Cooling Tower Calculator Formula
Understand the math behind the cooling tower calculator. Each variable explained with a worked example.
Formulas Used
Heat Rejected
heat_rejected = circ_flow * 4.18 * temp_rangeEvaporation Rate
evaporation = evap_rateBlowdown Rate
blowdown = evap_rate / (cycles - 1)Total Makeup Water
makeup = evap_rate + evap_rate / (cycles - 1)Variables
| Variable | Description | Default |
|---|---|---|
circ_flow | Circulation Flow Rate(L/s) | 50 |
temp_range | Cooling Range (inlet-outlet)(deg C) | 8 |
cycles | Cycles of Concentration | 4 |
circ_m3h | Derived value= circ_flow * 3.6 | calculated |
evap_rate | Derived value= circ_m3h * temp_range * 0.00085 | calculated |
How It Works
Cooling Tower Water Balance
Cooling towers reject heat through evaporation, requiring continuous makeup water.
Formulas
Heat rejected = m_dot cp delta_T
Evaporation rate = 0.00085 x Circulation (m^3/h) x Range (deg C)
The 0.00085 factor is an approximation for the fraction evaporated per degree of cooling. Blowdown removes concentrated water to limit mineral buildup: Blowdown = Evaporation / (Cycles - 1)
Makeup = Evaporation + Blowdown
Worked Example
50 L/s circulation, 8 deg C range, 4 cycles of concentration.
- 01Heat rejected = 50 x 4.18 x 8 = 1672 kW
- 02Circulation = 50 x 3.6 = 180 m^3/h
- 03Evaporation = 180 x 8 x 0.00085 = 1.224 m^3/h
- 04Blowdown = 1.224 / (4-1) = 0.408 m^3/h
- 05Makeup = 1.224 + 0.408 = 1.632 m^3/h
Frequently Asked Questions
What are cycles of concentration?
Cycles of concentration (CoC) is the ratio of dissolved solids in the circulating water to that in the makeup water. Higher CoC means less blowdown (saving water) but more risk of scaling and corrosion. Typical range: 3-7 cycles.
What determines the cooling range?
The range (inlet minus outlet temperature) depends on the heat load and water flow rate. A typical range for HVAC cooling towers is 5-10 deg C. Process cooling towers may have wider ranges.
What is approach temperature?
Approach is the difference between the cooling tower outlet water temperature and the ambient wet-bulb temperature. A typical approach is 3-5 deg C. Smaller approach requires a larger, more expensive tower.
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Open Cooling Tower Calculator