BCS

Electric Heating Elements

&

 Power Controller

#7000 Nichrome Heating Element - 1450 °F Max

If you are building Dave Gingery's "Lil' Bertha" foundry furnace, you will need a source of electric heating elements. BCS offers the required 120 Vac, coiled wire electric heating elements 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.

Electrical:  

A #7000 element will produce about 1,800 watts and draw about 15 amps at 120 Vac. 

A single #7000 element can be run on 120 Vac and controlled using a single #7002 type controller.  

Two #7000 (order the double pack #7001) elements can be connected in series across a 240 Vac supply, and will draw about 15 amps and deliver 3,600 Watts. A single #7002 type controller will control this configuration.

Two #7000 elements can also be connected to two separate #7002 type controllers and wired to two separate 120 Vac sources.  Each of the 120 Vac sources will draw approximately 15 amps.  For this reason each source must come from a different  20 amp circuit breaker.

Mechanical: 

The coil outer diameter is 7/32 inches, and the un-stretched length is 16 inches, not including the 2 1/2 inch pigtails on each end.   This element must be stretched to a minimum length of 23 inches in the coiled section.  The maximum stretched length is 64 inches in the coiled section.  This allows a gap between each turn of the element 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 6 inches or so of high temperature appliance wire between the outside connection of the furnace wall feed thru (long stainless steel screw works well here) and the controller.  This helps dissipate the heat from the wire so it won't damage the controller.

 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 each 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.

#7101 Kanthal Heating Element - 2300 °F Max

If you are going to be working with Brass or Bronze in addition to aluminum you may want to consider using these high temperature Kantal Elements.  The element material is rated to 2300 °F.

Electrical: 

One #7101 element will draw 13 amps at 120 Vac and will deliver 1550 Watts.

Two #7101 elements in series will draw 13 amps at 240 Vac and deliver 3100 Watts. 

Mechanical:  

As shipped the #7101 element is .270 in.O.D. x 34 in. long. It is made from .058 dia (Approx 15 AWG) Kanthal A-1 material. Each element MUST be stretched to a minimum of 60 inches, and should not be stretched more than about 140 inches.

Because 2 elements are required for 240 V operation, the total length for the two elements will be from 120 inches to 280 inches.

Thermal Considerations:

These elements are are capable of operating continuously at 2300 Deg. F.  A single element will deliver 1550 Watts and 2 elements will deliver 3100 Watts.  The manufacturers design calls for two of these elements to be used in series across 240 Vac for a furnace with an inner diameter of between 9 inches and 21 inches.  

The question then is how much power do you need for your furnace design?  Can you get by with one element running from 120 Vac and delivering 1550 Watts?  Or will you need 2 elements running from 240 Vac delivering 3100 Watts?  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.  In general 2 elements running from 240 Vac will be required to heat a small furnace to 2300 F.   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.

#7002 Power Controller

The #7002 power controller can be used with either 120 Vac or 240 Vac power connections, provided the load current is limited to 15 Amps by the heater elements. This means that for a 120 Volt connection you will need one control per element. On a 220 Volt connection you can use one control for 2 elements hooked up in series.

These controls are an "infinite range" type.  This means that the unit will provide power to the element that ranges from being full off to full on. Between the extremes the unit controls the power by cycling the voltage to the element on and off.  For lower settings the unit powers the element on for a short time, followed by a long off period.  For higher settings the unit powers the element on for a long time, followed by a short off period.

#7004 Knob

The knob kit contains the proper knob, adaptor and overlay for the #7002 controller. Black knob with silver overlay.

WARNING:

    The heating elements and controller 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.  

Heating Element Safety and Tech Data

Control Safety and Tech Data

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Revised: July 10, 2007