The term "Dew Point" can be a confusing term. It is used in the dry air business with the assumption that everyone knows what it means. To confound the confusion we also use the terms Pressure Dewpoint and Atmospheric Dewpoint. Let me try to explain what these terms mean and why they are important to consider in a compressed air system.
Dewpoint can be defined as the temperature at which moisture in the air will begin to condense if the air is cooled at constant pressure. At this point the relative humidity is 100%.
Pressure Dewpoint is the temperature at which water will begin to condense out of air at a given pressure. Pressure dewpoint is important when the goal is to keep moisture out of a piping system that might travel a long distance or outdoors.
Atmospheric Dewpoint is the temperature at which water will begin to condense out of air at atmospheric pressure. This is important when spec'ing an air dryer at the point of use which is usually low pressure.
Since air is a compressible gas, a pressure dewpoint indicates less water presence than an atmospheric dewpoint at a given dewpoint. That is, a pressure dewpoint of 35F at 100 psig holds less moisture than air at an atmospheric dewpoint of 35F. For a given pressure dewpoint, as you lower pressure, you lower the pressure dewpoint of the air.
Well put Dave, I look forward to more subjects about "DRY AIR"
Posted by: troy hogg | May 27, 2008 at 08:58 AM