30-05-2012, 05:07 PM
The ideal regenerative Rankine cycle
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The analysis of the Rankine cycle using the second law showed that the largest exergy destruction (major irreversibilities) occurs during the heat-addition process. Therefore any attempt to reduce the exergy destruction should start with this process.
When we analyze the Rankine cycle (Figure.2) we can notice that:
A considerable percentage of the total energy input is used to heat the high pressure water from T2 to its saturation (point a).
Closed feedwater heater ( no mixing)
The two streams now can be at different pressures since they do not mix. The water passes through in the tubes and steam surrounds the tubes. The steam is condensated and pumped by a condensate pump into the main feedwater line or it passes through a trap (a device that permits only liquids to pass through).
Advantages
- We don’t need a pump for each heater, since the pressure is different
Disadvantages
- More complex (internal tubing network)
- More expensive
- Less heat transfer performance, since the two fluids are not in contact.
Optimal number of feedwater heaters
All modern steam power plants use feedwater heaters (8 feedwater heaters). The optimum number of feedwater heaters is determined from economical considerations:
You add a feedwater heater only if it saves more fuel than its own cost (+ maintenance).
With a large number of heaters, it is possible to approach the Carnot efficiency but at an unjustifiably high cost.
- At which pressure, the steam must be extracted from the turbine?
The pressure at which the steam should be extracted from the turbine must be in such a way that for one heater, the steam should be extracted at the point that allows the existing feedwater heater temperature to be midway between the saturated steam temperature in the boiler and the condenser temperature. For several heaters, the temperature difference should be divided as equally as possible.