Environmental Problems Associated With The Use
of Cooling Towers
DAVID L. BRENCHLEY, Assistant Professor
School of Civil Engineering
Purdue University
West Lafayette, Indiana
PETER R. WESTLIN, Engineer
Environmental Protection Agency
Research Triangle Park, North Carolina
INTRODUCTION
The demand for energy continues to grow; at present the need for electrical energy is
doubling every seven years. This increase is due both to the population growth and the per
capita increase in consumption. This energy need must be produced by some sort of
thermodynamic process. Hence the second law of thermodynamics becomes of particular
interest. This law essentially limits the efficiency of powerplants to about 30 or 40 percent
and thereby necessitates rejection of 60 to 70 percent of the energy as low grade waste heat.
The people using energy conversion processes are therefore faced with the dilemma of how
and where to reject heat. They must maintain efficient design and yet still keep peace with
the populace. In the past, lakes and rivers have served as intermediate sinks for this waste
energy. The local result has been increased water temperatures often referred to as "thermal
pollution" or "thermal enrichment", depending upon one's point of view. Eventually the
water cools by transferring the energy to the atmosphere. Cooling ponds and cooling towers
are used to enhance this energy transfer process to the atmosphere and thereby relieve the
thermal stressing of local watercourses. Recent water quality standards (1) on water
effluents allow a 5 F (2.8K) increase in open rivers and streams. The allowable increase for
lakes is 3 F (1.7K). For estuaries and coastal waters a 1.5 F (0.8K) is allowed during the
months of June, July and August, while a 4 F (2.2K) is permitted during the rest of the year.
The maximum water temperature allowed in all cases is 90 F (306K). This type of regulation
makes it extremely difficult to use the watercourses as heat sinks. Typically condensers for
powerplant operations are designed on a 10 to 30 F (5.6 to 14.8 K) temperature rise. If the
condensers were to be designed for the 1.5 to 3 F (0.8 to 1.7 K) allowable increase, the water
pumping cost would be excessive. This obvious conflict thus explains the increased use of
cooling ponds and cooling towers. The cooling ponds seem to find application only in
certain circumstances because of the large land area requirements.
Cooling towers are devices which transfer the waste energy directly to the atmosphere.
The towers may be mechanical or natural draft and may be wet or dry. The most common
types are natural and mechanical draft wet-type towers. In these devices the hot water
comes in contact with the ambient air. The water is cooled by two mechanisms: 1) sensible
heat exchange and 2) latent heat of evaporation. During summer operations the cooling is
due almost totally to evaporation while under winter conditions, possibly one-third is due
to sensible heat exchange. At larger power plant installations these cooling towers discharge
thousands of gallons per minute of water to the atmosphere. Some of the water is in the
form of "drift", but mist eliminators generally limit this to 0.1 percent of the water flow rate.
A greater portion of the water leaves the tower as a vapor. The evaporative loss is on the
order of 1 percent of the water flow rate for each 10 F(5.6K) cooling of the process water.
Dry cooling towers rely completely on sensible heat exchange; the gas and liquid
streams do not come into direct contact. Large quantities of air must be moved through the
device in order to accomplish the necessary heat transfer. Like an automobile radiator, a
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