How is cooling tower efficiency calculated?

Cooling tower thermal efficiency is calculated as Eff (%) = [(Inlet Temp - Outlet Temp) / (Inlet Temp - Air Wet-Bulb Temp)] * 100. Typical industrial cooling tower efficiency ranges between 70% and 75%.

What is the difference between Cooling Range and Cooling Approach?

Cooling Range is the temperature difference between the warm water entering the tower and the cold water exiting (Ti - To). Cooling Approach is the temperature difference between the cold water exiting the tower and the ambient air wet-bulb temperature (To - Twb).

How much water is lost to evaporation in a cooling tower?

Evaporation loss can be estimated using the empirical formula: Evaporation Loss (kg/h) = 0.00085 * Water Flow Rate (kg/h) * (Ti - To in °C). This represents roughly 0.85% of the total water flow rate per 10°C of cooling range.

Air cooling towers : what is a cooling tower ?

What is a cooling tower and how it works ?

How to design a cooling tower ?

How to calculate the performance of an air cooling tower for design and troubleshooting activities ?

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1. Cooling tower principle of operation
2. How to design a cooling tower ? Cooling tower calculations
3. Interactive Cooling Tower Performance & Water Loss Calculator
4. Practical Plant Engineering Rules of Thumb & Design Limits

Air cooling towers are one of the most widespread process equipment in industries but are also present in regular building for air conditioning. This page explains the key concepts to design a cooling tower, or to monitor and troubleshoot its performances.

1. Cooling tower principle of operation

A cooling tower uses ambient air to cool down a flow of water that is used elsewhere in a factory to cool down another process.

The water to cool down is injected at the top part of the cooling tower and is then contacted with air blown at the bottom of the tower and going up. The air is creating an evaporation of some part of the water which, in consequence is cooling it down. This phenomena is known as evaporative cooling. This leads to a loss of water from the system which must be topped up.

There are actually 2 types of cooling towers based on how the air and the water are contacted : crossflow cooling towers and counterflow cooling towers.

Working principle of a crossflow cooling tower

Figure 1 : crossflow cooling tower (forced draft)

Working principle of a counterflow cooling tower

Figure 2 : counterflow cooling tower (forced draft)

There is also 2 sub-types which are natural draft of forced draft cooling towers. Most of the cooling towers are forced draft, which means that a fan is used to move the air, natural draft requires very high structure in order to have a difference of pressure in between the bottom of the tower and the top, which allows to create the draft, it is thus mostly found in large power plants or factories.

2. How to design a cooling tower ? Cooling tower calculations

2.1 Cooling tower efficiency

A cooling tower efficiency can be calculated thanks to the following formula :

\text{Eff (\%)} = \frac{T_i - T_o}{T_i - T_{wb}} \times 100

Equation 1 : cooling tower efficiency

With :

Eff(%) = efficiency of the cooling tower
T_i = water inlet temperature (°C or °F)
T_o = water outlet temperature (°C or °F)
T_wb = air wet bulb temperature (°C or °F)

Some orders of magnitude :

The difference of temperature in between the inlet and outlet temperature is usually around 5-8 °C
The efficiency of a cooling tower is usually in the range 70-75%

2.2 Cooling tower mass balance

Some water losses must be taken into consideration when designing a cooling tower, indeed, water will be lost through evaporation and through "windage" which means that the air will carry out of the tower some droplets.

The following orders of magnitudes can be used to calculate the water losses :

\text{Evaporation Loss (kg/h)} = 0.00085 \times \text{Water Flow (kg/h)} \times (T_i - T_o) \] \[ \text{Windage Loss (kg/h)} = 0.005 \times \text{Water Flow (kg/h)}

⚠️ ENGINEERING NOTICE & EDUCATIONAL DISCLAIMER: This interactive calculator is provided exclusively for preliminary estimation and educational purposes. It is not intended for detailed design or equipment procurement without certified vendor rating. No warranty, expressed or implied, is provided, and no liability is assumed.

Cooling Tower Performance & Water Loss Calculator

SI Metric US Customary

Water Inlet Temp, T_i (°C)

Water Outlet Temp, T_o (°C)

Air Wet-Bulb Temp, T_wb (°C)

Water Flow Rate, F_w (kg/h)

Cycles of Concentration (CoC)

Thermal Efficiency
54.55 %

Cooling Range (T_i - T_o)

6.00 °C

Cooling Approach (T_o - T_wb)

5.00 °C

Evaporation Loss

510.0 kg/h

Windage / Drift Loss

500.0 kg/h

Blowdown Water Loss

170.0 kg/h

Total Water Loss / Makeup
1,180.0 kg/h

💡 Industrial Best Practices & Engineering Rules of Thumb

Approach Temperature Limit: The economic limit for cooling approach ($T_o - T_{wb}$) is 3°C to 5°C (5°F to 9°F). Designing for an approach below 3°C requires an exponentially larger cell size and fan power due to psychrometric driving force limits.
Typical Range: Industrial cooling tower ranges ($T_i - T_o$) generally fall between 5°C and 10°C (9°F to 18°F).
Thermal Efficiency Norms: Well-designed mechanical draft cooling towers operate at 70% to 75% efficiency under design load conditions.
Drift Elimination: Modern high-efficiency drift eliminators reduce windage loss down to 0.0005% to 0.005% of circulating water flow, far below historical unmitigated rates of 0.3–0.5%.
Cycles of Concentration (CoC): Operating at 3.0 to 5.0 cycles is typical for industrial water treatment programs. Increasing CoC above 5 yields diminishing returns in water savings while significantly increasing mineral scaling and corrosion risk.