IMPACT OF ENVIRONMENTAL CONDITIONS ON THE PERFORMANCE OF OPEN MULTIDECK DISPLAY CASE EVAPORATORS

Transcription

1 MPACT OF ENVRONMENTAL CONDTONS ON THE PERFORMANCE OF OPEN MULTDECK DSPLAY CASE EVAPORATORS G. G. HEDNGER (), S. M. NASCMENTO (b), P. D. GASPAR (c) () (b) Eletrofrio Refrigerção Ltd, R João Chede, 1599, Cidde ndustril, Curitib - PR, , Brzil +55 (41) , () gustvo@eletrofrio.co.br, (b) suel@eletrofrio.co.br (c) University of Beir nterior, Engineering Fculty, Dept. of Electroechnicl Engineering, Covilhã, , Portugl , (c) dinis@ubi.pt ABSTRACT This pper reports the results of experientl tests perfored ccording to SO in n open ultideck disply cse with dul ir curtin in order to evlute the ipct of environentl conditions on the perfornce of its evportor. Experientl lbortory tests were perfored on two clite clsses usully found in Brzilin food stores (n.º 3: 25ºC nd 60% nd n.º 6: 27ºC nd 70%). n either test the xiu product teperture ws intined between 4 nd 5 C. The experientl results coprison shows the quntittive vrition of the frost fortion rte on the evportor coil (+37%) nd how this phenoenon ffects the distribution nd gnitude of ir flow, pressure drop, inlet nd outlet tepertures nd the coponents of the totl sensible (+10%) nd ltent (+46%) het lods. The nlysis of experientl dt provides vluble infortion to the evportor design bsed on in-store environentl conditions. 1. NTRODUCTON Knowing tht 28% of the energy consued in lrge superrkets is ttributed to copressors of refrigertion systes (ASHRAE, 2010), ny study iing to iniize this cost is of gret vlue nd iportnce. Concerning the refrigerted disply cses, the energy consuption is strongly connected to the environentl conditions of the store in which they re plced. The higher the teperture nd bsolute huidity of the bient ir, the greter the sensible nd ltent het lods which the cooling syste ust reove. The increse of ir huidity in the refrigerted spce increses the condenstion rte nd the frost fortion on the het exchnger, which in turn requires greter frequency of dily defrosting. This condition increses the het lod due the need to recover the preservtion teperture of food products fter defrosting. Additionlly, it prootes the teperture instbility of the exposed products. n generl ters, refrigerted disply cses re designed to operte in ir-conditioned superrkets. The interntionl industry considers suer clitic conditions to be ir teperture t 25ºC with 60% of reltive huidity (SO 23953:2005 clite clss n.º 3). This clite condition is considered stndrd for hoologtion of this type of equipent (criticl suer conditions). n Brzil, not ll stores hve ir conditioning, nd ore dverse ir teperture nd huidity conditions frequently occur. Thus, in these situtions, n ir teperture of 27 C with 70% of reltive huidity (SO 23953:2005 clite clss n.º 6) is usully considered. Tssou & Dtt (1999), Gs Reserch nstitute (2000), Gspr et l. (2007) nd Mousset & Libsig (2011) quntified the energy svings nd iproved perfornce in refrigerted disply cses operting in environentl conditions which re less dending thn clite clss n.º 3, inly tking into considertion lower levels of ir huidity which cuse to sller rte of frost fortion on the evportors, nd subsequent reduction in the ltent cooling lod. Kosr & Duitrescu (2005) conducted review relted with the influence of bient ir huidity on refrigerted disply cses. After copring the results of five uthors, they concluded tht reduction of ir huidity fro 55% to 35% in typicl superrket could sve thousnds of kwh in energy spent on nti-condenstion systes, electricl defrosting nd copressors. This infortion is of gret iportnce to the present study, lthough no studies were found which quntified the perfornce of refrigerted disply cses operting under severe operting conditions (Clite clss n.º 6). So, this rticle describes the experientl procedures for tests conducted on this clite clss, nd presents dt suited for the design of het exchngers, due to their relevnt role in iproving the perfornce of

