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Controlling heat created by gas (or oil) flames is far more
complex than that created by electricity, primarily because if
gas is shut off and turned back on, and it does not ignite, it
will create an explosive situation. The need is so common, for
heating buildings and for industrial processes, that a whole
industry, with laws and codes that must be obeyed, has developed.
If you are willing to spend the time and money, the industry will
help you develop your system and sell it to you. Fortunately,
this is not a rapidly changing industry, now that microcomputer
control is firmly in place, and it is likely that a system you
buy today will function on 3, 5, or 10 furnaces into the future.
Unfortunately, the system will be a serious capital investment,
easily costing $2000-2500 and up.
- From the point of view of the safety engineers, the
following are desired
- - The furnace must be clear of gas when ignition is
attempted so that the volume of gas is not ignited and
exploded and the burning occurs at the burners;
- - The flame must ignite within an appropriate time after
the gas is turned on and something must determine that
there is a flame and not just a lot of heat.
- During operation there must be a sensing that gas is
flowing, that air flow is adequate, and that temperatures
are neither too high or too low.
- If the electricity fails, the system must shut down the
flow of gas
- If the electricity comes back on, the system must do a
start up procedure if safe to do so.
- If the gas fails, the system must detect that and do a safe shut down
while setting an alarm.
The flame can go out by accident or it can be turned off to
reduce temperature. More often temperature control in a glass
environment is done by raising and lowering the amount of gas,
rather than shutting it off as is done in a household oven or
heating furnace. One of the difficulties is producing a system
that is efficient at many flow ratings and especially at the most
used ones. Burners often have only a few adjustment points at which they produce the
best flame.
- There must be a manual cutoff valve near the supply (tank
or exit from the main) and if the supply line
penetrates a wall, a shut off just inside the wall.
- There must be an electrically controlled cutoff valve
that will automatically shut down on power failure.
- There must be an individual control valve for each
furnace/gloryhole
- There must be a gas/air mixing method - which may just be
a pipe injecting gas into the air stream
- There should be a pressure switch to detect that gas is
available
- There should be an air flow detector to insure flow is
occurring
- There must be a method of igniting the gas/air at the
burner and/or a method of determining that the furnace is hot
enough to ignite the gas/air mix
- There should be a method of detecting the flame - either
a purple peeper that detects the UV in the flame or a
flame detector that works because the flame is ionized
and plain hot air is not.
- For temperature control, there should be a sensor in the
chamber, usually inside a shield to avoid oxidation or physical damage.
- A typical ignition sequence, whether controlled by a
microcomputer or a person is this:
- - Turn the air on or check that it is on and flowing
- Delay to allow the chamber to clear of any gas
- Check the gas pressure
- Turn on the gas
- Turn on the ignition system
- Delay and check that a flame has formed
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-If not, try again a specific number of times, then shut down and sound
alarm.
- A typical control cycle would be
- - If the flame has gone out - shut down and try to start
ignition cycle
- If the temperature in the chamber is low, increase or
turn on the gas flow, doing ignition if turned on.
- If the temperature in the chamber is high, decrease or
turn off the gas flow.
If the fuel is propane, there are another set of regulations as to where the
tank can be put on the property and how isolated it must be.
This site RegOŽ Products
LP-Gas Service Manual has a terrific set of tables and diagrams on placement
of tanks and pipe sizes with Btuh for various pressures and lengths.
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