performance evaluation of two stage air cooler in different speed

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International Journal of Mechanical Engineering and Technology (IJMET)

Volume 7, Issue 3, May–June 2016, pp.329–341, Article ID: IJMET_07_03_030

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PERFORMANCE EVALUATION OF TWO

STAGE AIR COOLER IN DIFFERENT

SPEED

MOHAMMAD UL HASSAN

Department of Mechanical Engineering,

NIT Patna, India

ABSTRACT

During summer use of khush curtains and water spraid over the khash

(fibre) curtain for air cooling is done. But the determination of the extent of

cooling is limited up to the wet bulb temperature in Conventional Cooler air is

passes at uniform rate through wetted pad. In this process humidity increases

sometimes which is not desirable. In Two Stage Air Cooler the primary air is

cooled by a contact surface which is maintained at lower temperature air and

water on the other side of the contact surface

After analysis it is found that in Two Stage Air Cooler Effective

temperature decreases COP increases up to 13 which are significantly higher

than normal cooler and conventional air conditioner. Refrigeration effect

increases. Makeup water recharges time increases for the same configuration

has been obtained for it is 24 hours 54 minutes and for conventional cooler it

is 15 hours 2 minutes. In two stage air cooler with cover noise has been

drastically reduce by the increase damping effect of covers Specific humidity

increase is significantly lower than that of in case of Conventional Cooler.

On the basis analysis it is recommended that two stage indirect air cooler

is energy effective, eco friendly, cost effective cooling system and with some

further modification it may be hope as replacement of air conditioner. In

direct cooler expected limit lower temperature was 22.6 where as

experimentally the temperature attains are 24. In indirect cooler system the

expected temp was 20.5 where as the actual temperature attain is 21.9 less

than the wet bulb temperature with lesser relative humidity than 100%)

Key words: Conventional Air Cooler, DBT, Heat Exchanger, Primary Pump,

Two Stage Air Cooler WBT

Cite this Article: Mohammad Ul Hassan, Performance Evaluation of Two

Stage Air Cooler In Different Speed. International Journal of Mechanical

Engineering and Technology, 7(3), 2016, pp. 329–341.

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Mohammad Ul Hassan


INTRODUCTION

Principle of evaporative cooling has been used since long back, during summer use of

khush (fibre) curtains and water sprayed over it for air cooling. Other example of

evaporative cooling are use of shurahi pot for water cooling, cooling tower, spray

ponds etc. But the determination of the extent of cooling is limited up to the wet bulb

temperature. In Conventional Cooler air is passes at uniform rate through wetted pad.

In this process humidity increases sometimes which is not desirable. In Two Stage Air

Cooler the primary air is cooled by a contact surface which is maintained at lower

than temperature of incoming atmospheric air. In both the process minimum

temperature can be achieved theoretically is wet bulb temperature (WBT) of the

incoming air.

PROPOSED MODIFICATION

The temperature of air decreases from ‘i’ to ‘o’ in Conventional Cooler. in indirect

evaporative cooler the temperature of ‘ i1’ is lower than temperature at‘ i’ and if

proceeding is done on the constant WBT Line then it goes up to‘O1’ which is lower

than the previous ‘O’. This indicates that if temperature of air at ‘ i ’is cooled to ‘

i1’prior to entry to main cooler by any means lower temperature can be achieved. This

suggests need of cooling of air prior or in other words two stages cooling. The

equipment based on this principle is designated as Two Stage Air Cooler and

theoretically one may reach up to the dew point temperature.

EXPERIMENTAL SETUP

The heat exchanger is fitted on the back panel of the cooler, the inlet is connected

with the secondary pump situated inside the cooler tank so that water supply from for

the exchanger is from the tank and outlet is connected with a pipe to discharge the

water in the same tank connected

Performance Evaluation of Two Stage Air Cooler In Different Speed


For measurement of temperature of cold water entering the heat exchanger a

thermometer probe is inserted in the inlet pipe and for measurement of hot water

temperature coming out of the heat exchanger a thermometer probe is inserted in the

outlet pipe. For measurement of air temperature at the inlet of heat exchange, outlet of

heat exchanger, outlet of cooler and ambient temperature four thermometers installed

at respective position. Also for the measurement of energy consumption at different

arrangement that is combined direct and indirect and direct only at various

speed/velocity energy meter was installed to get energy consumption in kwh.

