Page 205 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

CHAPTER

7
Industrial cooling systems

7.1 Introduction
All process industries require streams and equipment to be cooled. Process cooling up to ambient
temperature is carried out with cooling water and also with ambient air. Cooling water (CW) is the
most common process utility used for picking up this heat. Once-through systems use water from a
source like river, sea, or canal and after heat exchange, the warm water is returned back to the same
source or some other stream. Such systems in industrial scale are less popular due to limitations like
the unavailability of sufficient quantity of acceptable quality water throughout the year. Any chemical
added to this water for reducing algal growth or scale reduction also leaves along with the water
discharged.
Circulating cooling water system is the alternative to the once-through system. Many of the in-
dustries have changed over from their older “once-through cooling water system” to circulating
cooling water system and installed cooling towers. Air-cooled exchangers, also termed as fin fan
coolers, are frequently used for cooling and condensing process streams with ambient air as the
cooling medium. Fluid coolers are a case of circulating water surface cooler and cooling tower
combined as a single unit.
Drift and evaporation loss put together in a cooling tower (CT) is around 0.2% of the circulation
3
3
rate. In a 10,000 m /hr system, the same will be 20 m /hr
3
or 3360 m in a week, which is close to the size of a small
pond. The operation of this tower will dry up one such
Cooling Tower vs ‘Once through system’ pond full of water every week. As an alternative, if it is
3
possible to use a once-through system, 10,000 m /hr of
water would have been returned to the environment at a
temperature, which would be higher than its initial temperature by around 12 C. In the case of a

flowing stream, the warm water returned mix with the rest of the water and the stream temperature,
usually, rises by around 4e6 C that subsides within a reasonable distance downstream e say after

flowing 500 m and becomes almost imperceptible.
The foregoing analysis suggests that although the CT option is possibly more attractive from the
point of economy for the industry, it may not always be so from the environmental aspect.

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