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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 ofoperational 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.[1]

Their performance characteristics are

typically defined by the dew point.

Regenerative desiccant dryers, often called "regens" or "twin tower" dryers

Refrigerated dryers Deliquescent dryers[2] Membrane dryers

Water vapor is removed from compressed air to prevent condensation from occurring and to

prevent moisture from interfering in sensitive industrial processes.

Characteristics

A regenerative desiccant dryer typically delivers adew pointof between 40F(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.

Refrigerated dryer

Refrigeration dryers employ twoheat exchangers, one for air-to-air and one for air-to-

refrigeration. However, there is also a singleTRISABheat exchanger that combines both

functions. Thecompressorsused in this type of dryer are usually of the hermetic type and themost common gas used isR-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 whenthe compressor is OFF. When the refrigeration compressor runs, the large mass takes muchlonger to cool, so the compressor runs longer, and stays OFF longer. These units operate at lower

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dew points, typically in the 3540 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 heatexchangers.

Deliquescent dryer

A deliquescent dryer typically consists of a pressure vessel filled with a hygroscopic media that

absorbs water vapor. The media gradually dissolvesor deliquescesto form a solution at the

base of the pressure vessel. The liquid must be regularly drained from the vessel and new mediamust 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, andwaste gases such aslandfill gasanddigester gas.

The performance of a deliquescent dryer, as measured by outlet dew point, is highly dependenton the temperature of the air or gas being processed, with cooler temperatures resulting in better

performance.

Desiccant dryerThe 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 otherdesiccantmaterial. This desiccantmaterial 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 inwhich 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 40C (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 40C (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

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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 ofenergy 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 fromthe 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

Membrane dryerrefers to a dehumidification membrane that removes water vapor from

compressed air.

Typically, the compressed air is first filtered with a high-qualitycoalescingfilter. This filter

removes liquid water, oil and particulate from the compressed air. The water vaporladen airthen 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 sweepout 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 airdryers 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 modifiednitrogen membranesand pass air as well, usually changing the

composition of the compressed air by reducing the oxygen content. The only maintenancerequired 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

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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 belowered. 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 afixed 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