Air cooling towers : what is a cooling tower ?
What is a cooling tower and how it works ?
How to design a cooling tower ?
How to calculate the performance of an air cooling tower for design and troubleshooting activities ?
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1. Cooling tower principle of operation
2. How to design a cooling tower ? Cooling tower calculations
Air cooling towers are one of the most widespread process equipment in industries but are also present in regular building for air conditioning. This page explains the key concepts to design a cooling tower, or to monitor and troubleshoot its performances.
1. Cooling tower principle of operation
A cooling tower uses ambient air to cool down a flow of water that is used elsewhere in a factory to cool down another process.
The water to cool down is injected at the top part of the cooling tower and is then contacted with air blown at the bottom of the tower and going up. The air is creating an evaporation of some part of the water which, in consequence is cooling it down. This phenomena is known as evaporative cooling. This leads to a loss of water from the system which must be topped up.
There are actually 2 types of cooling towers based on how the air and the water are contacted : crossflow cooling towers and counterflow cooling towers.
Figure 1 : crossflow cooling tower
(forced draft)
Figure 2 : counterflow cooling tower
(forced draft)
There is also 2 sub-types which are natural draft of forced draft cooling towers. Most of the cooling towers are forced draft, which means that a fan is used to move the air, natural draft requires very high structure in order to have a difference of pressure in between the bottom of the tower and the top, which allows to create the draft, it is thus mostly found in large power plants or factories.
2. How to design a cooling tower ? Cooling tower calculations
2.1 Cooling tower efficiency
A cooling tower efficiency can be calculated thanks to the following formula :
Eff(%) = (Ti-To)/(Ti-Twb)*100
Equation 1 : cooling tower efficiency
With :
Eff(%) = efficiency of the cooling tower
Ti = water inlet temperature (°c or °F)
To = water outlet temperature (°c or °F)
Twb = air web bulb temperature (°c or °F)
Some orders of magnitude :
- The difference of temperature in between the inlet and outlet temperature is usually around 5-8c
- The efficiency of a cooling tower is usually in the range 70-75%
2.2 Cooling tower mass balance
Some water losses must be taken into consideration when designing a cooling tower, indeed, water will be lost through evaporation and through "windage" which means that the air will carry out of the tower some droplets.
The following orders of magnitudes can be used to calculate the water losses :
- Evaporation losses : 0.00085*(water flow kg/h)*(Ti-To)
- Windage losses : 0.005*(water flow kg/h)