Cooling of the
airstream is achieved by bringing it into contact with a cold surface. The cold
surface is a cooling coil. The cooling coil can be either a direct expansion
(DX) cooling coil which is the evaporator of a vapour compression chiller or it
may be a water coil similar to the heating coil described before. It differs from
the heating coil in that a mixture of water and antifreeze (glycol) circulates
through it rather than hot water. This mixture is cooled using a chiller.
Waste Heat Rejection
Vapour compression
chillers have been described previously. An alternative method of cooling called
absorption cooling is also described previously.
This section will look
at methods of condenser heat rejection. Chillers generate a large amount of waste
heat. In a domestic refrigerator, which we have used as an example of an everyday
chiller previously, the waste heat is simply allowed to enter the kitchen via
the condenser coil at the rear of the refrigerator. However, in air
conditioning systems the amounts of waste heat involved are too great and would
cause serious overheating in the plant room. Because of this the waste heat
must be safely rejected outside the building. There are three main ways in
which waste heat is removed from the condenser. These are by using; air cooled condensers,
evaporative condensers or water cooled condensers.
Air Cooled Condensers the figure have been described previously in their
application to split air conditioning systems. They are
predominantly used for smaller cooling loads (less than
100kW) although they are used for rejecting up to
three times this value mainly due to the fact that water is
not used in their operation. This means maintenance costs
are low.
Evaporative Condensers. Are similar to air cooled condensers except that their
heat rejection capacity is increased by spraying water
over the condenser coil (as shown in the figure). As this water
evaporates it absorbs heat. Cooling of the condenser
coil is therefore achieved by both sensible and latent
means. The practical implication of this is that the
unit has a smaller physical size for a given heat rejection
capacity than an air cooled condenser. The water
circulating over the condenser is treated to prevent bacterial
growth. In addition spray eliminators must be used to
avoid water droplets, which may be contaminated,
leaving the unit. Evaporative condensers can be used up to
500kW cooling capacity.
Water Cooled Condensers. Variations in ambient air temperature cause changes in the
efficiency of air cooled condensers (see IP2).
A more temperature stable heat sink is water. Water
cooled condensers make use of this by jacketing
the condenser in a shell which is filled with water
(as shown in the figure). The condenser passes
its waste heat to the water increasing its
temperature by about 5oC. The
water is then pumped to a water to water plate heat exchanger. Water from a large
nearby source, such as a canal, river, lake or sea
is also circulated through this heat exchanger having
first been strained and filtered. In this way the
condenser cooling water only makes thermal contact with
the heat sink water. The heat sink water having picked
up heat from the condenser circuit is returned to the
main body of water where the heat it carries is
dispersed. The condenser cooling water
leaves the plate
heat exchanger and returns once more to the condenser to
pick up more waste heat. The use of bodies of water
such as rivers and canals as a heat sink
is subject to water authority approval.
Cooling
Towers. In locations where there are no large bodies of
water that can be used as a heat sink, the water
cooled condenser is used in conjunction with a cooling
tower. A cooling tower is a device which cools the
condenser cooling water by evaporation before returning
it to the condenser to collect more heat. Figure shows a
forced draught cooling tower. It can be seen that
the condenser cooling water is allowed to tumble down through the device whilst air is forced upwards
through the cascading water by a fan.
The purpose of the tower is to enhance evaporative cooling of the condenser water. It does this by increasing the surface area of the water exposed to air. Evaporation is a surface effect so increasing the surface area of water in contact with the air increases evaporation. Water surface area increases are achieved in a number of ways such as by allowing the water to tumble down splash bars, by spraying or by running it over a PVC matrix. When the condenser water evaporates it absorbs latent heat from the water which is left behind. The effect of this is to cool the water which collects in the sump at the base of the tower. This is pumped back to the water cooled condenser to remove more waste heat.
Cooling
Tower Hygiene is an important area of concern since the
water temperatures in the tower are conducive to bacterial and algal growth. In
particular legionnaires disease, which is a form of pneumonia, has been associated
with wet heat rejection equipment. The legionella bacteria grow in the warm
water of the cooling tower. They escape from the tower as part of the mist
created by the flow of air and water through the tower. If ambient conditions
are suitable and the bacteria carrying droplets are breathed in by a
susceptible passer by a potentially fatal infection can occur. The problem is
avoided by using air cooled condensers. However, dry heat rejection uses
approximately 30% more energy for the same capacity as a wet method. Cooling
towers can be used safely with the following Precautions:
• Use of
spray eliminators to prevent the release of infected droplets
• The
tower should be built of easily cleanable materials such as plastics or epoxy
coatings with smooth surfaces. Access doors should be incorporated into the
tower to facilitate cleaning.
• The
tower should be positioned away from air intakes which could draw infected
droplets into the building through the air conditioning system.
• A program
of maintenance and cleaning should be carried out throughout the life of the
tower. This should include dosing the cooling water with bactericides. Chemicals
to prevent algal growth should also be used since algae tend to coat surfaces
and give the legionella a medium on which to grow.
