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typically have higher efficiency small sizes or low rotor speeds are not practical due to inherent leaks caused by clearance between the compression cavities or screws and compressor housing 5 they depend on fine machining tolerances to avoid high leakage losses and are prone to damage if operated incorrectly or poorly serviced rotary vane compressors edit eccentric rotary vane pump see also rotary vane pump 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 that is either 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 1 thus a series of increasing and 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 or 200 kpa or 29 psi for bulk material movement while oil injected machines have the necessary volumetric efficiency to achieve pressures up to about 13 bar 1 300 kpa 190 psi 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 rotary vane compressors can have mechanical efficiencies of about 90 13 rolling piston edit rolling piston compressor the rolling piston in a rolling piston style compressor plays the part of a partition between the vane and the rotor 14 rolling piston forces gas against a stationary vane two of these compressors can be mounted on the same shaft to increase capacity and reduce vibration and noise 15 a design without a spring is known as a swing compressor 16 in refrigeration and air conditioning this type of compressor is also known as a rotary compressor with rotary screw compressors being also known simply as screw compressors it offers higher efficiency than reciprocating compressors due to less losses from the clearance volume between the piston and the compressor casing it s 40 to 50 smaller and lighter for a given capacity which can impact material and shipping costs when used in a product causes less vibration has fewer components and is more reliable than a reciprocating compressor but its structure does not allow capacities beyond 5 refrigeration tons is less reliable than other compressor types and is less efficient than other compressor types due to losses from the clearance volume 5 scroll compressors edit mechanism of a scroll pump main article scroll compressor 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 17 18 19 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 between the scrolls due to minimum clearance volume between the fixed scroll and the orbiting scroll these compressors have a very high volumetric efficiency these compressors are extensively used in air conditioning and refrigeration because they are lighter smaller and have fewer moving parts than reciprocating compressors and they are also more reliable they are more expensive though so peltier coolers or rotary and reciprocating compressors may be used in applications where cost is the most important or one of the most important factors to consider when designing a refrigeration or air conditioning system this type of compressor was used as the supercharger on volkswagen g60 and g40 engines in the early 1990s when compared with reciprocating and rolling piston compressors scroll compressors are more reliable since they have fewer components and have a simpler structure are more efficient since they have no clearance volume nor valves and possess the advantages both of surging less and not vibrating so much but when compared with screw and centrifugal compressors scroll compressors have lower efficiencies and smaller capacities 5 diaphragm compressors edit main article 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 1 the degree of flexing and the material constituting the diaphragm affects the maintenance life of the equipment generally stiff metal diaphragms may only displace a few cubic centimeters of volume because the metal cannot endure large degrees of flexing without cracking but the stiffness of a metal diaphragm allows it to pump at high pressures rubber or silicone diaphragms are capable of enduring deep pumping strokes of very high flexion but their low strength limits their use to low pressure applications and they need to be replaced as plastic embrittlement occurs 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 on the right 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 reciprocating natural gas compressor was developed by sertco 20 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 dynamic edit air bubble compressor edit also known as a trompe a mixture of air and water generated through turbulence is allowed to fall into a subterranean chamber where the air separates from the water the weight of falling water compresses the air in the top of the chamber a submerged outlet from the chamber allows water to flow to the surface at a lower height than the intake an outlet in the roof of the chamber supplies the compressed air to the surface a facility on this principle was built on the montreal river at ragged shutes near cobalt ontario in 1910 and supplied 5 000 horsepower to nearby mines 21 centrifugal compressors edit a single stage centrifugal compressor a single stage centrifugal compressor early 1900s g schiele co frankfurt am main main article centrifugal compressor centrifugal compressors use a 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 1 22 23 their application can be from 100 horsepower 75 kw to thousands of horsepower with multiple staging they can achieve high output pressures greater than 1 000 psi 6 9 mpa this type of compressor along with screw compressors are extensively used in large refrigeration and air conditioning systems magnetic bearing magnetically levitated and air bearing centrifugal compressors exist many large snowmaking 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 centrifugal compressors are the largest available compressors offer higher efficiencies under partial loads may be oil free when using air or magnetic bearings which increases the heat transfer coefficient in evaporators and condensers weigh up to 90 less and occupy 50 less space than reciprocating compressors are reliable