Keeping dried compressed air dry

Ever wonder why there is moisture in compressed air lines even though a dryer is installed after the compressor? This moisture often shows up at any point-of-use, typically first thing in the morning. Liquid collects in drip legs and is expelled along with compressed air as it first begins to flow into a machine or process.


Key Concepts


  • Compressed air exiting a dryer aggressively seeks water vapor

  • Pinhole leaks allow moisture to enter compressed air systems

  • Use moisture removal devices at critical points-of-application

    Moisture invades
    Point-of-use cures

    Ever wonder why there is moisture in compressed air lines even though a dryer is installed after the compressor? This moisture often shows up at any point-of-use, typically first thing in the morning. Liquid collects in drip legs and is expelled along with compressed air as it first begins to flow into a machine or process. How did dried air get wet again?

    An obvious answer could be the dryer isn't working properly, but the likely cause for this moisture is leaks in the air lines. It could also mean the bypass piping installed around a dryer has been left open, or the block valve in that pipe is leaking (Fig. 1). Either condition allows moisture to bypass the dryer and pass downstream with the compressed air.

    Small pinhole leaks that develop in compressed air pipelines allow moisture to enter and deteriorate the dew point. These leaks can be caused by corrosion in the pipe or at nearly every fitting or connection along the pipeline. Once inside, this newly added moisture condenses as the compressed air cools while traveling through the pipelines.

    It is a wise investment to install a dryer after the compressor because an enormous amount of water vapor initially enters the system through the compressor intake. Getting the bulk of this water out immediately helps reduce the opportunity for corrosion within a piping system.

    Moisture invades

    How can moisture enter through small leaks since there should obviously be compressed air passing out through such openings?

    Water and air will always seek to reach equilibrium, which can be described as the point where the saturation ratio or relative humidity reaches 100% at any given temperature and pressure. Dalton's Law of Partial Pressures describes this relationship.

    Since water is always trying to reach equilibrium with air, dry, unsaturated air can be considered an unstable gas. This concept causes liquid water to evaporate into dry air, increasing the saturation ratio.

    The dryer installed in a compressed air system creates an unstable gas that aggressively seeks water vapor from any source. As long as the dew point (the temperature at which the saturation ratio reaches 100%) remains below the air temperature, any available water will continue to evaporate into the compressed air. This explains the why water vapor wants to reenter the compressed air lines.

    The how is also based on circumstances related to classic physics principles. Compressed air always seeks to return to atmospheric pressure, which is its natural state.

    As compressed air exits through any leak, rapid expansion takes place, which absorbs heat from any source, the Joules Thompson effect, and chills the surface of the pipe near the leak (Fig. 2).

    Fig. 2. Pinhole leaks allow moisture to enter compressed air piping.

    Any water vapor that is present in the surrounding atmosphere condenses onto the cool, metal surface around the opening. This liquid water then migrates into the opening, diffuses along the metallic surface, and is drawn inside, into the dry air, where it evaporates. This increases the saturation ratio of that air. It only takes a small amount of water to substantially alter the dew point of compressed air.

    A refrigerated dryer normally produces air at a 40- F dew point, which means there are about 3 grains of water in each cubic foot of air (Fig. 3). But the temperature of the air is about 90 F, which means the air can hold as much as 15 grains per cubic foot. This air only has a 20% saturation ratio.

    There are 7000 grains of water in a pound and 8.33 pounds of water in a gallon, which means there are 456 grains in a fluid ounce of water. One ounce of water entering the pipeline, can fully saturate 38 cu ft of air at 90 F, or substantially elevate the saturation ratio in more than 100 cu ft of this compressed air.

    The situation is even more pronounced when the compressed air has been dried to a -40 F-dew point, where the compressed air only contains 0.04 grains of water in each cu.-ft. Air this dry is produced using desiccant dryers or membranes.

