What Role Does Compressed Air Play In A Marine Diesel Engine?

Have you ever thought about diesel engines in ships, tankers, or any other marine fit-outs, and how they are started? Equivalent in size to a four-story building, the main propulsion engine in ships is started with the help of compressed air at pressure of 30 bar. As you may or may not know, a ship is a moving mini-city or a mobile power plant, meaning that it has all facilities within, sometimes better than what we have offshore.

How many onboard compressed air systems do ships have?

These moving marine fit outs that are pre-designed in dry ports come with very advanced onboard compressed air systems. They typically have four to eight compressors onboard, sometimes even ten depending on how large a ship is. The compressors take suction from the engine room atmosphere which is normally under a slight positive pressure. They compress air in different stages and fill up huge air containers, which act as accumulators. They often compress air up to 30 bar, and keep the air containers filled up at all times. Air compressors are of varying capacity and are utilized as per the requirements onboard. The number of air containers and their capacity depend on the size and power of the engine.

What function does compressed air has on a ship?

The uses of compressed air onboard are quite diverse. Every ship has what is commonly referred to as a ‘First Start’ or ‘Dead Start’ arrangement. When the ship is totally “dead”, that is, without any power, meaning that no machineries are running and there is no compressed air in the air containers to start the auxiliary diesel engine (generator engine), then a provision is given to get the air containers filled up to bring back the ship to safe, normal working condition. In most cases, this is provided either by an emergency air compressor which normally is driven by a smaller diesel engine, or electric motor that gets its power supply from and emergency generator. Some of the many uses of compressed air onboard ships include:

• Economizing exhaust gas and for soot blowing

• Fire alarm operations and valves closing

• For sea and fresh water hydrophores

• For instrumentation and automation of different machineries

• For deck services and for carrying out chipping

• Pneumatic tools operations

• For general cleaning and engine room service

• For starting power generator or auxiliary diesel engine

• For starting the main propulsion engine

The layout of onboard compressed air system

The compressed air system onboard normally comes with four to ten compressors. There is also an emergency air compressor, but which is normally not counted as used machinery. Here are these compressors in a summary:

Service air compressors

They are mostly utilized for pressing up the control air container and hence for controlling the quality of air. Since they produce pure air, they are used for different automation purposes in the pump room of oil tankers and general ship’s engine room.

Topping up air compressors

They are used when the ship is sailing in mid-seas where the consumption of air is less. In this case, they are comparable of lesser capacity than the main compressors.

Main air compressors

They are utilized when a ship enters or leaves a port, but mostly for manoeuvring purposes. They are of a higher capacity than all the rest put together.