Recovering energy!

Published:  26 February, 2014

PWE spoke with Philip McArragher, technical manager for HPC Compressed Air Systems, who explains there is up to 96% recoverable energy achievable from screw compressor installations.

In view of the ever-increasing cost of energy, efficient use of it is not just a sound economic measure but an ecological necessity as well. Compressor manufacturers, like HPC, offer many possibilities and expert advice to interested parties, but one potential and cost effective opportunity that is still largely ignored is the recovery of the heat produced by the compression process.

It is a fact that today’s screw compressors primarily generate heat. Although this statement may come as a surprise, the truth is that 100% of the electrical energy input to a compressor is turned into heat. The action of compression charges the air in the compressor with potential energy. This energy is given up at the point of use by the compressed air expanding and drawing heat from the surroundings.

However, up to 96% of this energy is recoverable says Philip McArragher, technical manager for HPC Compressed Air Systems: “The major proportion of the energy recoverable as heat, about 76%, is found in the compressor cooling fluid/oil; approximately 15% is in the compressed air itself and up to 5% is given up by the drive motor to the cooling air. In a fully encapsulated oil-cooled rotary screw compressor package even the losses from the electric motor can be recovered as hot air. This brings the total proportion of input energy available as recoverable heat up to an impressive 96%. Of the remaining energy, 2% radiates away from the compressor package and 2% remains in the compressed air.”

Energy conscious compressed air users wanting to improve the efficiency and economy of their compressed air plant can choose from a number of heat recovery methods:

Air/Space Heating

The simplest and most direct method of recovering the heat generated in an oil cooled rotary screw compressor is by direct use of the cooling air that carries away the heat from the airend, oil cooler, motor, etc. This heated air can be ducted away for use as space heating in warehouses and workshops or it can be used for other applications such as drying, heat curtains or pre-heating combustion air.

When the heated air is not required a manual or automatic flap/louvre can be used to discharge it to atmosphere. In some instances, this flap can be thermostatically regulated to maintain a constant, set temperature. McArragher says this space heating method allows 96% of the electrical energy consumption of a screw compressor to be recovered with considerable cost savings. For example, even with a relatively small 18.5 kW compressor you could easily recover enough energy to heat a small unit or office area.

Water Heating

An alternative option of heat recovery from compressors is hot water. This can be recovered for various purposes either from an air-cooled or water-cooled compressor package with a heat exchanger installed in the airend cooling oil circuit. Plate or fail-safe heat exchangers are employed, depending on whether the water is used for heating, laundry, showering, production or commercial cleaning purposes. Water temperatures of up to a maximum of 70°C can be achieved with these heat exchangers. McArragher says HPC’s experience shows that for compressor packages upward of 18.5 kW capacity the additional costs for these heat recovery systems can be realised within just two years (conditional on correct design).

Considerations of Reliability

Normally, the compressor's primary cooling system should not be used both for cooling and as a heat recovery system. HPC’s reasoning behind this, McArragher says, is that, should the heat recovery system fail, then the cooling of the compressor and the production of compressed air would be endangered. The safest method is to fit an additional heat exchanger in the compressor purely for heat recovery. In the event of it failing, or if no hot water is required, the compressor can revert to its primary air or water cooling system and so continue operation. The supply of compressed air is then ensured.

It is important to realise that each application and compressed air installation is different and should be treated accordingly. The path to maximum energy savings is the efficient and proper application of compressor control systems and energy saving techniques. Recovering the heat of compression for a useful purpose is an intelligent way of reducing the cost of compressed air production and improving energy efficiency at the same time. The effort and additional investment involved is reasonably small and, depending on individual applications, can be recovered over a relatively short period of time depending on the purpose to which the heat is put and the method of recovery chosen.

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