||This article needs additional citations for verification. (July 2009)|
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.
There are various types of compressed air dryers. Their performance characteristics are typically defined by the dew point.
- Regenerative desiccant dryers, often called "regens" or "twin tower" dryers
- Refrigerated dryers
- Deliquescent dryers 
- Membrane dryers
Water vapor is removed from compressed air to prevent condensation from occurring and to prevent moisture from interfering in sensitive industrial processes.
- A regenerative desiccant dryer typically delivers a dew point of between −40 °F(−40 °C) and −100 °F (−73 °C)
- A refrigerated dryer delivers a dew point not lower than approximately 35 °F (2 °C)
- A deliquescent dryer delivers a dew point suppression that fluctuates with air temperature. Typically this suppression is 20 °F below the compressed air temperature.
Refrigeration dryers employ two heat exchangers, one for air-to-air and one for air-to-refrigeration. However, there is also a single TRISAB heat exchanger that combines both functions. The compressors used in this type of dryer are usually of the hermetic type and the most common gas used is R-134a. The goal of having two heat exchangers is that the cold outgoing air cools down the hot incoming air and reduces the size of compressor required. At the same time the increase in the temperature of outgoing air prevents re-condensation.
Most manufacturers produce "cycling dryers". These store a cold mass that cools the air when the compressor is OFF. When the refrigeration compressor runs, the large mass takes much longer to cool, so the compressor runs longer, and stays OFF longer. These units operate at lower dew points, typically in the 35–40 °F range. When selected with the optional "cold coalescing filter", these units can deliver compressed air with lower dew points.
Some manufacturers are marketing compressors with built-in refrigeration dryers, but these have had a mixed acceptance in the market.
Commonly a coalesing prefilter is installed immediately upstream of a refrigerated dryer to remove lubricating oil and other contaminants that have the potential to foul the dryer's heat exchangers.
A deliquescent dryer typically consists of a pressure vessel filled with a hygroscopic media that absorbs water vapor. The media gradually dissolves—or deliquesces—to form a solution at the base of the pressure vessel. The liquid must be regularly drained from the vessel and new media must be added. The media is usually in tablet or briquette form.
Deliquescent dryers have no moving parts and don't require electrical power for operation. Common applications therefore often involve remote, hazardous, or mobile worksites. Deliquescent dryers are used for removing water vapor from compressed air, natural gas, and waste gases such as landfill gas and digester gas.
The performance of a deliquescent dryer, as measured by outlet dew point, is highly dependent on the temperature of the air or gas being processed, with cooler temperatures resulting in better performance.
The term "desiccant dryer" refers to a broad class of dryers. Other terms commonly used are regenerative dryer and twin tower dryer, and to a lesser extent adsorption dryer.
The compressed air is passed through a pressure vessel with two "towers" filled with a media such as activated alumina, silica gel, molecular sieve or other desiccant material. This desiccant material attracts the water from the compressed air via adsorbtion. As the water clings to the desiccant, the desiccant "bed" becomes saturated. The dryer is timed to switch towers based on a standard NEMA cycle, once this cycle completes some compressed air from the system is used to "purge" the saturated desiccant bed by simply blowing the water that has adhered to the desiccant off.
The duty of the desiccant is to bring the pressure dew point of the compressed air to a level in which the water will no longer condense, or to remove as much water from the compressed air as possible. A standard dew point that is expected by a regenerative dryer is −40 °C (−40 °F), this means that when the air leaves the dryer there is as much water in the air as if the air had been "cooled" to −40 °C (−40 °F). Required dew point is dependant on application and −70 °C is required in some applications. Many newer dryers come equipped with a dew dependant switching (DDS) which allows for the dryer to detect dew point and shorten or lengthen the drying cycle to fulfill the required dew point. Often times this will save significant amounts of energy which is one of the largest factors when determining the proper compressed air system.
The regeneration of the desiccant vessel can be during three different methods:
- Heatless "pressure-swing" drying, which uses part of the dry compressed air coming from the other vessel to dry the desiccant in the vessel being regenerated at lower pressure. 17-20% purge rate 
- Heated dryer, which uses a hot air blower, so there is no loss of compressed air. >7% Purge Rate.
- Heat of compression, which can only be used with an oilfree compressor.
Membrane dryer refers to a dehumidification membrane that removes water vapor from compressed air.
Typically, the compressed air is first filtered with a high-quality coalescing filter. This filter removes liquid water, oil and particulate from the compressed air. The water vapor–laden air then passes through the center bore of hollow fibers in the membrane bundle. At the same time, a small portion of the dry air product is redirected along the outside surface of the fibers to sweep out the water vapor which has permeated the membrane. The moisture-laden sweep gas is then vented to the atmosphere, and clean, dry air is supplied to the application. The membrane air dryers are designed to operate continuously, 24 hours per day, 7 days per week. Membrane air dryers are quiet, reliable and require no electricity to operate.
Some dryers are non-porous, which means they only permeate water vapor. Non-porous membranes' drying power is only a function of flow rate, pressure. The sweep flow is strictly controlled by an orifice and is not a function of temperature.
Porous membranes are modified nitrogen membranes and pass air as well, usually changing the composition of the compressed air by reducing the oxygen content. The only maintenance required is changing the prefilter cartridge twice a year. The performance of porous membranes are dependent on temperature as well as operating pressure and flow.
Membrane air dryers depress the incoming dew point. Most dryers have a challenge air dew point and pressure specification. So if the inlet dew point is lower than the specified challenge air then the outlet dew point is even lower than specified. For example, a dryer could be rated at a −40 °F dew point with a challenge of +70 °F dew point and 100 psig. If the incoming air has an inlet dew point of only 32 °F, the outlet dew point will be somewhat less. Pressure also plays a role. If the pressure is higher than the rated specification then the outlet dew point will be lowered. This lowering of the outlet dew point is due to the longer residence time that the air has inside the membrane. Using the spec above, an operating pressure of 120 psig will yield a lower outlet dew point than specified. The extent of the improvement is dependent on the nature of the membrane and could vary among manufacturers.
Dew point suppression is not a feature of refrigerated dryers, as they chill the incoming air to a fixed temperature, usually 35 °F. So a lower dew point challenge will not yield a dew point lower than 35 °F.
Membrane air dryers are used in pneumatic components, spray painting, laser plenum purge, air bearings, air spindles, medical equipment, air guns and pneumatic brakes for vehicles and trains.
- Drying air for use in commercial or industrial processes that demand dry air:
- Truck and Train Air brake systems.