The vapor compression cycle is the primary cycle used in commercial refrigeration systems.
- Understanding Boiling Points
- Step 1: Evaporator
- Step 2: Compressor
- Step 3: Condenser
- Step 4: Expansion Device
Step 2: Compressor: The next step is where the refrigerant gas is compressed by the compressor, which raises the temperature and pressure of the gas. The compressor is where the work takes place. The compressor is also the driving force that moves the refrigerant through the vapor compression cycle and prepares the refrigerant before it enters the condenser. It is important that the refrigerant gas is raised to a temperature that is above the temperature of the fluid in the condenser. This will allow heat to be transferred from the refrigerant to the condenser fluid. The compression of the refrigerant gas occurs isentropically, meaning that there is no change in entropy. Since the compressor is not completely efficient there will be an increase in enthalpy as the heat generated by the compressor is transferred to the refrigerant gas.
Entropy - a measure of the amount of disorder in a thermodynamic system.
Enthalpy - a measure of the total energy in a thermodynamic system (sensible and latent energy).
The engineer should be knowledgeable of the 5 different types of compressors and their advantages and disadvantages, in order to determine when they should be used. The five types of compressors are centrifugal, scroll, reciprocating, screw and rotary. A brief overview of the different types of compressors is shown below.
- Rotary: The rotary type compressor compresses refrigerant gas through positive displacement.
Positive displacement simply means that the pressure of the gas is increased by reducing the volume.
- Scroll: Similar to the rotary type compressor, the scroll compressor uses positive displacement to increase the pressure of the gas.
- Screw: Similar to the rotary type compressor, the scroll compressor uses positive displacement to increase the pressure of the gas. The screw compressor consists of two interlocking screws. The gas moves through the screw from the beginning thread to the end thread, increasing the pressure as it moves to the discharge side.
- Reciprocating: A reciprocating compressor compresses gas through positive displacement. A piston type movement compresses gas as it enters the cylinder.
- Centrifugal Centrifugal compressors are not like positive displacement compressors, these compressors rely on a rotating impeller to use its centrifugal force to move the gas to the outside diameter of the rotating impeller, which increases the velocity of the gas. The increased velocity is then translated into increased pressure.
Another distinction between compressors is made between hermetic, semi-hermetic and open drive compressors.
Hermetic is most often recognized when used in the phrase “hermetic seal”, which means airtight.
Hermetic: A hermetic compressor is airtight. The compressor and motor are located in a welded container, so no refrigerant can escape. Since the motor is located in the same enclosure as the compressor, the compressor needs to account for the motor heat.
Open Drive: An open drive compressor indicates that the compressor and refrigerant are located in an enclosure and out of the enclosure is a shaft connecting it to a motor. The motor is outside of the enclosure and the heat is lost to the space and not to compressor.
Semi-Hermetic: A semi-hermetic is similar to a hermetic compressor, except the motor and compressor are located in a mechanically sealed container, which can be opened without cutting into the enclosure unlike the hermetic compressor.