Lesson 35 Cooling And Heating Load Calculations - Estimation Of Required Cooling/Heating Capacity

Transcription

1 Lessn 35 Cling And Heating Lad Calculatins - Estimatin Of Required Cling/Heating Capacity Versin 1 ME, IIT Kharagpur 1

2 The specific bjectives f this chapter are t: 1 Intrductin t lad calculatins (Sectin 351) 2 Differences between cnventinal cling and heating lad calculatin methdlgies (Sectin 352) 3 Methds f estimating cling and heating lads n buildings such as rules-fthumb, semi-empirical methds etc (Sectin 353) 4 Cling lad calculatins using CLTD/CLF methd (Sectin 354) 5 Estimatin f the cling capacity f the system (Sectin 355) 6 Heating lad calculatins (Sectin 356) At the end f the lecture, the student shuld be able t: 1 Explain the differences between cnventinal cling and heating lad calculatins 2 List cmmnly used methds fr estimating cling lads 3 Estimate the internal and external cling lads n a building by separating sensible and latent parts using CLTD/CLF methd frm building specificatins, design indr and utdr cnditins, ccupancy etc 4 Estimate the required cling capacity f the cil by taking int accunt the bypass factr f the cil, ventilatin requirements etc 5 Explain briefly the prcedure fr estimating heating lads 351 Intrductin: As mentined in an earlier chapter, heating and cling lad calculatins are carried ut t estimate the required capacity f heating and cling systems, which can maintain the required cnditins in the cnditined space T estimate the required cling r heating capacities, ne has t have infrmatin regarding the design indr and utdr cnditins, specificatins f the building, specificatins f the cnditined space (such as the ccupancy, activity level, varius appliances and equipment used etc) and any special requirements f the particular applicatin Fr cmfrt applicatins, the required indr cnditins are fixed by the criterin f thermal cmfrt, while fr industrial r cmmercial applicatins the required indr cnditins are fixed by the particular prcesses being perfrmed r the prducts being stred As discussed in an earlier chapter, the design utdr cnditins are chsen based n design dry bulb and cincident wet bulb temperatures fr peak summer r winter mnths fr cling and heating lad calculatins, respectively Versin 1 ME, IIT Kharagpur 2

3 352 Heating versus cling lad calculatins: As the name implies, heating lad calculatins are carried ut t estimate the heat lss frm the building in winter s as t arrive at required heating capacities Nrmally during winter mnths the peak heating lad ccurs befre sunrise and the utdr cnditins d nt vary significantly thrughut the winter seasn In additin, internal heat surces such as ccupants r appliances are beneficial as they cmpensate sme f the heat lsses As a result, nrmally, the heat lad calculatins are carried ut assuming steady state cnditins (n slar radiatin and steady utdr cnditins) and neglecting internal heat surces This is a simple but cnservative apprach that leads t slight verestimatin f the heating capacity Fr mre accurate estimatin f heating lads, ne has t take int the thermal capacity f the walls and internal heat surces, which makes the prblem mre cmplicated Fr estimating cling lads, ne has t cnsider the unsteady state prcesses, as the peak cling lad ccurs during the day time and the utside cnditins als vary significantly thrughut the day due t slar radiatin In additin, all internal surces add n t the cling lads and neglecting them wuld lead t underestimatin f the required cling capacity and the pssibility f nt being able t maintain the required indr cnditins Thus cling lad calculatins are inherently mre cmplicated as it invlves slving unsteady equatins with unsteady bundary cnditins and internal heat surces Fr any building there exists a balance pint at which the slar radiatin (Q slar ) and internal heat generatin rate (Q int ) exactly balance the heat lsses frm the building Thus frm sensible heat balance equatin, at balanced cnditin: ( Q slar + Qint ) sensible = UA(Tin Tut ) (351) where UA is the prduct f verall heat transfer cefficient and heat transfer area f the building, T in is the required indr temperature and T ut is the utdr temperature Frm the abve equatin, the utside temperature at balanced cnditin (T ut,bal ) is given by: T ut,bal (Q slar + Qint ) sensible = Tin (352) UA If the utdr temperature is greater than the balanced utdr temperature given by the abve equatin, ie, when T ut > T ut,bal, then there is a need fr cling the building On the ther hand, when the utdr temperature is less than the balanced utdr temperature, ie, when T ut < T ut,bal, then there is a need fr heating the building When the utdr temperature exactly equals the balanced utdr temperature, ie, when T ut = T ut,bal, then there is n need fr either cling r heating the building Versin 1 ME, IIT Kharagpur 3

