Technologies for Drying Compressed Air: Aftercoolers and Coalescing Filters

Compressed Air Application - Parker HannifinSeveral technologies are available to remove condensed water and water vapor from a compressed air distribution system including aftercoolers, coalescing filters, air dryers, pressure reduction, water traps, and drip legs. This discussion will focus on aftercoolers and coalescing filters.


An aftercooler reduces the temperature and water content of the compressed air. All compressed air systems should have an efficient aftercooler installed immediately after the compressor to remove condensed water from the air distribution system. This is important because when, for instance, air enters a 3500 scfh compressor at 68°F (20°C) and exits at 100 psig (7 bar) and 248°F (120°C), it releases about 13 gallons (67 liters) of condensed water per day into the air distribution system.

There are two basic types of aftercoolers, air-cooled and water-cooled. The type of aftercooler you choose will depend on the application.

When the compressed air leaves the aftercooler it is saturated, and any further cooling of the air will result in condensation. Coalescing filters will remove this condensed water.

Coalescing Filters

Coalescing filters remove water droplets, compressor lubricant and particles — but not water vapor — from the compressed air stream. In the coalescing process, air enters the filter housing and flows through the filter cartridge passing from the inside element (cartridge) surface to the outside.  Coalesced liquid (water and oil) collects in the bowl where it is drained, and clean air exits the housing through the outlet port. Depending on the application, coalescing filters may be required at multiple points in the compressed air distribution system.

Where to Install Coalescing Filters

A typical compressor installation consists of a compressor, an aftercooler, and a receiver tank.  A coalescing filter should be installed downstream of the receiver.  In systems with an efficient aftercooler, the distance from the receiver to the filter is unimportant.  However, personnel responsible for maintaining the compressor usually also maintain the filter, so it is often installed immediately downstream of the receiver for convenience.  A mainline filter may be required just prior to the first distribution line manifold. Also, to ensure that any liquid condensate in the distribution system is completely removed, a high efficiency coalescing filter offering 99.99% removal of 0.01 micron sized particles and droplets should be installed at the point of use.  

Filter Specifications for a Standard System

One stage filter cartridge system - recommended for general purpose applications.
Filter Housing - sized by flow rate, but port size must be equal to or larger than the line size.
Automatic drain - required.
Differential pressure indicator - recommended to monitor pressure drop.

When instrument quality air is needed at the point of use, it is good practice to also install a second stage as a final filter, even if a main line filter has been used upstream. This ensures the removal of any remaining dirt and oil in the distribution lines, as well as any water that has condensed downstream from the main filter. If there is a pressure regulator at the point of use, install the filter immediately upstream from the regulator. Alternatively, a filter regulator assembly can be used to replace the existing regulator.

Filter Specifications for Applications Requiring Clean Air (such as pneumatic instruments)

Two stage filter cartridge system - appropriate for instrument air and other critical air requirements.
Filter housings- sized by flow rate, but port size must be equal to or larger than the line size.
Automatic drain - required.

Even when the application is not particularly sensitive to impurities in the air, such as an air-driven tool, it is still good practice to install a filter at the point of use.  This will help to reduce maintenance costs and eliminate unexpected downtime. Here, the use of a single stage filter with an automatic drain is recommended.

Drying Compressed Air Guide Image - Parker HannifinRead this white paper to learn more about the importance of drying compressed air.






This is part two of a four part series on drying compressed air.

Facts You Need to Know About Drying Compressed Air - part 1

Can Pressure Reduction Be Used to Dry Compressed Air? - part 3


Compressed Air Treatment Team Member - Parker HannifinThis series was written by Allan Fish, Product Manager, Parker Hannifin.




Other related posts on compressed air and gas treatment:

The Importance of Using an Air Receiver Tank in a Compressed Air System

What You Need to Know About Coalescing Filtration 

Are You Ready for an Audit? Is Your Plant Compressed Air in Compliance with GFSI, SQF, and BRC Codes? Part 1 of 6

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