Nomenclature

DBT = Dry bulb temperature

WBT = Wet bulb temperature

A1 = area of inlet of cooler

A3 = area of outlet of cooler

T1 dbt = dry bulb temperature of air inlet to heat exchanger

T2 dbt = dry bulb temperature of air after passing through heat exchanger

Mohammad Ul Hassan


T3 dbt = dry bulb temperature at outlet of cooler

T1wbt = wet bulb temperature at inlet of heat exchanger

T3wbt = wet bulb temperature at outlet of cooler

RH1 = Relative humidity at inlet to heat exchanger

RH3 = Relative humidity at outlet of cooler

W1 = specific humidity at inlet of heat exchanger

W3 = specific humidity at outlet of cooler

h1 = specific enthalpy at inlet to heat exchanger

h3 = specific enthalpy at outlet

Tw1 = Inlet temperature of water

Tw2= Outlet temperature of wate

Sw= specific heat of water

Sa = specific heat of air

DATA COLLECTION AND CALCULATION

Data was collected on various days on different time. All the parameter required for

calculation and performance evaluation of two stage air cooler needed has been

arranged in tabular form for convenience in visualisation and graph plotting.

Calculation of Velocity

Velocity was calculated at five different points then average of them is taken for both

inlet and outlet for high speed, medium speed and low speed respectively. The

velocity of air may vary for different atmospheric conditions

Table 1 Velocity at inlet of cooler at different points in different speed

Side Inlet Outlet

Points High speed

(m/s)

Medium

speed

(m/s)

Low speed

(m/s)

High speed

(m/s)

Medium

speed

(m/s)

Low speed

(m/s)

1 3.70 3.10 2.80 4.20 3.45 3.15

2 3.10 2.80 2.50 3.30 3.10 2.70

3 2.75 2.40 2.10 1.10 0.90 0.80

4 3.20 2.90 2.50 1.20 1.10 1.00

5 3.00 2.80 2.60 1.20 1.20 1.10

Average 3.15 2.80 2.50 2.20 1.95 1.75

For performance evaluation and analysis experiment is performed in for different

cases as following

Case A. Two stage air Cooler

In this process both cooler pump and auxiliary pump was switched on air is first

cooled sensibly then evaporative cooling is done, side panel of cooler is covered with

insulating medium so that there is no mixing of hot air from side panel.



Table 2 DBT and WBT at different points at different velocity for two stage air Cooler

Speed Average Velocity DBT WBT

1 3 1 2 3 1 3

High 3.15 2.20 31.5 27.5 21.9 22.6 20.50

Medium 2.80 1.95 31.5 27.4 22.2 22.9 20.7

Low 2.50 1.75 31.5 27.4 23.2 23.2 21.0

Case B. Conventional Cooler

In this process auxiliary pump is switched off so only evaporative cooling of air is

done. Side panel of cooler is covered with insulating medium so that there is no

mixing of hot air from side panel.

Table 3 DBT and WBT at different points at different velocity for Conventional Cooler

Speed Average Velocity DBT WBT

1 3 1 3 1 3

High 3.15 2.20 31.5 24.0 22.5 21.4

Medium 2.80 2.00 31.5 24.3 22.8 21.7

Low 2.50 1.80 31.5 24.5 23.1 21.9

Table 4 Calculation of various parameters for Two stage Air Cooler

Speed Velocity

(m/sec)

Dry bulb

temperature ( )

Wet bulb

temperature ( )

Relative

humidity (%)

1 3 1 2 3 1 3 1 3

High 3.15 2.2 31.5 27.5 21.9 22.6 20.5 46.40 87.37

Medium 2.8 1.95 31.5 27.4 22.2 22.9 20.7 47.92 87.46

Low 2.5 1.75 31.5 27.4 22.5 23.2 21.0 49.46 87.55

Speed Dew point

temperature

Specific humidity

(kg/kg )

