Posted by Neon John on July 24, 2008, 2:17 pm
>I thought the low temp raise from inlet to outlet was about protecting the
>fish/plants in the river. I'm not even sure all the water passing through
>the plant goes over the condenser, I think there is some "post mixing" to
>cool the condenser water before release. (but this would vary from place to
>place)
No, low temperature rise long predates any eco-nazi crap. Two considerations.
One, the lower the pressure in the condenser, the less drag on the low
pressure turbine wheels. In large plants, the diameter of the biggest wheel
is over 35 feet and the tips travel at supersonic speed (at atmospheric
pressure). The condenser operates at the vapor pressure of the water at
whatever temperature exists in the hot well and around the tubes. Therefore
it is desirable to keep the temperature in the condenser as low as possible.
This is actually a very significant effect. Some plants have to re-rate when
the river water is hot because of this effect.
I should add that loss of condenser vacuum is a serious event to a turbine.
Most turbines can survive sudden loss of vacuum at speed but it's very
stressful. The drag load is sufficient that during shuntdown, the condenser
vacuum is carefully broken to slow the turbine fairly rapidly. A turbine
operating in a vacuum will coast for hours if not artificially slowed.
The second consideration is overall efficiency. The higher the delta-T across
a Carnot cycle system, all else being equal, the higher the efficiency.
Material strength and corrosion set the upper limit of steam temperature so
lowering the other end's temperature is the only way to increase the delta-T.
In most plants, perhaps all, this is a secondary consideration to turbine
drag.
As for post mixing, that's a plant-specific and river-specific consideration,
as is diversion to the cooling towers where they exist. Some of the discharge
limits are silly. At Sequoyah, for instance, the cooling towers were an
afterthought. The discharge temperature was forecast to exceed a new,
arbitrarily selected number by something like a half a degree. Several
hundred million spent to build the towers and several megawatts diverted from
power production to run the pumps to feed the towers on account of a made-up
number that small.
Such is the cost of the new state-enforced religion, environmentalism.
John
--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com <-- best little blog on the net!
Tellico Plains, Occupied TN
You can't turn [MS] shovelware into reliable software by patching it a whole
lot. -Marcus Ranum
Posted by stu on July 24, 2008, 9:51 pm
> >I thought the low temp raise from inlet to outlet was about protecting
the
> >fish/plants in the river. I'm not even sure all the water passing through
> >the plant goes over the condenser, I think there is some "post mixing" to
> >cool the condenser water before release. (but this would vary from place
to
> >place)
> >
> No, low temperature rise long predates any eco-nazi crap.
It can't have or you wouldn't be complaining about being forced to a higher
delta-T. :)
Two considerations.
> One, the lower the pressure in the condenser, the less drag on the low
> pressure turbine wheels. In large plants, the diameter of the biggest
wheel
> is over 35 feet and the tips travel at supersonic speed (at atmospheric
> pressure). The condenser operates at the vapor pressure of the water at
> whatever temperature exists in the hot well and around the tubes.
Therefore
> it is desirable to keep the temperature in the condenser as low as
possible.
"as low as is economic"
Yes.
> This is actually a very significant effect. Some plants have to re-rate
when
> the river water is hot because of this effect.
Yes
> I should add that loss of condenser vacuum is a serious event to a
turbine.
> Most turbines can survive sudden loss of vacuum at speed but it's very
> stressful.
Why is this? does the sudden reduction in flow have a "water hammer" like
effect?
> The second consideration is overall efficiency. The higher the delta-T
across
> a Carnot cycle system, all else being equal, the higher the efficiency.
> Material strength and corrosion set the upper limit of steam temperature
so
> lowering the other end's temperature is the only way to increase the
delta-T.
> In most plants, perhaps all, this is a secondary consideration to turbine
> drag.
Yes
> As for post mixing, that's a plant-specific and river-specific
consideration,
> as is diversion to the cooling towers where they exist. Some of the
discharge
> limits are silly. At Sequoyah, for instance, the cooling towers were an
> afterthought. The discharge temperature was forecast to exceed a new,
> arbitrarily selected number by something like a half a degree. Several
> hundred million spent to build the towers and several megawatts diverted
from
> power production to run the pumps to feed the towers on account of a
made-up
> number that small.
You are saying that the "eco-nazi crap" forced lower temp rise inlet to
outlet on the plants.
So the plants would prefer to have a higher rise in outlet temp
Isn't that what I said?
> Such is the cost of the new state-enforced religion, environmentalism.
