The air-quality benefits of evaporative cooling

Published:  20 March, 2005

FIG 1
Fig. 1: Evaporative cooling can significantly reduce the indoor air temperature in UK weather conditions.

Hot, dry air is not at all comfortable, which is why ALAN BERESFORD argues that evaporative cooling has an important role to play in increasing indoor air quality.

sing evaporative, or adiabatic, cooling to reduce the temperature of work-space air can lead to important improvements in air quality. This article explains the principle of evaporative cooling as it is applied in the UK and describes the valuable benefits that it will bring.

How evaporative cooling works.

An evaporative cooler can significantly reduce indoor air temperature in UK weather conditions (Fig. 1). An evaporative cooler both reduces the temperature and smoothes out the variations that would otherwise be experienced. Working in an environment where the air temperature is less than 25°C is far preferable to one where it can exceed 35°C.

To appreciate this performance it is useful to have an understanding of the ambient-air conditions over a 24 hour cycle in the uk. a typical ‘hot’ day has the temperature and relative humidity profile shown in Fig. 2. An evaporative cooler can take air at 30°C/50% RH and pass it over wet filter pads to cool it to 24°C.

Fig. 2: Temperature and relative-humidity for a typical ‘hot’ day in the UK.

Relatively ‘dry’ air naturally takes up water which changes from a liquid state to a gaseous state. the energy required to achieve this, the latent heat of evaporation, is taken from the air itself, which is thereby cooled. This is usually illustrated by a psychometric chart which shows the relationship of air and water and can be seen in Fig. 3.

Fig. 3: Air with a constant energy content cools down as it relative humidity is increased by passing it through an evaporative cooling.

It can be seen from the psychrometric chart that the air follows a line of constant enthalpy (energy) from 30°C, 35%RH to a point close to the saturation line. The final performance is related to the efficiency of the pad material and the velocity of the air across it. A modern evaporative-cooling pad is made of a cellulose base material and constructed in corrugated honeycomb structure (Fig. 4) to provides high wetability from a large surface area.

Fig. 4: The mechanism of evaporative cooling is based on passing hot, dry air over wet filter pads to cool it and increase its relative humidity.

BENEFITS OF EVAPORATIVE COOLING
Comfort

The comfort of someone exposed to ambient conditions depends principally on temperature, relative humidity, air flow, clothing and work rate.

The first and most obvious benefit of evaporative cooling is an increase in comfort due to the lower temperature when the air is used to cool a building. A common concern is the associated increase in relative humidity. To understand this effect a concept of ‘apparent temperature’ is used.

The combination of actual temperature and relative humidity can be translated into an ‘apparent temperature’ using a formula devised by R. G. Steadman in 1979 (Fig. 5).

Fig. 5: Reducing temperature has a far greater effect on comfort than the corresponding rise in relative humidity.

The effect of temperature reduction on comfort is far greater than is the rise in relative humidity. By taking the conditions from ‘extreme caution’, which are found in many situations in the UK, to temperatures below 25°C can have a profound effect on the comfort of an individual in both industrial and commercial operations.

Static electricity

Low relative humidity, typically below 40%, can increase static electricity. This can either cause discomfort for people or problems with processes. An example is printing, where large sheets of paper will not feed into printing presses and finishing equipment at the design operating speed due to static electricity problems. Installing evaporative cooling will both reduce the temperature and increase the relative humidity to reduce these static problems and improve productivity.

Legionnaires’ Disease

A fundamental part of air quality is its safety. Where a water system exists, consideration must be given to legionella bacteria. There are reported to be over 30 million evaporative coolers installed world-wide and there has never been a case of Legionnaires’ Disease associated with a wetted-medium 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 to transmit the disease.

When the water was monitored in a cooler sump in the UK the results showed that its temperature stayed under 20°C, even though air temperatures entering the cooler approached 40°C. This can be explained by the water circulating at the wet-bulb temperature of the air.

The design of an evaporative cooler must take into consideration the air speed over the filter pads. Excessive air velocity across the pads, typically 3 m/s, may result in water being drawn off the pads and droplets being introduced into the air flow. An evaporative cooler with a wetted medium is normally designed with an air flow below 2 m/s to give aerosol-free operation.

With reference to the HSE ‘Approved code of practice Legionnaires’ Disease: The control of legionella bacteria in water systems L8’, a system where the water is below 20°C and no aerosols are produced is not considered a risk. When coupled with the latest process controls, an evaporative cooler is a very low risk piece of equipment.

Air cleanliness

As air passes over the filter pads, larger polluting particles such as pollen grains are filtered out. Because an evaporative cooler is used as part of a ventilation system, fresh air which is being continuously brought into the building and processed by the system is far healthier than that found with systems which recycle air.

The environment

The final aspect of air quality to be considered is the effect on the environment. A conventional refrigeration-based air-conditioning system adversely affects the environment by the amount of energy it consumes and the use of potentially damaging refrigerant gases. An evaporative cooler will typically have only 10% of the electricity consumption of an equivalent refrigeration-based air cooler. It uses only pure water, which is practically harmless to the environment.

Conclusion

An evaporative cooler is an extremely simple device which can, in a UK climate, provide positive benefits to the air quality in a building. The decrease in temperature and the use of fresh, clean air make for more comfortable and cleaner conditions. The low energy use and absence of refrigerants make evaporative cooling very environmentally friendly. Thus it can be seen that evaporative cooling has a real role to play in improving air quality in modern buildings.

Alan Beresford is a director with Work Space Cooling Ltd, Risby,

Bury St Edmunds, Suffolk IP28 6QR.



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