4 Fr residential buildings (with fewer internal heat surces), the balanced utdr temperature may vary frm 10 t 18 C As discussed befre, this means that if the balanced utdr temperature is 18 C, then a cling system is required when the utdr temperature exceeds 18 C This implies that buildings need cling nt nly during summer but als during spring and fall as well If the building is well insulated (small UA) and/r internal lads are high, then frm the energy balance equatin (352), the balanced utdr temperature will reduce leading t extended cling seasn and shrtened heating seasn Thus a smaller balanced utdr temperature implies higher cling requirements and smaller heating requirements, and vice versa Fr cmmercial buildings with large internal lads and relatively smaller heat transfer areas, the balanced utdr temperature can be as lw as 2 C, implying a lengthy cling seasn and a small heating seasn If there are n internal heat surces and if the slar radiatin is negligible, then frm the heat balance equatin, T ut,bal = T in, this implies that if the utside temperature exceeds the required inside temperature (say, 25 C fr cmfrt) then there is a need fr cling therwise there is a need fr heating Thus depending upn the specific cnditins f the building, the need fr either cling system r a heating system depends This als implies a need fr ptimizing the building insulatin depending upn utdr cnditins and building heat generatin s that ne can use during certain perids free cling prvided by the envirnment withut using any external cling system 353 Methds f estimating cling and heating lads: Generally, heating and cling lad calculatins invlve a systematic, stepwise prcedure, using which ne can arrive at the required system capacity by taking int accunt all the building energy flws In practice, a variety f methds ranging frm simple rules-f-thumb t cmplex Transfer Functin Methds are used in practice t arrive at the building lads Fr example, typical rules-f-thumb methds fr cling lads specify the required cling capacity based n the flr area r ccupancy Table 351 shws typical data n required cling capacities based n the flr area r applicatin Such rules-f-thumb are useful in preliminary estimatin f the equipment size and cst The main cnceptual drawback f rulesf-thumb methds is the presumptin that the building design will nt make any difference Thus the rules fr a badly designed building are typically the same as fr a gd design Versin 1 ME, IIT Kharagpur 4

5 Sln 1 Applicatin Office buildings: External znes Required cling capacity (TR) fr 1000 ft 2 f flr area 25% glass: 35 TR 50% glass: 45 TR 75% glass: 50 TR Internal znes 28 TR 2 Cmputer rms TR Htels Bedrms Single rm: 06 TR per rm 3 4 Restaurants Department stres Basement & grund flrs Duble rm: 10 TR per rm TR TR Upper flrs TR 5 Shps 50 TR 6 Banks TR 7 Theatres & Auditriums 007 TR per seat Table 351: Required cling capacities fr varius applicatins based n rules-fthumb (Crme and Rberts, 1981) Mre accurate lad estimatin methds invlve a cmbinatin f analytical methds and empirical results btained frm actual data, fr example the use f Cling Lad Temperature Difference (CLTD) fr estimating fabric heat gain and the use f Slar Heat Gain Factr (SHGF) fr estimating heat transfer thrugh fenestratin These methds are very widely used by air cnditining engineers as they yield reasnably accurate results and estimatins can be carried ut manually in a relatively shrt time Over the years, mre accurate methds that require the use f cmputers have been develped fr estimating cling lads, eg the Transfer Functin Methd (TFM) Since these methds are expensive and time cnsuming they are generally used fr estimating cling lads f large cmmercial r institutinal buildings ASHRAE suggests different methds fr estimating cling and heating lads based n applicatins, such as fr residences, fr cmmercial buildings etc Versin 1 ME, IIT Kharagpur 5

6 354 Cling lad calculatins: As mentined befre, lad calculatins invlve a systematic and stepwise prcedure that takes int accunt all the relevant building energy flws The cling lad experienced by a building varies in magnitude frm zer (n cling required) t a maximum value The design cling lad is a lad near the maximum magnitude, but is nt nrmally the maximum Design cling lad takes int accunt all the lads experienced by a building under a specific set f assumed cnditins The assumptins behind design cling lad are as fllws: 1 Design utside cnditins are selected frm a lng-term statistical database The cnditins will nt necessarily represent any actual year, but are representative f the lcatin f the building Design data fr utside cnditins fr varius lcatins f the wrld have been cllected and are available in tabular frm in varius handbks 2 The lad n the building due t slar radiatin is estimated fr clear sky cnditins 3 The building ccupancy is assumed t be at full design capacity 4 All building equipment and appliances are cnsidered t be perating at a reasnably representative capacity The ttal building cling lad cnsists f heat transferred thrugh the building envelpe (walls, rf, flr, windws, drs etc) and heat generated by ccupants, equipment, and lights The lad due t heat transfer thrugh the envelpe is called as external lad, while all ther lads are called as internal lads The percentage f external versus internal lad varies with building type, site climate, and building design The ttal cling lad n any building cnsists f bth sensible as well as latent lad cmpnents The sensible lad affects dry bulb temperature, while the latent lad affects the misture cntent f the cnditined space Buildings may be classified as externally laded and internally laded In externally laded buildings the cling lad n the building is mainly due t heat transfer between the surrundings and the internal cnditined space Since the surrunding cnditins are highly variable in any given day, the cling lad f an externally laded building varies widely In internally laded buildings the cling lad is mainly due t internal heat generating surces such as ccupants r appliances r prcesses In general the heat generatin due t internal heat surces may remain fairly cnstant, and since the heat transfer frm the variable surrundings is much less cmpared t the internal heat surces, the cling lad f an internally laded building remains fairly cnstant Obviusly frm energy efficiency and ecnmics pints f view, the system design strategy fr an externally laded building shuld be different frm an internally laded building Hence, prir knwledge f whether the building is externally laded r internally laded is essential fr effective system design Versin 1 ME, IIT Kharagpur 6

