A
scroll compressor is a device for compressing air or refrigerant. It is used in
air conditioningAn air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle...
equipment, as an automobile
superchargerA supercharger is an air compressor used for forced induction of an internal combustion engine.The greater mass flow-rate provides more oxygen to support combustion than would be available in a naturally aspirated engine, which allows more fuel to be burned and more work to be done per cycle,...
(where it is known as a
scroll-type superchargerThe scroll-type supercharger is a positive displacement orbiting-spiral supercharger. It is a compromise between the more rugged rotating lobe, and the more efficient sliding vane type superchargers, and is considered to offer the highest potential in regard to efficiency, noise and pressure...
) and as a
vacuum pumpA vacuum pump is a device that removes gas molecules from a sealed volume in order to leave behind a partial vacuum. The first vacuum pump was invented in 1650 by Otto von Guericke.- Types :Pumps can be broadly categorized according to three techniques:...
.
A scroll compressor operating in reverse is known as a
scroll expander, and can be used to generate
mechanical workIn physics, work is a scalar quantity that can be described as the product of a force times the distance through which it acts, and it is called the work of the force. Only the component of a force in the direction of the movement of its point of application does 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.
History
Léon Creux first patented a scroll compressor in 1905 in France and the US (Patent number 801182).
Creux originally invented the compressor as a
rotaryThe rotary engine was an early type of internal-combustion engine, usually designed with an odd number of cylinders per row in a radial configuration, in which the crankshaft remained stationary and the entire cylinder block rotated around it...
steam engineA steam engine is a heat engine that performs mechanical work using steam as its working fluid.Steam engines are external combustion engines, where the working fluid is separate from the combustion products. Non-combustion heat sources such as solar power, nuclear power or geothermal energy may be...
concept, but the metal casting technology of the period was not sufficiently advanced to construct a working prototype, since a scroll compressor demands very tight tolerances to function effectively. The first practical scroll compressors did not appear on the market until after
World War IIWorld War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
,when higher-precision machine tools permitted their construction. They were not commercially produced for air conditioning until the early 1980s.
Design
A scroll compressor uses two interleaving scrolls to
pumpA pump is a device used to move fluids, such as liquids, gases or slurries.A pump displaces a volume by physical or mechanical action. Pumps fall into three major groups: direct lift, displacement, and gravity pumps...
,
compressA gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
or pressurize
fluidIn physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....
s such as liquids and
gasGas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
es. The vane geometry may be
involuteIn the differential geometry of curves, an involute is a curve obtained from another given curve by attaching an imaginary taut string to the given curve and tracing its free end as it is wound onto that given curve; or in reverse, unwound. It is a roulette wherein the rolling curve is a straight...
,
archimedean spiralThe Archimedean spiral is a spiral named after the 3rd century BC Greek mathematician Archimedes. It is the locus of points corresponding to the locations over time of a point moving away from a fixed point with a constant speed along a line which rotates with constant angular velocity...
, 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.
Another variation is with flexible (layflat) tubing where the
archimedean spiralThe Archimedean spiral is a spiral named after the 3rd century BC Greek mathematician Archimedes. It is the locus of points corresponding to the locations over time of a point moving away from a fixed point with a constant speed along a line which rotates with constant angular velocity...
acts as a
peristaltic pumpA peristaltic pump, or roller pump, is a type of positive displacement pump used for pumping a variety of fluids. The fluid is contained within a flexible tube fitted inside a circular pump casing . A rotor with a number of "rollers", "shoes" or "wipers" attached to the external circumference...
, which operates on much the same principle as a toothpaste tube. They have casings filled with
lubricantA lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles and of distributing heat...
to prevent abrasion of the exterior of the pump tube and to aid in the dissipation of heat, and use reinforced tubes, often called 'hoses'. This class of pump is often called a 'hose pumper'. Furthermore, since there are no moving parts in contact with the fluid, peristaltic pumps are inexpensive to manufacture. Their lack of valves, seals and glands makes them comparatively inexpensive to maintain, and the use of a hose or tube makes for a low-cost maintenance item compared to other pump types.
Applications
- Air conditioner compressor
- Vacuum pump
A vacuum pump is a device that removes gas molecules from a sealed volume in order to leave behind a partial vacuum. The first vacuum pump was invented in 1650 by Otto von Guericke.- Types :Pumps can be broadly categorized according to three techniques:...
- Supercharger
A supercharger is an air compressor used for forced induction of an internal combustion engine.The greater mass flow-rate provides more oxygen to support combustion than would be available in a naturally aspirated engine, which allows more fuel to be burned and more work to be done per cycle,...
s for automobile applications, e.g. VolkswagenVolkswagen is a German automobile manufacturer and is the original and biggest-selling marque of the Volkswagen Group, which now also owns the Audi, Bentley, Bugatti, Lamborghini, SEAT, and Škoda marques and the truck manufacturer Scania.Volkswagen means "people's car" in German, where it is...
