During the compression process a high percentage of the consumed energy is transferred into heat. In an efficient compressor installation this heat is not lost, but used to save energy in other applications where heat is needed. About 70 % to 80 % of the energy used to run a compressor can be recovered and therefore saved in other places.
There are two main ways for heat recovery on screw compressors:
– use the warm cooling air flow coming from the compressor
– use the heat from the compressor oil circuit
Fig. 1: Compression heat in a screw compressor
Fig. 1 shows the different streams of heat out of an air cooled oil injected screw compressor. The heat from the drive motor, the oil cooler and the air aftercooler leaves the compressor in one stream of warm air, taking out 94 % of the generated heat. This cooling air flow can be used to heat rooms or production facilities. All that is needed is an air duct connected to the compressor cooling air outlet. Fig. 2 shows an example installation. This is the simplest method of heat recovery, immediately saving energy which otherwise would have to be used to heat the building.
Fig. 2: Example installation of heat recovery from cooling air
Another possible way for heat recovery is to take the heat from the oil circuit in the compressor with a heat exchanger. The advantage is that heat is then taken from the place where it is generated and from the hottest medium. Hot water with a temperature of up to 70°C can be generated which can be used in other places. This way of heat recovery is independent from the type of compressor cooling. One possible application would again be to supplement a central heating system whereby a significant amount of oil or gas for the heater can be saved.
Fig. 3: Saving potential for heat recovery (calculated at a price of only 0.80 €/l for heating oil)
Fig. 3 shows a table with possible saving potential when heat recovery is used. The saving potential is calculated for a single shift operation (2,000 op. h) and heat recovery only done during half of the year (1,000 op. h). In an application where the recovered heat is used in a three shift operation throughout the year, the saving potential is even six times bigger. In most cases the return on investment for heat recovery installations is very short. When building a new compressor station heat recovery should always be considered. That would also include choosing the compressor location as near as possible to a heat consumer.
