Furnace Shell Work

Control box 2

This afternoon I mounted the control box to the side of the steel shell with 1/4-20 UNC screws, washers, and ceramic standoffs, into rivnuts set into the shell itself.  

 

The ceramic standoffs were purchased several years ago to mount a heat shield to the wall behind a wood-burning stove and they were never used.  Hopefully having the controls mounted this way will minimize thermal bridging and promote air circulation around the control box keeping it cool.

Next I put 1/4-20 rivnuts around the shell at 90° intervals and I screwed 3-inch-long machine screws into these with double nuts on the end.  These come right up to the outside of the firebrick and should help keep the heating chamber centered in the shell.

Finally, I put garage door pulls on the outside of the shell 180° from each other.  

As soon as I get the armored conduit connected to the shell and the back of the control box, I will begin packing the shell with ceramic wool insulation and sealing it off with an annulus of 1/2-inch-thick ceramic fiberboard.

I want to get the controls wired up and begin testing - even before I get the door completed.

Control Box Work

This morning I drilled holes for the fuse holders and put a 20-amp fuse in each one.  The holder that I was going to use for the 1-amp PID controller fuse was defective, so I tossed it out.  I do not have a spare.

Next I marked and drilled holes to mount the Solid-State Relays (SSR).  The SSRs are in intimate contact with the aluminum control box and there are aluminum heat sinks mounted to the back wall of the control box.  The SSRs and the heat sinks have heat transfer compound applied to the surfaces facing the control box.

During testing I will run the furnace with the control box front panel off so that I can monitor temperatures inside.  If they get too hot, I will have to drill holes in the bottom and back of the box and install a 240-volt fan for forced air circulation.

High-Temperature Hook-Up Wire

With the heating chamber on the work table, I cut two four-foot-long pieces of fiberglass insulated 10 gauge hook-up wire to bring electric power to the heating elements.

I stripped the fiberglass insulation back about one inch and pushed a piece of heat-shrink tubing over the fiberglass.  I twisted the strands of wire together and put a stainless steel high-temperature wire terminal over the wires and crimped into place. 

Finally I pushed the heat-shrink forward over the end of the terminal and tightly shrunk it down over the fiberglass to keep it from fraying.

Repeat three more times.

I hooked the wire up to the stainless steel screw, nut, and washers at the end of the heater wire and tightened everything securely.

Next steps:  install carriage bolts to keep the heating chamber centered, install garage door pulls on opposite sides of the furnace shell, build the control box and lead the power and thermocouple wires to the controls.  Then I can pack the open space in the shell with ceramic wool insulation and seal it up with ceramic fiber insulation board.

Thermocouple

This afternoon I removed the heating chamber and floor from the furnace shell and turned the assembly over so that I could access the junction box that houses the thermocouple connections.

I stripped the high-temperature fiberglass insulation back about 1-1/2 inch to reveal the red and yellow wires.  I stripped the red and yellow insulation back about 3/8-inch to expose the chromel and alumel wires. 

On K-type thermocouples, the red wire goes to the negative (-) terminal and the yellow wire goes to the positive (+) terminal.

I put the exposed wire under the appropriate terminal and tightened the screws.  I wrapped about 2 feet of hook-up wire around my hand and stuffed that into the box in case it is needed for future repairs, and I threaded the rest of the wire through the rubber grommet and into the bottom of the furnace shell.

I put the cover on the junction box and turned the furnace shell and mobile base back over.

The thermocouple is a very robust device with zero moving parts.  It consists of 8-gauge nickel-chromium and nickel-alumel wires twisted and soldered together.  When the temperature changes, these two dissimilar metals create a very small voltage that the controller can sense and display as a temperature.  The temperature sensing occurs at the junction of the dissimilar metals - at the tip.

Next I put the 1/2" ceramic fiberboard back into the bottom of the shell, and then using some rope as a sling, carefully aligned the thermcouple with the hole in the bottom of the brick floor so that the thermcouple would penetrate the floor and up into the protective ceramic sheath.

Now I had to test the thermocouple before I went further.

I used the electric heat gun to direct heat at the ceramic sheath and the thermocouple while reading the resulting temperature on a portable reader.  It is clear that the sheath provides a delay before the sensor heats up and can register a temperature change.  Hopefully the furnace heats up so slowly that this delay will not matter.

The heat gun topped out around 350 °F (176 °C), so I took out a propane torch and continued heating up.  I heated up to 850 °F (454 °C) before I finished the test.  I am satisfied that the thermocouple will work satisfactorily as feedback for the furnace controller.

The thermocouple is important for two reasons:  1. It allows the controller to display the temperature inside the furnace, and 2. It provides feedback to the controller so that it may properly ramp temperatures up and down relative to the set point that has been entered.

Heating Chamber Top

Last night I cut the top of the heating chamber leaving a raised circular area 3/4-inch above the rectangular part of the firebrick.  I intend to cut a matching circle in the insulating ceramic fiberboard that will seal off the ceramic fiber insulation.  The ceramic fiberboard collar will be pressed down onto this raised circle of firebrick and be pressed into the steel furnace shell.

