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Dear Dave
You may have to resend this onto your groups as I seem to be excluded from most,
The term ultra-supercrtical is pure spin, since supercritical power plants were built in the UK and USA in the 1960s. They were seen as brave, but not wholly successful attempt since the pipe work suffered from severe thermal stress.
Supercritical implies a boiler pressure of over 221 bar and with the "water substance" leaving the boiler at over 374 deg C
In practice it is best to have the pressure somewhat higher than the critical.pressure of 221 bar.....If this is not done there are peculiar density changes in the region of the critical temperature of 374 deg C, since the water substance cannot make up it mind as to whether it has the properties of water or steam. It would be difficult to model the heat transfer because of the erratic changes in density,thermal conductivity and viscosity. Having a higher pressure gives a much more regular change in these functions.
Superheat is applied to the water substance and by the time it has reached a temperature of about 550 deg C, the water substance has got the normal properties of superheated steam.(it more or less follows the PV=RT law). However, because the pressure is so high the steam cannot be expanded through a steam turbine down to condenser pressures. If this was attempted the steam would have started to turn into water at a pressure of around 5 bar.
Hence the steam is partially expanded and then reheated back to a high temperature. By this time the pressure in the reheater has fallen to 40-60 bar. At this pressure the steam can be expanded down to condenser pressures. As the steam passes through the turbine the temperature drops, below about 100 deg C some of the steam condenses. Just before the condenser the steam will contain about 10% moisture.
In the Nordyllander plant because the condenser pressure is so low and the intial steam temperature is so high it is worth having two stages of reheat. Reheat does not improve efficiency that much. It does improve the specific output of the plant ( kW/ kg/sec steam flow)
Rehaet was actuallly first applled to the the North Tees A plant ( where my home town is) in 1922. to overcome the moisture problem, which was becoming a prblemas steam pressures were increasing.
Fred Starr
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