#7000: 120 Vac Nichrome Heating Element - 1400 Â°F Max
If you are building Dave Gingery's "Lil' Bertha" foundry furnace, you will need a source of electric heating elements. BCS #7000 offers the required 120 Vac, coiled wire electric heating element for the project. These elements are suitable for a furnace that can be used to melt metals that have melting points lower than about 1400 deg F. This means typically aluminum and zinc. Anything above this will melt the element.
The question then is how much power do you need for your furnace design? Unfortunately I can't tell you. It mostly depends on how much insulation the refractory provides and how hot you are trying to run the furnace. Other variables include the wall thickness, the chamber height, the sealing between the body and the top and bottom, the room air temperature, how hot you trying to run the chamber, etc. For specific furnace design information I will refer you to books by David Gingery and others who have designed furnaces and know how much power is required.
Stretching the element provides a gap between each turn to prevent shorting of one turn to the next. I have also noted that the elements tend to shrink slightly with the first firing and cooling cycle. It's better to have them stretched a bit on the long side so they stay in place.
The connections to the elements need to be mechanically and electrically secure. Stainless steel screws, washers and nuts have proven to be a good choice. It is also important to use High Temperature Hookup Wire between the outside connection of heating element and the controller. This helps dissipate the heat from the wire so it won't damage the controller. Also standard wire insulation will melt when used at higher temperatures.
I have heard from several furnace builders that making the connection from the heating element to the hook up wire is best done OUTSIDE of the furnace chamber. Run the pig tail of the element thru the wall and make the connection outside. This helps to prevent loosening of the connection by repeated heating and cooling cycles inside the chamber. Loose connections can cause the element to burn out at the connection.
I have used my furnace for 3 years with the original elements, but have friends who have had to replace their elements after only 6 melts. I have a hunch that the secret is to treat the heaters carefully by SLOWLY increasing the furnace temperature to the operating point over a period of 45 minutes to an hour the first time. I think that this avoids damaging thermal shock. The same for cooling down afterwards. Let the furnace slowly cool, with the top on. Also don't stretch the coils after they have been fired. The heating and cooling creates a hard scale on the element surface that will fracture if the element is stressed after the first use.
Another factor in element life may lie in the installation. I was careful about stretching out the coil to fit. Try to evenly stretch the element so that the coils are spaced about the same over the length. A bunch of coils closely spaced will create hot spots that may cause a burn out. I stretch my element by holding one end in a vice and pulling the other end by wrapping several turns of the pigtail around a screwdriver and pulling. (Wear safety glasses in case something lets go....). Stretch the element slowly and make frequent measurements of the length. It's tough to compress the turns back after stretching an element too long.
I've also learned a trick to stretch or shrink an element that has been previously fired. (Firing the element makes it hard and brittle, so trying to stretch or compress it will cause breakage.) Users have told me that heating the element with a propane torch prior to stretching, compressing or bending the element will help reduce breaks.
These heating elements are NOT INSULATED. The heating elements and wiring can be "live" even though the control is in the off position. NEVER come in contact with the heating element or other wiring while the power is connected. Unplug the power before coming near any part of the circuit. This unit MUST be operated from power that is protected by a Ground Fault Interrupter (G.F.I.) unit. Consult with a licensed electrician prior to applying power. You may be shocked, burned or killed by contacting electrical circuitry that is connected to the power source.