A Study of the Effect of Recirculation on an Air-CO 2 Evaporator Coil in a Secondary Loop of a Refrigeration System

Transcription

1 A Study of the Effect of Recrculaton on an Ar-CO 2 Evaporator Col n a Secondary Loop of a Refrgeraton System M. Ouzzane and Z. Adoun CTEC-Varennes, Natural Ressources Canada 1615, Lonel-Boulet Boulevard, Varennes, Québec, H3X 1S6, Canada Abstract Due to ts favourable thermo physcal and transport propertes, carbon doxde s among the natural fluds that represent a potental alternatve to synthetc refrgerants, namely HCFCs and HFCs, whch are harmful to the envronment (ozone depleton and greenhouse warmng effects). The obtaned results allow the collecton of detaled nformaton on ar and CO 2 across the col. The results have been compared wth those obtaned on our laboratory test bench and the agreement between the predctons and expermental data s very satsfactory. The analyss has been lmted to the evaporator col from the thermal hydraulcs pont of vew. The recrculaton rato, N has been vared n the range 1 to 4 and correspondng heat transfer coeffcents, nternal pressure drop and saturaton temperature varatons have been obtaned. Despte a substantal mprovement n heat transfer due to recrculaton (n the order of 180% for N=4), the col capacty remaned almost unchanged whle pressure drop has consderably ncreased and the correspondng saturaton temperature dropped from 1.7 o C for N=1 to 6.4 o C for N=4. Introducton Natural refrgerants ncludng CO 2 are currently proposed for the refrgeraton ndustry as alternatve solutons to the halogenated hydrocarbons whch are damagng to the envronment by ther greenhouse warmng effect. Even though CO 2 s tself a greenhouse gas ts global warmng mpact s very low when compared to synthetc refrgerants and the quanttes used by refrgeraton are neglgble n comparson to the huge amounts released by combuston for example. For all these reasons, combned wth ts very favourable thermo physcal and transport propertes, CO 2 s beng successfully rentroduced n commercal and ndustral refrgeraton. Carbon doxde may be used as a refrgerant (prmary flud, usually at low temperatures) or as a heat transfer flud wth phase change n secondary loops at medum temperatures. Ths becomes partcularly approprate as the loads become larger. Several research and development works are currently dedcated to ways of mprovng energy effcency n nstallatons where carbon doxde s used as a prmary refrgerant: t s the case of the mult-stage compresson systems [1] usng CO 2 or of the par NH 3 -CO 2 n a cascade confguraton cycle where CO 2 s the low temperature stage flud [2]. As a secondary flud, carbon doxde s beng ncreasngly used n supermarkets. In ths case the prmary loop may use ether ammona [3] or any currently accepted refrgerant such as R404a for example. It s mportant to pont out however that, CO 2 systems n general and

