Compressors - Rotary, Positive Displacement Type Compressors

Rotary, Positive 
Displacement Type 
Compressors

Rolling piston (fixed vane) type compressors: 

• Rolling piston or fixed vane type compressors are utilized in small refrigeration Systems (upto 2 kW capacity) like domestic refrigerators or air conditioners. 

• These compressors belong to the category of positive displacement type as compression is achieved by reducing the quantity of the refrigerant.

• In this sort of compressors, the shaft of the roller has its axis of rotation that matches with the centerline of the cylinder, however, it's eccentric with respect to the roller Figure. This eccentricity of the shaft with reference to the roller creates suction and compression of the refrigerant as shown in fig. A single vane or blade is positioned in the non-rotating cylindrical block. 
• The rotating motion of the roller causes a reciprocating motion of the only vane. 

As shown in Figure this sort of compressor doesn't require a suction valve but requires a discharge valve. The sealing between the high and low pressure sides has to be provided: 

• Along the road of contact between roller and engine block 

• Along the line of contact between vane and roller, and 

• between the roller and end-pates 

The leakage is controlled through hydrodynamic sealing and matching between the mating components. The effectiveness of the sealing depends on the clearance, compressor speed, surface finish and oil viscosity. Close tolerances and good 

surface finishing is required to minimize internal leakage. 

Unlike in reciprocating compressors, the tiny clearance volume crammed with 

high-pressure refrigerant doesn't expand, but simply mixes with the suction 

refrigerant in the suction space. As a result, the volumetric efficiency does not 

reduce drastically with increasing pressure ratio, indicating small re-expansion 

losses. The compressor runs smoothly and is comparatively quiet because the

refrigerant flow is continuous.

Multiple vane type compressors: 

• As shown in Figure in multiple vane type compressor, the axis of rotation coincides with the middle of the roller (O), however, it's eccentric with reference to the center of the cylinder (O’). The rotor consists of variety of slots with sliding vanes. 

• During the running of the compressor, the sliding vanes, which are normally made from non-metallic materials, are held against the cylinder thanks to centrifugal forces. 

• The number of compression strokes produced in one revolution of the rotor is adequate to the number of sliding vanes, thus a 4-vane compressor produces 4 compression strokes in one rotation. 

• In these compressors, sealing is required between the vanes and cylinder, between the vanes and therefore the refore the slots on the rotor and between the rotor and the end plate. 

• However, since pressure difference across each slot is merely a fraction of the total pressure difference, the sealing isn't as critical as in fixed vane type compressor. 

• This type of compressor doesn't require suction or discharge valves, however, as shown in Figure, check valves are used on discharge side to stop reverse rotation during off-time due to pressure difference. 

• Since there are no discharge valves, the compressed refrigerant is opened to the discharge port when it has been compressed through a hard and fast volume ratio, depending upon the geometry. 

• This implies that these compressors have a hard and fast built-in volume ratio. The built-in volume ratio is defined as “the ratio of a cell because it is closed faraway from the suction port to its volume before it opens to the discharge port”.

• Since no centrifugal force is present when the compressor is off, the multiple vanes will not be pressed against the cylinder walls during the off-period. As a result, high pressure refrigerant from the discharge side can flow back into the side and pressure equalization between high and low pressure sides take place.

• This is beneficial from the compressor motor point-of-view because it reduces the specified starting torque. However, this introduces cycling loss thanks to the entry of high and hot refrigerant liquid into the evaporator. 

•  Hence normally a non-return check valve is used on the discharge side which prevents the entry of refrigerant liquid from high pressure side into evaporator through the compressor during off-time, at an equivalent time there'll be pressure equalization across the vanes of the compressor. 

Characteristics of rotary, vane type compressors: 

• Rotary vane type compressors have low mass-to-displacement ratio, which in combination with compact size makes them ideal for transport applications.

• The compressors are normally oil-flooded type, hence, oil separators are required.

• Both single-stage (upto –40oC evaporator temperature and 60oC condensing temperature) and two-stage (upto –50oC evaporator temperature) compressors with the cooling capacity within the range of two to 40 kW are available commercially. 

• The cooling capacity is normally controlled either by compressor speed regulation or suction gas throttling. 

• Currently, these compressors are available for a good range of refrigerants like R 22, ammonia, R 404a etc.