Burner Choices
&
Getting Hot with Gas or Electricity
10/18/97, 5/1/2000, 2002-11-18, 2003-10-22, 2006-05-30, -11-30,
-12-14
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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.
[*I have learned recently that
Silicon Carbide elements, while suitable for high temps, require
adjustment of the voltage during the life of the elements,
increasing it to maintain power because the resistance changes so
much with aging and require lower voltage to start with (60 volts.) adding
complications, but becoming lower in cost and removing the size and temperature
limits.. 2006-05-30]
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.
Most of the considerations that apply to ceramics kilns also
apply to glass furnaces, such as spacing away from building walls.
But ceramic kilns are usually fired up to a temperature and then
shut down, while furnaces are often left at the holding
temperature (equal to cone 6 - 2100°F) for days and, when batch
is cooked, are maintained overnight at equivalent to cone 10-11 (2400-2500°F.)
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.
- WARNING-THIS IS NOT COMPLETE, DO NOT RELY ON IT FOR
DESIGN, IT IS AN OVERVIEW FOR GENERAL INFORMATION ONLY.
- At the fuel entry to the space (and/or at the tank) is a
manual cutoff valve which can easily be reached at all
times.
- At each branch to a burner there is a manual cutoff for
maintenance, etc. It is best if this is quickly
accessible. Some studios put in at the front of the hot
wall for quick access, especially for glory holes.
- For each burner there is a remotely driven (pneumatic or
electrical) valve that can provide cutoff of fuel. This
may be part of a valve that controls gas flow for
temperature control.
- There are one or more regulators that reduce pressure
from the main line to that useable by the burner.
- There may be a high and low pressure sensor to determine
if the gas is behaving.
- There may be gas/air mixer that proportions the two.
Mixers are available that once set, allow changing the
air flow and they will automatically keep the gas
properly flowing, about $200.
- At the burner, a flame sensor. This is totally vital
where the flame is shut off and turned back on later, as
in boilers, which require that all the gas be blown out
of the heating chamber before re-ignition is tried.
- At the burner, an igniter. Yes, a lot of furnaces and
glory holes are lighted with a burning piece of
newspaper, but if the furnace is to be left alone, it
probably has to have relighting ability.
- At the furnace, a temperature sensor. If the temperature
is to be controlled, a thermocouple or RTD is required.
It may also be used to determine whether a restart is
possible if an igniter has been left out of the setup. If
the furnace is hot enough to light the gas, the gas is
turned on, otherwise not, and the furnace chills down,
probably breaking the pot.
- A controller, to tie everything together. It is possible
to go both directions: a lesser setup where hard wired
sensors turn off valves or a greater setup where a
serious industrial controller handles the temperature and
the startup and the safety issues. A middle ground is one
simple controller that handles the temperature and
another fairly simple controller that handles safety and
restart.
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 |
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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.
Three methods of retaining the flame are used for glass heating.
- The lowest level is shaping the end of the pipe to swirl
the gas/air mix and nothing else. This can be noisy and
since the heat is formed inside the end of the pipe, the
iron pipe will be gradually destroyed; I use a reducing
coupling on the end to add swirl and act as a sacrificial
element that can be replaced. Use anti-seize compound,
sold at automotive supply places to keep spark plugs from
sticking in aluminum heads, for a high temp rust and
corrosion fighter.
- Second choice is an iron burner head shaped with many
smaller holes; these tend to reduce the noise and make
the flame stand off the head. The iron still is gradually
destroyed from the heat inside the furnace and the burner
must be aimed to keep iron flakes out of the molten glass.
- The highest tech is a ceramic head from Giberson or
Wilton. This is the only head that can be aimed straight
down at the glass from the heat of the crown and reduced
noise is claimed for the designs. Ceramic heads add $95-$150
to the basic cost of a burner or $25-40 in a swap for an
iron head on a venturi.
Weed Burner
Very high pressure propane, directly from the bottle, is injected
into the bottom of a can shaped burner shell where it turbulently
drags air through slots in the sides and bottom of the can. The
flame is retained on the end of the can due to secondary air
being sucked in as the flame leaves the can. Intended for used
outdoors, it is noisy and survives because of the air flow on the
outside. Raku potters sometimes use them for heating.
Cheap, Noisy, Special gas supply, about $30 (burner and handle
and hose) at propane supply places.
"Weed Burner"
This looks like a weed burner (above) in shape, but is often
chrome plated and well balanced with a short handle. Used for
blasting glass (a torch) and for patina work in bronze sculpture.
about $200. Has a small pilot flame for instant lighting and
delivers a blast of 250,000+ Btu.
Inspirated, Aspirated
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
Venturi
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]
Blower/Pipe Mixed
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.
Gas Follower
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
Preheated Air
If exhaust air is used to preheat intake air, then considerable
care must be used to add the fuel at the last moment as the
preheated air may be hot enough to ignite the fuel outside the
furnace. Preheating may be done by recuperative fittings (stainless
steel and silicon carbide tubes in chimney carry intake air
through exhaust heat) or by regenerative (exhaust flows over fire
brick which absorbs heat; later air flow is changed so intake air
flows over same brick getting heated while exhaust takes a
different route.)
Pro: Reuses heat, saving fuel cost
Con: Air flow path much more complicated, air/fuel mix
more complicated, costly, recup piping $1500-$2000.
Used by: Libby Owens Ford glassware manufacture (regen), Tom
Bloomfield & Penland new studio (recup) Charles Correll [Correll
Glass Studio, RR1, Box 150A, Conway MA 01341, 413-369-4283 FAX:
413-369-4769] sells recup parts for about $1700, whole furnaces
for $10k plus.]
Blower Mixed
Gas is injected at the blower throat, well back from the flame.
Pro: Allows placement of all controls well away from heat, better
mixing (longer path) of fuel/air
Con: Extra pipe filled with gas, more safety requirements,
providing a reduction flame requires an extra gas tube with
safety solonoid.
Used by: Bowling Green State U.
Source: UDC 3000 Honeywell - multiple set point controller; high
limit control-Honeywell; ultraviolet flame detection.
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