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Chapter: Mechanical and Electrical - Thermal Engineering - Air Compressors

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Technical Terms of Air Compressors

Mechanical and Electrical - Thermal Engineering - Air Compressors

TECHNICAL TERMS

 

After cooler - Heat exchangers for cooling air or gas discharged from compressors. They provide the most effective means of removing moisture from compressed air and gases.

 

Air-Cooled Compressors - Air-cooled compressors are machines cooled by atmospheric air circulated around the cylinders or casings.

 

Base Plate - A metallic structure on which a compressor or other machine is mounted.

 

Capacity - The capacity of a compressor is the full rated volume of flow of gas compressed and delivered at conditions of total temperature, total pressure, and composition, prevailing at the compressor inlet. It sometimes means actual flow rate, rather than rated volume of flow.

 

Capacity, Actual - Quantity of gas actually compressed and delivered to the discharge system at rated speed of the machine and under rated pressure conditions. Actual capacity is usually expressed in cubic feet per minute (cfm) at that stage inlet gas conditions.

 

Casing - The pressure containing stationary element that encloses the rotor and associated internal components of a compressor. Includes integral inlet and discharge corrections (nozzles).

 

Check valve - A check valve is a valve that permits flow in one direction only.

 

Clearance - The maximum cylinder volume on a working side of the piston minus the piston displacement volume per stroke. It is usually expressed as a percentage of the displaced volume.

 

Clearance Pocket - An auxiliary volume that may be opened to the clearance space to increase the clearance, usually temporarily, to reduce the volumetric efficiency of the air compressor.

 

Compressibility - A factor expressing the deviation of a gas from the laws of hydraulics.

 

Compression, Adiabatic - This type of compression is effected when no heat is transferred to or from the gas during the compression process.

 

Compression, Isothermal - Isothermal compression is a compression in which the temperature of the gas remains constant. For perfect gases, it is represented by the equation PV is a constant, if the process is reversible.

 

Compression, Polytropic - Compression in which the relationship between the pre-sum and the volume is expressed by the equation PV is a constant.

 

Compression Ratio - The ratio of the absolute discharge; press = to the absolute inlet pressure.

 

Critical Pressure - The limiting value of saturation pressure as the saturation temperature approaches the critical temperature.

 

Critical Temperature - The highest temperature at which well defined liquid and vapor states exist. It is sometimes defined as the highest temperature at which it is possible to liquify a gas by pressure alone.

 

Diaphragm - A stationary element between the stages of a multistage centrifugal compressor. It may include guide vanes for directing the flowing medium to the impeller of the succeeding stage. In conjunction with an adjacent diaphragm, it forms the diffuser surrounding the impeller.

 

Diaphragm Routing - A method of removing heat from the flowing medium by circulation of a coolant in passages built into the diaphragm.

 

Diffuser - A stationary passage surrounding an impeller, in which velocity pressure imparted to the flowing medium by the impeller is converted into static pressure.

 

Displacement - Displacement of a compressor is the piston volume swept out per unit time; it is usually expressed in cubic feet per minute.

 

Dynamic Type Compressors - Machines in which air or gas is compressed by the mechanical action of routing vanes or impellers imparting velocity and pressure to the flowing medium.

 

Efficiency - Any reference to efficiency of a dynamic type compressor must be accompanied by a qualifying statement which identifies the efficiency under consideration, as in the following

 

definitions.

 

Efficiency, Compression - Ratio of calculated isentropic work requirement to actual thermodynamic work requirement within the cylinder, the Inner as determined from the cylinder indicator card.

 

Efficiency, Isothermal - Ratio of the work calculated on an isothermal basis to the actual work transferred to the gas during compression.

 

Efficiency, Mechanical - Ratio of thermodynamic work requirement in the cylinder (a shown by die indicator card) to actual brake horsepower requirement.

 

Efficiency, Polytropic - Ratio of the polytropic compression energy transferred to the gas no the actual energy transferred to the gas.

 

Efficiency, Volumetric - Ratio of actual capacity to piston displacement, stated as a percentage.

 

Exhauster - This is a term sometimes applied to a compressor in which the inlet pressure is less than atmospheric pressure.

 

Expanders - Turbines or engines in which a gas expands, doing work, and undergoing a drop in temperature. Use of the term usually implies that the drop in temperature is the principle objective. The orifice in a refrigeration system also performs this function, but the expander performs it nearly isentropically, and is thus more effective in cryogenic systems.

 

Filters - Filters are devices for separating and removing dust and dirt front air before it enters a compress.

 

Flange Connection - The flange connection (inlet or discharge) is a means of connecting the casing to the inlet or discharge piping by means of bolted rims (flanges).

 

Fluidics - The general subject of instruments and controls dependent upon low rate flow of air or gas at low pressure as the operating medium. These usually have no moving parts.

 

Free Air - Air at atmospheric conditions at any specific location. Because the altitude, barometer, and temperature may vary at different localities and at different times, it follows that this term does not mean air under identical or standard conditions.

 

Gas - While from a physical point of view a gas is one of the three basic phases of matter, and thus air is a gas, a special meaning is assigned in pneumatics practice. The term gas refers to any gas other than air.

 

GasBearings - Gas bearings are load carrying machine elements permitting some degree of motion in which the lubricant is air or some other gas.

 

Volumetric Efficiency of the Compressor - It is the ratio of actual volume of air drawn in the compressor to the stroke volume of the compressor.

 

Mechanical efficiency - It is the ratio of indicated power to shaft power or brake power of motor.

 

Isentropic efficiency - It is the ratio of the isentropic power to the brake power required to drive the compressor.

 

Centrifugal compressor - The flow of air is perpendicular to the axis of compressor

 

Axial flow compressor - The flow of air is parallel to the axis of compressor

 

Compression - The process of increasing the pressure of air, gas and vapour by reducing its volume is called as compression.

 

Single acting compressor - The suction , compression and the delivery of air takes on the one side of piston

 

Double acting compressor - The suction, compresstion end the delivery of air takes place on both sides of the piston.

 

Multi stage compressor - The compression of air from initial pressure to the final pressure is carried out in more than one cylinder.

 

Application of compressed air - Pneumatic brakes, drills,jacks,lifts, spray of paintings, shop cleaning , injecting the fuel in diesel engine, supercharging, refrigeration and in air conditioning systems.

 

Inter cooler - It is a simple heat exchanger, exchanges the heat of compressed air from low pressure compressor to circulating water before the air enters to high pressure compressor. The purpose of intercooling is to minimize the work of compression.

 

Isentropic efficiency - It is the ratio of isentropic power to the brake power required to drive the compressor.

 

Clearance ratio - It is the ratio of clearance volume to the swept volume or stroke volume is called as clearance ratio.

 

Isothermal efficiency - It is the ratio between isothermal work to the actual work of the compressor.

 

Compression ratio - The ratio between total volume and the clearance volume of the cylinder is called compression ratio.

 

Perfect intercooling - When the temperature of the air leaving the intercooler is equal to the original atmospheric air temperature, then the inter cooling is called perfect intercooling.


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