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6
Evaporative Coolers
Wayne T. Hartshorn
Hart Environmental, Inc., Lehighton, Pennsylvania
6.1 DEVICE TYPE
Evaporative gas coolers use the controlled application of a liquid (usually water)
to a hot gas stream to reduce that gas streams temperature through the evapora-
tion of that liquid. The liquid is often applied in the form of an air-atomized mist
or fog.
6.2 TYPICAL APPLICATIONS AND USES
Evaporative coolers are designed to reduce a hot gas stream’s temperature to a level
suitable for further treatment. They are also used to “condition” the particulate
before capture in another device.
When a gas stream requires treatment by a device that is sensitive to gas tempera-
tures as well as gas humidity (such as a fabric lter collector), an evaporative gas
cooler is often used to reduce the gas stream temperature to a tolerable level above
the saturation temperature. Through the careful application of the liquid, the outlet
temperature can be reduced yet the bulk stream quality can be maintained safely
above the water saturation temperature and/or acid dewpoint.
The evaporative gas cooler is sometimes also used ahead of devices such as elec-
trostatic precipitators or spray dryers to temper or condition the gas stream before
particulate separation or gas absorption onto a sorbent. For boiler applications, the
addition of moisture often favorably reduces the resistivity of the y ash.
Evaporative coolers are often used as the rst stage of a gas cleaning system on
hot gas applications such as thermal oxidizers, incinerators, furnaces, calciners, and
kilns. Figure6.1 shows an evaporative cooler (to the right) ahead of a pulse type bag-
house equipped with dry lime injection on a medical waste incinerator. The evapora-
tive cooler reduces the ue gas temperature to less than 500°F to protect the lter
medium in the collector and to reduce the treated gas volume.
6.3 PRIMARY MECHANISMS USED
Evaporative coolers use the heat of vaporization of a liquid to extract heat from the
gas stream and thereby reduce the mixture temperature.
The evaporation rate is dictated by the temperature and humidity differential
between the desired outlet gas conditions and the given inlet gas quality. The droplet
48 Air Pollution Control Equipmenet Selection Guide
size produced by the evaporative cooling nozzles or spray system dictates the evapo-
ration time and therefore the physical size of the evaporative cooler.
6.4 DESIGN BASICS
Over the years much progress has been made in the further development and improve-
ments of air pollution control (APC) devices, such as electrostatic precipitators (wet
and dry), fabric lters (baghouses), scrubbers (wet and dry), as well as other types of
collection equipment. However, far less attention has been given to the cooling and
conditioning of hot-process gases before being treated in APC devices. Every APC
device installed on a high-temperature application is affected in some way by the
cooling technique used. Because of this effect, the area of cooling and conditioning
becomes signicant and indeed important when designing an overall gas handling
or pollution control system.
Evaporative cooling can be applied to hot-process gases in many industries and
applications. Some of those industries are ferrous and nonferrous metals, rock prod-
ucts, industrial and utility power, and incineration. When evaporative cooling sys-
tems are properly engineered, they can provide the most cost-effective method of
dealing with increased heat loads from these sources.
6.4.1 types of Gas CoolinG
The three most commonly used techniques for cooling hot-process gases are dilution
cooling, convection/radiant cooling, and evaporative cooling. Figure6.2 shows the
effect of evaporative and dilution cooling on resulting gas volume when cooling to
400°F. When selecting an APC device to be installed downstream of the gas cooling
system, it is important to note the lower gas volume that results using evaporative
cooling versus dilution cooling.
FIGURE6.1 Evaporative cooler on pulse type baghouse (Bundy Environmental Technology,
Inc.).

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