As we move rapidly into the 21st century our expectations and demands for comfort increase all the time Today, we work in air conditioned offices, drive air conditioned motor vehicles and most of us enjoy the comfort provided by some form of air conditioning system when relaxing at home.

In most cases these are expensive compressor type air conditioners that consume copious amounts of electrical energy, both expensive to our pocket, are a burden to our non-renewable energy resources. All over the world electricity costs are rising as governments grapple to meet the ever growing demand for power, and, of all our home appliances, conventional refrigerated air conditioners are one the biggest consumers of electrical energy.

Whilst our demand for summer comfort will not diminish there is a way of creating all the cooling comfort we require at a fraction of the electricity consumption of conventional air conditioning.

Evaporative Air Conditioning:

Ever wondered why we feel cooler by the sea or a large lake? Well, it's because of the water evaporation that is taking place; as hot air passes over the water natural evaporation occurs. During the evaporation process, heat from the hot air is absorbed into the water molecules and the temperature of the air is reduced.

To better understand how an evaporative air conditioner works, it is necessary to understand something about the properties of heat, air and water vapor.

Evaporative air conditioners are often called Evaporative air coolers or desert coolers. The most common type of evaporative air conditioner is the direct type in which the hot, outside air is cooled within the machine and forced into the building and exhausted outside again. But before we can discuss the cooling process we must understand a little of the nature of heat. Heat exists in two forms - sensible heat and latent heat.

Sensible heat is the heat we can feel, or sense. It is a property in nature that can be measured with a thermometer.

Latent heat is hidden heat that cannot be detected with a thermometer. The heat used to evaporate liquid water into water vapor (a gas) is called Latent heat of evaporation. As the liquid water changes its state into vapor (you cannot see water vapor) it absorbs heat from its surroundings. Evaporative air conditioning is only possible because of this natural phenomenon of latent heat.

Latent Heat comes from the surrounding air. That means that the surrounding air becomes cooler as it yields heat to the evaporating process.

Total Heat is the sum of latent heat and sensible heat. It is the total amount of heat in a room, made up of heat you can feel and heat you can't feel.

Total heat is measured in kilojoules (kJ). 1000kJ is approximately 1000 BTU's. The complete evaporation of one litre of water absorbs about 2000kJ of heat energy and that occurs within the evaporative cooling process without any external energy being supplied, which is why Evaporative Air Conditioners use a small amount of electrical power to operate. The only power that is required is for driving the fan and pump.

The Evaporative Air Conditioning Process:

In Direct Evaporative Air Conditioning machines the heat exchanging process is mechanised by means of a water pump that delivers water to heat exchanging media panels (cooling pads) and a motor driven fan that forces hot outside air through those media panels. These components combine to accelerate the natural heat exchange process.

During the process, some of the sensible heat from the air (the heat you can feel) is changed into latent heat (the heat you can't feel) when water in the Evaporative Air Conditioner is changed into water vapour.

This process of sensible heat changing into latent heat causes the hot air to become colder since some of its (sensible) heat has been used up as explained above. So the air temperature falls.

The cold air is then pumped into the building and is eventually exhausted from the building. It is NEVER re-circulated.

The amount of evaporation that takes place depends on the amount of water vapour that is already in the outside air, or, we say it depends on the humidity of the outside air. If the humidity is high not much evaporation can occur in the machine and therefore not much cooling will occur. Therefore Evaporative Air Conditioners are most suited to hot and dry climates.

Evaporative Air Conditioning Capacity

The most important information to know about an Evaporative Air Conditioner is how much it can reduce the temperature. You only need to know three pieces of data to make the calculation the ambient dry-bulb temperature, the ambient wet-bulb temperature and the saturation efficiency of the machine. The two temperatures can be found by using a special combined thermometer that has one thermometer with a dry bulb and one with a wetted bulb using a water-soaked sock over the bulb. The instrument is called a psychrometer.

The saturation efficiency of the machine should be available from the manufacturer, but 80% is a good average. The temperature reduction of the air supplied by an Evaporative Air Conditioner is the difference between dry and wet bulb temperatures multiplied by the efficiency. For example, the dry and wet bulb temperatures on a given day were 35 C dry bulb and 22 C wet bulb; machine efficiency was 80%. The temperature reduction is 35 22 = 13 C x 80% = 10.4 C, and therefore the air supplied to the building is 35 10.4 = 24.6 C.

In the above statement the figure of 80% saturation efficiency was used. More superior designed evaporative air conditioners which yields higher efficiencies and therefore deliver bigger temperature reductions and very high cooling effects. In the above example, the air supplied to the room could be as low as 23.0C, by using the more efficient machines.