does a 300lbs furnace cost more to run?

[Mike Kaplan]

I have a 150lbs furnace that runs on gas and wondering if a 300lbs furnace costs a lot more to run?

[Hugh Jenkins]

My general rule is square root of increased size. So you can do approx. four times the glass for twice the fuel. In your case I would estimate a 40% increase. Door size, fire box volume, better insulation, and of course a recuperator are all factors in improving the fuel efficiency when building a new furnace.

[Mark Wilson]

i am sorry but this square root concept does not make any sense to me. you need a minimum of 2 killowatts of power per cubic foot of heated space. so if your heated volume doubles, so does your requied amount of power. also, larger crucibles are deeper, so that it takes longer for the bubbles to rise to the surface. thus it takes a bit longer for the glass to fine out. so it takes more power, and longer to fine out leading to higher operational costs.

now if you have a furnace that is already large enough to fit a 300 pound crucible, but it only has a 150 pound crucible in it at the moment, then your costs won't double, because you are already heating a large volume. but it still takes the same amount of energy per pound of glass to melt it and fine it out. so if you double the number of pounds of batch that needs to be melted, it will take double the amount of energy to melt it and fine it out.

[Steve Stadelman]

My experience is in Hugh's corner. My 300lb furnace needs about 25%-30% more power to run than my 100lb furnace.

My 600lb furnace runs for approximately twice what my 100lb furnace does and will melt 500lb of nice glass overnight with a plenty deep pot.

I have plenty of furnace configurations at about 2000 Watts per cubic foot but it is not as easy to glibley achieve as you would think.

[Ken Peterson]

my theory is it has to do more with the inside wall surface area. Twice the inside wall surface area, twice the heat loss, twice the energy to keep it hot given all other things are constant. But there is the whole thing with the smaller furnaces having proportionally more outside wall surface area than larger furnaces. Ok..... maybe there's no super easy rule. I think the problem is pretty complex. The square root thing is sounding better to me the more I ramble.

[Pete VanderLaan]

I agree with Steve and Hugh and it is based on experience, not theory.

[Hugh Jenkins]

You do not have to double the interior volume and even less the exterior (heat loss) surface of a furnace to double the crucible or tank size. Often the head height above the glass can stay about the same. Also it is not the same thing to have a large furnace and compare melting with different size crucibles. And lastly operating a furnace and melting are not the same in that melting a cold mass only occurs for a small part of the furnace operating time. It spends a lot more time in most situations just sitting there at a set temp.

You do need to know the energy need for melting a given mass when calculating the maximum power or fuel need for a furnace, but that is needed only during actual melting. The rest of the time you operate well below this max.

[Pete VanderLaan]

There is also the circumstance where the furnace becomes too small for the burner installation and the gas never has the time and space to adequately combust before exiting the furnace. I have seen more than my share of expensive chokers that were profoundly inefficient.

[Ken Peterson]

pete, I agree with them too. I'm just saying the dynamics of the whole thing isn't quite that simple. I believe experience is stronger than theory, but theory together with experience is better than just experience or just theory right?

I trust experience more than theory when it comes down to it. But I like to talk about theory. Without theory we'd be doing the same thing forever.

It's odd that I feel the need to defend myself, but I did get sense that you wanted to discredit what I said through your post.

[Teresa Ueltschey]

This is Matthew by the way. You also need to look if by going to a larger furance you are charging less. Seven years ago we went from a 150lbs furance(gas), which I had to charge 3 times in a two week period. Then we built a 300lbs furance (gas) and we charged only once a week. We saw no increase in our gas cost using the same amount of glass a week. Of course it was a new furance with all new castable and insulation, but even after a few years will were still running the same cost (until gas went though the roof).

[Rollin Karg]

Originally posted by Pete VanderLaan
I agree with Steve and Hugh and it is based on experience, not theory.

I agree with Pete on this one. Over the years as our furnaces doubled in size then doubled again, the cost increased but not near double. At the end of our gas era we had meters on each furnace and like Matt we found that charging doubled the daily operating cost. Making them bigger seems to help a lot in the cost per pound game. Its all for nothing though if you can't sell the glass or pay the bill.