ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 26-2012 AIR-TO-AIR ENERGY RECOVERY EQUIPMENT.pdf

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1、26.1CHAPTER 26AIR-TO-AIR ENERGY RECOVERY EQUIPMENTApplications . 26.1Basic Thermodynamics 26.2Airflow Arrangements 26.4Additional Technical Considerations. 26.5Performance Ratings 26.8Types and Applications of Air-to-Air Heat Exchangers. 26.8Comparison of Air-to-Air Energy Recovery Systems. 26.20Eco

2、nomic Considerations 26.21Energy and/or Mass Recovery Calculation Procedure 26.23Symbols 26.27IR-TO-AIR energy recovery is the process of recovering heatA or/and moisture between two airstreams at different tempera-tures and humidities. This process is important in maintaining accept-able indoor air

3、 quality (IAQ) while maintaining low energy costs andreducing overall energy consumption. This chapter describes varioustechnologies for air-to-air energy recovery. Thermal and economicperformance, maintenance, and related operational issues are pre-sented, with emphasis on energy recovery for venti

4、lation.Energy can be recovered either in its sensible (temperature only) orlatent (moisture) form, or combination of both from multiple sources.Sensible energy can be extracted, for example, from outgoing air-streams in dryers, ovens, furnaces, combustion chambers, and gas tur-bine exhaust gases to

5、heat supply air. Units used for this purpose arecalled sensible heat exchange devices or heat recovery ventilators(HRVs). Devices that transfer both heat and moisture are known asenergy or enthalpy devices or energy recovery ventilators (ERVs).HRVs and ERVs are available for commercial and industria

6、l applica-tions as well as for residential and small-scale commercial uses.Air conditioners use much energy to dehumidify moist airstreams.Excessive moisture in the air of a building can result in mold, aller-gies, and bacterial growth. ERVs can enhance dehumidification withpackaged unitary air cond

7、itioners. Introducing outdoor or ventilationair is the primary means of diluting air contaminants to achieveacceptable indoor air quality. ERVs can cost-effectively provide largeamounts of outdoor air to meet a buildings minimum ventilationrequirements as prescribed in ASHRAE Standards 62.1 and 62.2

8、.Types of ERVs include fixed-plate heat exchangers, rotarywheels, heat pipes, runaround loops, thermosiphons, and twin-towerenthalpy recovery loops. Performance is typically characterized byeffectiveness; pressure drop, pumping, or fan power of fluids; crossflow (i.e., amount of air leakage from one

9、 stream to the other); andfrost control (used to prevent frosting on the heat exchanger). Recov-ery efficiency, the ratio of output of a device to its input, is also oftenconsidered. In energy recovery ventilators, effectiveness refers to theratio of actual energy or moisture recovered to the maximu

10、m possi-ble amount of energy and/or moisture that can be recovered.Fluid stream pressure drops because of the friction between thefluid and solid surface, and because of the geometrical complexityof the flow passages. Pumping or fan power is the product of thefluid volume flow rate and pressure drop

11、. Economic factors such ascost of energy recovered and capital and maintenance cost (includ-ing pumping power cost) play a vital role in determining the eco-nomic feasibility of recovery ventilators for a given application.APPLICATIONSAir-to-air energy recovery systems may be categorized accordingto

12、 their application as (1) process-to-process, (2) process-to-comfort,or (3) comfort-to-comfort. Some typical air-to-air energy recoveryapplications are listed in Table 1.In process-to-process applications, heat is captured from theprocess exhaust stream and transferred to the process supply air-stre

13、am. Equipment is available to handle process exhaust tempera-tures as high as 870C.Process-to-process recovery devices generally recover only sen-sible heat and do not transfer latent heat, because moisture transferis usually detrimental to the process. In cases involving condensablegases, less reco

14、very may be desired to prevent condensation andpossible corrosion.In process-to-comfort applications, waste heat captured fromprocess exhaust heats building makeup air during winter. Typicalapplications include foundries, strip-coating plants, can plants,plating operations, pulp and paper plants, an

15、d other processingareas with heated process exhaust and large makeup air volumerequirements.Although full recovery is usually desired in process-to-processapplications, recovery for process-to-comfort applications must bemodulated during warm weather to prevent overheating the makeupair. During summ

16、er, no recovery is required. Because energy issaved only in the winter and recovery is modulated during moderateweather, process-to-comfort applications save less energy annuallythan do process-to-process applications.Process-to-comfort recovery devices generally recover sensibleheat only and do not

17、 transfer moisture between airstreams.In comfort-to-comfort applications, the energy recovery devicelowers the enthalpy of the building supply air during warm weatherand raises it during cold weather by transferring energy between theventilation air supply and exhaust airstreams.Air-to-air energy re

18、covery devices for comfort-to-comfort appli-cations may be sensible heat exchange devices (i.e., transferring sen-sible energy only) or energy exchange devices (i.e., transferring bothsensible energy and moisture). These devices are discussed further inthe section on Additional Technical Considerati

