A cooling tower is an economical solution to cooling large volumes of water with minimal energy conusmption. A tower system is usually used to cool heat loads with 85°F water. This is the optimum operating temperature for hydraulic oil, chiller condensers (to cool refrigerants), and auxiliaries such as mold temperature controllers or air compressors. However the advantages and limitations of cooling towers must be understood before the equipment is selected for or applied to process cooling.

How Cooling Towers Work: A cooling tower system works through evaporative cooling. By evaporating a small part of the process water, the temperature of all the process water is lowered. This is done by spraying fine water droplets through a stream of upwardly moving air. The more contact time of the air and water, the greater the amount of evaporative and heat transfer.

For evaporation to occur, the air stream must absorb some of the water droplets. The higher the level of humidity, the less air is able to absorb water and, as a result, the less efficient the cooling tower. Typically, cooling tower systems’ capacities are rated to lower 95°F water to 85°F at 78°F wet bulb. Wet-bulb temperature is the lowest air temperature possible for evaporation, so the water temperature cannot drop below the prevailing wet bulb temperature of the air.

Each tower must be specifically sized for the geographic area’s prevailing Summer wet bulb temperature. Even in colder climates, a tower’s cooling capability is usually set at no colder than 70°F during Winter months. High efficiency mechanical draft towers cool the water to within 5 or 6°F of the wet-bulb temperature, while natural draft towers cool within 10 to 12°.

Types of Cooling Towers: There are three basic types of towers:

• Forced Draft
• Induced Draft
• Natural Draft

A forced draft tower has a sensor that thermostatically controls the fan. The sensor monitors the process water temperature as it leaves the tower. The fan starts or stops when the process water temperature rises either above or below the desired set point.

An induced draft has a fan in the wet air stream to draw air through the baffles. This type of tower is generally recommended for industrial processes.

A natural draft tower has no mechanical means to create airflow. In this case, water is pumped to the tower, enters a manifold with nozzles, and is sprayed at a high pressure that induces a draft of air. The finely sprayed water contacts the free-flowing air to cause evaporation.

Factory assembled cooling towers are available in numerous sizes starting at less than 5 tons and reaching several hundred tons. Larger capacities are designed by connecting several units together.

Cooling Tower Piping Systems: There are two basic types of evaporative cooling tower systems:

• Open loop
• Closed loop

Open Loop Systems: A conventional open cooling tower system has an outdoor cooling cell. As water cascades through the cell, the cool water flows into a tank. The tank is typically located indoors to avoid freezing. To improve temperature stability, a dedicated pump for tower water recirculation and a baffle in the tower water reservoir are installed, along with a process water pump, so the cooling tower system becomes a two-pump system.

With a two-pump system, warm process return water is isolated to one side of the baffled tank. The circulation pump circulates this warm water through the tower where it cools. The tower cascades water to a second, cold-water sump from which the process pump circulates water back to the process.

A major drawback of open loop cooling tower systems comes from dust and airborne contaminants entering through the open tower. These contaminants can foul down-stream equipment causing poor heat transfer at the process.

Filtering tower water can remove dust and dirt particles, but dissolved minerals remain. With the addition of water treatment chemicals, many (but not all) minerals precipitate out of the solution. Removal is achieved through mechanical filtration such as a sand filter.

Closed-Loop Systems: This system combines the economies of an open cooling tower with the heat transfer efficiency of a refrigeration system. Water-related problems common with open cooling towers are eliminated with the closed-circuit system because once the water is filtered and chemically treated, it remains pure as long as there is no leakage to replace with make-up water.

A closed-circuit system is similar to a conventional cooling tower except that a heat-exchanger is used to isolate the process water from the tower water and to effect heat transfer between them without process water contamination. Another difference involves the reservoir. In a closed loop system, the tower reservoir is built either with two completely separate compartments or two separate tanks. One compartment or tank holds process water that is piped to the process and back to its separate compartment or tank without coming in contact with the tower water. The compartment or tank holds the tower water. In a completely separate loop, the tower water circulates to the tower cell and back to the reservoir compartment or tank.

Tower Capacity Control: Three methods are available for controlling the capacity of a cooling tower:

• Fan cycling: A thermostat senses the temperature of water leaving the tower, and multi-speed fan motors adjust the amount of air delivered as the load changes.
• Dampers: Thermostatically operated dampers are built into the tower to control the air volume; as the load decreases, the damper closes and restricts airflow through the unit.
• Water volume sprayed: The capacity of a tower is related to the flow rate of water passing through it. A modulating valve regulates the amount of water sprayed in relation to load fluctuations. Another method involves a thermostatically-controlled pump that stops spraying water as the load decreases and restarts the pump when greater cooling capacity is needed.