Affordable and efficient - cool
May 1st 2005

Alan Beresford, director Work Space Cooling looks at how evaporative cooling works, how it is applied in industrial and commercial buildings and its efficiency

There is an increasing demand for cooling in factories and commercial buildings. In many cases ventilation systems are unable to cope or refrigeration based air conditioning systems are too expensive to install and operate evaporative cooling. At up to 10 times more energy efficient than air conditioning and typically only 25% of the capital cost evaporative cooling is now providing affordable solutions to these problems.

The principles of evaporative cooling Evaporative coolers are typically installed as part of a ventilation scheme. The input air is passed though an evaporative cooler that can give, on a typical UK hot day, up to 8°C of cooling. This obviously makes a considerable difference to the internal conditions of the building.

A modern evaporative cooler is constructed of plastic and contains a fan that draws air across filter pads that are kept wet by a water circulation system as shown in diagram 1.

As the air passes over the pads, water evaporates into the warm, dry air. The energy required to evaporate the water comes from the air which in turn cools down. This process is often referred to as adiabatic cooling.

The efficiency of Evaporative Cooling In practice the actual performance over a three day hot period in the UK is shown in diagram 2. It can be seen that the greatest cooling occurs when the ambient temperature is at its highest. This is because in the UK the climate is such that the minimum relative humidity coincides with the maximum temperature. This means that more water can evaporate into the air and results in greater cooling. The 45kW of cooling relates to a 4 m3/s cooler that will absorb approximately 1.25kW of electricity. A conventional refrigeration based air conditioning system, running at a COP of 3.5 would require 13kW to produce the same cooling effect. At this point the evaporative cooler is 10 times more efficient than a refrigerated system.

Maximum efficiency and lowest running costs are also achieved by EcoCooling evaporative coolers by the use of a control system to regulate the temperature. When connected to a thermostat the air flow rate is automatically adjusted to maintain a temperature level. Since 90% of the electricity usage is by the fan this minimises the running costs. Simple communications allow EcoCooling evaporative coolers to be integrated into existing control systems.

Water is at the heart of all evaporative coolers and is also a component of the running cost. Its control is vital in maintaining an efficient and hygienic cooler. A 4 level water probe salinity control system optimises the bleed rate continuously to prevent scaling at minimum cost. An integral alarm system prevents overflow and other alarm conditions which may lead to wastage of water.

For a typical hot day in the UK an EcoCooling evaporative cooler, operating at 4m3/s can provide an average of 26kW of cooling for less than 10p per hour. This could typically cover an area of 200 square metres.

Installation Evaporative coolers, as mentioned earlier, are installed as part of a ventilation scheme. It is also possible to retro-fit evaporative coolers onto existing ventilation systems.

Legionnaires Disease Whenever evaporative cooling is discussed consideration must be given to its safety in relation to Legionnaire’ disease. There are reported to be over 30 million evaporative coolers installed world wide and there has never been a case of Legionnaire’ disease associated with a wetted medial evaporative cooler. This can be attributed to two aspects of their operation – the temperature of the water in the cooler is too low for Legionella bacteria to thrive and the absence of aerosols which are the transmission media for the disease.

A modern evaporative cooler is considered low risk because the temperature of the circulating water follows the wet bulb temperature of the ambient air. When this water temperature was monitored the results showed that the water does not rise above 20°C which is the threshold for legionella growth.

This, coupled with the design of air velocity over the pads that avoid any droplet formation, determines that this type of evaporative cooler is a low risk device.

An evaporative cooler is an extremely simple device that can, in a UK climate, provide significant cooling to increase the comfort of modern industrial and commercial buildings. The decrease in temperature and the use of fresh, clean air make conditions more comfortable and clean. The low energy use and absence of refrigerants make evaporative cooling environmentally friendly. It can therefore be seen that evaporative cooling has a real role to play in improving the comfort in modern buildings.

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