Marine Air compressor

What is Air compressor ?

It is a machine which compresses the air at Ambient temperature(15 degree Celsius) & atmospheric pressure (1 bar ) to higher pressure which that it’s density increase.

Safety cut out fitted on the air compressor

1.Lubricating oil pressure low cutout :-It is provided such that if lub oil pressure falls down. The parts which are lubricated like liner, Piston, main bearing, bottom end bearing might get worn out.
2.Bursting Disc:-It is given in intercooler and after code in the waterside so that if any highly pressurised air tube burst, the cooler shell will not be pressurised, bursting disc will burst and liberate all the water, indicating air bursts.

3.Non return valve in delivery line.

4.high temperature alarm

At-around 90 degree Celsius, the alarm sound denoting the rise in air temperature.

5.fusible plug

Generally made of tin, aluminium, and Bismuth is fitted in inter and aftercooler to release excess air,when temperature rises up to 121 degree Celsius due to rise in air temperature.

6.discharge unloader

Used for unloaded starting of compressor and draining at continuous level and interval.

Starting main air compressor manually

A) change the switch to manual position on the switch board.B) check the lube oil sump level and conditions) open both the stage drain valve) open the compressor discharge discharge valve and feeling Valvo valve of air bottle.E) open cooling water system valves.F) Turn the compressor flywheel by hand ( 1 tURN)G) start the motor after draining the moisture shut the drain valve.H) check the motor ampere consumed) check the pressure gauge reading) Frequently drain the moisture)When air bottle in full, open drain valve and stop the compressor.

use of compressed air onboard Vessel

A) starting of compression ignition type diesel engines,namely main engine & generator engines.
(pressure 20 to 35 bar )

B)General maintenance such as cleaning of filter, cooler tubes etc.(7 to 8 bar)

C) operating pneumatic tools such as hand grinder, pneumatic chisel
(7 to 8 bar)
pneumatic hand drill,neu matic portable pump etc.

D) signalling equipment such as a air horn.

E) pneumatic controllers such as pneumatically controllers,temperature controller, pressure controller, viscosity controller etc. (4 to 5 bar)

F) hydrophore system (7 to 8 bar):- depending of on the consumption vessels may have a set of Main air compressor producing compressed air of 35 bar pressure.
low pressure is generated by passing the high pressure air through reducing valve ,set at desired pressure.

On board vessel there are normally two no. Of air compressor for filling main air bottle from which compressed air is available for various purposes.There is also an emergency air compressor for feeling auxiliary air bottle from which compressed air is available for starting auxiliary engine in case of emergency. main and emergency air compressor are reciprocating type. In some ship mainly tanker there is an working air compressor ( rotary type) Feel Up and working air reservoir.

Reason for excess carbon deposit on the valve

A)Lube oil carry over due to defective strapper ring.B)overheating of the valve due to lack of cooling) dirty suction fielder)Leaky valve.

Two stage compressor

Sketch above shows a two stage reciprocating air compressor.

Air is sucked into the L.P (low pressure ) cylinder during the downward movement of low pressure piston.
The air passes in the low pressure cylinder through a suction filter and L.P suction valve. During the downward stroke L.P discharge valve will remain closed.

As the L.P piston moves up, pressure inside the cylinder rises which causes the L.P suction valve to close and L.P discharge valve opens. AIR from L.P cylinder is discharged through intercooler to H.P cylinder during the downward movement of HP Piston.

Upward movement of HP piston compresses the air in the H.P cylinder and the compressed air passes through the HP discharged valve and then through intercooler to Air reservoir.Relief valves are fitted to each stage to limit the pressure rise to 10% of design pressure.

A high temperature cut out of the fusible plug limit the H.P discharge to 21 degree Celsius.Intercooler are also fitted with bursting disc or relief valve on the water side.

Normal parameter of air compressor

L.p discharge pressure :4 bar

HP discharge pressure :3o bar

intercooler inlet air temperature: 130 degree Celsius intercooler

outlet air temperature: 35 degree celsius

aftercooler inlet air temperature: 130 degree Celsius

aftercooler inlet air temperature: 35 degree celsius

Difference between single and multi stage compressor

The basic difference is that how many times that air get compressed between inlet valve and reservoir

Multistage advantage:-

A) dual stage compressor produces higher air power

B) reduction of thermal stress

C)work done in compressing the air is reduced and energy can be saved.

D) Increase compressor efficiency.

E) small machine and better balancing.

F)effect of moisture can we handle.

G) prevents air and mechanical problem as the air temperature is controlled.

H)compression approached isothermal.

Air bottle

it is a large container acting as a reservoir to store compressed air supplied by the main air compressor of ship at high pressure around 28-30 bar.
This compressed air is very important of start main engine or auxiliary engine.

purpose of air bottle:-

high pressure is used for initial starting of marine I.C engines.It also supplies control air to the main ebgines.service air is supplied from the air bottle.if the quick closing valve are Air operated, safety air is supplied through air bottle.Spring air for exhaust valve is provided by air bottle
Air bottle mounting :-

A)Filling or inlet valve

B)Outlet valve to main engine.

C)outlet valve to auxillary engine.

D) auxiliary connection :-(with isolating valve) -service air, safety air etc.

E)relief valve :- to relieve Express pressure

F)Drain valve:- it is fitted at the bottom of bottle to drain accumulated condenser from the reservoir.

G)Fusible plug:- fitted in the bottle in the bottle with the separate connection leading out of the engine room so that in the event of fire,thus plug Will melt and receive all the air to the outside atmosphere.

H)Manhole door:-for carrying out inspection.

safety devices on air bottle:-

A) relief valve

B)pressure gauge

C)pressure switch

D)Elliptical door

E)drain valve


[ compressor 1.1 to 1.4]

(Pressure is less than 5 bar)

–It is a positive displacement machine used to move gas or air and their speeds are higher than fans( used to handle large volume flow of air with pressure ratio varying to 1.1 )


A) ventilation purposes of engine pump room,pump room enclosed space etc .

B) supplying air for combustion chamber

C) supply and remove the air from engine and pump room i.e remove the contaminated air and give the fresh air. examples – Soot blower

Difference between blower and compressor


A) equipment which is capable of blowing air/gases( scroll fan)

B) capable of providing kinetic energy to the gases.

C)compression ratio 1.1 to 1.4(Pressure less than 5 bar)

D) It removes airs.

E) consists of a rotatory mechanism, so it can rotate in both directions.


A)Equipment which compresses air/gas or fluid.and used in Air- conditioning system.

B) capable of increasing pressure or interval energy.

C)compression ratio more than 1.4 (more than 5 bar)

D)It can remove air.

E)It consists both rotating & reciprocating mechanism but,it can rotate in one direction.

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