On Fri, 16 Aug 2013 14:30:22 +0100, The Natural Philosopher
wrote:

On 16/08/13 13:10, The Other Mike wrote:
On Fri, 16 Aug 2013 07:47:15 +0100, "harryagain"
wrote:

There are almost unsurmountable problems associated with pumpimg very hot
water into steam boilers.
Depends what you mean by very hot. UK coal and nuclear stations have no
problems pumping water at 160 deg C. I'd call that more than a bit hot. Not
steam temperature at the boiler outlet temperatures but still 'very hot'

Especially steam locomotive boilers (or cars for that matter).
Clearly you don't know this.

Clue. Why do you suppose so much heat has to be dumped at thermal power
stations?
It's *nothing* to do with pumping water into boilers.

Exactly.

Its all do with the basic issues of using steam as a working fluid at all.

you can't get it THAT hot, and it has to come out the arse end at
something like 40-50 C.

Better efficiency comes from gas turbines, because the gases are like
1000C and the final steam circuit outflows are like 40-50C.

The amaxim efficiency of a thermal engine is simple.

1 - (outlet-temp/inlet-temp) where both are in degrees absolute.
So for a steam boiler at say 260C inlet to the turbines, and final
condenser at 40C, the numbers come out to

1-(313 /533) i.e. about 41%

Go to a CCGT at 1000C and it all looks different.

1-(313/1273) . i.e. 75%.

That's the maximum theoretical efficiency. In reality it tends to be
about 37% for top steam plant and 60% for a CCGT.

And of course the gas temperature (and effiiency) goes down in a CCGT
when its throttled back to make way for harries solar panels and
windymills, so burning yet more fuel for te same power output


Once condensate leaves the condenser it is heated in multiple stages of feed
heaters before it even gets to the boiler feed pump. The appropriate inlet
pressure, inlet temperature, and dissolved gas concentration are all what are
needed to avoid cavitation in the feed pump.

Exactly.


Your turbine steam inlet figures are way out compared to the standard in the UK

568 deg C / 168 bar is the norm for HP inlet for all 120MW units with final
stage steam temperatures of around 30 deg C.

That makes the ideal Carnot efficiency closer to 64% with as you say 37% being
about the norm for real life.


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