What exactly do refrigeration compressors do?

The compressor is often called the Heart of the refrigeration system. It pumps the refrigerant throught the system and circulates it again and again in cycles.


The function of any compressor is to establish a pressure difference in the system annd thus to create a flow of the refrigeration from one part of the system to the other.

Different types of refrigeration compressors

1. Based on the method of compression

  • Positive displacement compressors
    • Reciprocating type
    • Rotary type
    • Screw type
  • Dynamic compressors or centrifugal compressors

2. Based on the construction and operation

  • Single cyclinder
  • Multi Cylinder

3. Based on the location of prime mover

  • Independent or open type compressor
  • Semi-hermetically Sealed compressor
  • Hermetically sealed compressor


Positive displacement type compressors function by reducing the volume of gas in the confined space, thereby raising its pressure.

Reciprocating, rotary, and crew compressors are positive displacement type compressors.


This is the most widely used type, available in different sizes and its construction is similar to the reciprocating engine/pump, with pistons, cylinders, valves, connecting rods, and crankshaft.

Reciprocating compressors obtain their name from the back and forth motion of the piston in the cylinder. These are capable of moving fluids up to 100,000 cubic feet per minute(CFM) at up to 35,000 PSI (Where 1 bar = 14.5 PSI) depending upon the number of stages employed.

Open type reciprocating compressor

Open-type compressors are indirectly driven compressors. This has an exposed shaft to which the electric motor or other driver is attached externally.


  • The upper portion of the compressor consists of a piston-cylinder casing, suction valve and discharges valve.
  • The lower portion is the crank-case which is filled with lubricating oil.
  • The outer casing is made of either cast iron or aluminium.
  • The piston is connected to the crank-saft throught connecting rod and crank.
  • The suction and dischange valve are arranged on the sylinder head.
  • Gas seal is provided in the housing around the crank shaft to prevent air coming inside the housing and to prevent the leakage of refrigerant.
  • Lubricating of reviprocating compressor is accoumplised y either splash or forced lubricating.

Working Principle

Suction Stroke:

The low-pressure refrigerant vapour from the evaporator is drawn in through the suction valve during the suction stroke of the piston.

Discharge stroke:

The vapour gets compressed to high pressure and high temperature during the discharge stroke of the piston and is sent out of the compressor through the discharge valve to the condenser.


Hermetic compressors are manufactured with both compressor and motor sealed in casting as shown in. In this way, there is no possibility of refrigerant loss and the motor is cooled by the refrigerant.

Hermetic compressors are usually classified as:

  1. Serviceable type – repairing is possible.
  2. Non-serviceable type – repairing cannot be easily performed.


  • Hermetic types are usually direct-driven compressors, where the motor directly driven the compressor.
  • In this type, the motor and the compressor are enclosed in the same housing to prevent the leakage problem fully.
  • The cylinder is cast integral with the main body casing in which the stator is pressed.
  • The motor is located above the compressor operates in a vertical position whereas the compressor operates in a horizontal position. The permits, operation of the compressor in oil, simplifying the lubricating problem.
  • Separation of oil and liquid refrigerant is assured due to rotation of the rotor and the oil sluggery is eliminated.
  • The only connections to the compressor housing are suction and discharge fittings and terminals of the motor.

Working principle

Same as that of open type reciprocating compressor.


  • Semi- hermeticlly type compressoes are directly driven compressors.
  • In this type, the motor and compressor are in separate housing as shown.
  • The cylinder heads are bolted to the main body of the cylinder, which encloses the motor, and the compressor as in hermetically sealed compressor.
  • The opening of the hermetically sealed compressor is done by hacksaw cutting, but in semi- hermetically type, it is done by loosening of bolts.
  • Other constructional details and the working principles are as same as that of hermetically sealed compressor.

Differences between sealed and semi-sealed compressor

sealed compressor Semi- sealed compressor
Compressor and motor are enclosed in a single housing Compressor and motor are enclosed in separate housing
Housing is made by electric welding Housing is made with help of bolts and nuts
Repairing is difficult Easy to repair
service valve is provided No service valve
New version Old version


Rotary compressors are positive displacement, direct-driven compressors. Rotary compressor capable of moving fluids up to 50,000 CFM but only at very low pressures, less than 15 PSL.