2 refrigerted disply cses operting in dverse clite conditions while lwys ensuring sfe teperture levels of shelf products, s required by food sfety stndrds. 2. EXPERMENTAL PROCEDURE Tests were conducted in n environentl test chber (Figure 1) constructed ccording to SO (2005). The cooling equipent selected for the experients is n open verticl refrigerted disply cse ( ) with dul ir curtin (Figure 2 nd Figure 3) used for displying et products, which teperture should be intined between -1 C nd 5 C (SO :2005 clss M1). Figure 1 - Environentl test chber. () (b) Figure 2 - Open verticl refrigerted disply cse with dul ir curtin. Ech test period lsted 24 hours. Dt cquisition relted to ir teperture, reltive huidity, velocity nd pressure drop ws conducted t 1-inute intervls in order to nlyse vritions of these vlues over tie. To esure physicl preters on the R22 refrigernt side, dt ws collected ccording to the recoendtions provided by SO (2005). To esure physicl quntities on the ir side, sensors were positioned t 5 equidistnt points fro the ir inlet nd outlet long the length (y) of the evportor (Figure 2b). Thus, the evportor ws divided into 5 virtul control volues (CV), so tht ir teperture, huidity nd velocity in ech CV re considered constnts (Figure 3b). With this procedure, it ws possible to deterine the ir distribution in the evportor, nd which zones re ore or less likely to for frost.

3 The ss of wter condensed during defrosting ws nully collected nd esured during ech test period. The outlet ss flow of the liquid coolnt which pssed through the evportor during ech operting period ws onitored by Coriolis flow onitor. The nufcturer of the refrigerted disply cse recoends, for idel opertion, the evportion teperture to be -10 ºC, ensuring products teperture within the prescribed liited rnge between -1 ºC nd 5 ºC. nitilly, the refrigerted disply cse defrosts 10 ties per dy, nd ech defrost period lsts 12 inutes. The defrosting control syste consists of stopping the cooling syste while intining the fns in opertion in order to defrost the ice on the evportor surfce by the pssge of bient-teperture ir. For testing purposes, there is no controller liiting the copressor work, whether therostt tht ceses its opertion depending on teperture, or soe other echnis which turns off the copressor during the operting period. The copressor is only turned off when the defrosting process begins. The clitic conditions of the roo vry ccording to the clitic clss desired for the test. n ore plesnt clites, the product tepertures re lower, nd in wrer nd ore huid clites, the product tepertures increse. n order to ke fir coprison, the xiu teperture of the wrest product ws intined between 4 ºC nd 5 ºC regrdless the clitic condition. t ws possible to ke this djustent by siply incresing or decresing the usble overheting of the refrigernt fluid. n this wy, there ws reduction or increse in the required refrigernt flow, nd consequent reduction or increse in the het cpcity of the evportor ccording to the needs of ech clite clss. () (b) Figure 3 - Division of evportor into virtul CVs. 3. MATHEMATCAL MODEL The totl het lod of the cooling syste on the refrigernt liquid side, Q TOT, is deterined by clculting the difference in enthlpies between inlet, i r, i, nd outlet, i r, o, nd the ss flow of the liquid refrigernt, r, s described in SO (2005) nd shown in eq. (1). TOT r r,o r,i Q i i (1) n order to deterine the totl het lod nd its coponents on the ir side, the ir flow rtes nd the condenstion nd freezing rtes of wter for ech cooling cycle were deterined. Considering unifor rtes of outflow for ech CV, nd using the verge vlues for velocity, V, nd density,, of ir in the inlet nd outlet res of the evportor of cross section, A, obtined by experientl spling bsed on the principle of conservtion of ss, the ss flow of ir,, ws obtined by eq. (2). ρ V A (2)