7 As mentined befre, the ttal cling lad n a building cnsists f external as well as internal lads The external lads cnsist f heat transfer by cnductin thrugh the building walls, rf, flr, drs etc, heat transfer by radiatin thrugh fenestratin such as windws and skylights All these are sensible heat transfers In additin t these the external lad als cnsists f heat transfer due t infiltratin, which cnsists f bth sensible as well as latent cmpnents The heat transfer due t ventilatin is nt a lad n the building but a lad n the system The varius internal lads cnsist f sensible and latent heat transfer due t ccupants, prducts, prcesses and appliances, sensible heat transfer due t lighting and ther equipment Figure 351 shws varius cmpnents that cnstitute the cling lad n a building Slar radiatin thrugh paque surfaces Slar radiatin thrugh fenestratin Infiltratin Internal heat surces Heat transfer frm grund Fig351: Varius cling lad cmpnents Estimatin f cling lad invlves estimatin f each f the abve cmpnents frm the given data In the present chapter, the cling lad calculatins are carried ut based n the CLTD/CLF methd suggested by ASHRAE Fr mre advanced methds such as TFM, the reader shuld refer t ASHRAE and ther handbks 3541 Estimatin f external lads: a) Heat transfer thrugh paque surfaces: This is a sensible heat transfer prcess The heat transfer rate thrugh paque surfaces such as walls, rf, flr, drs etc is given by: Q paque = UACLTD (353) Versin 1 ME, IIT Kharagpur 7

8 where U is the verall heat transfer cefficient and A is the heat transfer area f the surface n the side f the cnditined space CLTD is the cling lad temperature difference Fr sunlit surfaces, CLTD has t be btained frm the CLTD tables as discussed in the previus chapter Adjustment t the values btained frm the table is needed if actual cnditins are different frm thse based n which the CLTD tables are prepared Fr surfaces which are nt sunlit r which have negligible thermal mass (such as drs), the CLTD value is simply equal t the temperature difference acrss the wall r rf Fr example, fr external drs the CLTD value is simply equal t the difference between the design utdr and indr dry bulb temperatures, T ut -T in Fr interir air cnditined rms surrunded by nn-air cnditined spaces, the CLTD f the interir walls is equal t the temperature difference between the surrunding nn-air cnditined space and the cnditined space Obviusly, if an air cnditined rm is surrunded by ther air cnditined rms, with all f them at the same temperature, the CLTD values f the walls f the interir rm will be zer Estimatin f CLTD values f flr and rf with false ceiling culd be tricky Fr flrs standing n grund, ne has t use the temperature f the grund fr estimating CLTD Hwever, the grund temperature depends n the lcatin and varies with time ASHRAE suggests suitable temperature difference values fr estimating heat transfer thrugh grund If the flr stands n a basement r n the rf f anther rm, then the CLTD values fr the flr are the temperature difference acrss the flr (ie, difference between the temperature f the basement r rm belw and the cnditined space) This discussin als hlds gd fr rfs which have nn-air cnditined rms abve them Fr sunlit rfs with false ceiling, the U value may be btained by assuming the false ceiling t be an air space Hwever, the CLTD values btained frm the tables may nt exactly fit the specific rf Then ne has t use his judgement and select suitable CLTD values b) Heat transfer thrugh fenestratin: Heat transfer thrugh transparent surface such as a windw, includes heat transfer by cnductin due t temperature difference acrss the windw and heat transfer due t slar radiatin thrugh the windw The heat transfer thrugh the windw by cnvectin is calculated using Eq(353), with CLTD being equal t the temperature difference acrss the windw and A equal t the ttal area f the windw The heat transfer due t slar radiatin thrugh the windw is given by: Q trans = Aunshaded SHGFmax SCCLF (354) where A unshaded is the area expsed t slar radiatin, SHGF max and SC are the maximum Slar Heat Gain Factr and Shading Cefficient, respectively, and CLF is the Cling Lad Factr As discussed in a previus chapter, the unshaded area has t be btained frm the dimensins f the external shade and slar gemetry SHGF max and SC are btained frm ASHRAE tables based n the rientatin f the windw, lcatin, mnth f the year and the type f glass and internal shading device Versin 1 ME, IIT Kharagpur 8

