The Refrigeration Cycle

The refrigeration cycle is undoubtedly the most important concept in the HVAC field. Air-cooled chillers, water-cooled chillers, cooling towers, air-cooled heat pumps, water-cooled heat pumps, condensing units, commercial refrigeration equipment, etc. All of these types of equipment rely on the same working principle, but differ slightly in application.

There are four basic components to a refrigeration cycle: the compressor, condenser, expansion valve, and evaporator. Circulating between these components is a compound known as refrigerant. Refrigerant is incredibly useful due to its low boiling point and high heat of evaporation. These qualities allow the refrigerant to change phase and transfer heat in a way that we can take advantage of. Common types of refrigerant include: R-22, R-134A, and R-410A.

Let’s take a look at the above figure. The refrigerant will start at point #1. At this stage the refrigerant exists as a low temperature, liquid-vapor mixture. To start the process, the refrigerant passes through the evaporator from point #1 to point #2. The refrigerant undergoes a process of evaporation. The evaporator is a simple heat exchanger. There will either be a fan to blow air over the evaporator or a circulating water loop to transfer heat into the refrigerant. The main goal is to take heat from the space we are conditioning and transfer it into the refrigerant. This process will boil the refrigerant into a saturated vapor. From point #2 to point #3, the refrigerant will pass through the compressor and undergo the process of compression. The compressor will take the warm vapor and squeeze it into a superheated vapor. From point #3 to point #4, the refrigerant passes through the condenser and undergoes condensation. The condenser, similar to the evaporator, is a simple heat exchanger. In the case of the evaporator, we were taking heat from the space we were conditioning and transferring it into the refrigerant. In the case of the condenser, we are doing the opposite. We are now rejecting heat from the refrigerant to the outside. This can be done through condenser fans (also known as an air-cooled system) or through a circulating water loop connected to a cooling tower (also known as a water-cooled system). There are advantages to using one system over the other, but that would be better served for a different article.

But we can already see how we cooled the space. We absorbed heat through the evaporator (inside) and rejected it through the evaporator (outside). Heat was simply moved from the inside to the outside. This process is different from something like a direct fired gas furnace. In that instance, the furnace is fighting to raise the temperature by adding more and more energy into the space. This results in efficiencies of less than 100%. Instead, by moving the heat, efficiencies can rise far above 100% (cooling output/energy input).

Back to the refrigeration cycle, to finish up. At point #4, the refrigerant exists as a saturated liquid. It then passes to point #1 through the expansion valve. This process of rapid expansion allows the refrigerant to cool to a liquid-vapor mixture. The refrigerant is now ready to pass through the evaporator again and absorb more heat. The purpose of the compression stage was to create enough of a pressure differential to allow for this rapid expansion and cooling (known as the Joule-Thomson effect). Each part of the refrigeration cycle plays an interconnected and critical role for moving heat from one place to another.

This whole process can also be reversed to move heat from the outdoors to the inside during winter. This is how a heat pump works. To understand how this works, simply place the condenser (heat rejection) inside and the evaporator (heat absorption) outside. You do not need to physically replace the equipment; heat pumps have a reversing valve that automatically reverses the entire flow.