4 Using the verge vlues of ir teperture nd reltive huidity t the inlet nd outlet res of the evportor, bsolute huidity of the ir in ech of the situtions ws esured using psychroetric concepts,, i nd, o, respectively. Thus, using eq. (3), the ss flow of condensed wter is deterined for the period in which the refrigertion ws turned on,, nd for the period in which the refrigertion ws turned off, period, w, EXP opertion period, w, DRAN w, DEF w, OP. The ss flow of wter collected in experientl spling during the defrosting, given by eq. (4) is the su of the ss flow of ice fortion on the evportor during the, nd w, DEF. A sll prt of the w, OP ws drined without freezing. This ss flow,, is deterined by eq.(5). w,i,o (3) (4) w, EXP w, DEF (5) w, DRAN w, OP The coponents of het lod cn be deterined by eq. (6) to eq. (10), ccording to GAS Reserch nstitute (2000).,COLD Qw,COND i,i i,o Q (6) w,cold Q (7) w,cond w,op hw p,w DEW FREEZE Q c T T (8) Q h (9) w, FREEZE Q c T T (10), COLD p, FREEZE The totl, sensible nd ltent het lods cn be deterined by eq. (11), eq. (12) nd eq. (13), respectively. Q TOT,COLD w,cond w,cold Q Q Q Q Q (11) w,freeze,cold Q LAT Q Q (12) w, COND w, FREEZE Q SENS Q Q Q (13),COLD w,cold,cold Correspondingly, two ethods of deterining the refrigertion het lod re obtined. The results of these two clcultion ethods re copred nd discussed. 4. ANALYSS AND DSCUSSON OF RESULTS 4.1 Air huidity, condenstion nd freezing of wter The rtes of condenstion nd freezing of wter between the evportor fins increse with the huidity of the bient ir where the refrigerted disply cse is operting. The i of experientl study ws to esure how representtive is the increse of bient ir huidity nd teperture on the rtes of condenstion nd freezing during the entire opertion nd defrosting period. The experientl results obtined for the test conducted for clite clsses n.º 3 nd n.º 6 re shown in Tble 1.

5 Tble 1 - Absolute vlues for condenstion nd freezing of wter in experientl tests on clite clsses n.º 3 nd n.º 6. Clite clss w, OP w, DEF w, DRAN w, TOT [kg/24h] [kg/24h] [kg/24h] [kg/24h] [kg/24h] n.º n.º ncrese [%] Copring the experientl results of clite clss n.º 3 nd clss n.º 6 shown in Tble 1, it cn be concluded tht: (1) the totl quntity of condensed wter in the evportor is 47% greter; (2) the quntity of wter which solidifies between fins is 37% greter; nd (3) the quntity of wter which drins during the operting period is 168% greter. Such nubers prove the significnt influence of clite conditions on the opertion of evportors regrding their frost fortion. The following sections detil other spects relted to the evportor perfornce. 4.2 Air distribution nd pressure loss in the evportor The distribution of irflow ws evluted t two seprte ties: when the evportor ws copletely clen, i.e. fter defrosting the coil nd ieditely before defrosting it. The percent distribution of irflow in the five virtul CVs of the evportor is shown in Figure 4. Figure 4 - Distribution of flow in the evportor. Fro the nlysis of Figure 4, it is stted tht fter defrosting, irflow is eqully distributed in ech CV for both clss n.º 3 nd clss n.º 6 (pproxitely 20% in ech CV). CVs 2 nd 4 re those possessing the gretest irflow circulting, nd oreover, the greter velocity of ir pssing between fins. The lower quntity of irflow circulting in CVs 1 nd 5 cn be ttributed to extreities effects (friction nd consequently pressure drop) on the evportor. The lower proportion of irflow circulting in CV 3 is ttributed to the structurl colun which restricts the pssge of ir nd restrins idel fortion of ir plenu in the centre of the evportor. When the evportor is redy to be defrosted, the percentge distribution of irflow is significntly different fro the initil esureent. This fct is ttributed to the irregulr fortion of frost. The irflow through CV 3 is less thn the other CVs. t cn be concluded tht the frost is ost likely to for in this region. For clite clss n.º 6, the reduction is ore significnt, s before defrosting only 7% of the irflow circulting in the evportor goes through this region, quntity uch lower thn the initil esureent of 19%. This drstic reduction suggests tht the frost fortion grows significntly s function of bient ir conditions. The increse in the percent irflow inly in CVs 2 nd 4 is due to these regions re less likely to for frost

