation lines, and determine (a) the rate of heat supply to the house, (b) the volume flow rate of the refrigerant at the compressor inlet, and (c) the COP of this heat pump.

A large refrigeration plant is to be maintained at 2158C, and it requires refrigeration at a rate of 100 kW. The condenser of the plant is to be cooled by liquid water, which experiences a temperature rise of 88C as it flows over the coils of the condenser. Assuming the plant operates on the ideal vapor-compression cycle using refrigerant-134a between the pressure limits of 120 and 700 kPa, determine (a) the mass flow rate of the refrigerant, (b) the power input to the compressor, and (c) the mass flow rate of the cooling water.

Reconsider Prob. 11107. Using EES (or other) software, investigate the effect of evaporator pressure on the COP and the power input. Let the evaporator pressure vary from 120 to 380 kPa. Plot the COP and the power input as functions of evaporator pressure, and discuss the results.

Repeat Prob. 11107 assuming the compressor has an isentropic efficiency of 75 percent. Also, determine the rate of exergy destruction associated with the compression process in this case. Take T0 5 258C.

A refrigeration unit operates on the ideal vapor compression refrigeration cycle and uses refrigerant-22 as the working fluid. The operating conditions for this unit are evaporator saturation temperature of 258C and the condenser saturation temperature of 458C. Selected data for refrigerant-22 are provided in the table below. T, 8C Psat, kPa hf , kJ/kg hg, kJ/kg sg, kJ/kgK 25 421.2 38.76 248.1 0.9344 45 1728 101 261.9 0.8682 For R-22 at P 5 1728 kPa and s 5 0.9344 kJ/kgK, T 5 68.158C and h 5 283.7 kJ/kg. Also, take cp,air 5 1.005 kJ/kgK. (a) Sketch the hardware and the T-s diagram for this heat pump application. (b) Determine the COP for this refrigeration unit. (c) The evaporator of this unit is located inside the air handler of the building. The air flowing through the air handler enters the air handler at 278C and is limited to a 208C temperature drop. Determine the ratio of volume flow rate of air entering the air handler (m3 air/min) to mass flow rate of R-22 (kgR-22/s) through the air handler, in (m3 air/min)/(kgR-22/s). Assume the air pressure is 100 kPa.

An air conditioner with refrigerant-134a as the working fluid is used to keep a room at 268C by rejecting the waste heat to the outside air at 348C. The room is gaining heat through the walls and the windows at a rate of 250 kJ/min while the heat generated by the computer, TV, and lights amounts to 900 W. An unknown amount of heat is also generated by the people in the room. The condenser and evaporator pressures are 1200 and 500 kPa, respectively. The refrigerant is saturated liquid at the condenser exit and saturated vapor at the compressor inlet. If the refrigerant enters the compressor at a rate of 100 L/min and the isentropic efficiency of the compressor is 75 percent, determine (a) the temperature of the refrigerant at the compressor exit, (b) the rate of heat generation by the people in the room, (c) the COP of the air conditioner, and (d) the minimum volume flow rate of the refrigerant at the compressor inlet for the same compressor inlet and exit conditions.

An air-conditioner operates on the vapor- compression refrigeration cycle with refrigerant-134a as the refrigerant. The air conditioner is used to keep a space at 218C while rejecting the waste heat to the ambient air at 378C. The refrigerant enters the compressor at 180 kPa superheated by 2.78C at a rate of 0.06 kg/s and leaves the compressor at 1200 kPa and 608C. R-134a is subcooled by 6.38C at the exit of the condenser. Determine (a) the rate of cooling provided to the space, in Btu/h, and the COP, (b) the isentropic efficiency and the exergy efficiency of the compressor, (c) the exergy destruction in each component of the cycle and the total exergy destruction in the cycle, and (d) the minimum power input and the second-law efficiency of the cycle.

Consider a two-stage compression refrigeration system operating between the pressure limits of 1.4 and 0.12 MPa. The working fluid is refrigerant-134a. The refrigerant leaves the condenser as a saturated liquid and is throttled to a flash chamber operating at 0.5 MPa. Part of the refrigerant evaporates during this flashing process, and this vapor is mixed with the refrigerant leaving the low-pressure compressor. The mixture is then compressed to the condenser pressure by the high-pressure compressor. The liquid in the flash chamber is throttled to the evaporator pressure, and it cools the refrigerated space as it vaporizes in the evaporator. Assuming the refrigerant leaves the evaporator as saturated vapor and both compressors are isentropic, determine (a) the fraction of the refrigerant that evaporates as it is throttled to the flash chamber, (b) the amount of heat removed from the refrigerated space and the compressor work per unit mass of refrigerant flowing through the condenser, and (c) the coefficient of performance.

A two-evaporator compression refrigeration system as shown in Fig. P11114E uses refrigerant-134a as the working fluid. The system operates evaporator 1 at 308F, evaporator 2 at 229.58F, and the condenser at 160 psia. The cooling load of evaporator 1 is double that of evaporator 2. Determine the cooling load of both evaporators per unit of flow through the compressor, as well as the COP of this system. The refrigerant is saturated liquid at the exit of the condenser and saturated vapor at the exit of each evaporator, and the compressor is isentropic.

Reconsider Prob. 11114E. The refrigeration system of that problem cools one reservoir at 2158F and one at 408F while rejecting heat to a reservoir at 808F. Which process has the highest exergy destruction?

A two-stage compression refrigeration system with an adiabatic liquid-vapor separation unit as shown in Fig. P11116 uses refrigerant-134a as the working fluid. The system operates the evaporator at 2328C, the condenser at 1400 kPa, and the separator at 8.98C. The refrigerant is circulated through the condenser at a rate of 2 kg/s. Determine the rate of cooling and power requirement for this system. The refrigerant is saturated liquid at the inlet of each expansion valve and saturated vapor at the inlet of each compressor, and the compressors are isentropic.

Which process of the cycle in Prob. 11116 has the greatest rate of exergy destruction when the low-temperature reservoir is at 2228C and the high-temperature reservoir is at 208C?

An aircraft on the ground is to be cooled by a gas refrigeration cycle operating with air on an open cycle. Air enters the compressor at 308C and 100 kPa and is compressed to 250 kPa. Air is cooled to 708C before it enters the turbine. Assuming both the turbine and the compressor to be isentropic, determine the temperature of the air leaving the turbine and entering the cabin.

Consider a regenerative gas refrigeration cycle using helium as the working fluid. Helium enters the compressor at 100 kPa and 2108C and is compressed to 300 kPa. Helium is then cooled to 208C by water. It then enters the regenerator where it is cooled further before it enters the turbine. Helium leaves the refrigerated space at 2258C and enters the regenerator. Assuming both the turbine and the compressor to be isentropic, determine (a) the temperature of the helium at the turbine inlet, (b) the coefficient of performance of the cycle, and (c) the net power input required for a mass flow rate of 0.45 kg/s.

An absorption refrigeration system is to remove heat from the refrigerated space at 28C at a rate of 28 kW while operating in an environment at 258C. Heat is to be supplied from a solar pond at 958C. What is the minimum rate of heat supply required?

Reconsider Prob. 11120. Using EES (or other) software, investigate the effect of the source temperature on the minimum rate of heat supply. Let the source temperature vary from 50 to 2508C. Plot the minimum rate of heat supply as a function of source temperature, and discuss the results.

A gas refrigeration system using air as the working fluid has a pressure ratio of 5. Air enters the compressor at 08C. The high-pressure air is cooled to 358C by rejecting heat to the surroundings. The refrigerant leaves the turbine at 2808C and enters the refrigerated space where it absorbs heat before entering the regenerator. The mass flow rate of air is 0.4 kg/s. Assuming isentropic efficiencies of 80 percent for the compressor and 85 percent for the turbine and using variable specific heats, determine (a) the effectiveness of the regenerator, (b) the rate of heat removal from the refrigerated space, and (c) the COP of the cycle. Also, determine (d) the refrigeration load and the COP if this system operated on the simple gas refrigeration cycle. Use the same compressor inlet temperature as given, the same turbine inlet temperature as calculated, and the same compressor and turbine efficiencies.

The refrigeration system of Fig. P11123 is another variation of the basic vapor-compression refrigeration system which attempts to reduce the compression work. In this system, a heat exchanger is used to superheat the vapor entering the compressor while subcooling the liquid exiting from the condenser. Consider a system of this type that uses refrigerant-134a as its refrigerant and operates the evaporator at 210.098C, and the condenser at 900 kPa. Determine the system COP when the heat exchanger provides 5.518C of subcooling at the throttle valve entrance. Assume the refrigerant leaves the evaporator as a saturated vapor and the compressor is isentropic

Repeat Prob. 11123 if the heat exchanger provides 9.518C of subcooling.

An ideal gas refrigeration system with three stages of compression with intercooling operates with air entering the first compressor at 50 kPa and 2308C. Each compressor in this system has a pressure ratio of 7, and the air temperature at the outlet of all intercoolers is 158C. Calculate the COP of this system. Use constant specific heats at room temperature.

Using EES (or other) software, investigate the effect of the evaporator pressure on the COP of an ideal vapor-compression refrigeration cycle with R-134a as the working fluid. Assume the condenser pressure is kept constant at 1.4 MPa while the evaporator pressure is varied from 100 kPa to 500 kPa. Plot the COP of the refrigeration cycle against the evaporator pressure, and discuss the results

Using EES (or other) software, investigate the effect of the condenser pressure on the COP of an ideal vapor-compression refrigeration cycle with R-134a as the working fluid. Assume the evaporator pressure is kept constant at 150 kPa while the condenser pressure is varied from 400 to 1400 kPa. Plot the COP of the refrigeration cycle against the condenser pressure, and discuss the results.

Derive a relation for the COP of the two-stage refrigeration system with a flash chamber as shown in Fig. 1114 in terms of the enthalpies and the quality at state 6. Consider a unit mass in the condenser.

Consider a heat pump that operates on the reversed Carnot cycle with R-134a as the working fluid executed under the saturation dome between the pressure limits of 140 and 800 kPa. R-134a changes from saturated vapor to saturated liquid during the heat rejection process. The net work input for this cycle is (a) 28 kJ/kg (b) 34 kJ/kg (c) 49 kJ/kg (d ) 144 kJ/kg (e) 275 kJ/kg

A refrigerator removes heat from a refrigerated space at 08C at a rate of 2.2 kJ/s and rejects it to an environment at 208C. The minimum required power input is (a) 89 W (b) 150 W (c) 161 W (d ) 557 W (e) 2200 W

A refrigerator operates on the ideal vapor compression refrigeration cycle with R-134a as the working fluid between the pressure limits of 120 and 800 kPa. If the rate of heat removal from the refrigerated space is 32 kJ/s, the mass flow rate of the refrigerant is (a) 0.19 kg/s (b) 0.15 kg/s (c) 0.23 kg/s (d ) 0.28 kg/s (e) 0.81 kg/s

A heat pump operates on the ideal vapor compression refrigeration cycle with R-134a as the working fluid between the pressure limits of 0.32 and 1.2 MPa. If the mass flow rate of the refrigerant is 0.193 kg/s, the rate of heat supply by the heat pump to the heated space is (a) 3.3 kW (b) 23 kW (c) 26 kW (d ) 31 kW (e) 45 kW

An ideal vapor compression refrigeration cycle with R-134a as the working fluid operates between the pressure limits of 120 kPa and 700 kPa. The mass fraction of the refrigerant that is in the liquid phase at the inlet of the evaporator is (a) 0.69 (b) 0.63 (c) 0.58 (d ) 0.43 (e) 0.35

Consider a heat pump that operates on the ideal vapor compression refrigeration cycle with R-134a as the working fluid between the pressure limits of 0.32 and 1.2 MPa. The coefficient of performance of this heat pump is (a) 0.17 (b) 1.2 (c) 3.1 (d) 4.9 (e) 5.9

An ideal gas refrigeration cycle using air as the working fluid operates between the pressure limits of 80 and 280 kPa. Air is cooled to 358C before entering the turbine. The lowest temperature of this cycle is (a) 2588C (b) 2268C (c) 58C (d) 118C (e) 248C

Consider an ideal gas refrigeration cycle using helium as the working fluid. Helium enters the compressor at 100 kPa and 178C and compressed to 400 kPa. Helium is then cooled to 208C before it enters the turbine. For a mass flow rate of 0.2 kg/s, the net power input required is (a) 28.3 kW (b) 40.5 kW (c) 64.7 kW (d) 93.7 kW (e) 113 kW

An absorption air-conditioning system is to remove heat from the conditioned space at 208C at a rate of 150 kJ/s while operating in an environment at 358C. Heat is to be supplied from a geothermal source at 1408C. The minimum rate of heat supply is (a) 86 kJ/s (b) 21 kJ/s (c) 30 kJ/s (d) 61 kJ/s (e) 150 kJ/s

Consider a refrigerator that operates on the vapor compression refrigeration cycle with R-134a as the working fluid. The refrigerant enters the compressor as saturated vapor at 160 kPa, and exits at 800 kPa and 508C, and leaves the condenser as saturated liquid at 800 kPa. The coefficient of performance of this refrigerator is (a) 2.6 (b) 1.0 (c) 4.2 (d) 3.2 (e) 4.4

Write an essay on air-, water-, and soil-based heat pumps. Discuss the advantages and the disadvantages of each system. For each system identify the conditions under which that system is preferable over the other two. In what situations would you not recommend a heat pump heating system?

Design a vapor-compression refrigeration system that will maintain the refrigerated space at 2158C while operating in an environment at 208C using refrigerant-134a as the working fluid.

Design a thermoelectric refrigerator that is capable of cooling a canned drink in a car. The refrigerator is to be powered by the cigarette lighter of the car. Draw a sketch of your design. Semiconductor components for building thermoelectric power generators or refrigerators are available from several manufacturers. Using data from one of these manufacturers, determine how many of these components you need in your design, and estimate the coefficient of performance of your system. A critical problem in the design of thermoelectric refrigerators is the effective rejection of waste heat. Discuss how you can enhance the rate of heat rejection without using any devices with moving parts such as a fan. 11142 The temperature in a car parked in the sun can approach 1008C when the outside air temperature is just 258C, and it is desirable to ventilate the parked car to avoid such high temperatures. However, the ventilating fans may run down the battery if they are powered by it. To avoid that happening, it is proposed to use the PV cells discussed in the preceding problem to power the fans. It is determined that the air in the car should be replaced once every minute to avoid excessive rise in the interior temperature. Determine if this can be accomplished by installing PV cells on part of the roof of the car. Also, find out if any car is currently ventilated this way.

It is proposed to use a solar-powered thermoelectric system installed on the roof to cool residential buildings. The system consists of a thermoelectric refrigerator that is powered by a thermoelectric power generator whose top surface is a solar collector. Discuss the feasibility and the cost of such a system, and determine if the proposed system installed on one side of the roof can meet a significant portion of the cooling requirements of a typical house in your area.

A refrigerator using R-12 as the working fluid keeps the refrigerated space at 2158C in an environment at 308C. You are asked to redesign this refrigerator by replacing R-12 with the ozone-friendly R-134a. What changes in the pressure levels would you suggest in the new system? How do you think the COP of the new system will compare to the COP of the old system?

A company owns a refrigeration system whose refrigeration capacity is 200 tons (1 ton of refrigeration 5 211 kJ/min), and you are to design a forced-air cooling system for fruits whose diameters do not exceed 7 cm under the following conditions: The fruits are to be cooled from 288C to an average temperature of 88C. The air temperature is to remain above 228C and below 108C at all times, and the velocity of air approaching the fruits must remain under 2 m/s. The cooling section can be as wide as 3.5 m and as high as 2 m. Assuming reasonable values for the average fruit density, specific heat, and porosity (the fraction of air volume in a box), recommend reasonable values for (a) the air velocity approaching the cooling section, (b) the product-cooling capacity of the system, in kgfruit/h, and (c) the volume flow rate of air.

In the 1800s, before the development of modern air-conditioning, it was proposed to cool air for buildings with the following procedure using a large pistoncylinder device [John Gorrie: Pioneer of Cooling and Ice Making, ASHRAE Journal 33, no. 1 (Jan. 1991)]: 1. Pull in a charge of outdoor air. 2. Compress it to a high pressure. 3. Cool the charge of air using outdoor air. 4. Expand it back to atmospheric pressure. 5. Discharge the charge of air into the space to be cooled. Suppose the goal is to cool a room 6 m 3 10 m 3 2.5 m. Outdoor air is at 308C, and it has been determined that 10 air changes per hour supplied to the room at 108C could provide adequate cooling. Do a preliminary design of the system and do calculations to see if it would be feasible. (You may make optimistic assumptions for the analysis.) (a) Sketch the system showing how you will drive it and how step 3 will be accomplished. (b) Determine what pressure will be required (step 2). (c) Estimate (guess) how long step 3 will take and what size will be needed for the pistoncylinder to provide the required air changes and temperature. (d) Determine the work required in step 2 for one cycle and per hour. (e) Discuss any problems you see with the concept of your design. (Include discussion of any changes that may be required to offset optimistic assumptions.)