Your Information on How Air Conditioners Work...

It is amazing when you really think about how air conditioners work. Scorching heat from the sun outside your personal space could already bake a cake. Heat inside is almost life threatening.

But switch on your air conditioner, a few noises here and there, and cool air rules.

But how air conditioners work? Where does all the cooling come from?


Pre-syndication on how air conditioners work:

In the absolute basics, air conditioners used for cooling, work under five basic building blocks. How? Through refrigerant:

These five processes, work together to:

An air conditioner system in its simplest form consists of:

  • A compressor

  • A condenser coil

  • A receiver and dryer

  • An expansion valve

  • An evaporator coil

  • Fans

  • Tubing to make the system, a closed loop system

  • Refrigerant

Yes, air conditioners work in a closed loop arrangement and filled with necessary quantity of refrigerant to cool the air. That is the secret (well, not anymore) of how air conditioners work out the cooling effect.

Fundamentally, ACUs (air conditioning units) have two sides. The high pressure side and the low pressure side.

  • The high pressure side consists of the discharge of the compressor, the condenser, and the inlet of the expansion valve.

  • The low pressure side consists of the outlet of the expansion valve, the evaporator, and the suction of the compressor.



Schematic summary of how air conditioners work:



How Air Conditioners Work



  1. High pressure side. Refrigerant gas' temperature rises up due to compression from the compressor

  2. Outdoor air is passed through a heat exchanger named, condenser. Condensation of refrigerant happens here.

    Refrigerant will be cooled down - to liquid state, and air will be heated up.

    Motion of air is forced by condenser fan

  3. Warm liquid refrigerant

  4. Expansion valve or device. The device that allows pressure build up from the compressor

  5. Next, on how air conditioners work, the low pressure side. Expanded liquid refrigerant. Liquid refrigerant will "see" low pressure area after expansion valve, thus it will start to form refrigerant mist

  6. Refrigerant evaporation and boiling will happen at the evaporator, as temperature within the room (7), is higher than the refrigerant's boiling temperature.

    Room's warm air, will be passed through the evaporator, and cooled down. Forced draft is helped by the blower

  7. Boundary of the room

  8. Superheated refrigerant vapour at about 10 oC (50 oF). Useful for compressor's cooling

  9. Last, and by no means the least - the compressor itself. Cycle starts back from (1).

That was the pre-syndication part of how air conditioners work. Now, getting down to real business of its operations.


We’ll start with the high pressure side:


The compression cycle

Let us start at the compressor, the hard worker in how air conditioners work topic. This air conditioner part is used to compress the refrigerant gas from evaporator coil’s outlet.

I will use the simplest form of compressor to illustrate its working principle; the reciprocating compressor, also known as piston compressor.

As the driver, which is usually an electric motor, rotates; the piston is set to move on a continuous linear motion between two positions, within the casing.

There are two spring operated valves located at the end of the gas chamber. One is a suction valve, and another is a discharge valve. The suction valve is arranged such that, it will only open towards the chamber, and discharge valve away from the chamber.

Let us take a starting point when the piston is closest to the valves. At this point, let us also assume that both valves are in closed position.

As the piston moves away from the valves quickly, a space of low pressure is created within the chamber. Due to the negative pressure difference between the chamber and outside it, the suction valve will open, allowing refrigerant gas from suction side to rush into the enclosed space.

This happens as the force from the pressure difference overcomes the spring force.

Due to the construction of the compressor’s shaft, the piston will be on a momentary stop at the end of the suction cycle. During this moment, pressure difference between suction line and gas chamber is very little, allowing the spring force to pull the suction valve towards closed position.

Now what happens? The piston starts to move towards the valves again.

Compression will happen, because both valves are closed; and the piston is quickly reducing the volume of the gas.

Déjâ vu! I know you would say that. Compression takes place, and positive pressure difference between the gas chamber and outside it, will force open the discharge valve.

As a result, air conditioner's refrigerant gas will be forced out through the discharge line.

Quite slow process isn’t? Not quite. It’s very (and very is an understatement) quick. I can’t explain as fast as a 1000 over rpm reciprocating compressor. That is 17 suctions and 17 discharges in one second.

