I missed the staff meeting but the minutes show Grant Erwin
wrote back on Fri, 28 Jan 2005 14:02:18 -0800 in
rec.crafts.metalworking :
OK, let's go in a slightly different direction. To summarize, I proposed
putting a water tank over a burner, said water tank having a steam valve
on top. As I understood it, this is dangerous because
a) when the valve is opened and pressure is lowered a catastrophic amount
of water will instantly convert to steam
b) if the tank gets real hot somewhere and is sloshed such that water hits
the hot spot, a catastrophic amount of water will instantly convert to steam
Did I get those right?
Sounds right. All though in a), it is if the pressure drop is too
great, such that the temperature of the water is above the boiling point
_at that pressure_. Not having a steam table handy, I will make it up as I
go: Water normally boils at 100 petrograds, and one lungfull of pressure,
However if you raise the pressure to 2 lungs, then water boils at 150
petrograds. So now you have a pot of water at 150 petrograds, and
everything is happy. Steam floats around, "choo-choo" and da kine. The
pressure cycles between 1.95 and 2.0 lungfulls and everything is "cool".
Then some fool (not you, some other fool) pops the valve and the pressure
drops to 1.8 lungfulls, and the water, which formerly was below the boiling
point is now above the boiling point, and wants to join in the steam biz,
expanding 400 fold in the process. Big spike in the pressure. If the
boiler can hold the 3 lung transitory pressure while things settle down,
well and good. If not, and say the safety vale pops off - bad things
happen. Pressure equalizes inside and out at 1 lungfull. Well, it would,
if the water temperature was 100 petrograds, but all that water is fifty
petrograds above the boiling point of water at 1 lung pressure. Weeee!
Water flashing into steam all over the place, rushing for that little hole.
IF the hole isn't big enough, impatient steam molecules start to back up,
pressure rises and a new hole is made. Catastrophically.
And then all that 150 petrograd steam starts to cool and transfer heat
as it drops to 100 petrograds, condenses (transferring more heat) and then
drops to the "ambient" temperature. Lot of hurt in that alone.
So how *do* you build a boiler so that neither of these terrible things happen?
In plain simple words, please, no math allowed. Imagine I'm a bright 10th
grade physics student, for example.
1) Over build. Build that bad boy so it will hold 3 "lungfulls" of
pressure, then put a gage which reads "2.01 Lungfulls" as Max "never
exceed", put a pop valve set to 2.5 lungfulls, then cross your fingers and
pray for success. In other words, whatever steam pressure you want should
be a fraction of the pressure the boiler can handle. Then inspect for weak
spots, regularly. (I hope that isn't too much math :-) )
2) make sure there is plenty of water to cover the firebox, especially
the "flame crown" where the fire touches the metal.
I've looked at boiler designs and they have a firebox which is in thermal
contact with boiler tubes which contain water which is converted to steam.
How does this differ from a water tank over a burner flame?
More surface area gets heated, the water heats faster, you have power
sooner. I've seen two basic designs: one with tubes of water in a firebox,
the other with Tubes from the fire box through the water. YMMV, a lot of
plumbing either way.
Back in "zee old country," we had flash heaters. rather than keep a
thirty gallon tank of water hot all the time, when you turned on the tap,
the flash heater would light up (bottled butane gas) and heat a number of
coils with the water flowing through them, and you had hot water which only
had to travel a couple meters from the heater to the tap, rather than
through the whole house.
Same principle.
tschus
pyotr
--
pyotr filipivich.
as an explaination for the decline in the US's tech edge, James
Niccol wrote "It used to be that the USA was pretty good at
producing stuff teenaged boys could lose a finger or two playing with."
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