Furnaces - Control Systems for the Heat Treatment Process

A-Level: Physics

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Furnace sequence control
Whether furnaces are of the batch or continuous design, there is often an element of sequential control to be provided by the control system. Very often this is a secondary consideration to the process parameter control. For convenience, the sequencing can often be incorporated into the process controller itself.

The range of furnace dependent issues that will influence the sequence control requirement:

 Work piece transfer
 Furnace door actions
 Furnace hearth or hood movement
 Furnace transfer mechanisms
 Quench transfer
 Gas/Fan quench control
 Heat shield door actions
 Cooling water control

Elements for control system design explained:

 Furnace sequence control
 Temperature control and thermal profiling
 Furnace testing and audits
 Gas atmosphere control
 Vacuum atmosphere control
 Quenching systems
 Industry regulations
 Data management

Temperature control and thermal profiling
Accurate and repeatable temperature control is at the heart of most heat treatment processes. It is important that special measures are taken to control the process to the metallurgical requirements of the component. This requires accurate control for both the programming setpoints and the steady state temperature control over a wide range of temperature setpoints and furnace loadings.

To ensure repeatable and consistent performance across the widest range of furnace use, Eurotherm build into their control solutions special routines for gain scheduling, overshoot inhibition and ramp/dwell transitions. Control structures must also be incorporated to ensure that temperature control complies with the metallurgical requirements of the work piece and where necessary the control system should be designed to accommodate separate work-piece thermocouples.
Furnace testing and audits
Most thermal processing equipment is audited to ensure that parts are processed in accordance with the applicable specification.

To achieve maximum furnace loadings and equipment utilization, temperatures in a defined workload region must be within given tolerance and Temperature Uniformity Surveys (TUS) are often carried out to determine the degree of compliance.

Furnaces are grouped into different classes depending on the degree of (TUS) tolerance and workload treatments can be defined to be undertaken in furnaces of a particular accredited class.

Heat treatment furnaces are also controlled by a range of System Accuracy Tests (SATs), which define the type and accuracy requirements for control instrumentation and sensors.
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