Cooling Tower

Cooling Water System

How cooling tower works ?• Heat and Mass transfer• 1 kg of water on evaporation removes approximately 530 kcals of heat• The heat given up by the water falling inside the tower

equals the heat gained by the air rising through the tower

• The hot water entering the tower is distributed within the structure in a manner that exposes a very large water surface to the air passing through.

• Water distribution is accomplished by means of spray nozzles or distribution pans and by means of various types of “fill.”

• This fill increases the exposed water surface to maximize contact with the air, increasing the rate of heat transfer.

Cooling Tower Types

• Cooling towers fall into two main categories:  Natural draft and Mechanical draft. 

• Mechanical draft towers are available in the following airflow arrangements:

1. Counter flows induced draft.2. Counter flow forced draft. 3. Cross flow induced draft.

Cooling Tower Types

Cooling Tower Performance

Factors Affecting Cooling Tower Performance

Tower Size vs Approach

Thermal Performance• Thermal performance of an evaporative tower is primarily

dependent upon the entering air wet bulb temperature (WBT) and relative humidity (RH).

• The entering WBT is an independent variable that dictates cooling tower selection.

• It is both the theoretical limit to the leaving cold water temperature and the only air parameter involved in cooling tower selection.

• The difference between the WBT and the tower leaving or cold water temperature (CWT) is called the approach temperature or the approach.

• Approach temperatures generally fall between 5 and 20 F.• The difference between the tower leaving or hot water

temperature (HWT) and the CWT is called the range• Range, heat load, and WBT also affect tower size

Water losses in cooling tower

Water losses include evaporation, drift (water entrained in discharge vapor), and blow down (water released to discard solids). 

Drift losses are estimated to be between 0.1 and 0.2% of water supply.

Evaporation Loss = 0.00085 x water flow rate x (T1-T2)  

                               Blow down Loss = Evaporation Loss/(cycles-1)                                              where cycles is the ratio of solids in the circulating water to the solids in the make-up water

Total Losses = Drift Losses + Evaporation Losses + Blow down Losses     

Energy Saving Opportunities in Cooling Towers Follow manufacturer’s recommended clearances around cooling towers and

relocate or modify structures that interfere with the air intake or exhaust.

Optimise cooling tower fan blade angle on a seasonal and/or load basis.

Correct excessive and/or uneven fan blade tip clearance and poor fan balance.

On old counter-flow cooling towers, replace old spray type nozzles with new square spray ABS practically non-clogging nozzles.

Replace splash bars with self-extinguishing PVC cellular film fill.

Install new nozzles to obtain a more uniform water pattern

Periodically clean plugged cooling tower distribution nozzles.

Balance flow to cooling tower hot water basins.

Cover hot water basins to minimise algae growth that contributes to fouling.

Optimise blow down flow rate, as per COC limit.

Replace slat type drift eliminators with low pressure drop, self extinguishing, PVC cellular units.

Restrict flows through large loads to design values.

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