Dryers that stay cool, whatever the weather
15 Jan 2000
For the past 30 years, users of compressed air concerned about water in their systems and equipment have had two choices: install a refrigerant or desiccant compressed air dryer. In the last couple of years, however, a new choice has become available: the membrane air dryer. These have several advantages: no electricity needed, no moving parts, high reliability and low maintenance.
Membrane dryers' performance is usually expressed by a dewpoint figure - the temperature at which water forms in the compressed air. However, to fully understand their advantages, we should think of relative humidity (RH) and dewpoint suppression.
In a typical compressed air system, air leaving the compressor and aftercooler is at about 10 degrees C over ambient and will always be fully saturated (100 per cent RH). On a summer's day, the air entering a refrigerant dryer might be at 35 degrees C and the refrigerant dryer manufacturer will quote a dewpoint of typically 5 degrees C. This means the refrigerant dryer will cool the air (and therefore reduce the dewpoint) by 30 degrees C. Because of the way a refrigerant dryer works, it will only ever cool the air to about 5 degrees C and, therefore, with a 15 degrees C inlet it will only suppress the dewpoint by 10 degrees C. Therefore, refrigerant dryers are more effective in the winter than in the summer.
Relative humidity is a better way than dewpoint to consider the amount water in the air, as uncondensed water vapour can itself cause problems such as corrosion and promoting mould growth. By suppressing the dewpoint, a certain relative humidity sufficient to avoid these problems can be attained.
Membrane driers work by feeding air through hollow fibres which absorb moisture. A fraction of the dry air produced is fed back into the system to dry the fibres. The whole system works silently without moving parts or maintenance.
The great advantage of membrane dryers is that they reduce the dewpoint by a certain factor regardless of the inlet temperature; a dewpoint suppression enough to prevent corrosion of, for example, 40 degrees C, will be the same with an inlet temperature of 30 degrees C or of 15 degrees C. Membrane dryers can then be sized according to exact customer requirements and will then react automatically according to actual operating conditions. You can be sure that no moisture will form in your system in the hottest summer or the coldest winter.
