When air is compressed to approximately 1/7 of it atmospheric volume (for 100 PSIG applications), there is a rise in temperature during the compression phase. This temperature rise allows water vapor in the air to be held in suspension during the compression stage. As a general rule of thumb, air will hold almost double the amount of water vapor in suspension with every 20ºF in temperature rise. This temperature rise is normally anywhere from 170 to 400ºF (76 – 205ºC) depending upon the type of air compressor being used. After compression, the air will cool as it travels through piping, air receivers, and other devices. As the air cools, at some temperature level, (dependent upon the relative humidity of the air at the compressor intake), it will reach its saturation level (100% RH), and further temperature reduction past that point will result in condensate forming in the air line. Most air compressors are equipped with an air aftercoolers which may be an air-to-air, or an air-to-water heat exchanger. By reducing the air temperature immediately after the compression phase, condensing of water vapor will take place, and this condensate is then removed by a centrifugal separator and drain trap, or in the air receiver (in the case of smaller receiver-mounted compressors), where it is then drained off. In most cases, where air-cooled aftercoolers are used, the compressed air temperature will still be higher than normal room temperature, and it will be saturated. Therefore, as this air continues to travel through piping, fittings, surge tanks etc., additional condensate will form in the air lines.
What is “Dew Point” and what “Dew Point” do I require?
Simply stated, dew point is the temperature at which air is saturated and is holding in suspension, the maximum amount of water vapor possible. Any reduction in the sensible temperature below that level, must result in the formation of condensate. It is therefore necessary to achieve a compressed air “pressure” dew point level which is lower than any sensible temperature that the compressed air will be subjected to downstream in your compressed air system. As the compressed air pressure is reduced, and the air expands, the dew point level will also be lowered. This is because the captive water vapor in the air will now have a larger volume of air to distribute itself within. When discussing dew point, the level may be referred to as “Pressure” dew point or “Atmospheric” dew point. “Pressure” dew point refers to the saturation temperature at the existing pressure level of the compressed air, which is normally between 100 to 125 PSIG in most plant air applications. “Atmospheric” dew point is the saturation temperature after the air has been expanded to atmospheric pressure. Depending upon the dew point range you are working within, the difference may be between 30 to 45ºF between “pressure” and “atmospheric” dew points. Tables and charts are available for determining the actual difference based on your operating pressure and “pressure” dew point. In most applications however, the user should only be concerned with, and considering “pressure dew point”.