 

Last week I drilled a 3/4" hole for the protective thermocouple sheath and then I cemented the ceramic sheath in place with high temperature mortar.

Over the weekend I located the circle that on which the hole could be drilled for the thermocouple probe.  I did this by inserting a Sharpie marker into the open end of the thermocouple sheath and while holding the furnace floor suspended by ropes, lowered the floor into the furnace and twisted it in a circle to draw the circular arc on the bottom of the steel drum.  Then I carefully removed the floor and I drilled the hole through the bottom of the steel shell. 

 

I installed the thermocouple probe and mounted it to the bottom of the steel shell with self-tapping No. 6 screws.  Over the terminal end of the thermocouple, I mounted a 2"x4"x1-1/2" junction box and lid.  I fastened the junction box to the bottom of the shell with 1/4 - 20 screws and rivnuts. Inside the junction box I drilled another 1/2" hole for the hook-up wire to pass back into the furnace body.  I lined this hole with a rubber grommet to protect the wire from rubbing.  A small section of the mobile base had to be cut away with the grinder so that the junction box would fit.

 

I carefully lined up the thermocouple probe with the hole through the floor into the ceramic thermocouple sheath and lowered the floor into the shell.  Then I lowered the heating chamber into the shell and into place on the floor to test fit everything.

 

Today some 1"x1" braided ceramic wool gasket was delivered.  I intend to seal the body and lid with this material.  Also, some high temperature silicone u-section extrusion was delivered.  I am going to cover the sharp and rough edges of the steel drum with this soft material for safety.

Tomorrow evening the thermocouple hook-up wire should be delivered.  I will wire the thermocouple and test it.  Then I will re-install the furnace floor and heating chamber.

Heating Chamber Floor

This morning I laid out 8 insulating firebricks in a square that was over 18 inches per side.  I traced an 18-inch diameter circle on the top of the firebricks and then cut them into a rough circle with a regular saw.

Next I smoothed the edges with a wood rasp.  After cleaning up all of the scraps and dust, I got out the high temperature mortar and mixed some with water.  I painted the bricks with the thinned mortar to strengthen the brick and prevent it from soaking up so much mortar when I bond them together.

Later, after the mortar wash was dried, I spread the mortar onto the joints and fit the bricks together.  I clamped the bricks together tightly and left the assembly to dry overnight.

 

The firebrick floor will sit in the bottom of the steel drum on top of a 1/2-inch-thick circular piece of refractory fiberboard.

 

The space between the firebrick heating chamber and the steel shell (steel drum) will be filled with insulating ceramic wool.  The top of the wool and the top of the heating chamber firebricks will be covered with 1/2-inch-thick piece of insulating ceramic fiberboard.

A 3/4-inch hole needs to be sawn for the thermocouple well, and another hole needs to be cut and plugged with some aluminum foil.  In case a crucible fails and molten aluminum floods the bottom of the furnace, the ball of foil will melt and allow the molten charge to escape from the furnace cavity.

I had planned to cut carriage bolts to length and lock these in place with jam nuts on the steel furnace shell and use the ends of the bolts to hold the heating chamber centered in the shell, but now that I see that there is enough room around the heating chamber to stand a firebrick up, I will saw a brick down into quarters and mortar these short bricks to the floor at four corners (not against the bricks with electric terminals!) to hold the heating chamber roughly centered.

The heating chamber will not be mortared to the floor.  In the future I will have to replace the Kanthal heating element, and it will be much easier if I can lift the heating chamber out to do this.

Slow and steady progress! 

Steel Furnace Shell

This morning I put the steel drum that will enclose the furnace and its insulation up on the folding table.  

I measured 13-1/2 inches up from the bottom of the drum and made a line with silver Sharpie pen all the way around.  I secured the drum to the table top with ratchet straps. Then I got a 1-1/2 inch hole saw and cut four holes at 90° intervals tangent to this line.

Finally I used my power reciprocating saw to cut the drum apart along my line.  I used an angle grinder to clean up the cut and remove the sharp edges.

For the lid, I secured the drum to the table with a strap like before, but for the cutting I got my power sheet metal nibbler out of the toolbox.  This was the first time using it, and I was not sure it could cut steel this thick (18 gauge).  I was pleased with it.

I cut the lid right down the middle of the top rolling ring so that it would flare and hang over the straight edge of the body below.

If I had the tools, I would have hemmed the top edge of the body or rolled it into a beaded edge.  So my intention is to cover the top edge of the body shell with braided fiberglass insulating rope like that used for wood burning stove doors or oven doors.

Next steps:  cut a circular piece of ceramic insulating board and put it into the bottom of the body.  Cut and mortar insulating firebrick to make a furnace floor, this includes the sheath for the thermocouple.  Fasten the body to the mobile base with a bolt through the center.

I was going to line the lid with firebrick backed by ceramic wool insulation, but I think it will be lighter if I use insulation board and ceramic wool.  I will have to get some angle iron and pipe and weld up the pedal-actuated lid lifting mechanism.  This will be bolted to the body and to the lid.

A delivery of 10 gauge high-temp hook-up wire is coming on Thursday, so I also need to get the controls wired and tested.