Performanceanalysis of Regenerative Evaporative CoolerSnehalJ. PatilFYMEHP1711, ME HeatPower, Pimpri Chinchwad College of Engineering, Nigdi. AbstractRegenerative evaporative cooler isone configuration of an indirect evaporative cooler with the capability ofreducing the product air temperature lower than its wet bulb temperature andcloser to its dew point temperature witjout increasing moisture content in theair. This has improved the performance of EC system significantly by increasein effectiveness and cooling capacity. Also with higher energy efficiency ratiothe system is found to be affordable in terms of consumption of electricity.REC gives good performance in terms of the effectiveness and the coolingcapacity when the wet bulb depression of intake air increases. Moreover theperformance is better for lesser velocity of intake air.
Also with increase inthe working to intake air flow ratio from 0.1 to 0.5 there is great increase ineffectiveness found but for the ratio beyond 0.5 the cooling capacity getsreduced. Hence to achieve overall benefit the working to intake air flow ratiois taken as 0.
4 to 0.5. Furthermore, for compact size and light weightstructure the counter flow finned channel configuration is found to be the best one. With this trend forthe parameters in REC we can have great performance which makes the air coolingsystem more efficient and reasonable and that even without causing any kind ofharm to the environment.Keywords: regenerative, wet bulbtemperature, effectiveness, cooling, counter-flow,etc. Introduction Todaymost of the air cooling systems in application are based on the vaporcompression cycle. They are more popular because of low initial cost, stableperformance and longer life.
But along with this these vapor compression basedair conditioning systems consume great quantity of electricity due tocontinuous working of the compressor and also contribute to the global warmingby emitting CFC’s in the environment. On the other hand the Evaporative coolingsystems work on very natural way of cooling that is evaporation of water usingsensible heat of air. This consumes less electricity and also doesn’t harm theenvironment anyhow.Evaporativecooler (EC) is the air cooling system which works on the principle ofevaporation of water utilizing the heat in ambient air. Air taken in by usingfan is passed through the channel where it interacts with the water dropletssupplied and comes out from the outlet getting cooled. This happens by thevirtue of heat exchange between air and the water droplets.
Water extracts heatfrom the air and get evaporated and hence the temperature of air gets reduced.There are two major types of EC namely Direct EC (DEC) and Indirect EC (IEC).InDEC air interacts with water directly to get water evaporated and air itselfcools down. In IEC type the air – water direct contact is not made to occur butthe primary air is passed alone through dry channel and water and secondary airis passed in another channel over it. The main advantage of IEC is the moisturecontent of the product air does not increase and the dry, less humid air iscirculated in the room which maintains the comfort.
However, the traditionalIEC’s give only 55-75% wet bulb effectiveness 1 and this is not muchbeneficial. To improve the cooling capacity and performance of EC a new type ofIEC is suggested and tested which is Regenerative Evaporative Cooler (REC).This REC overcomes the limitation of cooling caused due to the ultimate wetbulb temperature of the entering air. REC has the capacity to lower the airtemperature closer to its dew point temperature. Inpast few years the comparative and numerical analysis of the different ECconfigurations with different modes of working has been carried out and isstill going on.
The performance and cooling capacity of the EC depends onvarious parameters as intake air temperature, specific humidity of intake air,air velocity, wb depression, fan speed, evaporation rate etc. in most of the studyand results the REC has come out to be very effective with more than 100% wetbulb effectiveness. RegenerativeEvaporative Cooler:Asmentioned earlier, REC is one kind of IEC that is capable of reducingtemperature of the intake air lower than its WBT and closer to its DPT.
Thesystem consists of number of pairs of dry and wet channels. Primary air passesthrough the dry channel. In the wet channel there is a thin water film whichgains heat from air to evaporate. The part of primary air is made to movethrough small opening to the wet channel and redirected towards outside and itthen goes out as exhaust air.
Here the secondary air that flows through the wetchannel has already been precooled in the dry channel. This gives the advantageof having an extra mode of heat exchange. Primary air exchanges heat with notonly the water present but also with the secondary air which is cooler thanintake air. Hence heat is transferred in both the forms latent and sensibleheat.
This increases the total heat being transferred and therefore thetemperature of the product air gets reduced even very close to its dew pointtemperature. Thereare two main configurations of REC as:2.1Counter-Flow REC Inthis type of configuration the part of primary air near to the outlet of EC isredirected through small opening in wet channel in reverse direction.
