All industrial cooling water systems function for one purpose — to remove unwanted heat. This is true whether the heat is generated in a critical process reactor or in an executive office building. In any case where unwanted heat is generated, it is necessary to design a suitable cooling system that is capable of absorbing this heat and transporting it to a point of rejection.
Industrial cooling water systems vary considerably in design, function and operation. It is customary to categorize cooling water systems into one of three types:
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Once thru
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Closed recirculating
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Open recirculating
In once thru cooling water systems, the water is pumped from its source through a point of heat exchange where it absorbs the heat and transports it to a point of discharge. Hence we have heat absorption and rejection with the water only being used once prior to discharge.
In a closed recirculating system, the water is continually recirculated through the system picking up heat at the point of heat transfer and rejecting the heat at a second point of heat exchange. This system is typical of an engine cooling system. In industry, closed chilled water systems for air conditioning systems are typical of this type of cooling system design.
Open recirculating systems are designed such that cooling water is pumped through a heat exchanger, absorbing heat in the process, and then transporting this heat to a cooling tower. At the cooling tower, the heat is rejected to the atmosphere by means of convective and evaporative cooling. Once cooled, the water is pumped back to the heat exchange site where the cycle continues. The primary benefit of the open recirculating system is that it permits the reuse of the cooling water prior to discharge.
Cooling Towers
The fundamental component of most open recirculating cooling systems is the cooling tower. A cooling tower is a heat rejection device used to lower the temperature of the return cooling water. The primary purpose of the tower is to cool water so it can be reused. In this sense, a cooling tower is a water conservation device.
Cooling towers are categorized as either natural draft or mechanical draft. Hyperbolic towers are natural draft towers. Air flow through the natural draft tower is created by the density differential between the warmer air inside the tower and the cooler outside air temperature. This is the same principle used in the design of a chimney.
Mechanical draft towers incorporate a fan to move the air through the tower. These towers are further subdivided into forced draft and induced draft design. Forced draft denotes “pushing” air through the tower whereas the fans on induced draft towers “pull” air through the tower. In either case, the overall requirement is to provide sufficient air flow to promote adequate and intimate air-to-water contact.
Mechanical draft towers are also referenced as cross-flow and counter-flow design. Counter-flow refers to air flow that is opposite to the water flow. Cross-flow refers to air flow that is perpendicular to the water flow.
Other cooling systems designs, such as spray ponds and evaporative condensers, are also considered open cooling systems. Spray ponds are large, shallow lagoons that are designed with a series of spray nozzles. Warm water is sprayed into the air where the heat is rejected by evaporative and convective cooling. The cooled water falls back into the collection basin ready for reuse.
In an evaporative condenser, the heat exchanger or condenser is placed directly inside the cooling tower. Water is sprayed over the outside of the condenser tube bundle to cool the liquid or gas inside the tubes. Evaporative condensers are commonly used in ammonia refrigeration systems.
The various cooling tower designs are fabricated from many materials. Early cooling towers were made from wood such as redwood and Douglas fir, but modern towers are available in galvanized steel, stainless steel, plastic, and concrete.
Bill Harfst
http://www.harfstassociates.com
Water Technology Report 