Subject: Re: Gas forge temps

From: [email-masked]
Date: Sat, 27 Jul 1996 10:32:46 -0400

I won't bother to quote Vance's extended, and quite enlightening discussion
of Bernoulli's principles, but I will pose the following
question/observation:

Vance's analysis is based on a high velocity gas jet entering an atmospheric
combustion chamber.  The velocity of the gas jet lowers the air pressure in
the general vicinity of the jet, naturally drawing air through the burner
along with it.  

The question is:  when you are using a forge with all of the doors closed, is
the internal pressure atmospheric, or something higher?  

With all of the combustion heating & expansion of the combustion & exhaust
gases trying to exit through small (relatively) orifices, with orifice
losses, etc., I am guessing that the atmospheric pressure inside the forge
climbs to somewhat above 14.7 psia.  This would result in back pressure
against the burner, loss of velocity, and breakdown of the Bernoulli effect.
 This can often be observed by loss of proper flame condition in naturally
aspirated burners (you can hear when it happens, and sometimes the loss of
combustion air is severe enough to extinguish the flame).

In this case, would not the blower allow you to maintain the proper air/fuel
ratio against substantial back pressure inside the combustion chamber without
having to open additional exhaust holes which might allow additional heat to
escape?

Pete Engle

---

From: Vance Burns <[email-masked]>
Date: Sun, 04 Aug 1996 22:31:04 -0500

Steven

Thanks for the kind works. Dad is hanging in there, but he wants to go
"home".

You wrote:
>If you direct the output of a blower into a box which has an exit
>hole in it, you are increasing the air pressure inside the box. This

Pressurized self sustaining mixtures are dangerous, besides the door
will eventually open. Is this pressure or flow? The blowers we use are
commonly squirrel cage. This type of blower's CFM is devastated by small
amounts of back pressure. One would need an axial blower to attain
measurable pressure, and the trade off is high noise (a vacuum cleaner)
and no tolerance for a vent of any appreciable size (if you are after
pressure). Of course "Tool Time Tim" might have a solution.

>means more oxygen available.

Remember oxygen beyond LPG's 14% ceiling will not support combustion. In
our forge the flame will not go out (15% oxy and LPG will not ignite in
the lab) but the scale will increase on the work piece. But there is an
up side. Under 1400 degrees, scale tapers off and the more air added,
the cooler the flame will burn. One could heat treat if the controls
will still function and the flame does not get too cold. 
 
>I think what you are saying is that adding a blower to the inlets of
>a naturally aspirated forge won't buy much. I'm not sure about that.

Depends in the device, but a well balanced setup will suffer if the
"balance" is changed.
 
>Lets take a different scene. You have a forge box which can mount
>either naturally aspirated burners or can be hooked to a bottle of
>optimum air/propane premix at high pressure. You should be able to
>easily achieve higher temps in the forge using the premix as you
>raise the pressure.

Good answer. Sure, your avoiding humidity and most of the drag. But your
refractory can only absorbe so much. Or just add a second burner. If you
could afford oxygen, go for it. The rest of the time we have to remember
the other freeloading gases. Air is 21% oxygen, 75% nitrogen, and some
change. In order to have a given temperature in the forge, the entire
gas volume, including the 79% of noncombustibles, must be heated too,
sapping vitally important BTU's.

Back to blowers. If excess air is added to a stable forge flame, the
cold unused air is a drag on the total BTU's because the unusable oxygen
and noncombustible nitrogen must ALSO be brought up to temperature (this
is getting depressing isn't it). If only it was the other way around,
79% oxy and 21% BTU wasting non-combustibles. 
But wait Monty, there's more... Even excess gas must be brought up to
temperature, expending more of those precious BTU's on everything except
the object to be forged, and creating a carbon monoxide problem. If
you're finding it difficult to reach a desired temperature, this might
be part of the problem. The balance is everything. 

Oh and one more thing. Excess anything is a catch-22 situation; while
the excesses drive you to increase flow to get more heat, the excess
flow reduces the temperature in the forge, etc, etc, but not the shop.
The relatively cool flue gasses are altering the environment in the
shop. The shop's ability to dissipate the heat is sort-of constant, but
the incremental addition of supper heated gasses shifts the efficiency
curve. In south Texas, that's a real problem. And of course there's
oxygen depletion.

Now... if the air supply is preheated, or the expended flue gasses
preheat it for you, there is not as much waste to heat the nitrogen.