6 nd therefore, s the resistnce to outflow is less, they becoe the preferred pths for ir. During the cooling period, the totl bsolute irflow decresed 24% for clite clss n.º 3 nd 40% for clite clss n.º 6. Figure 5 shows the ir pressure drop vrition before nd fter the fn bnk. The ir pressure drop is ore significnt for clite clss n.º 6 nd increses pproxitely 33% during the cooling period, while for clss n.º 3 the increse is pproxitely 23%. These results prove tht greter reduction of irflow in hot nd huid environentl conditions s consequence of greter frost fortion which is responsible for increse in resistnce to ir circultion between fins due to the decrese in free outflow re. The reduction in ir velocity is the in fctor in cooling cpcity loss during the cooling period (Aljuwyhel et l., 2008). Figure 5 - Air pressure drop vrition before nd fter fns. 4.3 nfluence of frost fortion on ir tepertures Figure 6 shows the vrition of the teperture of ir entering nd leving the evportor for clite clsses n.º 3 nd n.º 6. The ir teperture t the evportor outlet is lower for clss n.º 6. This is due to the different pretriztions needed to intin the xiu teperture of test pckges (product siultors) below the estblished 5 C liit. Greter cpcity to protect the refrigerted spce is necessry when the clite is ore dverse in order to intin product tepertures. Figure 6 - Air tepertures in the evportor. The circulting ir teperture nd the difference between the tepertures t the evportor inlet nd outlet increse s the defrost period ners. After defrosting, the ir is colder, nd the difference between the ir t

7 the evportor inlet nd outlet is less thn before defrost. Therefore, with the evportor free fro frost, the het trnsfer efficiency is t his xiu level. When the surfce of fins is covered with frost, the efficiency lowers. As frost fors over tie, there is n increse in the difference between the teperture of ir entering nd leving the evportor due to the decresed irflow on the evportor. This sitution ens cooling cpcity loss in the het exchnge due to reduced efficiency of the ir curtin, which llows greter quntity of externl ir to be tken in by the fns. As result, the teperture of the ir entering the evportor increses nd consequently increses the rte of frost fortion between fins. A substntil increse in the difference between ir entering nd leving the evportor is verified when the teperture nd huidity of the surrounding environent re greter (clite clss n.º 6). As the lyer of frost fored is thicker, the resistnce to het trnsfer is incresed, king the conduction/convection of het between the fluids ore difficult. Tble 2 shows ir tepertures vlues t two different ties: before nd fter defrosting the coil. Tble 2 - Air tepertures in the evportor. Period Clite clss n.º 3 Clite clss n.º 6 T,i T, o T T, i T, o T After defrosting Before defrosting ncrese [%] For clite clss n.º 3, the difference in the teperture of ir t the inlet nd outlet of the evportor increses 10 % nd 19% over clite clss n.º 6. Anlysing the dt in Figure 5 nd Tble 2, shows tht the teperture of ir leving the evportor, fter reching its highest vlue, strts reducing until the end of the operting period in clite clss n.º 6, gin proving the lrger reduction in the circulting irflow. 4.4 Het lod The totl het lod nd its coponents re shown in Figure 7. Figure 7 - Het lod nd its coponents. The het lod of the equipent incresed 25% fro clite clss n.º 3 to clss n.º 6. n clite clss n.º 3, the ltent het lod (condenstion nd freezing of wter) represents 34.6% of the totl het lod, with the reining 65.4% due to sensible het of cooling the ir, the wter nd frost. The ltent het due to freezing of ice represents only 3.9% of the totl lod nd the sensible het lod due to cooling the wter nd ice represents only 0.3% of the totl lod. For clite clss n.º 6, the ltent het lod represents 40.9% of the totl het lod, with the reining 59.1% due to sensible het of cooling the ir, wter nd ice. The ltent het due to freezing the ice represents only 4.3% of the totl lod nd the sensible het lod due to cooling wter nd ice represents only 0.5% of the totl lod.

