Experimental Research on Heat/Mass Transfer Features of Corrugated Plate Spray Humidification Air Coolers

Transcription

1 379 A publiction of CHEMICAL ENGINEERING TRANSACTIONS VOL. 6, 017 Guest Editors: Fei Song, Hibo Wng, Fng He Copyright 017, AIDIC Servizi S.r.l. ISBN ; ISSN The Itlin Assocition of Chemicl Engineering Online t DOI: /CET Experimentl Reserch on Het/Mss Trnsfer Fetures of Corrugted Plte Spry Humidifiction Air Coolers Xioqing Hung*,b, Dongling Zhng, Xu Zhng c College of Energy nd Power Engineering, Nnjing Institute of Technology, Nnjing 11167, Chin b School of Energy nd Environment, Southest University, Nnjing 10096, Chin c Institute of HVAC &Gs, Tongji University, Shnghi 01804, Chin hxq-101@163.com This pper studies the het/mss trnsfer fetures of brnd-new ir cooling device, the corrugted plte spry humidifiction ir cooler, under dry nd wet working conditions. Specificlly, the uthor compred the ir nd wter cooling effects of the sme nozzle type in different lyouts nd different nozzle types in the sme lyout. The TF6 nozzle type + 500mm 500mm lyout ws confirmed s the optiml working conditions for the het exchnger under wet conditions. The results show tht the spry incresed both the logrithmic men temperture difference (LMTD) of the het exchnger nd the ir-side het trnsfer coefficient, without obstructing the ir-side flow or dding to the ir resistnce. Moreover, the reltionship between the contct coefficient, ir mss velocity nd wter-ir rtio ws obtined through dt fitting. 1. Introduction Air cooling is one of the most effective wter-sving mesures in power sttions. However, the cooling effect is hevily influenced by the mbient temperture. Under the high mbient temperture in summer, the hot fluid outlet temperture of the ir cooler cnnot meet the process requirements. To solve this problem, evportive cooling hs been proposed to promote the ir side het trnsfer of the ir cooler. The typicl pplictions include hybrid (dry/wet) ir cooler, deluge-type ir cooler nd (pcking/spry) humidifiction ir cooler Among them, the spry humidifiction ir cooler cools down the inlet ir by sprying wter vpour through nozzle. Becuse of low initil investment, high rte of return nd reltively simple system, the spry humidifiction ir cooler ws selected s the object of this reserch (Xun et l., 007; Dun et l., 1998). Since the ir hs smll het trnsfer coefficient, the key ims of n ir cooler re enhncing het trnsfer, reducing flow resistnce nd incresing compctness. Therefore, the corrugted plte het exchnger, known for efficient het exchnge nd compct structure, enjoys bright mrket prospects. In this pper, severl experiments re crried out on corrugted plte spry humidifiction ir cooler. The cooler is brnd-new ir cooling device with structure entirely different from tht of trditionl tubulr ir cooler or finned tube ir cooler. Over the yers, Kchhwh, Stoitchkov, Armbruster, Yn, Wng, Kim, Cherif, Bell, Heyns nd others (Kchhwh et l., 1998; Stoitchkov nd Dimitrov, 1998; Armbruster nd Mitrovic, 1998; Yn, 1998; Wng nd Mmishev, 01; Kim et l., 011; Cherif et l., 011; Bell et l., 011) hve ll contributed to the reserch on the het/mss trnsfer nd flow resistnce fetures of evportive coolers. In terms of the het/mss trnsfer coefficient, the mss trnsfer coefficient is usully obtined through experiment, nd the het trnsfer coefficient through nlogy using the Lewis reltionship eqution.. Experimentl System nd Working Conditions The experimentl system comprises humidifiction system, hot fluid system, n ir system nd mesurement control system. The humidifiction system nd corrugted plte het exchnger (Figure 1) form the experimentl section. In the het exchnger, there re two rows of nozzles, ech of which hs two nozzles. The nozzles re rrnged in three different lyouts: (L W) 500mm 500mm, 500mm 75mm, nd Plese cite this rticle s: Xioqing Hung, Dongling Zhng, Xu Zhng, 017, Experimentl reserch on het/mss trnsfer fetures of corrugted plte spry humidifiction ir coolers, Chemicl Engineering Trnsctions, 6, DOI: /CET176064

