In the process of compression, air heats up and can hold large amounts of water vapor. As it travels downstream, the water vapor typically cools and condenses into liquid and causes corrosion in piping, components, air tools, and production equipment. Compressed air dryers are indispensable in most commercial and industrial compressed air systems, as they remove moisture that can increase maintenance and downtime in production equipment and possibly ruin products. In this two-part blog post we’ll take a look at maintenance components in compressed air dryers. Part two will cover maintenance for air treatment accessories such as aftercoolers, filters, and drains.
Refrigerated dryers are the most commonly used dryer type. They remove water from compressed air by cooling it so that the water vapor condenses and can be easily separated. They are fairly simple and very reliable, with relatively few service points. A refrigerated dryer consists of heat exchangers, a refrigeration system, a separator/drain, and a control system.
For those applications where extra dry air is needed, desiccant dryers can reach pressure dew points below the freezing point of water such as, -40°F, and even as low as -100°F. Rather than chill air, they rely on the hygroscopic properties of desiccant material to adsorb moisture onto its porous surface as compressed air flows over it. Typically, industrial desiccant dryers use pressure vessels filled with small alumina beads (or molecular sieve), which needs to be desorbed periodically using dried compressed air or a combination of compressed air and heat. They feature twin pressure vessels with interconnecting piping, valves, desiccant, controls, and possibly electric heaters and blowers. There are also simple single tower desiccant dryers, though these are usually much smaller point-of-use devices. All desiccant dryers depend on reliable pre-filtration to remove oils in order to function.
Like single tower desiccant dryers these are usually much smaller point-of-use devices. Membrane dryers use the selective porosity of special fibers to permeate water vapor to the outside of the membrane fibers, drying the compressed air flowing through it. Membrane dryers require a continual flow of purge air to remove the water from the fibers. They are entirely maintenance-free; however, they absolutely require reliable pre-filtration to function.
Maintenance checklist for compressed air dryers
Heat exchangers, like the condenser and evaporator of the refrigeration circuit, can have reduced heat transfer capacity if they are contaminated. When contaminated, the condenser gets clogged causing less efficient cooling of the refrigerant, while a clogged evaporator may cause excessive pressure drop across the dryer.
- For the condenser, when it operates less efficiently, the refrigerant doesn’t get cooled as much and will cause higher refrigerant temperatures, and thus lower evaporator temperatures and higher compressed air dew point. To prevent this, periodically clean the fins with compressed air or a bristle brush. If using compressed air, be sure to direct the air stream in a way that blows it out rather than deeper into the cooling fins. If using a brush, make sure it is soft enough not to bend or damage the fins of the heat exchanger.
- Evaporator contamination may be more difficult to spot since a visual check can’t really be done. Checking the differential pressure across the dryer would help indicate this. If the pressure drop is abnormally high, then that may mean there’s a blockage in the evaporator. Cleaning the blockage is difficult, but it does show that a filter before the dryer will be needed.
- Check the dryer temperature. There’s usually a green/red or green/yellow/red indicator. If in yellow/red, it's telling you that the evaporator temperature and thus the dew point of the compressed air is elevated, and indicates: 1) there is a problem in the refrigeration circuit, 2) the room temperature or compressed air temperature is too high, 3) the flow through the dryer exceeds its designed operating conditions, or 4) a combination of these. Regardless of the reason, the dryer is not drying the air to specification.
- Checking the refrigerant circuit requires the services of a qualified technician certified to work with refrigerants, but before calling them in, you can check 2 and 3 yourself. Hang a thermometer in the room or use an infrared thermometer. The infrared device can also be used on the piping to check the compressed air temperature going into the dryer (the temperature of the air in the pipe could be hotter than the pipe surface temperature). Also check your compressor specs to see how much volume is going into the dryer. If the room temperature is 100°F or less and the dryer is not being overfed, call for a service technician.
- Another maintenance checkpoint would be to spot leaks in the refrigeration circuit. This is usually done by a soap bubble test, or by using ultrasonic and infrared leak detectors. Leaks can appear very small at first and won’t affect the refrigeration system, but over time, when the system loses more refrigerant, operational problems will arise.
Because refrigerated dryers chill and condense water vapor into liquid water, it is vital that the drain valve is working properly to remove the condensate from the separator. Otherwise, it may simply be carried downstream with compressed air. On average a 100-cfm dryer generates about 6 gallons of condensate during a 3-shift operation, so it’s important to discharge it reliably. Drains should be checked periodically and serviced at least annually.
