Subject: Re: Gas forge temps

From: Vance Burns <[email-masked]>
Date: Fri, 19 Jul 1996 13:51:21 -0500

To: [email-masked]
Subject: Gas forge temps

>From: "David C. Hufford" <[email-masked]>
>A question about temperatures achieved in propane-fueled forges:
>all other things being equal, how much hotter does a forced-air
>forge get versus an atmospheric forge?  I realize that gas pressure,
>orifice size, and insulation contribute to the equation, but can
>someone provide typical temperatures reached by these two types of
>forges?  I believe my NC Tool atmospheric forge is suppose to max at
>about 2350 F; what is the typical temperature for a forced-air unit?
>D.C. Hufford

David

I've done some research on this and can speak to it as an informed 
amateur. 

Propane can sustain combustion in a mixture of 4~14% oxygen, 9 point 
something is optimum. Regardless of the method employed, natural or 
forced aspiration, 14% is the max. If you add more oxygen, and the gas 
flow is constant, the excess oxygen above 14% will remain unburned and 
interact with the surfaces in the forge, i.e. your metal will scale.  
Air supplied above or below the 9.?% optimum flame velocity will sap 
BTU's from the flame, ether in excess air or cold unburned gas. If the 
combustible gas does not "find" enough oxygen to mix with, the gas can 
exit the appliance unburned.  Yes, unburned. I also found it hard to 
conceive. In addition, you generate carbon monoxide and aldehydes.

Forced air burners (power burners) are compensating for poor burner 
design (per a combustion engineer).  By poor, I wish to imply physical 
and technological limitations. Power burners (normally) seem to be used 
to overcome high pressures in the combustion chamber.  Other exotic 
applications incorporate power burners. 

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. Logicaly, 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.

Vance

---

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

> 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]