James R. Stringer
The refrigeration cycle for all cooling systems generally follow the same guidelines be it for warehouses, skyscrapers, or fridges. We will be covering the compression refrigeration cycle in this article. Please see the above diagram to get a quick overview of the cycle and its major components.
Compressors remove low pressure vapour from the evaporator and increases the pressure, and thus temperature, of the vapour above the cooling medium of the condenser. The high pressure vapour refrigerant from the compressor then proceeds to the condenser. The three major types of compressors are reciprocating (positive displacement; like a car engine), rotary (positive displacement; like two gears interlocked while rotating), and centrifugal (dynamic; an impeller that sucks instead of pushing).
Condensers remove the heat from the now high pressure vapour provided from the compressor and facilitates a change of state from a vapour to a liquid. The basic types of condensers are air-cooled, water-cooled, air and water-cooled (evaporative).
Air-Cooled Condensers have refrigerant travel through tubes with fins that are exposed to air blown via a forced or induction fan. The air absorbs the heat from the tubes and fins, and rejected to the atmosphere.
A Shell and Tube Condenser is the most common water-cooled condenser. The steel shell has tubes running across the length of the chiller. Cooling water flows through the lower tubes and then makes a second pass through the upper tubes. High pressure refrigerant enters at the top of the shell and condenses as it contacts the cooled tubes, collecting as a liquid at the shell bottom.
Evaporative Condensers use both air and water to absorb the heat from the high pressure refrigerant vapour flowing through coils. Water is sprayed from above the coils down to a sump which has a pump that circulates the water back to the top. At the same time, a fan is drawing air from below, causing a counter flow to the water spray. The interaction between the air and water causes an evaporative effect with the water, producing a cooling effect from the absorption of latent heat.
A Liquid Receiver is required to provide a tank to collect liquid refrigerant as well as provide a location for changes in the refrigerant charge of the system as the volume varies depending on the system’s load requirement.
The Expansion Valve is a metering device that controls the flow of liquid refrigerant into the evaporator based on the rate of vapourization occurring in the evaporator.
Evaporators are where low pressure liquid refrigerant flows through tubing, absorbing the heat from the cooling medium and evaporates into a low pressure vapour. The evaporator may be located in a coil inside of an air handling unit, or in a straight bare tube shell above the shell and tube condenser.
Once the low pressure vapour refrigerant flows through the evaporator, it will then be removed by the compressor and the cycle continues. However, an Accumulator is located after the evaporator and before the suction of the compressor. The accummulator collects any liquid refrigerant that has not changed states and is heated by the vapour refrigerant overtime until it is vapourized. This helps with oil return back to the compressor and prevents liquid entering the compressor causing slugging; liquid entering the compressor might mean you’ll need a new compressor!
A Direct refrigeration system is where the evaporator is in direct contact with the material or space that is to be cooled. For example, an evaporator inside of an air handling unit absorbs the heat from outdoor air and supplies it to the building at a required supply temperature set point.
An Indirect refrigeration system is where a medium, such as water, is cooled by the refrigerant and the medium is circulated to the space that is to be cooled. For example. water from a cooling coil entering a chiller to reject the heat from a space it has provided cooling for. Another example of indirect systems would be for a hockey rink that needs the freezing effect for the ice surface by using a brine (salt water) that can flow while below the freezing temperature of water.
Bonus Picture!
References:
Power Engineering Third Class Edition 2.5; Part B2; Chapter 10: Refrigeration Principles and Systems