1. Air conditioning Introduction
Air Conditioning is just what the name suggests, air that has been conditioned to meet
the demands of the occupants of a home, office, factory or any internal space.
Air conditioning provides and maintains internal air conditions at a set temperature,
regardless of the time of year, the season or the country of residence.
Homes and offices are typically designed to include an internal air temperature of
19-23ºC (Women liking 23C and Men 21C on average) and relative humidity between
Heat energy is the motion of moving molecules and temperature is a measure of how
fast these molecules are moving.
At room temperature molecules are moving at the speed of a fired bullet
The more energy a molecules a gains (through heat energy maybe) the faster they
The Second Law of Thermodynamics says that Heat Energy always flows from a
warm region to a cooler one, never the opposite way around.
When molecules gain energy the start to move faster, if these faster moving molecules
come into contact with slow Colder (slow moving) molecules then they will start to
collide. These collisions cause the slow moving molecules to speed up and the fast
moving molecules to slow down. This changing of speed transfers energy from the
fast moving molecules to the slow moving molecules; this is in line with the second
rule of thermodynamics
When you make a cup of tea the water is boiling hot. If you decide to leave it a while
before drinking the it the Tea will cool down. This is because the temperature of the
tea (maybe 85 °C) is hotter than the surrounding room temperature. As we just learnt,
hotter molecules always become cooler (thermodynamics).The tea continuously gets
cooler until it gets to the rooms temperature.
2. In winter(or any season) if the room where you are drinking your tea is not kept
heated, the room will cool down, because the outside temperature is likely to be
If you placed an ice cube into a frying pan and heated it on a cooker hob the ice cube
The molecules inside the ice cube are slow moving (they don’t have much energy) as
heat is applied to the ice the molecules gain energy and vibrate more and become
faster moving. As the temperature of the ice hits 0°C the molecules inside the ice gain
enough energy to overcome the molecular interactions inside the ice holding them
together as a solid. The ice will then become a liquid (water).
When Ice (solid) changes state into water (liquid) at 0C there is no rise in
temperature this is called Latent Heat. It is This Latent heat (hidden Heat) that
overcomes the molecular interactions.
If the water in the frying pan was continuously heated still, the molecules inside the
water would start to become faster and faster moving.
When the faster moving molecules near the waters surface gain enough energy to
overcome the cohesive forces that hold them in a liquid structure, they break free.
The same process happens on all Earths oceans, seas, rivers, lakes etc every day
(the start of the water cycle). The sun heats up the molecules in the water and
gives them energy. The fast moving molecules near the water surface then break
free (evaporate) and become vapour.
3. Some Molecules above the surface water will randomly strike the water surface.
These fast moving molecules will randomly hit slow moving molecules in the water
and there by transfer energy to them. If they transfer enough energy the will slow
down enough to be capture back into the liquid water. This process is called
“The pressure of a gas of fixed mass and fixed volume is directly proportional to the
gas's absolute temperature”
This means that when you raise the temperature of a gas you also raise its
If you heated a sealed container with gas molecules inside the molecules
would gain energy and move quicker. As the molecules become faster moving
they hit the containers side more frequently which increases the pressure.
How does an Air con system Work?
All air conditioning systems work of the same basic principle. They take heat
and move it elsewhere.
Air conditioning systems take heat from, offices, room’s factories, aeroplanes
etc. and by taking the heat away it makes the room cooler.
A fridge works exactly the same way to keep all your food and beverages
How do Refrigerators work?
As mentioned above a fridge works by removing heat from a space and
dispersing it elsewhere. By removing the heat the space becomes cold.
Inside a fridge there are four main components that makes this process work
these are the:
• Expansion Valve
• Condenser Coil
• Evaporator Coil
4. The whole refrigerator cycle works on the process of evaporation.
For a liquid to become a vapour it needs energy, the liquid gets this energy
from the heat (in the fridge). The heat makes the molecules vibrate faster and
gives them enough energy to overcome the molecular force holding them
together and they become a vapour. The vapour uses the heat to leave the
liquid leaving the liquid to become colder
5. Evaporator Coil
After leaving the Expansion valve the liquid is at low pressure and starts to
evaporate into a Gas and become cool.
Remember that when the pressure of a gas raises so does the
Temperature. The lower the pressure the lower the temperature
This gas goes through the evaporator coil at a low temperature and takes in
all the heat from the fridge (or room if it is a air-conditioning system)
Remember the second rule of thermodynamics states that Heat Energy
always flows from a warm region to a cooler one, never the opposite
The expansion valve is an important component which regulates the amount
of liquid to enter the evaporator coil, this keeps the
The expansion valve allows a regulated amount of fluid to enter the
evaporator, which keeps pressure low and allows the liquid to expand back
into a gas state.
After leaving the Evaporator coil the vapour enters the compressor. The
compressor compresses the vapour, increasing its pressure and thereby
increasing its temperature.
Remember that when the pressure of a gas raises so does the
The compressor increases the pressure by pushing all the molecules
together. Because they are closer together the molecules hit each other more
often and this increases the pressure. With an increase in pressure comes an
increase in temperature.
Arriving at the condenser coil, from the compressor, the fluid has become as
high pressure and temperature gas. The high temperature gas then flows
through the condenser coil.
The condenser coil on a fridge is generally at the back on the outside of the
fridge. In air-conditioning systems the condenser coil would usually be outside
The temperature inside the coil is hotter than outside of it. Obeying the
second rule of thermodynamics, the heat leaves the coil and flows to the
cooler outside air molecules.
The gas molecules inside of the condenser coil lose heat and become cooler.
The molecules become slower as they lose energy and condense back into a
7. After leaving the Condenser coil the liquid will pass back through the
expansion valve to start the whole process again and again.