Recently we have blogged about Compressed Air Dryers and the different types of systems. We have reviewed the Desiccant and Refrigerant types of dryers, and today I will discuss the basics of the Membrane type of dryers.
All atmospheric air that a compressed air system takes in contains water vapor, which is naturally present in the air. At 75°F and 75% relative humidity, 20 gallons of water will enter a typical 25 hp compressor in a 24 hour period of operation. When the the air is compressed, the water becomes concentrated and because the air is heated due to the compression, the water remains in vapor form. Warmer air is able to hold more water vapor, and generally an increase in temperature of 20°F results in a doubling of amount of moisture the air can hold. The problem is that further downstream in the system, the air cools, and the vapor begins to condense into water droplets. To avoid this issue, a dryer is used.
Membrane Dryers are the newest type of compressed air dryer. Membranes are commonly used to separate gases, such as removing nitrogen from air. The membrane consists of a group of hollow fiber tubes. The tubes are designed so that water vapor will permeate and pass through the membrane walls faster than the air. The dry air continues on through the tubes and discharges into the downstream air system. A small amount of ‘sweep’ air is taken from the dry air to purge and remove the water vapor from inside the dryer that has passed through the membrane tubes.
Resultant dew points of 40°F are typical, and dew points down to -40°F are possible but require the use of more purge air, resulting in less final dry compressed air discharging to the system.
The typical advantages of Membrane Dryers are-
- Low installation and operating costs
- Can be installed outdoors
- Can be used in hazardous locations
- No moving parts
There are a few disadvantages to consider-
- Limited to low capacity systems
- High purge air losses (as high as 15-20% to achieve lowest pressure dew points
- Membrane can be fouled by lubricants and other contaminants, a coalescing type filter is required before the membrane dryer.
If you have questions about getting the most from your compressed air system, or would like to talk about any EXAIR Intelligent Compressed Air® Product, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.
Membrane Dryer Schematic – From Compressed Air Challenge, Best Practices for Compressed Air Systems, Second Edition