's G-LaderThe G-Lader is a scroll-type supercharger used in various Volkswagen Passenger Cars models. Its purpose is to increase the motive power output from the internal combustion engine attainable with a given engine displacement. Since it is not enough to simply inject more fuel, as this produces too...
.
Engineering comparison to other pumps
These devices are known for operating more smoothly, quietly, and reliably than conventional compressors in some applications. Unlike pistons, the orbiting scroll’s mass can be perfectly counterbalanced, with simple masses, to minimize vibration. (However, an orbiting scroll cannot be balanced if Oldham coupling is used.) The scroll’s gas processes are more continuous. Additionally, a lack of dead space gives an increased volumetric efficiency.
Rotations and pulse flow
The compression process occurs over approximately 2 to 2½ rotations of the crankshaft, compared to one rotation for rotary compressors, and one-half rotation for
reciprocating compressorA reciprocating compressor or piston compressor is a positive-displacement compressor that uses pistons driven by a crankshaft to deliver gases at high pressure....
s. The scroll discharge and suction processes occur for a full rotation, compared to less than a half-rotation for the reciprocating suction process, and less than a quarter-rotation for the reciprocating discharge process. However, reciprocating compressor have multiple cylinders (typically, anywhere from two to six), while scroll compressors only have one compression element. The presence of multiple cylinders in reciprocating compressors reduces suction and discharge pulsations. Therefore, it is difficult to state whether scroll compressors have lower pulsation levels than reciprocating compressors as has often been claimed by some suppliers of scroll compressors. The more steady flow yields lower gas pulsations, lower sound and lower vibration of attached piping, while having no influence on the compressor operating efficiency.
Valves
Scroll compressors never have a suction valve, but depending on the application may or may not have a discharge valve. The use of a dynamic discharge valve is more prominent in high pressure ratio applications, typical of refrigeration. Typically, an air-conditioning scroll does not have a dynamic disharge valve. The use of a dynamic discharge valve improves scroll compressor efficiency over a wide range of operating conditions, when the operating pressure ratio is well above the built-in pressure ratio of the compressors. However, if the compressor is designed to operate near a single operating point, then the scroll compressor can actually gain efficiency around this point if there is no dynamic discharge valve present (since there are additional discharge flow losses associated with the presence of the discharge valve as well as discharge ports tend to be smaller when the discharge is present).
Efficiency
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 so called under-compression losses that occur at high pressure ratio operation of the positive displacement compressors that do not have a dynamic discharge valve.
There is an industry trend toward developing systems operating on CO
2 refrigerant. While CO
2 has no ozone depletion potential and essentially no
direct global warming potential, it is very difficult to achieve a reasonable cycle efficiency using CO
2 as compared to other conventional refrigerants, without having substantial expenditures on enhancing the system with large heat exchangers, vapor injection options, expanders, etc. In case of CO
2, the reciprocating compressor appears to offer the best option, as it is difficult to design an efficient and reliable scroll compressor for this application.
The scroll compression process is nearly one hundred percent volumetrically efficient in pumping the trapped fluid. The suction process creates its own volume, separate from the compression and discharge processes further inside. 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. That remnant gas from the last cycle then occupies space intended for suction gas. 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.
Reliability
Scroll compressors have fewer "moving parts" than reciprocating compressors which, theoretically, should improve reliability. According to Copeland, a big manufacturer of scroll compressors, scroll compressors have 70 percent fewer moving parts, while comparing with the conventional reciprocating compressors.
In 2006 a major manufacturer of food service equipment, Stoetling, chose to change the design of one of their soft serve
ice creamIce cream is a frozen dessert usually made from dairy products, such as milk and cream, and often combined with fruits or other ingredients and flavours. Most varieties contain sugar, although some are made with other sweeteners...
machines from
reciprocatingA reciprocating compressor or piston compressor is a positive-displacement compressor that uses pistons driven by a crankshaft to deliver gases at high pressure....
to scroll compressor. They found through testing that the scroll compressor design delivered better reliability and energy efficiency in operation. However, many refrigeration applications rely on reciprocating compressors, which appear to be more reliable in these applications than scroll compressors. These applications include supermarket refrigeration and truck trailer applications.
Vulnerabilities
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.
