Compressed Air Best Practices - February 2008 - (Page 16)

| 02/08 Compressed Air Industry BULK HANDLING | COMPRESSED AIR AUDIT OF THE MONTH Dense-Phase Transport Wreaks Havoc on Compressed Air System compressor restored pressure in 15 to 20 minutes, suggesting that only half its capacity was needed. Reducing demand as described should be enough to keep this fifth compressor off-line during periods of high demand as well as keep the fourth compressor off-line — normally. But if not, we still had an ace to play. By talking with the Production Manager we had learned early on that this period of high demand resulted from an uninterrupted series of transport events. However, a small interruption (10 to 20 seconds) between the events could be tolerated, and calculations showed that interruptions even this short would make a big difference by reducing the average demand. The balance between supply and demand would shift in favor of supply, the pressure decay would be eliminated and the fifth compressor would not be needed. Brian proved this by taking manual control of the system. He found that he could support pressure with just three compressors at fullload and a fourth lightly loaded until that worst case series of dense-phase transport events lasting over two hours occurred. (We were able to identify from our measurements that this series of events occurred three times daily.) It was during these periods that we experienced the highest and most dynamic flow, and controlling the system was most difficult. During these periods, the fourth compressor went to full load, and a fifth had to be started. While Brian’s experiment actually provided more information than he realized at first, it definitely proved that the system behaved differently when under “intelligent” supervision. The local vendor was asked to make the adjustments. The network of local controls is now managing the system efficiently: only one compressor runs at part load, and at least one no longer starts at all. The shutdown timers were also adjusted to turn off compressors after just 2 minutes of running unloaded. This resulted in about two starts per hour for the trim compressor during the high demand periods and reduced unloaded run time by about 50%. One other fact that became evident from this experiment was that another compressor could be turned off (leaving just 3 compressors on-line) if we reduced the demand by just a little more, about 350 scfm. Demand was already down because pressure had been lowered from 115 psig to 80 psig, reducing artificial demand, but little had been done to fix leaks or eliminate inappropriate uses of compressed air. Addressing these sources of waste should suppress demand by more than the 350 cfm needed to turn off this fourth compressor. During his experiment, Brian had also observed that during the prolonged period of high demand, pressure on the supply side decayed slowly, and after about 15 to 20 minutes, a fifth compressor had to be started. But this Efficient Use of Energy So, the big question: was this project worth the effort? At least one of the compressors has been turned off, so Brian’s goal of providing a back-up compressor and sending the rental home has been accomplished. Plant-wide pressure is now very stable and at a much lower level, so the symptom of unstable pressure has also been resolved. But, wouldn’t purchasing and installing another compressor have provided a simpler solution? No, because it would not have accomplished the same results. In fact it would also have increased energy cost. First, let’s look at unstable pressure which is symptomatic of a poor balance between supply and demand. Adding a compressor would have increased supply, true, but not until the new compressor was running and loaded. The signals to start and load, unload and stop, are based on changes in pressure. Pressure must change for compressor controls to respond and the changes must be large enough 16 www.airbestpractices.com http://www.airbestpractices.com

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Compressed Air Best Practices - February 2008 Contents From the Editor Utility-Air News February Audit of the Month: Dense-Phase Transport System Wreaks Havoc on Compressed Air System Real World Best Practices: Material Conveying with Pneumatic and Vacuum Systems Industrial Vacuum Cleaners Use Compressed Air in Portland Cement Manufacturing Processes The Numatics Air Preparation Group The Ins and Outs of Vacuum Generators Dekker Vacuum Technologies Do Your Meetings Sabotage Your Profits? Resources for Energy Engineers Wall Street Watch Advertiser Index Job Market

Compressed Air Best Practices - February 2008

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