Page 206 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

204 Chapter 7 Industrial cooling systems

The present chapter discusses the design of cooling towers (CT) in which the circulating cooling
water comes in direct contact with air. The quality specification of cooling water is available in
Chapter 18 (Table 18.8).

7.2 Cooling tower
Wet Cooling Tower is the most common form of industrial cooling arrangement. The circulating
cooling water picks up heat from the plant and rejects it in the cooling tower. Hot water entering at
the top is distributed within the tower structure in a manner that exposes a large water surface to the
air passing through. Water distribution is achieved by spray nozzles or distribution pans. In most
cases, various types of fill media are used to increase the air-water contacting surface. Airflow can be
due to natural draft or by fans forcing in or sucking out the air. As water trickles through the fill
media, the air is blown across the fill to have direct contact with the falling water. A small part of the
water evaporates in the tower with its heat of vaporization (latent heat) being supplied from the rest
of the water that gets cooled. The cold water is circulated back to the plant. Since water comes in
direct contact with air in a cooling tower, this type of cooling system is calledanopen recirculating
cooling system.
Advantages of cooling tower includes -
• Can achieve lower water temperatures compared to other cooling methods. Achieved temperature
can be lower than the ambient air dry-bulb temperature if the air is relatively dry and typically to a

temperature around 4 C above the ambient air wet-bulb temperature.
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• Relatively cheap and efficient option for rejecting low-grade heat from the warm cooling water
returned from the process.
• Minimal freshwater requirement to make up water losses, thus promoting water conservation
• Lower environmental impact due to cut down on chemicals added to cooling water (as only a
small amount of chemical is lost with the water leaving the circulating water system).
• Close control and improved water quality that reduce fouling and corrosion tendency in process
equipment.
• Occupy a smaller plot space as compared to a fin fan air cooler used to remove the same heat duty.

7.2.1 Classification

Cooling towers are classified based on build, heat transfer method, airflow pattern, and the principle
used in creating the air draft. Fig. 7.1 shows the basis of classification and the different types of wet
cooling towers.

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Low-grade heat refers to the heat energy associated with a hot stream that has a relatively low temperature. Removing this
“low-grade heat” would involve a lower temperature difference (DT) with the cooling media. This calls for a large value of
“U A” for the heat exchange process designed to remove heat Q (Q ¼ U A DT). In Indian condition cooling water

returned from the processes is typically at a temperature of 45e47 C and the average ambient temperature is around 33 C.
Therefore, the warm water is cooled by direct contact with air in cooling towers that have high “U A.”