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| [The
following is an Technical Article published in the Newsletter of the
Glass Art Society, 18:3 2007. Minor updating was done May 2009] Gas or electric glass furnaces Finally my government has considered greenhouse gases to be a problem: it is time to look at the pros and cons of gas and electric glass furnaces. When I started building my studio, 15 years ago, I had several reasons for choosing to go for an electric crucible. First, since the studio was to be in my basement, I wanted to have the safest possible furnace. Electric meant no carbon-monoxide to worry about and less chance of fire. Secondly, since I was just starting out on a second career, I didn’t want to be burdened by the ongoing costs of a gas-guzzling furnace. And finally, I preferred to have a quiet studio. I built Polyphemus, a 100# pot furnace with 7-8in of insulation and silicon carbide elements which draw 48 A when new. The elements last up to 2 years if I’m gentle with them, and they are retired when the Ampere draw is in the twenties. A monthly electric bill is $200 to $300 in Seattle where the electric rate is a low, 6 cent a Kwh. When I do not plan to use Polyphemus daily, I can insulate the door with an insulating pad and reduce electric consumption to less than $5/day. Electric consumption for my studio for the entire last year was $1,500. This affordability surprises many, since the cost of energy (in BTU or kwh or calories) for gas is typically less than for electric. Although electric rates in Seattle are low, gas is still lower. The reason electric glass furnaces are economical is simple: you cannot build an efficient gas powered glass furnace. Gas furnaces transfer heat to the glass by having the gases over the glass hotter than the glass itself, and flue gases start there. So, up the flue go heated air, CO2 (a greenhouse gas), lots of heat, and, in the end, lots of money. There has been much buzz about recuperation of some of the flue heat, but in practice, few in Seattle operate with even 25% recuperation. A solution used in industrial glass settings is to separate out some of the nitrogen in air to provide the burners with gas and “air” enriched in O2. Consider the waste of having to heat up air, almost 80% of which is inert nitrogen, to 12,000C and then throwing it out! Another, lesser recognized, factor in consideration of efficiencies has to do with the combustion products of burning gas: CH2 + O2 = CO2 + H2O. The inside of the furnace has a high concentration of water and of carbon dioxide. When the temperature decreases to less than 100C, the water will start to condense into a liquid. Thus, any insulation which tries to make the outside surface of the furnace less than 100C will be waterlogged and an inefficient insulator. Further, the CO2 dissolves in this water, making carbonic acid. Carbonic acid is not strong, but nonetheless can be expected to aid the rusting of the metal support of the furnace. In Polyphemus none of the surface area is more than 65C, except of the area near the door; I don’t believe I could achieve that if the furnace were filled with saturated water vapor. It may be argued that using electric just means that the power company does the dirty work. I would argue that they can do it much more efficiently than I could in my little studio. Quiet, economical, safe and environmentally friendly: should all glass shops convert? No. Here are some complications: 1] Electrical blackouts can be trouble for electric furnaces. Areas which experience frequent blackouts need to have a back-up system 2] Invested furnaces are more cumbersome in electric and usually require more expensive elements. 3] Gas furnaces tend to be better at withstanding majorly abusive environments. Rodman G. Miller 2929 Mayfair Ave N Seattle rodman@rodmanstudio.com 206-283-3164 |