9 The Cling Lad Factr (CLF) accunts fr the fact that all the radiant energy that enters the cnditined space at a particular time des nt becme a part f the cling lad 1 instantly As slar radiatin enters the cnditined space, nly a negligible prtin f it is absrbed by the air particles in the cnditined space instantaneusly leading t a minute change in its temperature Mst f the radiatin is first absrbed by the internal surfaces, which include ceiling, flr, internal walls, furniture etc Due t the large but finite thermal capacity f the rf, flr, walls etc, their temperature increases slwly due t absrptin f slar radiatin As the surface temperature increases, heat transfer takes place between these surfaces and the air in the cnditined space Depending upn the thermal capacity f the wall and the utside temperature, sme f the absrbed energy due t slar radiatin may be cnducted t the uter surface and may be lst t the utdrs Only that fractin f the slar radiatin that is transferred t the air in the cnditined space becmes a lad n the building, the heat transferred t the utside is nt a part f the cling lad Thus it can be seen that the radiatin heat transfer intrduces a time lag and als a decrement factr depending upn the dynamic characteristics f the surfaces Due t the time lag, the effect f radiatin will be felt even when the surce f radiatin, in this case the sun is remved The CLF values fr varius surfaces have been calculated as functins f slar time and rientatin and are available in the frm f tables in ASHRAE Handbks Table 352 gives typical CLF values fr glass with interir shading Slar Directin the sunlit windw is facing Time, h N NE E SE S SW W NW Hriz Table 352: Cling Lad Factr (CLF) fr glass with interir shading and lcated in nrth latitudes (ASHRAE) c) Heat transfer due t infiltratin: Heat transfer due t infiltratin cnsists f bth sensible as well as latent cmpnents The sensible heat transfer rate due t infiltratin is given by: 1 At any pint f time, cling lad may be equated t the heat transfer rate t the air in the cnditined space If heat is transferred t the walls r ther slid bjects, then it des nt becme a part f the cling lad at that instant Versin 1 ME, IIT Kharagpur 9

10 Q s,inf = m c (T T ) = V ρ c (T T ) (355) p,m i p,m i where V is the infiltratin rate ( in m 3 /s), ρ and c p,m are the density and specific heat f the mist, infiltrated air, respectively T and T i are the utdr and indr dry bulb temperatures The latent heat transfer rate due t infiltratin is given by: Q l,inf = m h (W W ) = V ρ h (W W ) (356) fg i fg i where h fg is the latent heat f vaprizatin f water, W and W i are the utdr and indr humidity rati, respectively As discussed in an earlier chapter, the infiltratin rate depends upn several factrs such as the tightness f the building that includes the walls, windws, drs etc and the prevailing wind speed and directin As mentined befre, the infiltratin rate is btained by using either the air change methd r the crack methd The infiltratin rate by air change methd is given by: V 3 = (ACH)V / 3600 m / s (357) where ACH is the number f air changes per hur and V is the grss vlume f the cnditined space in m 3 Nrmally the ACH value varies frm 05 ACH fr tight and well-sealed buildings t abut 20 fr lse and prly sealed buildings Fr mdern buildings the ACH value may be as lw as 02 ACH Thus depending upn the age and cnditin f the building an apprpriate ACH value has t be chse, using which the infiltratin rate can be calculated The infiltratin rate by the crack methd is given by: V n 3 = AC ΔP m / s (358) where A is the effective leakage area f the cracks, C is a flw cefficient which depends n the type f the crack and the nature f the flw in the crack, ΔP is the difference between utside and inside pressure (P -P i ) and n is an expnent whse value depends n the nature f the flw in the crack The value f n varies between 04 t 10, ie, 04 n 10 The pressure difference ΔP arises due t pressure difference due t the wind (ΔP wind ), pressure difference due t the stack effect (ΔP stack ) and pressure difference due t building pressurizatin (ΔP bld ), ie, Δ P = ΔP + ΔP + ΔP (359) wind stack bld Versin 1 ME, IIT Kharagpur 10

11 Semi-empirical expressins have been btained fr evaluating pressure difference due t wind and stack effects as functins f prevailing wind velcity and directin, inside and utside temperatures, building dimensins and gemetry etc Representative values f infiltratin rate fr different types f windws, drs walls etc have been measured and are available in tabular frm in air cnditining design handbks d) Miscellaneus external lads: In additin t the abve lads, if the cling cil has a psitive by-pass factr (BPF > 0), then sme amunt f ventilatin air directly enters the cnditined space, in which case it becmes a part f the building cling lad The sensible and latent heat transfer rates due t the by-passed ventilatin air can be calculated using equatins (355) and (356) by replacing V vent BPF, where V vent is the ventilatin rate and BPF is the by-pass factr f the cling cil In additin t this, sensible and latent heat transfer t the building als ccurs due t heat transfer and air leakage in the supply ducts A safety factr is usually prvided t accunt fr this depending upn the specific details f the supply air ducts If the supply duct cnsists f supply air fan with mtr, then pwer input t the fan becmes a part f the external sensible lad n the building If the duct cnsists f the electric mtr, which drives the fan, then the efficiency f the fan mtr als must be taken int accunt while calculating the cling lad Mst f the times, the pwer input t the fan is nt knwn a priri as the amunt f supply air required is nt knwn at this stage T take this factr int accunt, initially it is assumed that the supply fan adds abut 5% f the rm sensible cling lad and cling lads are then estimated Then this value is crrected in the end when the actual fan selectin is dne 3542 Estimatin f internal lads: The internal lads cnsist f lad due t ccupants, due t lighting, due t equipment and appliances and due t prducts stred r prcesses being perfrmed in the cnditined space a) Lad due t ccupants: The internal cling lad due t ccupants cnsists f bth sensible and latent heat cmpnents The rate at which the sensible and latent heat transfer take place depends mainly n the ppulatin and activity level f the ccupants Since a prtin f the heat transferred by the ccupants is in the frm f radiatin, a Cling Lad Factr (CLF) shuld be used similar t that used fr radiatin heat transfer thrugh fenestratin Thus the sensible heat transfer t the cnditined space due t the ccupants is given by the equatin: ( N f peple)( Sensible heat gain / persn CLF Q s, ccupants = ) (3510) V with Versin 1 ME, IIT Kharagpur 11

