Scroll Compressors

A Scroll compressor is a type of rotary type compressor. They are a positive displacement machine that uses the compression action provided by two intermeshing spiral-shaped scrolls, in which one is fixed and the other orbiting, (ASHRAE, 2004).

According to ASHRAE’s HVAC Systems and Equipment Handbook, “Scroll compressors are orbital motion, positive-displacement machines that compress with two interfitting, spiral-shaped scroll members.”

Typical scroll compressors for automobile air conditioning system consists of the following components: Scrolls, Casing, shaft, bearing, refrigerant chamber, rubber seal, balancing spring, magnetic clutch and pressure valve

Construction of Scroll Compressor :

• It is rotary, orbital motion, and a positive displacement compressor with hermetic motor.
• The motor shaft bearings and the orbiting scroll journal bearings are lubricated by oil supplied by a centrifugal pump which is submerged in the oil sump.
• It has two inter fitting matching spiral-shaped scrolls assembled 1800 out of phase configuration to perform the gas compression.
• A scroll is an involute spiral open at one end of the vane and bound by a base plate on the other.
  When the two scrolls are placed together; they form a series of gas pockets between the two mating spirals.
• One of the two scrolls, the fixed scroll is held stationary, while the other, the orbiting scroll is made to orbit, but not rotates around a fixed point on the stationary scroll. (Refer Fig. )

• The flanks of the scroll remain in contact, although the contact locations move progressively inwards.
• Relative motion between the pair is prevented by interconnecting couplings; which maintains a relative angle between the pair of scrolls.
• The scroll tips are fitted with seals that ride on the surface of the opposing scroll and act like piston rings, providing a gas seal at the mating surface.
• The intake of the suction gas occurs at the outer edge of the scroll assembly and discharge occurs through the port at the center of the stationary scroll.
• The center of the scroll journal bearing and the center of the motor shaft of the drive assembly are offset and this offset imparts an eccentric or orbiting motion to the driven scroll.

Scroll Compressor Animation :

Working of Scroll Compressor :

• The orbiting motion causes the mated scrolls to form gas pockets and as the orbiting continues, the relative movement between the orbiting scroll and the fixed scroll causes the scroll to move towards the discharge port at the center of the assembly with constantly decreasing and increasing the pressure of the gas.

(i) First shaft revolution (Suction phase):

• When the shaft rotates, the orbiting scroll orbits and opens suction port creates space between two scrolls.
• At the completion of the first revolution, the scroll lateral surfaces again meet; forming gas pockets. [Refer Fig. (a)].

(ii) Second shaft revolution (Compression phase):

• Further rotation of shaft; by sealing suction gas in pockets of given volume at the outer periphery of the scrolls; and the volume of the gas pockets is progressively reduced and the gas is compressed.
• At the completion of the second revolution produces maximum compression of gas occurs. [Refer Fig. (b)].

(iii) Third revolution (Discharge phase):

During the third revolution of shaft; the compressed gas is discharged through the discharge port.

• At the completion of the third revolution, the volume of the gas pockets is reduced to zero, squeezing the remaining gas from the scrolls.
• In the complete cycle, the three phases intake, compression, and discharge occurs simultaneously in an on-going sequence. [Refer Fig. (c)].

Advantages of scroll Compressor:

Scroll compressor advantages are as follows, 

1) It has very high compression, mechanical and volumetric efficiency; thus it has a higher energy efficiency ratio also.
2) It has fewer moving parts and hence lower mechanical losses.
3) It can handle liquid refrigerant flooding back from the evaporator during fluctuating operating conditions and design to start at full load.
4) It does not require a crankcase heater and/or suction line accumulator.
5) It has almost laminar/uniform/continuous flow of refrigerant and hence lower noise level as well as no need for discharge muffler.
6) As the suction, compression, and discharge processes take place simultaneously; thus the smooth flow of gas reduces gas pulsations and lower the flow losses.
7) No metal to metal contact.
8) No suction and discharge valves; thus lower noise level and lesser chances of compressor ceasing & compressor motor failure.
9) Compact in size and weight hence easy to handle and maintain.
10) The system is perfectly balanced; thus extremely low vibrations & noise.

Disadvantages of Scroll Compressor :

Despite of all the above advantages scroll compressor has some disadvantages as follows,

• Compressor cannot be disassembled in the field for maintenance
• Incremental capacity control on systems with multiple compressors

Applications of scroll Compressor : 

Applications of scroll compressors are as follows, 

1) In residential and commercial refrigeration & air conditioning.
2) Central heat pump.
3) Automotive air-conditioning.

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