RACA Journal October 2020 | Page 23

Feature Continued from page 19 supply air temperature, the less air will be required to achieve the target required,” says Marillier. This is therefore calculated using the formula: T (air out) = T (Db air in) – [eff% x (T (Db air in) -T (Wb air in))] / 100 AN EXAMPLE: As an example, the average summer temperature for Gauteng, South Africa is 30˚C Db and 20˚C Wb (a difference of 10 20˚C). Now, assuming 80% evaporative efficiency, the achievable supply air temperature would therefore be 22˚C (10˚ temperature difference with a temperature drop of 8˚/80%). The average summer temperature for Durban, South Africa is 29˚C Db & 25.5˚C Wb (a difference of 3.5˚C). Assuming 80% evaporative efficiency, the achievable supply air temperature would therefore be 26˚C (3.5˚ temperature difference with a temperature drop of 3˚C/80%). From this example it is clear that evaporative cooling is not an option, nor will it be a feasible option in areas like Durban, with higher Wb temperature or higher relative humidity. The efficiency of the whole design is based firstly on the efficiency of the cooling system being used. Using an efficiently designed system, you can achieve temperature drops from ambient temperature of up to 22˚C in extreme hot and dry climates, where the Db temperature could be 48˚C and the Wb temperature of 20˚C as you would see in desert-type areas. There are different types of evaporative cooling, namely: • Direct (single stage) • 2 stage (Hybrid system using wet core and plate heat exchanges) • Indirect (Using an air to air heat exchange core). Air-Dale Engineering An installation in a furniture retail shop. Here you see the multi-outlet air discharge. For the purpose of this article the focus is mainly on direct or single stage evaporative cooling. “Most makes of evaporative coolers today have three airflow discharge designs that can be used for a specific application. Continued on page 23 Air-Dale Engineering A typical evaporative cooling unit installation feeding directly into ducting on the outside of the building. ASHRAE Figure 1: Engineering level controls to reduce the environmental risks for airborne transmission. Air quality is directly related to the the flow of air, especially in occurrences of pathogen outbreaks. Seeley International The efficiency of the whole design is based on the particular system option and size of units used. www.hvacronline.co.za RACA Journal I October 2020 21