Untangle Compressed Air Misconceptions
Jane Alexander | July 12, 2017
Where air compressors are concerned, what you believe may not be rooted in fact and could be costing your operation.
Compressed-air systems are often misunderstood. That’s why so many seem to be inefficient. The problem, according to compressed-air expert Ron Marshall, is that inefficiency constantly gnaws away at your bottom line through extra operating costs and, in some cases, reduced production-output capabilities. He knows what he’s talking about—and has been writing about it in Efficient Plant‘s pages for several years (see Learn More Box at the end of this article).
Marshall’s background includes, among other things, having been the first Canadian participant to qualify as a U.S. Department of Energy (USDOE) AIRMaster+ specialist and long-time involvement as a committee member and instructor with USDOE’s Compressed Air Challenge initiative (compressedairchallenge.org). He discusses several of the most widespread (and troubling) compressed-air misconceptions here.
Misconception: Compressed air is free.
“This one,” Marshall noted, “is a common belief among personnel who don’t have to pay a plant’s electrical bill.” In reality, using compressed air to drive any continuous mechanical operation can be very costly and inefficient. It takes about 7 to 8 hp of electrical power at the compressor to produce an equivalent of about 1 hp of mechanical output at a compressed-air-driven device, making it one of the most expensive energy consumers in your plant. Most of the energy ends up as heat in the compressor room, and often is simply blown into the atmosphere, warming the globe. Frequently, problems with the compressor control system, pressure restrictions, and extensive leakage make the 8:1 power ratio even worse.
When it comes to using compressed air to blow dust, turn an air motor, or agitate liquid, Marshall said it’s important to calculate energy-related costs associated with the operation. While it’s important to understand these costs yourself, it’s also important to educate others in your plant that compressed air is a very costly utility—and one that shouldn’t be wasted through inappropriate use.
Misconception: Fixing leaks saves big money.
According to Marshall, studies have shown that leaks waste between 20% and 30% of all the compressed air produced in industrial plants. Some plants with no leakage-detection-and-repair programs have even higher levels. Wasting 50% is not unheard of and, in some extreme cases, it can approach 80%.
“Considering the high cost of compressed air, leakage-reduction efforts have a high potential to reduce operating costs, Marshall stated, “but there’s a catch.” The energy savings you gain through leakage reduction is not always directly proportional to the flow reduction, but, rather, depends on the compressor control method. A very well-controlled system will turn the compressor energy down for every percent drop in leakage that is repaired. This would be a system with a good central compressor controller with multiple load/unload compressors, or a system with a properly sized and coordinated variable-speed-drive compressor.
“Some compressor control modes,” Marshall cautioned, “have less than optimum power-turndown capabilities, such as those running in blow-off or inefficient modulation mode, where the inlet flow to the compressor is choked off to control the output.” These compressors might only save 3% power for every 10% reduction in flow. Furthermore, this control mode causes an increase in pressure as the flow reduces, which causes the remaining compressed-air demand to consume more.
Misconception: Large pipe adds storage volume.
Adding storage volume to a compressed-air system can make it run more efficiently. Rather than clearing space for a large receiver tank in the compressor room, some people think they can install extra storage by upsizing their compressed-air lines. As Marshall noted, best-practice storage volume for load/unload compressors is between 5 and 10 gal./cfm of your trim compressor (the one that takes partial load). As an example, the recommended system storage for a100-hp compressor that puts out about 400 cfm would be between 2,000 and 4,000 gal.
Perhaps you want to upsize from 2-in. pipe to 3 in. to provide storage capacity. You should be aware that 3-in. pipe contains a volume of 38.7 gal./100 ft. To install the equivalent of a 2,000-gal. storage tank, you would need about 5,200 ft. of 3-in. pipe—about a mile of piping. So, should you upsize your piping? Marshall says yes, “but not to act as system storage.” Larger sized piping, he explained, reduces pressure loss in your system and that can allow a compressor to run at lower pressure, saving energy. If you do upsize piping, install a large receiver as well.
Misconception: Increasing pressure solves problems.