12 Table 353 shws typical values f ttal heat gain frm the ccupants and als the sensible heat gain fractin as a functin f activity in an air cnditined space Hwever, it shuld be nted that the fractin f the ttal heat gain that is sensible depends n the cnditins f the indr envirnment If the cnditined space temperature is higher, then the fractin f ttal heat gain that is sensible decreases and the latent heat gain increases, and vice versa Activity Ttal heat gain, W Sensible heat gain fractin Sleeping Seated, quiet Standing kmph Office wrk Teaching Industrial wrk 300 t Table 353: Ttal heat gain, sensible heat gain fractin frm ccupants The value f Cling Lad Factr (CLF) fr ccupants depends n the hurs after the entry f the ccupants int the cnditined space, the ttal hurs spent in the cnditined space and type f the building Values f CLF have been btained fr different types f buildings and have been tabulated in ASHRAE handbks Since the latent heat gain frm the ccupants is instantaneus the CLF fr latent heat gain is 10, thus the latent heat gain due t ccupants is given by: Q, ccupants ( Nf peple)( Latent heat gain / persn) l = (3511) b) Lad due t lighting: Lighting adds sensible heat t the cnditined space Since the heat transferred frm the lighting system cnsists f bth radiatin and cnvectin, a Cling Lad Factr is used t accunt fr the time lag Thus the cling lad due t lighting system is given by: Q s, lighting = ( Installed wattage)( Usage Factr) (Ballast factr) CLF (3512) The usage factr accunts fr any lamps that are installed but are nt switched n at the time at which lad calculatins are perfrmed The ballast factr takes int accunt the lad impsed by ballasts used in flurescent lights A typical ballast factr value f 125 is taken fr flurescent lights, while it is equal t 10 fr incandescent lamps The values f CLF as a functin f the number f hurs after the lights are turned n, type f lighting fixtures and the hurs f peratin f the lights are available in the frm f tables in ASHRAE handbks Versin 1 ME, IIT Kharagpur 12

13 c) Internal lads due t equipment and appliances: The equipment and appliances used in the cnditined space may add bth sensible as well as latent lads t the cnditined space Again, the sensible lad may be in the frm f radiatin and/r cnvectin Thus the internal sensible lad due t equipment and appliances is given by: Q, appliances ( Installed wattage)( Usage Factr ) CLF s = (3513) The installed wattage and usage factr depend n the type f the appliance r equipment The CLF values are available in the frm f tables in ASHARE handbks The latent lad due t appliances is given by: Q, appliance ( Installed wattage) (Latent heat fractin) l = (3511) Table 354 shws typical lad f varius types f appliances Appliance Sensible lad, W Latent lad, W Ttal lad, W Cffee brewer, 05 gallns Cffee warmer, 05 gallns Taster, 360 slices/h Fd warmer/m 2 plate area Table 354: Typical appliance lad (CP Arra) Fr ther equipment such as cmputers, printers etc, the lad is in the frm f sensible heat transfer and is estimated based n the rated pwer cnsumptin The CLF value fr these equipment may be taken as 10 as the radiative heat transfer frm these equipment is generally negligible due t smaller perating temperatures When the equipment are run by electric mtrs which are als kept inside the cnditined space, then the efficiency f the electric mtr must be taken int accunt Thugh the estimatin f cling lad due t appliance and equipment appears t be simple as given by the equatins, a large amunt f uncertainty is intrduced n accunt f the usage factr and the difference between rated (nameplate) pwer cnsumptin at full lads and actual pwer cnsumptin at part lads Estimatin using nameplate pwer input may lead t verestimatin f the lads, if the equipment perates at part lad cnditins mst f the time If the cnditined space is used fr string prducts (eg cld strage) r fr carrying ut certain prcesses, then the sensible and latent heat released by these specific prducts and r the prcesses must be added t the internal cling lads The sensible and latent heat release rate f a wide variety f live and dead prducts cmmnly stred in cld strages are available in air cnditining and refrigeratin handbks Using these tables, ne can estimate the required cling capacity f cld strages Thus using the abve equatins ne can estimate the sensible (Q s,r ), latent (Q l,r ) and ttal cling lad (Q t,r ) n the buildings Since the lad due t sunlit Versin 1 ME, IIT Kharagpur 13

