Performance the Evaporative cooling systems work on very

Performance
analysis of Regenerative Evaporative Cooler

Snehal
J. Patil

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FYMEHP1711, ME Heat
Power, Pimpri Chinchwad College of Engineering, Nigdi.

 

Abstract

Regenerative evaporative cooler is
one configuration of an indirect evaporative cooler with the capability of
reducing the product air temperature lower than its wet bulb temperature and
closer to its dew point temperature witjout increasing moisture content in the
air. This has improved the performance of EC system significantly by increase
in effectiveness and cooling capacity. Also with higher energy efficiency ratio
the system is found to be affordable in terms of consumption of electricity.
REC gives good performance in terms of the effectiveness and the cooling
capacity when the wet bulb depression of intake air increases. Moreover the
performance is better for lesser velocity of intake air. Also with increase in
the working to intake air flow ratio from 0.1 to 0.5 there is great increase in
effectiveness found but for the ratio beyond 0.5 the cooling capacity gets
reduced. Hence to achieve overall benefit the working to intake air flow ratio
is taken as 0.4 to 0.5. Furthermore, for compact size and light weight
structure the counter flow finned 
channel configuration is found to be the best one. With this trend for
the parameters in REC we can have great performance which makes the air cooling
system more efficient and reasonable and that even without causing any kind of
harm to the environment.

Keywords: regenerative, wet bulb
temperature, effectiveness, cooling, counter-flow,etc.

 

Introduction

Today
most of the air cooling systems in application are based on the vapor
compression cycle. They are more popular because of low initial cost, stable
performance and longer life. But along with this these vapor compression based
air conditioning systems consume great quantity of electricity due to
continuous working of the compressor and also contribute to the global warming
by emitting CFC’s in the environment. On the other hand the Evaporative cooling
systems work on very natural way of cooling that is evaporation of water using
sensible heat of air. This consumes less electricity and also doesn’t harm the
environment anyhow.

Evaporative
cooler (EC) is the air cooling system which works on the principle of
evaporation of water utilizing the heat in ambient air. Air taken in by using
fan is passed through the channel where it interacts with the water droplets
supplied and comes out from the outlet getting cooled. This happens by the
virtue of heat exchange between air and the water droplets. Water extracts heat
from 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).

In
DEC air interacts with water directly to get water evaporated and air itself
cools down. In IEC type the air – water direct contact is not made to occur but
the primary air is passed alone through dry channel and water and secondary air
is passed in another channel over it. The main advantage of IEC is the moisture
content of the product air does not increase and the dry, less humid air is
circulated in the room which maintains the comfort. However, the traditional
IEC’s give only 55-75% wet bulb effectiveness 1 and this is not much
beneficial. To improve the cooling capacity and performance of EC a new type of
IEC is suggested and tested which is Regenerative Evaporative Cooler (REC).
This REC overcomes the limitation of cooling caused due to the ultimate wet
bulb temperature of the entering air. REC has the capacity to lower the air
temperature closer to its dew point temperature.

In
past few years the comparative and numerical analysis of the different EC
configurations with different modes of working has been carried out and is
still going on. The performance and cooling capacity of the EC depends on
various parameters as intake air temperature, specific humidity of intake air,
air velocity, wb depression, fan speed, evaporation rate etc. in most of the study
and results the REC has come out to be very effective with more than 100% wet
bulb effectiveness.

Regenerative
Evaporative Cooler:

As
mentioned earlier, REC is one kind of IEC that is capable of reducing
temperature of the intake air lower than its WBT and closer to its DPT. The
system consists of number of pairs of dry and wet channels. Primary air passes
through the dry channel. In the wet channel there is a thin water film which
gains heat from air to evaporate. The part of primary air is made to move
through small opening to the wet channel and redirected towards outside and it
then goes out as exhaust air. Here the secondary air that flows through the wet
channel has already been precooled in the dry channel. This gives the advantage
of having an extra mode of heat exchange. Primary air exchanges heat with not
only the water present but also with the secondary air which is cooler than
intake air. Hence heat is transferred in both the forms latent and sensible
heat. This increases the total heat being transferred and therefore the
temperature of the product air gets reduced even very close to its dew point
temperature.

