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What is the dew point and how do you calculate it?

What is the dew point?

The dew point is a temperature where the air at constant barometric pressure no longer holds its water vapours and some of the moisture condensate to liquid water. This dew point is also called atmospheric dew point. When the dew point temperature is equal to air temperature it means that there is 100 % relative humidity.

The dew point and PSA operation

The correct dew point temperature is important for trouble free operation of oxygen/nitrogen generator, high dew point could cause serious damage of molecular sieve. If the inlet air contains too much of water there is no possibility of molecular sieve to regenerate enough and the purity of outlet gas (oxygen, nitrogen) decreases. Longer usage of inlet air with higher dew point causes a total damage of molecular sieve and its replacement becomes neccessary.

The dew point tempretatures depending on humidity and air temperature are ilustrated in graph below:

Following form allow you to calculate one of three variables (air temperature, dew point temperature or relative humidity); two of them must be known.

The air is a compressible gas and the dew point temperature changes with the pressure. More precisly it is not the pressure but volume that matters. When the gas is pressurized, the volume is reduced and the air has less capacity to hold moisture. We can say with the increasing pressure and reduced volume, the dew point temperature also increases.

Even in cold and dry environments is use of compressed air dryer very essential. In the following graph you can see the relation between the atmospheric dew point and pressure dew point at different working pressure.

Celsius
Kelvin
Fahrenheit



Example

Lets suppose there is 100% humidity and 0°C ambient temperature. The ambient dew point will equal also to 0°C, but the pressure dew point at 7barA will be over 30°C!