Mechanical Engineering Dictionary-Air Compressor

Important Terms and their Meaning of Mechanical Engineering-Air Compressor:

ABSOLUTE HUMIDITY – Actual quantity of water vapour in the air, usually expressed as so many grains of moisture in a cubic foot of air.

ABSOLUTE PRESSURE – Pressure measured from the true zero or point of no pressure.

ADIABATIC COMPRESSION – Compression of air without receiving or giving up heat.

AFTER COOLER – A type of surface heat exchanger in which compressed air is cooled after compression.

AIR – A gas consisting of mechanical mixture of 23.2% (by weight) of oxygen 75.5% nitrogen and 1.3% argon, 21 % (by volume) of oxygen, 78.06% nitrogen and 0.94% argon.

AIR COMPRESSOR – A machine (driven by any prime mover), which compresses air into a receiver to be used at a greater or shorter distance.

AIR COOLED COMPRESSOR – A compressor whose cylinder has cast integral numerous thin fins to form excess cooling surface exposed to a draught of cool air which forms the medium to carry off some of the heat of compression.

AIR ENGINE – A very small reciprocating engine driven by compressed air.

AIR EXHAUSTER – A suction fan, a vacuum pump.

AIR METER – An apparatus used to measure the rate of flow of air or gas.

AIR RECEIVER – A vessel into which compressed air is discharged, to be stored until required.

ANEMOMETER – An instrument for measuring the velocity of flow of a gas, either by mechanical or electrical methods.

ATMOSPHERIC PRESSURE – The force exerted by the weight of the atmosphere on every point with which it is in contact.

AXIAL COMPRESSOR – A multistage, high efficiency compressor comprising alternate rows of moving and fixed blades attached to a rotor and its casing respectively. The flow of fluid is essentially parallel to the axis of the compressor.

BOYLE’S LAW – At constant temperature, the absolute pressure of a gas varies inversely as its volume.

CENTRIFUGAL COMPRESSOR – A compressor designed to deliver large quantities of air or gas at low pressure, moved by centrifugal force generated by a fast revolving rotor.

CHARLE’S LAW – At constant pressure, the volume of a gas is proportional to its absolute temperature. At constant volume, the pressure is proportional to its absolute temperature.

CLEARANCE VOLUME EFFECT – Volumetric efficiency of the reciprocating compressor depends upon the clearance volume in the air cylinder. The greater the clearance volume, the greater the volume of the cylinder occupied by the clearance air which expands and prevents the entrance of free air during the early part of the admission stroke.

COMPRESSED AIR – Air forced into a smaller space than it originally occupied. When air is compressed both its pressure and temperature rise.

COMPRESSION CONSTANT – According to Boyle’s law, product of pressure and volume at any instant is constant, at constant temperature.

COMPRESSION EFFICIENCY – Ratio of the theoretical power required to compress the amount of air actually delivered to the actual power developed in the cylinder as shown by the indicator diagram.

COMPRESSOR OVERALL EFFICIENCY – Ratio of actual power developed in the air cylinder as shown by the indicator diagram to the power supplied to the compressor shaft.

CONSERVATION OF ENERGY – Energy can be transmitted from one body to another or transformed in its manifestations, but can neither be created nor destroyed.

DIRECT CONNECTED COMPRESSOR – A compressor in which the prime mover is attached direct to the compressor without any interposed transmission such as chain, belt etc.

DISPLACEMENT OF COMPRESSOR – The volume displaced by the net area of the compressor piston. This is the capacity of an air compressor, usually expressed in cum per minute.

DOUBLE ACTING COMPRESSOR – A reciprocating compressor in which compression occurs on either side of the piston during every stroke.

ECCENTRIC AND STRAP – An eccentric is a disc having its axis of rotation out of its centre. It is equivalent to a crankpin which is so large in diameter that embraces the shaft to which it is attached and dispenses with arms. Converts rotary motion into reciprocating motion.

EFFICIENCY – Ratio of the useful work performed by a prime mover to the energy expended, that is, the output divided by the input.

ENBLOCK CYLINDERS – Two or more cylinders cast integral, that is all in one casting.

FEATHER VALVE – A valve which consists of a strip of ribbon steel which covers a slightly narrower slot when the value is closed.

FINGER VALVES – Valves consist of narrow strips of stainless steel, fastened to the seat at one end and free to flex along their length. Suitable for light service.

FIXED COMPRESSOR – A compressor mounted upon a permanent base as concrete for service not requiring removal from place to place.

FREE AIR – Air at atmospheric condition at the point where a compressor is installed.

FREE AIR UNLOADER – An automatic device that varies the amount of air or gas being pumped.

HOPPER COOLED SYSTEM – A non (external) circulating system. The cylinder has an open water jacket of considerable volume.

