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These are: top discharge, side discharge and down discharge
– the most popular being the down discharge where the
unit is placed on a roof structure and the unit discharges
the airflow down into the building to be cooled,” says Chris
Toomey, Technical Manager at Seeley International.
“Most buildings can be cooled using evaporative
cooling, from a metal sheet building (non-insulated) to
a well-insulated energy-efficient building. The design of
applications needs to take into consideration that there
is particular ducting design, that applies to evaporative
cooling solutions, understanding that evaporative cooling
cools by bringing cooled air into the building and that
the same amount of air needs to be exhausted out of the
building. This high volume of air must be moved within
the building structure using correct design ducting and
diffusers. These ducts can be quite large compared to
typical refrigerated units,” says Toomey.
There are, however, some applications where direct
(single stage evaporative cooling) cannot be used that include closed
sealed laboratories, small individual rooms, hotel rooms (where there
is normally only one control per cooling unit and not individual room
control), multi-storey buildings, and any application where there is a
limited area to install the required ducting as this is essential for the
volume of air continually being moved.
SYSTEM SIZING
The heat load of a zone within a building is the sum of the heat
emanating from all the sources including solar radiation, convection,
conduction, machines, lighting and people – it is measured in watts.
The calculation for what evaporative cooling units’ requirements is
slightly different in the amount of heat load in the specific building that
you are designing to. Heat load calculations can be complex and various
calculation methods and software is available for this function. The
calculations to thus establish what methods for sizing are going to used,
need to be carried out correctly by an application engineer.
“The heat load calculations for evaporative cooling are simplified
by ignoring the latent heat. Our company has developed a heat load
calculation method where we are able to calculate the number of
evaporative coolers needed for a specific site. There are basically three
methods of sizing when using any form of evaporative cooling – they
are spot cooling, air change method, and then heat load method
calculations using the kW capacity of the evaporative cooler being
used,” continues Toomey.
Spot cooling is the simplest method and is the method of cooling
a target zone with a high velocity of cooled air that blankets the entire
zone to a height of about 2-3m, so people or products within that zone
are kept cool.
Spot cooling pays no regard to the actual heat-load of the zone, it
relies on a fixed air change rate of around 45 air changes per hour (ac/h)
to achieve its objective. The 45 ac/h is for direct, single stage units.
However, the indirect cooling technology will need substantially less ac/h
due to its supply temperature being lower than the ambient wet bulb.
ASHRAE
Figure 2: The direct relationship between relative humidity and health. Note the
healthy zone between 40 to 60% RH.
The air change rate method of sizing for selecting evaporative coolers
has long been the method of choice in the air-cooling industry. The
method uses air change rates that have been established over many
years by local companies in the Southern African market. The rates used
by most contractors are called the empirical data rates.
There’s no argument that this method of sizing is suitable for most
situations, however most application designs do not consider that
efficiency could have been achieved using better design methods.
This method also has some serious deficiencies as it assumes an
‘average’ building but an average building is hard to define – it may have
no significant heat loads and could also be well-insulated. Compare a
metal constructed warehouse to a warehouse built with brick, mortar
and insulation – these have very different parameters.
Further, this method makes no consideration for air coolers of
different efficiencies, and some manufacturers have similar airflows but
vastly different efficiencies. Quality manufacturers publish air change
rates for many cities around the world and these rates are calculated
using the correct design criteria, altitude and cooler efficiency and
therefore are quite different to empirical data.
The third is the heat load method. This method is also used in the air
conditioning industry where the building’s heat-load is calculated and
then the units that will be used are designed according to that particular
heat load.
In the case of single-stage evaporative cooling units, this method can
be more accurate than the methods already mentioned, but the correct
efficiencies and the correct cooling effect capacities in kilowatts (kW) of
the units must be established so that the correct number of units will be
installed for that specific site. Again, possibly a complex calculation.
“When using this method of design, you must take into
consideration how to remove the heat from the area concerned.
Evaporative air cooling does not re-circulate air. The air is cooled once
as it passes through the cooler. This cooled air must then be sized and
distributed in such a way as to remove the heat-load in the building.
A sufficient number of air changes must occur in order to expel that
www.hvacronline.co.za RACA Journal I October 2020 23