The Effects of Evaporative Cooling in Tropical Climate

Transcription

1 Americn Journl of Mechnicl Engineering, 2017, Vol. 5, No. 4, Avilble online t Science nd Eduction Publishing DOI: /jme The Effects of Evportive Cooling in Tropicl Climte Robert Poku *, Tokoni W. Oyinki, Ezenw A. Ogbonny Deprtment of Mrine/Mechnicl Engineering, Niger Delt University, Wilberforce Islnd, Byels Stte, Nigeri *Corresponding uthor: robertpoku21@gmil.com Abstrct The performnce of therml power plnts, chievement of humn comfort, preservtion of groceries etc. re generlly dversely ffected by poor environmentl conditions. In order to provide solutions to these chllenges, n evportive cooling system ws developed nd studied. The study ws imed t providing lower tempertures for the efficient performnce of mchineries nd humn comfort s well s lower temperture nd higher reltive humidity necessry for overcoming the bove dverse condition. The performnce of the cooler ws evluted in terms of temperture drop, cooler cpcity, sturtion efficiency nd fesibility index. The results showed tht evportive cooling is chievble with fesibility index of F* 10, when the difference between inlet dry bulb tempertures nd wet bulb temperture re greter, T 1 -T w = 11.5 C nd T 1 -T 2 = C with F*=9; T 1 -T w = 12 C nd T 1 -T 2 = C with F*=10 respectively. The results lso ffirmed tht cooler cpcity nd the sturtion index re higher where the fesibility indexes re comprtively low. Keywords: evportive cooler, fesibility index, sturtion efficiency, cooler cpcity Cite This Article: Robert Poku, Tokoni W. Oyinki, nd Ezenw A. Ogbonny, The Effects of Evportive Cooling in Tropicl Climte. Americn Journl of Mechnicl Engineering, vol. 5, no. 4 (2017): doi: /jme Introduction Evportive cooling occurs when ir, tht is not too humid, psses over wet surfce (humidifier). The movement of the ir cn be pssive, tht is when the ir flows nturlly through the pds (wet surfce) or ctive when it is mde to flow with fns or blowers [1]. It provides cold ir for gs turbine inlet ir, buildings cooling nd perishbles such s vegetbles nd fruits preservtion. It offers n lterntive to conventionl ir conditioners in hot, dry climtes with high system efficiency, low initil investment nd mintennce. It lso hs the dvntge of providing some level of cooling for frction of the energy consumption [2]. According to [3], regions with hot climtes, energy for spce cooling ccounts for over 60% of the totl energy used in buildings. Direct evporting cooling systems in low humidity zones typiclly relize n energy svings of 60 to 80% over refrigerted systems [4]. Reson being tht the only power consuming components of n evportive cooler re fns nd wter pumps. Energy svings with evportive cooling vry with humidity levels nd tempertures. It is lso n environmentlly friendly nd energy efficient cooling method tht only uses wter s the working fluid [4]. Physiologicl fctors such s respirtion involve het emission resulting in increse of temperture, nd this produces metbolic process nd decy [5]. Generlly, the loss of freshness of perishble commodities depends on the rte of respirtion in the storge spce [6]. And this results to rise in temperture. Evportive cooling therefore becomes n efficient nd economicl mens for reducing the temperture nd incresing the reltive humidity in storge spce. This effect hs been extensively tried for incresing good shelf-life of vegetbles. Extending the shell-life of vegetbles nd fruits, severl uthors hve recommended the proper storge of these products t lower tempertures. Proper storge mens controlling both the temperture nd