Next job was to prep the tank for the blanket and refractory. Fortunately, the way the tank had been manufactured from new there was a lip inside which I was able to cut into sections and bend to form a ledge around the lower section of the body. This workd out to be a good way to help secure the refractory.
I also put anchors in the body and base to secure the blanket when the refractory is being rammed into place, I used stainless rod inside the lid to help hold the refractory in place, the picture of the lid shows exposed ends of the stainless rods, I added more rod in a zig zag fashion linking the ends but it seems I neglected to take a photo of the final layout.
I turned up some tapered to plugs put in the ends of the tank to make the drain and exhaust ports. The brown material used to make the fillets around the plugs is synthetic clay.
The blanket was the next thing, I used two layers in the body but made one layer shorter than the other to ensure enough clearance was available on the ledge for the refractory to get large mounting surface.
The discs on the anchors are installed to hold the blanket in place while the refractory goes in, they fit over the anchors and are rotated ninety degrees to lock.
I used a plastic bucket as a plug for the body. I screwd the bucket to a sheet if MDF and threw in some lead weights and sand to make sure it was not going to move or deform during the ramming process. I used a thin layer of grease as a release agent where required on surfaces that came in contact with the refractory.
I used the chrome bar to hammer the refractory into place. Getting the refractory in is a very slow, hard job. The refractory I got was made by www.sila.com.au and supplied by www.pyrotek.info
To prepare the base for the refractory I turned up a plug which is for a drain hole should the crucible fail while inside the furnace. The plug had a slight taper on it so it released very easily after the refractory cured. I used some synthetic clay to make a radiused fillet at the ends of the plug which in operation is used to reduce noise of air flow through the hole, because the airflow through the drain hole in operation is almost zero anyway it was more a precautionary measure than a real requirement. The exhaust port also has radiused openings which was a necesity due to the higher air flow.
I melted wax into a pot to make a plug for the combustion chamber in the base, and used pvc pipe and clay to form recess for the burner and the lead for the flame.
The shape of the lid made it neccesary to weld legs to it and secure the lid to the trestles for the refractory installation. Because of the nature of the job it was impracticle to put a radius on the inside edge of the exhaust port so I rammed up the refractory then let it cure and used an angle grinder and die grinder to radius the edge and do a bit of a clean up of the dome on the inner face.
I was advised to construct the furnace with a seperate stand for the crucible because some crucibles when heated to opperating temperature can become sticky and bond to the surface on which they sit, this can usually be prevented by putting some paper between the stand and crucible but sooner or later there is sure to be a time when you forget to do it and it all turns pear shaped. With a sepperate stand for the crucible it can be much easier to deal with should this happen. The other benefits of a seperate stand is if the condition of the stand deteriorates through use it is a simple and cheap job to make another one, and if there is a need to change to a taller crucible a shorter stand can easily be made.
I used a plastic pot plant as a mould for the stand. The pot plant didn't have the flat bottom I wanted so I used sticky tape to seal off the drain holes and filled the the pot to the desired level with a mixture of sand and plaster of paris. When it set I lined it with a smear of grease and added the refractory. I formed channels in the base of the stand so the metal could flow to the drain hole in event of a crucible failure, and used a home made tool to make the base level. In hind sight I went about it the wrong way, I should have calculated the height of the stand and filled the pot with plaster to that level and used a plug to form the chanels and then troweld across the very top of the pot to elininate the need for the tool and to simplify the job.
When the refractory cured I poped it out of the pot and gave the plaster of paris a few sharp hits and it broke away cleanly to leave me with a finished stand.
When the base cured it was time to remove the plug. The drain port plug came out easily, but he combustion chamber plug, due to having no draft was very difficult to get out. I had to tip it on its side and smash and chisel it out bit by bit. I even had to use a heat gun to melt part of it away. The mold I used was a cooking pot which was the perfect size and matched the diameter of the body. When I origionally cast the plug I had to put it in the freezer so that the wax would shrink enough to get it out of the pot. I didn't think it would be quiet as difficult as this to get out of the refractory, it did actually make a nice form in the finished part though.
The only real issue I encountered was the way I finished the mating faces of the lid and the body created some irregularities which was a potential source of heat loss and flame leakage. Traditionally a common way to deal with poor joints is to bond a high temperature rope to one of the surfaces which forms a gasket. I didn't have any rope at the time so I tried something a little unorthodox, I mounted the body onto a rotary table and used a masonry saw to face off the offending surfaces. It actually turned out to be quiet successful.
Now for the fun part, the burner. The burner design was curtesy of Cameron McKeown http://home.iprimus.com.au/cmckeown/ which I understand is derived from design work by Ron Reil. Thanks for your help on the whole project Cam.
The thermocouple shows 1328 Deg. C. (2422 Deg. F.)but it did break through the 1350 (2462 Deg. F.) mark a few times when I was trying different settings with gas pressure and air fuel ratios. This was done using LPG from a Swap and Go bottle which I got from a service station (it's a BBQ bottle).