Importance of Wet Bulb Temperature(WBT) in Cooling Tower

Importance of Wet Bulb Temperature(WBT) in Cooling Tower

Wet bulb temperature(WBT) - Temperature measured by a wet bulb thermometer. If the bulb of a thermometer is wetted by muslin and the evaporation of moisture by a moving air with minimum velocity 5 mtr/sec, then the temperature recorded by the thermometer by the thermometer is WBT.

A wet bulb temperature measurements taken within 1.5 m of the air inlets of cooling tower and between 1.5 m and 2.0 m above the basin elevation of the cooling tower.

Approach - Difference between cooling water return temperature and the inlet wet bulb temperature of air. The minimum reasonable possible approach temperature is 2 Degree C, however 4 Degree C to 6 Degree C is more used by the industries.

The wet bulb temperature of the entering air has a very important for the performance of cooling tower. A lower wet bulb temperature produces a lower cooling water temperature.

For a given air wet bulb temperature, the cooling water return temperature has a significant effect on size of cooling tower. To cool water nearer to the wet bulb temperature of the air would require an extremely large size cooling tower. If we try to design cooling tower nearer to the air wet bulb temperature the tower size increases very speedily.

How does water cool in a cooling tower?

A cooling tower uses the advantage of the difference between dry and wet bulb temperature of air to cool water in a cooling tower. 

Water changes its state from liquid to vapor and heat transfer take place which is known as the latent heat of evaporation. this input energy must either be supplied from some heat transfer medium or be extracted from the surroundings. In cooling towers, hot water evaporates in the presence of moving air and due to that heat is extracted from the water and transferred to the moving air. This process is called evaporative cooling. Water droplet in cooling tower, surrounded by a thin film of air which is saturated and remains almost no any change by the passing air stream. There are three ways of heat transfer occurs

-          By evaporation, the majority of heat transfer takes place in the cooling tower around 70% to 75% due to evaporative cooling.

-          By conduction and convection, heat transfer takes place in the cooling tower due to conduction and convection may be as much as 25% to 30%.

-          By radiation, heat transfer due to radiation takes place is a very small proportion and it is usually neglected. 

Water evaporates in cooling tower because unsaturated air and moisture cannot coexist. Air forces moisture to reach equilibrium by making the water to evaporate (lose moisture ) by using its own internal energy adiabatically  Every particle of moisture that migrates to air carries the latent heat kj/kg which cools the water.
The other point that helps is the low specific heat or stored energy of air which is about four and a half times smaller than water. This makes heat always flow from water to air (high to low energy) and water to cool.
Because an approach temperature exists in a cooling tower the air in a cooling tower never reaches saturation or equilibrium which means that DBT can never reach WBT. The difference between DBT and WBT keeps pushing the cooling

" Approach temperature" is a loss of efficiency of a cooling tower it is also true that without " Approach temperature" there would be no driving force to cool water in a cooling tower

The diving force for cooling is (DBT - WBT) of air at air-water interface. Unsaturated air in the presence of water wants to reach an equilibrium. For that air requires moisture. To get the moisture air forces the water to release moisture

For this process, heat is required. Air itself has no such amount of heat with very small specific heat and no latent heat that can evaporate water. In this unstable peculiar situation, water is forced to release moisture to supply to air by cooling adiabatically using its internal energy

Since (DBT - WBT) is never zero because the approach temperature is never zero in a cooling tower, the cooling continues. (DBT - WBT) keeps pushing the cooling

Thermodynamics
Water sideH
Water = U water + W water
Air sideH Air = U air + W air,
H is enthalpy
Both W water and W air are = 0
W is work

(H water - H air) = (U water - U air)

The difference between the internal energy of water (U water) and the internal energy of air ( U air) drives a cooling tower

Heat and mass transfer
The moisture released by water carries 2256 kJ/kg of heat into the air.