There are several different types of compressor fittings in both National Pipe Taper (NPT) and metric sizes. There are also different styles of compressor fittings from straight and tee to elbow types. NPT compressor fittings are not sized according to the diameter of the fitting's opening. The sizing of these fittings refers instead to the taper rate of the threads. The sizes are based on an inch scale, with the size being represented as a fraction of an inch. There is no metric equivalent in this sizing style.
A few of the most common compressor fittings consist of the straight and elbow-type connectors. These compressor fittings are used in a variety of manners, from connecting an air hose to the compressor itself to attaching an air tool to an air line. Many times, the compressor fittings are a quick-release design, which allows an air hose to be connected to and disconnected from an air line in a matter of seconds. By utilizing a quick-detach style of compressor fitting, the air supply stored inside of the compressor's air tank can be saved rather than depleted whenever the air hose is removed from the air tank.
Some fittings are classified as high-flow fittings. These fittings are best when an activity, such as sanding a vehicle, is going to require a large quantity of air delivered at a steady rate of flow. Some fittings are known as swivel-type fittings and are used on air tools and other air attachments that might require the air feed line to be manipulated. Other fittings are designed specifically to attach water filters and oil separators to the output side of the air supply. These make it possible to spray paint without forcing water vapor or oil into the paint job.
On an air compressor that will not be used for painting, an automatic oilier can be a desirable attachment. This device mounts to a special fitting on the compressor and supplies a constant mist of oil into the airstream. This is used to extend the life of the air tools used with the air compressor. This also eliminates the requirement of oiling the air tools before and during each use. One of the most useful compressor fittings is a rotating fitting that allows an air hose to be wound up on a hose reel to be pulled out for use and rewound when not in use.