Pressure Dew Point (PDP) Explained in compressed air systems

The measure of how much water vapor there is in a gas is called dew point temperature. Not to be mixed up with pressure dew point.

The term “pressure dew point” is used when measuring the dew point temperature of gases or compressed air at pressures higher than atmospheric pressure. It is about the dew point temperature of a gas or air under pressure. It is important to know because changing the pressure changes the dew point temperature of a gas. Increasing the pressure increases the dew point temperature of the gas or compressed air.

The importance of dew point temperature in compressed air systems depends on the use or application of the air. In many cases dew point is not very important. For example when using portable compressors for pneumatic tools, tire filling systems and home applications. In some systems, dew point is only important because the pipes that carry the air run through systems outside of buildings, where a high dew point could result in condensation of the water vapour thus freezing , contamination and blockage of the pipes. In many modern factories, compressed air is used for a variety of purposes and equipment, some of which may malfunction if condensation forms on the internal parts. Some water sensitive processes such as paint spraying, which require compressed air may need specific dryness. More stringent applications such as medical and pharmaceutical processes may need treatment of water vapor and other gases as contaminants, requiring a very high level of purity.

Dew point temperatures in compressed air range from ambient down to -80 °C , sometimes even lower in special cases. Compressor systems without any air drying capability tend to produce compressed air that is saturated at ambient temperature. Even when only using an aftercooler.
Systems with refrigerant dryers pass the compressed air through a sort of cooled heat exchanger, causing water to condense out of the air stream and discharged by a condensate trap. These systems typically produce air with a dew point no lower than +5 °C .
Desiccant dryers adsorb water vapor from the air stream and can produce air with a dew point of -40 °C  to -70°C. In case of heatless regeneration methods of the desiccant dryer the temperature can reach -40°C. Desiccant dryers that are heat regenerated can go down to a -70°C pressure dew point.

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