A number of years ago, a metal fabrication plant started up an operation in an outbuilding to add to the production capacity of a larger plant. The new plant had several new processes that had never been tried out before, so the air loading was unknown. The plant had a couple of used air compressors available and a large air dryer, so they decided to install them rather than matching the equipment to a projected air loading.
The maintenance personnel who did the work had been to a Compressed Air Challenge Fundamentals seminar, so they knew the value of large storage receiver capacity and good compressor control. To go with a used 100-hp compressor, they installed 1060 gallons of storage so it could run in load/unload mode. The pressure band was also set wider than the standard 10 psi to reduce compressor cycling. The piping was sized well to minimize piping loss. And a surplus refrigerated air dryer was installed, sized at 850 cfm, which was thought to be enough for the existing load, plus future expansion of production many years down the road.
A second, much smaller, 20-hp compressor was also added to the plant for operation on evenings and weekends. This was also a load/unload style compressor that had an internal air dryer. Because this compressor had a “smart” control, it was set up to work with the large storage and run in start/stop mode to save energy. The controller shuts the compressor completely off between load cycles when the starts per hour do not exceed the maximum allowable frequency—this eliminates wasteful unloaded run time. Running this way saves significant operating costs during low loads. In fact, because air loading was much lower than anticipated, this worked so well the 100-hp compressor rarely ran, its operating time is less than 3% of the total system time. It produced an average of 1 cfm out of the average system flow of 33 cfm.
The facility had an air audit done that uncovered these many positive system attributes, but one thing stood out—the oversized non-cycling 850 cfm air dryer was running 24 hours a day, 7 days a week with an average air loading of less than 1%. This air dryer is consuming 4.5 kW compared with an total combined compressed air system power, compressors included, of only 13 kW. When calculated out, this means the dryer is consuming 35% of total system power! This is the cost of oversizing.
Had a cycling style dryer been used—even if it was oversized—the air drying system power consumption would have automatically reduced due to better response to lighter flows and reduced moisture loading. The use of a non-cycling dryer that is much larger than required is costing about $5,000 per year in additional electrical charges. At this rate it won’t take long to pay for a more efficient dryer.
Had this dryer been an oversized uncontrolled heatless desiccant dryer, the story could have been much worse.
Learn more about compressed air dryers in our next Compressed Air Challenge seminar in your area. Visit www.compressedairchallenge.org for more information.
By Ron Marshall for the Compressed Air Challenge
Leave a Reply