There
are two main configurations of REC as:

2.1
Counter-Flow REC

            In
this type of configuration the part of primary air near to the outlet of EC is
redirected 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 1

2.2
Cross-Flow REC

            In this configuration the secondary
air that is the part of the primary air is redirected to the wet channel in
cross direction to the primary one as shown in the following fig.

Fig.2.
Cross-Flow REC configuration 2

The
counter-flow type of regenerative evaporative cooler is proved to be more
beneficial with greater heat transferred and more cooling capacity.

3.
Performance indicators

3.1
Wet Bulb Effectiveness

It
is the ratio of difference between the dry bulb temperatures of intake and
product air to the difference between the dry bulb and wet bulb temperatures of
intake.

3.2
Cooling capacity

This
can be estimated from the given formula from ASHRAE std. 1

3.3
Energy Efficiency Ratio

It
is the ratio of cooling capacity to the total power consumption (fans and pump)
1

 

 

4.
Performance analysis:

4.1
effect of air WB depression

 

Fig.3.
performance as a function of inlet WB depression1

As
the wet bulb depression increases the cooling capacity, effectiveness and EER
of the system increases but after a certain point the product air temperature
is increased than the previous one.

 

4.2.
Effect of intake air velocity:

Fig.4.
Performance as a function of intake air velocity1

As
the air intake velocity increases the effectiveness goes on decreasing but the
cooling 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 ratio

Up
to 0.5 ratio of working to intake air the effectiveness increases significantly
and slightly if further increased. This is because as the working air flow rate
is more than the primary  air the heat
capacity and its ability to assimilate the water vapor increases. 1 But along
with this the cooling capacity goes on decreasing. Hence to get the overall
benefit the optimum working to intake air flow ratio has to be selected and it
is 0.4-0.5.1

4.4.
Effect of channel gap

Channel
gap of less than 3 mm gives better results with REC. But if gap is increased
then effectiveness and cooling capacity of REC unit decreases considerably.

4.5.
Compactness of the device

Amongst
the 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 one
has been proved to be the best one for compact size without compromising the
performance level.

The
finned channel type is proved to be the most compact and the lightest structure
among the three considered. The volume of this structure is found about 1/8
that of the flat plate type and moreover the weight can even be lowered to ½ of
the flat plate type.3

Conclusion:

REC
that is Regenerative evaporative cooler is the improvement in IEC which
overcomes the limitations in traditional IEC model that was giving less
effectiveness and cooling capacity keeping the moisture content in the intake
air constant throughout the process. Regenerative evaporative cooler uses the
part of cooled primary air as secondary air in the wet channel which causes
great increase in the total heat transfer and also the evaporation of water in
the wet channel.

REC
gives good performance in terms of the effectiveness and the cooling capacity
when the difference between the dry bulb and wet bulb temperature of the intake
air that is the wet bulb depression of intake air increases. Moreover the
performance is better for lesser velocity of intake air. Also with increase in
the working to intake air flow ratio from 0.1 to 0.5 there is great increase in
effectiveness found but for the ratio beyond 0.5 the cooling capacity gets
reduced. Hence to achieve overall benefit the working to intake air flow ratio
is taken as 0.4 to 0.5. Furthermore, for compact size and light weight structure
the counter flow finned  channel
configuration is found to be the best one. With this trend for the parameters
in REC we can have great performance which makes the air cooling system more
efficient and reasonable and that even without causing any kind of harm to the
environment.

References:

1.       ZhiyinDuan,
Changhong Zhan, Xudong Zhao, Xuelin Dong, Experimental study of counter flow
regenerative 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 indirect
evaporative coolers in general and regenerative operation modes, Applied Energy
195 (2017) 268-277.

3.       Joohyun
Lee, Bongsu Choi, Dae Young Lee, comparison of configuration for a compact regenerative
evaporative 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 pad
materials, Sustainable Cities and Society, 1 (2011) 252-256.

5.       Demis
Pandelidis, Sergey Anisimove, William M. Worek, Performance study of counter
flow indirect evaporative air cooler, Energy and Building, 109 (2015) 53-64

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