Energy savings with heat recovery
In view of ever-increasing energy prices, efficient use of energy is not only important for the environment, but is also increasingly becoming an economic necessity. Compressor manufacturers are able to provide various solutions in this regard, such as heat recovery with rotary screw compressor systems for example.
1. Compressors primarily generate heat
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.
2. Choices of heat recovery
Compressed air users interested in further increasing the effi ciency of their compressed air supply system can choose between various heat recovery options:
a) Air heating
The simplest and most direct method of recovering the heat generated in a fl uid-/oil-cooled rotary screw compressor is by using the heat from the compressor system’s warmed cooling air. This heated air is 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 fl ap, or louvre, discharges it into the open. The louvre can be thermostatically regulated to maintain a constant, set temperature. The space heating method allows 96 % of the electrical energy consumption of a rotary screw compressor to be recovered. It is well worth is too: even small systems, such as a 7.5 kW compressor, can easily generate enough recyclable heat energy to warm a typical family home.
b) Hot water
Hot water for various purposes can be recovered from either an air-cooled or water-cooled rotary screw compressor package by means of 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 or showering, production or commercial cleaning purposes. Water temperatures of up to 70 °C can be achieved with these heat exchangers. Experience shows that for compressor packages upward of 7.5 kW capacity the additional costs for these heat recovery systems amortise within two years. This of course depends on correct planning.
3. Considerations of reliability
Normally, the compressor’s primary cooling system should never be used both for cooling and as a heat recovery system. The reason for this is that if the heat recovery system fails then compressor cooling and, as a result, compressed air production would be endangered. The safest method is to install an additional specialised heat exchanger within the compressor station that is solely responsible for heat recovery. Compressor operation and reliability are therefore safeguarded in the event of a fault: if heat is not dissipated by the heat recovery system’s fl uid / water heat exchanger, the compressor can revert to its primary air or water cooling system and so continue operation.
4. Up to 96 percent usable energy
The major proportion of the energy recoverable as heat, about 76 %, is found in the compressor cooling oil, approx. 15 % in the compressed air itself and up to 5 % is lost through drive motor heat losses. In a fully encapsulated fl uid-/oil-cooled rotary screw compressor package even the losses from the electric motor can be recovered as heat energy if appropriate cooling is used. This brings the total proportion of input energy available as heat up to a startling 96 %. Of the remaining energy, 2 % radiates away from the compressor package and 2 % remains in the compressed air (pic. 1).
Recovering the heat of compression for a useful purpose is an intelligent way of improving the economics of compressed air production and benefi ting the environment at the same time; the effort involved is relatively small. The investment is quickly recovered depending on local circumstances, the purpose for which the heat is used and the method of recovery chosen.