Pressure reduction combined with filtration can be used to remove condensed liquids from saturated compressed air. To reduce pressure in an application, components such as pressure regulators, vortex tubes, expansion vessels, and receiver tanks may be used. A coalescing filter is commonly installed on the high pressure side of the line to remove liquid water, but not water vapor, from the compressed air.
The Pressure Reduction Process
Almost all air supplies are regulated from a higher line pressure to a lower line pressure at the point of use. As such, it is possible to take advantage of the “drying” effect of pressure reduction; however, the pressure reduction process does not remove all of the moisture from the compressed air.
Air at lower pressures holds more water vapor than air at higher pressures (at the same temperature). Therefore, less water vapor condenses out of the air at the reduced pressure. Table 1 shows the drying effect of reducing the pressure of air saturated with water from 90 psig (6 bar) to 45 psig (3 bar) at 68°F (20°C). In air systems with small line sizes and low flows, the air downstream from the pressure regulator cools slightly after expansion, and quickly warms to room temperature.
Note that if the application requires drying air to a specific dewpoint or if water vapor in the air can be harmful to the process, an air dryer is required.
|Air In||Air Out|
|Pressure||90 psig (6 bar)||45 psig (3 bar)|
|Temperature||68°F (20°C)||68°F (20°C)|
|Dew Point||68°F (20°C)||52°F (11°C)|
Preventing Water Condensation
For pressure reduction to have the intended drying effect, as shown in the table above, all condensed water must be removed from the air entering the pressure regulator. If liquid water enters the regulator, it will evaporate when the pressure is reduced, and the air exiting the regulator then has a dewpoint of 68°F (20°C). Therefore, any cooling downstream would cause further condensation.
The solution to the condensed water problem (in a nonfreezing environment) is to install a coalescing filter with an automatic drain immediately upstream from the pressure regulator. The filter will remove the liquid water, but not water vapor, before the air enters the regulator, enhancing the drying effect of pressure reduction.
This is part three of a four part series on drying compressed air.
This series was written by Allan Fish, Product Manager, Parker Hannifin.
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