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Salvaging first forge build


Selkirk

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Good Morning All,

I built my first forge over the winter but after running a few times I'm seeing a number of problems with how I built it. I'm hoping that now that I understand the operation and my needs better I can tweak it rather than start over.

The first problem I need to overcome is forge temperature. I have a hard time reaching yellow heat. It takes at least an hour to warm up enough to be able to reach that temp.  From my reading here it sounds like my choice of hard fire bricks on the bottom is a big mistake.

Currently I have a 1/2" top coat of 3000F castable to protect the brick. I'm thinking of adding a 1" thick layer of 2800F castable that I already have. It's a little better insulating (0.45W/mk) and will reduce the total volume of my forge. This would bring it down from 575cuIn to about 500cuIn. I just received some Matrikote for my final layer as well. 

Any thoughts on whether or not this is likely to be worth the time and effort? On whether or not a fresh layer of castable will bond to an existing lightly used surface without falling apart?

Thanks in advance for your thoughts!

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An hour to reach yellow heat is a massive issue.  It is more than just hard firebrick in the floor.  So lets start with your full design?  Including burner and regulator along with some video of the forge running and how you tune oxygen?

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I'd be happy to elaborate. I can get orange within about 15 minutes. But then for full yellow it takes a while and I really have to crank the gas. I've cranked it up to 10psi after 2 hours and couldn't get any more than a yellow on some quarter-round. 

It's a Forced air ribbon burner. I've got a large squirrel cage blower with a home made wooden damper to tune. I'll probably upgrade to a gate valve soon because it's touchy.

For Gas it's a high pressure regulator which I run through an .035" MIG tip. I have 30+ inches and one 90-degree bend before the ribbon burner. I run 2" pipe all the way to the burner.

I've attached pictures of the blower/gas injection as well as a photo of the inside of the forge after I covered the wool with Satanite.

The video was taken at about 20-30 minutes of start-up and was probably about 3-5psi. The clearly visible blue flame goes away after about 30+ minutes as the chamber gets hot.

Another comment on insulation... I've got two 1" layers of wool behind the satanite but I was paranoid and really went to town with rigidizer on each layer. Is it possible to compromise thermal conductivity severely but  over-rigidizing? The outside of my forge gets *very* hot.

Thanks!

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Do you see steam when you shut the forge down?  If you oversaturated your blanket and then did not dry it that could be a problem and it also heats the outside up a lot because its hot steam.  Beyond that possability I dont know what I see looks good.  Can you post a video of it running please.  Hmmm.   What size propane tank are you using?

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It definitely steamed on the first fire, but not since. 

I tried to post a video but it doesn’t seem to be working. I will have to look into that.

20 lb propane tank.

My personal theory is that I have a whole list of minor issues. Poor insulation, slightly too large chamber and doors that don’t seal well. It’s forced air and propane, so I feel pretty good about getting those BTUs into the forge but my amateur guess is too many of those BTUs are just heating my garage.

that is why I am thinking about another inch of insulating castable on the floor and the Matrikote. But I’m definitely worried about what is going to happen with those two different layers.

Does this theory sound off base?

Thanks Binesman.

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Honestly I think the 20lb tank is your problem.  You heat up fast because the tank isn't frozen yet then it freezes and cant push the gas to heat the forge faster.  Try a bigger tank or get a converter to link 2 - 20#.  But to test the theory before you spend money put your propane tank in a bucket full of water and see if it acts better (I think you will still get freeze up with the amount you are drawing but it will take a good 20 minutes longer) 

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That actually brings up a related question I had. The first five or 6 times I used the forge I definitely saw dropping pressure and had to continually open the regulator to maintain pressure. I had heard about tanks icing and could feel the frost on the outside of my tank. So I did some reading on the thread in this forum and decided to try something a little novel.

