Scroll Compressor Review

 
A scroll compressor (also called spiral compressor, scroll pump and scroll vacuum pump) is a free standing involute spiral which is bounded on one side by a solid flat plane, or base.
When a scroll compressor is operated in reverse rotation, it is known as a scroll expander, and can be used to generate mechanical work from the expansion of a fluid, compressed air or gas.
 
Scroll compressors are often used in residential HVAC heat pump and air conditioning systems and a few automotive air conditioning systems employ a scroll compressor instead of the more traditional rotary, reciprocating, and wobble-plate compressors.
Scroll Compressor Animation
 
The fundamental shape of a scroll is the involute spiral which is refered to as a "scroll set". The scroll set is the same profile used in gear teeth. An involute scroll set is a curve traced by a point on a thread kept taut as it is unwound from another curve. The curve that the thread is unwound from, that is, used for scrolls, is a circle.
 
A scroll set, is the fundamental compressing element of a scroll compressor. One scroll component is indexed or phased 180 degrees with respect to the other to allow the scrolls to mesh, as shown in Fig. 1. Crescent shaped gas pockets are formed bounded by the involutes and the base plates of both scrolls. As the moving or orbiting scroll is orbited about the fixed scroll, the pockets formed by the meshed scrolls follow the involute spiral toward the center and diminish in size (the motion is reversed for an expander or air motor). The orbiting scroll is prevented from rotating during this process to maintain the 180 degree phase relationship of the scrolls.
 
The compressor or vacuum pump's inlet is at the periphery of the scrolls. Air is drawn into the compressor as the inlet is formed as shown in Fig. 1 b, c, and d. The entering gas is trapped in two diametrically opposed gas pockets, Figure 1 a, and compressed as the pockets move toward the center.
 
The compressed gas is exhausted through the discharge port at the center of the fixed scroll. No valves are needed since the discharge is not in communication with the inlet. Figure 1 shows the scroll positions as the line connecting the centers of the two scrolls is rotated clockwise, illustrating how gas pockets diminish in size as the orbiting scroll is orbited.
 
Figure 1
Advantages:
 
Scroll refrigerant compressors, air compressors and vacuum pumps have the following advantages:
  •  Scroll compressors can achieve high pressure. The pressure ratio is increased by adding spiral wraps to the scroll. Pressures as high as 100 to 150 psig can be achieved in a single-stage air compressor.
  • Scroll compressors are true rotary motion and can be dynamically balanced for smooth, vibration-free, quiet operation.
  • There are no inlet or discharge valves to break or make noise and no associated valve losses.
  • Scroll compressors can be oil flooded, oil lubricated, or oil free. Due to the unique orbital motion, the rubbing velocities of the sliding seals are significantly less than piston rings or vanes for comparable speeds. Rubbing velocities are typically 30 to 50% less, resulting in greater durability.
  • Air is delivered continuously, therefore there is very little inlet or discharge pulsation and associated noise.
  • The scroll compressor has no clearance volume that gets re-expanded with associated losses. The compression is continuous.
  • Noise levels 3 to 15 dBA lower than other compressor technology are typical.
Disadvantages:
 
Although scroll compressors continue to expand into larger and smaller sizes, there are limitations. Since the scroll has a leakage path at the apex of the crescent shaped pockets, there are limits to how small a scroll compressor can be as a function of discharge pressure. Large displacement scroll compressors become large in diameter and the moving or orbiting scroll becomes massive. The maximum centrifugal force generated by the orbiting scroll gives a practical maximum size in a singlestage scroll.
 
Scroll compressors are more vulnerable to introduced debris, as any debris need to pass through at least two closed compression pockets. The scrolls that operate without radial and/or axial compliance are even more prone to the damage caused by foreign objects. However, scrolls do not have suction valves, which is one of the most vulnerable parts of the reciprocating compressor to liquid flooding.
 
Scroll compressors utilize different methods of protection inside the compressor to handle difficult situations. Some scroll designs utilize valves at different points in the compression process to relieve pressure inside the compression elements.

Applications:
 
Scroll compressors can primarily be used in those applications where its advantages are of benefit, specifically low vibration and noise, and durability. Although scroll compressors can be cost competitive, if cost is the most important factor, alternative technology should also be considered.
  • Residential air conditioning
  • Automotive air conditioning
  • Superchargers for reciprocating engines (E.G. Volkswagen)
  • Process controls
  • Pneumatic controls
  • Laboratory
  • Home health care
  • Medical and hospital
  • Computer peripherals
  • Optical equipment
Scroll compressors can be used where vane or reciprocating compressors are used. They can be dry or oil lubricated.
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