One of the most satisfying parts of being a compressed air system auditor is resolving compressed air system reliability issues. This article exposes a seldom, if ever, mentioned problem that can occur when air dryers are dedicated to air compressors. It examines a real-world application and discusses the action taken to remedy the situation.
Dryer Not Maintaining Pressure Dew Point
A portion of the general contractor’s project was to purchase and install a lubricated rotary screw air compressor along with a heatless desiccant dryer, a 1,060-gallon wet air receiver, and “no-air loss” auto drains. The air compressor was rated for 518 acfm at 110 psig, while its dedicated dryer was rated for 500 scfm at inlet conditions of 100⁰F (38°C) and 100 psig.
The equipment was installed indoors at sea level and the no-air loss auto drains were installed on the air receiver and the dryer pre- and after-filters. The reader would be correct to question the dryer sizing along with specifying an auto drain for the dryer’s after-filter. The project specifications required the dryer to deliver air at -40⁰F (-40oC) pressure dew point (PDP). The equipment was commissioned by a local distributor owned by the air compressor manufacturer. Three weeks after the commissioning the dryer wasn’t maintaining -40⁰F (-40oC) PDP, which was a problem for the contractor because they wouldn’t receive their final payment until it met the specification.
The distributor responded quickly and found the mufflers plugged along with an eight-psig drop across the pre-filter. The service person cleaned the mufflers, removed the balance line on the pre-filter’s no-air loss drain because it was connected downstream of the pre-filter, and adjusted the purge pressure. The pre-filter element was clean so it was assumed that the differential indicator was bad.
Return Visits Yield Few Results
The PDP never recovered so the service person returned two weeks later. He reinstalled the balance line on the pre-filter no-air loss auto drain, moved the control air for both drains from the dry side of the dryer to its wet side and decided to order a new pre-filter and differential gauge. When he left the site the PDP was improving while the dryer was cycling with its upstream air compressor turned off, but it never reached -40⁰F (-40oC) PDP.
The service person returned nine days later and replaced the pre-filter and differential gauge. The service person also replaced the pre-filter’s no-air loss drain with an electronic drain, but he left the float drain in the pre-filter housing. Float drains should be removed whenever an auto drain is installed. A factory engineer thought the dryer inlet temperature may be too low for the dryer to function properly. In addition, the service person noticed that while the dryer was delivering air at -23⁰F (-31⁰C) PDP the system PDP was only at -7⁰F (-22⁰C). Therefore, the service person theorized the downstream PDP may be affecting the dryer’s PDP reading.
The service person returned again nine days later and replaced the pre- and after-filters with larger ones and verified that the purge orifice was correct. It appears the original filters were undersized because the new ones have the same model number as shown in the dryer’s parts manual. He also disconnected the new equipment from the system and vented its air to the atmosphere but the PDP didn’t improve so he decided the desiccant should be replaced.
The service person returned once again 18 days later to change the dryer desiccant. He installed enough desiccants to increase the dryer’s rated capacity to 600 scfm. He didn’t find any sign of oil or water in the dryer’s towers. He also found the oil level in the air compressor to be okay. After the dryer was put back online the PDP read -54⁰F (-48⁰C), but three days later it was worse than the required -40⁰F (-40⁰C).
Dew Point Problem Persists
A week later the service person brought the air compressor manufacturer’s system engineer along to review the system. The service person changed the purge mufflers, increased the dryer purge pressure, and installed an air coupon at the inlet to the air compressor to test the quality of the air flowing into the air compressor. The system engineer discussed several options with the contractor but they didn’t want to proceed with any of them until receiving the results from the air coupon.
The service personnel returned three weeks later to change out the dryer purge mufflers and download the air compressor’s SD card. The data on the SD card showed that, at times, the air compressor’s discharge pressure dropped to 90 psig. A month and a half after the air coupon was installed it showed airborne contaminants so the manufacturer and the distributor said all warranty claims would be denied. Studies have shown that approximately 80% of airborne contaminants are washed out of the air by the air compressor lubricant. While the airborne contaminants shorten the life of the lubricant; thereby, requiring more frequent oil changes, if they even affect the dryer performance, they aren’t going to affect it that quickly.
In addition, the manufacturer’s system engineer said the engineering firm had undersized the air compressor and dryer. The specified 500 scfm dryer was undersized; however, after the distributor increased the size of the dryer to 600 scfm by installing additional desiccant, it was sized properly. They said the air compressor was undersized because it couldn’t maintain a discharge pressure of 100 psi, but the low discharge pressure was actually due to the setpoints of the other air compressors. So six months after commissioning the air compressor and dryer, the dew point issue remained unresolved and the contractor could not get any more support from the distributor.
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Remote System Assessment Only Goes So Far
Approximately two months later the contractor called and asked for my help. Due to the COVID-19 pandemic it was decided to attempt to resolve the issue remotely, even though most compressed air system issues can’t be resolved remotely. After reviewing the communications between the engineering firm, the equipment supplier, and the contractor the following facts were gathered:
- There was a no-air loss auto drain installed on the air receiver, but the others had been removed.
- The system contained four air compressors each having a dedicated dryer.
- All the air compressors were air-cooled lubricated rotary screw air compressors that exhausted their hot air into the air compressor room.
- The new air compressor’s discharge temperature sometimes reached 110⁰F (43⁰C).
- The new air receiver and dryer, located approximately 50 feet downstream of the new air compressor were installed in a separate room where the ambient temperature was 87⁰F (30⁰C).
