MARINE ENGINEERING SYSTEMS
Compressed air remains a vital resource onboard a ship and is used for various applications and at various pressures, from the operation of pneumatic tools, to the starting of main propulsion engines. Compressed air on-board ships are generally used for the following applications:
- High Pressure Starting Air greater than 20 bar. This high pressure compressed air is used for the barring over of the Prime Movers and Diesel Generators.
- Low Pressure Control Air of approximately 5 bar. This low pressure compressed air is used for operating pneumatically activated actuators.
- Low Pressure General Service Air of approximately 6 – 8 bar. As its name states, this low pressure compressed air is used for general service i.e. cleaning, inflating, operation of tools etc.
The compressed air generated by an air compressor is stored in High Pressure or Low-Pressure receivers for use by its consumers and depending on its application. As the consumers makes use of the compressed air, the pressure in the receivers are reduced triggering the air compressors to start-up and fill the receiver back to the required pressure.
Air Compressor Types
Four types of air compressors can be found on-board a ship, namely:
- Centrifugal Air Compressors
- Rotary Vane Air Compressors
- Rotary Screw Air Compressors
- Reciprocating Air Compressors. These compressors are the most widely used in marine applications and will therefore be the only type of air compressor to be discussed here.
Reciprocating Air Compressors
Out of all the reciprocating air compressors available in the marine built environment and shipbuilding industry, the 2-Stage Reciprocating Air Compressor is probably the most commonly found on board ships.
With reference to the below diagram depicting the major components of a 2-Stage reciprocating air compressor, this type of compressor compresses air in two stages as follows:
- Stage 1: The ambient air is drawn via the first stage delivery valve due to the suction created by the first stage suction valve, and compressed by the first stage piston.
- Stage 1b: The first stage air then passes through an inter-stage cooler. The purpose of the Inter-stage cooler is to reduce the air temperature, thus reducing the volume, after the 1st stage compression, and increasing the mass of air for the second stage compression. To this extent, the work done is saved and the air compressor efficiency is increased.
- Stage 2: Already compressed and cooled air is delivered to the 2nd stage piston via the second stage delivery valve with suction created by the 2nd stage suction valve. Here it is compressed to a higher, final pressure. The stage two-cylinder is a smaller cylinder than stage 1 to produce higher pressure.
During the compression process, condensate forms in the compressor due to the humidity in the air being compressed, therefore each air compressor will have a drain valve to periodically drain the condensate. It has happened in the past where personnel does not drain the condensate, causing damage to the air compressors and shortening its lifespan. Some air compressor manufacturers resolve this issue by installing one condensate draining point per stage with automatically actuated drain valves.
The Control Compressed Air System
The below diagram shows a simplified control compressed air system diagram for the provision of control air to its various users and/or mechanisms on-board a ship. Control Air is mostly used for operating pneumatic valves, as well as for controlling vital functions of the ship’s engines i.e. engine speed, direction of running and starting/stopping/reversing. Due the important nature of the control air, it needs to be as clean as possible. Thus, filters for the removal of particulates and oil, as well as water traps and drains for the removal of condensate is placed at various positions in the system. Various isolation valves are also fitted throughout the system to isolate separate sections in the event of repair.
With reference to the above diagram, the following constitutes a general control compressed air system:
1 – An air intake filter removes all large particulates from the ambient air preventing any unnecessary damage or blockage to the compressor.
2 – In marine applications, the primary compressor will mostly be a multi-stage reciprocating air compressor. A pressure relief valve would be fitted just after the compressor to prevent overpressure damage downstream of the compressor outlet.
3 – The compressed air discharged from an air compressor is generally hotter than the ambient air temperature. Compressed air at these temperatures contains large quantities of water in vapour form. As the compressed air cools this water vapour condenses into a liquid form. To remove as much moisture from the air as possible, air needs to pass through an Aftercooler to remove all water vapour by means of condensation. An automatic moisture separator and drain trap is installed directly downstream of the Aftercooler to separate the water/moisture from the cooled down air.
4 – The air receiver stores the compressed air at a high pressure. Usually 40 bar.
5 – High pressure air is tapped from the high-pressure air receiver, passes through a particle filter to remove any particles, like rust, that might have dislodged in the lines or the air receiver. The high-pressure air then passes through an air dryer. It does exactly what its name entails, it dries the high-pressure air from the remaining water vapour that might be in the system at this point. Any remaining oil contaminants are removed after this stage by means of a fine oil filter.
6 – Now that the air is free of particulates, moisture and oil, it can be distributed to the various users on-board the ship. Not all users however operate on high pressure 40 bar air. Control air, for example, needs to be reduced to approximately 5 bar for use. The high-pressure air is then stored in the specific user’s air receiver. It passes through another filter to remove in particulates or contaminants that might have dislodged in the air receiver. Finally, the pressurized air passes through a pressure regulator to reduce the pressure down to the required 5 bar.
7 – Finally, the air is passed to the end-users and/or mechanisms making use of the compressed air.
The Compressed Air Start System
The air start system of the ship’s main engines makes use of a similar compressed air system supply, but just at a much higher pressure than that of control air. The diagram shown below is an example to illustrate the application of compressed air for engine starting.