The dash blue line is the carnot cycle. It was Sadi Carnot (1796-1832) introduced the concept of the cycle but can not be applicated.
Rankine Cycle / Superheating
The basic cycle is the 'classical' Rankine Cycle.
In the case of the base cycle the vapour condensates during the expansion. The condensation will be noticed by liquid droplets surounded by vapour. In the case of turbines as expansion device the droplets will hit the blades with high velocities and cause errosion. In order to avoid this disadvantage, the vapour is often superheated so that the no condensation occurs during the expansion (see Ts diagram below).
Variation of the upper process temperature
The process efficiency increases as more heat is added and the condensation heat is left constant. Therefore by raising the main steam temperature and pressure the efficiency is raised. However the maximum values are limited by material strength as well as by working fluid properties (decomposition)
Variation of the condensation temperature
By reducing the condensation temperature less heat is lost and therefore the process efficiency is increased.
A reduction of the condensation pressure below ambient pressure is possible if the cycle is completely closed. The minumum possible temperature then is close to ambient which generates very low pressure, depending on the working fluid.
In some cases however the rejected heat might be used. This increases the process efficiency as well but means that the condensation temperature should not fall under a certain level.
Reheat
By reheating the fluid after having been expanded to saturated steam one can reduce the superheating whilst the vapour remains relatively dry throughout the expansion device. At the same time the mean heat addition temperature and therefore the efficiency is increased.
Large power plants often apply two or three stages of reheat.
Regeneration
Regenerative preheating means the preheat the liquid working fluid by the expanded or partially expanded vapour. The efficiency is increased because the external heat addition temperature is increased. The preheating by partially expanded vapour is called feedwater heating and is explained in the follwing two paragraphs. The preheating by the fully expanded vapour is called full flow regeneration or often just regeneration. This kind of regeneration requires that the temperature of the expanded vapour is above the condensation temperature. This is always the case for so called 'drying' fluids with positive saturation slope in the Ts diagram. It also can be achieved by adequate superheating. The vapour leaving the expansion device is passed through a heat exchanger (regenerator, recuperator) in order to preheat the compressed liquid.
Open Feed Water
In an open feed water heater partially expanded flow is extracted from the expansion device and mixed with the compressed liquid in order to preheat it. The compressed liquid and the extracted vapour must be at the same pressure. A first pump compresses the liquid to the pressure of the vapour. A second pump compresses the pre-heated fluid to the vaporization pressure.
Closed Feed Water
In a closed feed water heater partially expanded flow from the expansion device is used to preheat the compressed liquid by the use of an heat exchanger. Therefore the pressure of the compressed liquid and the extracted vapour don't need to have the same pressure. The condensed vapour is feed back at into the condedsed fluid at a point of lower pressure.
Methods to increase the process efficiency are:
• Increasing the main steam temperature and pressure
• Reducing the condensation temperature and pressure
• Multistage reheating
• Regenerative preheating of the feed water
From http://www.orc-forum.org/Process/Cycle_Design.html
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