- The plant only operated four days a week.
- The plant operated the new air compressor along with one other air compressor.
- On the weekends the plant shut off the new air compressor but they didn’t shut off the new dryer.
- Over the weekends the plant continued to operate the larger air compressor in order to support their nitrogen generator.
Our initial thoughts were:
- The dryer may be operating in energy savings mode, which reduces the PDP the dryer is capable of producing.
- The dryer’s purge exhaust mufflers may be plugged.
- The dryer may be undersized.
- One or more of the drains aren’t working.
- Condensation may be causing moisture and oil to reach the desiccant. Condensate can reach the desiccant if it slugs the pre-filter, or if it condenses between the pre-filter and the dryer inlet.
- The purge pressure may be set too low.
The contractor investigated these possibilities and found:
- The dryer was working in fixed mode and there wasn’t any backpressure on the offline towers.
- The dryer inlet temperature varied between 95⁰F (35⁰C) and 100⁰F (38⁰C), which meant the 600 scfm dryer was large enough for the existing dryer inlet conditions.
- The wet air receiver was found nearly full of condensate because the no-air loss drain wasn’t working. This suggested that condensate may be slugging the prefilter.
After removing the air receiver’s auto drain and draining the water out it was decided to try and regenerate the desiccant once again. The contractor shut off the air compressor over the weekend and increased the purge airflow to help with the regeneration. The PDP improved to -42⁰F (-41⁰C) by the end of the weekend, but shortly after the air compressor was put back online it worsened.
Site Visit Reveals Potential Problem
We decided a site visit was required and the following would need to be done in preparation for the visit:
- We couldn’t risk having anyone questioning our PDP readings so we had our two dew point monitors calibrated and shipped to the site along with a flow meter and large exhaust muffler.
- The contractor would order new desiccant and filter elements, replace the float drains with ¼-turn ball valves, and insulate the piping between the pre-filter and the dryer inlet.
In order to get familiar with the compressed air system I arrived a day prior to the plant shutting down. I found:
- The setpoints of the other air compressors were set too low for the system to maintain a dryer inlet pressure of 100 psi so the plant increased their setpoints.
- Oil in the main header that was installed near the floor below the dryer discharges.
One of our calibrated dew point monitors was installed downstream of the dryer and it read between -25 (-31.6⁰C) and -35⁰F (-37.2⁰C) PDP, which appeared to agree with the dryer’s dew point monitor.
The next day the contractor removed the desiccant from the towers and no moisture or oil was found in the towers. The old desiccant appeared to be slightly grayer than the new pure white desiccant. Then sections of the piping upstream of the pre-filter and downstream of the after-filter were removed so the flow meter and exhaust muffler could be installed. At the same time the pre- and after-filter housings were removed so the filter elements could be replaced.
It was at this time we noticed oil on the after-filter element and we began to suspect the cause of the dryer problem. There wasn’t any oil in the piping upstream of the dryer, but we did find oil in the downstream piping. At this point we were pretty sure the reason dryer couldn’t maintain -40⁰F (-40⁰C) PDP was because the plant didn’t shut off the dryer when they shut off the air compressor.
Oil in the compressed air piping downstream of the dryer pointed to the potential problem with the desiccant dryer’s inability to maintain the proper PDP.
With the upstream air compressor shut off the dryer’s purge air had to come from the main header that contained oil. Hence, the purge air from the main header flowed backwards through the after-filter leaving oil residue on the after-filter element and then into the offline tower contaminating the desiccant before it was exhausted out through the mufflers. However, we still had to prove our theory.
Oil also found on the after-filter element meant the plant didn’t shut off the dryer when it shut off the air compressor. This strongly suggested it was the reason the dryer wasn’t maintaining the proper PDP.
Theoretical Solution Met with Success
In order to prove our theory we completed the test setup after installing the new desiccant and then opened the valve upstream of the exhaust muffler, until the flow meter upstream of the dryer read 500 scfm, and the new air compressor stopped cycling. To our disappointment the dew point monitor on the dryer only read -41⁰F (-41⁰C) PDP. We installed our dew point monitor downstream of the dryer, but upstream of the ¼-turn ball valve used to exhaust the air to the atmosphere.
Shown is the test setup used to help validate the cause of the dryer problem.
To our surprise the PDP read -80⁰F (-62⁰C). The PDP of compressed air improves when it’s expanded to a lower pressure so we weren’t sure the -80⁰F (-62⁰C) PDP reading was accurate. In order to verify the PDP reading our second calibrated dew point monitor was installed just upstream of the dryer’s dew point monitor. Our second monitor also read -80⁰F (-62⁰C) PDP so we knew the dryer’s dew point monitor had failed. The dryer maintained -80⁰F (-62⁰C) PDP throughout the three-day test and when the dryer’s new dew point monitor arrived and was installed it also read -80⁰F (-62⁰C) PDP.
A second calibrated dew point monitor verified the accuracy of the PDP reading and showed the oil had damaged the dryer’s dew point monitor.
After the site visit the plant installed a check valve, downstream of the after-filter and started shutting down the dryer whenever they shut down the new air compressor. It’s been a few months since the test was conducted and the dryer is still maintaining -80⁰F (-62⁰C) PDP.
About the Author
Chris Beals is President of Air System Management, Inc. He is also a founding member of the Compressed Air Challenge and has been solving compressed air system problems for 22 years. Chris can be reached at email: email@example.com, tel: 303-881-8870. All photos courtesy of Air System Management, Inc.
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