2 when combned wth secondary loops are commonly labelled as less effcent than synthetc refrgerants ones and t s rghtly so wthout optmsed desgn. Carbon doxde beng n many respects largely dfferent from the conventonal refrgerants n terms of propertes, the general ntent s to adapt components and operatons desgns n order to take advantage of these propertes. In parallel to ths effort, several commercal nstallatons usng volatle carbon doxde n secondary loops have been bult. Investgatons of solutons and procedures known to enhance performance for other refrgerants, whch may also be benefcal wth CO 2, among them crculaton, are performed on such nstallatons. Crculaton n refrgeraton systems s well known and s common practce n large nstallatons, partcularly wth ammona. Ths technque s now beng proposed n more modest szes as well, by nsertng an accumulator n the system. Such confguraton makes better use of the evaporator surface wth an enhanced heat transfer and an mproved overall performance. In these systems a separator supples lqud refrgerant to the evaporator whle saturated vapour s fed to the compressor. At the evaporator ext saturated refrgerant n the state of lqud-vapour mxture s sent back to the separator (X<1). The overall mpact here s a performance mprovement through a refrgerant flow rate ncrease (wth a correspondng nternal heat exchange mprovement) and a reducton or elmnaton of superheat at the compressor sucton. One of the man benefts of recrculaton appears therefore to be the enhancement of the heat transfer coeffcent on the refrgerant sde, due to two-phase flow condtons and velocty. In prncple ths mpacts postvely on performance. However ths mprovement s accompaned by saturaton pressure and related temperature varatons, resultng n property values reductons. These opposng effects are expected to lead to an optmum crculaton rato wth tradtonal refrgerants and ammona. Usng an actual nstallaton to obtan data however, mposes lmtatons on ther range and on the extent of the correspondng parametrc analyss that wll be derved. In order to get round ths problem, t s the purpose of ths study to nvestgate the mpact of the crculaton practce on heat transfer and heat exchangers performance, through modelng and test bench data. In the frst nstance ths wll be lmted to the evaporator. It s the purpose of ths study to apply the same concept of crculaton rates for CO 2, where an optmum may exst as well. Modelng and soluton procedure A mathematcal model based on the conservaton equatons of mass, momentum and enthalpy has been developed for ar col desgn and analyss, CO 2 beng the evaporatng flud. The col s represented by a set of tny control volumes and the soluton procedure of the resultng equatons was based on the forward marchng technque. Key parameters such as refrgerant propertes and flow rates, col geometry, ar temperature and approach veloctes havng a sgnfcant nfluence on performance have been used. The model attempts to best represent CO 2 evaporaton by selectng the most approprate correlatons and nformaton data, currently avalable n the lterature [4, 5, 6]. Confguratons handled by the model nclude staggered co-current and counter-current cases. The tube layout across the col s an mportant parameter nfluencng overall performance. The basc elements of a col used n the model are an elementary control volume of the fnned tube and a tube crcut, ths latter beng schematcally represented n Fgure 1. A typcal secondary loop refrgeraton col crcut may thus have the geometrc confguraton represented n Fgure 1, and staggered type

3 crcuts are the most common. A refrgeraton col s generally an arrangement of several such crcuts. Ar flows on the outsde, across the fnned col and carbon doxde flows nsde the tube. The alumnum fns assembled on the copper tube n ths example are of wavy, rectangular shape but other fn types may be adapted. It s assumed that the col s completely free from frost accumulaton wth unform flow on the arsde. On the refrgerant sde, one dmensonal flow and steady state condtons are assumed to be establshed. Nrow CO 2 Nlgne Ar Fgure 1. Evaporator col confguraton Energy and momentum balances appled on a control volume result n equatons: Q.. = m a ( Ha Ha ) = m co2 ( Hco Hco ) Q = Ug. π.d ou. ΔL. ΔT lmtd (1) (2) Ug Aou = hn A n + 1 hou D AouLn D + 2πλ. ΔL t ou n 1 (3)

4 ΔT lmtd and Ug are respectvely, the logarthmc mean temperature and the overall heat transfer coeffcent referenced to the external tube area; they are defned as: A ou ( Af + A b ) Nf * ΔL =, and An = π.d n. ΔL For dry surfaces assumed here the convectve heat transfer coeffcent, expressed n terms of the J-factor depends on a number factors ncludng the col geometry, the fn type and the manufacturng technques. The resultng predctve capabltes of the avalable correlatons reman lmted. The same general comments are also vald for pressure drop correlatons. For the present case, the recent Wang et al. [7] correlatons for heat transfer and pressure drop for wavy fn cols are used. These were selected because they rely on a large data banks used n prevous establshed correlatons and new expermental data. Ar propertes are calculated usng the standard psychometrc relatons [8]. The operatng condtons expected for a secondary loop wth volatle refrgerants, where a certan amount of crculaton wll be present are essentally free from sub cooled and/or superheated zones. In ths case, the correlatons used to account for heat transfer coeffcents are those for two-phase flow, developed by Bennet-Chen and modfed by Hwang et al. [6]. These are based on the superposton prncple, whch conssts of assumng that the heat exchange coeffcent s gven by the equaton: h = h nb + h bc (4) h nb s a coeffcent based on nucleate bolng only and h bc s the contrbuton of the convectve heat transfer only. Lnear pressure losses are calculated by the equaton: f 2 Δ PL =. ΔL.v tp(+ 1) + (v tp(+ 1) v tp() ) G (5) 2Dn Wth G beng the mass flow rate per unt area and f, the frcton coeffcent determned on the bass of a homogeneous model approach reported n [9]. Pressure drop calculaton n bends s based on the correlatons due to Chsholm [10] for two-phase flow: 2.2 B = R ξl 2 + D cu n (6) ξ l s the frcton coeffcent for a pure lqud and R cu s the radus of curvature.