Mass flow rate of

air (kg/sec)

Specific volume

(m3/Kg)

1 3 1 3 1 3 1 3

High 18.62 19.70 0.01346 0.01443 0.3540 0.3563 0.8809 0.8844

Medium 19.13 20.20 0.01392 0.01472 0.3114 0.3154 0.8815 0.8556

Low 20.33 20.22 0.01437 0.01501 0.2805 0.2830 0.8821 0.8569

Speed

Specific enthalpy

(KJ/Kg)

∆H

(kJ/kg

)

∆w

(kg/kg )

Make

up

water

(kg/hr)

Area

(m2)

1 3 (1-3) (3-1) 1 3

High 66.10 58.67 7.43 9.67*10-4 1.2441 0.099 0.1384

Medium 67.26 59.71 7.55 8.00*10-4 0.9083 0.099 0.1384

Low 68.43 60.43 7.67 6.40*10-4 0.6462 0.099 0.1384

Mohammad Ul Hassan


Speed Energy

consumption

Kwh

Time to refill

water tank

(hr)

Carnot COP COP

High 0.201 24:54 30.71 13.20

Medium 0.196 34:07 31.72 12.00

Low 0.191 47:54 32.46 11.36

Table 5: Calculation of various parameters for Conventional Cooler

Speed Velocity

(m/sec)

Dry bulb temperature

Wet bulb

temperature

Relative

humidity (%)

1 3 1 2 3 1 3 1 3

High 3.15 2.2 31.5 31 24.0 22.5 21.4 45.89 79.56

Medium 2.8 2.0 31.5 31 24.3 22.8 21.7 47.41 79.70

Low 2.5 1.8 31.5 31 24.5 23.1 21.9 48.95 80.53

Speed Dew point

temperature

Specific humidity

(kg/kg )

Mass flow rate of

air (kg/s)

Specific volume

(m3/kg)

1 3 1 3 1 3 1 3

High 18.44 20.25 0.01331 0.01493 0.3527 0.3535 0.8841 0.8612

Medium 18.96 20.57 0.01376 0.01524 0.3145 0.3193 0.8813 0.8624

Low 19.47 20.93 0.01422 0.01559 0.2806 0.2885 0.8819 0.8635

Speed

Specific enthalpy

(kJ/kg )

∆H

(kJ/kg )

∆w

(kg/kg )

Make up

water

(kg/hr)

Area

(m2)

1 3 (1-3) (3-1) 1 3

High 65.72 62.12 3.60 1.62*10-3

2.0616 0.099 0.1384

Medium 66.87 63.20 3.67 1.48*10-3

1.7012 0.099 0.1384

Low 68.04 64.30 3.74 1.37*10-3

1.42282 0.099 0.1384

Speed

Energy

consumption

Kwh

Time to refill

water tank

(hr)

Carnot COP COP

High 0.183 15:02 39.6 6.9540

Medium 0.178 18:13 41.29 6.5833

Low 0.173 21:47 42.5 6.2369

RESULT AND DISCUSSION

Variation of Temperature at Outlet of Cooler with Fan speed

For data ref table no 4 and 5



Effects

Temperature attained in Two Stage Air Cooler is lower than Conventional Cooler.

As velocity/speed increases degree of cooling is increase

Reason

In Two Stage Air Cooler the air is pre-cooled sensibly in the heat exchanger so the

better cooling is performed this departure of temperature may be better explain by

psychometric chart.

Increased velocity provides increase of Reynolds no thence increase heat transfer

coefficient and better cooling.

In high velocity/speed some water droplets may not find sufficient time for

evaporation and at the exit air is sensibly cooled

This departure may be considered due to water spray at wet bulb temperature

provides sensible cooling of air which has already humidified adiabatically.

Mass Flow Rate at Different Speed

Data ref table no 4 and 5

21.9 22.2

22.5

24 24.3 24.5

20

21

22

23

24

25

High Medium Low

Min

imum

tem

per

ature

att

ain

Speed

Minimum temperature attain v/s speed

Mohammad Ul Hassan


Effect

As the velocity decreases mass flow rate also decreases

Reason:

Mass flow rate decreases with decreased velocity because area of inlet and outlet is

constant so mass flow rate varies according to velocity.

