Subject: Re: Gas forge temps

From: [email-masked]
Date: Fri, 26 Jul 1996 18:35:26 EDT

>Date: Fri, 19 Jul 96 17:03:00 MDT
>From: "Steven O. Smith" <[email-masked]>
>To: [email-masked]
>Subject: Re: Gas forge temps

[email-masked]: you wrote:

>I either don't buy all of this, or misunderstand:
>> The burner design is important, but the real losses occur in the
design
>> of the forge. Propane at optimum combustion (optimum everything,
>> really) reaches temperatures of 2950 degrees, and you want to
"store"
>> as much as possible. You've heard it said the flame does not heat
the
>> metal, but that is misnomer. Logically, the flame provides all
the heat.
>> The refractory forges (furnace/hearth) we use are accumulating
radiant
>> reflectors. The flame can be smaller if the accumulator is
efficient
>> enough.
>>
>> Whew. OK, simply put. It doesn't matter. You can't get better
>> performance out of hydrocarbon gas with more air. You're dumping
a
>> bunch of BTU's into your forge, but the efficiency of the forge
design
>> determines how "hot" the forge gets.
>
>The point of a blower is to put more air in so you can raise the
>propane flow. This gives more btu's. The more btu's, the closer
your
>forge temperature approaches the max of 2950. This is the same as
>saying that more insulation (and keeping the doors shut) gets you
>closer to 2950F at the same btu input.
>
>You can get closer to theoretical max temps with _either_ more
btu's
>or more insulation/efficiency.
>
>Steven O. Smith
>[email-masked]

Sorry for the slow reply; wife's father is in a bad way.

Thanks for the comments. I hope I can clarify:

The blower is (wish we could use italics) compensating for
impediments to free air flow and,  should I have implied it
previously, not a sign of ignorance, but a sign of design trade
offs.

The BTU's are a constant, you can't have more using a blower. The
ratio stays the same (wait a minute and hold that thought).
Efficiency is paramount, unless you have money to burn. If a forge
is a real leaker, you'll have to pump in lots of air/gas to keep up
with the losses. But then what does propane cost...

The ratio of air to fuel is constant and a blower will not

beneficially affect that relationship. Recall what Dom Bernoulli
taught us, if you increase speed of a fluid the pressure decreases.

Recall the old shop hand vacs that used compressed air? Put a small
tube about one third midway into a larger tube and orient the air
flow out the end. Hook up your compressor and add a cloth bag to the
compressed air exit end of the tube. If pressurized air is allowed
through the small tube, you get a vacuum on the other end.
Strangely enough, even though your pumping lots of volume, the
pressure drops and the 14 psi ambient air rushes in the other end to
try to balance the equation. Viscous air drag and kinetic energy
does the rest. Try this one; stick your compressor nozzle in a large
funnel  and hold a ping-pong ball into the inverted funnel. Or the
puzzler; put a sewing pin into a piece of stiff paper, like a 3x3
made from a 3x5 card. Blow air through a tube (we used a empty
sewing spool in school) and the 3x3 card will be pressed against the
end of the pressurized tube.

Let's set this up: nature abhors a vacuum, and ambient air is 14
psi. Your burner is naturally aspirated; venturi or inspirated, no
blower. As the pressurized gas leaves the orifice at a high speed
there appears to be a mass to volume increase (and there is). The
other end but the Bernoulli principle says when the gas is moving
through the semi-static ambient air, it  has lower pressure. The air
necessary for combustion is drawn to this low pressure area in an
attempt to equalize it. The linear inertia causes primary air  to
follow the gas stream.

If you lower the gas stream pressure (turn up the gas flow rate)
more surrounding ambient air is automatically drawn into the
current. I presume this is not a exceedingly flat ratio across all
flow rates, but commercial manufactures of inspirated burners for
boilers can demonstrate a wide range of control in flame regulation
that require no adjustments.  The primary air adjustment on un-blown
burners address the 4~14% ratio of gas to air.  Once you have
twiddled with the primary air shutter (you're compensating for
altitude and atmospheric conditions) the primary air will follow the
gas delivery rate. Home-made blowers, on the other hand, require you
to tinker with the gas to air pressure relationships.

As I mentioned previously, if the burner design is faulty, a fan CAN
make the design functional. But which principle would you rather
utilize? Flying with Bernoulli or flying on an X15. :<)

Thanks,
Vance

 * * * * * * * * * * * * * * * * * * * *
*  Message From : BURNS, VANCE A.       *
*  Location     : US-HOUSTON(IS3) LAN   *
*  KOMAIL ID    : FVAB94  (IS3     )    *
*  Date and Time: 07/26/96  18:34:57    *
 * * * * * * * * * * * * * * * * * * * *

---

From: "Steven O. Smith" <[email-masked]>
Date: Fri, 26 Jul 96 17:20:36 MDT

Dear Vance,

Hope things are better with your family.

I still don't buy it. Admittedly, what little I learned about
Bernoulli was a long time ago.

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 means
more oxygen available.

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.

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.

I'd like to understand this better. Keep talking.

--
Steven O. Smith
[email-masked]