Are My Pulse Jet Dust Collectors Wasting Compressed Air?

Compressed Air Manifold Cracks and Separations.

Pulse jet dust collectors are common air/material separators in the food industry serving as dust collectors, bin vents, and pneumatic conveying filter/receivers.  The biggest complaint I’ve heard from plant managers and plant engineers about these is that “these collectors don’t make us any money”.  While that is true, they can COST a plant a significant amount of money if they aren’t maintained.  Wasted compressed air is one of the worst offenders, as it not only costs the plant in energy costs associated with creating and conditioning the air, but also in premature bag failure from improper cleaning, production downtime, and inefficient dust collection leading to increased housekeeping requirements, and other many issues.

How can you tell if your pulse jet units are wasting your compressed air? To understand some of the issues that can cause wasted compressed air, we need to first look at how the units operate. Then we’ll look at simple ways of discovering, fixing, and preventing the issues that cause the compressed air to be wasted.

 

Basic Operation of Pulse Jet Dust Collectors

The unit takes in dust-laden air and exhaust clean air. The dust collects on the outside shell of the filters. Pulse jet units use compressed air to clean the dust off of the filters. Air volume and pressure requirements will vary. The cleaning cycle starts when the timer board or controller fires the solenoid valve. When this happens, the solenoid actuates the diaphragm valve, which opens and pushes air into the pulse tube. The air is directed into the inside top of the filters and “walks” its way down the filters, causing the dust to loosen and fall off the outside of the filters. This cleaning keeps the system operating within the designed parameters.

Over the last thirty years or so, I have seen thousands of installed pulse jet units. A vast majority of them had issues that wasted compressed air in some way. These issues fall into two general categories: Maintenance Issues and Operational Issues.

 

Maintenance Issues

Faulty Diaphragm Valves – Worn-out springs or cracked, brittle diaphragms lead to air leaking into the cleaning tube, but not at the correct pressure to clean the filters. Depending on whether the pilot or main spring and diaphragm are damaged, the leak can be continuous, wasting hundreds of cubic feet of compressed air per hour.

The easiest way to diagnose these issues is to listen to the collector. Is the pulse a sharply defined “pop” sound, or more of a “shhhh” sound. Anything other than a “pop” indicates that you have issues inside a diaphragm valve. To determine the correct one without waiting for the system to pulse again or disassembling the valve, you can manually pulse the filter. Most solenoid valves that actuate the diaphragm valves have an opening on the bottom that you can stick a paperclip into and manually pulse the valve. Make sure to let the manifold recover pressure before moving to the next solenoid. 

Leaks – There are numerous ways a pulse jet cleaning system can leak. The most obvious are holes in the manifold; disconnected, cracked, or broken compressed air lines; loose, cracked, or broken flex connections; and loose pipe unions or compression couplings on the pulse pipes. A less obvious leak would be disconnected pulse pipes (either left off after a filter change or blown off during operation). 

Exterior leaks can be found in a number of ways. Listening to the collector while it is in operation and during a shutdown.  You can usually hear air leaks. This might not pinpoint the leak, but it can give you a reason to investigate more closely. There are several portable detectors available that listen for and map leaks from a distance. You can often see the results of leaks.

A clean air plenum with pulse tubes blown out of their couplings. Author’s Note: I apologize for the picture quality. The clean air plenum was very dusty, and the airborne dust created by opening the door was reflected in the camera flash.
Visual evidence of leaks between a diaphragm valve and the compressed air header.

Other maintenance issues that can lead to wasting compressed air include low air pressure, water in the compressed air, and incorrectly aligned pulse tubes. Even if you have a compressed air dryer in your air compressor room, you can have water in your air outside at the collector. Cleaning with wet air can prematurely impinge the filters.  The very act of cleaning the filters is a waste of compressed air if you are blinding them instead of cleaning them. Incorrectly aligned pulse tubes also do not clean the filters (again, wasting the air), and can actually blow holes in the filter media leading to further problems.

These issues are a little more difficult to discover. Telltale signs of water in your compressed air line or header include filter differential pressure increasing more rapidly than expected and/or not decreasing when the filters are being cleaned and also moisture freezing in the compressed air lines or header. You can verify this by draining the header and checking the filters for signs of moisture.  Signs of incorrectly aligned pulse tubes include filter differential pressure increasing more rapidly than expected and/or not decreasing when the filters are being cleaned, a sudden drop to near zero differential pressure (also a sign of a hole in a filter or filters). Verification of this requires inspecting the pulse tubes in the clean air plenum. Low air pressure is wasting air by not efficiently cleaning the filters while still using large quantities of air to have little to no impact. Pressure requirements vary by manufacturer, model, and size of pulse jet dust collectors, but a general rule for this is 80-100 PSIG of clean, dry compressed air.

A misaligned pulse tube blew a hole in this bag filter in less than a month.

