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