How does Air Conditioning work?
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Lets first look some principles of operation.
Latent Heat
When a liquid boils (converts to gas) it absorbs heat without
raising the temperature of the resulting gas. When the gas condenses (converts
back to a liquid), it gives off heat without lowering the temperature
of the resulting liquid. This is called Latent Heat.
Relative Humidity
The amount of moisture (water vapour content) that the air can hold
is directly related to the air temperature. The more heat there is in
the air, the more moisture the air can hold. The lower the moisture content
in the air, the more comfortable you feel. Removing the moisture from
the air lowers its relative humidity and improves personal comfort.
Effects of Pressure on Boiling or Condensation As the pressure
is increased on a liquid, the temperature at which the liquid boils (converts
to gas) also increases. Conversely, when the pressure on a liquid is reduced,
its boiling point is also reduced. When in the gas state, an increase
in pressure causes an increase in temperature, while a decrease in pressure
will decrease the temperature of the gas.
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Air conditioning is the process by which air is cooled and dehumidified.
The air conditioning in your car, your home and your office all work the
same way. Even your refrigerator is, in effect, an air conditioner. While
there are many physical principles that relate to air conditioning, this
article sticks to the basics. It explains the general concepts of automotive
air conditioning, the components used and what you need to know to keep
your car's A/C system working properly. Basically, air conditioning systems operate on the principles of evaporation
and condensation.
Here's a simple example of evaporation. Imagine that you're swimming around
in your neighbour's backyard pool on a summer day. As soon as you get
out, you start to feel cooler. Why? The water on your body starts to evaporate
and turns into water vapour. And as it evaporates, it draws heat away
from your body, and you get goose bumps. Brrr! Now let's say your neighbour
hands you a big glass of ice-cold lemonade. You take a sip and set it
down on a table. After a minute or two, you notice that water has collected
on the outside of the glass. This is condensation. The air surrounding
the glass becomes cooler when it encounters the cold glass, and the water
vapour the air is carrying condenses into water. Both of these examples
occur at normal atmospheric pressure. But higher pressures can also change
a vapour (or a gas) into a liquid. For example, if you look at a typical
butane cigarette lighter, you can see liquid inside it. But as soon as
you push down on the button, butane gas comes out. Why? The butane is
under high pressure inside the cigarette lighter. This high pressure causes
the butane to take liquid form. As soon as the butane is released and
it encounters normal atmospheric pressure, it turns back into a gas. Now
hold the lighter as you release the gas and feel it get cold - that is
because as the liquid turns to gas it requires latent heat and it draws
this heat out of you hand and the environment making it feel cold.
The reverse is true too - latent heat is given up when gas is
compressed into a liquid.. Hold a bicycle pump in your hands after
inflating a tyre an feel how hot it is. Although you can not compress the
air enough to become a liquid you can feel the released latent heat being
given up into your hand. OK, those are the basic ideas. But how do they
apply to making your car's vents blow cool air? The principles of
evaporation and condensation are utilized in your car's A/C system by a
series of components that are connected by tubing and hoses. There are six
basic components: the compressor, condenser, receiver-drier, thermostatic
expansion valve, the evaporator and the life-blood of the A/C system, the
refrigerant. Refrigerant is a liquid capable of vaporizing at a low
temperature. In the past, R-12 refrigerant was used in cars. But this
chlorofluorocarbon (CFC) is said to be harmful to the earth's ozone layer.
Consequently, all vehicles built after 1996 use R-134A, a more
environmentally friendly refrigerant. In the case of the Scorpio all the
new range had R134a from the start. The refrigerant differs from R12 in
that it is about 10% less efficient in cooling and has smaller molecules.
Here's how an air conditioning system and its components work.
Step One: The compressor is the power unit of the A/C system. It is
powered by a drive belt connected to the engine's crankshaft. On the
Scorpio you can see it on the right hand side of the engine bay and
although the drive belt is permanently rotating the front pulley, the
pulley is connected to the compressor by a magnetic clutch. So it spins
freely with the clutch disengaged and drives the compressor when engaged.
When the A/C system is turned on, the compressor 'compresses' the
refrigerant vapour heating it up and pumps it under high pressure to the
condenser.
Step Two: The condenser is a device used to change the
high-pressure refrigerant vapour to a liquid. It is mounted ahead of the
engine's radiator, and it looks very similar to a radiator with its
parallel tubing and tiny cooling fins. If you look through the grille of a
car and see what you think is a radiator, it is most likely the condenser.
The hot vapour from the compressor arrives and the fins help the air
flowing through the condenser remove heat from the refrigerant, changing
it to a much cooler liquid state. When the car is stationary the radiator
cooling fans perform the task of moving air through the
condenser.
Step Three: Refrigerant moves to the receiver-drier. This is the
storage tank for the liquid refrigerant. It also removes moisture from the
refrigerant. Moisture in the system can freeze and then act similarly to
cholesterol in the human blood stream, causing blockage. The
receiver-drier is used on the high side of the Scorpio system and uses a
thermal expansion valve. This type of metering valve requires liquid
refrigerant. To ensure that the valve gets liquid refrigerant, a receiver
is used. The primary function of the receiver-drier is to separate gas and
liquid. The secondary purpose is to remove moisture and filter out
dirt.
