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WA900-3EO
An air conditioning unit operates on a num-
ber of different principles. Following is a brief
discussion of each of these principles and how
they are used in an air conditioner unit.
TRANSFER OF HEAT
To understand the principle of heat transfer, we
must first understand that cold is de- fined as the
lack of heat. Like water, which always flows,
downhill heat always flows down a temperature
scale from a warm level to a colder one. When
we hold a cold glass of drink, heat always flows
from our warm hands to the colder glass.
LATENT HEAT
Latent heat is the hidden heat that cannot
be found with a thermometer.
If we were
to put a kettle full of water on a burner and
measure the water temperature with a ther-
mometer, the temperature would rise until the
water starts to boil.
At this point the temperature will read 100
o
C
and stop rising. Even if the flame were to be
turned up the temperature of the water would
remain at 100
o
C.
Where does the heat go? The water ab- sorbs the
heat as it changes to steam. The heat just
described is called the latent heat of
vaporisation. There is one other instance where
heat seems to disappear. This second
kind of latent heat is known as the la- tent heat
of fusion (which means melting). Whenever we
think of something boiling we
instinctively think of it being very hot. However
this is not true in every case. Some substances
will boil violently while sitting on a block of ice.
Any liquid will absorb large quantities of heat at
its boiling point temperature without get- ting
any warmer.
It should be noted that the boiling point of water
is 100
o
C only if it is subjected to the air pressure
at sea level. The boiling point temperature will
actually decrease at higher elevations.
TEMPERATURE AND PRESSURE
It has already been pointed out that the boiling
temperature of water decreases as elevation
increases. This is because atmo- spheric pressure
decreases with an increase
in elevation.
Conversely an increase in pressure
raises the boiling temperature of water.
It is also true that if we apply pressure to
compress steam we increase its tempera- ture
without increasing the heat it contains. It is
useful to think of temperature as a mea- sure of
heat intensity. Temperature is not a measure of
heat quantity.
So far we have discussed steam but the
same principle applies to all vapours.
If we apply pressure to compress vapour we
increase its temperature.
BASIC AIR CONDITIONER OPERATION
There are four basic components in every air
conditioner system -
»
Evaporator
»
Compressor
»
Condenser and
»
Expansion Valve
The refrigerant in the evaporator boils into a
vapour at a temperature well below the air
temperature of the compartment.
As it does, it soaks up a lot of heat without
getting any hotter. The compressor then ap-
plies pressure to the vapour to increase its
temperature.
Heat is now transferred from the warm high-
pressure vapour in the condenser to the cooler
surrounding air. As the vapour cool, it
condenses into a liquid at high pressure; the
high-pressure liquid refrigerant passes to the
expansion valve at the entrance to the evaporator.
At the valve orifice the pressure is lowered and
the refrigerant enters the evaporator as a low-
pressure liquid. The cycle then starts all over
again.
PRINCIPALS OF AIR CONDITIONING
OVERVIEW
