The state of a gas is specified by pressure, volume and temperature and can be changed in different ways.
| Isochoric change of state: | Volume remains constant |
| Isobaric change of state: | Pressure remains constant |
| Isothermal change of state: | Temperature remains constant |
| Adiabatic change of state: | Without exchanging heat with the surroundings |
Isothermal and adiabatic changes of state are of particular significance for compressors. Isothermal1 calls for the lowest and adiabatic2 for the highest expenditure of energy when compressing gases. The true compression process takes place polytropically, i.e. the line of compression runs in between the adiabatic and the isothermal one. The closer compression, approaches the isothermal mode, the lower.the amount of work which has to be expended in order to compress the air or gas.
Figure 3.1.1
Fig. 3.1.1 shows an idealised operating process of a displacement compressor in simplified form.
1 exponent n = 1
2 exponent x = 1.4
As far as thermodynamic change of state is concerned, two principal theorems apply. The first principal theorem describes the effect of evolution of work and heat in accordance with enthalpy. The second principal theorem describes the heat exchange of enthropy. Using static thermodynamics, the first principal theorem can be incorporated into the second principal theorem. When air is being compressed, a change of state with increasing enthropy is carried out. This change of state takes place polytropically. A special polytropic change of state used in compressors is the adiabatic change of state without change of heat content. The technical work needed to bring about the change of pressure arises from the difference between the enthalpies as between one state and the other. The different changes of state can be shown as an area (see Fig. 3.1.1) in the pxV diagram. The approximation to isothermal single stage compression, the course of line 1-2, will be the closer, the more isobaric intermediate stages are used. Two and three stage compressor installations form part of present day technology.
Work used for adiabatic or isothermal compression is necessary in order to compress the air to reach the higher pressure level. This work expenditure, therefore, represents no loss of energy. Isobaric intermediate and final cooling, as well as the cooling of the oil and water circuits, represent the energy loss element of the required expenditure of energy, if the heat, which has to be extracted, is passed on, unused, to the surroundings. Roughly estimated, about 94% of the energy fed to a compressor is lost as waste heat. Only about 6% is delivered with the compressed air. Whether the necessary waste heat represents a real loss of energy is up to the operator of the installation. It is part of the present day state of the art to recover heat. The quantity of heat in question is hereby transmitted to cooling media and conveyed to the most varied points of use. With an oil injected screw compressor, for instance, about 72% of the waste heat is given off via the oil cooler, 13% of waste heat through the aftercooler and 9% from the drive motor. This heat is available e.g. for heating water and/or generating hot air. With oil free compressor systems, the heat of compression is used for the desorption of the adsorption dryer (see section 5).
