1 Introduction
The discharge of water at elevated temperature is often described as thermal
pollution. It is produced by industries such as electric power plants, pulp and
paper mills, chemical facilities, and other process industries that use and subsequently discharge water. Even if the discharged water is merely elevated in temperature, allowing it to return to streams, rivers, lakes, and other waters can dramatically
alter the native environment.
Often, elevated water temperature will be detrimental to native species of plants
and animals. In such instances, alternate, invasive, and possibly undesirable species
will be supported by the elevated water temperature. As water warms, the solubility
of oxygen decreases. In addition to promoting competitive species of plants and
animals, warmed waters may lead to aesthetic and odor problems if anaerobic
conditions are created. For these and many other reasons, the discharge of elevated
process water is widely regulated by governmental agencies in developed nations.
The fundamental principle of evaporation of water to achieve cooling is widely
utilized in nature. In the human body, sweat glands produce perspiration (water),
which cools the body by evaporation. The fact that this cooling is more effective
during a 95
F day in Arizona than an afternoon of the same temperature in Florida is
widely understood. This concept is illustrated by the following example.
Problem
Given a pail full of water, change the water in the pail into vapor.
Solution
Several possible methods are possible to achieve this goal:
1. Let the pail stand until all the water in the pail has evaporated
2. Obtain a second, empty pail, and pour the water into the empty pail. Repeat this
process again and again between the pails. In this way, one will intuitively
increase the water/air interaction, so evaporation of the water will be enhanced.
3. If possible, blow air over the surface of the water in the pail. A simple house fan
will suffice. Again, intuitively, evaporation of the water is enhanced, and the
moving air will carry away the water vapor as fresh air is pushed across the water.
4. And, per the previous mention of Arizona vs. Florida, low humidity air will
support the desired evaporation of water much more efficiently than high
humidity air.
The purpose of this chapter is to discuss cooling ponds and cooling towers.
Essentially, a cooling pond is choice “a” in the previous example. A cooling tower
combines choices 2–4.
Cooling ponds are the simplest choice to lower the temperature of a warm water
discharge stream and may be the least costly alternative. Such a solution to a water
cooling problem relies entirely upon contact of the pond water surface with the
surrounding environment. As such the warm water will slowly approach the desired
outlet temperature. Ponds are designed to be either recirculating or once-through type.
198
Y.-T. Hung et al.
The discharge of water at elevated temperature is often described as thermal
pollution. It is produced by industries such as electric power plants, pulp and
paper mills, chemical facilities, and other process industries that use and subsequently discharge water. Even if the discharged water is merely elevated in temperature, allowing it to return to streams, rivers, lakes, and other waters can dramatically
alter the native environment.
Often, elevated water temperature will be detrimental to native species of plants
and animals. In such instances, alternate, invasive, and possibly undesirable species
will be supported by the elevated water temperature. As water warms, the solubility
of oxygen decreases. In addition to promoting competitive species of plants and
animals, warmed waters may lead to aesthetic and odor problems if anaerobic
conditions are created. For these and many other reasons, the discharge of elevated
process water is widely regulated by governmental agencies in developed nations.
The fundamental principle of evaporation of water to achieve cooling is widely
utilized in nature. In the human body, sweat glands produce perspiration (water),
which cools the body by evaporation. The fact that this cooling is more effective
during a 95
F day in Arizona than an afternoon of the same temperature in Florida is
widely understood. This concept is illustrated by the following example.
Problem
Given a pail full of water, change the water in the pail into vapor.
Solution
Several possible methods are possible to achieve this goal:
1. Let the pail stand until all the water in the pail has evaporated
2. Obtain a second, empty pail, and pour the water into the empty pail. Repeat this
process again and again between the pails. In this way, one will intuitively
increase the water/air interaction, so evaporation of the water will be enhanced.
3. If possible, blow air over the surface of the water in the pail. A simple house fan
will suffice. Again, intuitively, evaporation of the water is enhanced, and the
moving air will carry away the water vapor as fresh air is pushed across the water.
4. And, per the previous mention of Arizona vs. Florida, low humidity air will
support the desired evaporation of water much more efficiently than high
humidity air.
The purpose of this chapter is to discuss cooling ponds and cooling towers.
Essentially, a cooling pond is choice “a” in the previous example. A cooling tower
combines choices 2–4.
Cooling ponds are the simplest choice to lower the temperature of a warm water
discharge stream and may be the least costly alternative. Such a solution to a water
cooling problem relies entirely upon contact of the pond water surface with the
surrounding environment. As such the warm water will slowly approach the desired
outlet temperature. Ponds are designed to be either recirculating or once-through type.
198
Y.-T. Hung et al.