A fresh look at adiabatic cooling
September 1st 2007 A fresh look at
A new, more efficient approach to indirect adiabatic cooling may provide an
ideal solution where new or additional comfort cooling is required. Chris Ree
of Hoval explains why
Many technologies evolve and
improve and this is certainly the
case with a newly patented design
of indirect adiabatic cooling that offers
efficient cooling for factories and
warehouses, either on a stand-alone basis
or as a boost to existing cooling plant.
This is achieved through the use of a
combination of indirect cooling and a
patented pre-cooling design, which
delivers 20% increased cooling efficiencies
compared to other methods, a coefficient
of performance (COP) of up to 7 and does
not increase the humidity of the air. For
instance, for a power input of just 3kW the
new AdiaVent offers a cooling capacity of
up to 21kW with an air flow of 6000m3/h,
and the ability to add up to 20% fresh air
to the supply air before cooling.
How it works
For those who are not familiar with
adiabatic cooling, this refers to the
humidification of air under adiabatic
(isocaloric) conditions, so that energy is
neither added nor removed. The heat
necessary for evaporation is taken from
the air, which consequently cools down.
Adiabatic cooling can be either indirect
or direct. With direct adiabatic cooling the
air flow supplied to the space is
humidified, so this causes an increase in
ambient humidity and also poses a risk of
contamination of the supply air.
An alternative approach is to implement
indirect adiabatic cooling, where outside
air is used to cool recirculating air which is
extracted from the space and then
returned, after cooling, as supply air. In this
scenario, the outside air is humidified and
then used to cool the recirculated air via a
heat exchanger before being expelled
back to the exterior.
With single stage indirect adiabatic
cooling, as described above, the
humidification and cooling occurs at the
same time and in the same place and can
achieve an adiabatic degree of efficiency
of about 76%. As discussed below, this
performance can be improved even further
through the use of pre-cooling.
The efficiency of the process depends
on both the quality of the humidification
and the quality of the heat transfer, so it
follows that any improvements to overall
efficiency are addressed through these
parameters.
In the case of humidification, this will
depend on the nature of the system used
and spray humidifiers have proved
themselves to be reliable and sturdy in
many applications. In single stage indirect
adiabatic cooling, spray humidifiers are
combined with plate heat exchangers and
sometimes two are connected in series.
Or, in the case of smaller air flow volumes,
counter-current heat exchangers may be
used.
Innovative approach
Consideration of all of the variables has
led to the innovative, and subsequently
patented, concept of using a second heat
exchanger as a pre-cooler for the cooling
air. In the AdiaVent, the air to be cooled is
recirculated and returned to the
conditioned space while outside air is
effectively used as the refrigerant. As such,
it is drawn through a filter and pre-cooled
in a plate heat exchanger (the pre-cooler).
This pre-cooled air then flows into a
second cross-flow heat exchanger where it
cools the recirculating supply air more
efficiently than would be the case if it had
not been pre-cooled.
The outside air is then returned to the
outside; while condensed water returns to
a reservoir to be recirculated (one of the
scientific peculiarities of adiabatic cooling
is that the temperature of the water does
not influence the cooling effect). Such
recirculated water is subjected to
continuous anti-Legionella measures such
as ultra-violet disinfection.
The water reservoir is emptied
automatically at the end of each day of
operation and the airflow is maintained
through the plate heat exchangers purging
all moisture out of the system.
This use of pre-cooling makes it
possible to reduce the cooling limit
temperature and increase efficiency by
around 20%, compared to coolers not
using this patented approach.
Where required, AdiaVent can be
supplied with an integral direct expansion
coil and condensing unit to supplement
the adiabatic cooling during times of peak
cooling demand. Onboard automatic
controls ensure that the adiabatic cooling
is used to its maximum potential so that
energy performance is optimised. More articles from Hoval Limited: | 