5 The pressure loss s then expressed as: ρ 2 Δ Ps = ΔP l (B.X(1.0 X) + X ) (7) ρl ΔP l s for lqud only and calculated usng the followng equaton: ΔP l l 2 ξ.m = 16 ρ π co2 2 4 Dn (8) The calculaton procedure uses a marchng technque n the reverse drecton of the refrgerant flow, n order to handle counter-current heat exchanger stuatons. The ncremental nature of the soluton procedure permts parameters determnaton locally along the col. Changes n flow drecton after each pass are detected, tube passes and return bends are counted. By callng the NIST database after each step, propertes and parameters condtons are constantly updated on the refrgerant sde. Smlarly on the arsde, ar flow and psychrometrc condtons are progressvely updated. Startng wth ar nlet condtons and gven the col geometry and characterstc dmensons, a guess on CO 2 outlet condtons s made to ntate the calculaton procedure. The model then successvely determnes the values of all the relevant parameters step by step along the tube length for both fluds. Test bench and valdaton The expermental set-up shown n Fgure 2, comples wth ASHRAE standards for forced ar coolng and heatng cols [11]. Brne coolng col Blower

6 Ar CO 2 Condenser L2 Electrc heater L3 TE MT TE PT Separator/Recever CO 2 L1 PT ΔP TE CO 2 test col TE Electrc heater PT FT ΔP MT TE PT Pressure measurement TE Temperature measurement MT Dew temperature measurement FT Mass flow rate measurement ΔP Pressure drop measurement Fgure 2. Schematcs of expermental set up. Carbon doxde s the workng flud n loop (L1), whch contans the test secton consstng of a CO 2 -ar col wth alumnum wavy fns and copper tubes. The loop s well nstrumented for the purposes of heat, mass transfer balances and flud flow. For a flexble control of temperature and capacty, a brne loop (L2) was used for CO 2 condensaton. Loop (L2) s connected to a mechancal refrgeraton system (L3) to control ts temperature. Loop (L1) s located n a closed room wth two compartments correspondng to nlet and outlet of the col: ar s blown through a duct enclosng the col, from one compartment to the other. Both compartments are equpped wth temperature, pressure, flow and dew pont sensors set n accordance wth ASHRAE standards. The col s 0.22 m deep wth a face area of 0.61m x 0.32 m. The confguraton employed n ths partcular nstance has eght rows of ten tubes at 4 FPI, arranged n one crcut. The model was valdated aganst expermental results both from manufacturer s data and from experments on a test bench. On the arsde, the predctons compare farly well wth data from Bohn Heat Transfer [12] and from our own test bench for varous condtons. Inlet ar and CO 2 temperatures and CO 2 mass flow rate have been vared respectvely n the range of -10 C to -22 C, -23 C to -27 C 14.7 g/s and 44.2 g/s. Equally on the refrgerant sde, comparsons were made n terms capacty, pressure drops, temperatures and other relevant operatonal ndcators and a sample of these s shown n Table 1.

