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.
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