The importance of pressure dew point

Countless applications require compressed air. And every one of them comes with specific quality standards. Some require ultra-clean air, others don’t. Many are somewhere in the middle. The cleanliness of compressed air is determined by the presence (or absence) of three types of contaminants: dust, oil, and moisture. When it comes to measuring and controlling moisture content, the pressure dew point (PDP) is the most important indicator. 

What is pressure dew point

Pressure dew point
The PDP is the temperature at which water vapor in compressed air changes to a liquid form (condensation). The air all around us contains moisture. When it is compressed in a compressor, it becomes very warm and saturated with water vapor. As the air cools down, this vapor will condense. This condensation can then become free-flowing water.

Having water in your compressed air system is never a good thing, but depending on the application, it can be catastrophic. Large amounts of water can cause bacteria or mold to form. It can also lead to corrosion in the piping system and pneumatic tools. No less problematic, it can push moisture into compressed air. Food & beverage companies cannot use this air to package or enhance food products. Pharmaceutical or medical companies cannot use humid air in hospitals or for any medicinal applications. Body shops can’t use air that contains water to paint vehicles because it ruins the final product. In short, condensation can compromise both the reliability of an air system and the integrity of its products. This makes controlling PDP an important goal.

How do I prevent compressed air condensation?

In order to monitor the PDP and keep air dry, most compressed air applications rely on a dryer. Dryers can be integrated (built-in) in the compressor, or they can be standalone units. Air dryers lower the dew point of compressed air, making it more difficult for vapor to condense into water. This keeps air dry and piping and hoses free of any moisture, and potential for corrosion and bacteria.

The three most important types of compressed air dryers are refrigeration dryers, desiccant dryers, and membrane dryers. Which one is best depends on the ambient temperature and on the PDP needed. 

Refrigeration dryers

AC 200-630 VSD heroshot

Refrigeration dryers rely on the same cooling principle as typical air conditioning units in homes across the world. Using a refrigeration system and passing compressed air through a heat exchanger, the air is cooled to approximately 3-4°C/40°F.

As the air cools, water droplets begin to form in the air. The moisture-laden air then undergoes a mechanical separation process where the liquid or “condensate” is separated from the air stream. The air is then rewarmed using the incoming air. This lowers the relative humidity and traps any remaining moisture in a vapor form.

Refrigeration dryers are the ideal solution if...

  • The ambient temperature is less than 50°C/122°F,
  • The main goal is to prevent condensation,
  • And the required PDP is around 3°C-4°C/37°F-39°F.

Adsorption dryers

PB 760-7400 HE

A desiccant or adsorption dryer uses desiccant material to adsorb and remove the humidity from compressed air. This type of dryer consists of two towers. While one tower is drying air, the other is removing the moisture from already saturated desiccant. This so-called regeneration process prepares the desiccant for a new drying cycle.

An adsorption dryer is the preferred solution for applications that require very dry air (PDP of -40°C/-40°F or -70°C/-94°F), as it can reach pressure dew points as low as -70°C/-94°F. That makes adsorption dryers ideal for very cold climates, as they can help avoid ice formation in pipes and applications. They are also widely used in applications such as mold prevention, medical applications, textiles, and food factories.

Membrane dryers

Essiccatori a membrana PMD 1-7

A membrane dryer consists of thousands of membrane fibers. As the saturated compressed air enters and moves through the fibers, the moisture in it in penetrates through the membrane wall. That's because there is a concentration difference between inside and outside the membrane. The permeated moisture is vented to the atmosphere, while the dry air continues on through the fibers and exits the dryer.

Membrane dryers contain no moving parts and require no electricity, which keeps their operational costs and maintenance needs low. They do not change the compressed air temperature.

Membrane dryers can be used for:

  • Flow rates up to 200 m³/h,
  • Applications with a +3°C/ -20°C PDP, but also lower.

Pneumatech PDP control

As the air treatment and gas generation expert, Pneumatech is well versed in PDP control. Our product offer contains a complete range of desiccant and refrigeration dryers, as well as different PDP meters. In addition, our representatives are always happy to answer your general air quality and specific dryer questions. They can also help you select the compressed air dryer that best meets your needs. So don't hesitate to contact us today!

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