Thermodynamic cycle

As a cyclic process is known in thermodynamics a sequence of state changes of a working fluid (liquid, vapor, gas - generally called fluid ) which periodically occurs, wherein again the initial state, characterized in that the state variables (see also Fundamental equation thermodynamic potential ) as u is a pressure, temperature and density, is achieved. There are technical processes, mostly for the conversion of heat into work (eg in internal combustion engines ) or for heating and cooling by expending of work (heat pump, fridge).

Two fundamental examples (mathematics)

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Crucial for a cyclic process (often called cycle), is that the return path is different from the way in which the state from the initial state. The state diagrams of the most widely used are the PV diagram, the TS diagram hs diagram and the PH diagram ( the latter in particular for cooling process ). In the first two diagrams thus an area is surrounded corresponding reversible processes in the cycle work. However, this applies only for ideal comparison processes. The real technical processes are not reversible (see dissipation) and the surface is then magnified by the dissipated work.

Right and left processes

There are legal processes and links processes, depending on whether the state diagram in a clockwise or undergo vice versa. When legal process ( clockwise) a portion of the supplied high-temperature heat is converted into work, the other part is dissipated at a lower temperature. The difference is the cycle work (see energy balance for cycles ). The extraction of work in the legal process comes from the fact that is compressed at low temperature, ie at low pressure ( workload ) and expands at high temperature and thus at high pressure, the fluid under labor tax. The amount of volume expansion of the working is thus greater than that of the compression. When links process, in contrast, everything turns around so that under workload heat is conveyed from a colder to a warmer reservoir. Particularly large specific cycle work is achieved when taking place within the process of phase change between liquid and gas, because of the difference in volume is particularly high. This makes you look in the steam power plant advantage. Since liquid (water ) is almost incompressible, eliminates the compression work and labor for conveying the liquid to the boiler at high pressure ( boiler feed pump) is relatively low.

Open and closed processes

A further distinction of cycles is given by the difference in heat. If this internally by burning incorporated of fuel, such as the internal combustion engine or the aircraft engine, the cycle is open, because a charge exchange between exhaust gas and fresh air must be done. A fundamental difference from a thermodynamic point of view, not because the atmosphere can be regarded as a large heat exchanger. The process in the sample picture is a closed two heat exchangers. Such processes can be used (for example, helium as the coolant and the working fluid ), for example in a nuclear power plant with a gas-cooled reactors.

With the computational and graphical representation of the processes one has a theoretical tool, both for the formulation of statements, as well as the technical implementation in the design of thermal engineering machinery and equipment. For example, the Born- Haber cycle is used to calculate the reaction energy (or enthalpy ) of a process step or the binding energy of a chemical compound, when the energies of the other process steps are known in chemistry.

To assess the efficiency of a cyclic process are the ideal comparison processes. These in turn are compared with the ideal theoretical cycle, the Carnot cycle, which has the maximum possible efficiency. It identifies what the second law of thermodynamics - at infinitely high technical effort - allowed. Practically, it is not accessible.

Cycles

Entropy (disambiguation) Temperature–entropy diagram Water vapor Steam-electric power station Born–Haber cycle Atkinson cycle Ericsson cycle Mixed/dual cycle
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