Refrigerant Cycle In Air Conditioner System

air conditioner refrigerant cycle
Schematic diagram and photos of refrigerant cycle

This article is a general understanding of what happens in a compression refrigerant cycle so that you know where to look at in case your air conditioner is not working right. Read about absorption refrigerant cycle in this article, and refer to this article for 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 components that serve the same purpose. There can be a slight difference in shape or design.

In the diagram above

Referring to the diagram above, refrigerant cycle involves movement of refrigerant from point 1, to 3, 4, 5, 6, and then back to point 1. Component 1 is the compressor, 3 condenser coil, 4 filter drier, 5 expansion device, and 6 evaporator coil.

The components are joined using metal tubing, 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.

Components 1, 2, 3, 4, and 5 enclosed in the triangle with black line constitute the condenser unit. Components 6 and 7 in the blue triangle form the evaporator unit. These condensing and evaporator units have their control circuitries attached inside their individual cabinets as shown below:

outdoor and indoor unit control circuitry
Cut out images of condenser and evaporator units’ circuitries

Compressor motor

ac compressor
Piston 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:
  • Discharge line carries compressed refrigerant to the condenser coil.
  • Return line carries low-pressure refrigerant that flows back into the compressor.
  • Charging line is used to charge refrigerant into the system.
To explain what happens during the refrigerant cycle, let us look at two of the thermodynamic laws that make air conditioning possible.
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 refrigerant cycle begins and ends at the compressor. The compressor pump draws vaporous refrigerant from evaporator coil through return line, compresses and pumps it to the condenser coil.

Refrigerant is the refrigeration gas that circulates in an AC system.


Condenser fan

fan-blade and motor
Condenser fan

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. Because ambient air is still, condenser fan facilitates its movement around the coil, hence improving cooling efficiency.

Condenser Coil

outdoor heat exchanger
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. As the refrigerant 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 (in this case the refrigerant) to a low temperature one (in this case air surrounding condenser).
Inside the condenser, vaporous refrigerant condenses to liquid form. Condensation is a vital aspect of the refrigerant cycle because, in the stage that follows, which is 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 about cleaning your condenser to know why it is important to keep condenser coil dust-free for an optimum heat exchange. Refrigerator and AC condensers do similar jobs.


refrigerant filters
Two types of AC filters. Screw connection and solder connection.

Component 4 is the refrigerant filter. It is connected in series with the refrigerant-carrying tube, 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. This can damage the compressor.

The filter also blocks semi-solid impurities that could block expansion device.

There are different types and sizes of filters. 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). A 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

split unit ac 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. For this boiling to occur, the refrigerant absorbs and use 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, how to clean your AC air filter, why a clean evaporator coil ensures a good AC performance.

Evaporator fan

indoor heat exchanger blower
Cylindrical evaporator fan 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 vaporised 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. Immediately low-pressure refrigerant enters the evaporator, it begins to boil.

3. Refrigerant in the evaporator absorbs and use 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 control the AC operation.

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