and cost less to maintain since less components are exposed to wear and only generate minimal vibration but their initial cost is higher require highly precise cnc machining the impeller needs to rotate at high speeds making small compressors impractical and surging becomes more likely 5 surging is gas flow reversal meaning that the gas goes from the discharge to the suction side which can cause serious damage specially in the compressor bearings and its drive shaft it is caused by a pressure on the discharge side that is higher than the output pressure of the compressor this can cause gases to flow back and forth between the compressor and whatever is connected to its discharge line causing oscillations 5 diagonal or mixed flow compressors edit 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 an axial rather than radial direction 24 comparative to the conventional centrifugal compressor of the same stage pressure ratio the value of the speed of the mixed flow compressor is 1 5 times larger 25 axial compressors edit an animation of an axial compressor main article axial compressor axial compressors are dynamic rotating compressors that use arrays of fan like airfoils to progressively compress a fluid they are used where high flow rates or a compact design are required the arrays of airfoils are set in rows usually as pairs one rotating and one stationary the rotating airfoils also known as blades or rotors accelerate the fluid the stationary airfoils also known as stators or vanes decelerate and redirect the flow direction of the fluid preparing it for the rotor blades of the next stage 1 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 a compressor will not function unless fitted with features such as stationary vanes with variable angles known as variable inlet guide vanes and variable stators the ability to allow some air to escape part way along the compressor known as interstage bleed and being split into more than one rotating assembly known as twin spools for example 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 compressors are used in medium to large gas turbine engines natural gas pumping stations and some chemical plants hermetically sealed open or semi hermetic edit a small hermetically sealed compressor in a common consumer refrigerator or freezer typically has a rounded steel outer shell permanently welded shut which seals operating gases inside the system in this case an r600a refrigerant there is no route for gases to leak such as around motor shaft seals on this model the plastic top section is part of an auto defrost system that uses motor heat to evaporate the water compressors used in refrigeration systems must exhibit near zero leakage to avoid the loss of the refrigerant if they are to function for years without service this necessitates the use of very effective seals or even the elimination of all seals and openings to form a hermetic system these compressors are often described as being either hermetic open or semi hermetic to describe how the compressor is enclosed and how the motor drive is situated in relation to the gas or vapor being compressed some compressors outside of refrigeration service may also be hermetically sealed to some extent typically when handling toxic polluting or expensive gasses with most non refrigeration applications being in the petrochemical industry in hermetic and most semi hermetic compressors the compressor and motor driving the compressor are integrated and operate within the pressurized gas envelope of the system the motor is designed to operate in and be cooled by the refrigerant gas being compressed open compressors have an external motor driving a shaft that passes through the body of the compressor and rely on rotary seals around the shaft to retain the internal pressure the difference between the hermetic and semi hermetic is that the hermetic uses a one piece welded steel casing that cannot be opened for repair if the hermetic fails it is simply replaced with an entire new unit a semi hermetic uses a large cast metal shell with gasketed covers with screws that can be opened to replace motor and compressor components the primary advantage of a hermetic and semi hermetic is that there is no route for the gas to leak out of the system the main advantages of open compressors is that they can be driven by any motive power source allowing the most appropriate motor to be selected for the application or even non electric power sources such as an internal combustion engine or steam turbine and secondly the motor of an open compressor can be serviced without opening any part of the refrigerant system an open pressurized system such as an automobile air conditioner can be more susceptible to leak its operating gases open systems rely on lubricant in the system to splash on pump components and seals if it is not operated frequently enough the lubricant on the seals slowly evaporates and then the seals begin to leak until the system is no longer functional and must be recharged by comparison a hermetic or semi hermetic system can sit unused for years and can usually be started up again at any time without requiring maintenance or experiencing any loss of system pressure even well lubricated seals will leak a small amount of gas over time particularly if the refrigeration gasses are soluble in the lubricating oil but if the seals are well manufactured and maintained this loss is very low the disadvantage of hermetic compressors is that the motor drive cannot be repaired or maintained and the entire compressor must be replaced if a motor fails a further disadvantage is that burnt out windings can contaminate the whole systems thereby requiring the system to be entirely pumped down and the gas replaced this can also happen in semi hermetic compressors where the motor operates in the refrigerant typically hermetic compressors are used in low cost factory assembled consumer goods where the cost of repair and labor i...
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