I got a tub of water and threw the tank in along with a coil of roof de-icing wire. It's meant to melt snow off areas where it builds up too much. I think it's may be 300-500 watts of warming. After about 2 hours of run time that water was like 98F. Like a nice warm bath. And amazingly as my forge warmed up I had to continually dial down the pressure in order to prevent excess 'dragon's breath' from coming out the front of my forge. But I can't understand what effect I'm seeing there. I didn't change my air output so why would a warmer tank running at the same pressure result in more dragon's breath? Unless my gauge also changed performance as my garage warmed up or something?

The last two times I've used a tub of cool water without the warming coil and it has actually been even more stable. It really is quite surprising how effective just plain ol' water is! The bottom line is that the water did help a bit so your hunch was totally legitimate however even with that yellow heat is still not easy for me.

Perhaps I'm being unrealistic with the amount of gas required to heat and maintain that large a chamber to yellow heat? How many BTU's do other folks need to use to operate at yellow heat?

Thanks again for your consideration Binesman.

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Water bath for the propane tank does not fix the problem. You need a larger volume of propane to operate the forge.  

The larger propane tank does not cost you money. The larger tank will allow you to work until you get tired.  If you run out of propane, then you have been shut down until you get more fuel.  This usually happens when you are in the middle of a project and working on a deadline.

You do not go to the grocery store and put only enough fuel into the tank to just get half way to home.  

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Binesman's water bath suggestion wasn't a guess unless he's not sure if it works himself. What you described is the propane being over drawn, it gets slushy then freezes up because at the rate we draw gas the liquid has to boil to maintain tank pressure. A boiling liquid is shedding the most energetic (warmest) molecules, leaving the least energetic, (coldest). Making the phase change from liquid to solid and vise versa takes considerable btus. 

Cold water is just as effective as warm / hot water, propane doesn't freeze much above -40 f. keeping it around 32f. is perfect. Warming it much above to say 98f. produces a burst of high pressure but it drops off rapidly because the liquid doesn't have to boil, tank pressure is already high. This causes a rapid psi drop untill it falls below the psi required to prevent the liquid from boiling then the liquid cools quickly until it reaches equilibrium. 

Buy a 100lb. tank and don't worry about it. It may start to slush up, if it does wrap your heat wire around the bottom of the tank. Just DO NOT LEAVE IT ON ALL THE TIME!! If you plug it in after an hour or so of forging and leave it on for an hour or so things will be good. 

Part of the problem you have getting your forge as hot as you want is those tall gaping openings on flat floors. The flame reaches the floor and takes the path of least resistance, straight out the openings. Make sense? If you put an IFB laying flat across the openings as a sill, quite a bit of heat will be redirected back into the forge chamber. I don't know what you intend to make that requires openings that tall but I'd try closing off the tops and leaving maybe 6" height max. 

Sliding IFB or kiln shelf for baffles rather than sealed doors will make a tremendous difference. 

I have a strong suggestion for you. Do NOT make more than ONE change between tests. If you start shot gunning ideas and often unfounded theories you can't know what change made what difference, you're trading an effective method of problem solving for blind luck. Luck works sometimes though, you just won't get many of the knowledgeable builders willing to play a losing game. 

Frosty The Lucky.

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5 hours ago, Selkirk said:

How many BTU's do other folks need to use to operate at yellow heat?

We have a GACO MR750 burner in our 20 pound tank forge and it is rated at 243,130 BTU at 20 psi. Our forge reaches high yellow at 15 psi 210,555 BTU hr. and pegd the pyrometer at 2250° F. (actually melted the bulb so it's hotter than that).

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IIRC there're about 91,000+ BTUs per Us gallon. Not trusting my memory I looked and I was close, 91,547 per Us gallon.

I'm pretty sure it varies, propane isn't a single pure gas or it'd cost like the dickens to refine. 

Frosty The Lucky.

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Thanks for all the responses gents. I would like to ask a follow up question about the tank size because I am having a hard time wrapping my head around this. Unless I misunderstood, the suggestion is a larger propane volume will improve fuel supply to get the forge hotter? I was under the impression that if my little 20# tank was putting out a given PSI then my energy output was also steady regardless of whether or not the tank was freezing or not?

