Marine compressors are integral to the proper functioning of your vessel. Many pieces of equipment on-board your vessel are dependent on the compressed air that the marine compressor generates. This compressed air serves many purposes such as air filtration, nitrogen generation, and offshore drilling exploration. If you care about your vessel, you should care about the inner workings of these compressors.
There are 4 types of marine compressors: the reciprocating air compressor, the rotary screw compressor, the rotary vane compressor, and the centrifugal compressor.
Reciprocating Air Compressor
The reciprocating air compressor, also known as piston air compressor, is the most commonly used. This is a positive displacement compressor; this means that the compression is done through decreasing the volume of the container that the air is in.
There are two main types of reciprocating air compressors: the single stage and the two stage.
The single stage works by using a crankshaft to control the piston displacement. As the piston retracts, increasing the volume of the air container, air flows in through the inlet valve. Then, when the piston pushes up, the volume of the container decreases and the air is compressed. Now, after being forced out through the output, the air is bottled up and ready for use.
The two stage piston works in a similar way. However, instead of one piston, there are two, both controlled by the crankshaft. After the air is compressed by the first piston, instead of being immediately forced out, it goes through an intercooler, which cools the air using water. Next, the second piston compresses the air again. Finally, the air is ready to be bottled.
Because the two stage air compressor includes additional steps, it’s more expensive than the single stage. Resultantly, the one stage is normally used for small scale work whereas the two stage is used for large tasks.
Rotary Screw Compressor
Just like the reciprocating air compressor, the rotary screw compressor is a positive displacement compressor.
The rotary screw compressors make use of two screws spinning in opposite directions. A vacuum is created because of this motion, causing air to get stuck between the screws. During their rotations, the screws both compress the air between them and move it towards the output area, where it’s ready to be bottled and used.
These compressors require relatively low maintenance in comparison to other marine compressors.
Rotary Vane Compressor
The rotary vane compressor works in a similar fashion to the rotary screw compressor, essentially using vanes – wide and rotating blades – instead of screws.
The air first comes through an input valve and gets stuck between the vanes. The middle point of the vanes is off-center and the blade lengths are uneven, so as the air moves with the blades, it has less space and gets pressurized. In this compressed state, the air goes through the output and is ready for use.
Just like the rotary screw compressor, this type of compressor is also easy to use and maintain.
Unlike the reciprocating air, rotary vane, and rotary screw compressors, the centrifugal compressor doesn’t use positive displacement to compress the air. Instead, it implements dynamic displacement, which increases the speed of the air and then restricts its motion to build pressure.
Air is first brought to the center of a spinning impeller, similar to a fan, where it is pushed radially outwards due to centrifugal force present. The air speeds up as it moves around the casing, until it hits a diffuser. Because the movement of the fast air is restricted by the diffuser, pressure builds. From here, the pressurized air is ready to be bottled.
The centrifugal compressor is mostly used for large scale tasks.
To understand more about marine compressors, check out our “Marine Equipment Series: Understanding Marine Compressors“