Temperature Controller for Furnaces

Temperature Controller for Furnaces: On-Off vs Proportional Control

A furnace temperature controller must do more than indicate temperature. It must convert the measured signal into an action that changes the heat supplied to the furnace. The following discussion explains early switching instruments, potentiometric control systems, and the mechanisms used to regulate fuel-fired and electric resistance furnaces.

1. Early On-Off Temperature Controllers

A mercury switch provided one solution to the contact problems found in early control instruments. It consisted of a sealed glass tube containing gas and a small quantity of mercury. Two electrical leads were sealed into one end of the tube. Their material was selected so that it would not form an alloy with the mercury. A compliant depressor mechanism tilted the tube in response to the galvanometer mechanism. When the end containing the leads moved downward, the mercury bridged the leads and completed the circuit.

Another design used a vane-operated controller without direct electrical switching contacts. A small, lightweight metal vane was attached to the instrument pointer and positioned between two fixed oscillator coils. Movement of the vane disturbed the oscillator circuit and caused a relay to operate.

These instruments were suitable for on-off, or two-position, control where temperature-control requirements were not particularly demanding. The switching arrangements described here did not provide continuously proportional control.

2. Why Proportional Control Is Not Always the Better Choice

Proportional control is often regarded as a more advanced method, but its suitability depends on the heating process. The source gives the example of a long-cycle annealing furnace heated from below.

At the beginning of the heating cycle, flames or hot gases enter the combustion chamber beneath the furnace at high velocity. This promotes circulation and helps produce a more uniform temperature. Once the surface of the workpiece reaches a specified temperature, the heat supply must be reduced.

With continuously reduced firing, however, the gas velocity may fall. Instead of maintaining the intended circulation, the hot gases can rise along the furnace walls, leaving the lower region inadequately heated. In the example described, two-position firing periodically restores the higher-velocity flow and can therefore provide better circulation than continuously operating at a low firing rate.

The lesson is that a furnace temperature controller must be considered together with burner performance and furnace circulation, rather than selected solely by the apparent sophistication of its control method.

3. Potentiometric Measurement and Signal Amplification

In the earlier indicating instruments, the electromotive force generated by a thermocouple was balanced indirectly by the mechanical action of the galvanometer hairspring. In a potentiometric instrument, the thermocouple voltage is instead balanced against a reference voltage supplied by a standard cell or another constant-voltage source.

The opposing voltages are applied through a modified Wheatstone-bridge arrangement. The circuit resistance is adjusted so that the bridge current becomes zero at the selected reference condition. A departure from that condition produces a very small current.

To operate the furnace-control mechanism, this small signal must be converted and amplified. In the arrangement described, a chopper converts the direct-current signal into a square-wave alternating signal. Electronic stages then amplify it sufficiently to operate a control motor.

Depending on the installation, the motor adjusts a valve, moves a hydraulic jet pipe or pneumatic flapper, or drives a sliding contact along a slide-wire resistor.

4. Rebalancing the Measuring Circuit

The source describes a stable response in which small temperature fluctuations following a disturbance gradually die away. Recovery is not instantaneous.

As the motor moves the contact along the slide-wire resistor, it changes the resistance between the reference source and the bridge. This restores electrical balance in the measuring circuit. Each measured temperature corresponds to a particular contact position, which also provides the temperature indication.

The setpoint can be changed by adjusting the linkage between the balancing motor and the slide-wire mechanism. This is comparable to changing a mechanical linkage in a liquid-level controller to alter the selected level.

Later instruments described in the source replaced the slide wire and moving contact with arrangements that avoided sliding electrical contact. Examples included paired variable multi-plate capacitors and wires whose electrical resistance changed with applied tension.

Figure 109 — Signal conversion and amplification in the temperature-measuring system. Visible labels include: thermocouple, converter or chopper, input transformer, AC supply, moving coil, permanent magnet, voltage amplifier, power amplifier, conversion stage, slide-wire resistor, and AC motor.

5. Hydraulic and Pneumatic Control of Fuel-Fired Furnaces

In proportional control of a fuel-fired furnace, the control equipment regulates the effective flow area of a valve or a group of valves. An electric motor can operate the valves, although hydraulic and pneumatic mechanisms were also commonly used in the systems described.

In a hydraulic controller, a movable jet pipe directs oil toward the dividing edge between two receiving ports. These ports connect to opposite ends of a piston cylinder. Deflecting the jet changes the distribution of oil between the ports and produces piston movement.

In a pneumatic controller, compressed air enters a chamber at a controlled rate and escapes through a small nozzle. A flapper is positioned near the nozzle opening. Moving the flapper closer to or farther from the opening changes the escaping airflow and therefore the pressure inside the chamber.

The temperature-control mechanism changes the flapper position, converting a small instrument movement into a pneumatic pressure signal.

6. Variable-Impedance Control of Electric Resistance Furnaces

An electric resistance furnace does not regulate its heat supply by changing fuel flow through a valve. In the historical arrangement shown in Figure 110, the heating current is controlled by a variable impedance.

A winding with an iron core is connected in series with the furnace heating element. Changing the magnetic condition of the core changes the winding’s impedance and therefore the current supplied to the heater.

An adjustable direct current passes through a separate control winding associated with the same core. Increasing this control current drives the core toward magnetic saturation. The effective inductive impedance decreases, allowing a greater heating current to flow. Reducing the control current increases the impedance and reduces the heat input.

This provides a means of regulating resistance furnace power through a relatively small control signal.

Figure 110 — Proportional control of an electric resistance furnace using variable impedance. The diagram identifies the heating element, thermocouple, saturable-core reactor, magnetic amplifier, automatic offset-correction unit, temperature controller, and slide-wire resistor. It also shows a 220/440 V AC heating supply and a 110 V supply for the associated control equipment; these are values from the original diagram, not installation specifications.

7. Why Fast Instruments Do Not Eliminate Thermal Lag

The instruments described could respond rapidly to a temperature change at a resistance-sensing element or thermocouple measuring junction. A fast electrical response, however, does not eliminate the physical delay between the furnace environment and the sensor.

Protective tubes surrounding the sensing element take time to heat and cool. Consequently, the measured temperature can lag behind a changing furnace temperature even when the indicating and amplifying equipment responds quickly.

The heat-supply system can introduce another delay. In pulverized-coal firing, for example, fuel takes time to travel from the feeder to the burner. A change in feeder operation therefore does not immediately produce a corresponding change in heat release.

Technical clarification: Rebalancing a measuring bridge is not the same as eliminating furnace temperature error. Pure proportional control can retain a steady-state offset; integral or reset action is used to correct that offset. These functions should not be confused when interpreting historical furnace-control diagrams. Eurotherm control principles