Cooling Tower Tonnage Calculator — Free Online Calculator
A cooling tower rejects both the evaporator load from the chiller and the heat of compression added by the compressor — typically 1.25× the chiller's cooling capacity. This calculator uses the simple 500-GPM formula to find the tower's heat rejection capacity in BTU/h and cooling tons, calculates the approach temperature to wet-bulb, and flags two critical design errors: an approach below 5°F and a tower capacity smaller than the total chiller reject load.
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Heat Rejection
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Approach
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See the breakdown
Heat rejection formula: Q = 500 × GPM × ΔT. Approach = leaving cold water − ambient wet-bulb. Tower selection requires approach ≥ 5°F at peak wet-bulb conditions.
Cooling tower design benchmarks
| Metric | Conservative | Target | Minimum |
|---|---|---|---|
| Approach temperature | 8–12°F | 5–8°F | 5°F absolute minimum |
| Condenser ΔT (range) | 8–12°F | 10°F | 5°F |
| GPM per ton rejected | 2.5–3.5 | 3.0 | 2.0 |
| Tower tons to chiller tons ratio | 1.35:1 | 1.25:1 | 1.15:1 |
The cooling tower heat rejection formula
The 500-GPM formula is derived from the specific heat of water (1 BTU/lb/°F) and the density of water (8.34 lb/gal). For water at standard temperature, the heat rejection rate equals 500 times the flow rate in GPM times the temperature range across the tower. The tower must reject not just the building's cooling load but also the heat of compression added by the chiller compressor — making the tower's required capacity roughly 1.2–1.3 times the chiller's cooling tonnage.
Why 500 × GPM?
The factor 500 comes from 8.34 lb/gal (water density) × 60 min/hr. This gives BTU/h when multiplied by GPM and temperature difference. It is exact for water at ~60°F and accurate to within 1% across the typical condenser water temperature range of 75–105°F.
Approach vs. range
Range is the temperature difference across the tower (hot entering minus cold leaving). Approach is the difference between cold leaving water and ambient wet-bulb. Range is set by the heat load and flow rate; approach is set by the tower's size (fill area, fan energy). A tighter approach requires a larger, more expensive tower.
Heat of compression
A chiller compressor adds work (heat) to the refrigerant circuit, and all of that work ultimately becomes heat that the condenser and cooling tower must reject. For a COP-5 chiller, the compressor heat equals 20% of the cooling load, so the tower must reject 120% of the chiller's rated tons.
Wet-bulb and tower capacity
Cooling tower capacity is a function of wet-bulb temperature, not dry-bulb. On a humid day (high wet-bulb), the driving force for evaporation decreases, reducing the tower's capacity. Towers should always be selected at the design wet-bulb temperature for the site (typically the 1% or 0.4% ASHRAE wet-bulb design condition).
Worked examples
Three scenarios showing an adequate design, a tight-approach violation, and a severely undersized tower.
200-ton chiller plant — 600 GPM, T_hot=95°F, T_cold=85°F, WBT=78°F, COP=5.0
Result: Tower capacity (250 tons) > required (240 tons) — adequate design.
Tight approach design — 400 GPM, T_hot=90°F, T_cold=83°F, WBT=80°F, COP=4.5
Result: Tower is thermodynamically unrealisable at this wet-bulb. Increase cold-water setpoint or select a larger tower.
Undersized tower — 200 GPM, T_hot=95°F, T_cold=85°F, WBT=75°F, chiller 200 tons, COP=5.5
Result: Tower (83 tons) < required (237 tons) — severe under-capacity, compressor high-pressure trip.
Frequently asked questions
Common questions about cooling tower sizing and condenser water system design.
What does a cooling tower do in an HVAC system?
A cooling tower removes heat from the condenser water loop of a water-cooled chiller. The chiller transfers building heat to condenser water, which flows to the tower where evaporative cooling rejects the heat to the atmosphere. Because the chiller compressor also adds heat, the tower must reject roughly 1.2–1.3 times the building's cooling load.
What is the 500 × GPM × ΔT formula?
This formula calculates the rate of heat rejection from a cooling tower in BTU/h: Q = 500 × GPM × ΔT. The factor 500 equals 8.34 lb/gal (water density) × 60 min/hr. Multiplied by flow (GPM) and temperature range (°F), it gives the heat rejection rate in BTU/h. It assumes water at standard density — accurate to within 1% across typical condenser water ranges.
What is the approach temperature of a cooling tower?
Approach is the difference between the cold water leaving the tower and the ambient wet-bulb temperature: Approach = T_cold − WBT. It measures how close the tower gets to the thermodynamic limit. A minimum approach of 5°F is the practical lower bound; closer approaches require exponentially larger tower fill areas and are rarely cost-effective.
Why is a minimum 5°F approach required?
Cooling towers work by evaporating water — the wet-bulb temperature is the thermodynamic floor for leaving cold water temperature. As approach decreases toward zero, the required tower size increases exponentially. In practice, 5°F is the economic and mechanical minimum; designing below this results in an impractically large tower with extreme fan energy.
How do I account for chiller compressor heat?
The compressor adds heat equal to its work input: Q_compression = chiller cooling load ÷ COP. For a COP-5 chiller, this adds 20% to the reject load. The tower's total required capacity is: Q_required = Q_evaporator × (1 + 1/COP). A common rule of thumb is 1.25 tower tons per chiller ton for a COP-4 machine.
What causes a chiller high-pressure trip?
If the cooling tower cannot reject enough heat, condenser water temperature rises above the chiller's high-pressure setpoint, triggering a compressor trip on high discharge pressure. Common causes: undersized tower, high ambient wet-bulb, fouled tower fill, low water flow, or strainer blockage. This calculator's under-capacity warning flags this condition at design.
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