14 surfaces varies as a functin f slar time, it is preferable t calculate the cling lads at different slar times and chse the maximum lad fr estimating the system capacity Frm the sensible and ttal cling lads ne can calculate the Rm Sensible Heat Factr (RSHF) fr the building As discussed in an earlier chapter, frm the RSHF value and the required indr cnditins ne can draw the RSHF line n the psychrmetric chart and fix the cnditin f the supply air 355 Estimatin f the cling capacity f the system: In rder t find the required cling capacity f the system, ne has t take int accunt the sensible and latent lads due t ventilatin, leakage lsses in the return air ducts and heat added due t return air fan (if any) 3751 Lad n the system due t ventilated air: Figure 352 shws a schematic f an air cnditining system with the cling cil, supply and return ducts, ventilatin and fans The cling cil has a by-pass factr X Then the cling lad n the cil due t sensible heat transfer f the ventilated air is given by: Q s,vent vent = m (1 X)c (T T ) = V ρ (1 X)c (T T ) (3512) vent p,m i where mvent and V vent are the mass and vlumetric flw rates f the ventilated air and X is the by-pass factr f the cil The latent heat lad n the cil due t ventilatin is given by: p,m i Q l,vent = m (1 X)h (W W ) = V ρ (1 X)h (W W ) (3513) vent fg i vent fg i where W and W i are the humidity ratis f the ambient and cnditined air, respectively and h fg is the latent heat f vapurizatin f water 3542 Lad n the cil due t leakage in return air duct and due t return air fan: If there is leakage f air and heat frm r t the return air duct, additinal capacity has t be prvided by the cling cil t take care f this The sensible heat transfer t the return duct due t heat transfer frm the surrundings t the return duct depends n the surface area f the duct that is expsed t utside air (A expsed ), amunt f insulatin (U ins ) and temperature difference between utdr air and return air, ie, Q s,duct = U A (T T ) (3514) ins exp sed i The amunt f sensible and latent heat transfer rates due t air leakage frm r t the system depends n the effectiveness f the sealing prvided and the Versin 1 ME, IIT Kharagpur 14

15 cnditin f the utdr air and return air Since the lad due t return air duct including the return air fan (Q return duct ) are nt knwn a priri an initial value (eg as a fractin f ttal building cling lad) is assumed and calculatins are perfrmed This value is mdified at the end by taking int accunt the actual leakage lsses m e = m vent Return fan Return duct lsses Q s,r, Q l,r m rc A/C Rm t i, W i, h i Cling cil Supply fan m s m vent Supply duct lsses Q t,c =Q s,c +Q l,c By-pass (X) Fig352: A typical summer air cnditining system with a cling cil f nn-zer by-pass factr and return fan pwer cnsumptin Nw the ttal sensible lad n the cil (Q s,c ) is btained by summing up the ttal sensible lad n the building (Q s,r ), sensible lad due t ventilatin (Q s,vent ) and sensible lad due t return air duct and fan (Q s,retrun duct ), that is: Q = Q + Q + Q (3515) s,c s,r s,vent s,return duct Similarly the ttal latent lad n the cil (Q l,c ) is btained by summing up the ttal latent lad n the building (Q lr ), latent lad due t ventilatin (Q l,vent ) and latent lad due t return air duct and fan (Q l,retrun duct ), that is: Q = Q + Q + Q (3515) l,c l,r l,vent l,return duct Finally the required cling capacity f the system which is equal t the ttal lad n the cil is btained frm the equatin: Re quired cling capacity,q = Q + Q (3516) t,c s,c l,c One can als calculate the sensible heat factr fr the cil (CSHF) and draw the prcess line n the psychrmetric chart and find the required cil Apparatus Dew Versin 1 ME, IIT Kharagpur 15

16 Pint Temperature (cil ADP) frm the abve data as discussed in an earlier chapter As mentined, the methd discussed abve is based n CLTD/CLF as suggested by ASHRAE It can be seen that with the aid f suitable input data and building specificatins ne can manually estimate the cling lad n the building and the required cling capacity f the system A suitable safety factr is nrmally used in the end t accunt fr uncertainties in ccupants, equipment, external infiltratin, external cnditins etc This relatively simple methd ffers reasnably accurate results fr mst f the buildings Hwever, it shuld be nted that the data available in ASHRAE handbks (eg CLTD tables, SHGF tables) have been btained fr a specific set f cnditins Hence, any variatin frm these cnditins intrduces sme amunt f errr Thugh this is generally taken care by the safety factr (ie, by selecting a slightly versized cling system), fr mre accurate results ne has t resrt actual building simulatin taking int accunt n all relevant factrs that affect the cling lad Hwever, this culd be highly cmplex mathematically and hence time cnsuming and expensive The additinal cst and effrt may be justified fr large buildings with large amunt f cling lads, but may nt be justified fr small buildings Thus depending upn the specific case ne has t select suitable lad calculatin methd 356 Heating lad calculatins: As mentined befre, cnventinally steady state cnditins are assumed fr estimating the building heating lads and the internal heat surces are neglected Then the prcedure fr heating lad calculatins becmes fairly simple One has t estimate nly the sensible and latent heat lsses frm the building walls, rf, grund, windws, drs, due t infiltratin and ventilatin Equatins similar t thse used fr cling lad calculatins are used with the difference that the CLTD values are simply replaced by the design temperature difference between the cnditined space and utdrs Since a steady state is assumed, the required heating capacity f the system is equal t the ttal heat lss frm the building As already mentined, by this methd, the calculated heating system capacity will always be mre than the actual required cling capacity Hwever, the difference may nt be very high as lng as the internal heat generatin is nt very large (ie, when the building is nt internally laded) Hwever, when the internal heat generatin rate is large and/r when the building has large thermal capacity with a pssibility f string slar energy during day time, then using mre rigrus unsteady apprach by taking the internal heat surces int accunt yields significantly small heating small capacities and hence lw initial csts Hence, nce again depending n the specific case ne has t select a suitable and ecnmically justifiable methd fr estimating heating lads Versin 1 ME, IIT Kharagpur 16

