It’s common knowledge (and the Ideal Gas Law) that compressing a gas raises is temperature. Compressors must be air- or water-cooled, and compressed gases are commonly cooled to condition them for use.
Less well known is the fact that all of the energy required to compress a gas is theoretically available for recovery as heat: the compressed gas leaving the compressor system at room temperature contains no more energy than the room-temperature air entering the compressor.
“It’s possible to extract, by heat transfer, an amount of energy from the compressed air that is equivalent to the amount of energy the electric motor placed into the compressed air,” says Bill Scales, P.E., CEO, Scales Industrial Technologies (www.scalesair.com). “This might appear to be a paradox, but confirms the first law of thermodynamics and the principle of the conservation of energy, which states that energy can neither be created nor destroyed; it can only change form.”
So how can compressed air perform work to power plant equipment? “When the compressed air expands, it draws energy from its surroundings equivalent to 20% to 25% of the energy we put in,” says Wayne Perry, technical director, Kaeser (www.kaeser.com). “It robs it from the atmosphere or the devices where it expands.”
Compressor manufacturers and savvy system engineers are well aware of the potential for energy recovery, and have increased their offerings of equipment, accessories and know-how to maximize energy recovery ROI. “One of the better methods to improve the overall efficiency of a compressed air system is to recover this rejected heat,” says Scales. “However, the availability of the heat and the opportunity to recover and use it are two different matters.”
Recovery: Practical versus possible
For a sense of how much energy might be available, consider that one horsepower equals 2,545 BTU/hr. “Although most rotary screw and reciprocating air compressors are sold in nominal horsepower sizes, they generally operate at loads that are 10% higher than their motor nameplate rating at rated compressor discharge pressure and full capacity output,” Scales says. “Therefore, a 100-hp air compressor (110 brake hp) generates almost 280,000 BTU/hr at full load. In addition, the electric motor with an assumed efficiency of 93% will dissipate an additional 19,600 BTU/hr.”
The potential to recover this energy was recently driven home by TÜV’s certification of Atlas Copco’s “Carbon Zero” compressor as capable of recovering 100% of the input electrical energy. The challenge is to recover energy from as many as possible of the compressor’s heat-producing components, including compression elements, oil cooler, intercooler and aftercooler (Figure 1).
Figure 1. The challenge is to recover energy from as many as possible of the compressor’s heat-producing components, including compression elements, oil cooler, intercooler and aftercooler.
Recoveries nearing 100% are possible only under rather ideal conditions. “We’re recovering 100% only under very specific conditions: 40°C, 70% RH, 20°C inlet water,” says Dave Hebert, product marketing manager, oil-free, Atlas Copco (www.atlascopco.us). “Typical recoveries are in the mid-90s.” The TÜV test results depend on extracting heat of condensation from input air humidity to compensate motor losses.
“If you have a need for the heat, you can recover 90% to 95% of it,” says Perry “It takes about $50,000 per year to power a 100 hp compressor, so a $10,000 heat exchanger can pay back in three months.”
Real-world project paybacks depend on the cost of alternative energy sources (typically natural gas), how well you can match energy supply and demand, the type(s) of compressors, and the complexity of the project.
“Depending upon the type of compressor, method of cooling and radiant heat losses, it is possible to recover energy in the form of heat transfer that is equivalent to 50% to 90% of the total energy input,” says Scales.
The most common industrial compressor is the lubricant-injected rotary screw supplied as a packaged compressor, which makes it easier to recover the heat. “In this type of compressor, approximately 80% of the heat is rejected in the lubricant cooler,” Scales says. “Most of the remaining heat is rejected in the aftercooler with a small percentage in the form of radiated heat from the compressor housing and lubricant separator receiver.
“In a two-stage lubricant-free rotary screw compressor, almost all the rejected heat is evenly divided between the aftercooler and intercooler,” Scales says. “In two-stage, water-cooled reciprocating compressors, the intercooler and aftercooler might each reject 40% of the heat, and the cylinders a total of 20%. A centrifugal compressor might have each intercooler and aftercooler share almost equally in the heat load.”