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975 pound day tank rebuild web chronicles
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Day 5 - Monday
This is the last day for this furnace. We pretty much finished the rebuild today. This is the furnace at the beginning of the day - a photo I omitted yesterday. We have mostly detail work to finish. Note that the bricks on the top three rows are not mortared together. I've done this because the furnace is going to expand and if I glued the IFB's in the top rows together they would crack open like a dead bear. At this point the flue is off, the burner is sitting on the marver, and there is no sheet aluminum (.065" 5052) around the top.
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This is the insulation of the main arch with meatball - Greencast 19L. Slate is looking hungrily at this material which covers 5 layers of 1"x8# frax. The first two rows are the HTZ frax - a bit higher temperature and less shrinkage. The last three rows of frax are the HP(standard 2300). The frax is covered with about 1-1/2" of the meatball. This not only adds a touch of insulation to the crown, it also seals the frax in.
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Meatball gets mixed up much wetter than you would expect. A 25lb bag takes nearly 5 gallons of water- the exact opposite of the Intracast MZ that we use for the door(2.5 pints/50#!) I like to trowel the meatball until it's smooth, thereby allowing you to sweep the dust off the crown if you've got compulsive cleaning issues.
We had our first real screw up today. Somehow I managed to under-supervise the casting of the door and I let a 3" thick door get cast instead of a 4" door. Normally this wouldn't be such an enormous deal but with this door the extra inch seems to make a difference, more durability and the additional weight helps the door hang better. It's not that there isn't room for improvement on this door though, we will probably sit down and work on some ideas in the next couple of days. This photo is the door 15 seconds before all the fresh castable hit the bottom of the dumpster, sad.
Casting a new door was actually a snap. We cleaned out the mold with a little soap, dried it out and put in more Vaseline. (I'd prefer thick yellowish axle grease but this was all the studio had, and in abundance!) The door is cast with Intracast MZ. It needs to be mixed up very carefully, take your time, mix only one bag at a time, and don't try to add the water all at once. When I mix it I notice that the castable reaches a certain condition while you are slowly adding water (1/2 cup every 2 minutes while vigorously mixing with hoe) that, remarkably enough, resembles deer turds. I suggest stopping at the deer turd stage and mixing for an additional 3 minutes and then, if you add about another 1/2-3/4 cup of water the castable comes together, the turds disappear and when you giggle your container it all flows, just like a low cement castable is supposed to. This is the perfect amount of water.
We got the flue bricks finished. Here Slate fits some of the last of the KX-99's in the base of the flue. The bricks in the top are 2-1/2" 2,800° IFB's. It's critical not to mortar any of the horizontal joints on the IFB's. If you do, you'll get a long vertical crack up the flue, due to the heat gradient between the bottom and the top. I usually stack up all the flue bricks and then clamp on the angle Iron and weld it all together. In this case that's more or less what we did except for the brick you see here. Also note that the IFB's extend above the angle iron on the top. There is a possibility that in the future this flue will be refitted with a recuperator model. There is research that needs to be done to design a great recuperator for this furnace. When that happens, it would be great to do see a recuperator flue instead of this.
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Reattaching the combustion system was as simple as putting the burner back in, tightening the gas union, hooking back up the cooper lines and plugging in the UV eye and spark. Yes, that is a PVC air line you notice there. It's been there for 10 years and seems to be in great condition, no heat damage that I could find. The other shops here have steel duct that was much more difficult to install and doesn't have the sewer system look. While it's probably better to do steel ducting for the combustion air, I'm not altogether convinced that if you have good hood ventilation this PVC is just as good.
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Well, that's about it. We put in the two Type R Thermocouples, scrubbed everything twice and have now only to wait for our door to cure - about 3 days to be on the safe side with Intracast. We can't weld in the sill yet and will wait until the last minute to put in the sill bricks, but other than that, that's all it takes to rebuild one of these.
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At one moment today I was thinking - how can it be that this furnace is so much simpler to build than it is to make a simple cylinder in glass or even to make a good blow pipe?" Part of the answer is that there is no difficult to acquire prerequisite skill, only the capacity to focus on the tasks at hand and honest attention to every detail. As a team we are not actual very experienced with this, it's Chad's 3rd furnace and Slates 2nd. They really did most of the work along with the Studio's techs Harry, Shawn and Carl. I mostly stood around, pointed at things, ran back and forth to the bathroom, and said "what?" a lot.
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Thanks for checking this out,
Fred Metz
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