Air compression is an inefficient way to do work. The thermodynamics are unforgiving — you're compressing a gas, and most of the energy you put in ends up as heat rather than pressure. A 37kW compressor might deliver 11kW of useful pneumatic power; the other 26kW is heating the air, the oil, and eventually the compressor room.
Heat recovery systems capture some or most of that waste heat and put it to useful work. The payback calculations are often surprisingly good.
Where the Heat Goes
On a typical oil-injected rotary screw compressor, roughly 80-94% of input electrical energy can be recovered as useful heat. The energy is distributed approximately as follows:
- 80-85%: carried in the cooling air (air-cooled machines) or cooling water (water-cooled)
- 5-10%: in the compressed air discharged to the system
- 2-5%: radiated to the machine room
Water-cooled machines offer the highest recovery potential because the heat is concentrated in the cooling water at a useful temperature (60-90°C outlet). Air-cooled machines can also be recovered from, but at lower temperatures and typically through ducting the cooling air to a useful location.
Air-Side Heat Recovery
The simplest approach on an air-cooled machine is to duct the hot cooling air to where it's useful. A compressor room heat recovery kit — essentially insulated ducting with a motorised damper and thermostatic control — routes the 40-60°C exhaust air to an adjacent space that needs heating in winter, and diverts it outside in summer.
This approach requires no modification to the compressor. The capital cost is low (£2,000-6,000 for a typical installation) and payback is quick if the space has significant heating demand.
The limitation is that warm air at 40-50°C has limited uses beyond space heating. You can't use it to heat water to useful temperatures or for process heat.
Water-Side Heat Recovery
Water-cooled machines, and air-cooled machines fitted with a water-cooled option or oil-water heat exchanger, can produce hot water at 60-80°C. This temperature is useful for domestic hot water preheat, process water heating, or as a feed to a low-temperature heating circuit.
The oil-water heat exchanger retrofit is available from most major manufacturers for their standard ranges. A 55kW water-cooled compressor running 4,000 hours per year can produce around 170,000 kWh of hot water energy annually. At current gas prices, replacing gas heating with recovered compressor heat saves roughly £7,000-9,000 per year.
Capital cost for a water-side recovery installation including heat exchanger, pump, controls and a buffer vessel is typically £8,000-18,000. Payback of 1-3 years is common for sites with significant hot water demand.
What You Need to Make It Work
Heat recovery requires a genuine heat load to absorb the recovered energy. If your site has no hot water requirement, no adjacent space heating demand, and no process heat requirement, the economics don't work — you can't store the energy and you can't benefit from it.
Sites with canteens, locker rooms with showers, washing facilities, or process water requirements are natural candidates. Manufacturing sites with drying ovens, rinse tanks or washing machines are worth evaluating carefully.
Before specifying a recovery system, meter your current hot water or space heating consumption to establish the load. Match the potential recovery output to that load. An oversized recovery system that produces more heat than you can use doesn't deliver the promised savings.