Membrane air dryers fill a gap

Over the past few years pneumatic equipment and pneumatic dependent processes have become more sensitive to contaminants such as oil, water, and pipe scale. Thus, there is a growing requirement for better quality compressed air than most air pretreatment systems currently offer.

02/15/2003


Key Concepts
 
  • Membrane dryers provide a level of compressed air purity between those of refrigerant and desiccant dryers.

  • Coalescing filters must be inline before all dryers.

  • Variable sweep reduces the amount of regeneration air used in membrane dryers.

Sections:
Dryer types
Pretreatment
Coalescing filters
How membrane air dryers work
Piping


Over the past few years pneumatic equipment and pneumatic dependent processes have become more sensitive to contaminants such as oil, water, and pipe scale. Thus, there is a growing requirement for better quality compressed air than most air pretreatment systems currently offer.

All compressed air dryers are sized based on the maximum output of a compressed air system delivering air at 100 F and 100 psig with 100 F ambient temperature. For most applications air dryers are not required to operate under these extreme conditions. Also, the majority of compressed air systems do not operate at maximum output or capacity.

Refrigerant and desiccant air dryers, sized to meet these operating conditions, are designed to run continuously regardless of system demands, even though actual system requirements are far less stringent. The result is significant operating costs wasted on energy and maintenance in addition to the long-term wear and tear on refrigerant compressors, cooling systems, drains, and other components.

Dryer types

The refrigerant air dryer is one of the mainstay technologies for treating compressed air. It typically reduces the dew point of compressed air to a range of +40 F to +45 F. Over time, as cooling coils become fouled with oil and other contaminants, heat transfer performance declines. Unless the refrigerant dryer is monitored closely and maintained on a regular basis, the outlet dew point rises, resulting in water and oil condensing downstream at the points of use.

Until now, refrigerant and desiccant dryers have been adequate for treating compressed air for most applications. Refrigerant technology may not be adequate for those applications and processes requiring higher purity and more consistent air quality. Desiccant technology, with dewpoints from -40 F to -100 F, is sometimes more than what is required. Membrane technology fills the purity gap that exists in between these two technologies. It removes contaminants and water vapor to a consistent -40 F to +35 F dew point.

Pretreatment

The outlet air from a compressor is extremely hot and saturated with water vapor, oil droplets, and oil vapor. It is important to cool the air, condense these bulk contaminants, and remove them from the system prior to introducing the air to any dryer.

Components recommended for all membrane dryers include an efficient aftercooler — either air-to-air, if the ambient air is relatively cool, or a liquid aftercooler — to cool the compressed air to 100 F or lower. Another important component is a separator immediately downstream of the aftercooler. This ensures that all condensed water and oil are removed from the system prior to the air dryer, since this large volume of contaminants can overwhelm high-efficiency coalescing filters (Fig. 1).

Coalescing filters

Prior to entering the membrane module, compressed air should pass through a series of high-efficiency coalescing filters to remove oil and water droplets and particulate contamination down to 0.01 microns with an efficiency of 99.99%. Liquid removed by the filters continuously drains into the bottom of the housing, where it is automatically emptied by an autodrain assembly.

Coalescing filters are comprised of several layers of microfiber matrixes which vary in diameter and density. This design allows for extremely efficient liquid removal as well as the capture of solid contaminants. The cartridges are 90% void which reduces overall differential pressure drop, lowers operating costs, and allows for significant solids holding capacity. Although the coalescing cartridges are high-efficiency in design and performance, they also offer a long life. The typical useful life of a coalescing cartridge is 6-12 months, depending on air purity.

How membrane air dryers work

Air leaving the final coalescer is laden only with water vapor, which is removed in the membrane module. Water vapor in the compressed air is removed by the principle of selective permeation through a membrane.

The membrane module consists of bundles of hollow membrane fibers, each permeable only to water vapor. As the compressed air passes through the center of these fibers, water vapor permeates through the walls of the fiber, and dry air exits from the other end of the fiber bundle (Fig. 2).

Membrane dryers incorporate thousands of small, hollow fibers sealed in an aluminum or polymer-based module. The fibers are extremely durable, with burst strengths exceeding 400 psig to 500 psig. A single membrane module can contain thousands of fibers ranging in length from 3 in. to 30 in., depending on flow and purity requirements. These fibers are made from polymers designed to hold up to the harshest conditions found in a compressed air system.

Performance of dehydration membrane dryers varies widely. Several membrane designs require as much as 25% to 30% sweep air to remove the collected water vapor on the outside walls of the hollow fiber, which translates into very high operating costs. Other membranes reduce oxygen concentration in the outlet dry air in an uncontrolled and unpredictable way, which may affect downstream applications, particularly breathing air applications.

A small portion of dry air (regeneration flow) is redirected along the length of the membrane fiber to carry away the moisture-laden air that surrounds the membrane fibers (Fig. 3). This process is referred to as countercurrent flow. The dry air exiting the module end is then piped to the application.


Sophisticated technology can monitor air consumption and automatically adjust the regenerative sweep flow as required. The variable sweep system results in significant energy savings and low operating costs with no fluctuation in output.

Piping

The only requirement downstream of the membrane air dryer is clean, oil-free piping. If the piping is old and has been contaminated with water and oil, these contaminants will continue to migrate to end-use points for several years after the dryer is installed. It is possible to try flushing the system; however, the best remedy is to replace the piping between the dryer and sensitive end-use points or install coalescing end-use point filters.

Membrane technology does not produce condensate. Water vapor is not condensed and drained; it is swept off the fibers and vented to the atmosphere. This offers a significant advantage, because an average 100 cfm compressor system can produce up to 1800 gal of oily condensate per year, which has to be disposed of in accordance with strict regulations at a significant cost.

Membrane dryers require minimal maintenance. Merely change the coalescing filter cartridges on an annual basis, a simple 5-min. task done without tools.

More Info: The author is available to answer questions on compressed air membrane dryers. Mr. Fish can be reached at 800-343-0051 x736. Article edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, jfoszcz@reedbusiness.com





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