exterior heat temps on forge.

[Titian]

Hi there I'm new here. maybe a silly question however was wondering what temps other peoples forges are reaching on the outer shell. Mine seems in be reaching 60-120 C on the out side (seems a little hot). Just built a new forge with a forced air ribbon burner. I put 40mm insulation with 50mm castable refractory inside. Its cranking hot inside. My last forge which while very in efficient and not so hot inside but had far less heat on the outer shell to the touch, it was lined with wool and refractory brick . My other question is does castable refractory (depending on type) generally transfer heat to the outside shell more that refractory brick? just wondering if maybe I cured it incorrectly. If anybody could;d help that would be great.

Cheers.

[Latticino]

Need more info to begin to evaluate.  40 mm of what kind of insulation (and how was it installed)?  50 mm of what kind of castable?  Any thermal bridges to the skin, or is it fully lined with insulation and the cast layer not touching the skin at all?  How are your doors designed (when the exhaust heat exits, does it pass over the skin of the forge)?  Have you thoroughly baked your castable and "buttered" insulation (moisture boiling off will steam heat your skin right up to 100 deg C)?

[Titian]

Hi, thanks for getting back to me me I really appreciate it. 

So it's 40mm ceramic fibre board not sure exact specs, and used what is labeled Desecrete, salesman said it is rated to 1600 C. Made the skin from 4mm plate. insulation is ceramic fibre board and is covered every where except around the ribbon burner. there is a small gap, a fraction of a mm in the ceiling around the burner entry point. I know some heat is escaping there perhaps its just traveling through the skin from there.

After casting I left it for 5 days or so until no moisture present. I probably cooked it a bit quicker than instructions (100 C per hour) as I couldn't keep the burner low enough. Just kept Turing it on and off and not sure how accurate my thermometer is. No cracks or crumbling in refractory surface. I did assume the steam would heat the shell but steam has stopped and its the same. 

I haven't buttered the surface ( not sure what that is) Doors are made of 8mm plate welded shut around 40mm of ceramic fibre board. 

If the refractory is cured too rapidly does it change its properties or is it only risk of cracking? Also do you think its just a case of sloppy sealing perhaps?

I appreciate you're time.

[Buzzkill]

First the disclaimer:  I haven't used fibre board or the specific castable refractory you've used.  However, the last 2 or 3 gas forges I've built tend to get uncomfortably warm on the outer surface of the shell after running for an hour or so.  I can place my hand on the surface briefly (about a second or so) with no burns or damage, but I can't rest my hand on the shell for any length of time before it's too uncomfortable.  I'd guess the temperature is between the upper and lower limits you gave.

I will also say it depends a little on what I'm doing.  Running at forge welding temperatures for extended periods of time does tend to produce a higher outer shell temperature than general forging temperatures.

If you aren't seeing any cracks in the refractory then you most likely did a satisfactory job when curing it.  Don't worry, it will probably crack at some point anyway. The difference between ambient temperature and forging temperature is a significant thermal shock that eventually takes a toll on almost everything that is subjected to the extremes.

If it were me I'd look for any obvious spaces between the insulation and the shell or any other route for heat to go places I didn't want it to go.  If I found anything I'd probably fill gaps with extra fibre blanket and/or refractory.  If I didn't find anything obvious I'd just "forge on" and try to avoid touching the shell when it's at high temps.

[Frosty]

Welcome aboard Titian, glad to have you. If you'll put your general location in the header you might be surprised how many members live within visiting distance. Lots of questions and answers have a strong location component, things that are easily available to me may be completely unavailable to you.

I don't know any specifics about the materials you describe but 50mm. of hard refractory seems to be an awful lot. The stuff I use I apply about 8-10mm thick and that's over ceramic blanked. Ceramic board is harder and more durable so shouldn't require as thick a coat of hard refractory. Bear in mind here, I do NOT know about what you have so I could be completely wrong. 

I don't think your forge is getting unreasonably hot, mine carries a little better insulation and it gets too hot to touch for more than a short second. It WILL burn you if you held your had against if for a couple seconds. 

From your description It doesn't sound like your forge shell is getting too hot. 

How about some pictures of your forge, burners, etc. a pic of the entire burner as mounted and another pic into the forge right after lighting and while it's up to temperature.

Frosty The Lucky.

[Irondragon Forge ClayWorks]

Like Frosty, my forge made out of an old 20 pound propane tank has 2 inches 50 mm of ceramic blanket with 3/4 inch 19 mm of castable refractory over it and soft fire bricks as doors. The shell will get too hot to touch after an extended burn (welding temp).

 Looking at it I think most of the heat is from the dragons breath and migrates back to the rest of the shell. It's never caused any problems though. You can see here where the high temp paint is burned around the mouth of the forge.

[Titian]

Thanks for getting back to me buzzkill, frosty and iron dragon. Ran it today for a few hrs. And was running smoothly. Not sure how hot it goes but very hot. Think all is well with this forge. Here are some pics for you frosty. Didn’t get one at full temp as I was busy working. Got one as it was warming up though.  I used a 1 mm migtip as the entry point for the fad. Is this enough or should I go bigger? 

Inlet for gas not fad. Sorry. Thanks so much for the advice guys. 

Edited April 28, 2021 by Mod30
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[PAR]

Hi...I just built my first forge...styled after the Edmonton forge utilizing two Larry Zoller side arm burners...it was recommended by a metal artist who urged me to make it out of solid refractory...it has 2.5" of Super Heatcrete 30...needles to say I can cook breakfast on it 6 hours after its been fired.  I just purchased 2600 C Kaowool and fabricated one solid end insulated with the kaowool. The forge works great after a 15 min warm up. I'd appreciate any comments on insulating the outside of the steel shell with two inches of kaowool covered in a 20 guage steel shell tack welded to the existing shell on each side of the burners. Thanks in advance for your comments.  Peter

 

 

 

(Wrong photos removed)

Edited April 28, 2021 by Mod30
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[Buzzkill]

6 hours ago, Titian said:

 I used a 1 mm migtip as the entry point for the fad. Is this enough or should I go bigger? 

Since you are using a blown burner this is mostly irrelevant.  Too small an orifice can limit the gas flow and therefore your max temp, but you do not seem to have that issue.  The only real value of a small gas inlet orifice like a mig tip on a blown burner is creating pressure in the supply line that you can reference with a gauge which will allow you to "dial in" specific temperatures that you can repeat easily.  You still have to adjust the air supply properly, so even that is of limited value.

If you're getting the temps you want then there is no need to change it.   If you can't get the temps you desire then you may want to use a larger gas supply orifice.

[Titian]

6 hours ago, Buzzkill said:

 

If you're getting the temps you want then there is no need to change it.   If you can't get the temps you desire then you may want to use a larger gas supply orifice.

For now seems pretty hot but have only used it for a few hours so far. Also I have to install a more consistent air source. At the moment just have a makita battery blower and cable tie. So have a small dedicated forge blower on the way. Then I can workout if everything is fine. Thanks for you’re comment about the mig tip makes a lot of sense.