COMPRESSED AIR - Textile industry
English word Pneumatic is derived from the greek word 'pneuma' meaning "breath".Pneumatic control system operate on a supply of compressed air, which must be made available in sufficient quantity and at a pressure to suit the capacity of the system. A compressor is a machine which takes in air, gas or vapors at a certain pressure and delivers the fluid at a higher pressure.
Everything on earth is subjected to the absolute atmospheric pressure(pa), this pressure cannot
be felt.The prevailing atmospheric pressure is therefore regarded as the base and any deviation is termed "gauge pressure".
Absolute pressure = Atmospheric pressure + gauge pressure
Absolute pressure is approximately one bar greater than the gauge pressure.
Charecteristics of interest on a compressor are,
Delivery volume or capacity of the compressor
Compressor capacity is usually expressed as air volume at ambient conditions at the compressor intake, namely in units of metercube per minute or litres per minute.
Compression ratio is expressed by the discharge pressure measured in the generally accepted unit of bars. Compressors should be installed in a separate room. Special care is required to ensure that the compressors will be able to take in air that is preferably cool but above all dry and substantially dustfree.
At locations where clean suction air is not available, the installation of a separate intake filter can answer this requirement. Piping leading from the filter to the compressor intake should be amply dimensioned. In this way it is also possible for clean suction air to be supplied to a multiple number of compressors via a common intake duct.
Unnecessary costs in the production of compressed air can be avoided by functional and expert planning
Clean condition of the suction air is one of the factors decisive to life of a compressor. Warm and
moist suction air will result in increased precipitation of condensate from the compressed air.
The amount of moisture condensing out of compressed air is a function of the relative humidity of the
intake air and the temperature. Relative humidity is the amount of water vapor present in a given volume of air, whereas the humidity at saturation is the total amount of water vapor which that same volume of air can absorb at the given temperature.
One metercube of compressed air is only capable of holding the same amount of water vapor as one metercube of atmospheric air.
Discharge pressure of the compressor should not be appreciably higher than the working pressure required for operation of the pneumatic control devices.
Delivering air at higher pressure will cost more for compression and will cause higher losses at leakage points.
Air receivers are instlaled directly downstream of the compressors to receive the compressed air delivered, thereby balancing out pulsations in the air flow. Mostly they are also intended to serve as storage reservoirs for the overall air mains, thus additionally helping to cool the compressed air and separate condensate before it is distributed further. In large compressor systems an aftercoller incorporating a moisture separator will be installed between compressor and receiver so that a large part of the condensate will be removed before the air enters the receiver. Size of the receiver is governed by the rate at which compressed air is consumed and the capacity of the compressor.
Volumetric capacity of the receiver in metercube equals the delivery capacity of the compressor in metercube per minute.But it is cheaper to use an air receiver or accumulator whose capacity is too large than one too small.
Pressure-volume product is calculated by multiplying the volumetric capacity of the receiver in liters
by multiplying the volumetric capacity of the receiver in liters by the working pressure in bars.
pressure-volume product = p * v
Air receivers should be installed outdoors(preferably on the shady side of the building). This contributes to better cooling of the compressed air and thus better separation of condensate, while avoiding overheating of an enclosed space that might be too small. Good ventilation must be provided if the receiver is setup indoors.
Air accumulators are secondary receivers installed at intermediate locations to equalise pressure variations within the system so as to ensure that operating pressure is as constant as possible for all consumers. Such intermediate accumulators should be provided for each of several consumers. Pressure drops in long lines are thus compensated and flow velocity in the piping can be maintained at the optimum.
Without intermediate accumulator, sudden large consumption of air may cause temporary breakdown of line pressure, resulting in abnormally high flow velocities in the air main, excessive cooling of air, and thus increased condensation at these points.
AIR MAIN is the piping system into which the compressed air is led from the receiver. It is permanently installed system of interconnected pipes carrying the air to the connections for the various consumers. Main criteria to be considered are,
pressure drop in piping
tight joints throughout the main
PIPE SIZE: Pipe sizing is governed by
permissible flow velocity
permissible pressure drop;
number of flow restrictors in piping
length of piping
"Rate of flow", that is the air consumption rate, is a quantity that must be determined in advance by the
planning engineer. Flow velocity and pressure drop are closely related to one another. Roughness of the inside walls of piping and the number of fittings installed will also affect pressure drop.
"Flow velocity" of compressed air in the mains should be between 6 and 10 m/sec. Every effort should be made to keep the velocity below 10 m/sec. Pipe elbows, valves, reducers and hose couplings cause the flow velocity to rise above the permissible figures at many points. Temporary increase in flow velocity also on actuating devices using air at a high rate.
"Pressure drop" should preferably not exceed 0.1 bar. Another measure used in practice is 5% of
"Flow restrictions" are formed in air mains by the valves, bends and tees installed. For calculation of
the inside pipe diameter such restrictions must therefore be converted to the equivalent pipe length, which is then added to the remaining pipe length of the main.
Permanently installed air mains piping should be accessible from all sides. Horizontal runs of air pipe
should be slope downwards 1-2 % in the direction of flow. Vertical main lines should not terminate at
a consumer take-off, but should run further down so that condensate precipitated in the main will
collect at the lowest point of the branch line where it can be drained off and will not pass to the consumer.
Air mains are preferably constructed of steel pipe with welded joints. In the long run welds are more
durable than any screwed joint. A drawback associated with welding is the formation of scale during welding, with the tendency of the weld to rust in time. The advantage of welding pipe lies in the tight sealing of joints and lower cost.
Isolating valves(gate valves) must be installed to divide the air main into sections so that it will
not be necessary to shutdown and depressurize the entire main when maintenance or repairs become necessary.
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