A basic understanding of what happens in compression refrigeration
Refrigeration cycle is the movement of refrigerant that transfers heat from one heat exchanger to another in a refrigeration system. The two heat exchangers are the condenser and evaporator. See the photo above.
Refrigerant is the chemical substance that refrigeration systems use. Ammonia, propane and carbon dioxide are examples of refrigerants in refrigeration systems.
Refrigeration is the technology behind the workings of air conditioners, heat pumps, and refrigerators.
There are two main types of refrigeration cycles – compression refrigeration and absorption refrigeration cycles. But I talk about compression refrigeration cycle in this article.
While a compression refrigeration cycle system has a compressor that compresses and pumps refrigerant in the system, absorption refrigeration cycle uses an external heat source to heat refrigerant to boiling point.
I have used a split phase air conditioner system to expound on this subject.
Even though all air conditioner systems have components that function in a similar way, yours could be slightly different in shape and design.
Let’s refer to the refrigeration cycle in the diagram above:
The refrigerant moves from point 1 to 3, 4, 5, 6, and then back to point 1. Component 1 is the compressor, 3 condenser coil, 4 refrigerant filter, 5 expansion device, and 6 evaporator coil.
Metal tubing joins together the five components in a closed circuit that forms a refrigerant flow path. Copper is the most used metal because it has a better resistance to oxidation.
As the refrigerant moves inside the tubes, it changes state from high pressure-high temperature to low pressure-low temperature. The change of state between point 3 and 6 is what causes air conditioning effect.
Components 1, 2, 3, 4, and 5 enclosed in the black line rectangle make up the condenser unit. Components 6 and 7 in the blue rectangle form the evaporator unit. These condenser and evaporator units have their electric control circuitries attached inside their individual casings as shown below:
Components in a refrigeration cycle:
Compressor motor is the heart of a compression refrigeration cycle. As the name denotes, it compresses and pumps refrigerant in the air conditioner tube system. The compressor has three refrigerant lines:
- Discharge line carries compressed high-pressure 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 a refrigeration cycle, let us look at two of the thermodynamic laws that make refrigeration possible.
Law one: “… in a closed system, you can neither create nor destroy energy, but can change it from one form to another.”
Law 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’s suction line draws vaporous refrigerant from evaporator coil through the return line, compresses it, and pumps it into the condenser coil.
Component 2 is the condenser fan that blows ambient air across the condenser tube. Ambient air is still, and its temperature is lower than that of the refrigerant in the condenser coil. The fan facilitates the air movement to enhance cooling.
Component 3 is the condenser coil, also known as 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 outside environment. 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 into liquid form. Condensation is a vital aspect of the refrigeration cycle because, in the stage that follows, which is the evaporation stage, liquid refrigerant uses every bit of heat energy around the evaporator to boil and evaporate. Hence, a space void of heat energy is created.
Streamlined condenser fins direct airflow across the condenser coil. Read about cleaning your condenser to know why keeping your condenser dust-free ensures optimum heat exchange.
Component 4 is the refrigerant filter drier. It is a copper or steel container with desiccant.
It is connected in series with the refrigerant tube. Its position is just before the refrigerant enters the expansion device. Its duty is to absorb – actually adsorb – water and moisture that could accidentally find its way into the refrigerant line. Moisture or water has the potential to condense and freeze, blocking the metering device. Such a blockage can damage the compressor.
The filter also blocks semi-solid impurities that come from internal wear and tear. Such could block the metering device if it is a capillary tube.
There are different types and sizes of filters for domestic and commercial applications.
Component 5 is the metering or expansion device. It restricts refrigerant flow to reduce the pressure of refrigerant entering 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 is a variable-opening type. The size and type of your installation will determine which metering device you use. But they both do the same work of refrigerant restriction.
Component 6 is the evaporator coil. Also known as the 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 boiling to occur, the refrigerant absorbs and uses heat energy from the space around the evaporator coil. Heat deficiency occurs where refrigerant has drawn heat. The result is cold temperature around the coil.
Component 7 is the evaporator fan. The refrigerant in evaporator tube absorbs heat energy around the tube. Then the fan facilitates circulation of the remaining low-temperature air around the evaporator coil into the conditioned space.
This split-unit evaporator fan is different from the condenser fan in design. It is cylindrical and aligns with the evaporator coil, stretching from one end of the evaporator coil to the other.
Refrigeration cycle summary
- The compressor suction line draws vaporized refrigerant from the low-pressure side of the evaporator. It compresses and pumps the high-pressure high-temperature refrigerant into the condenser coil.
With the pump having done some work of compression to the refrigerant, the now hot refrigerant loses the heat it absorbed from the evaporator to the outdoor environment.
- Expansion device restricts liquid refrigerant flow into the evaporator, reducing its pressure. Immediately low-pressure refrigerant enters the evaporator, it begins to boil.
- The refrigerant in the evaporator absorbs and uses 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.
- At the end of the evaporator coil, the vaporized refrigerant is sucked back into the compressor and the refrigerant cycle starts again. The conditioned room becomes cooler and cooler in the process.
If an installer sets up an air conditioner with precision, it will go on for years before any major breakdown occurs. What is important, though, is regular maintenance. Just as your car needs regular service, your air conditioner does too.
A functional thermostat and other controls keep your air conditioner working within its capability range.