It’s been a while since the last time I write an article about utility system. In this post I want to share you how to calculate cooling tower makeup water.
Cooling tower must be made up because the water level decrease by the time during operation. There are aspect that contribute to calculation of cooling tower makeup water. Those are:
- Evaporation loss
- Drift loss
- Blow down
Total cooling water makeup water requirement = evaporation loss + drift loss + blow down.
Let’s take a look one by one how to calculate each component.
Evaporation loss is loss of water due to evaporation. It is calculated by this equation:
Evaporation loss = 0.00085 Wc (T1-T2)
T1 – T2 = inlet water temperature minus outlet water temperature (oF)
0.00085 is evaporation constant (rule of thumb). The actual evaporation rate is a function of season and climate.
Drift is entrained water in the tower discharge vapor. Drift loss in cooling tower is a function of drift eliminator design.
Drift loss = 0.02% water supply [for new development in eliminator design]
Blowdown is a portion of circulating water that is discharged in order to lower solids concentration due to evaporative cooling. The requirement of blowdown is related with cycle of concentration (COC). COC is best described as ratio of chloride content in circulation water and in makeup water.
Blowdown = [ Evaporative loss – (COC – 1) x Drift loss] /(COC – 1)
Cycle of concentration is normally 3-4 cycles. When cycle of concentration below 3 cycles, quantity of water to be discharged will be large.
Let say we have cooling tower with the following data:
Inlet water flow rate = 10 000 gpm
Inlet water temperature = 100 oF
Outlet water temperature = 85 oF
Drift loss = 0.02%
Cycles of concentration = 5
Recall again, cooling tower makeup water = evaporation loss + drift loss + blowdown. Let’s calculate each component.
- Evaporation loss = 0.00085 x 10 000 x (100-85) = 127.5 gpm
- Drift loss = 0.02% x 10 000 = 2 gpm
- Blowdown = [127.5 – (5-1) x 2]/(5-1) = 29.87 gpm
- Cooling tower makeup water = 127.5 + 2 +29.87 = 159.37 gpm
Perry’s Chemical Engineers’ Handbook