Basics of Main Air compressor on board Ship

Main Air compressor on ship is used to produce service, control and starting air. In the following article we explain the introduction, working and maintenance done on Main Air compressor.

What is a compressor?

A compressor is a machine capable of compressing and delivering the air at desires pressure and is driven by a prime mover(Engine or a electric motor). A compressor takes in atmospheric air, compresses it and delivers the high pressure air to a receiver(storage vessel) from which it may be conveyed by a pipeline.

Applications of compressed air on Ship:

• For control air and service air application on board ship.
• For various automation and controls, general service applications.
• For starting main and auxiliary engines
• To supercharge IC Engines
• painting and spraying.
• For operating pneumatic hand tools such as rivet sets, drills..
• To operate air brakes in automobiles , ram lifts and pneumatic conveyors.
• To blow air.
• To transmit power for operation of machines.
• To clean machines and work shop floor

Types of Compressors: 

Types of air compressors in use:

1. Reciprocating  Compressor
2. Centrifugal Compressor
3. Screw Compressor
4. Rotary compressor
5. Diaphragm compressor

Classification of Air Compressors -

1.According to working: Reciprocating compressors, and (b) Rotary compressors.

2.According to action: Single acting compressors, and (b) Double acting compressors.

3.  According to number of stages: (a) Single stage compressors, and (b) Multi-stage  compressors.

Main Components in Compressed Air Systems:

• Intake air filters
• Inter-stage coolers
• After coolers
• Air dryers
• Moisture drain traps
• Receivers

Reciprocating Compressor:

The first commercial piston compressors were built in the middle of the last century, and evolved from the steam engines which provided the prime mover. Construction at first was double acting, but there was difficulty in maintaining gas-tightness at the piston rod, so the design evolved further into a single-acting machine.

The stroke/ bore ratio has diminished to the point of becoming fractional because of improvements in valve design and manufacture. Machines of four, six and eight cylinders arranged in either V or W formation are common. These are arranged in a multi bank configuration with two, three or four connecting rods on the same throw of the crankshaft to give a short, rigid machine.

This construction gives a large number of common parts – pistons, connecting rods, loose liners and valves through a range of compressors, and such parts can be replaced if worn or damaged without removing the compressor body from its installation.

Working Cycle

The general form of positive displacement compressor is the piston type, being adaptable in size, number of cylinders, speed and method of drive.

It works on the two-stroke cycle, as the piston descends on the suction stroke, the internal pressure falls until it is lower than that in the suction inlet pipe, And the suction valve opens to admit air from atmosphere. At the bottom of the stroke, this valve closes again and the compression stroke begins. When the cylinder pressure is higher than that in the discharge pipe, the discharge valve opens and the compressed air passes to the air bottle. Clearance air left at the top of the stroke must re-expand before a fresh charge can enter the cylinder.

Single stage Compression

1. It suffers the following drawbacks:
2. The size of the cylinder will be too large.
3. Due to compression, there is a rise in temperature of the air. It is difficult to reject heat from the air in the small time available during compression.
4. Sometimes, the temperature of air, at the end of compression, is too high. It may heat up the cylinder head or burn the lubricating oil.

In order to overcome the above mentioned difficulties, two or more cylinders are provided in series with inter-cooling arrangement between them. Such an arrangement is known as multistage compression.

Advantages of Multistage Compression

Following are the main advantages of multistage compression over single stage compression.

1. The work done per kg of air is reduced in multistage compression with intercooler as compared to single stage compression for the same delivery pressure.
2. It improves the volumetric efficiency for the given pressure ratio.
3. The sizes of the two cylinders (i.e. high pressure and low pressure) may be adjusted to suit the volume and pressure of the air.
4. It reduces the leakage loss considerably.
5. It gives more uniform torque, and hence a smaller size flywheel is required.
6. It provides effective lubrication because of lower temperature range.
7. It reduces the cost of compressor.
8. In commercial shipping industry, highest air pressure requirement is about 30 bar (M/E starting air).

It is a standard practice to use two stage reciprocating compressors on board ship to satisfy all compressed air requirements on board ship.

Operation and maintenance:

Compressors must always be started in the unloaded condition otherwise pressures build up rapidly producing very high starting torques. During running there is an accumulation of oil carried over from the cylinders and water from moisture, precipitated in the coolers.

The emulsion is collected in separators at cooler outlets and these must be drained off regularly, to reduce carry over. This is extremely important, first to prevent any large quantity of water and oil emulsion reaching a subsequent compression stage and causing damage to a further stage and secondly to reduce the amount carried over to the air receivers and starting air lines.

Moisture in air receivers can give rise to corrosion and despite the proper operation of compressor cooler drains, a large amount tends to collect, particularly in humid conditions or wet engine rooms. It is good practice to check air reservoir drains regularly to access the quantity of liquid present.

In extreme conditions, drains may have to be used daily to remove accumulated emulsion. This is very important if air for control systems is derived from the main receivers, to prevent problems with the reducing valve, moisture traps and filters.

Moisture traps for the control air system also require regular checking and possibly daily draining. A compressor is unloaded before stopping by opening first and second stage drains.

Effect of Choked Inlet Filter

Safety Devices:

1. Relief valve fitted to every stage
2. Fusible plug   (melts at 120°C)
3. Bursting disc or relief valve for cooling jacket
4. Alarms and cut outs for:
• High air temperature
• High cooling temperature
• Low lubricating oil pressure

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Author Amit                                                                            Article Requested By: Abhishek Verma

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