Preventing Maintenance Issues

One way to prevent some of these issues is to install a water trap with an automatic dump in the compressed air line just prior to the compressed air manifold on each pulse jet dust collector. This not only prevent the issues discussed previously, but also eliminates the need to heat the compressed air lines on the dust collectors to prevent the lines from freezing in winter.

A preventive maintenance program should include daily (or once per shift) checks of filter differential pressure on each pulse jet unit, looking and listening to each collector while online to check the pulsing sound and frequency, and checking each filter cleaning manifold compressed air pressure before and after pulses, along with weekly, monthly, and longer-term PM checks. An efficient and effective way to accomplish this is to make one person on each shift responsible for the dust collectors. After a very short time, they will be familiar with the differential pressure trends and the sounds the collector makes when it’s operating properly and when it has issues.  When we do maintenance training, we recommend that the designated person come from a utility-type position. This position gives the selected person a bump in pay as well as a path for growth in the facility by making them a part of the maintenance division.

 

 

Operational Issues

Long or Short Pulses – As with the pressure and volume requirements, different manufacturers recommend different pulse times.  When we troubleshoot issues with the cleaning system on pulse jet collectors, we recommend 300 – 500 milliseconds.  This is enough time to allow a solid “pop” of air, without an additional “Shhhh” with it.  Any air that is used after the initial pop is wasted, as the reduced pressure available after the initial “pop” does not increase the cleaning.  Shorter pulses may not allow enough air to clean the filters all the way to the bottom, causing premature filter impingement. As previously stated, inefficient cleaning is a waste of compressed air. Pulse time can be set on the timer board or controller.

Pulsing Too Frequently – We are often asked, “What is the correct delay between pulses?”  Our answer is always “Long enough for the manifold to recover to the correct pressure.” It seems simple, but we frequently see a combination of pulsing too long and too frequently. If the compressed air manifold does not refill back to the right pressure, the result is an ever decreasing manifold pressure and inefficient cleaning, which is (again) a waste of compressed air. Pulse delay time can be set on the timer board or controller.

Pulsing Sequence – This one may seem counterintuitive, but pulsing the filters in row order is a waste of air. For example, you have a pulse jet collector that has 10 rows of filters. Your pulsing sequence starts at row 1, then rows 2, 3, etc.  After row 1 has been cleaned, the static pressure required to pull air through this row of filters is less than the other rows. Air will always take the path of least resistance. When you pulse row 2, the dust is carried back to row 1 by the air being filtered, which basically negates some of the cleaning you just did on row 1. The sequence continues.  Inefficient cleaning of the filters is the result and is a waste of compressed air (a repetitive theme in this article).

Pulsing by row order can be detected by listening to the collectors while they are cleaning.  To change the order, re-land the wires on the timer board. We recommend moving at least two or three rows away if possible. For instance, in a filter with 10 Rows, our recommended sequence would be 1, 4, 7, 10, 2, 5, 8, 3, 6, 9, repeating.

Pulsing Continuously vs Pulsing On-Demand – The majority of pulse jet filters start their cleaning cycles when the filter in placed online, and these pulses continue until the filter is shut down. This is necessary for some materials, and all that is required is a timer board, smart controller, or a sequencing loop in a PLC. For damp, sticky, or hygroscopic materials, this may be required; however, if your material does not require continuous pulsing, pulsing on demand is an option.  Most filter media manufacturers consider that a filter with a “cake” of material makes the filtration more efficient. If the filter is continuously pulsing, this is not possible because the “cake” of material is always being removed. 

Pulsing continuously can waste air by cleaning the filters when they don’t need it. It can also cause premature filter failure. To change to pulsing on-demand, you will need a differential pressure gauge that can be set at two different pressures (a high differential pressure to start the cleaning sequence, and a low differential pressure to stop the cleaning) and have relays capable of starting and stopping power to the timer board, or switching discrete inputs to a smart timer or PLC. We typically recommend using 4” w.c. as the starting point for pulsing and 3” w.c. as the stopping point. The actual pressure can vary based on the application, but a 1” w.c. range is always recommended.

It is impossible to cover every possible way that compressed air can be wasted in and around pulse jet dust collectors in this article, but I hope this has given you some additional tools to use to save time and money in your facility.

 

 

About the Author

Kevin Cardwell, CFEI, CFPS is the Operations Manager for Airdusco Engineering and Design Services, LLC.  In over 23 years at Airdusco, and 30 total years in pneumatic conveying and dust collection, he has designed and started-up hundreds of systems, and has diagnosed and solved system and equipment issues in hundreds more. He can be reached at 901-362-6610 x 3104 or kcardwell@airduscoeds.com.

About Airdusco

Airdusco Engineering & Design Services, LLC is a specialty engineering and consulting firm based in Memphis TN with focus and expertise in the following areas: Combustible Dust Consultation & Dust Hazards Analysis, Audits of Existing Dust Collection Systems, Engineering and Design Packages and Technical Support & Field Services. www.airduscoeds.com.

To read similar articles on Compressed Air End Use System Assessments visit https://airbestpractices.com/system-assessments/end-uses.

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