19、ons.When outdoor air humidity is low and the building space has anappreciable latent load, an ERV can recover sensible energy whileThe preparation of this chapter is assigned to TC 5.5, Air-to-Air EnergyRecovery.Table 1 Typical Applications for Air-to-Air Energy RecoveryMethod ApplicationProcess-to-

20、processandProcess-to-comfortDryersOvensFlue stacksBurnersFurnacesIncineratorsPaint exhaustWelding exhaustComfort-to-comfort Swimming poolsLocker roomsResidentialOperating roomsNursing homesAnimal ventilationPlant ventilationSmoking exhaust26.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)possi

21、bly slightly increasing the latent space load because of watervapor transfer within the ERV. It is therefore important to determinewhether the given application calls for HRV or ERV.HRVs are suitable when outdoor air humidity is low and latent spaceloads are high for most of the year, and also for u

22、se with swimmingpools, chemical exhaust, paint booths, and indirect evaporative coolers.ERVs are suitable for applications in schools, offices, residencesand other applications that require year-round economical preheat-ing or/and precooling of outdoor supply air.BASIC THERMODYNAMICSThe second law o

23、f thermodynamics states that heat energy alwaystransfers from a region of high temperature to one of low temperature.This law can be extended to say that mass transfer always occurs froma region of high vapor pressure to one of low vapor pressure. The ERVfacilitates this transfer across a separating

24、 wall (shown by a thick hor-izontal line in Figure 1) made of a material that conducts heat and ispermeable to water vapor. Moisture is transferred when there is a dif-ference in vapor pressure between the two airstreams.On a typical summer day, supply air at temperature, humidity, orenthalpy of x1a

25、nd mass flow rate msenters the ERV, while exhaustair from the conditioned space enters at conditions x3and m3. Be-cause conditions at x3are lower than conditions at x1, heat and masstransfer from the supply airstream to the exhaust airstream becauseof differences in temperature and vapor pressures a

26、cross the separat-ing wall. Consequently, the supply air exit properties decrease, whilethose of the exhaust air increase. Exit properties of these two streamscan be estimated, knowing the flow rates and the effectiveness of theheat exchanger.ASHRAE Standard 84 defines effectiveness as = (1)Thermody

27、namics of Heat Recovery VentilatorsFrom Figure 1, the sensible effectiveness sof a heat recoveryventilator is given ass= (2a)where qsis the actual sensible heat transfer rate given byqs= sqs,max(2b)where qs,maxis the maximum sensible heat transfer rate given byqs,max= Cmin(t3 t1) (2c)whereqs= sensib

28、le heat transfer rate, kWqs,max= maximum sensible heat transfer rate, kWs= sensible effectivenesst1= dry-bulb temperature at location 1 in Figure 1, Cms= supply dry air mass flow rate, kg/sme= exhaust dry air mass flow rate, kg/sCmin= smaller of cpsmsand cpemecps= supply moist air specific heat at c

29、onstant pressure, kJ/(kgK)cpe= exhaust moist air specific heat at constant pressure, kJ/(kgK)Assuming no water vapor condensation in the HRV, the leavingsupply air condition ist2 = t1 s(t1 t3) (3a)and the leaving exhaust air condition ist4 = t3+ s(t1 t3) (3b)Equations (2), (3a), and (3b) assume stea

30、dy-state operating con-ditions; no heat or moisture transfer between the heat exchanger andits surroundings; no cross-leakage, and no energy gains or losses frommotors, fans, or frost control devices. Furthermore, condensation orfrosting does not occur or is negligible. These assumptions are gen-era

31、lly nearly true for larger commercial HRV applications. Note thatthe HRV only allows transfer of sensible heat energy associatedwith heat transfer because of temperature difference between theairstreams or between an airstream and a solid surface. These equa-tions apply even in winter, if there is n

32、o condensation in the HRV.The sensible heat energy transfer qsfrom the heat recovery ven-tilator can be estimated fromqs= mscps(t2 t1) = Qsscps(t2 t1) (3c)qs= mecpe(t4 t3) = Qeecpe(t4 t3) (3d)qs= smmincp(t1 t3) (3e)whereQs= volume flow rate of supply air, m3sQe= volume flow rate of exhaust air, m3ss

33、= density of dry supply air, kgm3e= density of dry exhaust air, kgm3t1, t2, t3, t4= inlet and exit temperatures of supply and exhaust airstreams, respectivelymmin= smaller of msand meBecause cpsand cpeare nearly equal, these terms may be omittedfrom Equations (1) to (4).Sensible heat exchangers (HRV

34、s) can be used in virtually allcases, especially for swimming pool, paint booth, and reheat appli-cations. Equations (1) to (3e) apply for both HRVs and ERVs withappropriate selection of x1, x2, x3, and x4.Thermodynamics of Energy Recovery VentilatorsThe ERV allows the transfer of both sensible and

