Tag Archives: cooling towers

PERFORMANCE OF THE COOLING TOWER

NATURAL-COOLING-TOWER-PERFORMANCE

PERFORMANCE OF THE COOLING TOWER. ARE YOU FORTUNATE OR CHEATED?
Did you ever feel fortunate that your cooling tower’s performance was more than the designed value? Or felt cheated by your manufacturers the cooling was less than what it promised?
If you did, then you’ve been knowing the concept of its operation all wrong!
Sounds surprising? Yes! There’s always a misconception on this topic and here is the reason why your cooling tower does so.
The cooling tower uses the principle of evaporation and diffusion as the heat from the source is removed by the contact of the working fluid (water) with the air. At this point, some amount of water evaporates and it also takes away the latent heat from the rest of the fluid. To be exact, the temperature dips much lower than the Dry Bulb Temperature (temperature of air as measured by thermometer).
An easy calculation to determine the cooling rate would be a simple mathematics of temperature of hot water minus temperature of the cold water. But this isn’t the correct way to determine the cooling capability or the performance of any cooling tower. There’s an expression for the heat load:
*Heat Load [Btu/hr] = 500 x Water Flow Rate [GPM] x Range [oF]
(1 kW = 3412.142 btu/hr)
(1 GPM = 0.227 m3/hr)
Here heat load refers to the heat source or the system for which a cooling tower is assigned. So, the tower is neither a heat sink nor a heat source. So heat load is obviously independent of the cooling tower. The main point here is that even the water flow rate is independent of the tower as it is determined by the pressure drop and the number of pumps engaged for the supply. So, if all the parameters are independent, there would not be any effect on Range temperature, no matter what type of cooling tower we use.
Therefore, the thermal capability of a cooling tower can never be calculated on terms of the amount of heat rejected or evaporated.That is, when the heat rejected is less by any maintenance factor, you’ll be taking it as if you are cheated by the manufacturer and when due to more heat rejection, you’ll feel fortunate as the tower range of cooling will be more than the designed value! (By the formula, Performance = hot water temp. – cold water temp)
Then how is it calculated exactly?
The performance of the tower is best calculated by the level at which the heat is rejected i.e. the temperature of the cold water at the time of the circulation, just before it comes in contact with the heat load.
*[Source: Jalal Engineering, oshuja@cyber.net.pk]