2 380 75mm 75mm. The hot fluids flow into the het exchnger from the top right corner nd flows out from the top left corner. Under dry working conditions, the ir cools down the hot fluid directly from outside of the plte bundle. Both the ir nd the hot fluid re subject to cross-flow het exchnge. Under wet working conditions, the nozzles on the side of plte bundle open up nd eject fluid t the ir inlet. Figure illustrtes the structure of the experimentl section of the het exchnger. The hot fluid flows in the plte bundle through 6.4mm-wide chnnel, nd the ir flows outside the plte bundle lso through 6.4mm-wide chnnel. The 0.7mm-thick corrugted plte is mde of stinless steel. The spirl pressure nozzle TF4 nd ir tomizing nozzle AM4 (Figure 3) were selected for the experiments. The experiments were conducted under both dry nd wet working conditions. The dry conditions include vrible ir volumes (VAV) nd vrible wter volumes (VWV). In ddition to the VAV nd the VWV, the wet conditions lso involve different lyouts of the sme nozzle type, nd different nozzle types in the sme lyout. Returned therml fluid Supplied therml fluid Nozzle Het exchnger plte bundle Spry wter supply Pre-treted ir inlet Figure 1: Corrugted plte spry humidifiction ir-cooler Figure : Experimentl section of the corrugted plte het exchnger Figure 3: TF4 nd AM4 nozzles 3. Comprison of Spry Cooling Effect This section compres the cooling effects of the sme nozzle type in different lyouts to determine the optiml lyout, nd contrsts the cooling effects of different nozzle types in the sme lyout to identify the nozzle for optiml humidifiction. 3.1 The sme nozzle type in different lyouts Bsed on previous reserch, three types of lyouts were selected to determine the optiml lyout, nmely, 500mm 75mm, 500mm 500mm nd 75mm 75mm. The ir cooling effects of the three lyouts re compred in Figure 4, where the wet-bulb depression of inlet ir is the cooling limit, nd the spry mount is closely relted to contct re between wter vpour nd the ir. The ir cooling effect ws enhnced with the increse in the product of wet-bulb depression nd spry

3 mount. The 500mm 500mm lyout yielded the best cooling effect, followed in descending order by 75mm 75mm nd 500mm 75mm. Hence, 500mm 500mm is the best lyout in terms of ir cooling effect. 381 Figure 4: Air cooling effects of different lyouts Figure 5: Wter cooling effects of different lyouts The wter cooling effects of the three lyouts re compred in Figure 5, where the het trnsfer is driven by the logrithmic men temperture difference (LMTD). The wter cooling effect ws enhnced with the increse in the product of the LMTD nd spry mount. The 500mm 500mm lyout yielded the best cooling effect, followed in descending order by 75mm 75mm nd 500mm 75mm. Hence, 500mm 500mm is lso the best lyout in terms of wter cooling effect. The experiment results show tht 500mm 500mm is superior to 75mm 75mm nd 500mm 75mm in both ir cooling effect nd wter cooling effect. 3. Different nozzle types in the sme lyout To determine the optiml nozzle for humidifiction, this section dopts the 500mm 500mm lyout ccording to the experimentl results in Section 3.1, nd select the nozzle types of TF6 4 (two rows, ech contining nozzles) AM4 4, nd TF6 +AM4. The ir cooling effects of different nozzle types re compred in Figure 6. It cn be seen tht the ir cooling effect ws enhnced with the increse in the product of wet-bulb depression nd spry mount. The TF6 4 bosts the best cooling effect, followed in descending order by AM4 4 nd TF6 +AM4. Hence, TF6 4 is the best nozzle type in terms of ir cooling effect. Figure 6: Air cooling effects of the different nozzle types Figure 7: Wter cooling effects of different nozzle types The wter cooling effects of different nozzle types re compred in Figure 7. As shown in the figure, the wter cooling effect ws enhnced with the increse in the product of the LMTD nd spry mount. TF6 4 nd TF6 +AM4 exhibited better cooling effect thn AM4 4. The experimentl results show tht TF6 4 outperforms AM4 4 nd TF6 +AM4 in both ir cooling effect nd wter cooling effect. To sum up, the TF6 nozzle type + 500mm 500mm lyout ws confirmed s the optiml working conditions for the het exchnger under wet conditions.

