Electrical Resistance Heat Treating

Electrical Resistance

The traditional low-voltage Electrical Resistance method of heat treating uses Flexible Ceramic Heaters, connected to a control unit by use of Secondary Cables.  A thermocouple is used to monitor the material temperature and provide feedback to the control unit and temperature recording system.  The control unit can come in 3 different forms:

1)    Twin Heat Module that is powered by a welding machine

2)    Heat Treatment Console that can be powered by a 3 phase supply

3)    Heat Treatment Rig, a fully self-contained unit with a Generator

Flexible Ceramic Heaters:

These heaters have been called Chicklets, FCP’s (Flexible Ceramic Pad’s), CMH’s (Ceramic Mat Heaters) and CP’s (Ceramic Pads).  Regardless of the name used they all provide the same function.

The industry standard for North America has been an 85 Volt heater that covers approximately 120 sq in.  Each heater is designed to draw 45 amps at 85 Volts, generating 3600 watts(3.6kw) of power when connected in a standard parallel circuit.

The heaters are constructed by hand with 20’ of high resistance Nickel Chrome wire providing the body of the heater. This Ni-Cr wire is then tig welded on both ends to 9”  lower resistance to create “cold tails”. The wire is then woven through die-pressed ceramic beads, with 90 amp camlock connectors on each end.  Ceramic beads are widely available is various chemical compositions and grades of quality.  Typically the higher quality beads have a higher percentage of sintered alumina.

 

Secondary Cables:

Secondary cables often referred to as Triple Cable Sets (TCS) consist of 2 power carrying cables, and 1 type K compensating cable.

The standard construction for a 3 – heater TCS would use 2 welding cables (#2), and a shielded 20 gauge soild wire Type K compensating lead.  These cables would bundled together using a high quality filament taped to prevent separation.  Some lower quality cables have been constructed using electrical tape which tends to fall apart quickly, or with a smaller gauge wire that is susceptible to over heating. TCS can be built at any length, with 50’ and 100’ being most common.  3 – heater TCS are terminated with 300 Amp Male Camlocks and Type K Male Thermcouple Plugs on one end, and females on the other.

3-Way Splitters are used with 3 – heater TCS to allow heaters to be connected in a parallel circuit.  They are constructed of 3 x #6 welding cables approximately 4 feet long with 1 x 300 amp male camlock and 3 x 90 amp female camlocks.

Smaller single heater TCS use a #6 welding cable, and are constructed in the same manner as the 3 – heater TCS using 90 amp camlock in place of the larger ones.

Control Units

 

Twin Heat Module (THM):

This control unit consists of 2 controlled outputs designed to power a maximum of 3 heaters per output.  The THM needs 110 volts to run the control system, and a welding machine power supply with a minimum of 300 amp @ 100% Duty Cycle.  Inverter welding machines are not recommend for use with the THM where PWHT is required without a test run first.  The lower Open Circuit Voltage of Inverter type welding reduces the output of the heaters, and may not provide sufficient heat for PWHT applications.

The Twin Heat Module is available with or without a built-in Temperature Recorder. The THM is a favorite among portable welding operations to provide Welding Pre-heat for Heavy Equipment Repair.

 

Heat Treatment Console (Console):

The Heat Treatment Console control unit is typically found with 6 controlled outputs, and a 65-70kva power supply.  With a 3 Phase 600-550 / 480-440 power supply the console will step down the voltage to 85VAC and increase the available amperage to 135 amps on each output giving the console enough power to run 18 heaters at 100% duty cycle.

While consoles typically come as 6-ways, are also found built as 9-ways, 12-ways, 18-ways, and 24-ways.  Consoles can be custom build with any # of output’s, but for the use of 3-phase power and to avoid concerns with phase balancing they are normally built with outputs in multiples of 3.

 

Heat Treatment Rig (Rig):

These “Rigs” are sometimes trailer mounted, or on a 5-Ton Truck Unit. Similarly to the Console they are usually built with outputs in multiple’s of 3.  12-way, 18-way and 24-way’s are the most common configurations, but they can be custom built as well.

The primary difference is that these Rigs come with a self-contained Gen-Set.  The rigs can have transformers to step down the voltage like consoles, or some times in a less efficient manor the gen-set itself lowers the voltage by using single phase 110 Volts with a voltage reducing regulator.  Rig’s without transformers generally loose 15-25% of its total KVA potential.

Truck mounted Rig’s have control rooms for the technician’s to monitor and control the Field Heat Treatment process.  Trailer Rig’s may or may not have a control room.

The Following Equipment is often used to perform the above-mentioned applications:

 
  • Electrical Resistance
  • High Velocity Combustion
  • Induction
  • Furnaces
  • Pin Welding
  • Insulating Materials
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