8 5. CONCLUSONS This experientl study seeks to nlyse the conditions of bient ir in the fortion of frost on the surfce of evportors of refrigerted disply cses. Experientl tests re designed for clite clsses n.º 3 nd n.º 6. The in conclusions of this study re: (1) the frost fortion significntly ffects the distribution of ir in the evportor due to its irregulr fortion in fins; (2) the quntity of ir circulted in the disply cse is decresed over tie due to the reduction of free re for ir outflow nd consequent increse of pressure drop; (3) the reduction in the evportor's het cpcity during the operting period is inly due to the reduction of irflow; (4) to intin the se conditions for preserving products under different clitic conditions, it is necessry to chnge the teperture t which the ir leves the evportor in such wy tht the degree of protection offered by the ir curtin cn be controlled; (5) clitic conditions significntly influence the energy consuption of refrigertion equipents, since the totl het lod increses s clitic conditions becoe ore dverse, nd the increse in ltent het lod is greter thn the increse in sensible lod; (6) the het lods due to the cooling of wter nd ice re not significnt. NOMENCLATURE A re (²) Subscripts CV control volue ir c p het specific to constnt pressure (kj/kg K) COLD cooling h ltent het (kj/kg) COND condenstion i specific enthlpy (kj/kg) DEF defrosting ss flow (kg/s) DEW dew Q het lod of cooling (kw) DRAN drin T teperture ( C) EXP experientl V velocity (/s) FREEZE freezing bsolute huidity (kg wter /kg ir ) i inlet ice Greek sybols LAT ltent specific ss (kg/³) o outlet T difference in teperture ( C) OP opertion r refrigernt fluid SENS sensible TOT totl w wter REFERENCES [1] ASHRAE 2010, ASHRAE Hndbook: Refrigertion, Aericn Society of Heting, Refrigerting nd Air-Conditioning Engineers, nc. [2] SO , "Refrigerted disply cbinets - Prt 2: Clssifiction, requireents nd test conditions", nterntionl Orgniztion for Stndrdiztion. [3] Tssou S., Dtt D. 1999, nfluence of superrket environentl preters on the the frosting nd defrosting of verticl ultideck disply cbinets, ASHRAE Trnsctions 105(1): [4] GAS Reserch nstitute 2000, nvestigtion of reltive huidity ipcts on the perfornce nd energy use of refrigerted disply cses, Finl Report. [5] Gspr, P., Pitr, R., Gonçlves, L. 2007, Estudo experientl do ipcto d vrição ds condições do r biente no desepenho globl de expositores refrigerdos bertos, Engenhri 2007 novção e Desenvolviento. [6] Mousset, S., Libsig, M. 2011, Energy consuptions of disply cbinets in superrket, 23 rd nterntionl Congress of Refrigertion, D:282. [7] Kosr, D., Duitrescu, O. 2005, Huidity Effects on Superrket Refrigerted Cse Energy Perfornce: A Dtbse Review, ASHRAE Trnsctions 111(1): [8] Aljuwyhel N., Reindl D., Klein S., Nellis G. 2008, Experientl investigtion of the perfornce of industril evportor coils operting under frosting conditions, nt. J. Refrig. 31(1):