Evaluating Performance Characteristics of Cooling Towers

In this post I want to share with you how to evaluate performance characteristics of cooling towers. The performance characteristics of various types of towers will vary depending on height, fill configuration, and flow arrangement. Please be noted that this post is for basic knowledge only. Accurate characteristics of specific tower performance should be consulted with cooling tower manufacturer.

This post will cover the following performance characteristics of cooling tower:

• Effect of varying wet bulb temperature on cold water temperature
• Effect of varying cooling range on cold water temperature
• Effect of varying water circulating rate and heat load on cold water temperature
• Effect of varying wet bulb temperature, range, and water circulating rate on cold water temperature
• Effect of varying fan HP (horsepower) input on cold water temperature

The performance of cooling tower will be evaluated using the following nomograph.

Let’s assume that the initial/basis of operation of cooling tower is as follows:

• Flow = 1000 gpm
• Hot water temperature = 105^oF
• Cold water temperature = 85^oF
• Cooling range = 105 – 85 = 20^oF
• Wet bulb temperature = 75^oF

Effect of Varying Wet Bulb Temperature on Cold Water Temperature

What is new cold water temperature when wet bulb changes from 75^oF to 60^oF with flow and range remaining constant?

Answer

Enter nomograph at 85^oF cold water temperature and 75^oF wet bulb temperature (green line), then vertically down to 60^oF wet bulb temperature (orange line). The new cold water temperature is 76^oF.

Effect of Varying Cooling Range on Cold Water Temperature

What is new cold water temperature when cooling range is changed from 20^o to 30^o, or 50% increase in heat load, with gpm and wet bulb temperature held constant.

Answer

Enter nomograph at 85^oF cold water temperature, go horizontally to 75^oF wet bulb temperature, vertically to 20^oF temperature range, horizontally to 30^oF temperature range, vertically downward to 75^oF wet bulb temperature. The new cold water temperature is 87.5^oF.

Effect of Varying Water Circulating Rate and Heat Load on Cold Water Temperature

What is new cold water temperature when water circulation is changed from 1000 gpm to 1500 gpm (50% change in heat load at constant range).

Answer

Please be noted that varying water rate over wide ranges may require modification to the distribution system.

Enter nomograph at 85^oF cold water temperature, go horizontally to 75^oF wet bulb temperature, vertically to 20^oF temperature range, horizontally to performance factor of 3.1. The new performance factor at new flowrate is obtained:

3.1 x (1500 gpm/1000 gpm) = 4.65

Enter nomograph at performance factor 4.65, go horizontally to 20^oF temperature range, and vertically down to 75^oF wet bulb temperature. The new cold water temperature is 91^oF.

Effect of Varying Wet Bulb Temperature, Range, and Water Circulating Rate on Cold Water Temperature

What is new cold water temperature when wet bulb changes from 75^oF to 60^oF, temperature range changes from 20^oF to 25^oF, gpm changes from 1000 gpm to 1250 gpm.

Answer

Enter nomograph at 85^oF cold water temperature, go horizontally to 75^oF wet bulb temperature, vertically to 20^oF temperature range, horizontally ready performance factor 3.1. Then, new performance factor at 1250 gpm is obtained:

3.1 x (1250 gpm/1000 gpm) = 3.87

Enter nomograph at performance factor 3.87, go horizontally to 25^oF temperature range, and vertically down to 60^oF wet bulb temperature. The new cold water temperature is 82^oF.

I hope you find this post useful.

References:

• GPSA section 11 Cooling Towers