A reciprocating compressor can run in either direction and still function properly, whereas a scroll compressor must rotate in one direction only in order to function. This can be important during extremely short periods of power loss when a scroll compressor may be forced to run backward from the pressure in the discharge line. Only single phase scroll compressors would continue to run in reverse after the power comes back on. If this happens, the scroll compressor will stop pumping. Running scroll compressor in reverse for several minutes would normally not damage the compressor. The
three-phaseIn electrical engineering, three-phase electric power systems have at least three conductors carrying voltage waveforms that are radians offset in time...
compressor, as compared to single phase compressors, would revert to operation in a forward direction at the end of a short power interruption. However, it is important to properly wire the three-phase compressor during the initial installation. If during the installation the polarity is inadvertently reversed then the three-phase compressor would run backward and the damage to the compressor may result if it goes unnoticed for long period of time. Interestingly, one of the ways to mitigate the flooded operation of the compressor on start up, is to actually run the compressor for several minutes in the reverse direction before turning the compressor in the forward direction. The short reverse run on the start up would expel any liquid accumulated inside the compressor pumping element back into the crankcase, as well as preheat the liquid stored in the crankcase by dissipated motor heat. Expelling the liquid from the pumping element and preheating any liquid refrigerant in the crankcase prior to initiating the normal run in the forward direction significantly alleviates problems with the flooded start.
Size
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. Thus the scroll mechanism is itself more tolerant of liquid ingestion but at the same time is more prone to experience it in operation. Small size of a scroll compressor and quiet operation allows 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.
Partial loading
Until recently, scroll compressors could only operate at full capacity when powered. Modulation of the capacity was accomplished outside the scroll set. 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-cycles the normal refrigeration cycle and allows some of the partially compressed gas to return back 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.
Recently, scroll compressors have been manufactured that provide part-load capacity within a single compressor. These compressors change capacity while running.
Reciprocating compressors often have better unloading capabilities than scroll compressors. Reciprocating compressors operate efficiently in unloaded mode when flow to some of the cylinders is completely cut off by internal solenoid valves. Two stage reciprocating compressors are also well suited for vapor injection (or so called economized operation) when partially expanded flow is injected between the first and second compression stages for increased capacity and improved efficiency. While scroll compressors can also rely on vapor injection to vary the capacity, their vapor injection operation is not as efficient as for the case of reciprocating compressors. This inefficiency is caused by continuously changing volume of the scroll compressor compression pocket during the vapor injection process. As the volume is continuously being changed the pressure within the compression pocket is also continuously changing which adds inefficiency to the vapor injection process. In case of a two stage reciprocating compressor the vapor injection takes place between the two stages, where there is no changing volume. Both scroll and reciprocating compressors can be unloaded from mid-stage compression, however reciprocating compressors are also more efficient for this mode of unloading than scroll compressors, because the unloaded port dimensions in case of scroll is limited by the internal port size, which would not be the case for a reciprocating compressor where unloading again occurs from between the two stages.
EmersonEmerson Electric Company is a major multinational corporation headquartered in Ferguson, Missouri, United States. This Fortune 500 company manufactures products and provides engineering services for a wide range of industrial, commercial, and consumer markets.Emerson is one of the largest...
manufactures a scroll compressor that is capable of varying the refrigerant flow as per requirement. Instead of fixing the scrolls together permanently, the scrolls are allowed to move apart periodically. As the scrolls move apart, the motor continues to turn but the scrolls lose the ability to compress refrigerant, thus motor power is reduced when the scroll compressor is not pumping. By alternating the two different working states: the loaded state and the unloaded state. A solenoid contracts and expands the rotating scroll and/or the fixed scroll, using axial compliance. The controller modifies the load time, and the unload time, matching the capacity of the compressor to the load requested. This type of scroll compressors while offering variable capacity control , normally down to 20% of the full flow, can suffer from a significant loss of efficiency especially toward the lower range of the capacity control.
See also
- Vacuum pump
A vacuum pump is a device that removes gas molecules from a sealed volume in order to leave behind a partial vacuum. The first vacuum pump was invented in 1650 by Otto von Guericke.- Types :Pumps can be broadly categorized according to three techniques:...
- Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
- Pump
A pump is a device used to move fluids, such as liquids, gases or slurries.A pump displaces a volume by physical or mechanical action. Pumps fall into three major groups: direct lift, displacement, and gravity pumps...
- Involute
In the differential geometry of curves, an involute is a curve obtained from another given curve by attaching an imaginary taut string to the given curve and tracing its free end as it is wound onto that given curve; or in reverse, unwound. It is a roulette wherein the rolling curve is a straight...
- Involute gear
The involute gear profile is the most commonly used system for gearing today. In an involute gear, the profiles of the teeth are involutes of a circle. In involute gear design contact between a pair of gear teeth occurs...
- Compressed air battery
A compressed air battery stores energy in the form of compressed air and uses it to generate electricity.. A compressed air battery uses air to drive a scroll expander or other device, which in turn drives a conventional generator, to produce electricity...
External links