EVAPORATIVE COOLING

The purchase price and operating costs of these units are generally much lower than for refrigerative air conditioners. However, their effectiveness is limited by humidity. They are suitable for hot, drier climates, particularly inland regions. Evaporative coolers cannot be used for heating.

Evaporative air conditioning provides three basic functions.

1. Cooling

Outside air is drawn towards a fan and directed through wet filter pads. This process lowers the air temperature by evaporation of water in the filter pads, and filters and humidifies the air at the same time. Whilst the air is cooler, the increase in humidity may result in uncomfortable conditions. A high initial humidity will limit the cooling effect.

2. Air Circulation

The air passes through the room and must then be discharged outside: it is not circulated. The movement of a large volume of air is essential for effective operation, and hence there must be adequate door or window openings to allow for the free flow of air.

3. Filtering

All units have filter pads which need to be washed clean at the end of summer season.

Types of Evaporative Systems

Portable

Portable evaporative coolers are mounted on trolleys and can be moved easily from room to room. Units must be periodically filled with water. A small pump is used to keep the filter pad moist, and air drawn through the moistened pad by an electric fan is cooled by evaporation.

Fixed ('Through-the-wall' or window )

These units should be installed in a wall which receives the prevailing summer winds. The water supply is plumbed to the unit and is automatically maintained at an appropriate level. A filter ensures that the concentration of dissolved salts in the water tank does not reach undesirable levels.

Ducted

Single inlet - Cooled air is delivered through one central grille and its flow through the house is controlled by fan speed and the opening doors and windows. Ducts may be located in the walls or roof.

Multiple inlets - The unit is generally mounted on the ridge of the roof (about 1.2 x 1.2 m cubed) with inlet louvers on all four sides. Cooled air is directed through ducts to individual inlets in each room. This system is more expensive to install but provides better air distribution. To achieve maximum efficiency duct work should be thermally insulated.

Evaporative Cooler Rating

Evaporative coolers are usually rated in cubic metres per hour, or litres per second (air delivery) and a size should be chosen that will deliver sufficient air to fill the room in 1.5 minutes. The discharge opening at the other side of the room should have an area of 1 square metre for each cubic metre per second of rated air delivery (1 cubic metre = 1,000 litres).
Common features of evaporative coolers include variable speed fans, directional outlet louvers and water level gauges.