Evaporative Cooler

Evaporative Cooling

Edited: B J Wernick PrEng BScEngLast Updated: 07 February 2008

What is evaporative cooling?

Try this quick and easy experiment.

On a warm and windy day, stand with exposed dry skin beside a pool. Yes, you may use a swim suit.

Now jump into the pool and stand with wet skin in the same air.

When dry, you may feel quite comfortable but with a wet body and you will experience the effects of evaporative cooling.

But, you already knew that...

The wet-bulb temperature

In the field of air-conditioning, we are all familiar with the wet bulb thermometer.

My whirling hygrometer and Gaël de Guichen demonstrating its use from his plane seat.

As dry air goes over the wet bulb, it becomes saturated at the muslin cloth around the bulb. In order to saturate the air, water must be evaporating from the muslin cloth. Since evaporation is a boiling process, energy is needed and this energy comes from the water. As energy is extracted from the

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Evaporative Cooler

water, its temperature drops. This lower temperature that we see on the wet bulb thermometer is known as the wet bulb temperature.

Simplification of the psychrometric process of the wet bulb

On the psychrometric chart, we can plot the process of the air as it goes over the wet bulb. Relatively dry air with a dry bulb temperature of db and a humidity of w experiences an increase in moisture and a simultaneous decrease in temperature to saturation.

No matter where we start on the blue line, the process will always follow the same path to the saturation line. The temperature at saturation is the wet bulb temperature.

From this, we can see that the entering air wet bulb temperature is an important parameter in evaporation. In addition, the starting humidity sets a limit to the maximum cooling available.

The above adiabatic saturation process is actually the thermodynamic wet bulb temperature and only achieved by means of an adiabatic saturator. See the section on psychrometric processes for a full description.

Simple Evaporative Cooler

So, how do we make this evaporative cooling principle work for us in practice?

The most obvious method is to circulate water across an air stream. Spray nozzles at the top of the duct increase the surface area of the water and improve the potential for evaporation.

Single stage evaporative cooler

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As you might suspect, the efficiency of the evaporative cooler depends on the method used to increase the air-to-water contact area. You could use a spray nozzle to create micron sized droplets or allow the water to splash through a fill material.

The rating is based on how close point 2 gets to the saturation line.

e = (t1 - t2) / (t1 - t4)

e is known as the contact factor of the evaporative cooler.

Typical values published by Gatley are:

2-Stage Evaporative Cooler

Now we can start getting a bit smarter.

In this concept, we add a sensible cooling coil into the airstream before the evaporative cooler. This makes is a 2-stage process. The chilled water could come from any source but it does make sense to use evaporative cooling to cool the water as well.

Industrial air washer using multiple opposing spray banks to achieve intimate mixing of the air and water

95 to 98%

Rigid media. (12 inch fill)88 to 91%

Residential evaporative media (plastic mesh referred to as hogs hair)

50 to 60%

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Evaporative Cooler

2-Stage evaporative cooler, using a discrete cooling tower to cool the water

By pre-cooling the entering ambient air, we now have air with a lower wet bulb temperature that will go into the direct cooling section. This means that we will be able to achieve a lower supply air temperature.

A further advantage is that the absolute humidity of the supply air will be lower than the simple single stage evaporative cooler.

2-Stage Evaporative Cooler made by Protek

Neels Claasen and Piet Hickley of Protek in South Africa have taken a further step to improve the performance of the 2-stage evaporative cooler and patented the following design.

Exploded model of IWAC 2-stage evaporative cooler

The resulting design is based on the cost benefit of using a 2nd stage in the cooling tower. The Protekdesign packages the above concept into a very neat box with the range named IWAC (Integrated Wet bulb depression Air Cooler).

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Evaporative Cooler

IWAC model evaporative cooler by Protek

Psychrometric chart showing (exploded model) 2-stage evaporative cooler

In the above psychrometric chart, you can see clearly that the leaving air at point 3 would be lower than the temperature of a single stage process starting at point 1.

When to use an evaporative cooler

An advantage of the evaporative cooler is that it provides a reasonably comfortable temperature in hot weather. By making use of the energy of evaporating water, supply air temperatures of around 16 degC are possible.

Clearly, this process needs a hot-dry ambient to be effective.

Compared to the mechanical cooling alternative, the benefit is a lower energy bill. In addition,

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Evaporative Cooler

maintenance is easier and the process is inherently environmentally friendly.

With the recent world-wide concern about global warming, evaporative cooling is a very attractive method of cooling residential and commercial applications.

Selection of Evaporative Cooler

The easy way to select an evaporative cooler is on the basis of air volume and supply air temperature.

For example, Protek publishes the following quick selection table for sea level:

For 1-stage, you must add 2 to 8 degC to the above values. Refer to the Protek catalog for more detail.

EXAMPLE LOAD CALCULATION: WHAT IS THE CAPACITY OF AN IWAC MODEL 15 WITH AN ENTERING TEMPERATURE OF 32/20 DEGC (DB/WB).

At the specified temperature, the leaving dry bulb is 17.8 degC. The airflow if the IWAC 15 is 3.65 cms. This means that the sensible cooling is: Qs = ma Cpm (dbi - dbo) = 3.65 x 1.2 x 1.023 x (32 - 17.8) = 63.6 kW

Note that the air density of 1.2 kg/m3 was used.

2-Stage evaporative cooler leaving air dry bulb temperature at sea level, degC

Entering dry bulb temperature, degC30 32 34 36 38

Entering

wet bulb

temperature

17 13.8 13.2 12.7 12.1 11.6

18 15.2 14.6 14.1 13.7 13.3

19 16.5 16.1 15.6 15.2 14.7

20 17.8 17.3 16.9 16.4 16.6

21 19.2 18.8 18.4 17.9 17.4

22 20.5 20 19.5 19.2 18.7

Airflow of the Protek IWAC evaporative cooler model, m3/s

Model cms Model cms

12 2.75 45 11.0

15 3.65 50 12.8

20 4.60 60 14.5

25 5.50 75 18.3

30 6.40 90 21.9

35 7.30 105 25.5

40 9.10 120 29.2

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Empirical Formula

Single stage unit:

dbsa = dboa - 0.9 (dboa - wboa)

wbsa = wboa

Two stage unit:

db2 = dboa - 0.8 (dboa - wboa)

wb2 = plot on psychrometric chart

dbsa = db2 - 0.9 (db2 - wb2)

wbsa = wb2

The figure below gives an easy supply air temperature selection for a 2-stage Protek IWAC evaporative cooler around South Africa.

Supply air temperature reference for South Africa (courtesy of Neels Claasen, Protek)

References

Neels Claasen, personal correspondence, January 2008.

Stoecker, W.F and Jones, J.W "Refrigeration and air conditioning", 2nd ed McGraw-Hill, 1982.2.

1.

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ASHRAE Handbook CD, 2002 to 2005.

Donald P Gatley, "Understanding Psychrometrics", Published by ASHRAE, 2005.

Kuehn, et al. "Thermal Environmental Engineering", 3rd ed. Prentice Hall, 1998.

Tim Padfield. http://www.padfield.org/tim/cfys/gael/gael.php

Thanks to Neels Claasen of Protek for information and permission to use his catalog material in this article.

For more information, call Protek directly at

Protek, PO Box 1943Halfway House, 1685tel 011 462 1752fax: 011 462 1753email: sales@protekhcm.co.za

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