Cooling Tower Description - Typical Values of Design Variables

            A cooling tower is a special type of heat exchanger in which the warm water and the air are brought in direct contact for evaporative cooling. It must provide a very good contact of air and water in terms of the contact area and mass transfer coefficient of water vapor while keeping the air pressure drop low. 

Figure 1 A single deck spray pond cooling system.

           In the early years of industrial development, cooling of warm water for reuse was done in “spray ponds”. In a spray pond, as the name implies, a spray system located about six to eight feet above the water surface creates small droplets of warm water that cool down by evaporation in contact with air. This is simple and easy process but required a large pond area. It is inefficient (the effective heat transfer coefficient is about 3.5 Btu/h.ft².°F or 20 W/m².K), and creates the problem of entrainment and carryover of water droplets by air.

     

         The first cooling tower was built in the USA by George Stocker in 1898. It was a wooden packed tower. Many changes in the design and construction of cooling towers have occurred since then.

Figure 2 First cooling tower built by George. J. Stocker.

Factors Governing The Operation of a Cooling Tower:

The important factors that govern the operation of a cooling tower are:

• The dry bulb and wet bulb temperatures of the air.

•  The temperature of warm water.

• The efficiency of contact between air and water in terms of the volumetric mass transfer coefficient and the contact time between the air and the water.

• The uniformity of distribution of the phases within the tower.

• The air pressure drop.

• The desired temperature of the cooled water.

Classification of Cooling Towers:

             A tower is irrigated at the top through nozzles. A large air-water contact area and contact time are offered by a typical tower packing. Air may enter the tower driven by a density gradient (natural draft), may be pushed into the tower (forced draft) at the base or sucked into the tower (induced draft) assisted by a fan. Several types of cooling towers have been designed on the basis of the above factors and operating strategies. A number of models of each type are offered by the manufacturers. The most important types are classified in the figure below.

Figure 3 Classification of cooling towers.

Typical Values of Design Variables:

          The practical values of the important design variables and parameters of cooling tower are given below.

• Range = 8 – 15°C.

• Approach = 5 – 6°C.

• Evaporative loss = 1.5 – 2%.

• Drift loss = 0.02 – 0.2%.

• Overall volumetric gas phase mass transfer coefficient = 1500 – 3000 kg/h.m².∆Y’.

• Packing depth = 5 -10 m for splash packing, 1.5 -2 m for film pack.

• Liquid mass flow rate = 2 – 5 gallons/min.ft².

• L/G ratio = 0.75 – 1.5.

• Free space above the drift eliminator = 2 – 3 m.

• Height of the fan stack (or fan cylinder) = 3 – 5 m.

• Spacing of corrugated sheets in a film pack = 19 – 25 mm.

Thank you for reading.