- Check separator and drain function regularly by testing the drain for condensate removal. There are several types of drain mechanisms and some are more vulnerable to clogging than others. Some drains have test buttons to help check their function. Though mostly water, condensate may have oils and particulates that can build up and interfere with the drain mechanism.
- If there is a strainer to trap debris, it should be opened and cleaned out. If they are heavily contaminated, it could indicate that the drain mechanism needs attention.
- The drain supplier can provide a maintenance kit to replace the serviceable parts of the drain. This is usually a simple task.
For most refrigerated dryers the controls are very reliable and typically do not need much attention; however, it would be worthwhile to periodically check that the controller readings, settings, setpoints, and alarms are correct.
Desiccant dryers are extremely effective at removing moisture as a vapor but not as a liquid. So a cyclonic/centrifugal moisture separator with a reliable drain valve might be needed in-line before the dryer if large amounts of liquid water are present. The separator itself may need no maintenance, but as noted above, the drain valve does. See the tips above in the Separator/drain section.
Coalescing pre-filtration is essential to protect the desiccant from oils that can coat the desiccant and prevent it from adsorbing water vapor. If kept clean, desiccant can be used for several years, but once contaminated with oil, it must be replaced with fresh desiccant. Desiccant replacement is expensive and time consuming.
- Check pressure drop across the coalescing filters (you may have two) frequently
- Change the filter cartridge when pressure drop exceeds permissible operating limits–they usually have differential pressure gauges. If there is no gauge, change the filter on a schedule. It's far less expensive than the costs of changing desiccant and fixing problems downstream.
- Check the drain on the filter(s) for proper operation and service them at least once a year.
- As noted above, desiccant is very sensitive to oil. Also, over time it breaks down from friction as compressed air flows through it in one direction and then the other. Not only does the desiccant wear down (and have less surface area to attract moisture) it creates dust that flows downstream.
- The desiccant can be periodically sampled. There are usually ports on each of the towers (for replacing desiccant) that can be used for the sample. Examine the beads for proper size (e.g. ⅛ or ¼”) and also for discoloration that indicates oil contamination.
- Replace desiccant according to the operating manual (or immediately if it's oily or worn down).
- Periodically check for dew point if the dryer is not equipped with a dew point monitor by connecting a dew point sensor with a sampling chamber after the dryer. This will ensure that the dryer is operating correctly, and that the desiccant still has an appropriate adsorption capacity. Dew point measurements may need to be taken at multiple times or durations to ensure the dryer produces good dew points throughout the drying cycle.
- There should be a dust/particulate filter immediately after the dryer to catch desiccant dust. Ideally, it will have a differential pressure gauge.
- Check pressure drop across the dust filter frequently.
- Change the filter cartridge when pressure drop exceeds permissible operating limits. If there is no gauge, change the filter on a schedule.
Proper valve operation is critical for desiccant dryer operation.
- Valves should be repaired according to the manufacturer’s recommendation to ensure proper operation. A higher than normal pressure gauge reading on the pressure vessels during “purge” cycle, or excess pressure drop across the dryer, can indicate excessive leaks from the valves (wearing valve seals), or improperly functioning valves.
Control systems on desiccant dryers consist of either reliable solid-state timers or a PLC.
- Both provide means to manually step through a complete operating cycle and verify proper valve sequence and pressures during each step of the cycle.
- Check message history to review any warnings or alarms. Make sure all set points and alarms are set correctly, including the clock. If communication to PLC is used, ensure connection is operational.
- Check purge mufflers for excessive desiccant dust, indicating desiccant breakdown. Replace the muffler when necessary.
- Check the purge line for obstruction (like clogged mufflers) and excessive pressure drop. This can usually be detected by a degraded dew point on the dew point sensor or high-pressure alarms during the regeneration cycle.
Heater and temperature controls (for heated dryers)
- Check operation and temperature settings of the thermostat to avoid heater element burnout. Check the safety temperature switch for proper operation.
Blower (on blower purge dryers)
- Service blower motor per instruction manual. Grease or replace motor bearings as necessary.
- Clean or replace the blower air inlet filter.
In part two we will cover additional components of your air treatment system that also require regular maintenance and attention. In the meantime, here are some additional resources: