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Heat ExchangersA superheater brings 2.5 kg/s of saturated
Chapter 4, Problem 96HP(choose chapter or problem)
Problem 96HP
Heat Exchangers
A superheater brings 2.5 kg/s of saturated water vapor at 2 MPa to 450°C. The energy is provided by hot air at 1200 K flowing outside the steam tube in the opposite direction as the water, a setup known as a counterflowing heat exchanger (similar to Fig. P4.95). Find the smallest possible mass flow rate of the air to ensure that its exit temperature is 20°C larger than the incoming water temperature.
FIGURE P4.95
Questions & Answers
QUESTION:
Problem 96HP
Heat Exchangers
A superheater brings 2.5 kg/s of saturated water vapor at 2 MPa to 450°C. The energy is provided by hot air at 1200 K flowing outside the steam tube in the opposite direction as the water, a setup known as a counterflowing heat exchanger (similar to Fig. P4.95). Find the smallest possible mass flow rate of the air to ensure that its exit temperature is 20°C larger than the incoming water temperature.
FIGURE P4.95
ANSWER:
Solution 96HP
Step 1 of 3:
A superheater makes the saturated water to 450°C. The energy is supplied by the hot air which is flowing in the opposite direction to the flow of water. We are going to find the mass flow rate of air.
The initial temperature of air T1 = 1200 K
The initial pressure of water Pw = 2 MPa = 2000 kPa
The final temperature of water T4 = 450°C
The initial temperature of water T3 = 212.42°C (From the table B.1.3 for 2 MPa)
Since the final temperature of air is increased by 20°C as that of the initial temperature of water.
The final temperature of air T2 = 212.42°C + 20°C = 232.42°C = 506 K
The mass flow rate of water w = 2.5 kg/s