The rotary compressor has a rotor eccentric to the casing; as the rotor turns, it reduces the gas volume and increases its pressure.

Rotary compressors are classified as:

  1. Rolling piston type or stationary blade type.
  2. Rolling vane type or rotating blade type
  3. Screw compressor



  • The shaft is eccentric with the cylinder wall and when it is rotated, a continuously changing point of the roller is close to the cylinder wall.
  • A single blase, which is mounted ion cylinder, is always in contact with the roller by means of a spring.
  • Suction and dischange side of the compressor are separate by a sprng loaded valve.
  • The wholw assemblt is enclosed in a housing filled with oil and remain submerged in oil.


The low-pressure suction vapour gets into the increasing space formed between the roller and cylinder wall when the high side of the roller revolves.

The low-pressure vapour already there ahead of the high side of the eccentric roller is compressed and sent out through the discharge port and valve.


  • In this type, the roller is mounted on an eccentric shaft with four vanes.
  • The vanes slide within the roller (rotor) but keep contact with the cylinder by means of springs.
  • The assembly of rotor and the vanes is off-center with respect to the cylinder.
  • The while assembly is enclosed in a housing filled with oil and remains submerged in oil.


The low-pressure gas enters the cylinder and is imprisoned between two or more blades as the blades rub against the wall of the cylinder.

As the rotor revolves, the low-pressure gas is gradually compressed into high-pressure high-temperature conditions. Now the gas, occupying very little space comes out of the compressor through the exhaust port.



Rotary screw compressor also belongs to the category of positive displacement compressors. This type of compressor has become popular in recent years due to its reliability, efficiency and cost. IT is generally used in larger capacities of up to about 1000 tons of air conditioning.


  • This type of compressor consists of two helically grooved rotors which rotate in a housing.
  • The male motor consists of lobed and is normally the driving rotor.
  • The female rotor consists of fullies and is normally the driven rotor.
  • There are three basic continous steps in screw type compressors, such as
    • Suction
    • Compression and
    • Discharge


When the male rotor turns clockwise, the refrigerant enters into the space between pair housing, and when re-meshing starts, the volume is decreased and the pressure rises.

The charge is moved helically and compressed until the trapped volume reaches the discharge end.


These types of compressors are non-positive displacement type compressors, which are also called dynamic compressors.

Centrifugal compressors function by increasing the kinetic energy (velocity) of the gas, which is then converted to increased pressure by reducing the velocity.



  • This type of compressor has vane impellers rotating inside a casing, similar to a centrifugal pump.
  • These are capable of moving fluids at up to 150,000 CFM at up to 5000 PSI, depending upon the number of stages.
  • A single stage centrifugal compressor mainly consists of four components.
  1. Inlet Casing – to accelerate the fluid to the impeller inlet.
  2. Impeller – to transfer the energy to the fluid in the form of static pressure and kinetic energy.
  3. Diffuser – to convert the kinetic energy at the impeller outlet into pressure energy.
  4. Volute casing – to collect the fluid and to further convert the kinetic energy.


The suction vapour enters the rotor impeller through the intake port it’s taken out at the periphery of the impeller due to centrifugal action. The high velocity and thereby high pressure are thus imparted to the entering vapour y the impeller wheel.

The vapour from the impeller then enters different blades where the kinetic energy of the vapour further gets converted into high-pressure vapour & passes through the volute casing to the outlet point.

Differences between reciprocating compressor and rotary compressor

Reciporcating compressorRotary compressor
1. Compression is done by the reciprocating action of the piston.Compression is done by the rotary motion of the moving part.
2. Single and multi-cylinder are available.Only one casing.
3. Used in industrial and domestic refrigerators.Used only in domestic refrigerators.
4. Refrigerant enters through the suction valve.Refrigerant enters through the check valve.

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