Heat Recovery from Air Compressors

Compressing air makes heat. About 94 percent of the kWh you put into a 75 kW screw compressor leaves as heat. The other 6 percent ends up in the compressed air. If the heat goes out the rooftop fan and the boiler downstairs is burning gas to make process water, that is money walking past itself.

Heat recovery is one line on the compressed air cost ledger.

Where the heat goes

Oil-injected screw at full load. About 72 percent of input power into the lube oil cooler. About 13 percent into the intercooler on a two-stage machine. About 9 percent into the aftercooler. Motor surface and friction account for the rest. Numbers shift a few points by machine generation but the rough split holds.

The 72 percent in the oil is the easy target. Oil temperature into the cooler runs 80 to 95 °C. Hot enough to make process hot water with no heat pump in the loop. The cooler discharge side is what gets recovered.

Air recovery

Cheapest path. Replace the cooler exhaust hood with a duct. Run the duct into the warehouse, the loading dock, the staff break area. Winter offsets gas heat. Summer, a damper diverts the exhaust back outside.

Duct length matters. Static pressure adds load to the cooling fan. Atlas Copco's GA series datasheets list maximum allowable external static pressure on the cooling fan, usually 50 to 80 Pa. Beyond that the cooler runs hot and the unit derates. Long duct runs need a booster fan.

Cold-climate plants on single shifts win on this. Warm-climate summer-heavy production loses.

Water recovery

Plate heat exchanger between the compressor oil circuit and a process water loop. Cold water in, hot oil out, hot water out, cool oil out. The heat exchanger sits on the compressor oil discharge side. The existing oil-water cooler stays in place as a trim cooler in case the process loop cannot absorb the full heat load.

Recovery rate to the water side hits 90 to 94 percent of input power on a properly sized retrofit. That is the textbook number from Kaeser's KHE module brochure and Atlas Copco's ER module documentation.

Output water temperature lands around 70 to 80 °C. Boiler feed preheat takes it. Washroom hot water takes it. Process tank heating, food plant CIP rinse, brewery hot liquor tank, all good. Anything needing above 90 °C falls short. District heating mains at 110 °C is too hot.

Recovery rate by compressor size

Below 22 kW the math rarely works. Annualized recoverable energy is roughly 324,000 kWh on a 75 kW machine running 6,000 h/yr at 80 percent load. On a 15 kW machine in the same conditions it is 65,000 kWh. The capital cost of the heat exchanger and piping does not scale down proportionally.

22 to 75 kW. Marginal. Depends on local fuel cost and how close the heat sink is.

75 to 250 kW. Standard practice in well-run plants. Payback usually 1.5 to 2.5 years.

Above 250 kW. Almost always worth it. Centrifugal machines have higher-grade heat available because intercooler stages run at 130 to 180 °C.

Payback

A 75 kW oil-injected screw runs 6,000 hours a year at 80 percent load. That is 360,000 kWh of input. 90 percent recoverable to a water loop. 324,000 kWh useful heat available.

Displacing $0.40/m³ natural gas at 90 percent boiler efficiency, that is roughly 34,000 m³ of gas displaced. Around $13,500 in fuel savings each year.

Equipment cost varies. Bare plate heat exchanger with controls and pump runs $3,000 to $8,000. Plumbing to the heat sink runs anywhere from $2,000 for a 30-meter run in an open shop to $20,000 for a longer run with insulation through occupied space. Payback 1.5 to 3 years on most retrofits.

When it does not work

Tropical-climate plants without process heat needs. The heat has nowhere to go.

Mismatched timing. Compressor runs night shift, hot water needed during day. Storage tank required, costs another $5,000 to $15,000 plus space.

Oil-free machines with low-temperature aftercoolers. Recoverable heat below 50 °C is not useful for most processes without a heat pump.

Two-shift schedules with one-shift heat demand. Recoverable energy gets dumped half the time.

Plants planning relocation in 2-3 years. Don't bother.