reltive humidity of the storge spce [7]. Reson being tht the storge of vegetble crops is one which requires high reltive humidity nd low ir temperture. Evportive cooling device will llow for the pproprite cooling tempertures of between 0 C to 21 C [8] necessry to reduce deteriortion process. The performnce of gs turbine plnts generlly, re temperture dependent. Therefore, with verge environmentl tempertures of Niger Delt between 20 C to 28 C [9]. This temperture rnge is obviously fr too high when compred to the ISO conditions of temperture, 15 C nd reltive humidity of 60% [10]. The effects of high environment temperture mens the performnce of the gs turbine plnts re dversely ffected. One effective technology employed to reduce the environmentl tempertures is evportive cooling. According to [11], evportive cooler is generlly becoming more effective where tempertures re high, reltive humidity is very low, wter is vilble for the purpose nd ir is conductive. It is n economicl mens of reducing tempertures nd incresing humidity in n enclosure where the humidity is comprtively low [12]. 2. Evportive Cooling Methods The principle underlying evportive cooling is the conversion of sensible het to ltent het with

2 Americn Journl of Mechnicl Engineering 146 consequent decrese in spce temperture s shown in Figure 1 nd Figure 2. Bsiclly, evportive cooling could be chieved either through direct nd indirect evportive Cooling. Direct evportive cooling (DEC) involves the movement of ir through moist medium where evportion nd cooling occurs. This cool nd moist ir is then llowed to move directly to where it is needed. Wheres, the indirect evportive cooling (INDEC) uses some form of het exchnger tht employs the cool, moist ir produced through the direct method, to lower the temperture of drier ir. This cool, dry ir is in turn used to cool the environment while the cool moist ir is expelled. The dry bulb temperture is reduced, T(A) to T(B) while the humidity of the ir is incresed from A to B s illustrted in Figure 2. The process is considered dibtic (no het gin or loss) becuse sensible het is converted to ltent het in the dded vpour. The ltent het follows the wter vpour nd diffuses into the ir [14]. As long s evportion tkes plce, the exit ir temperture will be lower thn tht of the mbient. However, there re fctors tht ffect the performnce of evportive cooling system which hve been determined by severl reserchers. Among them re environmentl conditions (inlet temperture nd reltive humidity), pd mteril, pd thickness nd density, pd ir fce velocity nd wter flow in pds [15]. This reserch, therefore seeks to investigte the performnce of direct evportive cooler in tropicl climte s in Niger Delt, Nigeri. 3. Mterils nd Methods Figure 1. Schemtic Digrm of Direct Evportive Cooler [13] An evportive cooler ws loclly constructed s seen in Figure 3. The pd is of cellulose type nd is porous enough to llow the flow of ir through it. The pd hs thickness, t of 0.06m; Height (H) nd width (W), 0.48m nd 0.36m respectively. The wter distribution system consists of tnk, pipe nd overhed tnk. Two wter tnks were instlled, one on the top of the cooler nd the other t the bottom. The top tnk with squre bse of side 55cm, depth of 19cm with cpcity of litres contins wter tht is fed to the distribution system. Then the collection tnk of 75cm squre cross-section nd 14.5 deep t the bottom of the cooler collects excess wter from the pd. The fn locted in the middle of the wll of cbin opposite the pd end cretes negtive pressure inside below tht of the tmosphere. This results in inflow of