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AirCompressed air is used for starting main and auxiliary
diesels, operating whistles or typhoons, testing pipe lines
(e.g.CO2 fire extinguisher) and for workshop services. Air is
composed of mainly 23% oxygen ,77% nitrogen by mass and since these
are near perfect gasses a mixture of then will behave as a near
perfect gas, following Boyles and Charles laws
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Air compressorWhat is isothermal compression?Compression at
constant temperature.
What is adiabatic compression?Compression under constant
enthalpy condition.
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Multi-stage compressionIf we had an indefinite number of stages
of compression with coolers in between each stage returning the air
to ambient temperature, then we would be able to compress over the
desired range under near isothermal condition.This of course is
impracticable so two or three stage compression with interstage
& cylinder cooling is generally used when relatively high
pressors have to be reached.
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Shaded area show saved work during compression
To prevent damage, cylinders have to water or air cooled
&clearence must be provided between piston & cylinder
head
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Volumetric efficiencyVolumetric efficiency is the measure of
compressor capacity It is the ratio of actual volume of air drown
in each suction stroke to the stroke volume
Volumetric efficiency = Actual volume of air drown Stroke
volume
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Theoretical method to calculate clearance volume
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Practical method to calculate volumetric efficiencyRemove
suction discharge assembly from the unit & place a small loose
ball of lead wire on the piston edge, then rotate the flywheel by
hand to take the piston over top dead center. Remove & measure
the thickness of lead wire ballPut crank on top dead center, slack
or remove bottom half of the bottom half bearing. Rig a clock gauge
with one contact touching some under part of the piston. Take gauge
reading. Then by using suitable lever bump the piston, i.e. raise
it until it touches the cylinder cover. Take another gauge reading,
the difference between the two readings gives the mechanical
clearance
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Three stage compression
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Reciprocating air compressors at sea are generally two or three
stage types with interstage cooling The pressure & temperature
at various points would be roughly as follows:
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Drains Use to remove the accumulated moisture from the air
storage unitFilters Use to remove the suspended foreign particles
from the intake airRelieving devices It limits the outlet pressure
after each stage of compression
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LubricationCertain factors govern the choice of lubricant for
the cylinders of an air compressor, these are:Operating temperature
Cylinder pressure Air condition
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Operating temperatureAffects oil viscosity & deposit
formation. If the temperature is high this results in low oil
viscosity, very easy oil distribution, low film strength, poor
sealing & increased wear. If the temperature is low oil
viscosity would be high, this causes poor distribution, increased
fluid friction & power loss.
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Cylinder pressuresIf these are high the oil requires to have a
high film strength to ensure the maintenance of an adequate oil
film between the piston rings & the cylinder walls.
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Air condition Air contains moisture that can condense out.
Straight mineral oils would be washed off surfaces by the moisture
& this could lead to excessive wear & possible rusting. To
prevent this a compounded oil with a rust inhibitor additive would
be used.
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Two-stage compressor These compressors are generally of the
reciprocating type, with various possible arrangements of the
cylinders or they could be a combination of rotary first stage
followed by reciprocating high pressure stage.
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Compressor valvesModern valves are somewhat more streamlined
& lighter in order to reduce the friction losses & valve
inertiaMaterial used in the construction are generally :
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Valve seat0.4% carbon steel hardened & polished working
surfaces Valve Nickel steel, chrome vanadium steel or stainless
steel,hardened & ground, then finally polished to a mirror
finishSpringHardened steel (N.B all hardened steel would be
tempered)
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Effects of leaking valvesFirst stage suction Reduced air
delivery, increased running time & reduced pressure in the
suction to the second stage if the suction valve leaks badly it may
completely unload the compressorFirst stage deliveryWith high
pressure air leaking back in to the cylinder less air can be drawn
in, this means reduced delivery & increased discharge
temperature
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Second stage suction High pressure & temperature in the
second stage suction line, reduced delivery & increased running
time Second stage deliveryIncreased suction pressure in second
stage, reduced air suction & delivery in second stage.delivery
pressure from first stage increased.
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Regulation of air compressorStart stop controlA pressure
transducer attached to the air receiver set for the desired max-min
pressures would switch the current to the electric motor either on
or off. It must be remembered that the starting current for an
electric motor is about double the normal running current.
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Constant running controlThe compressor runs continuously at a
constant speed & when the desired air pressure is reached the
air compressor is unloaded in some way so that no air is delivered
practically no work is done in the compressor cylinders
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Figure shows diagrammatically a compressor unloading valve
fitted to the compressor suction. When the discharge air pressure
reaches a desired value it will act on the piston causing the
spring loaded valve close shutting off the supply of air to the
compressor
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Automatic drain
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Water under pressure at the inlet the disc will lift, allowing
the water to flow radially across the disc from A to the outlet B
when the water is discharged & air now flows radially outwards
from A across the disc,the air expands increasing in velocity
ramming air in to sea & the space above the disc, causing the
disc to close on the inletIn order that this pressure can fall
& the trap reopen, a small groove is cut across the face of the
disc communicating B & C through which the air slowly leaks to
outlet
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Starting air compressorsThese are required to charged the
starting air receivers & supply compressed air for veriety of
uses safety regulations demand at least two compressors one must
have an independent drive for an emergency use. With the high
pressure demanded & to obtained the best efficiency at full or
part load, reciprocating compressors are invariably used these are
two stage compressors, electrically driven Piston for each stage
may be arranged in line, either as a trunk piston or cross head
alternatively, to reduce size & weight a tangent arrangement
with one piston mounted above the other & driven from a common
cranck may be used
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Second stage piston is smaller than first stage for
corresponding reduction in the volume of air with a common piston
stroke Each stage has a light, spring loaded non return suction
& delivery valves of low inertia stainless steel
platesWater-cooled heat exchangers are fitted to act as an
intercooler,which cools the air after first stage compression &
an after cooler to cool it again after the second stage the maximum
air temperature at a compressor dicharge must not exceed 93 C &
a fusible plug or alarm may be fitted
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OverhaulingBefore overhaulthe makers handbook should be read
& special instruction must be adhered to. Ensure that all
receivers are fully charged & that remaining air compressors
are operational this will provide for any possible emergency in
which compressed air is required