Details Of A Marine Compressor (For Trainee Marine Engineers)

Air Compressor:

On a ship an air compressor is used to compress air and store it in tanks for various uses. These use these could range from simple processes such as cleaning and drying of filters to tasks such as starting the auxiliary engines and the main engines of the ship.

Types: 

There may have some others type but following types are widely used on board ship.

1. Screw Type

2. Reciprocating Type : a. Single Stage b. Multi stage

Here we will discuss about reciprocating type air compressor. 

Operating Principle:

The basic construction of a reciprocating compressor is similar to that of an internal combustion engine. It consists of the familiar piston and cylinder arrangement, having inlet and outlet valves for the entry of normal air and the exit of compressed air respectively. The other component is present in the bigger engines such as the connecting rod and the crankshaft are also their in the reciprocating compressor. This arrangement is depicted clearly in the diagram shown alongside and you can see the various parts which have been described here. The more the power required from the compressor, the greater the number of cylinders in it.

For easy understanding watch this Video

Uses of compressed air :

               25 – 30 bar

Ø Starting of main & auxiliary engines

Ø   Boiler soot blowing


4 – 7 bar

Ø Service air/ Cleaning Purpose.

Ø Whistle air

Ø Pneumatic tools

Ø Life boat, pilot ladder

1.5 – 2 bar

Ø   Instrumentation & control

Emergency air compressor: 

Emergency air compressor is used for starting auxiliary engine at the time of an emergency or when the main air compressor has failed for filling up the main air receiver. This type of compressor can be motor driven or engine driven. If motor driven, it should be supplied from emergency source of power.

Rules-Regulation-Requirements: 

Compressors

·         Minimum 2 or more compressors.

·         Sufficient capacity to charge air receivers within 1 hour from atm. to max. Pressure to provide.

·         6 starts for non reversible engine

·         12 starts for reversible engine

1 emergency air compressor & receiver for dead ship condition

Air Receivers

·        At least 2 air receivers with sufficient capacity without replenishment and able to provide

·         6 start for non reversible engine

·         12 start for reversible engine 

Safety Devices:

1. Bursting discs are fitted on the cooler shells (At water side).
2. Relief valves are fitted to discharge side for every stages.
3. Moisture drain valves (unloader) are fitted at each cooler side
4. Cooling water failure alarm.
5. Low L. 0 pressure alarm.
6. Delivery air high temperature alarm on after cooler outlet (Max 93°C).

Unloader Valve:

At starting this valve must be opened, this reduced the starting torque and

clear out any accumulated moisture and oil in the system.

Different between relief valve and bursting disc: 

Pressure relief valve

• Excess pressure is released by opening the valve.
• It opens at 10% over working pressure.
• Valve lift is proportional to excess pressure build up.
• Valve setting pressure can be altered by spring tension .

Bursting disc

• Pressure is released by bursting the disc.
• It permanently damaged.
• It bursts at setting pressure.
• Setting pressure cannot be altered in place.

Fusible plug:

When the temperature high (above 105 °C) Pressure is released by melting (fusing) the

metal.

• It cannot be used next time. ( permanently damage)
• Release all content or pressure to empty

Advantages of Multistage Air Compressors:

• More stages are needed to increase the required final pressure.
• Easier to control the air temperature.
• Reducing in air compressor size.
• Lubrication problem does not exit.
• Reduced the thermal stress.
• Lower work done to compressing air.
• Improve compressor efficiency. 

Why inter-cooler is fitted in Main air compressor?

1. Reduced air temperature, volume and increased air density for next stage

2. So increased Volumetric efficiency and compressor efficiency.

3. Due to reduced temperature give better lubrication for cylinder and piston rings

4. Drain valves are fitted from which water and excessive oil can be drained out, to prevent air bottle corrosion and starting air explosion and fouling of inter coolers and pipe.

5. Save the work done.

Advantages of after cooler:

• Reduce final discharge air temperature thus air bottle seizure can be avoid.
• To reduce air volume after it has been compressed to the final pressure.
• So greater amount of air could be stored in air bottle.
• Increase volumetric efficiency.

Safety devices on main air bottle:

• Pressure relief valve.
• Fusible plug .(if safety valve is not directly fitted on the bottle}
• Pressure gauge.
• Low air pressure alarm.
• Moisture drains valve.

bumping clearance:

This is a clearance must be provided between the piston top and the cylinder head when the piston is top dead center. It must be kept as small as possible for achieving the best compressor efficiency.

• To prevent mechanical damage to the compressor.
• To provide for thermal expansion and
• To provide necessary space for valve operation.

Suction and Discharge valve:

These are multi-plate valves made up of stainless steel and are used to suck and to discharge air from one stage to another and to the air bottle. Proper assembling of valves is very important for efficient operation of the compressor.

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References: 

1. Youtube 

2. Marineinsight. 

3.imimg.com 

4. Bright-Hub-Engineering 

5. Hand note of Mr Tar.

6.  Marine Engineers' World