Simple steps keep compressed air clean, dry

When it comes to compressed air systems, equipment performance is only as good as the quality of the air itself. Unfortunately, it is a fact of life that air compressors produce high-pressure air that is both wet and dirty.

By Doug Ward , La-Man Corp., Port Orange, FL April 1, 2000


Learning Objectives

  • Proper selection and care of air dryers eliminates maintenance headaches by removing liquids and particles before they cause harm.
  • There are four types of dryers: refrigerated, chemical or deliquescent, regenerative or desiccant, and membrane or mechanical.
  • Filters and dryers work together to remove water from air and prevent component contamination.

When it comes to compressed air systems, equipment performance is only as good as the quality of the air itself. Unfortunately, it is a fact of life that air compressors produce high-pressure air that is both wet and dirty. Air dryers and filters keep your compressed air system operating efficiently, but only if they are properly maintained.

Any atmospheric air contains some moisture and dirt. No matter how small the amount of contaminants initially, they are concentrated when the air is compressed. As the air heats up, its ability to hold water vapor increases. When the air begins to cool as it travels downstream, the vapor condenses into liquid.

Among the possible consequences of this condensation are leaking seals, rusty or scaling pipelines, premature wear of moving components, and similar problems that can lead to subpar operation or even equipment failure. Plant personnel can prevent many of these maintenance headaches by selecting the right type of air dryers and filters to remove the annoying liquid and particles-and by maintaining them properly.

Four types of dryers

Most compressors incorporate an aftercooler to reduce the temperature of the compressed air. Aftercoolers remove about 70% of the water vapor. Air dryers are often installed to reduce the moisture content further. They lower the dewpoint of the compressed air to prevent water droplets from forming downstream. There are four major types of dryers:


Chemical or deliquescent

Regenerative or desiccant

Membrane or mechanical.

The refrigerated dryer operates on basic heat exchanger principles (Fig. 1). It removes moisture from the compressed air by lowering the air temperature to the point at which condensate begins to form. This water and oil vapor condensate is then separated and drained off, while the dried air continues downstream. Normally, a prefilter is required to prevent contaminants from entering the heat exchanger.

Refrigerated dryers are either cycling or noncycling. A noncycling dryer runs continuously. A cycling dryer saves on energy costs by using an intermediate fluid (a water/polypropylene/glycol mixture) to cool and dry the air. When the load is low, the compressor shuts off until the temperature of the fluid rises to a predetermined point.

Dryers with heat exchangers must be supplied with clean, cool water. Water hardness, which reflects contaminant levels, should be checked regularly. Water quality should also be monitored and chemical treatments applied, if necessary.

In contrast to the popular refrigerated type, chemical or deliquescent dryers are least common. They use a desiccant (typically salt, urea, or calcium chloride) to adsorb moisture and large particles. Essentially, wet air enters a pressure vessel (or tower) containing a bed of desiccant tablets. As the air slows and changes direction inside, large water droplets and particles are separated out and the desiccant bed adsorbs any additional moisture.

Similarly, a regenerative or desiccant dryer also uses a desiccant (either silica gel or activated alumina beads) to adsorb moisture and large particles, which are subsequently expelled during regeneration (Fig. 2). The arrangement consists of two towers, each fitted with a desiccant bed, switching valves, and piping to control the airflow. Air flows alternately between the two towers, typically in 5-min cycles. As one bed dries the air, the other regenerates. This type of dryer is available in both heated and unheated versions.

A desiccant dryer requires both a prefilter and an afterfilter. The first protects the sensitive desiccant, which would otherwise be quickly destroyed by contaminants. The second prevents the carryover of desiccant to downstream components.

The simplest and most economical dryer is the membrane or mechanical type. It uses a textile filter made up of thousands of individual fibers to trap large particles and cause moisture to form large droplets (coalesce). These particles and droplets collect at the filter’s base and are drained off. Water vapor passes through the filter to a sweep chamber, where it is vented.

Variations may be made to mechanical dryers. One example is a combination filter/separator that incorporates a prefilter into the mechanical dryer assembly (Fig. 3). This two-stage device removes almost all water droplets and contaminants and has a 5-micron nominal rating.

Mechanical systems are typically installed at the point of use (as opposed to desiccant-type dryers, which are placed near the air compressor to capture water vapor). Here, air temperature has cooled sufficiently to permit water droplets to form and be captured by the system.

Maintenance needs

Maintenance requirements vary with dryer type. Refrigerated air dryers enjoy wide use and have relatively low maintenance requirements. Dust should be removed from the condenser fins and the drain valve serviced regularly. The prefilter should also be checked and replaced as needed.

