Options for clean, dry, oil-free compressed air

Nov. 4, 2004
It takes more than point-of-use filters to remove dirt, oil, and water from compressed air.

Bob Gleason
Parker Hannifin Corp.
Cleveland, Ohio

In-line desiccant dryers provide dew points to 40°F, but require regular maintenance.

Heatless regenerative dryers, such as these units from Parker's Wilkerson Operations, automatically switch airflow between two desiccant towers to ensure a constant supply of dry air.

A membrane dryer, shown here with a coalescing filter, takes advantage of the different size of oxygen, nitrogen, and water molecules to deliver moisture-free air.

The need for cleaner compressed air is apparent to anyone trying to increase the productivity of sensitive pneumatic applications. Today, the modern manufacturer has needs for high-speed spindles, air gauging, air transfer, bagging, and other pneumatic controls where clean, dry, oil-free compressed air is essential.

While particulate filters are the right choice for combating dirt and liquid water and oil, they do not lower dew points or remove oil aerosols. Dew point, a measure of relative humidity, is the temperature at which water begins to condense as compressed air cools. As long as the compressed air temperature does not drop below the established dew point the system will remain dry.

What is not commonly known is that liquid oil aerosols must be removed, along with the water, for a system to be truly clean and dry. These small submicron aerosols can cause just as much damage as wet air. The reason is that the oil is burnt, acidic, and will condense inside pneumatic components causing sticking and downtime. Thus, the demand for clean, dry air is the impetus behind the burgeoning use of coalescing filters and dryers.

Most plants maintain a 37°F dew point, which is usually adequate for industrial equipment. But, when using more-sensitive equipment, a lower dew point may be required. The solution is an in-line, point-of-use dryer. Three different types of air dryers fill the bill, with each having advantages and disadvantages.

In-line desiccant dryers. In these units, air passes through a chemical compound, such as silica gel, which adsorbs moisture from the air. There is a physical limit as to how much water it can hold, and these dryers may reach their saturation point rather quickly. Maintenance must be done on a daily basis, so a better fit for these units is an intermittent airflow application. Dew points can be down to 40°F.

Heatless regenerative desiccantdryers. This "twin-tower" design has two tubes, or towers, of desiccant. Moist incoming air passes through one tower and is dried. Most of the airflow continues downstream, but a small portion of the dry air is used to purge moisture from desiccant in the other tower. An automatic valve continuously switches airflow between towers, ensuring that the air traveling to the equipment is always dry. Dew points are as low as 100°F atmospheric and they require little maintenance. The units require a particulate filter and coalescing filter on the inlet side and a post filter for desiccant dust. The dryers also need an electrical connection for the switching valves and have a set purge rate.

Membrane dryers. This type of dryer "wicks" water molecules out of the air stream. Similar to a single, inline desiccant dryer, the membrane dryer is effectively a plug-and play unit that does not require additional electrical or drain connections. Unlike desiccant dryers, however, membrane dryers do not require routine maintenance to keep operating.

Membrane-dryer operation is based on the size difference between water, nitrogen, and oxygen molecules and the difference between inlet and atmospheric pressures. Inlet air passes through a large bundle of tubular membranes, similar to porous drinking straws.

The tube IDs are sized to pass all molecules in the air system. The pores in the sides of the tubes, however, are sized only for water molecules — which are smaller than nitrogen and oxygen molecules. Some of the airflow is purged from the pressurized system to atmospheric pressure, and it surrounds the membranes before leaving the dryer assembly. This pressure differential between-the inside and outside of the tubes, combined-with the fact that only water molecules pass through the pores, wicks water molecules from the high-pressure inlet air and out through the atmospheric-pressure purge.

Dew points for membrane dryers range from 4 to 40°F. Membrane dryers use a portion of the airflow — up to 20% of the volume — as they purge air necessary to remove water molecules. A particulate and coalescing filter should precede the membrane dryer.

Air dryers and the proper filters play a critical role in improving productivity and eliminating downtime. Understanding point-of-use air dryers and filter options can add to the bottom line.

Parker Hannifin Pneumatic Div.,
(269) 629-2550, parker.com

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