Air conditioners give you much-needed thermal comfort
during a scorching summer.
air conditioners help to maintain the room temperature at an optimal level.
They also help to remove airborne particles
and humidity from the room.
Let’s find out how these devices work.
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Let’s start with a very simple approach
to understanding the functioning of an air conditioner.
An air conditioner has two connected coils
with continuous flowing refrigerant fluid inside them.
The coil inside the room is called the evaporator.
And the coil outside the room is called the condenser.
The fundamental principle of the air conditioner is simple:
Just keep the evaporator cold,
more specifically colder than the room temperature,
and the condenser hot,
more specifically hotter than the surroundings.
With these conditions,
the continuously flowing fluid will obviously absorb the heat from the room
and eject it out to the surroundings.
This is the fundamental rule of an air conditioner.
Let’s see how this rule is implemented in practice.
To achieve this objective,
2 more components are needed inside your air conditioner:
A compressor and an expansion valve.
As you can probably guess,
the compressor increases the pressure of the refrigerant.
Here you can see a working example of a reciprocating-type compressor.
The compressor handles the refrigerant in its gas state
so that as it compresses the gas,
the temperature rises along with the pressure.
The temperature at the compressor outlet
will be far higher than the atmosphere temperature.
Therefore, if you pass this hot gas through the condenser heat exchanger,
you can easily eject the heat.
A fan in the condenser unit makes this task easier.
During this heat ejection phase,
the gas gets condensed to a liquid.
And the expansion valve is fitted at the exit of the condenser.
The purpose of the expansion valve
is to restrict the refrigerant flow,
thus reducing the pressure of the fluid.
Here comes the main trick.
You might be aware that it is possible to boil a liquid
just by reducing the pressure around it.
This phenomenon happens inside the expansion valve as well.
As the pressure drops,
one part of the refrigerant liquid is evaperated.
However, for this evaporation to happen,
some energy should be supplied to it.
This energy comes from within the refrigerant.
So its temperature drops.
This is how the cold refrigerant is produced inside the air conditioner.
This low-temperature refrigerant
should be at a temperature lower than the room temperature.
So by passing the room’s air over the evaporator coil,
the room temperature will drop.
The refrigerant gets converted to vapor during this heat absorption process.
You might’ve heard the term called ‘ton’ associated with the air conditioners.
The ‘ton’ represents how much heat
the evaporator can absorb from the room.
In simple words,
it represents the air conditioner’s heat-removal capability.
In this way,
we have achieved the fundamental rule of an air conditioner.
The temperature is lower than the room temperature in the coil
inside the room.
And the temperature is more than the atmosphere temperature in the coil
outside the room.
In an actual air conditioner,
the compressor sits near to the condenser,
and the expansion valve sits near to the evaporator.
There are some practical issues with this design.
Near to the evaporator coils,
the air temperature would be quite low.
This would lead to water condensation on the evaporator coils.
That’s why we need a pipe to remove this water condensate.
Modern air conditioners use scroll compressors
instead of the reciprocating type.
You can see how the compression process happens from this animation.
They are silent and they have good speed-control.
Do you know how your air conditioner is able to maintain
an almost steady room temperature in respective workload?
Modern air conditioners use a variable-speed motor technology,
called ‘inverter technology’,
for better temperature control.
Just by adjusting the motor speed,
the compressor speed, the refrigerant flow rate,
and the cooling capacity can be controlled accurately.
Here comes an interesting design challenge for the air conditioner.
The compressor of an air conditioner
is designed to handle only vapor,
and the small fraction of the liquid content can affect its performance
and damage the compressor.
For these reasons,
it is desired that the evaporator convert
all of the liquid to vapor,
and even increase the temperature of the vapor
by 5 to 8 degrees celsius after the conversion.
How does it make sure that the fluid
which enters the compressor is purely vapor?
This condition is maintained by a special kind of expansion valve
called a ‘thermostatic expansion valve’.
The abbreviation is TXV.
The TXV is similar to the expansion valve we saw earlier.
But here, the temperature of a bulb controls the needle.
The bulb is connected to the evaporator exit,
the refrigerant inside the bulb
is separated from the main refrigeration cycle by a diaphragm.
When the bulb is hot,
the refrigerant inside it will be evaporated
and the needle will move down.
We know the sudden cooling of the refrigerant
is achieved across the expansion valve
with the help of the restriction.
The narrower the restriction,
the cooler the outlet refrigerant will be.
Let’s consider a bad situation for the compressor.
Assume the evaporator temperature is not so low,
so the evaporator absorbs very low heat,
and all the liquid in it will not convert to vapor.
Thus the refrigerant which leaves for the compressor
will not be super heated.
This low temperature at the evaporator exit
will immediately be sensed by the bulb,
and will cause the needle to move upwards
and make the restriction narrower.
The narrower restriction causes a good temperature drop.
Thus the evaporator will receive much cooler fluid
and it can absorb a great amount of heat.
This will make sure that all the liquid will be converted to vapor.
Thus the complex task of having a vapor-only condition
that the compressor in that is automatically maintained by the TXV.
Let us clear up a common misconception at this stage.
TXVs do not control the room temperature.
They just make sure that the compressor receives the refrigerant in pure vapor form.
The refrigerant and flow rate in the room temperature
are controlled by the speed of the compressor.
Due to their simplicity and efficiency,
TXVs are widely used in the industrial and domestic applications.
To learn about TXVs and hundreds of other cooling topics,
please check out Danfoss-learning,
a free training and certification hub brought to you by DANFOSS.
or click the link in the video description.