17 Questins and answers: 1 Which f the fllwing statements are TRUE? a) Steady state methds are justified in case f heating lad calculatins as the utside temperatures during winter are nrmally very lw b) Steady state methds are justified in case f heating lad calculatins as the peak lad nrmally ccurs befre sunrise c) Steady state methds are justified in case f heating lad calculatins as the utside temperature variatin is nrmally lw during winter mnths d) Neglecting internal heat surces while calculating heating lads underestimates the required capacity Ans: b) and c) 2 Which f the fllwing statements are TRUE? a) An all year air cnditining system has t be switched frm winter mde t summer mde as the utdr temperature exceeds the utdr temperature at balance pint b) An all year air cnditining system has t be switched frm summer mde t winter mde as the utdr temperature exceeds the utdr temperature at balance pint c) The utdr temperature at balance pint increases as the amunt f insulatin increases d) The utdr temperature at balance pint decreases as the amunt f insulatin increases Ans: a) and d) 3 Which f the fllwing statements are TRUE? a) Methds based n rules-f-thumb are nt always useful as they are nt based n practical systems b) Methds based n rules-f-thumb are nt always useful as they d nt distinguish between a gd building design and a bad building design c) Methds based n Transfer Functin Methd are nt always useful as they d nt yield accurate results d) Methds based n Transfer Functin Methd are nt always useful as they are cmplex and time cnsuming Ans: b) and d) 4 Which f the fllwing statements are TRUE? a) An internally laded building requires a system with variable cling capacity b) An externally laded building requires a system with variable cling capacity c) An auditrium is a gd example f an internally laded building d) A residence is a gd example f an internally laded building Versin 1 ME, IIT Kharagpur 17

18 Ans: b) and d) 5 Which f the fllwing statements are TRUE? a) External lads cnsist f nly sensible cmpnents, where as internal lads cnsist f bth sensible and latent cmpnents b) Bth external and internal lads cnsist f sensible as well as latent cmpnents c) Fabric heat gain cnsists f bth sensible and latent cmpnents d) Heat transfer due t ccupancy cnsists f bth sensible and latent cmpnents Ans: b) and d) 6 Which f the fllwing statements are TRUE? a) Cling Lad Factr is used fr radiative cmpnent nly b) Cling Lad Factr is used fr bth radiative as well as cnvective cmpnents c) The value f cling lad factr always lies between 0 and 1 d) The cling lad factr increases as the thermal capacity f the walls increase Ans: a) and c) 7 Which f the fllwing statements are TRUE? a) Infiltratin lad is a part f the building lad b) Infiltratin lad is nt a part f the building lad c) Infiltratin rate increases as the pressure difference acrss the building decreases d) Infiltratin rate is uncntrllable Ans: a) 8 Which f the fllwing statements are TRUE? a) Ventilatin is t be cnsidered while calculating lad n the cil, nt n buildings b) Ventilatin is t be cnsidered while calculating lad n the cil, nt n buildings nly when the by-pass factr f the cling cil is zer c) Ventilatin is t be cnsidered while calculating lad n the cil, nt n buildings nly when the by-pass factr f the cling cil is nn-zer d) Lsses in supply duct are a part f the building lad, while lsses in return duct are a part f the cil lad Ans: b) and d) 9 A building has a U-value f 05 W/m 2 K and a ttal expsed surface area f 384 m 2 The building is subjected t an external lad (nly sensible) f 2 kw and an internal lad f 12 kw (sensible) If the required internal temperature is 25 C, state Versin 1 ME, IIT Kharagpur 18

