A tire shop was having problems with its small 15-hp air compressor. It had served the shop well over many years, but it was failing due to its age—and it needed to be replaced. Because it was a single stage reciprocating compressor, the plant employees were not sad to see it go. It had always bothered them due to its high noise level. When choosing a replacement, the company owner became interested in a much quieter type of compressor, the lubricated rotary screw style, which was said to be a more efficient way to produce compressed air.
After installing the new compressor, the shop owner was very pleased by the low noise level of the new unit, but he became concerned because the new compressor was running all the time. The previous reciprocating compressor ran in a start/stop mode where it only turned on to fill up the system after compressed air was used. In all, it usually ran only a few hours per day, as the average shop air demand was low. The new compressor, however, never shut off and the owner suspected that this meant it was less efficient that the previous unit.
The owner contacted his local power utility and asked that the system be audited for energy efficiency. Instruments were placed on the system and it was determined that the plant had very low average compressed air flows and higher peaks when pneumatic tools were operated, typical for a service shop. The readings showed that the production and air drying of the compressor air was very inefficient and that the system was producing three times the amount of compressed air than was actually used.
The compressed air auditor explained that the system was inefficient because the new rotary screw compressor was installed without the proper amount of storage receiver capacity. Rotary screw compressors are typically not allowed to start and stop more than 6 times per hour by the on-board compressor controllers, so as to prevent damage to the drive motors. Rather than shutting off when the required shop pressure was achieved, the compressor unloaded but continued to run, consuming 35% of its full load power, even though it was producing no air. Most of the hours were in the unloaded condition.
There was another problem, too. The new air compressor was installed with a new membrane style air dryer. The owner had assumed that because this dryer style had no moving parts, it would be more efficient that an electrically powered refrigerated style. Unfortunately, due to its characteristics, the membrane dryer consumed a constant flow of purge air of about 20 cfm average, even while the average shop flow was only 8 cfm. The dryer was determined to be a poor match for the load profile of the shop.
Fortunately, the controller on the new compressor was a type that could be programmed to run in start/stop mode, just like a reciprocating compressor, if the frequency of compressor cycles is low. A large storage receiver was installed to slow down the cycles and a new refrigerated dryer replaced the membrane unit. Savings due to these measures were 60% of the original power consumption and the compressor operating hours were substantially reduced saving even more on maintenance costs.
By measuring their system and learning how it was operating, some easy-to-implement energy efficiency measures could identified. These measures allowed this shop owner to properly apply equipment that matched the characteristics of his load profile, saving him substantial operating costs.
Learn more about compressed air measurement in the next Compressed Air Challenge webinars. Visit www.compressedairchallenge.org for more information.
By Ron Marshall for the Compressed Air Challenge.
Filed Under: Pneumatic equipment + components