CHAPTER 2 | Cooling Tower Types

Cooling towers are often categorized by the way they make air and water interact. The counterflow tower has air and water passing in opposite directions... The water falls vertically down while the air travels vertically up.

In the crossflow tower, water flow remains vertically down while the air flow is horizontal. When a crossflow tower is constructed with two opposing air streams joining in a common plenum, the design is called a double flow, crossflow tower.

Another distinction involves the location of the fan as in 'blow thru'' or 'draw thru' and is determined by whether air is pushed into or pulled out of the tower.

Counterflow towers tend to be the most compact. This is because the coldest air is in intimate contact with the entire cross section of water just before it falls into the basin. Less space is needed because of this increased efficiency and lack of plenum space required for cross flow cooling towers. The down side, though, is the increased fan horsepower resulting from air flow in direct opposition to the water flow.

Crossflow towers enjoy considerable popularity primarily due to their operational cost savings. They often have the lowest initial cost as well plus a simple, easy to maintain design.

Blow thru designs generally have easier mechanical component access because the moving parts are located at the base. They exhibit a reduced corrosion potential because they handle dry, ambient air instead of the saturated air of the draw thru arrangement.

The type of fan -centrifugal or propeller type- can further categorize tower offerings. Centrifugal fans have the advantage of quiet operation and can also be used in conjunction with oversized motors to overcome the resistance to airflow imposed by connected ductwork or tight installations.

Conversely, prop fans are more noisy and generally lack the ability to handle duct work but display the highly desirable characteristic of consuming approximately half the horsepower of a centrifugal fan for the same thermal capacity.

Yet another distinction is made between factory and field assembled towers. Factory assembled towers are limited by the practicality of transporting over size loads to the job site by trucks with maximum dimensions of 14'W x 12'H x 48'L. [This does not mean that a tower can ship in one piece with these dimensions because 48' length and 12' height are not mutually acceptable to the freight carrier.] Large factory assembled cooling towers ship in multiple sections for assembly at the job site.

When designing over seas projects, be sure to check the size restrictions imposed by the local authorities. Highways and bridges can be less capable in other areas of the world. Be careful not to ship a cooling tower abroad only to find it cannot be transported from the dock to the job site. It may be better to design the project with towers built locally. Export fees, ocean freight, increased risk of shipping damage, etc. are eliminated and repair parts availability is greatly enhanced. Some domestic manufacturers have overseas manufacturing facilities. The biggest single cell, US manufactured crossflow tower is about 1,050 nominal tons. For counterflow, it's approximately 1,350 nominal tons. Both must be broken into several pieces for transport. Multiple factory assembled cooling towers are often used but there comes a point where field erected cooling towers become more practical. They are built on site of wood, steel, fiberglass or concrete typically on a concrete basin although fiberglass or steel basins can be provided on smaller models.

Very large projects such as nuclear power plants can employ hyperbolic cooling towers. They are characterized by their distinct shape much like a tall cylinder with a tight belt around the waist. Such towers have the advantage of not requiring any fans, motors, gear boxes, etc.. The design consists of a ring of wet deck surface arranged in a crossflow fashion encircling the base of the tower. The center contains a large, hyperbolically shaped chimney. Any warm air at the base of the chimney rises and accelerates as the cross section diminishes creating a negative pressure that draws additional air through the wet deck fill. The tall stack insures against recirculation (See Ch.4). Such towers run largely on their own with capacity increasing along with the cooling requirement. Pumping costs would be quite high for conventional cooling towers built to the proper scale for such projects making one more advantage apparent... The low elevation of the hot water distribution basins offers substantial pump horsepower savings.

The designer must realize that each project is unique and that the site requirements should be matched to the most desirable characteristics of the available towers. Selecting a tower based solely on first cost or energy consumption is typically not the best approach.