Pete:
>The question is:  when you are using a forge with all of the doors
>closed, is the internal pressure atmospheric, or something higher?

Higher, even with the doors open. Regardless of the method, you have to
have adequate ventilation or the process stops. This procedure creates a
lot of volume you'd rather have out of the forge. Inadequate ventilation
of the forge increases back pressure and reduces flow. Excessive flue
products spilling into the air can also deplete the oxygen content of
the atmosphere near the forge, and your lungs.

When we input one cubic foot of gas we need 10 cubic feet of air for
combustion (ventilate the shop!). Water vapor is produced and will
(must!) vent if it doesn't come in contact with something cooler than
the vapor. Co2 is produced as is hydrogen. (note: LPG(s) have a flame
velocity of appx 1 foot per second and hydrogen is about 10 feet per
second. Shame its so expensive. Remember that when you go see "Chain
Reaction". Also watch for the scene where Keanu passes the sign
"Bessemer Project" in the beginning. It looks like a legitimate
Chicago historical site). 

>With all of the combustion heating & expansion of the combustion &
>exhaust gases trying to exit through small (relatively) orifices,
>with orifice losses, etc., I am guessing that the atmospheric
>pressure inside the forge climbs to somewhat above 14.7 psia.  This
>would result in back pressure against the burner, loss of velocity,
>and breakdown of the Bernoulli effect. This can often be observed by

This is true of any type of inefficient forge, forced or natural
aspiration. 

>loss of proper flame condition in naturally aspirated burners (you
>can hear when it happens, and sometimes the loss of combustion air
>is severe enough to extinguish the flame).
>
>In this case, would not the blower allow you to maintain the proper
>air/fuel ratio against substantial back pressure inside the
>combustion chamber without having to open additional exhaust holes
>which might allow additional heat to escape?
>Pete Engle

We need to learn how to reuse the heat because it has to vent.

We have plenty of pressure already, so why do we need additional back
pressure? The Bernoulli effect is at the jet and neck of the venturi and
it is not feeble. The air fuel ratio will not change. The operational
end of the tube experiences rapidly heating and expanding gasses. It is
interesting to measure the input volume compared to the exhaust. 

You are already fighting a pressure problem. Just like barometric
pressure, the expanding gas creates a high pressure cell at the burner
end, and while it can't stop the flow (that's good), the cell can't
expand into a sealed container before the expansion will blow back
through the burner tube; flame velocity is inverted. No blower can
overcome that scenario. The pressure cell must vent at the correct rate,
regardless of the input pressure, or the combustion process will stop.
The expansion will continue for sometime; notice how hot your tubes get
after you shut off the gas?

I understand what you are getting at. Outside of theory, we just want
high temperature so we can avail the metal of the implied heat,
regardless of the method (within constraints). Radiant heat from
combustibles is most effective when we take advantage of high flame
velocity (more on that in a second). Flame temperatures are a property
of fuel burn velocity (a constant based on the type of gas; molecular
weight in hydrocarbons), and does not normally change. You want the
combusting gasses to stay in the forge as long as they can "give up"
heat to the surfaces in the forge. If the gas is forced out prior to the
near completion of the process it is just heating the shop. Cut the
losses and it increases the temperatures. I'm stepping out here but
recall the ammunition burn factors? You want the powder to finish the
majority burn before the projectile exits the barrel.

Also, if the flame velocity of the fuel selected (feet per second) is
sufficient, the temperature will be perfect. If the gas mixture velocity
is too high (over blown) the temperature will not be correct.
Propane/oxygen is fast enough, acetylene/oxygen is very fast. We don't
need a lot of oxy/acetylene to get the job done. It's a matter of
economics.

When you have your blown burner outside of the forge, you can really
crank up the flame. Try this; put the blower on a mount and max out the
fuel/air pressure. Stand a comfortable distance in front of the flame
and take a mental reading of the subjective feel of the flame after all
the components are at temperature. Now set the flame for low noise and
stable flame with no burner lift. Stand in exactly the same spot and
note the "feel" of the flame. Let us know what you discover.

Thanks
Vance

PS. I am looking for info that will help me understand emissive
frequencies of burning gasses and refractories. The radiation
(emissivity) of refractories is picked up easier by metals that the
emissivity from a flame. Refractories have a limited bandwidth and ratio
to size-surface area of radiation absorbability or capacity.

Should you know of a source for this information, please let me know.