On the whole most of us are quite aware of the considerable savings that can be accomplished by wise use and recovery of energy. One way that a plant can save substantially is to capture the energy that an electric motor adds to the compressed air from the air compressor. As much as 80% to 93% of the electrical energy used by an industrial air compressor is converted to heat. A properly designed heat recovery system can capture anywhere between 50% to 90% of this energy and convert it to useful energy.
The heat recovered is sufficient in most cases to use in supplemental ways such as heating water and space heating, however generally there is not enough energy to produce steam directly.
Packaged air cooled rotary screw compressor lend themselves easily to heat recovery, supplemental heating or other hot air uses very well due to their enclosed design. Since ambient air is directed across the compressors aftercooler and lubricant cooler where the heat can be easily collected from both the compressed air and the lubricant.
Packaged coolers are normally enclosed cabinets that feature integral heat exchangers and fans. This type of system only needs ducting and an additional fan to minimize back pressure on the air compressors cooling fan. This arrangement can be controlled with a simple thermostat operated vent on a hinge and when the extra heat is not required it can be ducted outside the facility.
The recovered energy can be used for space heating, industrial drying, preheating aspirated air for oil burners or other applications requiring warm air. Typically there is approximately 50,000 Btu/Hr of energy available from each 100 SCFM of capacity (at full load). The temperature differential is somewhere between 30°F – 40°F above the air inlet temperature and the recovery efficiency is commonly found to be 80% – 90%.
We all know the old saying there is “no free lunch” and that principle applies here. If the supply air is not from outside the plant a drop in the static pressure could occur in the compressor cabinet thereby reducing the efficiency of the compressor. If you choose to use outside air for makeup, you might need some return air to keep the air above freezing to avoid compressor damage.
Heat recovery is generally not utilized with water cooled compressors since an extra stage of heat exchange is required and the efficiency of recovering that heat is normally in the 50% – 60% range.
To calculate annual energy savings:
Energy Savings (Btu/Yr) = 0.80 * compressor bhp * 2,545 Btu/bhp-hour * hours of operation.
If we consider a 50 HP compressor:
.080 * 50bhp * 2,545 Btu/bhp-hour * 2080 hrs/year = 211,744,000 Btu/yr
Where 0.80 is the recoverable heat as a percentage of the units output, 2,545 is the conversion factor.
Cost savings in dollars per year = [(energy savings in Btu/yr)/Btu/fuel) x ($/unit fuel)]/primary heater efficiency.
Photo courtesy of CC BY 3.0, https://en.wikipedia.org/w/index.php?curid=32093890