7 Table 1. Model versus expermental results Power (W) Outlet Qualty ΔP (kpa) Outlet Temp. ( o C) Ar CO 2 CO 2 CO 2 Ar CO 2 CASE 1 CASE 2 CASE 3 CASE 4 CASE 5 CASE 6 Model % Experments % Model % Experments % Model % Experments % Model % Experments % Model % Experments % Model % Experments % Analyss In the followng analyss and dscusson of data, two sets of results wll be consdered. The frst set comes from experments performed on a test bench wth col characterstcs specfed n the prevous secton. The second set of results comes from smulatons usng the model after ts valdaton as descrbed above. The model requres specfcatons of fn and tube geometry, the confguraton and the operatng condtons. Snce the purpose of ths nvestgaton s to study the evaporator col under the condtons of crculaton, the operatng condtons are adjusted so that only two-phase flow exsts (0 < x < 1). The range of crculaton ratos covered expermentally was from 1 to 5.5 approxmately. The col geometrc specfcatons are those reported n the above secton and the operatng condtons as well as resultng performance parameters are recaptulated n Table 2.

8 Table. 2. Measurement results for dfferent crculaton ratos CO 2 Ar Crculaton rato N=1.0 N=1.23 N=1.50 N=2.90 N= m CO2 (g / s) Tn (C) Pn (kpa) X (%) (ext) ΔP (kpa) Tar n ( C) m ar (g/s) Capacty (kw) On the arsde temperature and flow rate were mantaned approxmately constant. On the CO 2 sde saturaton condtons (pressure and temperature) were also mantaned approxmately constant whle the flow rate was progressvely ncreased. Assumng neglgble heat losses to the surroundng, performng a heat balance on ar and CO 2 allows estmatng col capacty and ext qualty, whle pressure drop results from drect measurement. The col capacty appears to be quas-constant despte some fluctuatons around 1.5 kw at these condtons. Bearng n mnd however that the ampltude of these fluctuatons s wthn the uncertanty range of the measurements and because of the lmted number of data ponts t s not possble at ths stage to dentfy a varaton tendency. The pressure drop on the other hand ncreases very rapdly wth crculaton rato. In order to nvestgate further ths effect, a typcal refrgeraton case for supermarket condtons has been smulated. The col operatng condtons and the correspondng results are summarzed n Table 3 for range of crculaton ratos from 1 to 4. Here agan, nternal pressure drop ncreases very rapdly, practcally ncreasng n the same proportons as the crculaton rato N, wth a correspondng decrease n the saturaton temperature through the col. There s however no major gans n capacty, whch goes through a maxmum at N=2 (an ncrease of 6.2% relatve to base). Even though the varatons here are more substantal n comparson to the prevous case they stll reman small and more nvestgaton s necessary. Recrculaton affects postvely the refrgerant sde heat transfer coeffcent, as s shown by Fgure 3. Ths s due to the combnaton of good thermo physcal propertes of CO 2 and the ncreasng flow rates. The ncrease s at least 85% when gong from N=1 to N=4. Fgure 4 represents the overall heat transfer coeffcent n terms of qualty for the same condtons but now the maxmum local ncrease of the heat transfer coeffcent s no more than 10%.

9 Table. 3. Calculaton results for dfferent crculaton ratos (T co2 =-30 C, Tar n =-24.0 C, mar = (kg / s),l=90 m) CO 2 Crculaton rato m CO2 (g / s) X (%) (ext) ΔP (kpa) Q (kw) ΔT CO Fgure 3. Internal heat transfer coeffcent dstrbuton for dfferent crculaton ratos. Fgure 4. Global heat transfer coeffcent dstrbuton for dfferent crculaton ratos. Ths s not surprsng snce the controllng factor here s the ar sde heat transfer, whch s known to be lmtng. As ponted out earler, pressure drops ncrease very rapdly as shown on Fgure 6 as functon of col length. Up to 90% of ths pressure drop occurs n the frst half of the col, partcularly for the lower crculaton ratos, where lower refrgerant qualtes are prevalng. Ths pressure drop s necessarly accompaned by a reducton n the saturaton temperature. Ths well represented by Fgure 7. Ths shows that at the col ext the temperature varaton s as much as 6.4 o C for N=4 as compared to N=1 for whch case the temperature drop s 1.7 o C. In contrast heat transfer mprovement provded by the crculaton rato s at most 0.5 o C when varyng N from 1 to 4 all along the col length, as Fgure 7 shows. As already mentoned, ths has to do wth the overall heat transfer coeffcent whch s essentally lmted by the ar sde heat transfer. The ar temperature drop across the col s shown to be sgnfcant however. It s n the order of 4 o C for N=4 and results from a combnaton of crculaton rate and excessve col tube length.

