Compressors - DOC
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Compressors 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. Liquids are relatively incompressible, so the main action of a pump is to pressurize and transport liquids. Types of compressors The main types of gas compressors are illustrated and discussed below: Centrifugal compressor Figure 1: A single stage centrifugal compressor Centrifugal compressors use a muskan rotating disk or impeller in a shaped housing to force the gas to the rim of the impeller, increasing the velocity of the gas. A diffuser (divergent duct) section converts the velocity energy to pressure energy. They are primarily used for continuous, stationary service in industries such as oil refineries, chemical and petrochemical plants and natural gas processing plants. Their application can be from 100 horsepower (75 kW) to thousands of horsepower. With multiple staging, they can achieve extremely high output pressures greater than 10,000 psi (69 MPa). Many large snow-making operations (like ski resorts) use this type of compressor. They are also used in internal combustion engines as superchargers and turbochargers. Centrifugal compressors are used in small gas turbine engines or as the final compression stage of medium sized gas turbines. Diagonal or mixed-flow compressor Diagonal or mixed-flow compressors are similar to centrifugal compressors, but have a radial and axial velocity component at the exit from the rotor. The diffuser is often used to turn diagonal flow to the axial direction. The diagonal compressor has a lower diameter diffuser than the equivalent centrifugal compressor. Axial-flow compressor An animation of an axial compressor. Axial-flow compressors are dynamic rotating compressors that use arrays of fan-like aerofoils to progressively compress the working fluid. They are used where there is a requirement for a high flow rate or a compact design. The arrays of aerofoils are set in rows, usually as pairs: one rotating and one stationary. The rotating aerofoils, also known as blades or rotors, accelerate the fluid. The stationary aerofoils, also known as a stators or vanes, decelerate and redirect the flow direction of the fluid, preparing it for the rotor blades of the next stage. Axial compressors are almost always multi-staged, with the cross-sectional area of the gas passage diminishing along the compressor to maintain an optimum axial Mach number. Beyond about 5 stages or a 4:1 design pressure ratio, variable geometry is normally used to improve operation. Axial compressors can have high efficiencies; around 90% polytropic at their design conditions. However, they are relatively expensive, requiring a large number of components, tight tolerances and high quality materials. Axial-flow compressors can be found in medium to large gas turbine engines, in natural gas pumping stations, and within certain chemical plants. Reciprocating compressors A motor-driven six-cylinder reciprocating compressor that can operate with two, four or six cylinders. Main article: Reciprocating compressor Reciprocating compressors use pistons driven by a crankshaft. They can be either stationary or portable, can be single or multistaged, and can be driven by electric motors or internal combustion engines. Small reciprocating compressors from 5 to 30 horsepower (hp) are commonly seen in automotive applications and are typically for intermittent duty. Larger reciprocating compressors well over 1,000 hp (750 kW) are still commonly found in large industrial and petroleum applications. Discharge pressures can range from low pressure to very high pressure (>6000 psi or 41.4 MPa). In certain applications, such as air compression, multi-stage double-acting compressors are said to be the most efficient compressors available, and are typically larger, noisier, and more costly than comparable rotary units. Rotary screw compressors Rotary screw compressor Rotary screw compressors use two meshed rotating positivedisplacement helical screws to force the gas into a smaller space. These are usually used for continuous operation in commercial and industrial applications and may be either stationary or portable. Their application can be from 3 horsepower (2.2 kW) to over 1,200 horsepower (890 kW) and from low pressure to very high pressure (>1200 psi or 8.3 MPa). Rotary vane compressors Rotary vane compressors consist of a rotor with a number of blades inserted in radial slots in the rotor. The rotor is mounted offset in a larger housing which can be circular or a more complex shape. As the rotor turns, blades slide in and out of the slots keeping contact with the outer wall of the housing.  