Air Compressor & Receiver

Uses of compressed air

25 – 30 bar

1. Starting of main & auxiliary engines 
2. Boiler soot blowing 

4 – 7 bar

1. Service air 
2. Whistle air 
3. Pneumatic tools 
4. Life boat, pilot ladder 

1.5 – 2 bar

1. Instrumentation & control

Installation Requirements

Compressors

1. Minimum 2 or more compressors 
2. Sufficient capacity to charge air receivers within 1 hour from atm. to max. pressure to provide 
3. 6 starts for non reversible engine 
4. 12 starts for reversible engine 
5. 1 emergency air compressor & receiver for dead ship condition 

Air Receivers

1. At least 2 air receivers with sufficient capacity without replenishment and able to provide 
2. 6 starts for non reversible engine 
3. 12 starts for reversible engine 

Pressure Relief devices

1. Relief v/v to be fitted after each stage of compression 
2. Relief v/v to be fitted on the air receiver 
3. Relief v/v or bursting disc to be fitted on inter & after coolers 

Air Temperature

1. Limited to 93 C to prevent explosion 
2. Fusible plug or high temp cut out provided as safety measure 

Pressure Test:
Cylinders, cylinder covers, inter & after coolers are tested by hydraulic pressure to twice their working pressure.
Casing of intercooler and after cooler is hydraulically tested to 1.5 times the cooling water pressure

Multistaging & Intercooling

Advantages:

1. Compression work is reduced 
2. Vol efficiency and hence compressor capacity is increased 
3. It approximates isothermal compression 
4. Vol of air delivered is reduced; as a result reduced size of cylinder, piston and delivery pipes is required 
5. Reduced air delivery temp gives less thermal &mechanical loading and better lubrication of piston & cylinder 
6. Due to less temp, suc & del valves remain clean 
7. Facilitates moisture separation 
8. Use of intercooler allows smaller receiver 

Clearance Volume

To provide for thermal expansion and prevent the piston from striking cylinder cover, a small clearance, called bumping clearance is maintained between cylinder cover & piston .

1. Bumping clearance must be as small as possible 
2. High bumping clearance reduces vol effn. of compressor 
3. Reduces compressor capacity & increases disch air temp. 
4. Bumping clearance is effected by bearing wear & gasket thickness 
5. Low bumping clearance can cause mechanical damage 
6. Bumping clearance must be checked after overhauling 
7. Clearance volume is about 6% of swept volume

Factors affecting volumetric efficiency

1. Clearance volume - gaskets , bearing wear down 
2. Valves – dirty, leaky, spring tension 
3. Leakage past piston rings 
4. Insufficient cooling water due to dirty coolers 
5. High cooling water temp. 
6. High air inlet temp. 
7. Throttling of air intake due to 
8. Dirty intake filter 
9. Insufficient suction v/v lift 
10. Strong suction v/v spring 

Air Filter
Material – felt or paper, metal gauze or nylon strands

1. Remove contaminant to prevent 
2. abrasive wear of piston rings, valves 
3. deposit on valves casing source of ignition 

Very dirty filter will cause

1. Reduction in suction pressure 
2. Increase in compression ratio 
3. Increase in discharge temperature may cause coke deposit 
4. Less air flow and danger of overheating coke deposits 
5. In extreme case disch. temp.may exceed auto ignition temp of LO, resulting in an explosion

Suction & Delivery Valves

1. Low inertia automatic valves which operate on low diff pressure .
2. Material used for Valve seat – 0.4% carbon steel hardened & polished surface Valve plate – Nickel or stainless steel hardened & polished Spring – tempered hardened steel 
3. Usually of same type, only differences are spring load and the valve lift 
4. Excessive lift causes impact & v/v breakage 
5. Late closing of v/v affects volumetric efficiency 
6. Coke deposits make v/vs leaky & sticky causing overheating reduce vol. efficiency 

Piston

1. Trunk type piston is used in single acting compressor 
2. Tandem piston is used in multistage compact design 
3. Aluminium alloy material is used for good friction properties high strength with low coeff. of thermal expansion .
4. Piston rings material is alloyed cast iron 
5. HP piston rings are made thicker 

Bearings
Thin shell type in modern compressor Requires L.O. pressure above 2 bar Conventional thick shell can run even at 1.0 bar

Crankshaft
Usually forged carbon steel integral with counter weight Cast steel being fatigue prone is not used Inter-cooler & After-cooler

Unloader

Prevents compressor to start on load .

Some of many methods to unload compressor cylinder 

1. throttling of suction 
2. speed variation 
3. by-pass discharge to suction 
4. depression to hold suction v/v plates on their seats 

Last two methods are most widely used in marine compressors.

Air Receiver

1. Size depends on engine requirements 
2. Should have sufficient capacity to give

•  12 consecutive starts for reversible engine 
• 6 consecutive starts for non reversible engine

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 Reference:

 brighthubengineering.com

marineengineeringonline.com

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