Industry, Manufacturing

Understanding PDP and Why Dry Air is Good Air


Dry compressed air is more effective and efficient compressed air, and determining the water content in a system is critical to its performance and durability.

“It’s a dry heat.”

It’s an idiom that is repeated everywhere from sitcoms to T-shirts, an optimistic outlook on an otherwise oppressive atmospheric condition. What is dry heat, and why do people favor it over wet heat (also known as humidity)? Essentially, greater humidity retains sweat on the skin. And since sweating is the way our bodies cool themselves, more humidity equals feeling warmer. Therefore, a dryer heat, despite possibly reaching a warmer environment temperature-wise, keeps you feeling cooler.

Why does this matter, and what is the relevance? All atmospheric air contains varying amounts of moisture based upon the temperature – higher air temperatures can hold more water, while air at lower temperatures is not capable of holding as much. This is true in compressed air systems as well, and if our bodies are any indication of the ways excess water and humidity can impact performance, imagine how this impacts a compressed air system (which can’t step into the shade with a cold glass of lemonade for some brief respite). That’s why, to avoid disturbances and disruptions due to water and humidity in the pipes, compressed air must be dried with after-coolers and drying equipment.

The water content in compressed air is measured in PDP, or Pressure Dew Point. This is the temperature at which water vapor (a gas) condenses into water (liquid state) at the current working 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.

Selecting a primary type of drying equipment should be determined by the PDP, not the atmospheric dew point (the dew point in the air, not within the compressed air system, which the PDP measures). The more a dew point needs to be lowered, the greater the investment (and operational cost) it takes to get it there. PDP already has a higher dew point than that of atmospheric dew point (a PDP of -4ºF at 100psig is equal to -39.5ºF at a 0 psig atmospheric pressure). Additional cooling leads to greater water condensation; therefore, a filter isn’t an effective means of removing moisture – or lowering the dew point – at colder temperatures. That’s why determining drying equipment should be based on PDP alone. Five ways to effectively dry compressed air include:

  • Cooling plus separation
  • Over-compression
  • Membranes
  • Absorption
  • Adsorption drying

Moisture can be damaging. Valves, motors and even the end product can be impacted by excessive moisture in a compressed air system. Lubricants can wash away, leaving parts and pipes prone to rusting. In cold-weather operations, water can also freeze and expand, creating even greater damage. Corrosive operations are ineffective operations and are more expensive to operate, in both ongoing maintenance and the potential for downtime and manufacturing shutdowns.

Lowering dew point can take considerable cost and effort, but the alternative is much more costly. That’s why using PDP, and the proper methods to achieve it, is such an important part of a compressed air system, because optimal PDP equals optimal performance.