High Temperature Vacuum Furnace

High temperature vacuum furnaces are used to manufacture silicon carbide tubes and rings, which are used in high quality bearings. Due to the high working temperature of up to 1675°C and the silicon atmosphere thermocouples can only last for a few cycles. As these thermocouples are made from Tungsten they are very expensive. To reduce the T/C wastage a pyrometer is used at higher temperatures. Normally T/C’s are used for temperatures up to 1200°C and pyrometers from 1100°C. This gives a 100°C switchover range between the T/C and the pyrometer. However, there can also be a requirement to use thermocouple control on the full cycle in certain processes which go above 1200°C.

A 2604 single loop programmer is used utilising its Toolkit blocks. Three PV inputs are used. The thermocouple is connected in parallel to two inputs, with one input (IP1) ranged 0-2000°C and the other (IP2) ranged 0-1200°C.

The Pyrometer input, which is a 4-20mA linear signal representing 1100°C to 2000°C is connected to the third PV input (IP3).

Using “Toolkit Blocks” we calculate Analogue 1 using the switchover function between IN2 and IN3. The result (AOP1) is then used as an input Analogue 2, which dependent on the state of a digital input will connect either IN1 or AOP1 to the actual process variable.

Other features include:

Alarms on PV1 at 1090°C to open the sight glass window for the Pyro, and at 1210°C to retract the T/C when using Pyrometer mode.

A 0-10Vac retransmission of PV1 to a chart recorder.

Digital inputs for run and reset.

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