Program Control of Aluminum Melting Furnaces


Charge temperature


So far, we have only discussed the problem of maintaining a constant charge temperature. Once the problem of raising the charge surface temperature to a given value and then maintaining it constant is solved, the problem of automatically changing the temperature according to a pre-specified pattern or cycle presents no insurmountable difficulties.


Remember that the galvanometer or potentiometer in automatic temperature control has a manual adjustment component used to adjust the desired temperature (the given value or control point) to be achieved and maintained. In exactly the same way, a motor or clock, through an appropriate reducer driving a cam, can be used to drive the temperature control adjustment component according to the desired pattern, thereby causing the temperature to vary according to a given time cycle.

 

Automatic temperature control


Figure 113 Schematic diagram of an instrument that causes the furnace temperature to vary with a predetermined period over time.
1—Pyrometer pointer, 2—Contact; 3—Spring; 4—Cam that drives the contact arm; 5—Reduction gear.
Group: 6—"Temperature increase" relay; 7—"Temperature decrease" relay. In this instrument, to change the length of the time cycle, simply change the gear ratio; or to alter the relationship between temperature and time within a cycle, simply replace the cam. Alternatively, an automatic timer can be used instead of a cam. This automatically cuts off the fuel, air, or electricity supply after the specified heating time, allowing the charge to cool.

 

These gears and cams cannot be selected or manufactured arbitrarily. They must be adapted to the material and size of the material being heated. Some mild steel ingots should also be heated slowly at low temperatures. The heating rate can only be increased after reaching a certain temperature.


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