Insulation Question

[IgnrntNewb]

I've read over and over again that two 1" layers of ceramic fiber insulation is all you want to lay down (with refractory, rigidizer and whatnot...). And I understand (I think...) that you don't want to use more than that because it creates more of a heat sink...? Please correct me if I'm wrong. (My username almost certainly applies here) But what do you do if you need to bring your inside diameter down in size to achieve the overall cubic inch range that you had planned to match your burners? 

The empty inside diameter of my tank is about 13-3/4" with a length of about 14" inches (after I cut it in half). I need to get as close to a 6" inside diameter as possible to achieve the 300-400 (roughly) cubic inches that my two 1/2" burners will heat. The only way I can see to do this is to add at least one more 1" layer of kaowool, and layer the refractory thick enough between the layers of wool to achieve my final inside diameter. Is this not a good idea or is this one of those "gotta do what you gotta do" scenarios?

Thanks in advance!

[ThomasPowers]

Your wrong.  Insulating refractory is not a heat sink.  It is quite possible to add more with no issues save that the efficiency goes up; but it's a matter of diminishing returns for the cost of extra insulation.

[Latticino]

I've recently done some calcs  that indicate that you don't get appreciably more thermal efficiency from more than 2.5" of 8# density high temperature ceramic blanket insulation.   The relatively low density of the blanket prevents it from being a significant heat sink, though any mass will retain some heat.  The castable insulation refractory  (Kastolite or equal) is more dense than the blanket.  As such is a poorer insulator, and more of a thermal mass, making it a bit of a heat sink. Not as much of one as a hard brick, or heavy castable refractory like Mizzou, but still what I like to call a thermal battery.  That is why it should be on the inside of your forge where it can both protect the blanket and provide some "smoothing" of the internal temperature profile when you make changes like opening the forge door or putting in large masses of cold stock (if one cares about such things ), not in between layers of blanket.  This is clearly evident in a forge with a castable floor when you put in cold stock. It sucks the heat right out of the floor, which you can see if you move it before the stock heats up to general internal forge temperature. 

[ThomasPowers]

I noticed that when I relined my forge and put a coating of castable refractory in it.  Took longer to come up to forging temp; but then maintained temps much better afterwards.

[G-son]

In theory… If you add extra layers of insulation just to reduce the internal size, those outer layers of insulation should never see the very high internal temperatures, so I'm guessing a cheaper, lower temperature insulation could be used there.

[Latticino]

Correct.

[Frosty]

21 hours ago, IgnrntNewb said:

Is this not a good idea or is this one of those "gotta do what you gotta do" scenarios?

It's a perfectly workable idea, it just won't make the forge significantly more efficient. 

Don't let all the discussion your question generates get to you, we just never seem to tire of talking about lining forges, efficiency as opposed to effectiveness, etc. Some of us have been making burners for years and others have been making and using various types of furnaces and kilns for years. Latticino is or was a professional glass guy and worked with glowing heat daily. 

It is a good question and we LOVE good questions, even if we've talked about them many times over the years. Something new comes up almost every time. 

If we lose you, ask again, someone will be sort of maybe concise answering. 

Frosty The Lucky.

[Jasent]

Might be cheaper to just find a different shell than add extra wool or refactory.  

[IgnrntNewb]

It's hard to get any cheaper than "free", which is how much that compressor tank cost me. I mainly wanted to make sure it wasn't a completely terrible idea. At this point, I'm just trying to use what I have to get my first forge built. The more efficient the better, obviously, but again... I'm trying to use what I currently have. 

On 12/11/2019 at 4:20 PM, ThomasPowers said:

Your wrong. 

One of the few times I'm happy to be wrong. Perhaps I misread or misunderstood what I was reading about having more than two layers of wool. I appreciate your response!

Thanks!

On 12/11/2019 at 6:22 PM, Latticino said:

That is why it should be on the inside of your forge where it can both protect the blanket and provide some "smoothing"...

...not in between layers of blanket.

