Taken from Wiki
"A compressed air dryer is a device for removing water vapor from compressed air. Compressed air dryers are commonly found in a wide range of industrial and commercial facilities.
The process of air compression concentrates atmospheric contaminants, including water vapor. This raises the dew point of the compressed air relative to free atmospheric air and leads to condensation within pipes as the compressed air cools downstream of the compressor.
Excessive water in compressed air, in either the liquid or vapor phase, can cause a variety of operational problems for users of compressed air. These include freezing of outdoor air lines, corrosion in piping and equipment, malfunctioning of pneumatic process control instruments, fouling of processes and products, and more.
to read full article http://en.wikipedia.org/wiki/Air_dryer
There are various types of compressed air dryers. Their performance characteristics are typically defined by the dew point.. Below are some attachments of switch bed desiccant dryers.