Cooling tower design

Cooling towers are heat removal devices for industrial processes. They are defined as any open water recirculation device that uses fans or natural draft to draw or force air to contact and cool water by evaporation. Cooling towers minimise the thermal pollution of natural water heat sinks and allow the reuse of circulating water.

Cooling towers can be characterised by the way in which air is moved. For example, mechanical draft towers use mechanical fans to move air through the system. These towers implement either a forced or induced draft mechanism.

Natural draft cooling towers use natural convective airflow moving up the stack to cool water. These systems are very large (+400 feet high) and can exceed flows of 500,000 gpm (gallons per minute). Due to their size, they are used mostly by power utility companies. Loop Type

In addition to the draft and flow design, cooling towers can also be either closed or open loop.Open loop systems are enclosed structures which distribute warm water over a "fill" with packing material. The fill provides a vastly expanded air-water interface for heating of the air and evaporation to take place. The water is cooled by direct contact with air that passes over it.

The cooled water is collected in a cold water basin below the fill from which it is pumped back through the system. The heated and moisture laden air leaving the fill is discharged to the atmosphere at a point remote enough from the air inlets to prevent its being drawn back into the tower. Closed loop systems work in a similar way to refrigeration systems.

The cooling fluid (usually water or a glycol mixture) is contained inside a closed piping system and evaporative cooling occurs by running water over the pipe containing the heated water.

Air is drawn through the recirculating water cascading over the outside of the hot tubes, providing evaporative cooling similar to an open cooling tower.

Operation of the indirect cooling towers is therefore very similar to the open cooling tower with one exception: the process fluid being cooled is contained in a "closed" circuit and is not directly exposed to the atmosphere or the recirculated external water. Materials of Construction

There are a number of materials that can be used to construct each component of a cooling tower, depending on the design and application.

Metals used in towers include copper for heat transfer and piping, aluminium for fan blades, and galvanised steel or coated carbon steel for structural components. Certain dissolved components in water have a tendency to cause metals to corrode, and extra corrosion resistance requires the use of stainless steel or nonmetal materials to extend the life of the component.

Fiberglass reinforced polyester (FRP) is a hard lightweight material that is corrosion resistant and can withstand heavy loads. It can be used for the construction of major body parts of the tower and smaller system components. This material is used in systems where chemicals in the cooling water would be highly corrosive to metals.

Polymers such as polyvinyl chloride (PVC), polyethylene, and polypropylene are favoured as non-structural components in towers where the water's corrosion potential inhibits the use of metals. Wood is a common building material used in the construction of expansive cooling towers.

It is most cost effective in designs requiring a lot of material, where the field labour for construction becomes less of a factor. Wood suffers from leaching (loss of wood preservative chemicals by flowing water), which can lead to degradation of the material over time. Concrete is used as the primary material in natural flow cooling towers.

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