I'm almost certain I'm not understanding this correctly. Are you saying that I should NOT put castable refractory in between each 1" layer of kaowool? Should I only use the castable as a protective layer over the final layer of kaowool? If this is the case, than I've definitely been misunderstanding a lot of what I've been reading about insulating a forge.

Thank you all for taking the time to reply. This would be a miserable endeavor without guys like you willing to walk guys like me through the process.

[JHCC]

4 hours ago, IgnrntNewb said:

It's hard to get any cheaper than "free", which is how much that compressor tank cost me. I mainly wanted to make sure it wasn't a completely terrible idea. At this point, I'm just trying to use what I have to get my first forge built. The more efficient the better, obviously, but again... I'm trying to use what I currently have. 

Don't fall into the "I have to use what I already have" trap. Sometimes a little bit of patience can win you something that's even better than what you have, and you'll end up saving time, money, and aggravation.

For example, your "free" tank isn't really free if you end up having to buy more ceramic wool to fill up the interior space.

[Latticino]

6 hours ago, IgnrntNewb said:

I'm almost certain I'm not understanding this correctly. Are you saying that I should NOT put castable refractory in between each 1" layer of kaowool? Should I only use the castable as a protective layer over the final layer of kaowool? If this is the case, than I've definitely been misunderstanding a lot of what I've been reading about insulating a forge.

That's right. No castable in between layers of blanket. You can rigidize with colloidal silica, if you want, but even that is not required.   The castable refractory belongs on the inside.

[IgnrntNewb]

2 hours ago, JHCC said:

Don't fall into the "I have to use what I already have" trap.

You just quoted exactly what I said in my previous post, and nowhere did I say "I have to use what I already have". I said I'm trying to use what I already have. And I suppose I can take that a step further and say I'm using what I already have. The wool is bought and paid for and I already got more than I needed in case I make any mistakes. Also, being that I'm cutting the tank in half, I'll have more than enough wool left over to play with a second forge project at some point. 

20 minutes ago, Latticino said:

That's right. No castable in between layers of blanket.

Thank you for clearing that up, Latticino. I was about to waste a lot of refractory...

[JHCC]

14 minutes ago, IgnrntNewb said:

nowhere did I say "I have to use what I already have".

That wasn't quoting you; it was describing a trap that's easy to fall into. Just a word of caution from someone who's been there more than once.

[Frosty]

John's just pointing out an all to easy to paint yourself into, corner. Old compressor tanks are commonly thought of as "perfect" forge shells but that's the CORNER you see guys on Youtube espousing as THE thing.

There are much better things to do with a compressor tank, especially a long narrow one. Say cut one end off and maybe weld a small wheel rim to the other and make a quench tank from it. Welding a hinge and the cut end back would make a good lid for fire control and keeping critters out. 

Do similar to it only fill it with perlite and use it to anneal. 

Frosty The Lucky.

[IgnrntNewb]

Ah, I see... My mistake. 

[Frosty]

Ah, it's not so much of a mistake, it's easy to write and think the reader is thinking what you are. We all do it and knowing the other guy makes it more likely we'll at least get close to what the writer actually means. That's why I was able to interpret John's post, coincidentally while he was writing a follow up. 

It's all good brother, we're all students here.

Frosty The Lucky.

[Irondragon Forge ClayWorks]

If you stop by an auto repair shop and ask if they have any empty Freon tanks you could have, you may be surprised. My shop gave me three tanks for free and were happy to be rid of them. They make excellent forge shells in the size you want.

[ThomasPowers]

I have a handful of empty "party balloon" He tanks hanging in the rafters of the shop to give to folks to make forges from.  Many of them were free the rest were 20 US cents a pound. The shell is probably the easiest thing to source. 

I probably should make an annealing "tank"; though generally I just leave stuff in the propane forge overnight when I shut it down. They mine Perlite in this town!

Just a reminder that this forum is read by folks all over the world and so while the question is *yours* the answer goes out to the entire world and so sometimes get expanded to handle cases that don't apply to you; but may apply to others...Safety warnings are also commonly repeated even to those who may not need them---I know a person who can safely do fire gilding; but I'll stomp all over any suggestions for us normal folk to try it! (Requires a mercury scavenging hood to do so sanely!)