dry ir through the wet pd to the cbin where the cooling is required. As the ir pssed through the wet pd into the cbin, it gve out its sensible het nd get cooled. Prt of the wter in the wet pd then gined ltent het nd evportes. The loction of the fn in the middle ensures tht the cool nd humid ir is uniformly distributed within the cbin. The pd is continuously kept wet by the wter trickling on the pd from the wter tnk through the perforted pipe equipped with stop vlve. The bottom tnk collects excess wter from the pd which is mde to circulte through the tnk mnully. Figure 2. Psychometric Chrt Illustrting Evportive Cooling [16] The bsic components of DEC re: pd, pump, fn nd sump. The pd is mde wet with wter from the sump tnk by pump which sends wter up the top of the pd from where it grdully trickles down. Wrm, dry nd low humidity outdoor ir is drwn with fn through wet filter pd. The pd filters the ir of impurities nd the wter gins sensible het from the incoming dry ir nd converts it to ltent het tht evportes the wter within the pds. Figure 3. Isometric View of the Cooler

3 147 Americn Journl of Mechnicl Engineering The study ws crried out in the Southern costl re of Nigeri. Ambient tempertures nd reltive humidity were tken t prticulr times. Hence, the properties of dry ir (C p, ʋ nd k) re bsed on the mbient tempertures vlues. The exct vlues were obtined by interpoltion between dry bulb tempertures of 20 C nd 40 C Figure 4. The Rectngulr Cellulose pd 3.1. Performnce Evlution One criterion for evluting the cooling pd is to clculte the cooling efficiency. The sturtion or cooling efficiency of the evportive pd is clculted s: ε ha 1 w T = 1 exp = mcp T1 Tw Where: T is the difference between the entering nd exit dry-bulb tempertures, T 1 -T 2. Fesibility index (F*) is defined s: * F = Tw T1 Tw. 2 The smller the F*, the more efficient the evportive cooling. It indictes the evportive cooling potentil to give therml comfort. (T 1 -T w ), wet bulb depression; T 1 inlet dry bulb temperture; T w wet bulb temperture. The greter the difference between the two tempertures, the greter is the evportive cooling effect. According to [14], F* 10 for comfort cooling; 11 F* 16 for relief (lenitive) cooling nd for F*>16 not recommended for the use of evportive cooling systems. Totl wetted surfce re (Aw) of the rectngulr pd is given s: A = A H W t 3 w 1. According to [17], the outlet ir dry bulb temperture cn be clculted s: T = T ε T T w The cooling cpcity is then given s: Q = mcp T1 T2 * The quntity of wter, Q w needed (consumption) by the evportive cooler is clculted from: 1 w Q = m ω ω 6 m 2 1 = H W V ρ 7 Where: Q = cooling cpcity (kj/h) m = ir flow rte (kg/s) Cp = Specific het of ir (J/kgK) V = velocity of ir t inlet of pd (m/s) ω 2 nd ω 1 re determined from the psychometric chrt. ρρ= density of ir (kg/m 3 ) A 1 = Wetted surfce re of pd mteril per unit volume m 2 /m 3. h 1 = Convective het trnsfer coefficient (W/m 2 K) cn be clculted from the Nusselt number: h N k u = [1] 8 lx Where k, therml conductivity is the property of mteril (ir) to conduct het nd it is function of ir temperture. The vlues of k re determined from the therml conductivity of ir ginst the corresponding temperture vlues. l x is known s the chrcteristic length nd ccording to [18] is given s: H W t lx = 9 A lx 0.8 N 0.1 Pr 3 u = Re. t Reynolds number, R e is given s: Re w V lx 10 = 11 υ Where ʋ is the kinemtic viscosity nd the Prndtl number, Pr is lso given s: Pr υ = 12 Where is the therml diffusivity which mesures the rte of het trnsfer of mteril from the hot side to the cold side. It is mthemticlly expressed s: k 2 = m / s ρ Cp T w =wet bulb temperture (K) H = height of pd W = width of pd t = thickness of pd k = therml conductivity (W/(m K) Tble 1. Vlues of Prmeters used in Clcultion Prmeters Units A 1 220m 2 /m 3 t 0.06m H 0.48m W 0.36m V 4.0m/s AA ww 3.04m 2 llll m m 0.69ρρ kkkk/ss 13

