dear shop owner

Shop report: Common air system problems and recommendations

We recently visited a high-end specialty metal fabricator who asked us to assess their compressed air system because the users reported a number of issues that were slowing them down and reducing the quality of their work. The shop had the following air system equipment:

  • Three good quality light industrial 5 hp piston compressors
  • Each was mounted on an 80-gallon tank with a timed electric drain trap
  • All fed into one refrigerated dryer rated for 50 cfm at 100°F inlet temperature
  • Several hundred feet of ½" copper pipe for distribution
  • Two dozen point-of-use filter canister filters with drip legs

Below is a detailed report to the shop management on what we found. Because the issues are very common, we thought it would be useful to publish it. It’s been modified to remove specific product recommendations and any trace of the customer’s identity.

Dear Shop Owner:

Thank you for your time and the opportunity to review your air system.

Your air system needs attention and is causing productivity issues throughout the fabrication and paint areas. Tools are not getting the pressure or flow needed to work effectively. Team members report their tools are not operating well, and they have to wait till others stop using air. This is likely caused by a combination of insufficient flow (cfm) from the compressors and pressure drop through the piping network. They also mentioned issues with air quality that impact productivity. Water and oil in the air are reducing tool life, requiring extra maintenance and early replacement. In the paint booth, the dedicated drying and filtration equipment is not maintained. Water and oil cause paint and clearcoat defects that require more time and materials. The compressed air that is released from the tools contains excess water and oil in a mist that people are breathing.

Compressor room

Excessive oil on the outside of compressors and the compressor room floor suggests excess leaking. It’s likely the pumps should be rebuilt or replaced. Team members do not recall any service being performed. I noted the motors are much newer than the compressors. This is a sign of lack of lubrication in the pumps and excess heat in the room. Both the oil and the premature motor replacement are likely due to high heat from multiple causes:

  • Running longer than designed duty cycle
  • High amperage draw which can occur if pumps and valves are not moving freely
  • Tanks have timed electric drains that are not working properly. Water building up inside the tanks makes the compressors run longer (and therefore hotter) than they are designed to
  • The compressor shed temperature might not be excessive in the winter, but is probably very hot in warmer months as the room is not well ventilated, is uninsulated, and has a very low ceiling. 

Safety concern

shop issues

Apart from the significant effect on productivity from insufficient air and downtime, I see a fire hazard. There’s a lot of oil on the floor. Cardboard is being used as a floor mat and it's soaked through. Further, the wiring to the ventilation fan is exposed.

Air quality issues

The shop air is wet and gunked up with oil. This is an indication of worn out cylinders/piston rings. The results are increased tool wear and maintenance (and creates an unpleasant mist coming out of the tools). The drip legs and filter housings at the many points of use are full of water and emulsified oil, respectively.

If condensate traps on the compressors are not working properly, water is not getting drained out as it should. As a result, water builds up in the tanks and prevents the storage needed to let the compressors operate within their allowable duty cycle. Piston compressors need to periodically turn off and cool down.

When compressors run hot from lack of cool down, moisture in the air cannot cool enough to condense in the tank.  Excess moisture vapor travels downstream. You have a type of drain that runs on timers and solenoids. Even if they are working properly, they have to be adjusted seasonally because the amount of moisture in the air changes throughout the year. Even when properly set, they often waste air (they are timed leaks). There are more effective drain types that operate automatically when there is water to remove.

The type of dryer you have will not cool the air and remove the moisture if the compressed air temperature is above 100F (see equipment tag photo below). Even under proper conditions, piston compressors typically operate over 200°F (250-300°F is common). Yours may be running even hotter because of the drain problem mentioned above. The red indicator on the dryer confirms it is not operating within its design parameters (see photo).

air quality issues

Emulsified oil (like Crisco) seen in the point of use filters is affecting the tools. Many of these filter housings have no filters in them at all, and in others rolls of toilet paper are being used as filter elements, in an apparent cost saving effort. (Note: toilet paper is ineffective in trapping oil and water.) The color may indicate molding, which could affect worker health (as will oil mists coming from the tools). This suggests the whole piping network is clogged with this substance.

The desiccant cartridge in the paint booth is long overdue for replacement. This is confirmed by both the built-in service indicator and a note on the paint room door indicating that the filters have not been serviced in nearly 3 years.


You have copper piping, which is an excellent choice.  It doesn’t rust or scale and it's smooth, which allows air to move more freely throughout the system with less pressure drop.   But the main header appears to be ½ inch and that’s too small for the flows you seem to need. Small diameters restrict flow and increase air velocity. Increased air velocity may sound good but it isn’t. It tends to draw out more oil from the crankcase and it reduces the effectiveness of dryers. Further, your effective pipe diameter is probably even smaller from a build up of emulsified oil.

Low-cost options

Here are some minimum cost improvements that could help:

  • Replace dryer with a high inlet temperature refrigerated dryer
  • Change filters in the paint booth now and on more routine basis
  • Clean out the bowls of the point of use filter housings, install proper coalescing filter elements in them and establish a routine replacement schedule
  • Tear down and rebuild the compressor pumps to reduce leaking and improve air flow (although it may be less costly and time consuming to replace them).
  • Install a storage tank (80 gallons) at the end of the room away from the compressors
  • Install new automatic drains (not timer drains) on the compressors to maximize storage, reduce run time and reduce wasted air.
  • Hanging a thermometer in the room to keep an eye on compressor room conditions

These may help, but in truth several problems are interlinked. Changing the dryer type won’t fix the oil from the compressors. Putting in coalescing filters at every point of use will reduce oil getting into the tools, but it will be expensive and it won’t clean out the oil that has likely built up in the piping and may be constricting air flow to your tools. Repairing the leaky compressors will reduce but not eliminate oil carry-over. It may improve performance but won’t overcome the too-small piping. Replacing the timer drains on the compressors could help reduce excess run time but not the stuffy compressor room.

Maximum impact recommendations

  • Replace all three piston compressors with 2 rotary screw compressors. Rotaries make more air per hp than pistons. They have far less oil carry over and operate much cooler, which will improve moisture removal. Unlike pistons, rotaries have cooling systems that let them as much as needed without overheating.
  • Increase the size of the main header to 1” to accommodate more flow and reduce air velocity (even ¾" will be an improvement). This will reduce pressure drop throughout the system and increase contact time in the dryer for more effective moisture removal.
  • It is not practical to clean out the piping and the main line is too small for the flows you require.
  • Install a proper dryer (based on total flow of compressors installed)
  • Install a coalescing oil removal filter just after the dryer to greatly improve air quality throughout the fab shop and paint/body area. This will allow you to remove the point of use canister filters.
  • Establish a routine maintenance program

The combination of more flow and better air quality will improve productivity and reduce maintenance on tools. It will also save time and materials in the paint booth. If you do this you can remove the point of use filters in the fab shop (which aren’t working anyway). Point-of-use filtration and drying in the paint booth is still a good idea (but needs to be maintained).

Compressor sizing

If I had to make a recommendation without more information, I would suggest two 7.5hp screw compressors to replace your three 5hp piston units.  Together, they would produce more air than you get now (perhaps much more). But because of the type and condition of your compressors, we cannot tell how much air (CFM) they are actually making. If you want to move forward with replacing the compressors, we should do a pump up test to know your current capacity. Then we’ll have a better basis for sizing new machines.

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