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**

Evaporation loss is loss of water due to evaporation. It is calculated by this equation:

**Evaporation loss = 0.00085 Wc (T _{1}-T_{2})**

Where:

T_{1} – T_{2} = inlet water temperature minus outlet water temperature (** ^{o}F**)

**0.00085** is evaporation constant (rule of thumb). The actual evaporation rate is a function of season and climate.

**Drift Loss**

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**

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.

**EXAMPLE**

Let say we have cooling tower with the following data:

Inlet water flow rate = 10 000 gpm

Inlet water temperature = 100 ^{o}F

Outlet water temperature = 85 ^{o}F

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**

**Reference:**

**Perry’s Chemical Engineers’ Handbook**