Cooling Tower Water Use Calculator
Enter tower flow, hot and cold water temperatures, drift, cycles of concentration, and operating hours. The calculator estimates current and proposed makeup water, blowdown, annual savings, and the conductivity setpoint for the proposed cycles.
Annual makeup water saved612,000
- Cooling range (°F)
- 10
- Estimated evaporation (gpm)
- 8.5
- Estimated drift (gpm)
- 0.05
- Current blowdown (gpm)
- 4.2
- Current makeup water (gpm)
- 12.75
- Proposed blowdown (gpm)
- 1.65
- Proposed makeup water (gpm)
- 10.2
- Proposed conductivity setpoint (µS/cm)
- 1,800
- Makeup water reduction
- 20%
- Zero-blowdown cycle limit
- 171
Current versus proposed cycles
| Case | Cycles | Blowdown gpm | Makeup gpm |
|---|---|---|---|
| Current | 3 | 4.2 | 12.75 |
| Proposed | 6 | 1.65 | 10.2 |
Uses the field estimate E = 0.00085 × circulation rate × cooling range.
Annual gallons use the entered equivalent full-load operating hours.
How to use this calculator
- Enter the recirculating tower flow and the hot-water and cold-water temperatures.
- Enter the drift-eliminator rating as a percent of circulating flow.
- Enter current and proposed cycles of concentration approved for the water treatment program.
- Enter makeup-water conductivity and annual equivalent full-load operating hours.
- Read the annual makeup water change, blowdown rates, and proposed conductivity setpoint.
How the cooling tower water balance is calculated
A cooling tower loses water three main ways: evaporation, drift, and intentional blowdown. Evaporation removes heat. Drift is liquid water carried out of the tower by the air stream. Blowdown is the controlled discharge used to keep dissolved solids from concentrating beyond the allowed cycles.
The calculator uses the common field estimate for evaporation:
E = 0.00085 × Q × (Th - Tc)
Q is circulating flow in gallons per minute, Th is hot-water temperature in degrees F, and Tc is cold-water temperature in degrees F. Drift is calculated from the drift rating:
D = Q × d ÷ 100
Cycles of concentration compare dissolved solids in tower water with dissolved solids in makeup water. For a target cycle value C, the allowed non-evaporative discharge is:
L = E ÷ (C - 1)
Because drift is already a non-evaporative discharge, intentional blowdown is the remaining part of that limit:
B = max(0, L - D)
Makeup water is the sum of evaporation, blowdown, and drift:
M = E + B + D
The same balance is run once at the current cycles and once at the proposed cycles. Annual makeup water saved is (Mcurrent - Mproposed) × 60 × hours. If proposed cycles are lower than current cycles, the result is negative because the change increases water use.
What changes the result most
Cooling range and circulation rate set the evaporation estimate, so they usually dominate total makeup water. Cycles affect the blowdown share: moving from 2 to 4 cycles saves more water than moving from 8 to 10 cycles. Drift matters most when the drift rate is high relative to evaporation, because drift can replace all intentional blowdown and make a high cycle target physically unreachable.
Conductivity setpoint and feasibility
The proposed conductivity setpoint is estimated as makeup conductivity × proposed cycles. Conductivity is only a proxy for dissolved solids, but it is the practical controller signal used by many tower systems.
The zero-blowdown cycle limit is C0 = 1 + E ÷ D. If the proposed cycles are above that limit, drift alone removes too much water for the tower to concentrate to that target. With zero drift, the limit is mathematically unbounded.
What this calculator leaves out
The estimate does not include leaks, basin overflow, splash-out, side-stream filter backwash, unmetered draw-off, seasonal load changes, or chemistry limits. The 0.00085 evaporation factor is useful for field planning, but measured makeup and blowdown meters are better for operating decisions.
Worked example
Suppose a tower circulates 1,000 gpm, enters the tower at 95°F, leaves at 85°F, has 0.005% drift, currently runs at 3 cycles, and is proposed to run at 6 cycles. Makeup conductivity is 300 µS/cm and the tower runs 4,000 equivalent full-load hours per year.
The cooling range is 10°F. Evaporation is 0.00085 × 1,000 × 10 = 8.5 gpm. Drift is 1,000 × 0.005 ÷ 100 = 0.05 gpm.
At 3 cycles, required non-evaporative discharge is 8.5 ÷ (3 - 1) = 4.25 gpm, so blowdown is 4.20 gpm and makeup is 12.75 gpm. At 6 cycles, blowdown is 1.65 gpm and makeup is 10.20 gpm. Annual makeup water saved is (12.75 - 10.20) × 60 × 4,000 = 612,000 gallons. The conductivity setpoint is 300 × 6 = 1,800 µS/cm.
Common questions
How do you calculate cooling tower makeup and blowdown water?
Estimate evaporation from circulating flow and cooling range, then calculate the non-evaporative discharge allowed by the cycle target. Drift counts as part of that discharge, so intentional blowdown is the allowed discharge minus drift, not below zero. Makeup water is evaporation plus drift plus blowdown.
What are cycles of concentration?
Cycles of concentration compare dissolved solids in tower water with dissolved solids in the incoming makeup water. If makeup water conductivity is 300 µS/cm and tower water is controlled near 1,500 µS/cm, the tower is operating near 5 cycles.
How is a cooling tower conductivity setpoint calculated?
A simple conductivity setpoint is makeup-water conductivity multiplied by the target cycles of concentration. The value is approximate because conductivity does not identify which ions are present, so the final setpoint should be checked against the treatment program and water analysis.
Does drift reduce the required blowdown rate?
Yes. Drift removes liquid water that contains the same dissolved solids as tower water, so it functions like uncontrolled blowdown in the mass balance. If drift is high enough, the calculated intentional blowdown can fall to zero.
Why can a proposed cycle target be impossible even with zero blowdown?
A tower can only concentrate water if evaporation removes water faster than non-evaporative losses carry dissolved solids away. When drift alone is larger than the discharge allowed by the target cycles, the tower cannot reach that target unless drift is reduced.