Up to three quarters of the lifetime costs of a typical compressed air system are attributed to energy. In addition, depending on utilisation, electrical power can account for up to 90% of the total costs of compressed air production. With the heightened electricity prices we have all seen in recent times, finding ways to optimise energy efficiency has never been so important.
Compressor operators have a distinct advantage when it comes to reducing energy consumption and costs, because in most cases the savings are already there for the taking! Amazing as it may seem, 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.
Crucially, up to 96% of the energy that is used to produce compressed air remains available for reuse as heat! Air- and water-cooled rotary screw compressors are the best-suited compressor technologies for efficient heat recovery. 76% of the energy consumed by these compressors is transferred to the cooling medium, 15% is transferred to the compressed air, with heat losses from the electrical motor accounting for up to 5%. Only around 2% of the energy is lost as radiant heat and about 2% remains as heat in the compressed air.
Putting all that hot air to good use
There are a number of heat recovery options available to compressed air users;
- Air heating
The simplest and most direct method of recovering the heat generated by a fluid/oil cooled rotary screw compressor is by using the heat from the compressor system's warmed cooling air. This heat can be ducted away to be used for space heating of warehouses and workshops.
The hot air can also be used for other applications such as; drying, heat curtains and pre-heating combustion air.
When the heated air is not needed, a manual or automatic flap - or louvre - discharges it into the open. The louvre can be thermostatically regulated to maintain a constant set temperature.
The space heating method allows for 96% of the electrical energy consumption of a rotary screw compressor to be recovered. When you consider that even a small 7.5 kW compressor could easily generate enough recyclable heat energy to warm a typical family home, it is well worth it!
- Water heating
Hot water for various purposes can also be recovered from either an air or water cooled rotary screw compressor package. This is achieved by installing a heat exchanger in the screw compressor block cooling oil circuit. The water can be used for heating, production or commercial cleaning, a laundry or even showering. Depending on what the water will be used for a plate or fail-safe heat exchanger will be employed with water temperatures of up to 70oC achievable. Experience has shown that for compressor packages upward of 7.5 kW capacity, the additional costs for these heat recovery systems amortise within two years.
Recovering the heat of compression for a useful purpose is a smart way to improve the economics of compressed air production whilst benefiting the environment at the same time. The effort involved is relatively small and depending on; local circumstances, the purpose for which the heat is used and the method of recovery chosen, the investment can be quickly recovered.