30 www. refrigerationandaircon. co. za RACA Journal I February 2026
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Human comfort has five components:
• Temperature
• Relative humidity
• Air flow
• Personal activity
• Personal clothing
Conventional air conditioners create comfort by changing temperature and humidity.
Evaporative air conditioners create comfort by changing temperature and air flow.
Human beings‘ feel’ the combination of temperature and humidity. As the humidity increases, they‘ feel’ hotter when the temperature has not changed at all. We call this‘ felt’ temperature,‘ effective temperature’.
Effective temperature( or apparent temperature):
• Apparent temperature is a measure of how‘ hot’ we‘ feel’ due to the combined effects of temperature and humidity
• The higher the relative humidity the hotter we‘ feel’ when there is no change in actual temperature
Cooling effect:
• This term applies to the cooling capacity of the equipment
• Cooling effect is measured in kW
• Che cooling effect is calculated after testing for the saturation efficiency and airflow according to standardised procedures and parameters
• It is a measure of the ability of an evaporative air cooler to cool a hot space down to a standard exit temperature
TYPES OF EVAPORATIVE COOLERS We find two main categories of evaporative coolers: 1. Direct evaporative air cooler: An evaporative air cooler in which the primary supply air is cooled and delivered directly into the building the cooled air has increased moisture content.
2. Indirect evaporative coolers: These have two air circuits, primary and secondary. The secondary air( ambient) is cooled and passed through a heat exchanger and exhausted to atmosphere. The primary air( from the room) is passed through the other side of the heat exchanger where it is cooled and delivered back to the room with no increase in moisture content.
Terms used in evaporative cooling:
Exit temperature:
• The dry-bulb temperature of the air exiting the building when an evaporative air cooling system is in operation
• It is slightly higher than the indoor temperature at the centre of the conditioned space
• The exit temperature is what is used to calculate the cooling effect( capacity) of evaporative air coolers
Heat-load:
• The heat load of a zone within a building is the sum of the heat emanating from all sources including solar radiation, convection, conduction, machines, lighting and people
• Heat-load is measured in watts
• Heat-load calculations are complex – various calculation methods and software are available
• Heat-load calculations for evaporative air cooling are simplified by ignoring latent heat
Saturation efficiency:
• The saturation efficiency of an evaporative air cooler is its ability to cool the incoming air down towards the wet-bulb temperature
• Typical saturation efficiency for high performance evaporative air coolers is 85 % to 90 %
Spot cooling:
• Spot cooling is the method of cooling a target zone with high velocity cooled air that blankets the entire zone to a
30 www. refrigerationandaircon. co. za RACA Journal I February 2026