There also seems to be a huge range in forge fuel requirements to operate at yellow heat. And I suppose that reflects the wide variety of forges and uses.

In terms of my forge doors they were simply not given any thought. I was completely preoccupied with the ribbon burner and insulation and didn’t consider how important a role they actually play. My first forge was coal so it wasn’t even something I had to worry about. When I run I have the back completely covered with IFB. Not a perfect seal but 90% blocked. I wedge a chunk of brick in the front to block 2/3 of the opening. Maybe still just not closed off enough?

if I had to do it again I would make a lower square end to the forge and just use bricks to seal off the ends. I do mostly decorative stuff so bends and scrolls all need to fit. The flexibility of the adjustable opening would be nice!

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I've got nothing against the science here. And I grasp the concept of the evaporation of the liquid as it maintains pressure. Even if it has been a minute since high school chem!

Early on I definitely experienced some freezing. But ever since I put my tank in water I have not lost pressure as the forge ran. If I am not losing pressure then doesn't that mean the evaporation is keeping up with consumption? And therefore I am maintaining the number of BTU's I am expecting? This was my understanding of this mechanism. And when I have run this way it still takes a while to reach yellow heat. Therefore a larger gas tank(while certainly convenient!) would not on it's own solve this, correct?

Not to say I'm against larger tank volumes. I was already considering that before dropping my single tank in water. But I feel like maybe the two problems got tangled up in this thread?

 

 

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To me the most likely culprits are the combination of the forge volume and the mass of solid refractory materials.  In the first post I believe you said you have a half inch of Kastolite over hard fire bricks on the floor.  Even though the Kastolite is somewhat insulating it will still absorb a fair amount of heat.  I experimented with different thicknesses of Kastolite 30 over 2 inches of fiber blanket in the past.  The thicker the Kastolite, the longer it would take the forge to heat up.  To whatever extent heat passes through the Kastolite, you have some serious heat sinks right under it on your floor.  

Decreasing the volume of the forge by adding yet more mass on the floor may or may not solve the problem.  It comes down to whether the additional mass will absorb more heat compared to the lower volume requiring less constant heat to maintain a given temperature.   It may be worth a shot.  There is an equilibrium reached between the heat produced and the combination of heat absorbed, transferred through the solids (or into your stock), and the heat lost through exhaust openings.  Obviously we'd prefer to shift the equilibrium further to the heat produced side and minimize the heat lost through exhaust openings, absorption of construction materials, and transference.

You'll have to decide whether the cost of that additional inch of refractory and the added difficulty in breaking out the floor are worth it in case it doesn't work.   If you get to the point where you have to replace the floor, I suggest building up the floor with as many layers of ceramic fiber blanket as needed to give you the final volume you desire (minus the half inch for the insulating castable refractory layer of course).

A more precise way to control the air flow will also be beneficial.  Fine tuning is hard to do with sliding mechanisms unless they are controlled by screws or other methods of producing very small adjustments.

There's nothing particularly bad or wrong in principle with using a mig tip for fuel delivery, but it is not needed for a blown burner.  The only real benefit is a restriction that small will provide a wider range on a pressure gauge to use as reference points once you get things running the way you want.  A potential down side is that if you have too small of an orifice for your fuel delivery and you hit the max pressure capability of your regulator, then you can't introduce more fuel per unit of time into the forge, which in turn limits the maximum heat you will be able to produce.  A larger mig tip will still provide enough restriction to provide a pressure reading on a gauge so you can quickly return to a previous setting, but allow more fuel to be delivered at a given pressure.

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Sorry I misread.  I thought you stated you were still loosing preasure while in the water bath.  Not that it had solved the issue.  Construction all looks good.  And the forge should run fine.  So that means your problem is in the burner.  Ive yet to make a ribbon burner so cant help a lot there.

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Thank you for your responses gentlemen. You insight is appreciated.

It sounds like I have several different options to test. I’ll be sure to follow frosty’s advice and change only one thing at a time!

I am sadly going to be away from my shop a bit. But when I get results I’ll be sure to share for those who may be interested.

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