Refrigerant Cycle In Air Conditioner System

refrigeration cycle image
Schematic diagram and photos showing refrigeration cycle

This article is for general user understanding of what happens in a compression refrigerant cycle so that a user can know where to look at in case an air conditioner is not working right. Read about absorption refrigerant cycle in this other article. Also, refer to this other article if you would like a more technical insight on refrigerant cycle.

I have used a split phase air conditioner system to expound on this subject. All air conditioner systems have the same components, albeit with some slight difference in shape or design.

In the diagram above

According to the diagram above, refrigerant cycle is the movement of refrigerant from point 1, to 3, 4, 5, 6, and then back to point 1. These components are joined using metal tubing which is mostly copper. As the refrigerant moves in the tubes, it changes state from high pressure-high temperature to low pressure-low temperature. The resultant refrigerant change of state at point 3 and 6 is what causes air conditioning effect. We’ll see about that shortly.

Component 1 is the compressor, 3 condenser coil, 4 filter drier, 5 expansion device, and 6 evaporator coil. Refrigerant flows in them.

Components 1, 2, 3, 4, and 5 enclosed in the black triangle constitute the condenser unit, which is that photo below the schematic diagram. And components 6 and 7 in the blue triangle, from the evaporator unit, which is the photo above the schematic diagram. These condensing and evaporator units have their control circuitries attached inside their individual cabinets as shown here below:

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Cut out images of condenser and evaporator units’ circuitries, with top covers removed

Compressor motor

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24,000 BTU AC compressor
Compressor motor is the “heart” of a compression system air conditioner. As the name denotes, it compresses and pumps refrigerant in the AC tubing system. The compressor has three refrigerant lines:
(a) Discharge line which carries compressed refrigerant to the condenser coil.
(b) Return line from where refrigerant flows back into the compressor.
(c) Charging line used to charge refrigerant into the system.
To help you understand what happens during the refrigerant cycle, let us look at two of the thermodynamic laws that make air conditioning possible. Simply put, in relevance to air conditioning:
Law number one: …in a closed system, you can neither create nor destroy energy, but can change it from one form to another.
Law number two: …heat moves from a high-temperature material to a low-temperature material. But with some “work,” heat can move in the reverse direction. 
So the compression refrigeration cycle begins and ends at the compressor. The compressor pump draws vaporous refrigerant from the evaporator coil through the return line, compresses, and pumps it to the condenser coil.
Refrigerant is the chemical substance that circulates in an AC system. Refrigerant behaves uniquely when under different temperature and pressure. 


Condenser fan

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Condenser fan that blows ambient air across the condenser coil fins

Component 2 is the condenser fan that blows ambient air across condenser coil. Ambient air temperature is basically lower than that of the refrigerant in the condenser coil. And because it is still, condenser fan facilitates its movement around the coil, hence improving cooling efficiency.

Condenser Coil

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Air Conditioner condenser coil
Component 3 is the condenser coil or outdoor heat exchanger. The compressor pumps high-pressure high-temperature refrigerant into this condenser. And as it flows from point a to b, its heat is rejected to the atmospheric air around the condenser. Remember heat moves from a high-temperature material to a low temperature one.
In the condenser, vaporous refrigerant condenses to liquid form. Condensation is a vital aspect of the refrigerant cycle because, in the next evaporation stage, liquid refrigerant needs to boil and evaporate to achieve desirable heat transfer.
Streamlined condenser fins play an important role in directing air flow across the condenser coil. Read in the Maintenance Procedures article why it is important to keep condenser coil dust-free for an optimum AC performance.


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Two types of AC filters

Component 4 is the refrigerant filter connected in series with the refrigerant-carrying tubing. It is placed just before refrigerant enters the expansion device. Its duty is to absorb moisture that could accidentally find its way into refrigerant line during installation. Moisture has the potential to condense and freeze, thereby blocking expansion device – a recipe for the danger that can damage the compressor.

The filter also blocks semi-solid impurities that could block expansion device.
There are different types and sizes of filters, and selection is done according to what fits a particular application.

Expansion device

expansion device images
Thermostatic expansion valve and capillary tube

Component 5 is the expansion device. It also goes by the name metering device. Its work is to restrict refrigerant flow thereby reducing refrigerant pressure as it enters evaporator coil.

Two examples of expansion devices are capillary tube and Thermostatic Expansion Valve (TEV). Capillary tube is a fixed opening type expansion device and TEV a variable opening type. Whichever is installed in a system depends on the application. But both do the same work of refrigerant restriction.

Evaporator Coil

indoor heat exchanger
Indoor heat exchanger with front cover removed to reveal the coil

Component 6 is the evaporator coil. Also known as indoor heat exchanger or indoor unit, it is where the air conditioner’s actual cooling effect takes place. What happens here is that as low-pressure refrigerant enters the evaporator from the expansion device, it begins to boil. And for this boiling to occur, the refrigerant absorbs heat energy from the space around the evaporator coil. Space from where heat has been removed remain heat-deficient, and thus the room temperature is lowered.

You can also read in the maintenance and service article why a clean evaporator coil ensures a good AC performance.

Evaporator fan

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Cylindrical evaporator fan exposed in the evaporator unit assembly

Component 7 is the evaporator fan that facilitates room air circulation across the evaporator coil, as refrigerant absorbs heat energy from that air.

This split-unit fan is different from the condenser fan in design but serves the same purpose. It is cylindrical and aligns with the evaporator coil, stretching from one end of the evaporator coil to the other end.

In summary

1. The compressor suction line draws vapourised refrigerant from the low-pressure side evaporator. It compresses and pumps the high-pressure high-temperature refrigerant into the condenser coil.
The now hot refrigerant loses that heat to the outdoor environment as it condenses into liquid inside the condenser.

2. Expansion device restricts liquid refrigerant flow into the evaporator, reducing its pressure as a result. And immediately low-pressure refrigerant enters the evaporator, it begins to boil.

3. Refrigerant in the evaporator absorbs heat energy around the evaporator coil to boil, leaving the space surrounding the coil without heat. Thus space where heat is removed is left cool. The conditioned room, that is.

4. At the end of evaporator coil, vapourised refrigerant is sucked back into compressor and the refrigerant cycle starts again. The conditioned room becomes cooler and cooler in the process.

To maintain the desired temperature of a conditioned room, installed air conditioner thermostat, and other controls, automatically do the work.

An air conditioner installed with precision should serve well for years without a major breakdown. What is important though is regular maintenance. Just as your car needs regular service, your air conditioner does too.

Watch this short video for another angle of refrigerant cycle explanation:


Now read: Air conditioner maintenance and service



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