Scuba diving is a lot of fun, I myself love scuba diving! Unfortunately I live in a part of the world where you can only see mud, sand, and all sorts of boring gray-looking fish. So whenever I am on holiday in a tropical country, I love to go scuba diving (like on Sipadan, Malaysia, the number one diving spot in the world! )
But this site is not about scuba diving, it is about compressors. What's the link? Compressed air!
Compressed air for scuba diving
Small scuba air compressor
To breath under water, scuba divers carry a air-tank with them filled with compressed air. For recreational diving this is normally normal atmospheric air, but compressed about 200 times to 200 bar (or 3000 psi). The scuba diver uses a pressure regulator (in their mouth piece) to bring back the air from the high pressure to atmospheric pressure (when at the surface) or the local underwater pressure, depending on the depth.
There is nothing special about the air. It's just normal atmospheric air which is compressed 200 times, so you can take more of it with you on your underwater trip. The compression is done by a scuba air compressor.
Scuba air compressor
Obviously, the job of the scuba air compressor is to compress the air from 1 bar atmospheric pressure to about 200 bar.
Of course, we don't want any oil, chemicals or dust to contaminate the air. We also want a compressor that can produce up to 200 bar of pressure at a reasonable flow rate, so it doesn't take too long to fill-up the scuba tanks.
Since the compressor is usually located at the back of the dive-shop. And the dive-shop is usually located on a nice, white, tropical beach... so compressor noises can be a problem.
In short: our perfect scuba air compressor produces clean, dry, oil-free air, and lots of it without making any sound. Of course, this is impossible, but let's check out the possibilities and issues that can come up when buying, installing or using a compressor for scuba air.
Compressor types
When shopping for a scuba air compressor, you will notice there are different types of air compressors, all with different price-tags. Most scuba air compressor air three- or four-stage reciprocating compressors. Their working principle is not different than other industrial reciprocating piston compressor, except that they have more stages, to be able to create the high pressure of 200 bar.
A basic distinction can be made between oil-lubricated and oil-free diving air compressors. The oil-free type uses special ceramic cylinder liners and rings which require no lubrication. The big advantage of course is that there is no compressor oil, so there is no chance of contamination of the breathing air. But, of course, they are a lot more expensive compared to oil-lubricated compressors.
The oil-lubricated type is the most common kind of compressor and much cheaper compared to oil-free compressor types. This type uses oil to lubricated the moving parts and the inside of the cylinder. This means that the breathing air comes in contact with the oil, and there will always be a little oil that leaves the compressor together with the air (called oil-cary-over).
This oil (although a very small amount per liter air) needs to be removed, for the obvious reasons. This is done by the use of special filters. There are filters for dust, water and oil.
Scuba air treatment
Breathing air for scuba diving coming directly from your compressor is not suitable for use directly. I needs to pass a set of filters first, to improve the air quality.
While air can be compressed, water cannot. So all the water vapour in the air that is sucked-in by the compressed will enter your compressed air system. This water needs to be removed to avoid filling your tanks with water instead of air :) (you'll be surprised how much water is created in a day).
There will be a water trap, sometimes together with a compressed air dryer (desiccant dryer) to remove most water from the air. The main point in removing the water from the air is to prevent your tanks from rusting.
After that water is removed, we need another filters to remove oil particles, CO and CO2. The oil obviously comes from the compressor. The CO and CO2 are sometimes sucked in by the compressor, from the outside air. When your scuba diving compressor is driven by a small petrol or diesel engine, there is a risk that it will suck-in the exhaust fumes. Always make sure that this is not possible and the compressor sucks in fresh air.
System layout
If you use the scuba air compressor for personal use, all you need to fill up your tank is a high-pressure hose between your compressor and your scuba tank.
If you own a scuba shop / diving center and you will a lot of tanks throughout the day, it might be a good idea to use a storage tank, together with a distribution board / filling stations.
You can fill up the big storage tank in the early morning. You could even do that at a higher pressure (compressor, tank and piping permitting of course) for extra storage capacity.
In the early morning, the outside air will be cold, giving you less problems with water in the compressed air, and theoretically higher compressor efficiency (=lower electrical / petrol bill). Plus, you don't need to run your compressor all day long, which means no annoying background noise. This way, scuba tanks can be filled fast and efficiently.