Cooling Tower Heat Rejection Calculator

Calculate heat rejection (BTU/hr & tower tons), approach temperature, range, evaporation loss, and makeup water requirement for industrial process cooling towers. Imperial units.

Free Tool · BTU/hr · Tower Tons · Approach Temp · Makeup Water GPM

Cooling Tower Parameters

Calculation Mode

Process cooling: enter actual hot/cold water temps from your heat exchanger or process cooler.

Water Temperatures

Hot Water In (T₁)

°F

Cold Water Out (T₂)

°F

T₁ = water entering tower (hot from process). T₂ = water leaving tower (cold to process). Typical range: 10–15°F for process, 10°F standard for HVAC.

Circulating Water Flow Rate

GPM gal/min

Total condenser water or process cooling water flow through the tower.

Design Wet-Bulb Temperature

WB °F

Design wet-bulb for your region. Common US values: Gulf Coast 82°F, Southeast 78°F, Midwest 75°F, Southwest 70°F, Pacific NW 65°F. Use 99.6% ASHRAE value for your city.

Water Quality

Cycles of Concentration (COC)

COC

Higher COC = less blowdown water wasted = better water efficiency. Typical industrial target: 4–6 COC.

Circulating Fluid

💧

Glycol reduces heat capacity — the correction factor adjusts BTU/hr accordingly.

Results

Heat Rejection (Q)

—

BTU/hr

Tower Tons

—

@ 15,000 BTU/hr/ton

Refrigeration Tons

—

@ 12,000 BTU/hr/ton

Cooling Range

—

°F (T₁ − T₂)

Approach Temp

—

°F above wet-bulb

Evaporation Loss

—

GPM (≈1% per 10°F range)

Blowdown

—

GPM at selected COC

Makeup Water

—

GPM total required

Ready Enter cooling tower parameters and click Calculate.

Detailed Summary

Heat Rejection
Heat duty Q	—
Tower tons (Q ÷ 15,000)	—
Refrigeration tons (Q ÷ 12,000)	—
Fluid correction factor	—
Temperature Analysis
Hot water in (T₁)	—
Cold water out (T₂)	—
Cooling range (T₁ − T₂)	—
Design wet-bulb (WB)	—
Approach (T₂ − WB)	—
Approach assessment	—
Water Balance
Circulating flow	—
Evaporation loss	—
Blowdown (COC)	—
Drift loss (est.)	—
Total makeup water	—
Annual makeup (gal/yr)	—
Inputs
Mode	—
Flow / fluid	—
Cycles of concentration	—

Live Cooling Tower Diagram — Updates With Your Inputs

Hot water in (from process)

Cold water out (to process)

Makeup water (replaces evaporation + blowdown)

Air intake (evaporative cooling)

The tower rejects heat by evaporating a small fraction of the circulating water (~1% per 10°F range). The approach temperature — how close T₂ gets to wet-bulb — determines tower efficiency. A smaller approach requires a larger, more expensive tower.

How Cooling Tower Heat Rejection Works

A cooling tower rejects heat by evaporating a small percentage of the circulating water into the airstream. For every gallon that evaporates, it carries away approximately 1,000 BTU of heat — far more efficient than sensible cooling alone. This is why cooling towers can cool water to within 5–10°F of the wet-bulb temperature, a thermodynamic limit that no dry cooler or air-cooled heat exchanger can match.

1 Heat Rejection (BTU/hr)

The fundamental equation uses GPM flow, temperature range, and the 500 constant (8.33 lb/gal × 60 min/hr × Cp=1.0). For glycol mixtures, multiply by the fluid correction factor.

Q = GPM × 500 × Range × CF GPM = circulating water flow 500 = fluid constant (water) Range = T₁ − T₂ (°F) CF = fluid correction factor (1.0 for water) Example: 500 GPM, 95→85°F Q = 500 × 500 × 10 × 1.0 Q = 2,500,000 BTU/hr = 167 tower tons = 208 refrigeration tons

2 Tower Tons vs. Refrig. Tons

A refrigeration ton = 12,000 BTU/hr (heat removed from chilled space). A tower ton = 15,000 BTU/hr — 25% higher to account for the chiller compressor's heat of compression that the tower must also reject.

Tower Tons = Q ÷ 15,000 Refrigeration Tons = Q ÷ 12,000 For a chiller system: Tower Tons ≈ Chiller Tons × 1.25 Why 25% more? Chiller compressor adds heat equal to ~25% of cooling capacity. Tower must reject BOTH the process heat AND compressor heat.

3 Approach Temperature

Approach = T₂ − Wet Bulb. This is the most critical tower design parameter. A 5°F approach requires a much larger (more expensive) tower than a 10°F approach. Never design for less than 5°F approach in most US climates.

Approach = T₂ − WB (°F) Assessment guide: ≤ 5°F → Very aggressive design, large tower, high cost 5–8°F → Efficient, good design 8–12°F → Conservative, economical 12+°F → Over-approach, may indicate undersized or fouled tower Cold water temp (T₂) can NEVER reach wet-bulb — thermodynamic limit.

4 Water Balance

Makeup water replaces three losses: evaporation (~1% per 10°F range), blowdown (to control mineral concentration), and drift (mist carry-off, ~0.001–0.002% of flow with modern drift eliminators).

