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

Air compressorWhat is isothermal compression?Compression at constant temperature.

What is adiabatic compression?Compression under constant enthalpy condition.

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.

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

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

Theoretical method to calculate clearance volume

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

Three stage compression

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:

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

LubricationCertain factors govern the choice of lubricant for the cylinders of an air compressor, these are:Operating temperature Cylinder pressure Air condition

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.

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.

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.

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.

Compressor valvesModern valves are somewhat more streamlined & lighter in order to reduce the friction losses & valve inertiaMaterial used in the construction are generally :

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)

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

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.

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.

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

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

Automatic drain

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

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

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

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