Determination of electric capacity for box type furnace:

Allowing for heat loss through furnace walls, the electric capacity can be approximated from the following formula and diagram.

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Temperature raising time and thermal capacity of processed material are not included in this formula.

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Series and parallel connections are generally used with EREMA elements. The maximum number of EREMA elements in series must limited to 2, or two-step serial connection must be used. If furnace chamber temperature exceeds 1350C parallel connection must be used. Open delta connection (single-phase 3 circuits / 6 wire delta) is recommended for three-phase connection.

  1. Single-phase parallel connection(Figure11)
  2. Single-phase two series and parallel connections(Figure12)
  3. Open delta connection(Figure13)
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Electric Power Control Devices:

As described earlier, resistance of EREMA elements increase with use. To maintain a given furnace wattage the voltage applied must be increased as the resistance increases. A voltage compensating device such as a thyristor or multi-tapped transformer will be needed. Thyristor control devices with a matching transformer are most commonly used for the following reasons: (1) Durability - no moving parts. (2) Accurate temperature control is possible because of its rapid response. (3) Temperature control fully automated with PID control (proportion, integration & differentiation) are readily available. (4) Compact and weighs less than the conventional power regulators. (5) It is easy to handle, reliable, and easy to maintain. In case of small capacity application, the thyristor control device may be used without a matching transformer. Particularly, if low-voltage and high-current are applied, incoming capacity can be reduced by installing a matching transformer with three to five-step tap on the secondary side of the thyristor.

Capacity of Power Control Device:

The capacity of an EREMA electric furnace is generally indicated by equipment power in KW, the capacity of the transformer / thyristor is indicated by apparent power of KVA. The capacity of a transformer has a 10% leeway. KVA = 1.1 Pm For example, the capacity of a transformer is 33KVA if equipment capacity of an electric furnace is 30KW. 15~20% leeway is provided for a matching transformer for thyristor. KVA = (1.15~1.20)Pm

Extent of Secondary Voltage Compensation:

As outlined in the section "Life of EREMA Heating Elements" heating elements should be replaced when the resistance increases 3 times the initial resistance for Types D, E and F and increase by about 1.7 times for Types for Type SG and SGR. Consequently, the extent of compensation outlined below becomes necessary. Types D,E & F 1.73 rated voltage Types SG & SGR 1.3 rated voltage

Caution in changing D-type to E-type:

If you have changed D-type to E-type applying the same voltage, you may get a higher load (Resistance D>E). The surface load at the heating portion will go up by approximately 8% if the same power is kept. For a thyristor control, check the allowable rating of the power control equipment. If the capacity adequate to accept a lowered resistance (higher current), it is OK to change the type.. For an On-Off control, it is necessary to decrease the power by approximately 8%. For this reason, you may need to change the tap voltage. In such the cases, make sure that the electric capacity is adequate.