In this Ask the Experts feature, expert instructors from the Compressed Air Challenge (CAC) tackle your questions on compressed air systems and associated technology.
The Compressed Air Challenge is a voluntary collaboration of industrial end-users; manufacturers, distributors, and their associations; trade organizations; consultants; state research and development agencies; energy efficiency organizations; and utilities. The CAC has one purpose in mind; helping you enjoy the benefits of improved performance of your compressed air system.
This month’s question: In your opinion, what are the most common problems plaguing most industrial compressed air systems?
Chris Beals (Denver CO, L1 Trainer): The number one problem is air leaks. Many plants don’t conduct air leak surveys until they must operate their backup compressors, while others conduct them only several years apart. After the leak survey is completed, most plants only repair the largest leaks which results in the number of leaks increasing over time. This is why it is suggested the plant assign an employee to attempt to fix the smaller leaks during the leak survey.
Many leaks must wait to be repaired during a plant outage and some require the piping to be completely depressurized, both of which are usually given a low priority during plant outages. Some plants have a full-time employee assigned to conducting air leak surveys and fixing air leaks; however, when additional thorough air leak surveys are done, the studies usually find that there are still a high number of leaks. The conclusion is that most of the leaks couldn’t be fixed until there was a plant outage and that not enough manpower is assigned to fixing leaks during the outage.
Paul Shaw (Berlin CT, L1&2 Trainer): I see plenty of systems and the top problems I usually see (beyond leaks) are inappropriate uses and excessive part load energy waste. Using air inappropriately comes from a lack of understanding about the actual cost of compressed air and thereby the cost of performing the process, whether the plant is doing it by design or simply for convenience.
An example is blowing off a product with a wide-open tube. This is extremely wasteful as a typical ¼-inch tube at 100 psig can use 30-40 cfm depending on tube length and connections. If the consumption is 30 cfm this may consume 8 kw and cost more than $10,000 annually for 24 hour / 7 day per week operation (at $0.15 per kWh). The alternative may be to use a blower with an air knife or perhaps an engineered nozzle, as both choices would reduce the wasted air by at least 2/3 and significantly reduce the cost.
Additional common demands wasting compressed air are aerating, cleaning, personal cooling, vibrating, and cabinet cooling, plus many other unique uses (people are innovative at wasting compressed air). These should all be reviewed with the aim of reducing or eliminating the compressed air uses by designing other methods that would do the same task at a lower cost, and in most cases these other methods end up being more effective.
Part load energy waste is another big area of waste I frequently see with compressed air systems. Usually, too many compressors are left online, either because of poor system control or a lack of understanding of the compressor controls and setpoints. There may also be a lack of maintenance, and/or fear of a possible shutdown with no plan on how to get another compressor on quickly when needed.
With many of these issues the problem or solution comes down to storage receiver capacity or the lack thereof. I have seen systems with an additional 300 hp compressor left constantly running simply because there was not enough storage to cover demand events that arose, causing just enough pressure fluctuation to keep the additional compressor on. Once the problem was understood additional storage was installed, the controls were adjusted, allowing the compressor to shut off automatically, only coming on from time-to-time as required.
Greg Ashe (Denver CO, L1&2 Trainer): Slightly undersized piping or hoses at various locations in the system is a very common problem, in part because it’s difficult to diagnose. Significantly undersized piping is usually caught immediately and replaced, but small piping can be installed in some locations for decades without notice, with problems only happening when there is a change in load (see Figure 1). The pipe’s capacity and pressure drop are based on the radius squared, which means that a slight difference in diameter has a drastic impact on performance for better or worse. For example, using 1-inch pipe instead of 1¼-inch pipe causes an additional 10 psi of pressure drop (100 cfm at 100 psig for a 500-foot straight run).
As pressure drop inevitably increases over time, the incremental “fix” is to increase the pressure set points on the compressor, thereby increasing power consumption and the leaks in the system, which are based on pressure in the pipe. The more efficient solution would be to reduce total air flow by reducing leaks or replacing the pipe that is undersized. To better understand where and when the excessive pressure drop is occurring, multiple pressure transducers connected to a data logger can be temporarily installed by a compressed air service provider to help isolate any areas of concern.
Gopalakrishnan Bhaskaran (Morgantown, WV, L1 Trainer): I find that most often screw compressors are unloading at high amperages and do not stay long in the unloaded condition due to lack of storage receiver capacity in the system, and perhaps pipe sizing issues are causing excessive pressure differentials. In some cases, the compressor amps swing wildly with a very short unloading time. When more than one compressor is in operation, often the compressors will each operate sharing loads, ramping up and down together while operating very inefficiently. The biggest problem, in some cases, is that even when the plant is not in production during the weekends, such inefficient operation will continue to occur.
Another problem I have noticed is that even though many compressors are on VFD drives, due to lack of storage and controls issues, the VFD is not being allowed to do the job it was intended to do. This often means the power consumption does not go down much at all, even in an unloaded state, wasting energy.
Ron Marshall (Winnipeg, MB, L1&2 Trainer): I am often asked where the most needed adjustment is required in getting a compressed air system running efficiently and effectively. I answer that the adjustment most needed is “between the ears of the compressor operators and compressed air users.” The problems listed in the above commentaries are all due to the lack of education about compressed air systems, resulting in the operators not knowing the cost of compressed air and what to do about lowering it. As a result, they continue to make poor decisions about system operation and abuse the expensive compressed air they produce.
