GENERAL PRINCIPLES OF EVAPORATIVE COOLING

On days when heat or humidity, or even excessive pollen, causes discomfort, people often head off to lakes or the seaside in search of relief. Evaporative Air Conditioning is, in many instances, capable of simulating the cooler conditions so often found at or near large expanses of water. It makes it possible to effectively cool homes, factories, warehouses, offices and many other areas. This type of air conditioner circulates hot outside air through a specially designed media that cools and cleans hot and dusty outside air, prior to feeding it into your premises. Listed below are some of the effects of an evaporative cooling system.

LOWERS ACTUAL AIR TEMPERATURE

An ordinary thermometer reads the actual air temperature and this is referred to as a Dry Bulb temperature.

Wet Bulb temperature is the lowest temperature that air can be cooled to by the evaporation of water.  It is the temperature that the air would be when reaching 100% humidity or saturation.

If the humidity of the air is below 100% (it is not saturated), evaporation will take place when water comes into contact with it. The air begins to absorb water. Any given volume of air can hold a certain amount of water vapour and the amount that it can still absorb depends on how much it is already holding. The term humidity describes how much water is already in the air, relative to the amount it is capable of holding. Air is saturated when it cannot hold any more water. Imagine it as a sponge, if the sponge held half as much water as it was capable of it would be 50% saturated. In the case of air, we would describe the Relative Humidity as being 50%.

Energy is required to change water from a liquid to a vapour. This takes the form of heat that is obtained from the air into which the water is being evaporated, as well as from the water itself. Both the air and the water are cooled during this process. Air entering an evaporative air-conditioner gives up heat and is cooled as water, which cascades down the cooling media, evaporates into it. During this process, the Dry Bulb temperature of the air passing through the cooler is lowered.

LOWERS EFFECTIVE TEMPERATURE

This is what people "feel" the temperature to be, and which is influenced by air movement over the body. It should not be confused with Dry Bulb temperature. When sitting in a breeze people feel cooler. Air blowing on a person increases the rate of evaporation of moisture from their skin. Heat for this process is gained from the air, but more importantly, it is also gained from the person’s skin. The person feels cooler, even though the dry bulb temperature of the air remains the same. One feels cooler, and this gives rise to the concept of effective temperature. Humidity also influences effective temperature. The more humid the air is, the less water it can hold and the rate at which it is able to evaporate water drops. Saturated air cannot evaporate water and hence cannot be cooled through this process. Saturated air may influence a persons effective temperature if it is warmer or cooler than the person, just as one’s hand is cooled by placing it on a cool surface.

It is important to mention that effective temperature varies from person to person. For example, large people tend to feel warmer than small people, when both are subjected to the same ambient conditions.

REDUCES BUILDING TEMPERATURE

Buildings in hot locations are warmed by the sun and by winds. Walls offer little cooling to any fresh air entering the building. An evaporative air-conditioning system offsets the effects of sun and wind by cooling walls from the inside. Cool walls add very little heat to the air being supplied into a room by an evaporative cooling system.

FULL FRESH AIR SYSTEM

Evaporative air-conditioning systems should not re-circulate air, as this will result in hot and humid conditions. Air from an evaporative cooler should be used once only and should move from the cooler, into the air-conditioned area, and then out of doors, windows, or specially designed exhaust outlets. Generally most applications require air exchange rates of between 25 – 35 per hour in order to achieve comfort conditions.

FILTERED AIR

Outside air often contains dust and pollen. When cooling, good filtration takes place, and most airborne dust and pollen particles will be removed from the supply air. The wet cooling media traps most of these particles which are washed down to the unit sump by the circulating water. Cool Breeze evaporative air conditioners are fitted with a built in wash and drain cycle which allows dirt to be drained away, so reducing the necessity for frequent servicing of the cooling unit.