This is shown in the following fig.Fig.1.working principle of REC (a)schematic of airflow path (b)thermal process of air treatment on Psychometric chart 12.2Cross-Flow REC In this configuration the secondaryair that is the part of the primary air is redirected to the wet channel incross direction to the primary one as shown in the following fig.
Fig.2.Cross-Flow REC configuration 2 Thecounter-flow type of regenerative evaporative cooler is proved to be morebeneficial with greater heat transferred and more cooling capacity.3.Performance indicators3.
1Wet Bulb EffectivenessItis the ratio of difference between the dry bulb temperatures of intake andproduct air to the difference between the dry bulb and wet bulb temperatures ofintake.3.2Cooling capacityThiscan be estimated from the given formula from ASHRAE std.
13.3Energy Efficiency RatioItis the ratio of cooling capacity to the total power consumption (fans and pump)1 4.Performance analysis:4.1effect of air WB depression Fig.
3.performance as a function of inlet WB depression1Asthe wet bulb depression increases the cooling capacity, effectiveness and EERof the system increases but after a certain point the product air temperatureis increased than the previous one. 4.2.Effect of intake air velocity:Fig.4.Performance as a function of intake air velocity1Asthe air intake velocity increases the effectiveness goes on decreasing but thecooling capacity and energy efficiency ratio is found to be increased. 4.
3.Effect of working to intake air ratio Fig.5.Performance as a function of working to intake air ratioUpto 0.5 ratio of working to intake air the effectiveness increases significantlyand slightly if further increased. This is because as the working air flow rateis more than the primary air the heatcapacity and its ability to assimilate the water vapor increases. 1 But alongwith this the cooling capacity goes on decreasing.
Hence to get the overallbenefit the optimum working to intake air flow ratio has to be selected and itis 0.4-0.5.14.4.Effect of channel gapChannelgap of less than 3 mm gives better results with REC. But if gap is increasedthen effectiveness and cooling capacity of REC unit decreases considerably. 4.
5.Compactness of the deviceAmongstthe 3 different configurations tested for REC unit as-(1) flat plate type, (2)corrugated plate type, and (3) counter flow finned channel type, the third onehas been proved to be the best one for compact size without compromising theperformance level.Thefinned channel type is proved to be the most compact and the lightest structureamong the three considered.
The volume of this structure is found about 1/8that of the flat plate type and moreover the weight can even be lowered to ½ ofthe flat plate type.3Conclusion:RECthat is Regenerative evaporative cooler is the improvement in IEC whichovercomes the limitations in traditional IEC model that was giving lesseffectiveness and cooling capacity keeping the moisture content in the intakeair constant throughout the process. Regenerative evaporative cooler uses thepart of cooled primary air as secondary air in the wet channel which causesgreat increase in the total heat transfer and also the evaporation of water inthe wet channel.RECgives good performance in terms of the effectiveness and the cooling capacitywhen the difference between the dry bulb and wet bulb temperature of the intakeair that is the wet bulb depression of intake air increases.
Moreover theperformance is better for lesser velocity of intake air. Also with increase inthe working to intake air flow ratio from 0.1 to 0.5 there is great increase ineffectiveness found but for the ratio beyond 0.5 the cooling capacity getsreduced. Hence to achieve overall benefit the working to intake air flow ratiois taken as 0.4 to 0.
5. Furthermore, for compact size and light weight structurethe counter flow finned channelconfiguration is found to be the best one. With this trend for the parametersin REC we can have great performance which makes the air cooling system moreefficient and reasonable and that even without causing any kind of harm to theenvironment.
References:1. ZhiyinDuan,Changhong Zhan, Xudong Zhao, Xuelin Dong, Experimental study of counter flowregenerative evaporative cooler, Building and Environment (2016). 2. Hui-Jeong Kim, Sang-Woo Ham, Dong-Seob Yoon,Jae-Weon Jeong, Cooling performance measurement of two cross-flow indirectevaporative coolers in general and regenerative operation modes, Applied Energy195 (2017) 268-277.
3. JoohyunLee, Bongsu Choi, Dae Young Lee, comparison of configuration for a compact regenerativeevaporative cooler, International Journal of Heat and Mass Transfer, 65 (2013)192-198.4.
J.K.Jain,D. A. Hindoliya, Experimental performance of new evaporative cooling padmaterials, Sustainable Cities and Society, 1 (2011) 252-256.
5. DemisPandelidis, Sergey Anisimove, William M. Worek, Performance study of counterflow indirect evaporative air cooler, Energy and Building, 109 (2015) 53-64