Thus, a series of decreasing volumes is created by the rotating blades. Rotary Vane compressors are, with piston compressors one of the oldest of compressor technologies. With suitable port connections, the devices may be either a compressor or a vacuum pump. They can be either stationary or portable, can be single or multi-staged, and can be driven by electric motors or internal combustion engines. Dry vane machines are used at relatively low pressures (e.g., 2 bar) for bulk material movement whilst oil-injected machines have the necessary volumetric efficiency to achieve pressures up to about 13 bar in a single stage. A rotary vane compressor is well suited to electric motor drive and is significantly quieter in operation than the equivalent piston compressor. Scroll compressor Mechanism of a scroll pump A scroll compressor, also known as scroll pump and scroll vacuum pump, uses two interleaved spiral-like vanes to pump or compress fluids such as liquids and gases. The vane geometry may be involute, archimedean spiral, or hybrid curves. They operate more smoothly, quietly, and reliably than other types of compressors in the lower volume range Often, one of the scrolls is fixed, while the other orbits eccentrically without rotating, thereby trapping and pumping or compressing pockets of fluid or gas between the scrolls. This type of compressor was used as the super charger on Volkswagen G60 engines in the early 1990's. Diaphragm compressor A diaphragm compressor (also known as a membrane compressor) is a variant of the conventional reciprocating compressor. The compression of gas occurs by the movement of a flexible membrane, instead of an intake element. The back and forth movement of the membrane is driven by a rod and a crankshaft mechanism. Only the membrane and the compressor box come in contact with the gas being compressed. Diaphragm compressors are used for hydrogen and compressed natural gas (CNG) as well as in a number of other applications. A three-stage diaphragm compressor The photograph included in this section depicts a three-stage diaphragm compressor used to compress hydrogen gas to 6,000 psi (41 MPa) for use in a prototype compressed hydrogen and compressed natural gas (CNG) fueling station built in downtown Phoenix, Arizona by the Arizona Public Service company (an electric utilities company). Reciprocating compressors were used to compress the natural gas. The prototype alternative fueling station was built in compliance with all of the prevailing safety, environmental and building codes in Phoenix to demonstrate that such fueling stations could be built in urban areas. Temperature Gas laws Compression of a gas naturally increases its temperature. In an attempt to model the compression of gas, there are two theoretical relationships between temperature and pressure in a volume of gas undergoing compression. Although neither of them model the real world exactly, each can be useful for analysis. A third method measures real-world results: Isothermal - This model assumes that the compressed gas remains at a constant temperature throughout the compression or expansion process. In this cycle, internal energy is removed from the system as heat at the same rate that it is added by the mechanical work of compression. Isothermal compression or expansion more closely models real life when the compressor has a large heat exchanging surface, a small gas volume, or a long time scale (i.e., a small power level). Compressors that utilize inter-stage cooling between compression stages come closest to achieving perfect isothermal compression. However, with practical devices perfect isothermal compression is not attainable. For example, unless you have an infinite number of compression stages with corresponding intercoolers, you will never achieve perfect isothermal compression. Adiabatic - This model assumes that no energy (heat) is transferred to or from the gas during the compression, and all supplied work is added to the internal energy of the gas, resulting in increases of temperature and pressure. Theoretical temperature rise is T2 = T1·Rc(k-1)/k, with T1 and T2 in degrees Rankine or kelvin, and k = ratio of specific heats (approximately 1.4 for air). R is the compression ratio; being the absolute outlet pressure divided by the absolute inlet pressure. The rise in air and temperature ratio means compression does not follow a simple pressure to volume ratio. This is less efficient, but quick. Adiabatic compression or expansion more closely model real life when a compressor has good insulation, a large gas volume, or a short time scale (i.e., a high power level). In practice there will always be a certain amount of heat flow out of the compressed gas. Thus, making a perfect adiabatic compressor would require perfect heat insulation of all parts of the machine. For example, even a bicycle tire pump's metal tube becomes hot as you compress the air to fill a tire. Polytropic - This model takes into account both a rise in temperature in the gas as well as some loss of energy (heat) to the compressor's components. This assumes that heat may enter or leave the system, and that input shaft work can appear as both increased pressure (usually useful work) and increased temperature above adiabatic (usually losses due to cycle efficiency). Compression efficiency is then the ratio of temperature rise at theoretical 100 percent (adiabatic) vs. actual (polytropic). In the case of the fire piston and the heat pump, people desire temperature change, and compressing gas is only a means to that end. Isothermal compression takes less work than adiabatic (isentropic) compression. This