Posted by daestrom on March 28, 2009, 8:38 pm
BWR's use saturated steam with very little moisture (quality above 99%)
leaving the reactor vessel. Some PWR's are the same, but some use a
steam-generator type (a 'once through steam generator') where after the
dryer the steam passes around the tubes carrying reactor coolant from the
hot leg. These can create a small amount of superheat (<50 F I believe).
Both PWR and BWR use slower turbines developed for saturated steam. Without
a lot of superheat, a lot of moisture is formed as the steam passes through
the stages and much of it has to be removed to avoid excessive erosion of
Posted by daestrom on March 28, 2009, 9:00 pm
Yep, usually mounted in the boiler depending on the design. Old style 'D'
boilers would have chevron dryers in the top (two long rows of them like an
upside down 'V') to remove the moisture. Nuc steam generators of various
designs use cyclone separators to remove most of the moisture, down to less
than 10% and then chevron style dryers to reduce it down below 1%. Railroad
locomotives pre-date some of that but space and weight considerations also
limit what can fit in the steam dome.
Early railroad just used saturated steam and it was important for the
engineer to open/close the cylinder drain petcocks appropriately to blow any
condensate out of the cylinders when starting (still is even with
superheat). Then shut them once moving and the cylinders were heated
enough. Later locomotives would take the steam from the dome and run it
though many long narrow u-tubes that were fit inside the upper rows of
fire-tubes. So the steam from the dome would travel in a pipe towards the
smokebox, enter the superheater header where it split and flowed in parallel
through these many superheater tubes, then returned to another header in the
smokebox and on to the cylinder via the inlet valves. The hot gasses from
the fire traveling along the fire tubes towards the smokebox would superheat
the steam in the u-tubes.
But superheaters don't like a lot of moisture either. If you have too much
carryover (moisture droplets entrained in the steam) going into the
superheaters, the moisture will of course evaporate but leave behind
scale/chemicals that can buildup inside the superheater tubes. Too much
scale builtup in a low spot where moisture collected during times of low
power and the tube metal behind the scale can overheat and cause a blowout.
Bad news, lots of repairs.
Another aspect of steam quality is the quality of the water chemistry. You
don't want oil actually in the boiler, that makes foam and the carryover
will cause the piston to try and compress water in the cylinder. Since
water doesn't compress very well, you get huge shock loading on the cylinder
end and broken cylinder heads. Of course commercial steam plants sample and
take several steps to control the chemistry of the water actually in the
boiler. Olden days with road engines it was more hap-hazard.
Makes sense. Turbines don't need lubrication except in the bearings which
are outside the steam space. Of course piston engines need lubrication of
parts outside the steam path as well. I was responding to a poster that
sugested an ICE engine block could be converted to a steam engine. I was
pointing out some of the pitfalls of such an undertaking. Keeping steam out
of the crankcase oil is one of them.
Posted by Ulysses on March 28, 2009, 11:17 pm
It sounds like the turbine would actually be more practical for home use for
generating electricity etc. Simpler perhaps anyway.
Posted by Curbie on March 25, 2009, 9:32 pm
Just a few additional points based on my opinion "for what that's
I'm assuming you know that 1 kW per m^2 is a rounded up number that
can only be realized like on mountain top, on the equator, at noon, on
a particular day, reality for most locations is far less.
I bring this up as an example of using the "Best Case" numbers in all
your calculations, I find that using "Worst Case" numbers get you
closer to reality. Spent time on stuff that most effects the results
most, use "Worst Case" and you'll probably be able to skip (or
postpone) the minutia.
For what it worth.
Posted by Morris Dovey on March 26, 2009, 12:53 am
"Worst case" numbers say to not waste time even trying.
"Best case" numbers provide an upper limit on what might, but probably
won't, be achieved with a best effort.
I'll go with the best case guesstimate, with a plan to learn from the
shortcomings, so that a second generation can come /closer/ to the upper
limit, and a third generation closer still, and...
DeSoto, Iowa USA