Marshall described what seems to be a common low-pressure-detecting instrument in many plants—a phone. “If compressed-air pressure gets low,” he said, “production will call and complain, and the compressor operators will jack up the pressure.” This may solve production issues, but it also causes a problem in the compressor room. That’s because, for every 2 psi of additional compressor discharge pressure, power input per unit of flow increases by about 1%. This manifests as higher power bills for the same amount of compressed air, and produces more heat inside the compressor, which can lead to shutdown, extra maintenance costs, and air-dryer problems.
“Many times,” Marshall said, “pressure problems are only temporary, often caused by production areas doing something strange with compressed air. They can also be caused by poorly sized piping, connectors, and hoses at the end use.”
Another problem with extra pressure is that all unregulated compressed-air consumers, i.e., leaks, will use almost 1% more air for every 1 psi in extra discharge pressure. This increases a compressor’s load, leading to more energy consumption.
Misconception: Change filters only if differential reads high.
Filter-pressure differential is one of the most common and largest single pressure restrictions found in compressed-air systems. According to Marshall, plant personnel often wait to replace filters until the differential-gauge needle (if there is one) points to a high level (often there are red and green indicators). Or, they simply ignore filter replacement.
“Replacing filters based solely on pressure differential,” he explained, “is problematic because most filters aren’t running at rated flow where the differential gauge is calibrated.” This means an occasional glance at a filter during average loads won’t reveal any problems. At high flows, though, the filter may develop excessive pressure differentials, causing low-pressure problems in the plant.
Misconception: Air dryers don’t improve efficiency.
Air dryers are another often-forgotten part of compressed-air systems with regard to energy efficiency. Marshall stressed that it’s important to choose air dryers that turn themselves down under light load conditions.
The choice of air dryer can save significant extra power, especially if your system has periods of light loading.
Misconception: Use VSD compressors anywhere, any way.
Marshall described variable-speed-drive controlled compressors (aka VSD or VFD compressors) as game changers for plants. “Where VSD compressors are applied properly,” he said, “the savings are substantially greater than with fixed-speed units, especially for single-compressor systems with significant periods of light load.” In fact, use of such units can often double the savings gained by other efficiency measures. Problems, though, can occur when these advanced technologies are misapplied or installed in poor locations.
It’s important to properly size and control VSD compressors when using them in multiple-compressor systems. The required pressure settings are different fixed-speed-compressor systems. Use of VSD units in applications with high temperatures, large amounts of ambient dust, and/or poor power quality is not recommended. In such cases, savings can be realized with fixed-speed compressors, using other types of capacity controls or with sequencing control.
Ron Marshall has spent more than 22 years working with compressed-air systems, first as an industrial systems officer with Manitoba Hydro, and since his retirement, as owner of a compressed-air consulting company. Contact him at firstname.lastname@example.org.
Rethink Your Purchasing Approach
When purchasing compressors, these questions are at the top of everyone’s list. The “obvious” answer is not always correct:
Can our plant save money by purchasing one large air compressor instead of two or three smaller ones? Not really. This approach may save on the initial purchase and ongoing maintenance costs, but these costs are typically only about 20% to 30% of the total life-cycle costs of running an air compressor. Over-sizing an air compressor will likely cost much more on the largest component of the lifetime operating costs: electricity input. For example, a single 200-hp load/unload compressor running full time at half capacity with a small receiver tank might consume $130,000/yr. of energy at $0.10/kWh. A well-designed system using two 100-hp units at the same average load would consume about $74,500/yr. in energy. Over 10 years the savings for choosing the two-compressor system would be worth more than half a million dollars.
Since all compressors are the same, can’t we just go with the least expensive? Not necessarily. The energy characteristics of air compressors vary with make and model. It’s wise to check out these numbers before purchasing a new compressor because the energy savings from choosing an efficient compressor can quickly offset any higher initial cost. Most large manufacturers have data sheets that show the power-versus-flow characteristics using a data format developed by the Compressed Air and Gas Institute (cagi.org). Look up the numbers before you buy any new compressor to ensure you are purchasing the most efficient unit. All CAGI members are required to publish their numbers. The effort could save you a bundle.