Fundamentals of Cooling Tower Design
Consider your design and operating conditions before specifying a cooling tower for industrial evaporative cooling.
Your cooling tower may be the most overlooked piece of equipment at your facility. If impropery selected or poorly maintained, it will cost you financially.
Your cooling tower may be the most overlooked piece of equipment at your facility. A cooling tower uses a combination of heat and mass transfer to cool process water. If improperly selected or poorly maintained, it will cost you financially, causing a loss in production due to increases in circulation water temperature and increased electrical operating costs. Emphasis must be placed on properly specified and designed cooling towers that require minimal maintenance.
Cooling towers come in all shapes and sizes. Cooling towers with fans are referred to as mechanical draft -- forced or induced draft, depending on fan location. Some cooling towers, used primarily in the utility industry, operate on a natural draft chimney principle without any fans. As their name implies, cooling towers employing this design are referred to as natural draft.
Most cooling towers are designed as simple wet cooling towers, but upon occasion, a tower will be designed to operate as a wet-dry cooling tower. A wet-dry cooling tower adds heat to the airflow prior to discharge through the cooling tower fan stack. The discharge air is warmed above the ambient dewpoint to eliminate any visible plume that could cause local environmental concerns or hazards to local roadways.
Most heating and cooling applications (HVAC) require cooling towers sized below 10,000 gal/min. Towers of this type, called package cooling towers, usually are mass produced in factories with galvanized steel structure and casing. This type of cooling tower is manufactured so it can be transported easily to the job site without special trucking permits.
Cooling towers requiring a thermal duty beyond the capabilities of a package cooling tower are larger, requiring them to be manufactured, shipped and assembled at the site. These are field-erected towers and generally are used in most industrial and utility applications. Field-erected mechanical draft towers can handle flow rates from 10,000 to 350,000 gal/min. Larger flows generally are required only for large utility applications and are best served by natural draft cooling towers.
Most towers employ the counterflow water flow, but crossflow designs are available. The differences between these two designs are governed by the relationship of air and water flow inside the cooling tower. In counterflow cooling towers, water falls by gravity over the heat exchanger media, and air passes vertically upwards. By contrast, in crossflow towers, water falls vertically by gravity over the heat exchanger media while air flows horizontally. Most counterflow cooling towers utilize a plastic film fill heat exchange media that reduces both pump head and horsepower costs; crossflow towers typically utilize a splash-type heat exchanger. However, it is possible to find either type of exchange media in both types of towers.
Selecting Fill Media
Care must be taken in the selection of cooling tower fill media. Because the circulating water flow in cooling towers is warm and oxygenated, it creates an ideal environment for biological growth if water chemistry is not kept under strict control. In addition to biological growth, dissolved solids, in favorable conditions of temperature and alkalinity, may precipitate within critical areas of the fill media and cooling tower piping. Both biological growth and dissolved solids precipitation -- if left unchecked -- will destroy the thermal capability of your cooling tower. To ensure the proper long-term thermal operation of your tower:
- Initiate and follow a rigorous tower water analysis and treatment plan.
- Maintain tower blowdown.
- Use antifouling fill media.
Both splash and film fill can act as antifouling fill media. Splash fill is used between the incoming ambient airflow and the water droplets formed as the tower water falls and impinges on a network of grids or laths. Film fill can be used where modules consisting of thermoformed plastic sheets create a large surface area for a thin film of cooling water.
Today, most cooling towers utilize film fill with vertical flutes called nonfouling film fill. This fill allows higher cycles of concentration for the tower water supply. Select your fill based on your circulating water's chemistry, the ability to clean your fill if it becomes plugged or fouled, and the need to maintain your tower's design cold water temperature.
While package cooling tower materials of construction generally are limited to galvanized steel (or stainless steel in special situations), many possibilities exist for field-erected structures. Field-erected towers can be constructed of Douglas fir, redwood, fiberglass, steel or concrete. Each material has many advantages and disadvantages. Wood towers offer the shortest life expectancy, leach the preservative chemicals (CCA or ACC) with which they are treated into your blowdown and tower sediment, and require a pH balance below 8.5, but they are relatively inexpensive to build and repair. Concrete towers will last more than 40 years, but they are the most expensive to build. Because of their cost, they represent only 2 to 3% of all field-erected towers.
One advance in cooling tower construction has been the supply of cooling towers built with fiber-reinforced plastic (FRP). Fiberglass has been used in cooling tower piping, fan stacks and siding for many years with great success due to its low maintenance requirements, resistance to moisture, and material properties that allow a range of water temperatures and pH.
Currently, the fastest growing segment of the cooling tower market is structures built with pultruded FRP sections. This inert inorganic material is strong, lightweight, chemically resistant and able to handle a range of pH valves. Pultruded fiberglass is produced by pulling many strands of fiberglass through a heated mold while creating a protective surface on the product. The final shape can be an I-beam, channel, angle, tube or panel, and all designs can be used in the cooling tower structure or decking. FRP is stronger than Douglas fir and redwood, and because it is available in long lengths, it allows a cooling tower to be designed and built with a minimum number of airflow obstructions. This enhances performance and reduces the number of connections and field labor erection costs. Fire-retardant FRP can eliminate the cost of a fire protection system, which can equal 5 to 12% of the cost of a cooling tower.
Sizing Your Tower
Many choices and decisions are required to properly size a tower. At minimum, be sure your specification to cooling tower manufacturers stipulates the following:
- Flow rate (gal/min).
- Total heat rejection (BTU/hr).
- Cold water temperature (°F).
- Hot water temperature (°F).
- Design wet bulb temperature (°F).
- Elevation above sea level (ft).
- Tower type (crossflow or counter- flow).
- Materials of construction.
- Fill media choice (film, splash orantifouling).
- Water quality.
- Noise limitations.
- Drift loss expected. (Standard is 0.008% of water flow.)
- Scope of supply. (Who is responsible for basin, external piping, electrical hookup, etc?)
- Evaluation factors ($/kW).
Table 1 compares relative costs for various types of cooling towers meeting specified criteria. The table provides relative capital costs and operating costs for different cooling tower designs. Cost comparisons are based on a cooling tower designed to handle 48,000 gal/min; 16°F (9°C) range; 13°F (7°C) approach; 72°F ( 40°C) wet bulb; 30 PSF wind velocity pressure at a seismic zone UBC 2 or less. The occupied ground area for each comparison has been kept constant to achieve comparable values. It is fair to assume that these comparisons apply to a range of cooling towers.
Your cooling tower is an essential piece of equipment in your plant, and the increasing cost and decreasing availability of process water can no longer be ignored. Most towers in operation today were designed with technology and components popular 20 years ago. It is possible to upgrade the performance of your tower with simple repairs and upgrades, or replace it with the latest technologies.
Designing and optimizing a new cooling tower is not a small job. Be sure to consider which materials of construction, fill type and sizing choices are best suited to your application.