Step Four: As the compressor continues to pressurize the system,
cool liquid refrigerant under high pressure is circulated from the
receiver-drier to the thermostatic expansion valve. The valve relieves
pressure from the liquid refrigerant so that it can expand and become
refrigerant vapour in the evaporator. Remember that when a liquid becomes
a gas it needs latent heat and so it extracts this from its surroundings
cooling them down. The expansion valve is the "brain" of the system. By
sensing the temperature of the evaporator, or cooling coil, it allows
liquid to pass through a very small orifice, which causes the refrigerant
to expand to a low-pressure, low-temperature gas. This "cold" refrigerant
flows to the
evaporator
Step Five: The evaporator is very similar to the condenser. It
consists of tubes and fins and is mounted inside the passenger
compartment. As the cold low-pressure refrigerant is released into the
evaporator, it vaporizes and absorbs heat from the air in the passenger
compartment. As the heat is absorbed, cool air will be available for the
occupants of the vehicle. A blower fan inside the passenger compartment
helps to distribute the cooler air. The evaporator serves as the heat
absorption component. The evaporator provides several functions. Its
primary duty is to remove heat from the inside of your vehicle. A
secondary benefit is dehumidification. As warmer air travels through the
aluminium fins of the cooler evaporator coil, the moisture contained in
the air condenses on its surface. Dust and pollen passing through stick to
its wet surfaces and drain off to the outside. On humid days you may have
seen this as water dripping from the bottom of your vehicle. Rest assured
this is perfectly normal.
The ideal temperature of the evaporator
is 32° Fahrenheit or 0° Celsius. Refrigerant enters the bottom of the
evaporator as a low pressure liquid. The reducing pressure and the warm
air passing through the evaporator fins causes the refrigerant to boil
(refrigerants have very low boiling points). As the refrigerant begins to
boil, it absorbs large amounts of heat from the air entering the passenger
compartment and this heat is then carried away with the refrigerant.
Several other components work in conjunction with the evaporator. As
mentioned above, the ideal temperature for an evaporator coil is 0
centigrade. Temperature and pressure regulating devices must be used to
control its temperature. In the de-ice switch has a temperature probe
mounted on the evaporator and cycles the compressor off when near to
freezing to prevent the evaporator from freezing up. A frozen evaporator
coil will not absorb as much heat.
Step Six: The
heat-laden, low-pressure refrigerant vapour is then drawn into the
compressor to start another refrigeration cycle.
As you can see,
the process is pretty simple. Just about every vehicle's A/C system works
this way, though certain vehicles might vary by the exact type of
components they have.
The best thing about air conditioning is that
all you have to do is press a button to make it work. Air conditioning
systems are pretty reliable. On a modern and relatively new vehicle, it is
rare to have problems. And if there are problems, they are pretty much one
of two things: No cool air or insufficient cool air. If you own an older
car and its A/C system doesn't seem to be working properly, here are some
general troubleshooting tips:
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Most A/C repairs are
best left to a specialist repairer or garage. Recharging the refrigerant,
in particular, requires special equipment that most people don't own.
There are a couple things you can do, however.
First, make sure to
have the system checked regularly according to your vehicle's owner's
manual - in the Scorpios case that's every 10,000 miles.
Second, if
you live in a place with a cold climate, it might not make much sense to
run the A/C during the winter months, but many A/C technicians recommend
running your A/C system regularly, because it contains a light mineral oil
in the refrigerant to keep the compressor properly lubricated. The general
rule of thumb is 10 minutes per month. The Scorpio heating, ventilation
and air conditioning systems also engage the A/C compressor when the
distribution control is in defrost mode.
So those are the basics
behind air conditioning. The next time you're riding along in your car and
you hit the A/C button, you can say, Boy, those evaporator tubes sure are
cold. It's all thanks to R-134A!
An air conditioner is basically a refrigerator without the insulated
box. It uses the evaporation of a refrigerant, like Freon, to provide
cooling. The mechanics of the Freon evaporation cycle are the same
in a refrigerator as in an air conditioner. According to the dictionary,
the term Freon is generically "used for any of various nonflammable fluorocarbons
used as refrigerants and as propellants for aerosols."
This is how the evaporation cycle in an air conditioner works The
compressor compresses cool Freon gas, causing it to become hot,
high-pressure Freon gas. This hot gas runs through a set of coils so
it can dissipate its heat, and it condenses into a liquid. The
Freon liquid runs through an expansion valve, and in the process it
evaporates to become cold, low-pressure Freon gas. This cold gas
runs through a set of coils that allow the gas to absorb heat and cool
down the air inside the building. Mixed in with the Freon is a small
amount of a lightweight oil. This oil lubricates the compressor.
Taken literally, air conditioning includes the cooling and heating of
air, cleaning it and controlling its moisture level: conditioning it to
provide maximum indoor comfort. An air conditioner transfers heat from the
inside of a building, where it is not wanted, to the outside. Refrigerant
in the system absorbs the excess heat and is pumped through a closed
system of piping to an outside coil. A fan blows outside air over the hot
coil, transferring heat from the refrigerant to the outdoor air. Because
the heat is removed from the indoor air, the indoor area is cooled. An air
conditioning system generally consists of five mechanical components:1. A
compressor, 2. A fan, 3. A condenser coil (hot), 4. An evaporator coil
(cool), 5. A chemical refrigerant.
Most central air conditioning systems include of a "hot" side, outside
your home, and a "cold" side, inside your home. The "hot" side generally
consists of a condensing coil, a compressor and a fan. The "cold" side is
usually located within your furnace. The furnace blows air through an
evaporator coil, which cools the air, and routes this cool air throughout
your home using a series of air ducts. The cleaning function of air
conditioners is performed by filters, which remove dust from the
air.
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alex@rackett.freeserve.co.uk