Multi-stack batch annealing
Not all stack annealers are single. Figure 2 depicts a multi-stack annealing furnace. This particular type of furnace is for high temperature (over 1000°C) annealing of electrical steel used for transformers and such specialist applications. Hydrogen is used for atmosphere gas and electrical heating is used for the furnaces.
The furnaces are laid out in eight batteries, each battery comprising four bases and each base comprising four stacks of two coils each.
For each battery of four bases, two electrical supplies are available, so that only two bases can be heated at any one time. Therefore, normally two bases would be being heated, one cooling and one being loaded or unloaded.
The furnaces, which are mobile in order to be used on any base, comprise four heating zones plus a fifth in the base floor. For each electrical supply, the five heating zones are controlled in on/off or time proportional mode. A supply control module selects and stores the analogue inputs from the appropriate bases depending on which supply is being used, and averages the two base temperature measurements for control.
Also provided by the control module are the following functions:
- The average zone temperature is calculated and used if one zone thermocouple fails
- A hydrogen permissive flag is set when all zones are above 610°C
- The power supplies’ digital inputs are checked (only one base must be selected for each supply, and only one supply must be switched to a base)
An additional control module provides health monitoring of the zones’ heating elements. The actual current flowing through each phase of each zone element is compared with the demand. If the current indicates an open circuit (high resistance) when on or a high current (welded contact) when off, a discrepancy alarm is flagged. All currents are summed to provide an instantaneous total power.
