Before understanding atmospheric and pressure dew point. It is important to understand dew point. Dew point is the temperature at which water vapour contained in a volume of air reaches saturation and condenses to form dew and gets separated from the volume of air. When the dew point temperature is equal to air temperature it means that there is 100 % relative humidity. Qty of water contain at dew point temperature can be calculated with psychometric chart keeping RH as 100% at dew point temperature.
Volume of compressed air varies with the change in air pressure. The variation in volume of air leads to change in dew point temperature.
When dew point is measured at atmospheric pressure dew point is called as ADP. (Atmospheric dew point).While “pressure dew point” is encountered when measuring the dew point temperature of air at pressures higher than atmospheric pressure. Low PDP means there are small amounts of water vapor in the compressed air; higher PDP values mean higher amounts of water vapor in the compressed air.
If we consider atmospheric pressure 1.03 bar(a) and 40 deg C dew point temperature, water holding capacity of 1 m3 of air is 48.9 g/m3. The water holding capacity drops to 6.29 g/m3 in case pressure is raised to 8 bar(a). It indicates as the pressure of compressed air increases,the moisture contained in the gas compresses and makes it dry.
This can be summed up by the following equation:
Water content at atmospheric pressure (g/m3) /Actual compression ratio = Water content at a given pressure (g/m3)
Relation between ADP and PDP is indicated in below chart:-
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