can be shown for air using w=integral(v*dP) from P1 to P2 (from Tds=dh-vdp, 2nd law, and the 1st law) and Pvn=c=P1v1n where n=1 for isothermal and n=k=1.4 for adiabatic. Integrating for Isothermal: w=-P1v1*ln(P2/P1) For adiabatic: c=(P1v1/P)^(1/1.4) w=-(P1*v1)^.714*int(P^.714,P,P1,P2)=-(P1*v1)^.714/.286*(P2^.286-P1^.286). Plugging in numbers or graphing clearly shows that the absolute value of w(isothermal)<w(adiabatic). Staged compression In the case of centrifugal compressors, commercial designs currently do not exceed a compression ratio of more than a 3.5 to 1 in any one stage (for a typical gas). Since compression generates heat, the compressed gas is to be cooled between stages making the compression less adiabatic and more isothermal. The inter-stage coolers typically result in some partial condensation that is removed in vapor-liquid separators. In the case of small reciprocating compressors, the compressor flywheel may drive a cooling fan that directs ambient air across the intercooler of a two or more stage compressor. Because rotary screw compressors can make use of cooling lubricant to remove the heat of compression, they very often exceed a 9 to 1 compression ratio. For instance, in a typical diving compressor the air is compressed in three stages. If each stage has a compression ratio of 7 to 1, the compressor can output 343 times atmospheric pressure (7 x 7 x 7 = 343 atmospheres). Prime movers There are many options for the "prime mover" or motor which powers the compressor: gas turbines power the axial and centrifugal flow compressors that are part of jet engines steam turbines or water turbines are possible for large compressors electric motors are cheap and quiet for static compressors. Small motors suitable for domestic electrical supplies use single phase alternating current. Larger motors can only be used where an industrial electrical three phase alternating current supply is available. diesel engines or petrol engines are suitable for portable compressors and support compressors used as superchargers from their own crankshaft power. They use exhaust gas energy to power turbochargers Applications Gas compressors are used in various applications where either higher pressures or lower volumes of gas are needed: in pipeline transport of purified natural gas to move the gas from the production site to the consumer. Often, the compressor in this application is driven by a gas turbine which is fueled by gas bled from the pipeline. Thus, no external power source is necessary. in petroleum refineries, natural gas processing plants, petrochemical and chemical plants, and similar large industrial plants for compressing intermediate and end product gases. in refrigeration and air conditioner equipment to move heat from one place to another in refrigerant cycles: see Vaporcompression refrigeration. in gas turbine systems to compress the intake combustion air in storing purified or manufactured gases in a small volume, high pressure cylinders for medical, welding and other uses. in many various industrial, manufacturing and building processes to power all types of pneumatic tools. as a medium for transferring energy, such as to power pneumatic equipment. in pressurised aircraft to provide a breathable atmosphere of higher than ambient pressure. in some types of jet engines (such as turbojets and turbofans) to provide the air required for combustion of the engine fuel. The power to drive the combustion air compressor comes from the jet's own turbines. in SCUBA diving, hyperbaric oxygen therapy and other life support devices to store breathing gas in a small volume such as in diving cylinders. in submarines, to store air for later use in displacing water from buoyancy chambers, for adjustment of depth. in turbochargers and superchargers to increase the performance of internal combustion engines by increasing mass flow. in rail and heavy road transport to provide compressed air for operation of rail vehicle brakes or road vehicle brakes and various other systems (doors, windscreen wipers, engine/gearbox control, etc). in miscellaneous uses such as providing compressed air for filling pneumatic tires. Hydrogen compressor A hydrogen compressor is a mechanical device that increases the pressure of hydrogen by reducing its volume. Compression of hydrogen gas naturally increases its temperature, due to Charles' Law. Hydrogen embrittlement occurs. Hydrogen compressors are closely related to hydrogen pumps and gas compressors: 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 hydrogen gas, whereas the main result of a pump raising the pressure of a liquid is to allow the liquid hydrogen to be transported elsewhere. Staged compression Since compression generates heat, the compressed gas is to be cooled between stages making the compression less adiabatic and more isothermal. For instance in a typical hydrogen compressor, the hydrogen is compressed in four stages. Types of hydrogen compressors Piston-Metal Diaphragm compressor Piston-metal diaphragm compressors are stationary high pressure compressors, 4 staged water cooled, 11~15kW 30~50Nm3/h 40MPa for dispensation of hydrogen. Thermal hydrogen compressor In a thermal hydrogen compressor the use of the thermal and pressure properties of a hydride to absorb low pressure hydrogen gas and release high pressure hydrogen gas.. absorption on ambient temperatures, on release the bed is heated with hot water. Electrochemical hydrogen compressor A multi-stage electrochemical hydrogen compressor incorporates a series of membrane-electrode-assemblies (MEAs), similar to those used in proton exchange membrane fuel cells. , this type of compressor has no moving parts and is compact. With electrochemical compression of hydrogen a pressure of 5000 psi is achieved. Pressure is believed to go beyond 10,000 psi to the structural limits of the design. Liquid nitrogen compressor A liquid ring pump is a rotating positive displacement pump. They are typically used as a vacuum pump but can also be used as a gas compressor. The function of a liquid ring pump is similar to a rotary vane pump the difference being that the vanes are an integral part of the rotor and churn a rotating ring of liquid to form the compression chamber seal. They are an inherently low friction design, with the rotor being the only moving part. Sliding friction is limited to the shaft seals. Liquid ring pumps are typically powered by an induction motor. Description of operation The liquid ring pump compresses gas by rotating a vaned impeller within an eccentric to a cylindrical casing. Liquid (usually water) is fed into the pump and, by centrifugal acceleration, forms a moving cylindrical ring against the inside of the casing. This liquid ring creates a series of seals in the space between the impeller vanes, which form compression chambers. The eccentricity between the impeller's axis of rotation and the casing geometric axis results in a cyclic variation of the volume enclosed by the vanes and the ring. Gas, often air, is drawn into the pump via an inlet port in the end of the casing. The gas is trapped in the compression chambers formed by the impeller vanes and the liquid ring. The reduction in volume caused by the impeller rotation compresses the gas, which reports to the discharge port in the end of the casing. History US Patent 1,091,529, for liquid ring vacuum pumps and compressors, was granted to Lewis H. Nash in 1914 . They were manufactured by the Nash Engineering Company in Norwalk, CT. Around the same time, in Austria, Patent 69274 was granted to Siemens-Schuckertwerke for a similar liquid ring vacuum pump. Recirculation of Ring-liquid Some ring-liquid is also entrained with the discharge stream. This liquid is separated from the gas stream by other equipment external to the pump. In some systems, the discharged ring-liquid is cooled via heat exchanger or cooling tower, then returned to the pump casing. In some recirculating systems, contaminants from the gas become trapped in the ring-liquid, depending on system configuration. These contaminants become concentrated as the liquid continues to recirculate, eventually causes damage and reduced life to the pump. In this case, filtration systems are required to ensure contamination is kept to acceptable levels. In non-recirculating systems, the discharged hot liquid (usually water) is treated as a waste stream. In this case, fresh, cool water is used to make up the loss. Environmental considerations are making such "once-through" systems increasingly rare. Types and Applications Liquid ring systems can be single or multi-stage. Typically a multi-stage pump will have up to two compression stages on a common shaft. In vacuum service, the attainable pressure reduction is limited by the vapour pressure of the ring-liquid. As the vacuum generated approaches the vapour pressure of the ring-liquid, the increasing volume of vapor released from the ringliquid diminishes the remaining vacuum capacity. The efficiency of the system declines as a result. Single stage vacuum pumps typically produce vacuum to 35 torr (mm Hg), and two-stage pumps can produce vacuum to 25 torr (mmHgA), assuming air is being pumped and the ring-liquid is water at 15°C (60F) or less. Dry air and 15°C sealant water temperature is the standard performance basis which most manufacturers use for their performance curves. These simple, but highly reliable pumps have a variety of industrial applications. One typical industrial application is the vacuum forming of molded paper pulp products (egg cartons and other packaging). Other applications include soil remediation, where contaminated ground water is drawn from wells by vacuum. In petroleum refining, vacuum distillation also makes use of liquid ring vacuum pumps to provide the process vacuum. Liquid ring compressors are often used in Vapor recovery systems. Liquid Ring Vacuum Pumps can use any liquid compatible with the process, provided it has the appropriate vapor pressure properties, as the sealant liquid. Although the most common sealant is water, almost any liquid can be used. The second most common is oil. Since oil has a very low vapor pressure, oil-sealed liquid ring vacuum pumps are typically air-cooled. The ability to use any liquid, allows the liquid ring vacuum pump to be ideally suited for solvent(vapor) recovery. If a process, such as distillation, or a vacuum dryer is generating toluene vapors, for example, then it is possible to use toluene as the sealant, provided the cooling water is cold enough to keep the vapor pressure of the sealant liquid low enough to pull the desired vacuum. The air compressors seen by the public are of 5 main types: To supply a high-pressure clean air to fill breathing apparatus cylinders To supply a moderate-pressure clean air to supply air to a submerged surface supplied diver To supply a large amount of moderate-pressure air to power pneumatic tools For filling pneumatic tyres To produce large volumes of moderate-pressure air for macroscopic industrial processes (such as oxidation for petroleum coking or cement plant bag house purge systems) For more detailed information, see Gas compressor. Most air compressors are either reciprocating piston type or rotary vane or screw. Centrifugal compressors are common in very large applications. 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. 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 rotary steam engine 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 practicable scroll compressors therefore did not appear on the market until after World War II when the development of higher precision machine tools permitted their construction and were not commercially produced for air conditioning until the early 1980s. Design A scroll compressor uses two interleaved 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. Another variation, is with flexible (layflat) tubing where the archimedean spiral acts as a peristaltic pump, that operates on much the same principle as a toothpaste tube. and have casings filled with lubricant 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 pump'. 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 Superchargers for Volkswagen's G-Lader. automobile applications, e.g. 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, Oldham coupling mass that ensures proper position of the orbiting scroll cannot be balanced and thus its presence still results in inherent scroll compressor vibration.= 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 compressors. 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. The more steady flow yields lower gas pulsations, lower sound, lower vibration, and more efficient flow. 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 airconditioning scroll does not have dynamic valves. 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 small additional discharge flow losses associated with the presence of the discharge valve). 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 not have a dynamic discharge valve. There is an industry trend toward developing systems operating on CO2 refrigerant. While CO2 has no ozone depletion potential and essentially no DIRECT global warming potential, it is very difficult to achieve a reasonable cycle efficiency using CO2 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 CO2 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. Accordindg to Copeland, a big manufacturer of scroll compressors, scroll compressors have 70 percent less 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 cream machines from reciprocating to scroll compressor. They found through testing that the scroll compressor design delivered better reliability and energy efficiency in operation. . However, many refrigeration application rely on reciprocating compressors, that 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 run in reverse during short power interruption. 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 phase compressor, as compared to single phase compressors, would continue to operate in a forward direction after 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. 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 operated 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 (called hot-gas bypass), vary motor speed, or provide multiple compressors and stage them on and off in sequence. Each of these methods has drawbacks: Hot gas bypass short-cycles the normal refrigeration cycle and allows some of the compressed gas to return directly to the compressor 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. 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. One method is to delay the start of compression. The beginning stages of compression are vented back to suction. This reduces the amount of fluid that will be compressed. The rest of the compression process is normal. 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. Emerson manufactures a scroll compressor under the "Digital Scroll" trade name that is capable of stopping discharge, intermittently. 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. Even though the capacity of the scroll compressor can be varied down to 10%20% of its normal capacity, the power consumption is still substantial. The digital scroll compressors are still not nearly as efficient as compressors operated by variable speed drive or as reciprocating compressor with solenoid valves blocking flow to some of the reciprocating compressor cylinders.