19 whether a cling system is required r a heating system is required when the external temperature is 3 C Hw the results will change, if the U-value f the building is reduced t 036 W/mK? Ans: Frm energy balance, (Q slar + Qint ) sensible (2 + 12)x1000 Tut,bal = Tin = 25 = 833 UA 05 x 384 Since the utdr temperature at balance pint is greater than the external temperature (T ext < T ut,bal ); C the building requires heating (Ans) When the U-value f the building is reduced t 036 W/mK, the new balanced utdr temperature is given by: (Q slar + Qint ) sensible (2 + 12)x1000 Tut,bal = Tin = 25 = 185 UA 036 x 384 Since nw the utdr temperature at balance pint is smaller than the external temperature (T ext > T ut,bal ); C the building nw requires cling (Ans) The abve example shws that adding mre insulatin t a building extends the cling seasn and reduces the heating seasn 10 An air cnditined rm that stands n a well ventilated basement measures 3 m wide, 3 m high and 6 m deep One f the tw 3 m walls faces west and cntains a duble glazed glass windw f size 15 m by 15 m, munted flush with the wall with n external shading There are n heat gains thrugh the walls ther than the ne facing west Calculate the sensible, latent and ttal heat gains n the rm, rm sensible heat factr frm the fllwing infrmatin What is the required cling capacity? Inside cnditins : 25 C dry bulb, 50 percent RH Outside cnditins : 43 C dry bulb, 24 C wet bulb U-value fr wall : 178 W/m 2 K U-value fr rf : 1316 W/m 2 K U-value fr flr : 12 W/m 2 K Effective Temp Difference (ETD) fr wall: 25 C Effective Temp Difference (ETD) fr rf: 30 C U-value fr glass ; 312 W/m 2 K Slar Heat Gain (SHG) f glass ; 300 W/m 2 Internal Shading Cefficient (SC) f glass: 086 Occupancy : 4 (90 W sensible heat/persn) (40 W latent heat/persn) Lighting lad : 33 W/m 2 f flr area Versin 1 ME, IIT Kharagpur 19

20 Appliance lad : 600 W (Sensible) W(latent) Infiltratin : 05 Air Changes per Hur Barmetric pressure : 101 kpa Ans: Frm psychrmetric chart, Fr the inside cnditins f 25 C dry bulb, 50 percent RH: W i = 9,9167 x 10-3 kgw/kgda Fr the utside cnditins f 43 C dry bulb, 24 C wet bulb: External lads: W = kgw/kgda, density f dry air = 1095 kg/m 3 a) Heat transfer rate thrugh the walls: Since nly west wall measuring 3m x 3m with a glass windws f 15m x 15m is expsed; the heat transfer rate thrugh this wall is given by: Q wall = U wall A wall ETD wall = 178 x (9-225) x 25 = W (Sensible) b) Heat transfer rate thrugh rf: Q rf = U rf A rf ETD rf = 1316 x 18 x 30 = 7106 W (Sensible) c) Heat transfer rate thrugh flr: Since the rm stands n a well-ventilated basement, we can assume the cnditins in the basement t be same as that f the utside (ie, 43 C dry bulb and 24 C wet bulb), since the flr is nt expsed t slar radiatin, the driving temperature difference fr the rf is the temperature difference between the utdr and indr, hence: Q flr = U flr A flr ETD flr = 12 x 18 x 18 = 3888 W (Sensible) d) Heat transfer rate thrugh glass: This cnsists f the radiative as well as cnductive cmpnents Since n infrmatin is available n the value f CLF, it is taken as 10 Hence the ttal heat transfer rate thrugh the glass windw is given by: Q glass = A glass [U glass (T T i )+SHGF max SC] = 225[312 x x 086] = 7069 W (Sensible) e) Heat transfer due t infiltratin: The infiltratin rate is 05 ACH, cnverting this int mass flw rate, the infiltratin rate in kg/s is given by: m inf = density f air x (ACH x vlume f the rm)/3600 = 1095 x (05 x 3x3x6)/3600 m inf = x 10-3 kg/s Versin 1 ME, IIT Kharagpur 20

21 Sensible heat transfer rate due t infiltratin,q s,inf; Q s,inf = m inf c pm (T T i ) = x 10-3 x x (43 25) = 151 W (Sensible) Latent heat transfer rate due t infiltratin, Q l,inf : Q l,inf = m inf h fg (W W i ) = 88125x10-3 x 2501x10 3 ( )=164 W (sensible) Internal lads: a) Lad due t ccupants: The sensible and latent lad due t ccupants are: Q s,cc = nf ccupants x SHG = 4 x 90 = 360 W Q l,cc = nf ccupants x LHG = 4 x 40 = 160 W b) Lad due t lighting: Assuming a CLF value f 10, the lad due t lighting is: c) Lad due t appliance: Q lights = 33 x flr area = 33 x 18 = 594 W (Sensible) Q s,app = 600 W (Sensible) Q l,app = 300 W (Latent) Ttal sensible and latent lads are btained by summing-up all the sensible and latent lad cmpnents (bth external as well as internal) as: Q s,ttal = = W (Ans) Q l,ttal = = 4764 W (Ans) Ttal lad n the building is: Q ttal = Q s,ttal + Q l,ttal = = W (Ans) Rm Sensible Heat Factr (RSHF) is given by: RSHF = Q s,ttal /Q ttal = / = 0889 (Ans) T calculate the required cling capacity, ne has t knw the lsses in return air ducts Ventilatin may be neglected as the infiltratin can take care f the small ventilatin requirement Hence using a safety factr f 125, the required cling capacity is: Required cling capacity = x 125 = W 15 TR (Ans) Versin 1 ME, IIT Kharagpur 21