Feature
However, development is a never-ending process, and efforts are
going on to develop more efficient compact, reliable and quiet
compressors. New types such as linear compressors, trochoidal
compressors, acoustic compressors, digital scroll, inverter
compressors and oil less compressors are being introduced in
refrigeration and air conditioning applications,” says Bell.
RECIPROCATING AIR CONDITIONER
COMPRESSOR
The reciprocating compressor uses a piston driven by a
crankshaft in a straight line back and forth motion to compress
the refrigerant. This rotary motion is achieved using an electric
motor and the construction is quite similar to that of an
automobile engine.
The piston moves up and down inside a cylinder. Vapour from
the suction line is moved through the intake valve as the pistons
move downward. As the piston moves upward, it compresses
the vapour refrigerant which is then pushed through the exhaust
valve into the condenser.
The compressor may have more than one cylinder which is
also known as a multi-cylinder compressor. The common ones are
the two-cylinder, four-cylinder and eight-cylinder compressors.
The compressor is available in semi hermetic and hermetic.
Semihermetic reciprocating compressors are used for
commercial and industrial refrigeration applications, while
hermetic reciprocating compressors are used for residential
refrigeration and commercial refrigeration applications.
“These are the most widely used in the commercial
refrigeration sector. A ‘hermetic’ compressor essentially means
that the housing has been sealed airtight,” adds Bell.
SCROLL
A scroll compressor has one fixed scroll which remains stationary
and another moving or orbiting scroll that rotates through the
use of a swing link. When this happens, the pockets of refrigerant
gas between the two scrolls are slowly pushed to the centre of
the two scrolls causing the reduction of the volume of the gas. It
is then discharged through the centre port to the condenser.
The advantage of a scroll compressor is that it has fewer
moving parts and less torque variation compared to the
reciprocating compressor. This advantage is translated to a
smooth and quiet operation.
The scroll compressor is also known as a scroll pump or scroll
vacuum pump. Scroll compressors are mainly used for residential
and commercial air conditioning as well as commercial
refrigeration applications.
SCREW
The screw compressor uses a pair of helical rotors where it traps
and compresses the refrigerant gas as the rotors revolve in a
IMD
cylinder. In HVAC, they are usually used in systems with 20-ton
capacity and above. The male rotor and the female rotor are built
inside the cylinder. The low-pressure refrigerant enters one end
of the compressor and the resultant high-pressure refrigerant is
discharged from the opposite end to the condenser.
Screw Compressors are used for larger industrial
refrigeration systems and medium capacity air conditioning /
process cooling chillers.
ROTARY
The rotary compressor can be divided into two types. One has
blades or vanes that rotate with the shaft. The other type has the
blade which remains stationary and is part of the compressor
housing assembly. In both types, the vapour from the suction line
is drawn into the cylinder through the suction port.
Rotary vane compressors consist of a cylindrical casing,
two openings – one suction and one discharge – and a rotor
positioned eccentrically with respect to the casing. Compression
occurs by refrigerant flowing into the chamber where, due to
eccentric rotation, there is a reduction in the desired volume.
As the blade rotates, trapped vapour in the space ahead
of the blade is compressed into high pressure gas after which
it is discharged to the condenser through the exhaust port.
The number of blades can range from two to eight in a single
system. These are mainly found in residential air conditioning
applications.
CENTRIFUGAL
Centrifugal compressors are usually used in large capacity
refrigerating systems. In this type of compressor, the refrigerant
vapour or gas is moved in a circular motion known as centrifugal
force. An impeller which is a disk with radial blades spins rapidly
inside this housing causing the gas to gain velocity.
Continued on page 21
A compressor
is used in
different
applications.
www.hvacronline.co.za RACA Journal I August 2020 19