Marine Air Compressor: Working and Types

Nowadays in various industries and even on the ship, we are using much equipment driven by compressed air, we call them pneumatic driven equipment, for example, drill machine, spray paint gun, grinder, chipping gun etc. At any place to avoid chances of fire we use pneumatic control system instead of electrical control. Onboard ship compressed air or high-pressure air having the number of uses like-

  • Starting of diesel engine
  • Control and instrumentation purpose
  • Cleaning equipment
  • Ship’s whistle/horn
  • Soot blowing of boiler

By considering the above mentioned important work we can say that the air compressor is the heart of the ship.

So in this blog let’s study about air compressor, different types, working principle.

What is the air compressor?

It is a device which is used to convert electrical energy into kinetic energy. Simply we can say that air compressor is a device which used to compress the air at a pre-set value in single or multistage. The initial atmospheric air will be available to us in pressurized form, i.e. having kinetic energy.

 

Working Principle

The function of air compressor is similar like that of an internal combustion unit, except it run by a prime mover and the internal combustion engine is itself is a prime mover. A piston moves in a cylinder with help of crankshaft provided with connecting rod. An electric motor(prime mover) is fitted to rotate the crankshaft. Air is sucking in the cylinder through the inlet valve and when the piston moves upward it starts compression. As pressure starts increasing inlet valve get closed and at a certain compression pressure delivery valve gets open and compressed air get discharge to next stage or air bottle.

Cycle of Operation

The whole operation will complete in 4 stages –

  1. Opening of suction valve or Suction of air

Here in this first stage, when the piston starts moving downward, the pressure inside the falls results in the opening of the suction valve. The suction valve is a spring plat loaded valve, allow opening at designed negative pressure and closed all the time. The open suction valve allows the air to draw into the cylinder.

  1. Closing of suction valve

When the piston goes up, start pressurizing the air. Because there is pressure inside the cylinder, the suction valve will close of spring pressure.

  1. Opening of delivery valve

Further upward movement of the piston increases the pressure inside the cylinder. When the pressure reached a pressure of designed opening pressure of the delivery valve, allow the opening of the delivery valve.

  1. Delivery of Air

The opening of the delivery valve allows the discharge of air to the second stage of the compressor or to the air bottle.

Cycle of Operation of Air compressor
Effect of bumping clearance in cyclic operation

Explanations of this graph are already given in my previous blog of bumping clearance of air compressor

To achieve high discharge pressure like 30 bar which we required to start the engine on board, we need a multistage air compressor.

Multistage Air Compressor and its Importance –

A multistage air compressor is one, which having more than one cylinder of different diameter. The air in the first stage get compressed and to the second stage via an inter-cooler. Inter-cooler helps to achieve a temperature very close to ambient temperature. This cooled air is passed to the second stage where it is again compressed. The compressed air of the second stage passed to the third stage via an inter-cooler or into the air bottle via an after-cooler. After-cooler is fitted after final stage delivery to achieve temperature close to the ambient temperature.

Having multistage compressor process completed close to adiabatic. Avoid the rise in temperature. Such high pressure of 30 bar is difficult to achieve in a single stage because the temperature will go very high. In multistage we follow pressuring and cooling then again pressuring helps to achieve high desirable pressure without high temperature.  Multistage also provide small size and better balance.

Graph of advantage of multistage compressor

Advantage of the multistage compressor

Advantages of inter-cooler –

  • The process is close to isothermal. Which reduces the work done, thus saving the power.
  • Volumetric efficiency is increased. As by reducing temperature, we can increase the mass of the air.
  • Reduces the air temperature which allows less mechanical problems
  • Suction and delivery valve remains clean.
  • Low temperature allows better lubrication of piston and cylinder.
  • It’s become easy to separate moisture through an inter-cooler drain.

 

Type of air compressor-

For various purposes basically, there is 5 type of compressor used:

  1. Positive displacement
  • Reciprocating or Piston-type air compressor
  • Rotary screw air compressor
  • Rotary vane air compressor
  1. Dynamic displacement
  • Centrifugal air compressor
  • Axial flow air compressor

Positive displacement air compressor –

In a positive displacement compressor, the air mass is drawn into the volume of a cylinder. The volume of cylinder decreases to compress the air and after compression because of pressure deliver valve will open, which deliver the air to the storage bottle. Once pressure reached at desire pressure in storage bottle, pressure switch at air bottle will stop the compressor.

  • Reciprocating or Piston-type air compressor
Reciprocating air compressor
Reciprocating air compressor

It is a type of positive displacement air compressor in which cylinder, piston, connecting rod and crankshaft arrangement is used to compress the air. Up and down movement of the piston into the cylinder such and compress the air. An electric motor is fitted to rotate the crankshaft.

  • Rotary screw air compressor –
Screw Air Compressor
Screw Air Compressor

It is a type of positive displacement compressor in which two screw arrangement are meshed each other and connected with timing gear. An electrical motor is fitted to rotate one gear which rotates another one. The screw having helical grooves becoming narrow at the end. These helical groove carries the air from suction to discharge side and narrowing of the groove allow the air gets compressed.

  • Rotary vane air compressor –
Rotary Vane Air Compressor
Rotary Vane Air Compressor

It is also a type of positive displacement air compressor. This one having an arrangement of rotor and vanes (blades) in the cylinder. Air is trapped between the vanes and get compressed. Vanes are adjustable according to the casing and rotor is eccentric (not concentric) with a casing center. When the rotor rotates center of it shifted which moves inside the casing and govern the opening and closing of the valves. let’s understand with the help of the diagram.

Dynamic displacement

Dynamic displacement type of compressor having a rotational part which imparts its kinetic energy to the air and volute casing arrangement converts this kinetic energy to pressure energy. Imparting of kinetic energy to air is done by centrifugal force. Centrifugal and axial flow compressor are the types of dynamic displacement compressor.

  • Centrifugal air compressor –

This type of compressor having an arrangement of impeller, diffuser and volute casing. Impeller rotates and imparts the kinetic energy to the air. The arrangement is similar to the centrifugal pump.

  • Axial flow air compressor

In this compressor air flows along the shaft mean axial to the shaft. An impeller is mounted at the suction side imparts its rotating energy to the air. The then flows axially, narrowness and volute casing converts its kinetic energy to pressure energy.

Axial Flow Air Compressor
Source: WIKIMEDIA

The main air compressor is one of the most important machinery on board, that’s why two main air compressor and one emergency compressor is provided. All important machinery like the main engine, auxiliary engine and boiler also require air for starting. let’s think of a dead ship, initial aim to press up the air reservoir and one by one start other machinery. of course the first one will be the generator. once electrical power is restored, we can start the other machinery one by one. because of the very basic and important use of air is called the nerve of the ship and compressor is the heart of the ship.

In my next blog, we will discuss the maintenance, troubleshooting and the performance of the air compressor.

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Abhay Kumar

He is working as engineer in Shipping Corporation Of India, member of Nielsons Educational Forum. He is alumni of Marine Engineering & Research Institute(MERI) Mumbai. He loves driving and writing blog in his free time. Specialty : Tanker ship

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