How does a rotary compressor works and the usages on a ship

Speaking to the air compressors,there might be many people have experiences of using them,but do all of you know exactly what is a rotary compressor used for on a ship?Let’s start with the working principle of a rotary compressor.

A rotary compressor is as simple as it looks,only a few components but it is a bit of hard to fully understand how the air goes through the rotors and then be compressed.Typically,a rotary compressor consists of two rotors named male/female rotors,kind of like the human being.When the air is drawn into the compressor through the inlet it is then to be compressed as the two rotors continues to rotate,a certain amount of air is trapped in the small space between the male and female rotors and with the continuous process of rotation the pressure of air is growing until it finally get to the outlet and then discharged into the air tank.

What is a rotary compressor used for on a ship?

There are a wide variety of usages a rotary compressor is applied on a marine ship,just like what it does on land,but there’s one thing that still remains uncleared,as we know a marine diesel engine is started by the compressed air but normally there will have an extra reciprocating compressor per engine(For offshore drilling units there are often more than three diesel engines) to help the diesel engine start to run,why wouldn’t the engine use the air from the tank that filled by the service compressors?To answer this question,you must have the basic idea that for a engine to start the required PSI the starting air must has is way higher than the compressed air used for the common air tools,that is why on a ship or a mobile offshore drilling unit there are two types of air tanks,one is for the engine and the other is for the common usages.Let’s break down this question,why a reciprocating compressor for engine start instead of a rotary compressor?In my guess,it is first due to the reciprocating compressor can attain a much higher air pressure than a rotary one,and the second reason is that the former one is slightly cheaper than the latter,am I right?

Regardless of the second question,if the starting air pressure a marine engine must has is as same as the common applications,then there is no need for the extra starting air compressor,that is what it is designed on the jack up I used to work on.There are also some other usages the compressors are applied to on a ship or oil rig which are seldom seen on land,such as air winches and quick ballast system,ballast pumps are seen as the normal ways to pump water in or out of the ballast tanks but for some Special Occasions we use compressed air for quick ballasting,I will write something about this system in the future.

Ijin Marine Limited is a repairing agent in Shanghai, providing all kinds of repair,supply,service etc in more than 300 ports of 80 countries.