Rev. 10/18/97, 5/1/2000, 2002-11-18, 2003-10-22, 2006-05-30, -11-30, -12-14, 2009-05-07
|Gas Control & Ignition|
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Choices in Heating a Furnace/Glory Hole Rev. 3/27/94, 12/14/94, 7/18/95
ELECTRICITY VS GAS - For a furnace or glory hole for
glassblowing, a burner is normally used as it is much usually more
economical (when element cost is included) to use gas over
electricity. Those people using electricity to melt glass report
that the size of the furnace must be limited* and the elements
must often be replaced each time the furnace is shut down. Since
the temperature of melted glass is very near the upper limit* of
most electric heating material (except carbon arc which requires
such complicated hardware no artist uses it) or silicon oxide rod
which requires heavy power) the elements must be heavy and well
supported. Being heavy means that higher power and current must
be provided, adding to the expense.
HEAT SUPPLIED - Burners supplying 90,000 Btu up to 350,000 Btu are be used. A British thermal unit (Btu) is the amount of heat needed to raise one pound of water one degree Fahrenheit. (about 1055 Joules in metric, 1055 watt-seconds) A common kitchen gas burner is about 8,000 Btu, the large "Super Burner" on our stove is 18,000. Turkey fryers blast off at 140,000-180,000 Btu. Wok burners are 16,000-40,000 Btu. 2006-11-30
BASIC BURNER OVERVIEW - For the burner to work fuel must be mixed with air. Since glass melting often takes place for months on end, safety features must be considered, including dealing with rare gas and more common electric power failures
Normally, the glory hole is heated only while the glass is
being worked and most often has manual controls, including a way
to produce a reducing atmosphere for striking, most often just
cutting off the air for a while.
COMBUSTION TRAIN - The entire flow of fuel to the burner, in a code approved arrangement, is prescribed by one or more standards from National Fire Protection Association (NFPA) industrial safety. Wading through the standards involves eliminating a lot of stuff that applies only to relatively low temperature burners - boilers and cooking ovens. Most suppliers of industrial burners will work with the buyer to design a safe package, given the Btu requirements, with the result costing $1500-2500 per burner train.
MINIMUM SETUP - At the lowest level, by not using any
electricity and providing a separate building with good ventilation, one
worker uses just a venturi for the furnace with fuel from a propane tank
with no safety additions. He also expects to come back one day and find it
burned; he keeps nothing in the building he can't replace and it is well
separated from other buildings
|POWER FAILURE - Normally, electricity is involved (see below)
so the system must allow for the electricity failing and for it
coming back on. Usually a valve is provided so gas is cut off
when the electricity is off. When the power comes back on, one of
the following things must happen:
the gas and blowers stay off, so the glass chills;
automatic restart is provided with control of ignition, gas and air so it works;
or temperature of the furnace is checked to see if it hot enough to relight the gas, the gas being turned on if it is and left off if it is not.
One problem is that a glass furnace is so hot that many methods of re-ignition that work with home gas heaters and stoves - peizo temp sensors and sparks or pilot flames and thermocouples - would be melted. One method is to use a UV flame detector that works from a distance. (Right above) Another is to provide a flame that needs no electricity, such as a small venturi, to inject a reliable restart flame into the furnace. Ransome Burners use the latter method. Another method of starting the burner is to use a long spark plug. This can be fired by the control system or fired continuously (and perhaps noisily.) The spark can be in the feed pipe (right) if there is not a ceramic head, otherwise the spark must be out in front of the burner. 2005-03-03
While working, control must be provided for the amount of air and the amount of gas. In a venturi, a valve controls the gas and the wasp-waist design of the venturi controls the air/gas mix. In a manual blower system, there are simply two valves, one for air, one for gas. When automatic control is provided various methods are used, the most primitive being a lower level maintenance burner that stays on all the time and an added heat burner that is turned on and off with valves. Other methods are mentioned below.
When gas is burned, the flame can be literally blown off the
end of the burner.
The pipe burner here if operated without a blower drags air along, the position of the orifice determining the amount of air. Unlike a venturi burner, there is no special shape to change the air pressure to increase air flow. Burners of this type are more common than might be expected as many low pressure burners like those used in BBQ pits and hot water heaters do not have the wasp waist. Okay with low pressure gas. 2006-12-14
A wasp waist tube with the gas inlet appropriately placed drags air proportionate to gas pressure. The wasp waist and gas velocity lowers air pressure to drag more air in. Used with high pressure gas. No blower.
Con: Requires high pressure gas (ounces in Natural Gas, psi LPG)
Requires free flow of air into and out of furnace, flue.
Hard to adjust quickly, as for reducing flame and back to normal
Not easily controlled automatically, usually not used in control applications
Pro: Not subject to power failures
Used by: Art Allison, preferred by Dudley Giberson in his notes
Source: Ransome Venturi Burners, Dedell Gas Burner & Equipment, RR.1.Box 2135, Newfane VT 05345; Giberson heads, Joppa Glassworks, Box 202, Warner NH 03278. Ceramic supply house. Ward Burner Systems - Power Burners, Raku Burners and Kilns showing the Ransome burners
[A. Brass attachment adaptor; B gas input pipe, threaded outside for adjustment; C air choke turns to open and close gap D restricting air; E waist at which orifice on pipe is adjusted to drag air in; F expansion/swirl flame retention chamber. 2006-11-30]
A small blower supplies air through 1 1/2"-2" pipe and gas is injected into the air stream. Air and gas are adjusted by manual valves. For safety, requires shut off of gas when power fails.
Pro: Very cheap to build from standard pipe parts; even with new blower costs only $40-50
Can use low pressure gas.
Con: Cost goes up quickly when safety features are included.
Can be touchy to adjust, subject to changes in both gas pressure and blower voltage.
Sources: Dayton blower from Grainger; pipe from almost any hardware store
Used by: Texas Tech at Junction TX, Mike Firth, Divas, described in Giberson's Notes.
An industrial valve is adjusted for the proportion of gas to air. It automatically adjusts the gas as the air volume is changed manually or by a controller.
Pro: Very precise control
Con: Valve costs on order of $250, controller costs $200+
Source: Eclipse low pressure proportional mixers, series LP, Eclipse Combustion, Rockford IL 61103 815-877-3031 and regional offices. http://www.eclipsenet.com/
Used by: Fire Island, Austin TX
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