Air conditioning is the process of cooling the ambient air in rooms/buildings
and at the same time controlling the relative humidity by adding
back the correct amount of moisture. This is to achieve a relative
humidity of around 50%, which is generally considered to be most
comfortable. When the air is cooled and no control of humidity takes
place the correct term is actually ‘comfort cooling’
but this is more commonly referred to as air conditioning. Air conditioning
/ comfort cooling are normally achieved by use of refrigerant based
systems which utilise two heat transfer coils. The coil, which dissipates
the heat, is the condenser coil and the coil over which the air
is cooled is the evaporator coil. The condenser can be installed
outside the building and the evaporator unit is floor, wall or ceiling
mounted inside the building.
A different type of cooler is the evaporative cooler – this
is not air conditioning.
It is a fan with a moist filter across it. The filter is kept moist
by a tank of water, which needs to be topped up regularly. Air passing
through the filter evaporates the water from the filter and the
latent heat of evaporation is taken from the air, so making it feel
cool. The air is, however, moist and if these units are used in
confined spaces then the humidity of the room will continue to rise
and as it reaches near 100% evaporation will stop and the unit will
stop working. These are best used in open, well-ventilated spaces
where a cool breeze is needed and the air is continually vented
to the outside.
There are different designs of air conditioner; most people in
the UK are familiar with the "split system". These can
be seen in many offices and server rooms where the evaporator is
either wall mounted or a ceiling cassette.
Where buildings have central air conditioning systems they can
be combined with heating systems and the fan coil units in the rooms
can then either heat or cool the area as demanded by the control
system or building management system. These systems can be very
complex in design whereas split systems are relatively simple and
ideal for retrofitting in buildings where no air conditioning has
been installed from the outset.
Refrigerant based air conditioning systems often have the facility
to operate in reverse. In this mode they are termed as a heat pump.
Heat pumps take heat from outside and discharge heat via the coils
in the indoor unit.
Whether in air conditioning or heat pump mode these types of systems
typically have a coefficient of performance (CoP) of about 3. This
means that a 3 KW air conditioner uses about 1 KW of power. Conversely
a heat pump provides about 3 KW of heat while using about 1 KW of
Where the installation of an air conditioning system is not practical,
or is not sensible due to short-term occupancy of the building,
temporary air conditioners can be used. Air conditioner hire is
common and these units can be split types or monobloc units.
Monobloc air conditioners have both evaporator and condenser coils
in the same casing and discharge heat outside by exhausting hot
air through a flexible duct, which is taken to a window or a vent
in the wall. These air conditioning units will remove moisture from
the air and so reduce humidity but they have no means of controlling
humidity. Split air conditioners have the condenser coil contained
in a housing inside the room and the separate evaporator is mounted
externally and is connected with the refrigerant hoses.