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Rotary Compressors:-
In rotary compressors, the vapour
refrigerant from the evaporator is compressed due to the movement of blades.
The rotary compressors are positive displacement type compressors. Since the
clearance in rotary compressors is negligible, therefore they have high
volumetric efficiency. These compressors may be used with refrigerants R-12,
R-22, R-114 and ammonia. Following are the two basic types of rotary
compressors
- Single
stationary blade type rotary compressor.
A single stationary blade type rotary compressor is shown in
Fig. 9.14. It consists of a stationary cylinder, a roller (or impeller) and a
shaft. The shaft has an eccentric on which the roller is mounted. A blade is
set into the slot of a cylinder in such a manner that it always maintains
contacts with the roller by means of a spring. The blade moves in and out of
the slot to follow the rotor when it rotates. Since the blade separates the
suction and discharge ports as shown in Fig. 9.14, therefore it is often called
a sealing blade. When the shaft rotates, the roller also rotates so that it
always touches the cylinder wall. Fig. 9.14 (a) to (d ) shows the various
positions of roller as the vapour refrigerant is compressed. Fig. 9.14 (a)
shows the completion of intake stroke (i.e. the cylinder is full of low
pressure and temperature vapour refrigerant) and the beginning of compression
stroke. When the roller rotates, the vapour refrigerant ahead of the roller is
being compressed and the new intake from the evaporator is drawn into the
cylinder, as shown in Fig. 9.14 (b). As the roller turns towards mid position
as shown in Fig. 9.14 (c), more vapour refrigerant is drawn into the cylinder
while the compressed refrigerant is discharged to the condenser. At the end of
compression stroke, as shown in Fig. 9.14 (d), most of the compressed vapour
refrigerant is passed through the discharge port to the condenser. A new charge
of refrigerant is drawn into the cylinder. This, in turn, is compressed and
discharged to the condenser. In this way, the low pressure and temperature
vapour refrigerant is compressed gradually to a high pressure and
temperature.
Hermetic Compressors
When the compressor and
motor operate on the same shaft and are enclosed in a common casing, they are
known as hermetic sealed compressors. These types of compressors eliminate the
use of crankshaft seal which is necessary in ordinary compressors in order to
prevent leakage of refrigerant. These compressors may operate on either
reciprocating or rotary principle and may be mounted with the shaft in either
the vertical or horizontal position. The hermetic units are widely used for
small capacity refrigerating systems such as in domestic refrigerators, home
freezers and window air conditioners. The hermetic sealed compressors have the
following advantages and disadvantages
Advantages
- The leakage of refrigerant is completely prevented.
- It is less noisy.
- It requires small space because of compactness.
- The lubrication is simple as the motor and compressor operate in a sealed space with the lubricating oil.
Disadvantages
1. The maintenance is not easy because the moving parts are inaccessible.
2. A separate pump is required for evacuation and charging of refrigerant. Crankshaft
Internal working of a welded hermetic compressor.
