[TheForge] photos under "inexpensive recuperative forge"

[Jerry Frost]

From: "Jeffrey Polaski" <jeff.polaski at rgs.uci.edu>


>>The potential problem with this kind of heat
>>exchanger/scavenger is overheating the air/fuel and
>>having it ignite in the burner tube. Controlling the
>>preheat by blocking the open pipe might work.


>Isn't that true of any recuperative forge? At least, 
>if it gets at all
>hot I'd think there would be a problem. Is there a 
>design where it's not
>a potential problem?


>Jeff Polaski


#1 For a recuperative system that preheats the intake 
air or mixed air/fuel it is.

#2 A recuperative system that recirculates the exhaust 
gasses through a gap between an inner and outer hard 
liner doesn't have this problem.

#3 A recuperative system that preheats the fuel doesn't 
have the problem unless you preheat the fuel to extreme 
temps. Preheating just the fuel uses so little of the 
exhaust heat it's hard to call it recuperative but it 
is.

I don't recall the flash temp for air/prop but think 
it's around 1,000f. (okay, I admit it I'm just feeling 
lazy right now or I'd look it up) If you don't control 
it you'll burn the burner out of your forge in short 
order. Minutes if it's properly tuned. It's easily 
controlled though so there's no reason not to build a 
recuperative system.

Of the two (#1) recuperative strategies, I prefer 
heating the air/fuel mix as you're doing. There are a 
couple advantages over preheating the intake air. 
Primarily it doesn't reduce the air density going into 
the burner. Heating the mix causes it to accelerate as 
it expands with little if any increase in back pressure 
so there's no reduction in fuel entering the forge. It 
is harder to control but I think it's worth the 
difference. The increased velocity will also increase 
dragon breath some.

If on the other hand you preheat the intake air it 
expands before meeting the fuel. This expansion will 
reduce induction in a naturally aspirated burner. The 
combination of less dense intake air and reduced 
induction reduces the amount of air available to the 
fuel so you must reduce the fuel or run too rich. The 
proper ratio will change as the forge and intake air 
heat up too so it may be running an oxidizing flame 
till it gets hot. This type is easy to control with a 
simple bypass around the heat exchanger.

I realize you're using a gun burner (blown) so 
induction isn't a factor.

A recuperative wall furnace (#2) is a lot more 
complicated to build and is more suited to large 
furnaces.

Preheating the propane on the other hand (#3) from what 
I've read looks like it's more effective than 
preheating the intake air or air/fuel mix.

One of the real beauties of these three recuperative 
systems is they're not mutually exclusive. It may take 
some tinkering (read more trouble than it's 
worth<grin>) but there's no reason you couldn't 
incorporate all three in one forge.

I know that was a lot more than you asked for but . . . 
Uh. . . Er. <grin>

Frosty
-------------------------------
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