Cooling Tower Sizing: Step-by-Step Calculation With a Worked Example

Cooling tower sizing starts with four numbers: water flow, hot water temperature, cold water temperature, and the local wet-bulb temperature. For a 30 m³/h tower cooling water from 42 °C to 32 °C at a 28 °C wet bulb, the heat load is about 348 kW. That calls for roughly 38,500 m³/h of air, a plan area of 2 to 3 m², and an estimated fan shaft power of about 2.7 kW.

Below is the full calculation, one step at a time. It is a preliminary method, good for checking a quote or planning a project. If you are still deciding which cooling system to use, start with our cooling systems comparison.

What Is a Cooling Tower and How Does It Work?

Hot water is sprayed over a fill (also called packing) while air moves through it. A little water evaporates and carries the heat away, and the rest drops into the basin colder than it came in.

In an induced draft tower, the fan sits on top and pulls air up through the fill. Drift eliminators catch droplets before the air leaves.

Design Inputs for the Example

We will size an induced draft tower for the conditions below. The wet-bulb temperature matters most, because it limits how cold the water can get.

Parameter	Value
Water flow	30 m³/h
Hot water in	42 °C
Cold water out	32 °C
Wet bulb	28 °C
Range	10 °C
Approach	4 °C

Step 1: Calculate the Heat Load

Cooling tower sizing begins with the heat load, the energy the tower must remove from the water. Use the following relationship:

• Mass flow: 1,000 kg/m³ × 30 m³/h = 30,000 kg/h
• Specific heat of water: 4.18 kJ/(kg·°C)
• Heat load: 30,000 × 4.18 × (42 − 32) = 1,254,000 kJ/h
• Divide by 3,600 to get kW: about 348 kW

Step 2: Check Range and Approach

Range is the temperature drop across the tower. Approach is the difference between cold-water temperature and entering-air wet-bulb temperature. A smaller approach means more fill area and a pricier tower. Manufacturers rarely guarantee an approach below about 3 °C (5 °F), according to Marley’s cooling tower basics guide, so 4 °C is reasonable but tight.

Step 3: Estimate the Air Flow

For a preliminary estimate, we can assume an air-to-water mass ratio. This is a simplifying assumption, not a universal cooling-tower design rule. Final airflow must be checked using the entering and leaving air conditions, a psychrometric heat-and-mass balance, and the manufacturer’s performance data. For this kind of duty, a ratio between 1.0 and 2.0 kg of air per kg of water is common. We adopt 1.5.

• Air mass flow: 1.5 × 30,000 = 45,000 kg/h
• Air density at 28 °C and 1 atm: about 1.17 kg/m³
• Air volume: 45,000 ÷ 1.17 = about 38,500 m³/h

Quick sanity check: each kilogram of air must absorb about 28 kJ (1,254,000 ÷ 45,000). That is realistic for these conditions.

Step 4: Find the Plan Area

The simplest check is water loading, which is the water flow per square meter of fill plan area. Typical values depend on fill type and approach.

Parameter	Typical range
Air-to-water ratio	1.0-2.0
Water loading	10-15 m³/h·m²
Fan pressure	100-250 Pa
Fan efficiency	0.5-0.7

At 10 to 15 m³/h per m², a 30 m³/h flow needs between 2 m² (30 ÷ 15) and 3 m² (30 ÷ 10). Because the 4 °C approach is tight, lean toward the larger area.

Step 5: Estimate the Fan Power

Estimate fan shaft power from the airflow, total fan pressure, and fan efficiency. With airflow in m³/h and pressure in Pa, use:

We adopt 150 Pa and a fan efficiency of 0.60.

• P = 38,500 × 150 ÷ (3,600 × 0.60)
• P = about 2.7 kW
• Across the 100 to 250 Pa range: roughly 1.8 to 4.5 kW

The 2.7 kW result is estimated fan shaft power, not electrical input. Select the motor only after accounting for fan-drive and motor efficiencies, service factor, and the manufacturer’s rated duty; a nominal 3 kW motor should not be assumed adequate from this estimate alone.

Step 6: Estimate the Pump Head and Power

Calculate the pump’s total dynamic head from the actual system: static lift, required pressure at the water-distribution nozzles, and friction losses in pipes, fittings, valves, and equipment. For this example, assume 8 m of total dynamic head and a pump efficiency of 0.65. Use Q in m³/s in the power equation. These are illustrative assumptions; verify them against the actual piping layout and nozzle requirements.

• P = ρ × g × Q × H ÷ η
• P = 1,000 × 9.81 × (30 ÷ 3,600) × 8 ÷ 0.65
• P = about 1.0 kW of pump shaft power

Step 7: Calculate Water Loss and Makeup

Evaporation removes the heat, so it also consumes water. Divide the heat load by the latent heat of vaporization, about 2,400 kJ/kg.

• Evaporation: 348 kW ÷ 2,400 kJ/kg = 0.145 kg/s, or about 0.52 m³/h (1.7% of flow)
• Blowdown at 4 cycles of concentration: 0.52 ÷ (4 − 1) = about 0.17 m³/h
• Makeup water: 0.52 + 0.17 = about 0.70 m³/h, ignoring drift

Cooling Tower Sizing Results at a Glance

Item	Result
Heat load	348 kW
Air flow	38,500 m³/h
Plan area	2-3 m²
Fan shaft power	2.7 kW
Pump shaft power	1.0 kW
Evaporation	0.52 m³/h
Makeup water	0.70 m³/h

In this cooling tower sizing example, the estimated fan and pump shaft powers total about 3.7 kW to reject 348 kW of heat. This is roughly 1.1% of the heat-rejection rate, not the system’s total electrical consumption.

Common Cooling Tower Sizing Mistakes

• Using dry-bulb instead of wet-bulb temperature
• Ignoring the approach, which drives tower size more than the range does
• Counting only piping losses when sizing the pump
• Forgetting makeup water and blowdown
• Designing for average weather instead of the hottest wet-bulb conditions

When to Bring In an Engineer

A hand calculation gets you to the right size range. Final selection also needs manufacturer curves, site wet-bulb data, water treatment, noise limits, and a safety margin. Our team handles this kind of work as part of building energy simulation and efficiency consulting. You can see everything we offer on our services page.

Cooling Tower Sizing FAQs

Sources and Further Reading

1. SPX Marley. Cooling Tower Basics, for range, approach and evaporation fundamentals.
2. ASHRAE. Standards and guidelines for HVAC system design.

Typical values for water loading, fan pressure and efficiency vary by tower design. This is an educational guide, not a substitute for a project-specific design.