What is a refrigerant compressor?
• A machine that is used to compress the vapor
refrigerant from the evaporator and to raise
its pressure so that the corresponding
saturation temperature is higher than that of
Classification of compressors
Acc. To method of
Acc. To no. of
Acc. to the method of
Acc. To the
location of prime
• A reciprocating
compressor or piston
compressor is a positive-
displacement compressor that
uses pistons driven by
a crankshaft to deliver gases at
• Applications include oil
plants, and refrigeration plants.
• Compressors 1-50 HP are
• Rotary compressors are positive displacement compressors.
• They are commonly used to replace piston compressors
where large volumes of high pressure air are needed,
either for large industrial applications or to operate high-
power air tools such as jackhammers.
• Rotary screw compressors use two meshing helical screws,
known as rotors, to compress the gas.
• Gas enters at the suction side and moves through the
threads as the screws rotate. The meshing rotors force the
gas through the compressor, and the gas exits at the end of
Typically, they are used to supply compressed
air for general industrial applications. Trailer
mounted diesel powered units are often seen at
construction sites, and are used to power air
operated construction machinery.
• It is a dynamic High Speed
compressor which depends on transfer
of energy from a rotating impeller to
• They achieve a pressure rise by
adding kinetic energy/velocity to a
continuous flow of fluid through the
rotor or impeller. This kinetic energy is
then converted to an increase
in potential energy/static pressure by
slowing the flow through a diffuse
• These types of compressors are also
designed for higher capacity because
flow through the compressor is
Direct Drive Compressors
• A direct drive compressor is an
compressor that can be hooked
right to a product that needs
air without having to fill an air
• They are also usually self
lubricating which means that
no oil is usually required.
• They tend to draw low
amperage and can handle
pressures around 110psi.
• provide compactness and
minimum drive maintenance.
Compressors can be flange
mounted or direct-coupled to
the power source.
Belt Drive compressors
• They are most commonly
used with electric motors
and internal combustion
• V-Belt drives provide
great flexibility in
load to power source
load and speed at
minimum cost. Belts
must be properly
shielded for safety.
Advantages & Disadvantages for Belt Drive
1. Belt drives are easier to install.
2. Absorb minor misalignments.
3. Relatively cheaper than direct drives.
1. Induces higher loads on compressor bearings and support system. Reduces life
by approx. 6-8 times.
2. May need additional drive supports to handle bending loads.
3. Requires relatively more frequent checks and maintenance.
4. Has lower transmission efficiencies.
• P.S. For direct drives just flip the pros & cons for the belt drives. Realistically, belt
drives are used for relatively smaller units and lower HPs, while direct drives are
used for larger units and higher HPs.
• A hermetic or sealed compressor is one
in which both compressor and motor
are confined in a single outer welded
• The motor and compressor are directly
coupled on the same shaft, with the
motor inside the refrigeration circuit.
Thus the need for a shaft seal with the
consequent refrigerant leakage
problem was eliminated.
• Hermetic compressors are ideal for
small refrigeration systems, where
continuous maintenance (replenishing
refrigerant and oil charge etc) cannot
be ensured. Hence they are widely
used in domestic refrigerators, room
air conditioners etc.
• With reference to the above figure, One can see
the cooper windings inside the outer shell and
also the refrigerant connections (copper pipes).
• Since, the motor is in the refrigerant circuit, the
efficiency of hermetic compressor based systems
is lower as the heat dissipated by the motor and
compressor becomes a part of the system load.
• Since the complete system is kept in a welded
steel shell, the hermetic compressors are not
meant for servicing. A variation of hermetic
compressor is a semi-hermetic compressor, in
which the bolted construction offers limited
Advantages and Disadvantages
1. Leakage of refrigerant removed
2. Less noisy and compact.
3. Lubrication is simple as the motor and
compressor operate in a sealed space with the
1. Maintenance is not easy coz the moving parts
2. A separate pump is reqd. for evacuation and
charging of refrigerant.
Semi Hermetic compressors
• A semi hermetic compressor is a sealed
mechanical device that is used to regulate gas
pressure by varying the volume of the gas.
• The way a semi hermetic compressor works is
that a motor is energized by current then in
turn rotates a crankshaft, during the down
stroke, a piston creates low pressure between
the cylinder head and the piston and during the
upstroke the compressed gas force a high
pressure on the outside system.
• Due to the semi-hermetic design such compressors are very compact and
occupy less space compared to open type machines thus saving expensive
building and plant room costs.
• The foot print and weight of a semi-hermetic machine is 70% of that of the
corresponding open type machine. These compressors are hence easier to
handle and do not require heavy foundations like open type machines.
• The common problem of refrigerant leakage through the shaft seal is not
face with the semi-hermetic compressor because the motor is directly
mounted on the compressor crankshaft.
• Semi-hermetic compressors do not require any pulleys and belts and have
no alignment problems and no wear and tear problem of the d rive
• Such compressors are also quieter and produce less vibration compared to
the open type. This reason alone makes them the ideal choice for roof-top