240-Volt Testing

This afternoon I assembled the 4-prong 14-30P plug and connected it to my big rubber covered 10-3 power cord.  At the other end I put connectors on the wires and held them apart using small vise-grip pliers.  I connected the volt-ohm meter to the two hot lines (the green wire is the ground) and I read 243 Volts.  

 

243 Volts permits the furnace to make 3.3 kilowatts.

The big 10 gauge copper wires were not going to fit inside the ceramic terminals with the Kanthal wire doubled up, so I removed the ceramic terminals and bent the wire around a stainless steel screw, nut, and washers (like I did to the middle of the element behind Brick 5).

I went upstairs and notified Bob so that he could come down and watch it being tested, and I got the non-contact infrared thermometer out for temperature readings.

We started with the whole assembly at 84 °F.  I plugged it in and watched the clock.  Within 3 minutes the coils began to glow a dull red.  30 minutes later the coils were over 1250 °F.  The hot face was over 1000 °F, and the back of the bricks was about 140 °F.

 

The fasteners and electric connections on the back of the bricks read about 130 °F.  Safe enough.  For the final assembly, I will be using 10 gauge high-temperature hook-up wire to bring the power from the control box to the heating elements.

I am convinced that with a lid and lots of insulation, this furnace will be able to melt aluminum for hobby metal casting.

Next steps:  cut the steel drum to size, build an insulated floor inside the drum, wrap the heating chamber in ceramic wool insulation, and wire the control box.

Heating Element Installation

This morning I installed the electric resistance heating elements in my shop furnace under construction.

I stretched the elements to 62.5 inches by fastening one end in the vise and stretching beside the tape measure.  After stretching to the correct length, I bent the uncoiled ends in one-half of their length and twisted them together.  This will make a lower resistance wire to hook onto and it will hopefully be cooler than the main element inside the heating chamber.

Next, starting in the middle, at Brick 5, I pushed the twisted end through the drilled hole and began pushing the coiled element into the groove cut in the bricks to hold it.  Around, around, around.  When the element ran out, I was on Brick 2 and still had about 5 inches to go!

So, I pulled the element back out, put it back into the vise, and stretched it 5 more inches.  I did this to the other element too.

Now I put it back into the heating chamber and it worked out this time!  I pulled the other twisted end through the hole drilled in Brick 1.  Then I flipped the heating chamber over, and started again with the other element.

I used the nose of the pliers to push the elements into the groove as deeply as they would go.  Because the ramping grooves did not always meet up exactly where the bricks stand next to each other, there was less space for the coiled element in a few places.  At three of these bricks I had to use Kanthal staples to hold the element back in the groove.

On the back of Brick 5, I wound the twisted ends of both elements around a stainless steel screw and nut and tightened them together.

On the back of Brick 1, I pushed the twisted ends of the elements through some porcelain insulated terminal blocks.

I checked the resistance of each element - 9.5 Ohms and 9.8 Ohms.  Then I checked the resistance of the entire heating element assembly in series - about 18 Ohms.  I did some calculations to find out how much current and power the furnace would use - 240 Volts / 18 Ohms = 13.33 Amps.   Safe enough, I will be plugged into a 30 Amp circuit.  13.33 Amps x 240 Volts = 3,200 Watts.

The plug has not been made up for 240 Volts yet, but I wanted to test the elements anyway, so I decided that I would temporarily plug into 120 Volts.  To be sure it would be safe, I did the same calculations for 120 Volts.  120 Volts / 18 Ohms = 6.67 Amps.  6.67 Amps x 120 Volts = 800.4 Watts.

I attached the elements to an old computer plug with primary wire and wire nuts and plugged it in.  It began to slowly warm up.  I used an infrared non-contact thermometer to test the coil temperature and the temperature of the back of the bricks.

With 120 Volts, it took about 35 minutes to bring the elements to 600 °F.  The back of the bricks was about 140 °F.  With 240 Volts it will be faster, but an electric furnace will never win any races.  The heating element did not get red hot this time.

Assembling the Hot Face

Exterior of the heating chamber.

Inside the heating chamber.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Today I applied a generous amount of high-temperature mortar to the sides of the fire brick and stuck them all together.  After I had all eight brick arranged in an octagon, I fitted worm-drive clamps to the outside of the bricks and tightened everything up.

Finally, I went through the groove with a screwdriver tip to clean out mortar squeeze out that might be blocking the groove.

I would like to stretch the heating element to final length and install it in the groove tomorrow and maybe do a test heat this week.

Next I will be cutting the steel drum and building the floor in the bottom of the furnace.

Cutting The Element Groove

Today I cut the 1/4-inch wide, 1/2-inch deep spiral groove that will hold the coiled heating element in the furnace wall.

I cut each groove one at a time on my folding table with my handheld trim router and a sacrificial 1/4" dado bit.  A 2x4 was clamped to one side to use as a fence.   I ran the router on the lowest speed and the firebricks cut as easily as butter.  Very soft.

After the grooves were cut, I cleaned the dust out and then widened the groove by about 1/16" so that the coiled heating element would easily sit all the way down in the bottom.

Doing this made A LOT of dust, but it wasn't as bad as I feared it would be.  I kept a fan blowing over the work table while I was cutting and filing.

Next I will drill the 1/4" holes at the beginning and end of the spiral groove and one hole at the very middle (in the middle of Brick 5).  These holes will be where the electrical leads pass through the bricks to the power supply on the outside.  The hole in Brick 5 is used for tying the two heating elements together in series with each other.

After the holes are drilled, I am going to mix up some high temperature mortar and paint the bricks.  This will make the surface a little more durable, and it will limit how much mortar is soaked up by the bricks when I actually mortar the joints together.

Heating Element Grooves - Plan 2

Spiral heating groove plan.

I haven't cut the grooves in the firebrick yet, and I have changed my mind about how the grooves will go.  I have decided that the grooves will ramp up the sides of the firebrick in a continuous spiral instead of having ramps on the terminal brick.

In this plan, you can see that Brick 1 will be the terminal brick.  The groove will ramp up continuously through 125 inches of groove with about 1.25 inches between rows.  There will be a hold drilled at the midpoint in Brick 5.  This hole is there to tie the two elements together.  This way one of the elements can be replaced without disturbing both elements.

I am going to cut these panels out and attach them to the face of the bricks to transfer the lines.  Then I will buy a couple of sacrificial cutters and use my small laminate router to cut these grooves.

Heating Element Grooves

Next I will cut grooves to hold the coiled electric resistance heating element.  The two elements that I have are 31 inches long.  They must be stretched to twice their original length, so I will have 60 inches x 2 elements, or 120 inches of element to support in the fire brick.  Since the circumference of the chamber is 24 inches, the element will have to go around the chamber 5 times.

The insulating fire brick are very soft and cut easily with a saw.  I am going to mark the bricks with pencil and then begin cutting the grooves away with two throwaway files that I got at Harbor Freight.  One of the bricks is special - that is the terminal brick.  The electric leads will be threaded through drilled holes at the beginning and end of the spiral groove at this brick.  Also, the grooves in this brick are inclined to carry the element up to the next level.

The groove needs to be a little more than 1/4-inch wide and a little more than 1/4-inch deep.  Wire staples will help hold the heating element in the groove after it is installed.

How It Turned Out

This is how the hot face turned out after running the fire brick across the table saw set at 45°.  Now I need to cut the grooves to hold the heating element, mortar the joints, and hold the whole assembly together with steel worm drive clamps.

 

The face turned out about 3.00 - 3.125 inches.  This works out to a cavity 24 inches in circumference and 7.6 inches diameter.

I am at home in Apex, NC.