air treatment maintenance

Maintenance for air treatment components, part 2: Aftercoolers, filters, and drains

In our last post, we gave you a maintenance overview for your compressed air dryers- including drains and filters integral to them. Now we turn to other air treatment components you may have that also require your attention.


The compression process adds a lot of heat inside the compressor. Aftercoolers reduce the air temperature back down close to ambient (inlet) levels. They are either air or water-cooled and eliminate up to 80% of all water vapor. This prepares the air for more effective drying and filtration downstream. Screw compressors built in the last two decades will likely have one built in, but older models may not have them, and extra coolers might be used also depending on the ambient conditions. Shop piston compressors may have fin cooling tubes. These help but do not bring the temperature down as far as those on screw compressors. 

Coalescing and particle filters

Filter technology has steadily advanced to provide reliable removal of solid particles, oil aerosols, water, and hydrocarbon vapors to produce air many times cleaner than the air going into your compressor. When selecting filters, ensure that the liquid loading (amount of water or oil) does not exceed the filter rating. Typically, starting with a bulk filter to remove contaminants at the aftercooler would be best, followed by a finer, and then a fine or extra-fine filter after the refrigeration or before a desiccant dryer. If any hydrocarbon vapors, tastes or smells need to be removed, install a carbon-activated filter after any dryers. Note that any liquid water going through a carbon filter greatly hinders its effectiveness.

Condensate drains

Condensate drains are vital in any clean air treatment system and are often overlooked. Malfunctioning drains will render dryers and filters ineffective since the materials they trap won’t be removed. Four styles of drains are available: manual drains, timed electric drains, float drains, and electronic demand drains. Manual drains should never be considered on the wet side of the system (before dryers) because separators or filters need to be drained up to 10 times an hour. Timed electric drains are not ideal since they must be set for the maximum drain demand, opening unnecessarily in low drain periods. They also create unwanted oil/water emulsions, which are hard to separate and dispose of economically. Electronic demand and magnetic-operated float drains offer the highest reliability and efficiency, since they only open when condensate is present and don't discharge compressed air. Electric drains may often include malfunction alarms to indicate drain failure.

Maintenance checklist for air treatment components

Air-cooled aftercoolers

  • Clean heat exchanger fins with compressed air or a bristle brush. Be careful not to damage or bend fins. Check the outlet temperature of the compressed air to make sure the cooler is cooling adequately.

Water-cooled aftercoolers

  • Check for proper water pressure, flow, and inlet/outlet temperatures.
  • Check the heat exchanger on the water and air side for fouling. Mineral deposits on the water side negatively influence heat transfer. Clean or flush with special cleaning solvent if necessary.

Moisture separators and filters

  • Check the separator element for pressure drop and inspect for damage and cleanliness. Separators typically don't have replacement elements but the separator can be cleaned.
  • Check the drain for proper function periodically. Rebuild mechanism if necessary.
  • Check pressure drop regularly by checking the pressure drop gauge, or use an electronic filter monitor to indicate optimum element replacement.
  • Replace filter elements if pressure drop is excessive, or replace on a schedule according to manufacturer recommendations (usually yearly).
  • If pressure drop is really low or zero, it may indicate a damaged element. Causes of damaged elements can be from high velocity air (low pressure or high flow), or excess pressure drop for long periods of time.

Vapor adsorbers

Activated carbon is often used to remove unpleasant tastes or smells. These filters should be installed downstream of a dryer and an extra-fine oil removal filter.

  • Check for any liquid water, which renders the carbon ineffective.
  • Carefully open the drain valve and check for smell. Change cartridge when smell is detected or after 1000 hours (cartridge-style filters) or after 10,000 operating hours (tower style filters) whichever occurs first. Refer to the manufacturer’s recommendations for carbon replacement.
  • Compressed air oil vapor tests can also be performed to ensure vapor adsorber performance.

In addition, on carbon tower style filters, a fine after-filter needs to be installed to remove carbon dust.

Condensate drains

  • Ensure drain is installed per instruction manual. Vent lines are often necessary to provide reliable operation.
  • Never feed more than one drain line into a drain.
  • If your drains have test functions, these should be checked routinely to confirm they are operating. 
  • Drains may cycle as many as 250,000 times a year.  They should be serviced at least annually. Most drains have rebuild kits or modules that are easily swapped out.
  • Where a pre-drain strainer has been installed, check and remove clogging material.  Note that newer high-quality demand drains usually do not require a strainer. 
  • Periodically clean out (with water or by blowing out with air) excessively large contaminants which did not pass through the drain (large contaminants can cause drain failure).

Condensate management system

  • Ensure that condensate is properly treated before being discharged into sewer systems. Failure to do so could result in severe penalties. Check your local laws and regulations about discharging the condensate.
  • Most oil-water separators are gravity-fed devices which have replaceable filter bags or elements. Check the instruction manual for the maintenance schedule. 
  • Check periodically for proper function and inspect for any foreign contamination.

It’s well worth the effort

Making these practices part of your maintenance routine will keep air treatment components working well and reduce compressed air service costs in the long run. More importantly, they will reduce excessive maintenance, repair and downtime on your pneumatic equipment – reducing scrap and maintaining high productivity and product quality. Whether you do it yourself or pay a compressed air service company, these efforts cost time and money, but far less than the alternatives. 

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