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ssors 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 is high compared to the value of the device and it would be more economical to just purchase a new device or compressor semi hermetic compressors are used in mid sized to large refrigeration and air conditioning systems where it is cheaper to repair or refurbish the compressor compared to the price of a new one a hermetic compressor is simpler and cheaper to build than a semi hermetic or open compressor thermodynamics of gas compression edit isentropic compressor edit a compressor can be idealized as internally reversible and adiabatic thus an isentropic steady state device meaning the change in entropy is 0 26 the enthalpy change for a flow process can be calculated 27 dh vdp tds isentropic ds is zero dh vdp non flow isentropic processes like some positive displacement compressors may use a different equation 28 dh pdv by defining the compression cycle as isentropic an ideal efficiency for the process can be attained and the ideal compressor performance can be compared to the actual performance of the machine isotropic compression as used in asme ptc 10 code refers to a reversible adiabatic compression process 29 isentropic efficiency of compressors η c i s e n t r o p i c c o m p r e s s o r w o r k a c t u a l c o m p r e s s o r w o r k w s w a h 2 s h 1 h 2 a h 1 displaystyle eta _ c frac rm isentropic compressor work rm actual compressor work frac w_ s w_ a cong frac h_ 2s h_ 1 h_ 2a h_ 1 h 1 displaystyle h_ 1 is the enthalpy at the initial state h 2 a displaystyle h_ 2a is the enthalpy at the final state for the actual process h 2 s displaystyle h_ 2s is the enthalpy at the final state for the isentropic process minimizing work required by a compressor edit comparing reversible to irreversible compressors edit comparison of the differential form of the energy balance for each device let q displaystyle q be heat w displaystyle w be work k e displaystyle ke be kinetic energy and p e displaystyle pe be potential energy actual compressor δ q a c t δ w a c t e l t a q a c t t 0 displaystyle delta q_ act delta w_ acteltaq_ act t geq 0 furthermore d s δ q a c t t displaystyle ds geq frac delta q_ act t and t is absolute temperature t 0 displaystyle t geq 0 which produces δ w r e v δ w a c t displaystyle delta w_ rev geq delta w_ act or w r e v w a c t displaystyle w_ rev geq w_ act therefore work consuming devices such as pumps and compressors work is negative require less work when they operate reversibly 26 effect of cooling during the compression process edit p v specific volume vs pressure diagram comparing isentropic polytropic and isothermal processes between the same pressure limits isentropic process involves no cooling polytropic process involves some cooling isothermal process involves maximum cooling by making the following assumptions the required work for the compressor to compress a gas from p 1 displaystyle p_ 1 to p 2 displaystyle p_ 2 is the following for each process p 1 displaystyle p_ 1 and p 2 displaystyle p_ 2 flow processes vdp all processes are internally reversible the gas behaves like an ideal gas with constant specific heats isentropic p v k c o n s t a n t displaystyle pv k constant where k c p c v displaystyle k c_ p c_ v w c o m p i n k r t 2 t 1 k 1 k r t 1 k 1 p 2 p 1 k 1 k 1 displaystyle w_ comp in frac kr t_ 2 t_ 1 k 1 frac krt_ 1 k 1 left left frac p_ 2 p_ 1 right k 1 k 1 right polytropic p v n c o n s t a n t displaystyle pv n constant w c o m p i n n r t 2 t 1 n 1 n r t 1 n 1 p 2 p 1 n 1 n 1 displaystyle w_ comp in frac nr t_ 2 t_ 1 n 1 frac nrt_ 1 n 1 left left frac p_ 2 p_ 1 right n 1 n 1 right isothermal t c o n s t a n t displaystyle t constant or p v c o n s t a n t displaystyle pv constant w c o m p i n r t l n p 2 p 1 displaystyle w_ comp in rtln left frac p_ 2 p_ 1 right by comparing the three internally reversible processes compressing an ideal gas from p 1 displaystyle p_ 1 to p 2 displaystyle p_ 2 the results show that isentropic compression p v k c o n s t a n t displaystyle pv k constant requires the most work in and the isothermal compression t c o n s t a n t displaystyle t constant or p v c o n s t a n t displaystyle pv constant requires the least amount of work in for the polytropic process p v n c o n s t a n t displaystyle pv n constant work decreases as the exponent n decreases by increasing the heat rejection during the compression process one common way of cooling the gas during compression is to use cooling jackets around the casing of the compressor 26 compressors in ideal thermodynamic cycles edit ideal rankine cycle 1 2 isentropic compression in a pump ideal carnot cycle 4 1 isentropic compression ideal otto cycle 1 2 isentropic compression ideal diesel cycle 1 2 isentropic compression ideal brayton cycle 1 2 isentropic compression in a compressor ideal vapor compression refrigeration cycle 1 2 isentropic compression in a compressor note the isentropic assumptions are only applicable with ideal cycles real world cycles have inherent losses due to inefficient compressors and turbines the real world system are not truly isentropic but are rather idealized as isentropic for calculation purposes temperature edit main article gas laws compression of a gas increases its temperature for a polytropic transformation of a gas p v n constant p 1 v 1 n p 2 v 2 n p 2 p 1 v 1 v 2 n p n 1 n t constant p 1 n 1 n t 1 p 2 n 1 n t 2 p 2 p 1 n 1 n t 2 t 1 displaystyle begin cases pv n text constant p_ 1 v_ 1 n p_ 2 v_ 2 n rightarrow frac p_ 2 p_ 1 left frac v_ 1 v_ 2 right n frac p frac n 1 n t text constant frac p_ 1 frac n 1 n t_ 1 frac p_ 2 frac n 1 n t_ 2 rightarrow left frac p_ 2 p_ 1 right frac n 1 n frac t_ 2 t_ 1 end cases the work done for polytropic compression or expansion of a gas into a closed cylinder w v 1 v 2 p d v p 1 v 1 n v 1 v 2 v n d v p 1 v 1 n 1 n v 2 1 n v 1 1 n p 1 v 1 n 1 n v 1 1 n v 2 1 n v 1 1 n 1 p 1 v 1 1 n v 2 1 n v 1 1 n 1 displaystyle w int _ v_ 1 v_ 2 pdv p_ 1 v_ 1 n int _ v_ 1 v_ 2 v n dv frac p_ 1 v_ ...