HVAC Systems ME 414
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This 21 page Class Notes was uploaded by Roman Jaskolski on Friday October 23, 2015. The Class Notes belongs to ME 414 at University of Idaho taught by Ralph Budwig in Fall. Since its upload, it has received 86 views. For similar materials see /class/227896/me-414-university-of-idaho in Mechanical Engineering at University of Idaho.
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Date Created: 10/23/15
Topch Cool Equlp doc 1er 414514 HVAC Systems Tupi 14 Cnnling Equipment Ref gem nn Cycles Vapor Cnmpressinn Cycle VCC Basrc components ofa mechamcal r compressror chller Transraor 1 2 c rsehtroprc compressror Transraor 4 1 crsobanc evaporatror occurs and evaporaaor superheaarrg occurs and compressor emcrehcy Frrctror losses and heatlossgam occurl all the components and plumblng 5 performance cm smgytmovsa pro WW h e W Example 10 l walks through the calculauohs for an ldeal VCC 1r addluon to coolmg molustrral space dlrectly walkrln freezers refugerator trucks an m Topicl4 Cool Equipdoc the air handlers The coupling of a VCC with waterrefrigerant heat exchangers is called a chiller Many different working uids 7 refrigerants 7 are available R123 R134a R22 We are fortunate to have access to actual refrigerant properties in EES Several types of compressors are in common use 7 reciprocating centrifugal screw rotary and scroll the scroll compressor was patented by Neils Young an inventor who retired to Boise in the 1990s The work input for an actual polytropic compression cycle pv constant is WUn n71 71 n is the polytropic exponent and in general it is not the ratio of specific heats Example 103 compares work in using a polytropic compression analysis in comparison with isentropic compression Specific characteristics of different compressors are related to their performance reciprocating compressors rpm bore stroke centrifugal compressors torque rpm rotor radius Example 104 uses vendor information for a reciprocal compressor in Figure 109 to find capacity and COP Example 105 looks at a centrifugal compressor The part load ratio PLR of a chiller is defined as the actual cooling load at any particular time divided by the capacity at full load as rated by the manufacturer 7 QLa t 7 Q5001 PLR QL All Q uz The power input to the compressor at part load is then Wm QM ABPLR CPLRY COPM where A B and C are curve fit constants from vendor data Table 104 includes these for sample chillers It is better to use PLR data from the manufacturer of the actual compressor of interest Example 107 illustrates the use of actual PLR data Absorption Cycle Absorption chillers AC have much lower power input demand than a VCC chiller because the compressor is replaced by a pump and a high temperature heat source The system also operates at pressures below atmospheric Small absorption refrigerators are common in RVs cabins without electricity and mobile medical units A propane burner Topch Cool Equlp oloc also common Flgure 10 4 on p 501 presents a schematrc for an AC showmg the n w l w But the compressor ls replaceol by a pump an absorber and a generator gure ms sanematc meng at ample hat Absolplloll cycle lame ten otme wncal nasnea We the svslem ts the same as hc v0 CVEKE 1 Wm row amt V TL concurct ohm lnnn am mner er pf wd lntheLrBr t m th r m r m ls the lowrpressure watervaporrn the evaporator and LlBrls the absorbant 1n ammoma systems ammoma ls the refngerant and water ls the absorbent 1n Flgure 10 4 for a HzorLlBr system the thermodynamrc states are ldennfled as follows schematac below of a solar powered HzorLlBr absorptaon chlller In thls schematlc the thermodynamlc states are ldennfled as follows Topicl4 Cool Equipdoc Note the counter intuitive result that because energy must be removed for absorption to occur the enthalpy of the waterLiBr solution at State 1 is less than the enthalpies of each component taken separately Because it is thermal energy not electrical power that is input to an AC the de nition of COP differs energy removed Qevap COP AC energy in Q gm Example 102 looks at a LiBr system Referring to Figure 104 m th m1 total mass balance thX2 rthl LiBr mass balance X refers to the mass fraction of LiBr in the uid Q39g m3h3 rh2h2 irhlhi generator energy balance Q39md 13013 424 condenser energy balance iwater only QM rhshs rh2h2 7 r39nlh1 absorber energy balance Q39mp 15025 424 evaporator energy balance 7 water only Q system energy balance TopmM Cool