Variation in change in enthalpy

Data ref table no 4 and 5

Effect

Change in enthalpy for incoming and outgoing air in combined arrangement is more

than direct cooling arrangement.

Change in enthalpy increases with decrease in speed for the same setup.

The change in enthalpy for Conventional Cooler should be zero because it is an

adiabatic cooling process.

Reason

The incoming air condition is same for both the arrangement but the outgoing air in

former case is much lower than Conventional Cooler so in case of Two Stage Air

Cooler arrangement change in enthalpy is more.

Refrigeration Effect

Data ref Table no 4 and 5

0

0.1

0.2

0.3

0.4

High Medium Low

0.3

56

3

0.3

15

4

0.2

83

1

0.3

53

5

0.3

19

3

0.2

88

5

Mas

s fl

ow

rat

e K

g/s

ec

Speed m/sec

Mass flow rate at different spped

Two stage air

cooler Conventional

cooler

7.43 7.55 7.67

3.6 3.67 3.74

0

5

10

High Medium Low

Chan

ge

in e

nth

alp

y

Speed

Change in enthalpy v/s speed

Two stage air

cooler

Conventional

cooler



Effect

Refrigeration effect for Two Stage Air Cooler is more than Conventional Cooler.

Refrigeration effect decreases with decreased speed.

Reason

Since the change in enthalpy in case of Two Stage Air Cooler is more than

Conventional Cooler only. So refrigeration effect in combined process is also more.

As mass flow rate decreases with decrease in speed so the refrigeration effect also

decreases with decrease in speed

Energy Consumption


Effect

Energy consumption for Two Stage Air Cooler is more than Conventional Cooler

only.

The energy consumption is decreases with decrease in velocity.

Reason

Energy consumption for Two Stage Air Cooler is higher than direct cooling only

because in former case along with fan and primary pump secondary pump is also

running but in direct cooling secondary pump is not running so energy consumption

is less.

2.6473 2.38127 2.27

1.2726 1.17183 1.07899

0

1

2

3

High Medium Low

Ref

riger

atio

n e

ffec

t

Speed

Refigeration effect v/s Speed

Two stage air

cooler

Conventional

cooler

0.15 0.16 0.17 0.18 0.19 0.2

0.21 0.2

01

0.1

96

0.1

91

0.1

83

0.1

78

0.1

73

Ener

gy c

onsu

mp

tion i

n K

wh

Speed m/s

Energy consumption v/s speed

Two stage

air cooler

Convention

al cooler

Mohammad Ul Hassan


Energy consumption by the cooler fan is less for lower speed so it decreases with

decreasing speed.

Variation of COP with Speed


Effects

1. The COP for Two Stage Air Cooler is more than the Conventional Cooler only.

2. The COP for the same set up decreases with the decrease in speed.

Reason

Instead of energy consumption is more in case of Two Stage Air Cooler the COP is

more because as compare to energy consumption the cooling is more in this case so

the refrigeration effect is also more hence COP is more in case of Two Stage Air

Cooler.

COP decrease with the decrease in speed in both the cases because effect of the

decrease in energy consumption with decrease in speed is less as compared to the

effect of decrease of refrigeration effect

Make up Water Requirement

As in cooler the cooling of air is by the process of evaporative cooling so the water

evaporated continuously so make up water is need after particular interval of time

according to arrangement and use of cooler.


13.206 11.995 11.3644

6.954 6.583 6.237

0

5

10

15

High Medium Low

CO

P

Speed (m/s)

COP v/s Speed

Two stage Air

Cooler

Conventional

Cooler

0

0.5

1

1.5

2

2.5

High Medium Low

1.2441 0.9083

0.5502

2.0616 1.7012

1.42282

Mak

e up

wat

er r

equir

ed p

er

hour

Speed

Make water requirement v/s Speed

Two stage Air

Cooler

Conventional

Cooler



Effect

Make up water requirement is more for Conventional Cooler than Two Stage Air

Cooler.