    This very dry air can easily regain a few grains of water through any opening and return to 40-F or 50-F dew point, which might not be suitable.

    If the entire compressed air supply is dried to -20 F or below, the piping must be completely free of any leaks, or the desired end result will not be achieved. Maintaining such a dry air condition could involve welding every connection throughout the entire pipeline.

    It is easy to see how the intrusion of a small amount of liquid water can affect the quality of the entire compressed air system. Each point-of-use continuously consumes compressed air, often using 10-50 cfm. With the opportunity for small leaks at every pipe fitting in a compressed air system, many ounces of condensed water could be continuously entering the compressed air stream.

    As compressed air travels through the pipeline, all of this available water is continuously evaporating and getting carried to every end point. Every night, or anytime that an air system stands idle, the temperature of the piping cools to match the ambient temperature and some of this moisture condenses, leaving liquid water to collect in every drip leg.

    Point-of-use cures

    A good solution for this problem requires the installation of filters, dryers, and purifying devices at each point-of-use in a compressed air system. These devices remove unwanted water, oil, and dirt immediately preceding the end use of the air. This also allows selecting a suitable purification device.

    Point-of-use devices include filter/regulator/lubricators (FRLs), mechanical separators, coalescing filters for moisture removal, dryers, vapor absorbers, and special application products (such as. breathing air panels, etc.).

    Many of these devices also require the use of some type of drain valve to expel any collected water. Drain valves can be manual petcocks, mechanical float traps, or solenoid-operated valves. Some electronic drains always retain a small amount of water to prevent air leakage during the discharge process.

    ISO standard 8573.1, Quality Classes, defines air quality at six levels (See Fig. 4 below). This global unit of measure for compressed air was developed in 1992 to help plant and process engineers specify desired air quality in terms covering solid particle content, moisture content or dew point, and oil content.

    Most applications, in which compressed air supplies power, require that any liquid water or oil, and large (>40

    For these applications, dew point considerations would only relate to any special atmospheric temperature constraints surrounding the point-of-use (such as when the piping or hose lines could be exposed to cold temperatures that might cause further condensation).

    FRLs and coalescing filtration products provide adequate purification for these applications without the need to add a point-of-use dryer. Compressed air used in breathing air systems need not be dry.

    Anytime there is concern for product spoilage (air coming in direct contact with a product), compressed air quality becomes an important issue. Solid particles and liquid moisture can contaminate painted surfaces or electronic circuits, spoil food, damage optic surfaces, clog dry-powder-conveying pipelines, and chemically alter pharmaceuticals, beverages, or photochemicals. A point-of-use dryer is required to remove this water vapor and a point-of-use absorbing filter would be needed to eliminate oil vapor (See Fig. 5 below).

    There is no need to overpurify compressed air. Simply install point-of-use purification devices just before each application, because moisture intrusion is inevitable and every application needs a different air quality. Clean, dry compressed air can be the most cost-effective source of power in a facility.

    Edited by Joseph L. Foszcz, Senior Editor, 630-288-8776,

    More Info The author is available to answer technical questions concerning this article. He can be reached at 800-3438-2463. The company web site is

    Fig. 4. Portion of ISO Standard 8573.1

    Quality classes

    Solids content, max. particle size, microns

    Moisture content, dew point

    Oil content, ppm



    -94 F




    -40 F




    -4 F




    38 F




    45 F

    No limit


    No limit

    50 F

    no limit

    Fig. 5. Required Compressed Air Quality


    ISO Solids class

    ISO Moisture class

    ISO Oil class

    Air cylinders, sand blasting ,large power tools, air motors, and most spray painting

    4 or 5

    N/A (no liquid)

    N/A (no liquid)

    Powder painting, sanders, powder conveying, air gauging, instruments, and controls


    4 to 6

    3 or 4

    Food packaging

    2 or 3

    2 to 4

    2 or 3

    Microchips and optics

    1 or 2

    1 to 3

    1 or 2

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