RACA Journal I February 2026 www. refrigerationandaircon. co. za 31
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height of two metres so that people working within that zone will always feel cooled.
• Spot cooling pays no regard to the actual heat-load of the zone; it relies on a fixed air change rate of 45ac / h( air changes per hour) to achieve its objective.
• Spot cooling is often the most cost-effective method of cooling in large industrial premises where all that is required is that certain workers benefit from the cooling rather than cooling the whole factory area.
SYSTEM DESIGN Method 1: Air Change Rate method The air change rate method has been used for decades in the industry for sizing of air coolers. It is quick and satisfactory in many cases.
The Air Change Rate method suffers from two disadvantages: 1. It completely ignores the cooling efficiency of the air coolers 2. It assumes an average building construction and heat load
However, it is important to know how the method is used.
To select an air cooler, try this method:
• Measure and calculate the total volume of the area to be cooled. Take the height from floor to air vent
• Select the recommended air change rate for the area concerned, air changes per hour
• Multiply the volume by the rate to find the total air to be supplied per hour
• Select an air cooler which has the desired air flow at zero static pressure
Method 2: Heat Load method
All air coolers have a rating in Kilowatts( kW). This allows the designer to apply well-known air-conditioning design procedures to calculate heat load, except that no calculation is done for latent heat load, and to then select air cooling equipment directly from the kW capacity ratings. Method 3: Spot Cooling method In many industrial and commercial projects it is either not practical, or it is not necessary, or it is too expensive to install full evaporative air cooling generally throughout the building. In these cases, the Spot Cooling method can provide an excellent Solution.
Spot Cooling systems provide cooled air into a specific‘ spot’ within a building where people or machines require cooling due to the high heat load in that location. The design for Spot Cooling follows a different procedure from the other methods. The object is to blanket the whole‘ spot’ with high velocity cooled air, so that any person moving anywhere within that spot can feel the benefit of the cool air. It is not necessary to erect walls around the spot.
The air itself acts as a curtain.
Air distribution:
• A poorly designed or poorly installed distribution system will create noise, cause excessive system energy consumption and deprive the customer of valuable air flow and cooling effect.
• Always aim for the simplest air distribution layout – the less bends and fittings you use the better.
• Minimise duct resistance and noise with the use of large radius bends, splitters, expanding transitions to reduce velocity, swept branch junctions, minimum use of dampers.
• Air coolers function efficiently up to 120 – 150Pa. Air cooler air flow curves against system resistance are available. Best to aim for 80Pa system resistance.
Guide for duct velocities:
• Main ducts: 8 – 12m / sec.
• Branch ducts: 5 – 8m / sec.
AIR FLOW Evaporative air cooling systems employ 100 % fresh air and the air is cooled once before being delivered to the room and then exhausted to outside. The air is NOT recirculated and re-cooled again and again.
In order to distribute the fresh cooled air to every corner of the building with minimum loss of temperature, it is necessary to pump large amounts of the cooled air into the building. The air movement this creates contributes to the feeling of human comfort since air movement is an essential element of feeling comfortable. Control of air movement velocity is determined by the selection of air outlet hardware. The air must be able to exit the conditioned space
Condensation: In some circumstances condensation might occur when moist air strikes surfaces that are at dew point temperature or below. Evaporatively cooled air has a higher dew point than ambient air and will therefore cause condensation more readily.
Condensation will occur more readily if there are insufficient exhaust openings in the building.
Ben, I hope that this helps with your understanding around evaporative cooling, they are widely used especially in hot areas
with low humidity. RACA
Grant Laidlaw
REFERENCES: 1. ACRA 2. Breezair / Seeley International voices
RACA Journal I February 2026 www. refrigerationandaircon. co. za 31