Cooling Tower Experiment: Range, Approach & Cooling Efficiency

The cooling tower experiment measures how effectively a cooling tower lowers water temperature by evaporative cooling, evaluated through three quantities — range, approach, and cooling efficiency. It is a key heat-transfer and thermal-engineering practical, because cooling towers reject waste heat in power plants, HVAC systems, and process industries.

Aim of the experiment

To study the performance of a cooling tower and determine its range, approach, and cooling efficiency.

Theory

A cooling tower cools warm water by bringing it into contact with air, so a small fraction of the water evaporates and carries away latent heat. Performance is judged by three quantities:

• Range = T₁ − T₂ (hot water inlet temperature minus cooled water outlet temperature)
• Approach = T₂ − T_wb (cooled water outlet temperature minus the wet-bulb temperature of the inlet air)
• Cooling efficiency η = (T₁ − T₂) / (T₁ − T_wb) = Range / (Range + Approach)

The wet-bulb temperature is the theoretical lowest temperature to which the water can be cooled; a smaller approach means a more effective tower.

Apparatus required

• Laboratory forced/induced-draft cooling tower test rig
• Water heater, circulating pump and flow meter
• Blower/fan with air-flow measurement
• Dry-bulb and wet-bulb thermometers (air inlet/outlet) and water inlet/outlet thermometers

Procedure

1. Fill the sump and start water circulation; switch on the heater to set the inlet water temperature.
2. Start the fan and allow conditions to stabilise.
3. Record inlet and outlet water temperatures, air dry-bulb and wet-bulb temperatures, and the water and air flow rates.
4. Compute range, approach, and cooling efficiency.
5. Repeat at different air-flow rates or heat loads and plot efficiency against the air/water ratio.

Result

Cooling efficiency rises with air flow up to a point, and the approach shrinks as air–water contact improves. Typical laboratory tower efficiency is 70–90%.

Applications

Cooling towers serve thermal power stations, HVAC chillers, refrigeration condensers, and process cooling. This links to the cooling side of the IC engine performance test, and complements the double-pipe heat exchanger and natural & forced convection experiments. See related terms in the engineering lab glossary.

Frequently asked questions

What is range and approach in a cooling tower?

Range is the temperature drop of the water (T₁ − T₂); approach is how close the cooled water gets to the air wet-bulb temperature (T₂ − T_wb).

Why can’t water be cooled below the wet-bulb temperature?

The wet-bulb temperature is the thermodynamic limit of evaporative cooling, so the cooled water can only approach it, never go below it.

What is a good cooling tower efficiency?

Most towers operate at 70–90%; a smaller approach indicates better performance.

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