[TheForge] Burner "flares"?

Fri Apr 26 22:55:01 2002

You're right Bruce, there is a lot of confusion regarding "flares".

In commercial inducers the entire length of the tube is tapered at a 1:12
ratio starting from the bore and ending at the throat. The purpose of
tapering the tube is NOT to act as a flame holder it's to increase induction
and hence the efficiency of the inducer. A 1:12 increase or decrease is the
maximum rate at which the tube can change diameter without creating harmful
turbulence. In the case of inducers the idea is to extract the maximum
efficiency so the steepest ratio is used. For other purposes lesser ratios
are used. For instance a shallower taper can simply compensate for friction
over a distance.

These things aren't normally used as burners commercially, they're most
commonly used to provide large volumes of low pressure gas or liquid using a
small volume of high pressure gas or liquid as a primary. They are in effect
non-mechanical superchargers and there are a number of basic types and
efficiency ranges. That they also make dandy supercharger/carburaters for
furnaces is a good thing.

The ones most typically seen on naturally aspirated / atmospheric / venturi,
etc. fired forges are linear inducers. In a linear inducer the intake air
enters in-line with the exhaust tube. If you remove any choke plates, etc.
you can look straight up the tube to daylight. Commercial versions will
induce at about a 19 - 20 : 1 ratio. A Bunsen burner is a good example of a
straight tube version.

Another version we're starting to see here is commercially known as a "Jet
Ejector" and are being called: Side Arm, Mongo, T-Rex, etc. They differ from
a linear inducer in that the intake(s) are at 90* to the tube. If you look
up the bore of a jet ejector you'll see the back end of the intake chamber.
Commercial versions induce up to around 27 : 1. Jet ejectors are commonly
used commercially to produce vacuum in quantity, pump large volumes of
liquid, etc. A Fisher burner is a jet ejector with a tapered tube and flame
holder.

Before you or anybody else asks. I have NO idea why a jet ejector is upwards
of 30% more efficient than a linear and have never gotten a satisfactory
answer to the question. (I could understand anyway <grin>)

When I first started designing a propane forge I'd planned on using a jet
ejector but the internet betrayed me. <sigh> I was surfing the IBM patent
server and ran into a 1930's patent for a propane forge using a plumbing
parts linear inducer burner so I changed plans and went with a linear.
Unfortunately a commercial linear induces only a little more intake air than
propane needs for a neutral burn and a home made version falls somewhat
short so I needed to add a choke plate. The jet ejector induces a lot more
intake air than needed for a propane burner so tolerances are much more
forgiving and allows the hobbyist to make a very hot burner easily.

Anyway, the tapered tube or flare increase the efficiency of the inducer.
Flares acting as a flame holder on straight tubes, is a result of the
pressure drop experienced as the gas passes through the flare, making the
flame reluctant to jump away from the end.

Frosty
------------------------
If it ain't forged
it ain't real.
Wrought iron is.
The FrostWorks

----- Original Message -----
From: "Bruce Freeman" <[email protected]>
To: <[email protected]>
Sent: Friday, April 26, 2002 12:05 PM
Subject: [TheForge] Burner "flares"?

These threads get so confused I'm not sure we're talking about the same
thing.  I probably missed a key posting.

Anyway, if by "flares" you guys are referring to a bell-shaped endpiece put
on a buner so that the burner will "hold" it's flame (and not go out), then
let me point out that such a device may be unnecessary.

If your burner is inserted into a gas forge, especially one lined with
refractory wool, then the forge lining will do quite nicely as a "glow plug"
to reignite the gas in the event the burner "blows out."  No flare or other
flame-retainer needed.

I don't use a flare on my burner in my homemade gas forge.  I ignite the
forge initially using a blow torch (for convenience), but seldom do this
twice.  Even if I turn off the gas for a few minutes, the forge retains
enough heat to reignite the flame just by turning on the gas. If the kaowool
is glowing, then it will ignite the gas.

You do need a flame holder if the burner is used out of the forge (with no
local hot surfaces to reignite the gas if the flame goes out).

Bruce
NJ

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