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, either in 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 of equipment; fouling of processes and products.
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
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 20F below the compressed air temperature.
Refrigeration dryers employ two heat exchangers, one for 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 to 40 degree 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 filled with an adsorbent media such as activated alumina, silica gel, molecular sieve or other desiccant material. The desiccant can bring the dewpoint of the water vapor in the air down to -40 degrees Celsius or below. This means that the air will not condense (deposition) water until it is cooled to -40 degrees Celsius. In practice two cylinders with desiccant are used; one is drying the air, while the other vessel is being regenerated. The switching of the vessels and the regeneration sequence is typically done automatically via solenoid operated valves.
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 generated at lower pressure.
- Heated dryer, this can use a hot air blower, so there is no loss of compressed air.
- Heat of compression. This can only be used with an oilfree compressor.
Membrane dryer refers to a dehumidication 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 drying power of the membrane is controlled by varying the compressed air flow rate and pressure. 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. 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.
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 & trains.
- Drying air for use in commercial or industrial processes that demand dry air:
- Truck and Train Air brake systems.
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