About 50,000 Btu/hr of thermal energy is available to be recovered for every 100 cfm of air provided during compression, Sustainable Plant reports.
Gas compression is a common process, used to provide compressed air for industrial plants as well as for air conditioning and refrigeration across industrial, commercial and residential applications.
But while the process aims simply to increase the pressure of a gas, between 80 and 90 percent of the input energy gets consumed in raising the gas temperature, the website reports. About 8 hp of electricity is needed to produce 1 hp of compressed air.
The temperature rise is not simply a process inefficiency – it’s a necessary byproduct of the physics involved. But, the article notes, more efficient processes will create less of a temperature rise. And in the case of air compression for refrigeration systems, the temperature increase is what makes the system work in the first place, because the increased temperature allows the unit to reject heat from the outside.
Typical recovery rates for waste heat from compression systems vary between 30 and 90 percent. Methods for recovery include ducting the air leaving an air-cooled aftercooler or lubricant cooler, or installing a short duct run to vent from the air compressor to a loading dock area.
Compressed air generators were recently included in a GE energy efficiency program with a projected payback period of under six months, implemented at 20 General Motors plants.
GM adapted GE’s Proficy Cimplicity software to tie the use of lights to the schedule of conveyers. The manufacturer then discovered other aspects of consumption that could also be tied to conveyer operations, including not just compressed air generators, but air supply houses, water and paint shop ovens.
And an Owens Corning plant in Santa Clara, Calif. achieved annual cost savings of $252,000 after implementing several energy-savings projects recommended by the Department of Energy’s Save Energy Now assessment, which identified compressed air systems as a major area for reducing energy use.
Some of the energy-saving measures at the plant included replacing a large compressor, retrofitting inefficient pumps, replacing valves and installing variable speed driveson the pumps, and installing variable frequency drives on the fans.
Picture credit: Hideya HAMANO