In this Ask the Experts feature, which will run regularly in Plant Services, expert instructors from the Compressed Air Challenge (CAC) will 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 facilities enjoy the benefits of improved performance of your compressed air system.
This month’s question: “What are the best strategies to reduce the energy consumption of my compressed air system?”
Paul Maguire (Natick MA, L1 Trainer): Reducing energy consumption in a compressed air system is crucial for both cost savings and environmental sustainability. Compressed air systems are notoriously energy-intensive, often consuming more energy than necessary due to inefficiencies. Implementing strategic measures can lead to substantial reductions in energy usage. Here are the most effective strategies to minimize energy consumption:
1. Lowering System Pressure: One of the most impactful strategies to reduce energy consumption is optimizing the system pressure. Compressed air systems typically operate at pressures higher than necessary, leading to excessive energy consumption. Reducing the operating pressure by just 2 psi can result in a 1% reduction in energy use. According to the U.S. Department of Energy (DOE), lowering system pressure from 120 psi to 100 psi can decrease energy costs by approximately 10%. The key is to balance the pressure requirements for various tools and equipment with the minimum pressure needed, thereby reducing the workload on compressors and saving energy.
2. Implement Leak Detection and Repair Programs: Leaks in compressed air systems are a significant source of energy loss. The DOE estimates that air leaks can account for up to 20-30% of the total system output. Regular leak detection and repair programs are essential for maintaining system efficiency. Using tools such as ultrasonic leak detectors or even simple soapy water solutions can help identify leaks. Addressing these leaks promptly can significantly reduce wasted energy. For example, a single 1/8-inch leak can waste approximately 37,000 cubic feet of air per day (26 cfm), leading to substantial energy costs over time. By investing in regular maintenance and repairs, organizations can improve system efficiency and achieve notable energy savings.
3. Upgrade to More Efficient Equipment: Upgrading to more energy-efficient equipment can have a substantial impact on overall energy consumption. Modern, high-efficiency compressors and variable speed drive (VSD) technology can significantly reduce energy usage compared to older models. VSD compressors adjust their speed based on demand, which avoids the inefficiencies of running at full speed constantly. According to a study by the Compressed Air and Gas Institute (CAGI), upgrading to a VSD compressor can yield energy savings of up to 35% compared to a fixed-speed compressor. Additionally, investing in high-efficiency air dryers and filters can further enhance system performance and reduce energy consumption. Though initial costs may be higher, the long-term savings and performance improvements often justify the investment.
Chris Beals (Denver CO, L1 Trainer): In order to save energy the compressors in the compressor station(s) have to be coordinated so they can reduce energy when demand side work reduces demand. Therefore, the first place to start is with the compressors in the compressor station.
There are two types of compressors – rotary screw and centrifugal. For rotary screw compressors, set the compressors’ pressure set points so all but one is fully loaded. The compressor that isn’t fully loaded is called the trim compressor. If the trim compressor is a constant speed rotary screw then it should operate in load/unload mode with sufficient storage so it doesn’t short cycle, as short cycling can damage a rotary screw compressor.
If the trim compressor is a variable speed drive (VSD) compressor then it needs to be sized either so its turndown covers the entire variation in demand or so its turndown range covers the standard cubic feet per minute (SCFM) capacity largest constant speed rotary screw compressor that will unload during the variation in demand. VSD compressors also require sufficient storage to reduce hunting that can reduce the life of its variable frequency drive. Installing compressor automation can further improve the system efficiency.
Blow off must be eliminated to save energy with centrifugal compressors. Given multiple centrifugal compressors, the best way to eliminate blow off is to install automation that allows load sharing across all the compressors. While tuning the control valves on a centrifugal compressor can help it react better to system events, large storage is often required, particularly between the compressor and a desiccant air dryer.
Gopalakrishnan Bhaskaran (Morgantown, WV, L1 Trainer): The best strategy to start examining energy saving potential in compressed air system would be to do effective concurrent data logging to determine the power draw from each compressor. If a plant has multiple air compressors, with or without a sequencer, then this collected data will provide significant insight and knowledge into the effectiveness of compressed air system operation.
For example, in a plant with four screw type air compressors all providing air to the same header, the power and/or current draw will indicate the unload/load status of each compressor over time. This information will help to ascertain if multiple air compressors are being loaded and unloaded in a manner that results in an overall low kW and air capacity loads, which would signify a significant energy conservation opportunity. The use of the DOE MEASUR tool for modeling the compressed air system can then be attempted to analyze the energy saving opportunities, in more effective sequencing of the compressors operation based on target pressure or cascading pressure bands.
The collected data in this regard will also reveal rapid load/unload cycling of the compressors; and, if any compressor is operated by VFD, it will reveal if the VFD is actually providing benefits as it intended. The effectiveness of the VFD controls on an air compressor is only as good as the effective integration of the supply and demand side.
I recommend also collecting short interval data on the compressed air pressures at various locations in the plant and superimposing them on the current/power profiles of the compressor operation to determine the changes in pressures with respect to time and with respect to compressor operations. Such efforts will lead to closer examination of the end uses and their air use patterns and examine their correlation with the power draw from various compressors at the same time periods.
Paul Shaw (Berlin, CT, L1&2 Trainer): Very often plant pressure is much higher than required, causing the plant extra operating costs. For the system pressure the operator should investigate what the REAL pressure needs are for the plant – they may find that the perceived operating pressure is much more than the required or needed pressure. I have frequently been in plants where the operating pressure is 125 psig or higher and by the time we are done with a study and implementation that has addressed the issues, we are able to reduce the plant’s operating pressure to 85 or 90 psig and hold it there. Some examples of the issues we uncover that require the higher pressure are an inadequate header sizing, inadequate pipe looping, high filter pressure drop, storage and lack of pressure control to provide a couple of examples. Implementing this provides a real energy benefit as a 2 psig decrease in system pressure realizes a 1% energy savings plus additional savings in the leak load if that portion has not been addressed.
Also, the operator needs to look at the supply side of the system and specifically the air compressors. How many compressors are on, are they required, are they integrated correctly, do they shut off automatically when they are not required? Also, is there one compressor in the system that provides excellent part load turndown being used as the trim compressor in a multi-compressor system, or is the primary compressor in a single compressor system? A compressed air system specialist can help with this along with datalogging to understand where the issues are, if any.
Ron Marshall (Winnipeg, MB, L1&2 Trainer): A lot of study typically takes place inside the compressor room, but often the largest potential cost savings benefit results if you poke your nose outside the door and look at how the air is used (or abused). Very seasoned auditors estimate that if aggressive and thorough investigations of the compressed air demand is done you can double the savings gained in the compressor room, often with little to no cost involved.
If you really want to optimize your system purchase an ultrasonic leak detector or an acoustical imager to help you find compressed air uses and leaks hidden in your production equipment. You will soon be amazed at all the innovative ways your compressed air users can think of the waste compressed air. Considering it takes about 10 units of energy to produce one unit of energy output on a compressed air device like an air motor, considerable energy can be saved by reducing or eliminating wasteful end uses.
And for plants that have been ignoring their leakage, it is very common to see leakage levels approaching or above 50 percent of the total compressed air system output. Your compressors need a rest – start an aggressive leak detection and repair program and reduce your load!
Want help in finding out what to do? Compressed Air Challenge’s Fundamentals and Advanced training can help you formulate a plan. More information at the Compressed Air Challenge website.