Many residential central 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…
- Cadet Mayank
A scroll compressor (also called spiral compressor, scroll pump and scroll vacuum pump) is a device for compressing air or refrigerant. It is used in air conditioning equipment, as an automobile supercharger (where it is known as a scroll-type supercharger) and as a vacuum pump.
A scroll compressor operating in reverse is known as a scroll expander, and can be used to generate mechanical work from the expansion of a fluid, compressed air or gas. Many residential central 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.
A scroll compressor uses two interleaving scrolls to pump, compress or pressurize fluids such as liquids and gases. The vane geometry may be involute, Archimedean spiral, or hybrid curves.
Often, one of the scrolls is fixed, while the other orbits eccentrically without rotating, thereby, trapping and pumping or compressing pockets of fluid between the scrolls. Another method for producing the compression motion is co-rotating the scrolls, in synchronous motion, but with offset centers of rotation. The relative motion is the same as if one were orbiting.
The isentropic efficiency of scroll compressors is slightly higher than that of a typical reciprocating compressor when the compressor is designed to operate near one selected rating point. The scroll compressors are more efficient in this case because they do not have a dynamic discharge valve that introduces additional throttling losses. However, the efficiency of a scroll compressor that does not have a discharge valve begins to decrease as compared to the reciprocating compressor at higher pressure ratio operation. This is a result of under-compression losses that occur at high pressure ratio operation of the positive displacement compressors that do not have a dynamic discharge valve.
The scroll compression process is nearly 100% volumetrically efficient in pumping the trapped fluid. By comparison, reciprocating compressors leave a small amount of compressed gas in the cylinder, because it is not practical for the piston to touch the head or valve plate. The reduction in capacity (i.e. volumetric efficiency) depends on the suction and discharge pressures with greater reductions occurring at higher ratios of discharge to suction pressures.
Figure 1. Principle of Scroll Compressor
Scroll compressors have fewer moving parts than reciprocating compressors which, theoretically, should improve reliability. According to Emerson Climate Technologies, manufacturer of Copeland scroll compressors, scroll compressors have 70 percent fewer moving parts than conventional reciprocating compressors.
In 2006 a major manufacturer of food service equipment, Stoelting, chose to change the design of one of their soft serve ice cream machines from reciprocating to scroll compressor. They found through testing that the scroll compressor design delivered better reliability and energy efficiency in operation.
Scroll compressors tend to be very compact and smooth running and so do not require spring suspension. This allows them to have very small shell enclosures which reduces overall cost but also results in smaller free volume. This is a weakness in terms of liquid handling. Their corresponding strength is in the lack of suction valves which moves the most probable point of failure to the drive system which may be made somewhat stronger. The small size and quiet operation of a scroll compressor allow for the unit to be built into high power density computers, like IBM mainframes. Scroll compressors also simplify the piping design, since they require no external connection for the primary coolant.
Figure 2. Horizontal Scroll Compressor
Partial Loading - Limitations
Until recently, a powered scroll compressor could only operate at full capacity. In order to achieve part-loads, engineers would bypass refrigerant from intermediate compression pocket back to suction, vary motor speed, or provide multiple compressors and stage them on and off in sequence. Each of these methods has drawbacks:
• Bypass short-circuits the normal refrigeration cycle and allows some of the partially compressed gas to return to the compressor suction without doing any useful work. This practice reduces overall system efficiency.
• A two-speed motor requires more electrical connections and switching, adding cost, and may have to stop to switch.
• A variable speed motor requires an additional device to supply electrical power throughout the desired frequency range. Also variable frequency drive associated with variable speed compressor has its own electrical losses, and is a source of additional significant cost and often is an additional reliability concern.
• Compressor cycling requires more compressors and can be costly. In addition, some compressors in the system may have to be very small in order to control process temperature accurately.
Figure 3. Components of Scroll Compressor
About 30% of Reefer Containers use Scroll Compressors mainly for their easy servicability. A scroll compressor used for Reefer Container weighs about 44 kgs compared to 125 kgs of a semi-hermetic compressor. This is particularly useful when servicing the Reefer Containers which are 2 or 3 tiers above the main deck.
Figure 4. Internal Parts of Carrier Scroll Compressor used in Refrigerated Containers
Limitations of Scroll Compressors
1. Since they are sealed units, field servicing of Scroll Compressors is not possible. Only replacement is recommended. The defective scroll compressors are landed with the manufacturers for reconditioning.
2. Scroll compressors are unidirectional compressors. Running in opposite direction may cause mechanical lockage, high current draw and burn out of the drive motors. Hence an additional phase reversal protective circuit is incorporated in the starting circuits. The incoming phase sequence is detected and corrected before going to the compressor.
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