35、latent heat, thelatter due to the difference in water vapor pressures between theairstreams or between an airstream and a solid surface. ERVs areavailable as desiccant rotary wheels and also as membrane plateexchangers; although other gases may also pass through the mem-brane (Sparrow et al. 2001a)

36、of membrane plate energy exchangers,it is assumed in the following equations that only the water vapor isallowed to pass through the membrane.From Figure 1, assuming no condensation in the ERV, the latenteffectiveness L of an energy recovery ventilator is given asL= (4a)where qLis the actual latent

37、heat transfer rate given byqL= LqL,max(4b)where qL,maxis the maximum heat transfer rate given byFig. 1 Airstream Numbering ConventionActual transfer of moisture or energyMaximum possible transfer between airstreams-qsqsmax,-mscpst2t1Cmint3t1-mscpet3t4Cmint3t1-=Cminmscps-Cminmecpe-qLqLmax,-mshfgw1w2m

38、minhfgw1w3-mehfgw4w3mminhfgw1w3-=Air-to-Air Energy Recovery Equipment 26.3qL,max= mminhfg(w1 w3) (4c)whereL= latent effectivenesshfg= enthalpy of vaporization, kJ/kgw = humidity ratios at locations indicated in Figure 1ms= supply dry air mass flow rate, kg/sme= exhaust dry air mass flow rate, kg/smm

39、in= smaller of msand meBecause the enthalpy of vaporization from Equation (4a) can bedropped out from numerator and denominator, Equation (4a) can berewritten asm= (4d)where mis moisture effectiveness, numerically equal to latent ef-fectiveness L, and mwis actual moisture transfer rate given bymw= m

40、mw,max(4e)where ms,maxis the maximum moisture transfer rate given byms,max= mw,min(w1 w3) (4f)Assuming no water vapor condensation in the ERV, the leavinghumidity ratios can be given as follows. The supply air leavinghumidity ratio isw2= w1 L(w1 w3) (5a)and the leaving exhaust air humidity ratio isw

41、4= w3+ L(w1 w3) (5b)The total effectiveness tof an energy recovery ventilator isgiven ast= (6a)where qtis the actual total energy transfer rate given byqt= tqt,max(6b)where qt,maxis the maximum total energy transfer rate given byqt,max= mmin(h1 h3) (6c)wheret= total effectivenessh = enthalpy at loca

42、tions indicated in Figure 1, kJ/kgms= supply dry air mass flow rate, kg/sme= exhaust dry air mass flow rate, kg/smmin= smaller of msand meThe leaving supply air condition ish2= h1 t (h1 h3) (7a)and the leaving exhaust air condition ish4= h3+ t (h1 h3) (7b)Assuming the stream at state 1 is of higher

43、humidity, the latentheat recovery qLfrom the ERV can be estimated fromqL= mshfg(w1 w2) = Qsshfg(w1 w2) (8a)qL= mehfg(w4 w3) = Qeehfg(w4 w3) (8b)qL= Lmminhfg(w1 w3) (8c)wherehfg= enthalpy of vaporization or heat of vaporization of water vapor, kJkgw1, w2, w3, w4= inlet and exit humidity ratios of sup

44、ply and exhaust airstreams, respectivelyThe total energy transfer qtbetween the streams is given byqt= qs+ qL= ms(h1s h2s) = Qss(h1s h2s)= mscps(t1 t2) + mshfg(w1 w2)(9)qt= qs+ qL= me(h4e h3e) = Qee(h4e h3e)= mecpe(t4 t3) + mehfg(w4 w3) (10a)qt= 60tmmin(h1s h3e) (10b)whereh1s= enthalpy of supply air

45、 at inlet, kJkgh3e= enthalpy of exhaust air at inlet, kJkgh2s= enthalpy of supply air at outlet, kJkgh4e= enthalpy of exhaust air at outlet, kJkgERVs can be used where there is an opportunity to transfer heatand mass (water vapor) (e.g., humid areas, schools, offices withlarge occupancies). Latent e

46、nergy transfer can be positive or nega-tive depending on the direction of decreasing vapor pressure.Depending on conditions, the supply airstream flowing through anERV may gain heat energy (+qs) from the adjoining stream, but loselatent energy (qL) if it transfers the water vapor to the adjoiningstr

47、eam. Heat and latent energy gain may be in the same or oppositedirection. The total net energy gain is the difference between qsandqL, as shown in Example 1.Example 1. Inlet supply air enters an ERV with a flow rate of 4.41 m3s at35C and 20% rh. Inlet exhaust air enters with a flow rate of 4.27 m3/s

48、at 24C and 50% rh. Assume that the energy exchanger was testedunder ASHRAE Standard 84, which rated the sensible heat transfereffectiveness at 50% and the latent (water vapor) transfer effectivenessat 50%. Assuming the specific heat of air is 1 kJ/(kgK) and the latentheat of vaporization to be 2560 kJ/kg, determine the sensible, latent,and net energy gained by the exhaust air.Solution:From the psychrometric chart, the properties of air at 35C and 20%rh areV1= 0.8