Equip do u m eeneensme Wnter cmuee WntEr uur lt13 Absorber en Exchanger Solar Powered Two shell HzoerBr Absorption System Blue WeakaBr soluuon Aqua strong LxBrsoluuonPmk Watersteam TopmM Cool Equrp doe Frgure 10 5 or p 503 rs very useful wrth the abrhty to fmd 2 ofthe 4 heeded values temperature pressure mass fraeuoh x of mar and ehtha1py xfthe other 2 are known 55 ms Ihs LlBr Enlha pyrwnnmve m ntagtam I rma 59 War I swim a 7 22 kJkg mam Inwule was Technomay W mpermsswnj m aumtpymrutthmt w t u nasuutmsmmo 01504006050055 am mu umsom Mm nxlmu x at mum humude mum vcc Here Qxs the heat supply rate m terms of steam produeuoh m the generator g mam c FLRZ arm HR The eohstauts A B and c are from eurve ts tomauufaeturer39s data It rs better to use aetua1 data from the mauufaeturer for part 1oad aua1ysrs Cnnling Tuwers tower Warm Waterfrom the eohdehser emtrs sprayedmto an outdoor an stream Topch Cool Equip doc Evaporauve coonng occurs anomnc cooled water s returnedto the condenser Energy total energy m the Water 1 to 4 ms called afarced rimHowe Ifthe fans are located at unc top ofthe tower nus called an buoyancy as unc auwarms to create a stack effect quotin hnn Wo I quothawu W 5 W Coohng tower overall mass and energy balances Coohng tower analysts munc effectiveness nncunod starts wnn an overall energy balance on a coonng tower Mn 7 NF MC V 7 TnsmnW WJCMTW y r y y m smusnn mass owrateofdrymnsconstant w 7mm rm unc mass ow rate ofwater changes because ofwaterthat is evaporatedmto the am however unc amount of Water evaporatedxs small audit is common to assume that unc er ow rate through unc tower is constant Topicl4 Cool Equipdoc rhva ma W7 7W1 mass ow rate of evaporated water Q39 lierNTU rR 7 MW R eff quot ofthe cooling tower ma hamm ha liR e7 39 glow2r Q is the heat rejection rate set by the condenser load of the chiller using the design cooling load from a cooling load analysis and the work input to the chiller s compressor rh C a a sat R 7 p capacity ratio mw pr NTU number of transfer units must be obtained from manufacturer s data C 7 hasatx hagam pasal T wx w0 The capacity is just the amount of heat transferred from the water QM y rthW TW 7 Tm capacity Range Tm iTw the range is the temperature drop in the water Approach Tm 7 Maw the approach is the difference between the water exit temperature and the entering air wb temperature The water ow rate is determined by the condenser specifications The temperature drop of the water across the cooling tower is found from the heat rejection rate and the water ow rate The outlet water temperature is recommended by the manufacturer The outdoor air conditions used in calculations are set by local weather the 1 outdoor wb temperature Example 106 illustrates this process Care must be taken to avoid standing water in the cooling tower as bacteria growth has resulted Topicl4 Cool Equipdoc Evaporative Coolers Evaporative coolers swamp coolers work well in hot dry climates Direct evaporative coolers work by pulling outdoor air through lters to remove particulates The air then passes through a media kept moist by water at a lower temperature than the air Water is evaporated into the air causing sensible and latent cooling Swap Tdt 7Td0 T114 Tw Q39Smyml pCFVAT rule of thumb for residences AT 8 F 44 C is the temperature difference between the room setpoint and the air exiting the cooler effectiveness of the evaporative cooler Evaporative coolers near Mountain Home Idaho used to increase gas turbine efficiency As the incoming air cools its density increases Hence for the same volumetric ow rate a greater mass of air is pulled into the turbine Moist air also improves combustion efficiency take ME529 Combustion and Air Pollution to learn more Topicl4 C001 Equipdoc Topicl4 Cool Equipdoc quotI39 l Close up of evaporative cooler media A criticism of direct evaporative coolers is that they humidity the air Which may not be desired but can be a merit in dry desert climates Indirect evapora ve coolers avoid this problem They are essentially 2stage coolers In the rst stage a direct evaporative cooler cools an air stream secondary air The cooled air stream is used in an airtoair heat exchanger to cool the air used in the HVAC system primary air An advantage of an indirect evaporative cooler is that it can use Topmm Cool Eqmp doc ns Figure 1022 Indirect evapora ve cooler system dwagvam Return aiv rem buuding ls opuonal Mam exhaust nix Optional buildmg remm 321 CM Cuo d an a building 13 lndimx cuula heat exchanger Tm 0 TH m a E TarTw W Tda Tw 5 m if 0m In TV Topicl4 Cool Equipdoc The capacitance rate C is the speci c heat of the air stream times its mass ow rate The capacitance rate for the building is that for the primary air The minimum capacitance rate is from the heat exchanger design Q5001 PF C1713 TN Twsecx Example 109 looks at a bin analysis for an indirect evaporative cooler Bin analysis of the economizer mode as applied to a Boise office building with a balance temperature of 40 F a supply temperature of 65 F and a return temperature of 77 F The cooling load including ventilations loads is 300 tons when the outside temperature is 100 F and there is a linear variation in cooling load with outside temperature Find the annual energy consumption with and without operation of the economizer mode Find the difference in cooling cost if electricity is being purchased at 010 per kWh Use the 300 ton chiller by manufacturer A on the text CD low Self preserving flow FIGURE 636 Demils of the eally development of a real jet 47S VISCOUS FLUID How v 5 1 FE NK WHITE 1 00 quot O 75 12 DE 050 experiments 0 25 Eq 6152 Eq 6153 0 005 010 015 03920 025 z x SEE EQUATION Ill To PRED 4 peaN gmemm 0504 CkVD gpmgno at 7amp7 Diffusers Registers amp Grilles Page 1 of 3 Diffusers Registers amp Grilles Round Ceiling Diffusers Diffusers have a stepdown face design for superior air distribution Made of stamped steel with a white nish Mor hardware is included Collars sold separately are used to connect the diffuse round duct They have an openring design for continuous I 39 v and are made of galvanized steel Dl user 00 Damper Dampers sold separately provide opento close adjust air ow They include an installation collar and operating kr Made of galvanized steel For 39 Duct Diffusers Collars Dan Dia O39all Dia Each ac 6quot 1Dquot 1988K31 804 1988K41 5 96 1988K 8quot 12 1988K32 9 36 1988K42 6 92 1988K3 10quot 14quot 1988K33 11 63 1988K43 8 77 1988K3 12quot 16quot 1988K29 1527 1988K4410 4O 1988K3 14quot 18quot 1988K34 17 83 1988K46 11 82 1988K4 Allln One Ceiling DiffuserDamper Assemblies Save on assembly time diffusers come with collar and damper already in place Both styles are made of white ABS plastic for impact abrasion and chemical resistance A molded onepiece base provides an airtight t The collar is adjustable and ts 6quot 7quot and 8quot duct Mounting hardware is included Style Overall Size Square 11 14quot x 11 14quot 2559K5 28 96 Round 12 18 Dia 2559K9 28 96 Square Perforated and GridStyle TBar Ceiling Diffusers Use in suspended Tbar ceilings All are made of stamped steel with a white nish and have a berglass housing with an R6 insulation value Diffusers t all collar sizes in tablet Overall size is reggae 23 34quot x 23 34quot and they t 24quot x 24 ceiling openings Perfor ted Diffuser Grid3tye Diffuser The perforated style is an economical choice The grid style Bottom View Bottom View diffuses a thin layer of cool air along the ceiling surface 80 as warm air rises it displaces the cool air which descends to cool your area snapon collar is required to connect the diffuser to round duct Dampers mount into the collar for airflow control l Diffuser with Collar Replacement Top View Diffusers Diffuser Faces Style Each Each Perforated 1837K21 31 93 1837K22 26 80 Grid 1837K45 43 95 1837K47 29 87 For SnapOn Duct Collars Dampers Dia Each Each 6quot 18525K44 5 96 18525K81 9 32 8quot 18525K45 6 57 18525K82 9 77 10quot 18525K46 6 86 18525K83 11 00 12quot 18525K47 7 7O 18525K84 12 25 Square TBar Ceiling Diffusers Use these diffusers in your suspended Tbar ceilings They re made of stamped steel with a white nish and inclu a 2quot high collar for connection to round duct Overall size is 23 34quot x 23 34quot and they t 24quot x 24quot ceiling openings httpWWW mcmastel comc gDisplCtlgPage aspxRequpCAIALOGampCtlngNb1610 i 4 32008 Diffusers Registers amp Grilles Page 2 of 3 i The insulated diffusers have a foil vapor blanket on the back for insulation and moisture protection at Dampers snap onto the diffusers to adjust air ow r2 For Insulated I Duct Diffusers Diffusers Dampers Dia Each ach E Diffuser Damper 6quot 17905K38 56 95 17905K54 63 47 18525K81 2 8quot 17905K39 56 95 17905K55 63 47 18525K82 S 10quot 17905K41 56 95 17905K56 63 47 18525K83 11 12quot 17905K42 56 95 17905K57 63 47 18525K84 12 14quot 17905K43 56 95 18525K85 11 r f These multidirectional registers have an adjustable damper for maximum air ow g control All are made of stamped steel with a white nish Overall size is 1 38quot greater than the duct size Mounting hardware is included For Duct Size Each OneWay Airflow 8quot x 4quot 18425K81 8 60 10quot x 4quot 18425K83 9 37 12quot x 6quot 18425K87 10 88 14quot x 6quot 18425K89 11 91 ThreeWay Airflow 10quot x 4quot 18425K65 7 99 10quot x 6quot 18425K67 8 55 12quot x 6quot 18425K69 9 42 FourWay Airflow 10quot x 10quot 18425K57 16 03 39 12quot x 12quot 18425K59 20 73 F I A g xy 14quot x 14quot 18425K72 28 52 16quotX16quot 18425K74 34 48 Floor Registers These registers are made of stamped steel and have an easy tooperate damper Overall siZe is 1 33quot greater than the duct size White Finish Brown Finish h E For Duct Size ac 2 14quot X 10quot 2004K21 5 23 2004K11 5 23 2 14quot x 12 2004K22 5 49 2004K12 5 49 2 14quot x 14quot 2004K23 6 97 2004K13 6 97 4quot x 10quot 2004K24 5 92 2004K14 5 92 4quot x 12quot 2004K25 6 59 2004K15 6 59 4quot X 14quot 2004K26 9 O5 2004K16 9 05 6quot X 10quot 2004K27 10 39 2004K17 1O 39 6quot X 12quot 2004K28 11 13 2004K18 11 13 6quot X 14quot 2004K29 11 7O 2004K19 11 70 WallCeiling Registers with Adjustable Louvers Control the air ow with the damper and adjustable louvers Registers are made of stamped steel with a white nish Overall size is 1 38quot greater than duct size Mounting hardware is include For Duct Size Each For Duct Size ac 8quot x 6quot 18395K62 16 82 14quot x 6quot 18395K72 29 51 10quot x 6quot 18395K64 17 71 14quot x 8quot 18395K74 30 96 12quot x 4quot 18395K52 18 38 16quot x 4quot 18395K76 31 21 12quot x 6quot 18395K66 18 81 16quot x 6quot 18395K83 32 44 12quot x 8quot 18395K68 22 62 16quot x 8quot 18395K78 36 62 TBar Ceiling Filter Return Grilles Grilles hold a 20quot x 20quot X 1quot lter not included see pages 652653 to httpwww mcmaster comctlgDisplCtlgPagelaspxRequpCATALOGampCtlngNb161011 4 32008 ME414514 T15 TwoPointmcd ME414514 HVAC Systems Topic 15 Controls Example of twopoint control Determine the zone temperature response under a twoposition onoff control for the system in the figure shown The cooling load for the zone is a sensible load of 100000 BTUhr The supply airflow rates is 20000 cfm 88900 lbhr and is supplied at 55 F The zone setpoint is 70 F and the deadband is selected at 4 F to allow the zone to remain withing the comfort zone The zone thermal capacitance is 10000 BTUF The temperature response ofthe zone is given by the solution of a transient energy balance assuming a lumpedcapacitance model F R d CZETZ QS 7 mCpTZ 7 TS m pV Here the zone temperature is higher than the supply air temperature CZ 10000 thermal capacitance of the zone F BTU BTU QS 100000 cooling load C 024 hr P lbF 1b m 88900 mass flow rate of dry air hr 3 ft V 20000 volumetric flow rate min TS 55F supply air db temperature TZ 7017 zone setpoint temperature AT 4F deadband When there is a cooling load and the air flow is off d CZ39ETZ Q5 The rate of change of the zone temperature during this time d T Qs T 7 Qs T 7 10 F The temperature in the zone will 5 z 39 C zit quot C zit T E increase at this rate if no Z Z ventilation air is provided The solution to this 1st order separable ODE T Q5 t Zlt CZ ME414514 T15 TwoPointmcd V thout any ventilation how many minutes will it take for the zone temperature to rise 4 degrees from the lowerto the upper limit for human comfort AT 24 mm Tzit When the ventilation air is supplied at the supply setpoint temperature the rate of change of the zone temperature is d T QS 7 mCpTZ7 TS E Z C Q 7 mC T 7 T TZ t M TZ t 7220041 The rate at which the temperature will C hr drop from the upper limit Fhr The solution of this 1st order separable ODE 1 71nQS mCpTS 7 TZ 1 1 dTZ dt t Q5 7 mCpTZ 7 TS mCp CZ CZ How many minutes will it take for the zone temperature to drop 2 degrees from the setpoint to the lower limit for human comfort AT 2 thalf 7T thalf 5454 mm AT TLiTzi7 THTZ ME414514 T15 TwoPointmcd j 01440 t jsec Qs temperature rise in time without ventilation T21 t TL J CZ i 0 662 ti isec 7mg temperature drop in time with Q Q mc T 7 T exp pt ventilation s s p s H C i Z T 1 T 22 s 1 L m39Cp J 74 T21 J F 72 Q T m 22i E F g 70 g Tzlj a 5 F N Tzli 68 F 66 0 10 20 30 40 50 60 70 80 t t J in 43 is 60 60 j 60 j 60 Time minutes Manama T15 Twopumt mm x 12 mm Gwen H125 7 Q5 m cp T5 7 TH exp m cp Fmdx 11 I47mm Mmipulmd valbxbk Comm Valuable Supply u new we zone umpemmm Ibermosm D sturhnnu mm mm Schemauc uf ma feedback cuntrm uup
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