4 Americn Journl of Mechnicl Engineering Result nd Discussion The constructed system ws tested in order to ssess its performnce. Some prmeters nmely tempertures (dry bulb inlet, wet bulb nd dry bulb outlet), sturtion efficiency, cpcity of the cooler nd fesibility index were of prticulr interest. The dily redings of tempertures (dry nd wet bulbs) nd reltive humidity of ir mesured t hourly intervls nd these re shown in Tble 3. Therefter, Tble 3 ws generted with dt from Tble 2 nd Tble 3. Vlues for F*, ε, T 2 nd Q were clculted nd tbulted. The clcultion ws mde simpler by C++ nd Microsoft excel. Tble 2. Temperture nd Reltive Humidity Redings with Time (Hrs.) Time Ambient Condition (Hrs.) T 1 ( 0 C) T w ( 0 C) RH (%) Tble 3. Clculted properties of ir on hourly inte Time (Hrs) Ambient Condition t T₁ ( C) Tw ( C) RH (%) ρ (Kg/m³) mₐ (kg/s) Cpₐ(kJ/kgK) k(w/mk) ν*10⁶(m²/s Re Time (Hrs) Ambient Condition t T₁ ( C) Tw ( C) RH (%) Pr Nu h F* ɛ T₂( C) T₁-T₂ ( C) Q(kJ/h) Discussion Figure 5 shows grph of the cooler cpcity ginst the exit dry bulb temperture. From the grph, it is obvious tht the cooler cpcity rises s the exit dry bulb temperture increses nd decreses with it. The grph shows tht t the mximum Q, T 1 -T 2 is higher nd F* is 9. Figure 6 is grph tht illustrtes the performnce of sturtion efficiency with the time. The performnce of n evportive cooler is ssessed by the sturtion efficiency. Numerous reserches hd proven tht sturtion efficiency of commonly used cooling pd mterils fll within 50% to 90% [19]. Figure 6 shows tht the efficiency is within the rnge peking t 59%. Figure 5. Illustrtion of Cooler Cpcity, Q(kJ/h) with Exit Dry Bulb Temperture, T 2 ( 0 C)

5 149 Americn Journl of Mechnicl Engineering cooler cpcity. Also, it cn be observed tht t these points both T 1 -T 2 nd T 1 -T w give the gretest vlues. 6. Conclusion Figure 6. Sturtion Efficiency ginst Time The objective of this work is to investigte the performnce of n evportive cooler with specified pd thickness in Niger Delt re of Nigeri. Prmeters for the performnce ssessment were sturtion efficiency, cooler cpcity, fesibility index nd the exit temperture. It cn be seen from the dily redings when the inlet dry bulb temperture is lowest erly in the morning (8:00hrs), F* is mximum, 18 nd with the two mximum tempertures of 34 C nd 32 C, the F* flls within the rnge of F* 10. The results showed tht evportive cooling is most efficient in periods of high mbient tempertures. And tht the greter the difference between T 1 nd T w, the greter the evportive cooling effects. Acknowledgements We humbly wish to cknowledge Mr Cnn, Ayibkrelyef for the insight, motivtion nd for providing the relevnt mterils needed for this work. References Figure 7. The Effects of time of the dy on RH, T 1 nd T Figure 8. Reltionship between Fesibility Index, Cooler Cpcity nd Exit Temperture The brs in Figure 7 depict the inlet dry bulb temperture, reltive humidity of the inlet ir nd the exit dry bulb temperture with the hourly redings. From Figure 7, it cn be seen tht though t the lowest inlet nd exit dry bulb tempertures, the humidity of inlet ir is higher t 08:00.m. However, t this time Tble 3 shows tht cooler cpcity ws the lest nd the vlue of F* is 18. This shows tht with, high reltive humidity nd low