Performance, Precision, Partnership

Maintenance for air treatment components, part 2: Desiccant dryers

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 the second part of this two-part blog post we’ll take a look at maintenance components in Desiccant Dryers. Read part 1 on Refrigerated Dryers, here.​ 

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.

 

Maintenance checklist for desiccant dryers

Moisture separators

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. 

 

Filtration

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.

Desiccant

  • 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.

Dust filter

  • 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.

Valving

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 system

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.

Purge mufflers

  • 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.

Membrane dryers

Membrane dryers themselves need no maintenance unless contaminated with oil (then the membrane fibre cartridge must be replaced). But they do require coalescing pre-filters and condensate drains which have maintenance requirements noted above.

  • Monitor the purge rate. If it seems too large, then a membrane fibre might have ruptured. An indication of this could be an increase in purge noise or pressure gets too low downstream without any increase in air demand. Membrane ruptures can be remediated by replacing the membrane bundle. Ruptures typically occur when the dryer pressurizes too quickly, and/or after years of cyclic operation.

 

Choose HPC KAESER for your compressed air needs


HPC and its Authorised Distributors are experienced and qualified to design, install, and tailor the compressed air system to meet your individual requirements while maximising efficiency. Peace of mind is also provided that all work will be in accordance with HPC guidelines and current legislation.

HPC and its Authorised Distributors are also able to offer a variety of service plan options tailored to your needs. Get in touch to speak to an experienced and knowledgeable HPC KAESER trained engineer about your compressed air needs. 

 

Speak to our team today