The makeup water requirement for the same set up decreases with the decrease in

speed for both the process.

Reason

Difference in specific enthalpy at inlet and outlet of cooler is more in case of

Conventional Cooler only because at outlet specific enthalpy is more in case of

Conventional Cooler.

For the same setup as the speed decreases the difference in specific enthalpy

decreases along with this the mass flow rate of air also decreases so the makeup water

require is also less.

Makeup Water Recharge Time

(May vary according to prevailing ambient condition)

As the makeup water needed so the tank has to be refill after particular interval of

time depending on the capacity of tank and use of cooler.

In this cooler the tank capacity is 31 litres so time to refill the tank for different

arrangement and different speed has been given below in tabular form.


Comparison of COP of Simple Air Cooler v/s Two Stage Air Cooler v/s

Conventional Window Air Conditioner

0:00:00 24:00:00 48:00:00

High

Medium

Low

24

:54

:00

34

:07

:00

47

:54

:00

15

:02

18

:13

21

:47

Time after which refilling require

Sp

eed

tme for reffiling of tank require v/s Speed

Conventinal

Cooler

APPLIANCES COP

Two Stage Air cooler 13.206

Simple Air Cooler 6.954

Window AC 2.846

Mohammad Ul Hassan


CONCLUSION

Following are the fruitful result outcome of indirect two stage air cooler their details

have been provided in result and discussion.

Effective temperature decreases

COP increases up to 13 which are significantly higher than normal cooler and

conventional air conditioner.

Refrigeration effect increases.

Makeup water recharges time increases for the same configuration has been obtained.

In two stage air cooler with cover noise has been drastically reduce by the increase

damping effect of covers

Specific humidity increase is significantly lower than that of in case of Conventional

Cooler.

RECOMMENDATIONS

Two stage indirect air cooler is energy effective, eco friendly, cost effective cooling

system and with some further modification it may be hope as replacement of air

conditioner.

In direct cooler expected limit lower temperature was 22.6 where as experimentally

the temperature attains is 24. In indirect cooler system the expected temp was 20.5

where as the actual temperature attain is 21.9 less than the wet bulb temperature with

lesser relative humidity than 100%.

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mass transfer coefficients in rigid imperegnated cellulose evaporative media,

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model for thermal and hydraulic calculations of indirect evaporative cooler

performance, ASHRATE Transactions, 97, Part 2, pp 852–865.

[3] Peterson J. L. (1993), An effectiveness model for indirect evaporative coolers,

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hydraulic performance of a modified two-stage evaporative cooler, Renewable

Energy, 7(2), pp 165–176.

[5] Maheshwari G. P., Al-Ragom F., Suri R. K. (2001), Energy saving potential of an

indirect evaporative cooler, Applied Energy, 69(1), pp 69–76.

0

5

10

15

Two StageAir

cooler

Conventional

Air Cooler

Window AC

13.206

6.954

2.846

CO

P

Appliances

COP for different appliances



[6] El- Dessouky H., Al—Zeefari A. (2004), Performance analysis of two stage

evaporative coolers, Chemical Engineering Journal 102(3), pp 255–266.

[7] Camrago J. R., Ebinuma C. D., Silveria J. L. (2005), Experimental performance

of a direct evaporative cooler operating during summer in Barzilian City,

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[8] Jain D., (2007), Development and testing of two stage evaporative cooler,

Building and Environment, 42(7), pp 25–54.

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investigation of two stage indirect/ Convetional Cooler system in various climatic

conditions. Building and Environment, 44(10), pp 2073–2079.

[10] Principle of Evaporative Cooling System, 2012,PDH online Course M231 (4

PDH)

[11] Mohd. Rehan Khan and Dr. Ajeet Kumar Rai, An Experimental Study of Exergy

In A Corrugated Plate Heat Exchanger. International Journal of Mechanical

Engineering and Technology, 6(11), 2015, pp. 16–22.

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[13] ASHRAE Handbook, 1995, chapter47, Evaporating Air–Cooling notes.