Propane vs. Natural Gas forges

[Latticino]

Just now, ClevelandGuy said:

 If I'm trying to heat the middle of a piece that's long and flat, it's often difficult to get it into the hottest part of the fire.  It's this type of thing I'm trying to address with the gas forge design.

 

That is what torches are for...  I have both an oxy-acetylene setup and a high end propane hand torch that puts out some serous heat (looks like a really good quality weedburner, but can't remember the manufacturer offhand)

[ClevelandGuy]

5 minutes ago, Latticino said:

One of the key drawbacks of a gas forge.  In addition to Buzzkill's excellent suggestions also consider door configurations like this one from Majestic:

 

Okay, assuming some scale for that pic, I'm thinking that forge is something like 36" long and about 6" wide.  So that's ~1300 cubic inches.  So at 540 BTUH per cubic inch, we're talking 702,000 BTUH, or 140,000 BTUH per burner?

[Latticino]

Actually according to the manufacturer (and I'm no supporter of majestic, just wanted you to see the side opening door) the interior volume is 864 cu inches.  At 450 BTUH/Cubic inch we are at around 389 MBH, or about 78 MBH per burner.  This is easily achievable.  I think the hand torch I mentioned above was rated around 150 MBH max output and Zoeller's 3/4" Z-burner is reported to be measured up to close to 120 MBH. 

[ThomasPowers]

I've not had that experience with coal forges.  I select where the hotspot is by how much air goes in and can push it above the firepot with a sufficient pile of coal and sufficient air flow.

A true clamshell propane forge with a slot all the way from end to end will handle scrolls.  With helixes you do have a problem with any forge unless you go huge.

I have seen 100+ year old pictures of ship building where the factory floor was once dirt and they piled huge piles of coke on the floor with big fans blowing into the pile to heat ship components for forge welding---no firepot! (lots of jib cranes though...)

[ClevelandGuy]

Okay, the math looks doable, all the way around.  I just have to come up with a suitable forge size and configuration for my work.  The comment about using firebrick may be the simplest and most effective way to accomplish things.

Just got off the phone with an engineer at my gas utility.  We'd spoken last year when I was considering the addition of a NG backup generator and he'd given me a price to upgrade our meter, which currently can source 400 MBH.  The next size up will go to 900 MBH.  We still envision the generator installation at some point (budgeted for 265 MBH) so with our home and garage furnaces, water heater, and kitchen range, that leaves something around 400 MBH for the forge.  So far I'm not seeing any dealbreakers.

[Mikey98118]

12 hours ago, ClevelandGuy said:

The subsequent statement of 450 BTUh per inch of cubic space inside the forge seems usable, although I don't understand the statement about it being related to a certain size or style of burner.  Required heat would be a target number, regardless of how you get the heat in there,

And you would be utterly wrong. Btu input has to do with POTENTIAL energy past through a given piece of equipment If one burner can only get the standard 2450 F flame out of propane, and another burner can get 3000 F from a much more compact flame, the amount of transferred energy into the equipment is going to be a whole lot different. And if a third kind of burner can put out a high heat milt-flame that slows way down in the first couple on inches out of the burner nozzle, it will transfer even more heat. So, yeah, the kind of burner matters a whole lot. 

[ClevelandGuy]

15 hours ago, Mikey98118 said:

And you would be utterly wrong. Btu input has to do with POTENTIAL energy past through a given piece of equipment If one burner can only get the standard 2450 F flame out of propane, and another burner can get 3000 F from a much more compact flame, the amount of transferred energy into the equipment is going to be a whole lot different. And if a third kind of burner can put out a high heat milt-flame that slows way down in the first couple on inches out of the burner nozzle, it will transfer even more heat. So, yeah, the kind of burner matters a whole lot. 

You misunderstood.  I didn't say that the type of burner is irrelevant to the amount of heat produced.  I said that the amount of heat needed is a given requirement that must be the starting point.  From that number you can use whatever burner you like, as long as it achieves that end and meets all your other requirements as well.

[Mikey98118]

Thenm you are quite right; of course, the devil still remains in those details

[Mikey98118]

There are several mitigating factors that you don't want to overlook, 'cuase they are all in your favor:

The square of the distance rule is  curtailed a whole lot in a forge; the better that forge is designed the greater that curtailment is. The  main goal in a modern gas forge is to turn it into a radiant oven. You lean into that goal, by employing more insulation; not less, and by using a high quality heat reflecting finish coat over the insulation. You place a baffle wall outside of the the exhaust opening, and paint it with the heat reflective coating, so that radiant heat is bounced back into the forge, instead of leaving along with the exhaust gases. You employ a ceramic multi-port flame nozzle on  your burner, to insure maximum flame heat in the shortest distance within the forge, followed by a rapid slowing of combustion gases toward the exhaust. You seal the burner opening, by stuffing ceramic blanket between the flame nozzle and the forge opening, to prevent secondary air from being induced into the forge by the flame. All these things should make your forge hot enough, even when burning natural gas.

But, supposing that, on occasion you want more heat than your NG forge  will give you? Then, on those rare occasions, you can use propane, propylene, or oxygen enrichment (no more than 10%) to get over the hump. We aren't big about calculations on this forum, because we have discovered that c-h-e-a-t-i-n-g beats good math every time

[Mikey98118]

On 6/28/2017 at 8:10 AM, ClevelandGuy said:

You could leave some kind of space to move the piece in/out, but then you're letting the heat escape and will have to counteract that.

It isn't a perfect answer, but the best idea I've seen so far, is that some guys install hinges and a latch on the working end of a forge, so that larger material can be moved in and out, on occasion. The expected plummet in forge temperature turns out to be minor.

And why would the heat loss be minor? Because the forge insulation, is itself a heat sink; not much of a heat sink, but more than needed to take care of rapidly recovering the loss from a door opening for thirty seconds or so.