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Old 03-20-2004, 12:32 PM
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Watt Loading for Dummies

I didn't stick through my engineering classes long enough to learn how to figure out the exact science and numbers behind watt loading, but I understand the basic physics pretty well. My understanding tells me that for given x furnace heating chamber space at y temperature I can use the same amount of energy to maintain y and have either one element running hot as blazes (obviously impractical) relative to the furnace temp y or a wall packed thick with elements (equally impractical and unnecessarily expensive) running barely above measured y air temp. My solution to maximizing element life was to get as much heating surface in the furnace as realistically possible (Steve's Watt loading equation tweaks this) to keep the wire temp as low as possible relative to the furnace chamber temp. I tended to run 12 5ohm 13ga 3/8ths OD coils while Steve prefers 12 3.5ohm 11ga 1/2" OD coils. Actually, on the last two furnaces I've sold I've designed them to run off of Steve's elements -- though I'm still trying to talk him into squeezing 4ohms into each element for even more heating surface (Steve's afraid at that point the coils become too tight - I'm going to be the test dummy).

As to suspending the wire inside the furnace to maximize efficiency; what small gains it would produce I fear would be lost because of the extra heat needed to compensate for the added size of the heating chamber (my furnace wall is close enough to the pot that if the elements were outside the wall they'd be nearly touching the pot). What I've done instead is cut a straight edged, square notch in the brick at an upward angle.

All the heat in my furnace has to pass
through the pot, there's no radiant source on the surface of
the glass, so the glass that trails off tends to trap small bubbles
that would dissapate much faster in a furnace that had heat
coming in from the top. This would be one of the worst flaws in
my furnace design but I did it to protect the elements from
the batch. in my furnace a gather at 2100 feels really cold
compared to glass coming out of a gas fired furnace that runs
colder temps. - Kurt
This covers at least two reasons why I dislike batch in a wire furnace. If you would cast a Mizu ring (my Thermal Flywheel™ as seen in photo) that sits on the open furnace (the crown and lid then sit on this), and allow a gap of even 1/2 inch between the top of the pot and the bottom of the ring, it will make a great radiant heat source to keep a low pot at temp. The other thing I don't like about batch in a wire furnace is the extra temps people use to fine out. For the last year, using cullet, I've never run the furnace over 2005f. For busy weeks (all of late) I'll toss in 20-30 pounds each night to re-top off the pot, it's fined and good to go the next morning and about once every 10-14 days I'll blow the pot low (1/4 full) and do a full charge. This freshly charged pot sits through the next day -- still at 2005f -- and on the morning after that I'm blowing perfect glass again.


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