There are three main types of air compressors. Which you need will depend on the air pressure you need and how portable your compressor needs to be.
Reciprocating compressors come in a range from 5 HP to 1000 HP. The smaller compressors are portable, sometimes being mounted on wheels. These may be petrol driven or electrically powered. They compress air by a series of pistons in cylinders. The pistons are driven by a camshaft. Reciprocating compressors are often used in Auto Body Repair shops.
Larger reciprocating compressors are gradually being replaced by simpler rotary screw compressors. A large reciprocating compressor can discharge air at 35 Mpa, more than 5,000 psi.
If you are looking for a compressor for continuous use, a rotary screw compressor may be suitable. These compressors use helical screws to compress the gas into a smaller volume. Portable and fixed rotary screw compressors are also available. These compressors vary in size from 5 HP to 500 HP. You will find rotary screw compressors that give you a low pressure gas output, or a high pressure output, up to 8 Mpa. These compressors are used to power pneumatic drills when workers are digging up roads and pavements.
For continuous, heavy industrial uses you need a centrifugal compressor. A centrifugal compressor uses a rotating disk with vanes. Output from centrifugal compressors can be as high as 70 Mpa, 10,000 psi. Snow blowing machines use this type of compressor. Superchargers and turbochargers in car engines are centrifugal compressors.
When you buy compressed air for diving, the air has been compressed using a three stage reciprocating compressor. The cylinders in the compressor are ceramic lined, so no lubrication is needed. This avoids the air in the cylinders being contaminated with lubricating oil. If a non-recommended lubricating oil is used it can undergo partial combustion and cause carbon monoxide to enter the air cylinder.
The compressed air is filtered through silica gel to remove water, then through activated charcoal to remove any traces of oil.
Sometimes air from a cheap, low powered compressor will be stored in a bank of large high pressure cylinders. Air from these cylinders is then used to quickly fill diving cylinders. The diving cylinders need to be filled slowly to prevent the compressed air inside them from getting hot. If the gas inside is hot, it will expand and the cylinder maximum pressure will be reached with less air inside. The gas cools and the cylinder may only be half full.