Variable affecting performance of CT Heat transfer – Cooling Tower
The rate at which heat is transferred in a cooling tower depends upon four factors:
- the area of the water surface in contact with air;
- the relative velocity of air and water;
- the time of contact between air and water;
- the difference between the wet bulb temperature of the inlet air, A, and the temperature of the returned water, R.
Item (1) depends upon the construction of the fill; (2) can be controlled by regulating speed of the fans; (3) is a function of (2) and the height of the tower; (4) is fixed by climate.
Wet bulb temperature can be measured with a sling psychrometer.
Under ideal conditions, when a stream of unsaturated air passes over a wetted surface water evaporates saturating the air and lowering the temperature of the remaining water. When the water becomes cooler than the air, sensible heat flows from the air to the water, eventually reaching equilibrium at the wet-bulb temperature, where the loss of heat from the water by evaporation is equal to the sensible heat. Thus, as water falls through a cooling tower, the latent heat of vaporization and the sensible heat approach each other so that in an infinitely high structure the temperature of the bulk water would be equal to the wet-bulb temperature of the entering air. In a finite tower, however, it is impossible to achieve zero approach (approach = Supply temperature – wet bulb temperature) because not all the water falling through the structure can contact fresh cool air.
One measure of the efficiency of a cooling tower is its approach, which is the difference between the temperature of the cooled water in the basin of the tower and wet-bulb temperature of the atmosphere.
The second measure of performance is the cooling range, which is the difference between the supply temperature and return temperature.
The amount o heat rejected by a cooling tower can be calculated from the cooling range and the recirculation rate. This is also known as heat duty.
1 BTU is the amount of heat required to raise the temperature of one pound of water one degree F. Therefore:
Heat duty, BTU/hr = gpm (circulation rate) >< 8.34 lb/gal >< Delta T deg F
Another important characteristic of a cooling towers performance is L/G, the liquid-gas mass transfer ratio: L/G = (water, kg/hr)/(air, kg/hr)