Cooling Tower Performance




The professional engineer must be able to properly design and size and select a cooling tower to fit the HVAC and Refrigeration application. Cooling towers are characterized by two terms the approach and the range. The range of the cooling tower is the difference between the entering and exiting temperatures of the cooling tower water.

Range=T_(water,in)-T_(water,out)

The approach or approach to wet bulb is the temperature difference between the water out and the wet bulb temperature of the air.

Approach=T_(water,out)-T_(air in,WB)

The approach is important because it describes the level of performance of the cooling tower. The smaller the approach the better the cooling tower is at providing cooling. The wet bulb temperature of the entering air is the lowest the temperature of the exiting water can reach. If a cooling tower has a 0 degree approach then the cooling tower is using all of the available heat exchange from the air to cool the water. Typical approaches are in the range of ~10 °F.

Cooling Towers for the Mechanical PE Exam

Approach also leads to another important term in determining the performance of cooling towers, called effectiveness. Effectiveness is a term used to describe how effective the cooling tower is at cooling the water or how close the actual temperature difference between the water temperatures in and out is to the maximum temperature difference. The maximum temperature difference that a cooling tower can produce is the difference between the water temperature in and the air wet bulb temperature.

Effectiveness=Range/(Range+ Approach)

The range is important because when used in conjunction with the water flow rate, the capacity of the cooling tower can be found. The capacity and the amount of cooling provided by the cooling tower are found by multiplying the flow rate of the cooling water by the difference in temperature at the inlet and outlet of the cooling tower, using the following equation, Q = mc∆T and for a simplified equation to use during the test, follow the derivation below. Cooling Towers for the Mechanical PE Exam

Cooling Tower Water Loss and Make-up

In a cooling tower, water is lost due to multiple sources such as evaporation, drift and blow-down. The first term, evaporation, is calculated through the following equation, where the assumption is made that the total heat loss is due to the heat loss through evaporation.

Cooling Towers for the Mechanical PE Exam

The second water loss is due to drift. Drift is the amount of water that is carried out through the airstream. Drift eliminators provided prior to the discharge are best described as a maze of baffles that the air must travel through before exiting to atmosphere. The drift eliminator trap the water droplets that get picked up by the exiting air and send the droplets back to the fill material. Typical water loss due to drift is less than 0.2%.

The third major source of water loss is due to blow-down. Blow-down is required because as water is evaporated it leaves behind the total dissolved solids (TDS), which increases the concentration of the TDS in the water. In order to bring the concentration of the TDS back to normal conditions so that it may be used safely with the equipment, the high concentrated TDS water is drained regularly and this is what is referred to as blow-down. The water is then replaced with fresh water and this is referred to as make-up water.