T 1 -T w nd T 1 -T 2, evportive cooling is not fesible. Figure 8 shows the reltionship between fesibility index, F*, exit temperture, T 2, cooler cpcity, Q. Thorough observtion of Figure 8 nd Tble 2 shows tht F* of 9 nd 10 gives the highest sturtion efficiency nd F* T2(0C) [1] Ndukwu, M. C., Mnuw, S. I., Olukunle, O. J. nd Oluwln, I. B. (2013) Mthemticl Model for Direct Evportive Spce Cooling Systems. Nigerin Journl of Technology (NIJOTECH) Vol. 32. No. 3. pp [2] Kulkrni, R. K., Rjput, S. P. S., Gutte, S. A. nd Ptil, D. M. (2014) Lbortory Performnce Of Evportive Cooler Using Jute Fiber Ropes As Cooling Medi. Interntionl Journl of Engineering Reserch nd Applictions, Vol. 4, Issue 12, pp [3] Dodoo, A., Gustvsson, L. nd Sthre, L. (2011) Building Energy-efficiency Stndrds in Life Cycle Primry Energy Perspective, Energy nd Buildings, Vol. 43, No. 7, pp [4] Foster, R. E. (2013) Evportive Air-Conditioning Contributions to Reducing GreenhouseGs Emissions nd Globl Wrming. Retrieved 10 Februry, 2017 from the World Wide Web: 7&rep=rep1&type=pdf. [5] Snchez-Mt (2003) New Recommendtion for Building in Tropicl Climtes, Building nd Environment. Vol. 28, pp [6] Liberty J. T; Okonkwo W. I. nd Echiegu E. A. (2013). Evportive Cooling: A Posthrvest Technology for Fruits nd Vegetbles Preservtion. Interntionl Journl of Scientific nd Engineering Reserch, Vol. 4, Issue 8, [7] Susn, D. S. nd Durwrd S. (1995) G Storing Fresh Fruits nd Vegetbles. Historicl Mterils from University of Nebrsk-Lincoln Extension. Retrieved 8 Februry, 2017 from the World Wide Web: commons.unl.edu/extensionhist/1042/. [8] Lerner, B. R., nd Dn, M. N. (2001) Storing Vegetbles nd Fruits t Home, Purdue University Coopertive Extension Service, West Lfyette, Retrieved 12 Jnury, 2017 from the World Wide Web: pdf. [9] Poku, R., Ogbonny, E. A. nd Oyinki, W. T. (2015) Thermo- Economic Anlysis of Evportive Cooling in Gs Turbine Plnt in Niger Delt, Nigeri. OSR Journl of Engineering (IOSRJEN), Vol. 05, Issue 03: pp [10] Cengel nd Boles (2006) Thermodynmic: An Engineering Approch; 5 Th Edition, McGrw Hill Compnies, Inc., 1221 dvnce of Americns, New York 10020, pp. 584.

6 Americn Journl of Mechnicl Engineering 150 [11] Rusten, E. (1985) Understnding Evportive Cooling, Volunteers in Technicl Assistnce. Technicl Pper #35. VITA, Virgini, USA. [12] Sushmit, M.D., Hemnt, D., nd Rdhchrn, V. (2008) Vegetbles in Evportive Cool Chmber nd in Ambient, Mcmilln Publi. Ltd., London nd Bsingstoke, pp [13] Xun, Y. M., Xio, F., Niu, X. F., Hung, X. nd Wng, S. W. (2012) Reserch nd Appliction of Evportive Cooling in Chin: A Review (I) Reserch. Renewble nd Sustinble Energy Reviews, Elsevier. Issue 16, pp [14] Wtt, J. R. (1963) Evportive ir conditioning. New York: The Industril Press, p.300. [15] Ndukwu, M. C. nd Mnuw, S. I. (2014) Review of Reserch nd Appliction of Evportive Cooling In Preservtion of Fresh Agriculturl Produce. Int J Agric & Biol Eng. Vol. 7, No.5: pp [16] Bhti, A. (2012) Principles of Evportive Cooling System. Retrieved 13 Jnury, 2017 from the World Wide Web: file:///c:/users/robert/appdt/locl/temp/m231content.pdf. [17] Wtt, R. J. nd Brown, W. K. (1994) Evportive Air Conditioning Hnd Book, 3 rd edn(the Firmne Press Inc, Liburn GA). pp [18] Cmrgo, J. R., Ebinum, C. D. nd Crdoso, S. A. (2003) Mthemticl Model for Direct Evportive Cooling Air Conditioning System. Engenhri Térmic, Curitib, Vol. 04, p [19] Kulkrni, R. K. nd Rjput, S. P. S. (2011) Comprtive Performnce of Evportive Cooling pds of lterntive Mterils. Interntionl Journl of Advnced Engineering Sciences nd Technologies, Vol. 10, Issue 2, pp