Cooling Tower Blowdown: Control & Optimisation

What is Blowdown?

As water evaporates in a cooling tower, dissolved solids (minerals) remain behind and concentrate. Without blowdown (also called bleed), the solids concentration would rise until scale forms on the fill and heat exchangers. Blowdown removes a portion of the concentrated water and replaces it with fresh make-up water, maintaining the solids concentration at a safe level.

The balance is controlled by 'cycles of concentration' — the ratio of dissolved solids in the tower water to the dissolved solids in the make-up water.

Cycles of Concentration

Higher cycles mean less blowdown, which saves water and treatment chemicals. But if cycles are too high for the water hardness, scale will form. The maximum safe cycles depends on the make-up water quality and the treatment chemicals used.

Calculating Blowdown Rate

The required blowdown is calculated from the evaporation rate and the desired cycles:

Example: A tower with an evaporation rate of 100 L/min and desired cycles of 4:

The total make-up water required is: Evaporation + Blowdown + Drift = 100 + 33 + 2 = 135 L/min.

Automatic Blowdown Control

Automatic blowdown control uses a conductivity sensor to measure the TDS in the tower water. When conductivity exceeds the setpoint (corresponding to the desired cycles), the controller opens the blowdown valve. When it drops, the valve closes. This maintains the cycles precisely at the target — saving water compared to continuous manual blowdown.

• Install the conductivity sensor in the tower basin or the return line — not in a stagnant area
• Set the conductivity setpoint based on the desired cycles and the make-up water TDS
• Clean the conductivity sensor monthly — scale deposits cause false high readings
• Verify the blowdown valve operates — check that it opens and closes fully when the controller commands it
• Calibrate the sensor quarterly using a standard solution

Optimising for Savings

• Increase cycles (safely) — every additional cycle reduces blowdown and saves water. Use scale inhibitors to safely increase cycles.
• Use side-stream filtration — removes suspended solids, allowing higher cycles and reducing biological fouling.
• Use acid treatment — lowering pH with acid allows much higher cycles (8–15), dramatically reducing blowdown. Requires careful control and safety equipment.
• Recover blowdown water — in some cases, blowdown can be treated and reused for non-potable applications.
• Monitor make-up water quality — if the source water quality changes, adjust the cycles accordingly.