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.

04/01/2000


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.

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:

  • Refrigerated

    •  

      • 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%%MDASSML%%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: