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There are many applications in furnace design where the designer is tasked with providing a thermally consistent flat zone within the furnace.

At the same time the furnace may require open access at either one or both of the ends for the introduction of products or atmosphere control.

Description of the process

The need for complex master slave configuration within the control system is deemed to be unnecessary and very often all that is required is a simple method of end zone tracking in order to maintain the desired length of stable temperature across the flat zone.

For many years this has been achieved using the differential thermocouple principle and the end slave zones have configured to display temperature offset in terms of their millivolt difference from the master zone.

With modern digital controllers it is possible by carefully selecting the input range and display range to achieve a very good approximation between millivolts and the required temperature scale. This allows the offsets to be controlled directly in engineering units.

Description of the solution

Master Zone

The Master zone can take any of the conventional configurations for single zone control, either operating in a simple closed loop mode from a thermocouple sited close to the furnace heaters or alternately taking a cascade input from a work piece thermocouple.

Slave end zones

It is possible to use any controller which can be configured to accept a linear input for this application.

However for low cost projects either a series 91 controller or the 2100 range of products are suitable. The controller is connected and wired in series opposition to two thermocouples, a master zone reference thermocouple and a slave zone thermocouple as shown in the attached sketch.

To determine the optimum range for the end zone controller proceed as follows

Determine the desired operating range of the furnace and thermocouple type to be used.
example. 1000’C – 1600’C type R.

Select the mid point for the operating range and determine from thermocouple reference tables the appropriate millivolts associated with two equal points close to the limits of the operating range. In selecting the points consideration should be given to the possible lag which may exist between the master and slave zones thus avoiding unwanted sensor break detection example.

Low limit of operating range = (1300’C (14.624 ) – 1000’C
(10.503)) = 300’C = 4.121mV

High limit of operating range = (1600’C (18.842) – 1300’C
(14.624)) = 300’C = 4.218 mV

The range should be configured symmetrically since this ensures that the display shows zero when both the master reference thermocouple and the slave thermocouple are at the same temperature. The figures in brackets would be used

Using the 2100 as an example the controller would be
configured as follows
Input type = mV
Input low = InP.L= -4.12
Input high = InP.H = 4.22
Value low = Val. L = -300
Value high = Val.H = 300

In use the end zones usually operate with their setpoints set to zero and this would normally ensure a good uniformity along the desired zone. However if a temperature survey of the furnace indicates poor uniformity, the end zones can be adjusted directly in engineering units to rectify the error.

Precautions and considerations

Good furnace design will assist greatly with the ability to provide correct temperature uniformity of the desired hot zone. Graded elements and the use of zone barriers is good practice.

Differential thermocouple end zones are not suitable for vertical multi-zone or short furnace with no other zone separation.

Ensure the range limits encompass the working range and accommodate possible end zone lags to avoid unwanted sensor break detection.

Errors may be observed when used in a programming mode with low setpoints outside the operating limits.

In high temperature applications where leakage in the furnace insulation causes high levels of common mode noise consider the following precautions

Use 2100 series instead of series 91 for better noise rejection and filter availability.

On 3 phase applications use separate master reference thermocouples for each slave zone.

This application provides a simple low cost solution to the requirement for slave zone control. Other methods using PDSIO and digital communications are available which overcome all the above precautions and provides complete accuracy over the tracking of all zones.

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