Evaporation (GPM) ≈ GPM × Range × 0.001 Blowdown (GPM) = Evaporation ÷ (COC − 1) Drift (GPM) ≈ GPM × 0.0002 (modern eliminators) Makeup = Evap + Blowdown + Drift Higher COC = less blowdown = less makeup water consumed

Tower Ton vs. Refrigeration Ton — The Most Common Sizing Mistake

The single most common cooling tower sizing error in industrial plants is using refrigeration tons (12,000 BTU/hr) instead of tower tons (15,000 BTU/hr) when selecting a cooling tower for a chiller application. If you size a tower for 100 refrigeration tons but the chiller produces 100 tons of cooling plus 25 tons of compressor heat, your tower is undersized by 25% before it starts running. Always size the cooling tower at 125% of the chiller's nameplate refrigeration tons, or use tower tons directly from the calculator above.

Worked Examples — 3 Real Industrial Scenarios

🔵 CNC Machining Facility

Mode: Process cooling Flow: 200 GPM Hot in: 95°F (from HX) Cold out: 85°F Wet bulb: 76°F COC: 4

Q: 1,000,000 BTU/hr
Tower: 67 tons · Approach: 9°F
Makeup: ~3.5 GPM

9°F approach is economical — standard counterflow tower selection. Makeup of 3.5 GPM at 4 COC means annual water use ~1.8M gallons. Drift eliminators critical near CNC equipment.

🟠 400-Ton Chiller Plant

Mode: Chiller condenser Chiller: 400 refrig. tons Flow: 1,200 GPM Hot in: 95°F (condenser out) Cold out: 85°F Wet bulb: 78°F

Q: 6,000,000 BTU/hr
Tower: 400 nominal tons
Approach: 7°F

400-ton chiller → select 400 nominal tower ton (= 500 refrig. tons × 1.25). 7°F approach at 78°F WB requires standard-size counterflow tower. Verify with CTI-rated performance at design WB.

🔴 Injection Molding Plant

Mode: Process cooling Flow: 800 GPM Hot in: 100°F (mold return) Cold out: 85°F Wet bulb: 80°F (Gulf Coast) COC: 3 (poor water)

Q: 6,000,000 BTU/hr
Tower: 400 tons · Approach: 5°F
Makeup: ~35 GPM

5°F approach at 80°F WB (Gulf Coast) is aggressive — requires large tower with good fill. At only 3 COC, blowdown is high: 35 GPM makeup vs 15 GPM at 6 COC. Invest in water treatment to reduce municipal water cost.

Approach Temperature Guide by Application

Application	Typical Range °F	Design Approach °F	Tower Selection Note
HVAC / Chiller (commercial)	10°F	7–10°F	Standard counterflow, 1.0–1.2 GPM/ton
Industrial process cooling	10–15°F	7–12°F	Counterflow or crossflow, depends on space
Injection molding	12–18°F	8–12°F	Higher flow, moderate approach acceptable
Hydraulic oil cooling	15–25°F	10–15°F	Larger range, smaller tower possible
Data center cooling	8–10°F	5–7°F	Aggressive approach — premium tower required
Power plant condenser	12–20°F	8–15°F	Very large towers, natural draft at >45,000 GPM

Frequently Asked Questions

Use the ASHRAE 99.6% design wet-bulb temperature for your city from ASHRAE Fundamentals Handbook or ASHRAE Weather Data Viewer. Common U.S. values: Miami 81°F, Houston 80°F, Atlanta 77°F, Chicago 75°F, Dallas 78°F, Phoenix 71°F, Los Angeles 68°F, Seattle 64°F. For critical process cooling, use 99.6% (exceeded only 0.4% of hours per year). For less critical applications, 99% is acceptable and results in a smaller, less expensive tower.

A refrigeration ton = 12,000 BTU/hr, representing heat removed from the chilled space. A tower ton = 15,000 BTU/hr, which is 25% larger to account for the heat added by the chiller compressor. When selecting a cooling tower for a chiller, always use tower tons. Sizing a tower in refrigeration tons will result in a 25% undersized tower — the most common cooling tower selection error in industrial plants.

As water evaporates in the cooling tower, minerals stay behind and concentrate in the basin. COC measures how concentrated the recirculating water is relative to makeup water. At 2 COC, you blowdown 100% of evaporation — extremely wasteful. At 6 COC, blowdown is only 20% of evaporation. Increasing from 3 to 6 COC can cut makeup water consumption by 30–40% — significant savings in water and sewer costs. However, higher COC requires better water treatment to prevent scale, corrosion, and biological growth.

Yes — select the appropriate fluid type to apply the correction factor. Glycol reduces the heat capacity of the fluid (lower Cp and higher SG partially offset each other), so a glycol loop transfers less BTU/hr per GPM than pure water at the same flow rate and temperature range. For 50% ethylene glycol, the correction factor is approximately 0.91, meaning you need about 10% more flow or range to achieve the same heat rejection as water. Note that glycol should NOT be used directly in an open cooling tower — it must pass through a heat exchanger to separate the closed glycol loop from the open tower water.