The importance of water in the cooling tower industry – Water (part 9)
HOW TO MEASURE THE EFFICIENCY OF A COOLING TOWER
In the design process of a cooling tower, there is a need to have at least seven data values or rather 7 variables. If only one of these values is changed, the result will be a cooling tower with different dimensions.
Some of this data or design values are questionable, such as the temperature of the air at wet-bulb temperature, that seems to shift every year for speculation reasons.
To measure the efficiency of each tower, there is a need to know the exact design data and the theory that allows designing the tower, hence, knowing the values detected by lab effective testing.
All this will be treated in another chapter with interesting considerations.
Now we shall discuss how to evaluate the efficiency of the cooling tower in a simple and practical way.
We shall use the method that will allow, at least, to compare the efficiency of the tower at the any given time with the efficiency of the tower at the time it was installed: in other words, the degradation of efficiency (if it exists).
The main element for cooling water is air. The amount of air is essential for the amount of heat to dissipate, temperature, etc.
The amount of air measurement is synonymous of tower efficiency.
The degradation over time can decrease the amount of air in the tower because there are occlusions within the filling material that block the passage of air. Occlusions may originate from collapsed material, limestone, dust, algae, etc.
The air will always circulate.
The masses of water and air inside the tower are huge. The water weight is about 15,000 to 30,000 kg/m2 of the tower. Thus, the water will always fall via preferential routes: holes or by laminating on the tower walls. The amount of air passing through the tower is of vital importance.
A quick way to know the efficiency of the cooling tower is to measure the amount of air all you need to is to measure the speed of the air incoming to the tower or outgoing from the tower.
The speed in m/s and is measured with an anemometer (a very simple tool with fan). This tool is used to measure wind speed, etc. Hence, the speed through the cross-section (m2 top view) of the tower MUST be between 2.5 m/s and 4 m/s.
If the speed is lower than 2.5 m/s, the amount of air through the tower is very low and the cooling is compromised. The tower will be not efficient!
This method is basically a rule of thumb, but it serves as an alarm that a more accurate review is required.