Atmospheric air entering a compressor always contains water vapor, or humidity. At 75°F and 75% relative humidity, for example, 20 gallons of water will enter a typical 25 hp compressor during one day of operation. When air is compressed, this water content is concentrated. While the air naturally heats up during compression, the water remains vaporized. When, however, the compressed air travels downstream and cools, the vapor condenses into liquid droplets. This water is unacceptable for many applications and can also contaminate an entire compressed air system and lead to corrosion, air leaks, pressure drops, and scale formation.
Commonly used dryer types include: refrigerated dryers, desiccant air dryers, deliquescent dryers, and membrane dryers.
These are the most economical type of dryer. Warm and saturated air from the air compressor is cooled to a temperature of 35°F to 50°F. At these temperatures, the water condenses and can be mechanically separated and discharged from the system. Air, now free of liquid moisture, can be reheated and discharged into the compressed air system. This air now has a 35°F to 50°F pressure dew point, which means the air temperature has to drop below this temperature before further condensation occurs.
These dryers are used in applications that require compressed air at dew points as low as -100°F. Through two identical drying towers, each containing a desiccant bed, air flows alternately. While one tower is on-stream drying, the other is off-stream being regenerated. Purge air is used to regenerate the desiccant.
Diameter and length of desiccant beds determine drying efficiency. Bed diameter controls air velocity through the bed. If velocity is too high, the desiccant will float or fluidize, causing desiccant degradation. Bed length determines consistency of the dew point: the bed must be long enough to ensure sufficient contact time between the wet air and the dry desiccant to each the proper outlet dew point.
Channeling, when an air stream finds a path through the bed and follows the path instead of flowing evenly throughout the bed, is often a problem with desiccant dryers. Channeling can be avoided by using stainless steel diffusers in the inlet and outlet of the desiccant towers and controlling air velocity through the desiccant bed.
Desiccant dryers are either cold regenerative or heat regenerative. In cold regenerative dryers, 15% of dried compressed air is diverted from the air outlet and is used to purge the wet desiccant bed. In heat regenerative desiccant dryers, purge air is heated to 300 to 400°F and directed through one of the desiccant towers. Depending on the heated dryer type (internally heated, externally heated, blower purge, etc.), only a small percentage of 1 to 7% of purge air is diverted from the dried air stream. Valuable purge air is saved, reducing operating costs up to 40% in applications over 500 cfm.
These utilize the newest technology for compressed air drying. The process is quite simple: compressed air passes through a bundle of hollow membrane fibers and the water vapor permeates the membrane walls. The dried air continues down the tubes and into the downstream air system. The main drawback is the relatively large amount of costly and unrecoverable compressed air (sweep air) lost through the membrane walls along with the water vapor.