Cooling amplitude definition
The cooling amplitude refers to the difference between the cooling tower outlet water temperature and the inlet air wet bulb temperature. A smaller cooling amplitude usually means a higher performance cooling tower.

Efficiency considerations
When evaluating cooling towers, its efficiency is also an important indicator. An efficient cooling tower can more effectively reduce the water temperature, thereby ensuring stable operation of the system.

Cooling tower capacity
The capacity of a cooling tower is usually measured in “kilocalories per hour” or “tons of cooling”. The calculation formula is: Cooling tower capacity = cooling water mass flow × water specific heat capacity × temperature difference. Generally speaking, a cooling tower with a larger capacity has a higher performance.

Make-up water calculation
The make-up water volume is an important factor to consider in the operation of a cooling tower. It includes evaporation loss, splash loss, and periodic discharge loss. The amount of water lost by evaporation can be calculated by the formula E = Q/600 = (T1-T2)×L/600, where E represents the amount of water evaporated (kg/h), Q represents the heat load (Kcal/h), 600 represents the latent heat of evaporation of water (Kcal/h), T1 and T2 represent the inlet and outlet water temperatures (℃), and L represents the circulating water volume (kg/h). The amount of water lost by splashing is usually about 0.1~0.2% of the circulating water volume, while the loss of water by regular discharge depends on factors such as water quality or solid concentration in the water, and is generally about 0.3% of the circulating water volume. Therefore, the total amount of make-up water M = E + C + D.

Cooling water flow
When the cooling tower is used for air conditioning, the temperature difference is usually designed to be 5℃. At this time, in order to maintain the stable operation of the system, the amount of make-up water required by the cooling tower is about 2% of the circulating water volume.
K·Q=C·M·ΔT
K: Estimation coefficient, usually 1.0
Q: Maximum cooling capacity of the unit, such as 640RT (refrigeration tons)
C: Specific heat capacity of water, usually 1.0 Kcal/kg·℃
ΔT: Supply and return water temperature difference, usually designed to be 5℃
M: Cooling water mass flow

For compression refrigeration units, the estimated coefficient K of its maximum cooling capacity is usually 1.3 times; while for absorption refrigeration units (such as lithium bromide units), the estimated coefficient K of its cooling capacity is 2.5 times.

Next, let’s take a specific project as an example. Assuming that a 640RT chiller is used, we can calculate the required cooling water flow and make-up water according to the above formula.
Make-up water m=M·2%=140kg/s·2%=2.8kg/s