Hornos eléctricos

Controllers for bright annealing furnaces

Find out how to achieve temperature uniformity of 1.5 ° C in a 3-zone metal heat treatment furnace with a multifunctional PID controller

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The 3850T multifunction PID controller in conjunction with the GRM-H Power Controller

The 3850T multifunctional controller together with the GRM-H Power Controller meet all the needs of the thermal process of bright annealing:

  • control of the preheating and heating areas;
  • automation of the system;
  • data recording for production batches – Real time clock (RTC);
  • variation management of the cold to heat resistance value of SiC heaters.

Overview

3850T controllers:  Performs PID control loops, interlocks logic functions, data recording, VNC remote access etc

GRM-H power controller: Controls non-linear Silicon Carbide (SiC) heaters; connected over primary of a transformer

Result: Achieved 1.5°C (34.7°F) temperature uniformity in a 3-heating zone furnace against expected 7.5°C (45.5°F) @ 750°C (1382°F) working temperature

The process

Bright annealing is a thermal process where the material is being heated from ambient temperature to a high temperature and then cooled at control heating and cooling rates to get desired property such as mechanical strength and surface finish. For the jewellery industry, the requirement is to get adequate strength, uniformity of colour and bright and shiny surface of the articles being treated.

Normally such annealing process is carried out in protective atmosphere to prevent oxidation of the material being heat treated. Typically, Mixture of H2 (Hydrogen) and N2 (Nitrogen) is used for bright annealing process. H2 at the high temperature inside the furnace generates a combustion that creates a reducing atmosphere inside the tunnel (muffle); in this condition the oxidation of the treated material is avoided. Similarly, N2 dilutes the level

A conveyor belt moves the articles being heat treated from preheating to heating and rapid cooling sections of oxygen in the tunnel which also prevents oxidation.

Figure 1 – Process scheme

The challenge

Furnace sections

  • Preheating Combustion heating by gas burner, controlled by PID closed loop at 750 °C (1382°F)
  • Heating and deoxidation Maintain the temperature at 750 ° C (1382°F) with electric heating and protection gas injection
  • Cooling A water tank for rapid cooling operations.

Control solution

  • Temperature control The heating – deoxidation section needs 3 numbers of PID controls to maintain temperature uniformity of 7.5°C (45.5°F)
  • Batch recording and alarm management Also need to record each batch along with process values, alarms and events
  • Logic control The plant needs to work on Auto and Manual mode and  the control system should perform plant interlocks, furnace start-stop, control of the conveyor belt and lifting devise to move the articles from furnace to cooling chamber

Electrical heating

Typical change of the resistance in SiC from cold to hot is 3:1. Also the aging of the heaters causes significant variation of the nominal resistance which is 10:1. Working voltage requirement is 70 to 200Vac. Need transformer of 400V: 70/200 Vac to drive the heating elements

Products benefits

Multifunction PID controller 3850T

  • Up to 16 PID control lops- self and auto tuning
  • Up to 32 dig I/O
  • Configurable custom graphic display
  • Pre-formatted instrumentation, recording, alarm pages
  • Extensive built in library of function blocks
  • Batch recording, encrypted file for data protection, export data in CSV, PDF formats
  • GET logic: Math and logic control
  • Communication: Ethernet Modbus TCP/IP and VNC for remote access

Power controller 1PH series GRM-H

  • Up to 120A 660VAC
  • Configurable control mode: Phase angle, burst firing, soft start
  • Configurable Control feedback: V, V², P, I, I²
  • Communication: Modbus RTU/ IO-Link

Common GF_eXpress configurator tool for both 3850T and GRM-H

Solution

3850T multifunctional controller along with GRM-H Power controllers meets the complete control requirement of this application

Pre heating & heating /deoxidation temperature control

Complete PID temperature loop control for both Pre-heating (1 zone PID loop) and heating and deoxidation (3 zones PID loop). The same control unit drives both combustion control for burners and power controller for electrical heaters using respective control algorithm. Each heating zone is controlled by three power controllers (tot. 9 single phase power controller GRM-H). Figure 2.

Figure 2 – Process control scheme

 

Result: 3850T together with GRM-H could achieve temperature uniformity of 1.5 °C (34.7°F)  of the furnace temperature against expected target of 7.5°C (45.5°F).

Furnace automation

  • 32 Dig I/O and GET logic execute plant interlocks (load–unload, doors open-close etc), drive conveyor belt, alarming control and various safety interlocks (Figure 4).
  • Control of lifting devise to immerse articles from heating section in to the water tank using GET logic.
Figure 4 – Math and Logic control

 

Batch recording and reports – Real time clock (RTC):

  • The data are displayed in graphic trend mode along with time, alarms and events.
  • All the process data are archived safely in internal memory of 3850T – can be saved either in standard (.CSV) or encrypted files and exported via USB or Ethernet. (Figure 3)
Figure 3 – Data monitoring

 

Electrical eating Sic elements:

GRM-H power controller to drive primary of transformer, providing phase angle and soft start, current feedback and VI control to compensate line voltage variation and change of nominal resistors value caused by aging.