Although they have no moving parts, chemical dryers require special attention, partly because of the corrosive nature of the desiccant. Although carryover of this salty solution is unlikely, it could wreak maintenance havoc on equipment should it escape downstream. Wastewater from any compressed air system should be disposed of carefully. With a chemical dryer, it can be particularly problematic. In some cases, drainoff may need to be specially treated prior to disposal. The desiccant must also be replenished frequently.

Desiccant dryers have some unique maintenance requirements. Even components designed for long life require periodic inspection. Controls also require regular adjustment. On unheated units, desiccant needs to be replaced approximately every 3—5 yr. Desiccant on heated units needs to be replaced more frequently, typically every year or two. In addition, prefilters and afterfilters should be routinely inspected and changed.

Membrane dryers have no moving parts and require little service or adjustment. An industrial-grade membrane lasts for years. When a combination filter/separator is used, the weep or float drain should be kept clear and filters checked regularly and replaced as necessary.

Impact of air filters

Mechanical filters work with compressed air dryers to remove water and other contaminants from the compressed air and prevent component contamination. Three types of filters are typically used:

  • Particulate

  • Coalescing

  • Adsorption.

Particulate filters are typically made of a fine mesh glass fiber, plastic fiber, or woven wire cloth. They remove large particles using centrifugal force, while smaller particles are strained out. The filter is rated by the largest size particle it will allow to pass.

Particulate filters work hand-in-hand with coalescing filters. Coalescing filters are high-efficiency filters that use a fine stainless steel mesh or woven fiber cloth (such as a cotton co-knit) to remove water and lubricants from the compressed air. They are often installed downstream of a particulate filter. These filters have efficiencies rated at 99.98% for 0.1 micron-size particles to 99.9999% for 0.01 micron-size particles.

Adsorption filters use activated carbon to remove gaseous contaminants from compressed air. They adsorb the oil vapor into the pores of the carbon granules and must be replaced once saturated with collected oil. They are point-of-use filters, which should be supported upstream by a coalescing filter. Typical uses for adsorption filters include sanitary environments, such as clean rooms, pharmaceuticals, and food and beverage manufacturing.

Using filters dramatically improves the performance and extends the life of compressed air systems. Following these four steps helps ensure the filtration system will function optimally:

  1. Confirm that all filters are the appropriate size

  2. Confirm that a coalescing filter is compatible with any lubricants used

  3. Ensure that the maximum pressure drop specified is not exceeded

  4. Monitor pressure drop and replace filter elements when necessary.

Improperly maintained filters are a primary cause of substandard performance and even early equipment failure. Plant personnel can prevent these problems by using a differential pressure gauge that indicates when a filter should be changed.

All filters cause a drop in pressure. As dirty air passes through a filter, collected particles block airflow passages. Over time, the filter becomes clogged and the pressure drop increases. Most manufacturers recommend replacing the filter element before the costs of the pressure drop exceed the replacement costs. Every 2-psi drop in pressure results in a 1% loss in hp. Gauges should be installed in a well-lighted location at an easy-to-view height. Some units feature visual indicators that show when the filter element needs to be changed.

Air dryer maintenance tips

Following these suggestions can help keep air dryers operating at top performance:

  • Locate units in well-ventilated locations

  • Keep drains clear

  • Dispose of wastewater properly

  • Do not operate dryer at pressures below its sizing pressure

  • Provide filtration to eliminate particulate matter

  • Inspect and change filters regularly

  • Replace expendable materials (desiccants, for example) as required.

Air dryer maintenance schedule

Here are five points to include in every filter and dryer maintenance schedule:

  1. Evaluate air quality annuallyMonitor filter pressure drop and replace filters as needed

  2. Clean air intake filters regularly

  3. Check and clear drains and moisture traps

  4. Replace expendable elements when required.

  5. Comparing air dryers

Refrigerated dryer Lowers air temperature to condense water vapor $10,000—$15,000 Medium Clean dust from condenser fins; check prefilter
Chemical dryer Uses chemical desiccant to adsorb moisture and large particles that are subsequently drained off $10,000—$15,000 Medium Replace desiccant as needed; special considerations for wastewater disposal
Regenerative dryer Uses chemical desiccant to adsorb moisture; will regenerate desiccant using heat or predried air $5,000 and up High Replace desiccant every 2—3 yr; adjust controls; check prefilter, afterfilter periodically
Membrane or mechanical dryer Uses fine mesh material to trap large particles and causes moisture to coalesce into a sweep chamber; sometimes combined with filter to separate finer particles and convert remaining moisture to water vapor &$100—>$3000 Low Check filter element

Comparing filters

Particulate General liquid, particle removal 2—7 None
Coalescing Extra fine particle removal 10 Particulate filter upstream
Adsorption Vapor removal (commonly sanitary environments) 1 Coalescing filter upstream