10 Fgure 5. Internal cumulatve pressure drop dstrbuton for dfferent crculaton ratos. Fgure 6. Saturaton temperature dstrbuton for dfferent crculaton ratos. Fgure 7. Saturaton temperature dstrbuton for dfferent crculaton ratos. Concluson A numercal model based on an ncremental soluton procedure has been developed. It has been valdated wth data from lterature and from experments performed on a dedcated test bench. The model has been used to study the recrculaton effect on ar/co 2 evaporator col wth a geometry that commonly used n supermarket. It has been shown that the ncreasng recrculaton rato leads to the mprovement of the nternal heat transfer whle pressure drop has consderably ncreased. Because of the hgh ar thermal resstance the capacty remaned almost

11 unchanged. Wth N=1, n comparson to other fluds, CO 2 presented a low pressure drop wtch resultng n a very low temperature gldes. References [1] A. Dspenza, C. Dspenza, V. La Rocca, D. Panno and G. Panno, Advanced Refrgeratng Plants based on Transcrtcal Cycles Workng wth Carbon Doxde for Commercal Refrgeraton, IIR Internatonal Conferences, Commercal Refrgeraton, Aug 31- Sept. 2, 2005, Vcenza, Italy. [2] S. Sawalha, J. Rogstam and P. O. Nlsson, Laboratory tests of NH 3 /CO 2 Cascade system for Supermarket Refrgeraton, IIR Internatonal Conferences, Commercal Refrgeraton, Aug 31- Sept. 2, 2005, Vcenza, Italy. [3] D. F. Pearson, Development of Carbon Doxde as a Volatle Secondary Heat Transfer Flud to Replace Glycols, 6 th IIR Gustav lorentzen Conference on natural Workng Fluds, Aug. 29- Sept.1, 2004, Glasgow, UK. [4] Corberan, J. M. and Melòn, M. G., Modellng of plate fnned tube evaporators and condensers workng wth R134a, Int. J. Refrg., Vol. 21, No4, pp , [5] Bredesen, M.; Aflekt, K.; Pettersen, J.; Hafner, A.; Neksa, P. and Skaugen, G., Studes on CO 2 Heat Exchangers and Heat Transfer, Worshop Proceedngs, CO 2 Technology n Refrgeraton, Heat Pump and Ar Condtonng Systems, Trondhem, Norway 13-14, May, , [6] Hwang, Y.; Km, B.H. and Radermacher R., Nov., Bolng Heat transfer Correlaton for Carbon Doxde, IIR Internatonal Conference, Heat Transfer Issues n Natural Refrgeraton, [7] Wang, C.C.; Hwang, Y.M. and Ln, Y.T., Emprcal Correlatons for Heat Transfer and Flow Frcton Characterstcs of Herrngbone Wavy Fn-and-Tube Heat Exchangers, Int. J. Refrg. Vol. 25, pp , [8] ASHRAE, ASHRAE Handbook of Fundamentals, SI Edton, USA, [9] Rohsenow, W.M.; Hartnett, J. P. and Cho, Y. I., Handbook of Heat Transfer, Thrd Edton, Mc Graw Hll, USA, ISBN , [10] Coller, J.G. and Tome, J. R., Convectve Bolng and Condensaton, 3 rd edton, Oxford Scence Publcatons, [11] Methods of testng forced crculaton ar coolng and heatng cols, ASHRAE Standard

12 [12] Bohn Heat Transfer Cols, Danvlle, Illnos HP750/3