INDICATED HORSE POWER – The actual power developed within a cylinder as calculated from the indicator diagram.

INDICATED WORK – The work of compression plus the work of expulsion of the air from the cylinder minus the work done on the piston by the pressure of the air during admission.

INLET LINE LOADER – An unloader that automatically opens and closes the inlet line under pressure variations in the receivers.

INTERCOOLER – A type of surface heat exchanger placed between two cylinders of a two stage compressor so that heat of compression generated in the first stage cylinder may be removed (in part or whole) from the air as it passes through the intercooler to the second stage cylinder.

ISOTHERMAL COMPRESSION – Compression of air at constant temperature. Law of compression is PV= constant.

KINETIC ENERGY – Energy due to momentum, that is, the energy of a moving body, which is equivalent to saying, dynamic inertia.

MECHANICAL EQUIVALENT OF HEAT – Relationship between the unit of heat and unit of work.

MEAN EFFECTIVE PRESSURE – The average resultant pressure acting on the piston during the stroke that is the effective pressure which compresses and discharges the air. This is the difference between the mean forward pressure and the mean back pressure.

MECHANICAL EFFICIENCY – The ratio of the air indicated horse power to the brake horse power supplied to the compressor shaft.

MULTISTAGE COMPRESSION – Dividing the compression of air into two or more stages so as to obtain the work saving due to a nearer approach to isothermal compression by using intercooling.

PISTON SPEED – The total distance travelled by the piston in one minute, not the actual velocity at any given instant.

PORTABLE COMPRESSOR – A small compressor which is easily moved from place to place.

POWER – The rate at which work is done, that is work divided by the time in which it is done. Unit of power is horse power equal to 4500 minute.

POWER DRIVEN COMPRESSOR – A compressor having a separate prime mover and connected by a suitable transmission such as a belt.

PRIME MOVER – An apparatus or mechanism whereby motion and force are received directly from some natural source of energy (fuel) and transmitted into some motion by means of which the power may be conveniently applied.

R (gas constant) – An experimentally determined constant which is equal to the mechanical work done by the expansion of unit weight of a perfect gas at a constant pressure while heat is added to increase its temperature by one degree centigrade.

RATIO OF COMPRESSION – Ratio of final volume to the initial volume during compression.

RECIPROCATING COMPRESSOR – A compressor having a piston which is made to move to and fro, that is forward and backward and thereby compresses and delivers air.

RELATIVE HUMIDITY – Degree of saturation of the air with water vapour as determined by the use of the wet and dry bulb thermometers.

RESISTANCE – The quality of not yielding to force or external pressure, that quality of a body which acts in opposition to the pressure of another.

ROTARY COMPRESSOR – A compressor having a vane rotor or its
equivalent mounted eccentrically in a stationary casing.

SEMI FIXED COMPRESSOR – A unit larger than the portable type where skids are used in place of being mounted on a truck, the adaptation being for service where frequent moving is not necessary.

SEPARATOR – Device through which the compressed air after being cooled in the after cooler, is sent so as to separate the moisture from the air by centrifugal force.

SINGLE ACTING COMPRESSOR – A reciprocating compressor in which compression takes place on one side of the piston during alternate strokes.

SINGLE STAGE COMPRESSOR – A compressor in which the compression cycle takes place in a single cylinder.

SLIPPAGE EFFICIENCY – The ratio of volume of air actually measured to the apparent volume accounted for by the indicator diagram.

SPECIFIC HEAT OF AIR – Amount of heat that is to be supplied to raise the temperature of 1 kg of air through 1 degree C.

SPECIFIC HEAT OF AIR AT CONSTANT PRESSURE – Total specific heat of air which is made up of (1) the internal work of raising the temperature of air, and (2) the external work of pushing away the atmosphere to make room for its expansion.

THROW OF THE ECCENTRICITY – Twice the eccentricity or the amount of reciprocating motion produced.

TWO STAGE COMPRESSOR – A compressor in which compression begins in one cylinder and is completed in the second cylinder. It divides the compression range between the two cylinders and permits cooling between the cylinders.

TWO STAGE CYLINDER – A cylinder of special construction with a step piston, the low pressure being at the top while the high pressure is formed around the trunk.

VALVE GEAR – The mechanism or combination of parts by which a reciprocating or to and fro motion is imparted to the valve from the rotary motion of the shaft.

VOLUMETRIC EFFICIENCY – The ratio of the actual number of cubic meter of free air (at 1.03 abs and 15° C) compressed per unit of time to the number of cubic meter of piston displacement during that time.

WATER COOLED COMPRESSOR – A compressor whose cylinder is water jacketed and through which flows a current of cold water which functions as a transmission medium to carry off some of the heat of compression.

WORK – The overcoming of resistance through a certain distance by the expenditure of energy.

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