The most common method of providing oil-free air aboard Navy ships is to use low- or high-pressure reciprocating oil-free air compressors. For the purpose of our discussion of the oil-free design, we will use the reciprocating low-pressure, oil-free air compressor as our example.

RECIPROCATING OIL-FREE LOW-PRESSURE AIR COMPRESSORS

For many years, the reciprocating types of air compressors had to use lubricating oil to reduce the friction and wear of piston rings sliding on the walls of the cylinders. Some of the oil, in the form of vapor, would be picked up by the airstream and carried to the outlet of the compressor. Like water vapor, oil vapor tends to collect in low places. When oil vapor mixes with water vapor, the resulting emulsion may deposit itself on machined surfaces, which generally become collecting points for dirt and particles of rust. This sludge formation can foul pneumatic valves, which must have very close clearances and operating tolerances. However, fouling of machine parts is minor compared to the potentially hazardous condition that results when oil vapor is entrained in the airstream of a high-pressure air compressor.

Figure 14-13.-Magnetic-type unloader.

From our earlier discussion, you should remember that air is mostly nitrogen (about 78 percent) and oxygen (about 21 percent). Nitrogen is an inert gas. When mixed with oil vapor, nitrogen is not hazardous. On the other hand, when oil vapor is mixed with oxygen in the air, two of the three elements needed for an explosion or fire have been combined. All that is needed to trigger this combination is heat. Where could the heat possibly come from? Think about it. When air is compressed, a lot of heat is produced. Now, lets consider what could happen when a careless operator quickly opens a valve in a high-pressure air system. The air and oil vapor mixture will flow rapidly into the next part of the system until it reaches the next obstruction, which will cause the air to be compressed. At this point, all three elements needed for an explosion are present-oxygen, fuel, and heat.

The danger associated with the use of lubricated air compressors has caused the Navy to replace low-pressure and high-pressure reciprocating air compressors with the oil-free type. Oil-free air compressors are designed to prevent any oil vapor from getting into the airstream in the compressor. (Navy standard oil-free compressors have not been developed for the medium-pressure range.) The design of oil-free air compressors was made possible by the development of new materials for piston rings. These materials do not require lubrication, only cooling by water. The design of the oil-free air compressor makes it possible for the compression stages to be separated from the running gear, which still uses oil for lubrication.