Yes
> John
>> 1) Understand this if you understand nothing else: A condenser system
is not
>> just about the convenience of returning feed water to the steam system!
A steam
>> plant operating without a condenser (exhausting spent steam directly to
the
>> atmosphere) is operating at a fraction of the efficiency compared to the
same
>> plant operating with a condenser.
>How much of "a fraction" are you talking about? I wouldn't have thought it
>would make that much difference. Although they would be upset about losing
>their wonderfully clean feed water.
Anyone done the maths on this one?
"at a faction" to me means something around say 20%. (although 9/10th is a
faction also)
Sorry for butchering your post John but the English language isn't one of my
greatest skills.... unfortunately the English language is the only one I
have lol
Stuart
Posted by daestrom on July 26, 2008, 12:07 pm
Neon John wrote:
>> I thought the low temp raise from inlet to outlet was about
>> protecting the fish/plants in the river. I'm not even sure all the
>> water passing through the plant goes over the condenser, I think
>> there is some "post mixing" to cool the condenser water before
>> release. (but this would vary from place to place)
>>
> No, low temperature rise long predates any eco-nazi crap. Two
> considerations. One, the lower the pressure in the condenser, the
> less drag on the low pressure turbine wheels.
Again with the 'drag' comment. When operating at full load the last stages
of the turbine aren't 'dragging' along in the vacuum. They are extracting
energy from the steam. The higher vacuum means that most of the stages will
be operating at a higher pressure and the work extracted from the steam will
be less. But there is no 'drag'.
The last turbine stages only 'drag' when the turbine is lightly loaded or
unloaded. If they were a 'drag' when operating normally, a good engineer
would remove the wheel and thus increase output.
<snip>
> As for post mixing, that's a plant-specific and river-specific
> consideration, as is diversion to the cooling towers where they
> exist. Some of the discharge limits are silly. At Sequoyah, for
> instance, the cooling towers were an afterthought. The discharge
> temperature was forecast to exceed a new, arbitrarily selected number
> by something like a half a degree. Several hundred million spent to
> build the towers and several megawatts diverted from power production
> to run the pumps to feed the towers on account of a made-up number
> that small.
I suspect there is more too it than that. After all, if all they wanted was
to reduce the outlet temperature they could have done that with a modest
increase in pumping power to the circ-water systems. If it was just an
'arbitrarily selected number' they could have just gotten a permit for an
extra 50 kgpm or so and beat that 1/2 degree. Sounds like you're just
repeating some myth you've heard.
With river-cooled plants the issue isn't the exact discharge temperature but
the total heat load on the river. The concern is the total MBTU load being
dumped into the river regardless of whether it's 500 kgpm @ 30 delta F or
600 kgpm @ 25 delta F.
The maximum heat load a river can bear is, of course, a function of the
river flow. Often plants can bypass their cooling towers in the spring when
river flow is higher and only need to use the towers in late summer when the
river flow is limited and the river can't bear the same heat load.
daestrom
Posted by daestrom on July 26, 2008, 12:17 pm
stu wrote:
<snip>
>>> 1) Understand this if you understand nothing else: A condenser
>>> system is not just about the convenience of returning feed water to
>>> the steam system!
> A steam
>>> plant operating without a condenser (exhausting spent steam
>>> directly to the atmosphere) is operating at a fraction of the
>>> efficiency compared to the same plant operating with a condenser.
>>
>> How much of "a fraction" are you talking about? I wouldn't have
>> thought it would make that much difference. Although they would be
>> upset about losing their wonderfully clean feed water.
> Anyone done the maths on this one?
> "at a faction" to me means something around say 20%. (although 9/10th
> is a faction also)
If you look at a steam Mollier diagram you can get a pretty good idea. If
an ideal turbine operates between 1000 psia steam (saturated) and a vacuum
of about 1 inhg (about 0.5 psia), with no reheat or other complications, it
can extract 442 BTU/lbm of steam. Discharging to atmosphere it can only
extract about 287 BTU/lbm.
How much the overall cycle efficiency drops depends on exactly what the
high-side temperature is. But for example if it were a nuc plant running
about 540 degF on the hot side, then the Carnot efficiency with a 100 degF
condenser would be about 44% (actually plants don't operate this good). If
the condenser operates at atmospheric pressure (212 degF), then the Carnot
efficiency drops to only 32.8%.
daestrom
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>fish/plants in the river. I'm not even sure all the water passing through
>the plant goes over the condenser, I think there is some "post mixing" to
>cool the condenser water before release. (but this would vary from place to
>place)