Piston compressor. How air conditioners work page

You might ask a question. “Ok fine, it draws in gas from suction line and discharges it to a discharge line. But if the discharge is to an open space, I won’t have any pressure build up!” You’re right.

That is where a form of restriction is required to allow pressure build up, but that is jumping the gun for the moment. Let’s move towards the next part in sequence. Believe me, how the sequence is arranged makes air conditioners work in correct manner.

That was how air conditioners work with the compression part. Over to...


The condensation world

Although I have not yet explained on how the high pressure is formed within air conditioners’ cycle, let us postulate that the pressure after the discharge line is high.

The rapid compression of the refrigerant will increase its pressure, and heat it up to approximately 40 oC (104 oF).

The exact figure depends on the compressor and refrigerant used. Nevertheless, air conditioner manufacturers have to select refrigerant and compressor, such that refrigerant temperature after compression will have higher temperature than the normal outdoor ambient temperature, for condensation to occur.

However, refrigerant after compression is still in gas form, until it meets the condenser.

A condenser is an air conditioner part, located after the compressor’s discharge line. It is essentially an equipment to aid heat exchange between a hot fluid (within the tube), and cooler fluid (outside the tube).

As the fluid passes through the condenser coil that is exposed to outdoor ambient air, heat is lost from gas, through condenser coil, fins and finally to ambient air.

Heat exchange depending on air convection, is just too slow. Hence a fan is located close to the condenser coil, to improve the condensation process.

Refrigerant looses heat, air gains heat.

The condenser coil is designed to allow enough cooling for the refrigerant to reach condensation point. This liquid refrigerant will go through a receiver. It is simply a storage tank and filtration unit, and useful during maintenance of major parts. Brumbaugh in his "HVAC Fundamentals. Volume 3: Air Conditioning, Heat Pumps, and Distribution Systems" book also stated that such receiver could be a separate tank.

Yes, finally it is in liquid form. High pressure liquid.

But why we have to compress it into liquid form? Keep reading through the next two sub sections.

This brings me to the next part of an air conditioner’s unit.


How air conditioners work in the expansion zone

Analogy

Imagine a very large number of people in the busy town of New York, on a Monday morning rushing to work. Everyone will be walking at a steady pace, until they meet the subway staircase. It is a bottleneck. Yes, the entrance will be slowed down, and people will start to pack up behind them. More and more commuters will be coming.

People at the back will give the "pressure" and people in the middle will feel the "pressure". However, once the staircase ends and wide passage is "seen", people in front will be spread out i.e. they have "seen" an area of low "pressure".

That is the principle of which the expansion valve operates. It is a restriction. The role is supremely important to allow refrigerant gas from compressor, to accumulate to a certain degree. Hence, it will create high pressure from the compressor’s discharge, all the way through the condenser, and inlet of the expansion valve.

This is the vital area of where the high pressure side and the low pressure side are separated.

The outlet part of the expansion valve is connected to the evaporator, which in turn, is connected to the suction side of the compressor.

Therefore, refrigerant at the outlet part of the expansion valve will meet low pressure side of the air conditioning cycle. As a result, it will begin to vaporize due to sudden release of pressure.

How air conditioners work next?


The low pressure side:


And it goes to the evaporator

You see, any substance will go through a heat exchange process as it heats up, cools down, condenses, boils or evaporates.

In this case the refrigerant evaporates from liquid to gas, and it needs energy in form of heat. It will take up heat from the surrounding area, outside the evaporator.

Refrigerant gains heat, surrounding area, vis-à-vis the surrounding air, looses heat. Absence of heat is what you and I know as COLD, or the cool sensation.

Did that answer your question on why we have to compress the refrigerant into liquid form? Yes. So that we can allow it to evaporate and draw in heat from its surrounding.

Hang on a minute, we have cool air already. How air conditioners work out the cool air to you? We put a nice little fan behind that evaporator and let it deliver the cool air to you. The temperature could go as low as 5 oC (41 oF).

That is how air conditioners work. Well, not just that. The cycle starts back at compressor's suction, as long as you have paid your electricity bills.

Phew. All the typing and explanation of how air conditioners work made me and the space quite warm over here. Let me switch on my air conditioner.

Now, that’s cool air just the way I like it!



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