4 38 4. Air-Side Flow Resistnce nd Het Trnsfer Fetures under Wet Conditions Under wet conditions, the flow resistnce drop on the ir side of the ir cooler incresed with the fce velocity (Figure 8). Bsed on the experimentl dt, the reltionship between the ir-side resistnce drop, fce velocity nd plte width cn be obtined s follows: Δ P = H v 0.3m/s v R = m/s (1) Figure 8: Air-side flow resistnce drop curve Figure 9: Reltionship between Re nd Eu The sufficient turbulence ws chieved when fce velocity reched 3m/s. Bsed on the experimentl dt, the following expressions cn be obtined: Eu = H Re R = Re () Figures 8 nd 9 show tht the resistnce drop curves nd the resistnce curves under wet conditions coincide with those under dry conditions. This mens the unevported droplets did not obstruct the ir-side flow, resulting in no reduction of ir flow re or increse in ir resistnce. Figure 10 depicts the reltionship between the ir-side Nu nd the ir-side Re. Under wet conditions, the reltionship between Nu nd Re on the ir side cn be obtined s follows: Nu = 0.03 Re R = Re (3) According to Figure 10, s long s the ir flow volume remins constnt, the ir-side het trnsfer coefficient under wet conditions ws much greter thn it ws under dry conditions; the het trnsfer coefficient ws 8~10 times s much s tht under dry conditions t the moment of sufficient turbulence, nd 8~16 times s much s tht under dry conditions before tht moment. The results indicte tht the spry lowered the drybulb temperture of inlet ir, nd pushed up the LMTD over the corrugted plte, but the ir-side het trnsfer coefficient ws incresed s unevported droplets crried on with the het bsorption inside the plte. 5. Air-Side Mss-trnsfer Fetures under Wet Conditions The spry cooling effect cn be influenced by vrious fctors, including but not limited to ir mss velocity, nozzle type, distribution density, tube dimeter, wter pressure, s well s the contct durtion, movement direction nd initil/finl prmeters of the ir nd wter. In typicl ir tretment processes, the most importnt fctors re ir mss velocity, wter-ir rtio nd nozzle structure. To depict the similrity between the ctul process nd the idel process with limited wter but sufficient contct durtion, the contct coefficient η ws introduced below:

5 383 t t η = t t s1 1 1 s1 (4) where t 1 is the dry-bulb temperture of inlet ir (); t s1 is the wet-bulb temperture of inlet ir (); t is dry-bulb temperture of outlet ir (). Insted of mthemticl clcultion, the contct coefficient cn only be determined through experiments. In the cse of the sme nozzle type in different lyouts, the contct coefficient cn be clculted by the following eqution: η = Avρ μ ' ' ( ) m n (5) where vρ is the ir mss velocity (kg/(m s)); v is the ir velocity (m/s); ρ is the ir density (kg/m 3 ); μ is the wter-ir rtio. The coefficient A nd indices m nd n cn be cquired through experimentl dt fitting. The spry wter flow is generlly expressed with the wter-ir rtio μ, tht is, the wter flow consumed in the tretment per kilogrm of ir: m = W kg ( wt er )/kg( i r ) G (6) where W is the ggregte spry wter flow (kg/s); G is the ir flow through the corrugted plte (kg/s). The experimentl dt re sorted bsed on Eqution (5). As shown in Figure 11, the contct coefficient incresed with the wter-ir rtio when the ir mss velocity reched vρ=3.6 kg/(m s). In other words, with the increse in wter pressure, the dry-bulb temperture declined stedily before the ir entered the corrugted plte. According to the experimentl results, the contct coefficient ws pproximtely 1, i.e. the cooling limit, t the ir flow of 15,55 m 3 /h nd the TF6 4 wter pressure of 0.55MP. (Under the cooling limit, dry-bulb temperture drops to the wet-bulb temperture of inlet ir. There is n optiml wter pressure despite the simultneous increse in wter pressure nd contct coefficient.) Figure 10: Reltionship between the ir-side Nu nd the ir-side Re Figure 11: Vrition in contct coefficient with the wter-ir rtio of TF Conclusions (1) Through experimentl dt fitting, the uthor obtined the flow resistnce nd het-trnsfer fetures on both the ir side nd fluid side under dry conditions, lying solid bsis for the design of corrugted plte ir coolers. () The TF6 nozzle type + 500mm 500mm lyout ws confirmed s the optiml working conditions for the het exchnger under wet conditions. (3) The spry lowered the dry-bulb temperture of inlet ir, nd pushed up the LMTD over the corrugted plte, but the ir-side het trnsfer coefficient ws incresed s unevported droplets crried on with the het bsorption inside the plte. The unevported droplets did not obstruct the ir-side flow, resulting in no reduction of ir flow re or increse in ir resistnce. (4) The reltionship between contct coefficient, ir mss velocity nd wter-ir rtio ws obtined by dt fitting before wter pressure reched 0.55MP.

6 384 Acknowledgement This pper is mde possible by the generous support from Nnjing Institute of Technology (Grnt No. YKJ01406) nd the Ntionl Nturl Science Foundtion of Chin (Grnt No ). References Armbruster R., Mitrovic J., 1998, Evportive cooling of flling wter film on horizontl tubes. Experimentl Therml nd Fluid Science, 18(3), , DOI: /S (98)10033-X Bell I.H., Groll E.A., Koenig H., 011, Experimentl nlysis of the effects of prticulte fouling on het exchnger het trnsfer nd ir-side pressure drop for hybrid dry cooler, Het Trnsfer Engineering, 3(3-4), 64-71, DOI: / Cherif A.S., Kssim M.A., Benhmou B., 011, Experimentl nd numericl study of mixed convection het nd mss trnsfer in verticl chnnel with film evportion, Interntionl Journl of Therml Sciences, 50(6), , DOI: /j.ijthermlsci Dun Z., Zhn C., Zhng X., Mustf M., Zho X., 1998, Indirect evportive cooling:pst, present nd future potentils, Renewble & Sustinble Energy Reviews, 16(9), , DOI: /j.rser Kchhwh S.S., Dhr P.L., Kle S.R., 1998, Experimentl studies nd numericl simultion of evportive cooling of ir with wter spry-i, Horizontl counter flow, Interntionl Journl of Het nd Mss Trnsfer, 41(), , DOI: /S (97) Kim M.H., Kim J.H., Choi A.S., Jeong J.W., 011, Experimentl study on the het exchnge effectiveness of dry coil indirect evportion cooler under vrious operting conditions, Energy, 36(11), , DOI: /j.energy Stoitchkov N.J., Dimitrov G.I., 1998, Effectiveness of crossflow plte het exchnger for indirect evportive cooling: Efficcité des échngeurs thermiques à plques, à cournts croises pour refroidissement indirect évportif. Interntionl Journl of Refrigertion, 1(6), , DOI: /S (98) Wng H.C., Mmishev A.V., 01, Het trnsfer correltion models for electrospry evportive cooling chmbers of different geometry types, Applied Therml Engineering, 40(40), , DOI: /j.pplthermleng Xun Y.M., Xio F., Niu X.F., Hung X., Wng S.W., 007, Reserch nd ppliction of evportive cooling in Chin: A review (I)-Reserch, Renewble & Sustinble Energy Reviews, 16(5), , DOI: /j.rser Yn W., 1998, Evportive cooling of liquid film in turbulent mixed convection chnnel flows, Interntionl Journl of Het nd Mss Trnsfer, 41(3), , DOI: /S (98)