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Preventing over temperature damage

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article image Eurotherm’s 2704 -- advanced control strategies.

HEATING elements, if not properly specified and controlled, can damage the product being manufactured, as well as damaging surrounding equipment due to overheating.

An all too common control strategy is to simply switch the heaters completely on to quickly heat a process to temperature.

In such circumstances, for an oven, the heat generated can quickly surpass the maximum temperature for the walls of the surrounding oven damaging them, resulting in expensive and repetitive replacement cost.

Eurotherm's high specification 2704, has the standard function blocks for advanced control strategies such as Cascade, Ratio and Override, required to such process applications.

Override control in the 2704 uses dual loop control, which allows the product to be heated quickly, while ensuring over temperature situations are prevented.

As an example, in the process of brick drying a thermal mass is heated to allow constant drying for the bricks. An aluminum block is used as the thermal mass, however, in heating the block to 700°C the heat inside the drying chamber must not rise above 1100°C, or the product would be destroyed and the oven damaged.

In control terms, the solution to this application is referred to as Override Control, whereby control allows a secondary control loop to override the main control output in order to prevent an undesirable operating condition.

Essentially two inputs are required to control a single output, depending on the state of the two inputs in relation to certain predefined conditions.

For the brick drying application, override control can be implemented with one thermocouple attached to the aluminum mass (main), and another situated close to the heating elements (override).

Control of the furnace during the heating up period is regulated by the override (heating elements) thermocouple, which provides a safeguard against overheating (>1100°C).

Control of the furnace is switched over to the thermal mass (main) thermocouple at a defined point, in this case when the temperature is nearing its target setpoint (700°C). The exact point of switchover is determined automatically by the controller, and will be dependent on the selected PID terms.

If a single loop controller is utilised to control the temperature of the aluminum, a temperature in excess of 1100°C could be reached in the oven during heating, which would destroy the product being manufactured and damage the oven.

On initial startup both PID control loops send control signals to the output heater element to provide 100 per cent output power to begin increasing the temperature of the aluminum.

During the heating event the oven will reach 1100°C well before the aluminum reaches the required 700°C. The PID control loop monitoring the oven temperature will begin to reduce output power to control the oven temperature to below 1100°C.

The PID loop in control for this override application is the loop with minimum output, thus the heater element takes over control and maintains the temperature of the oven.

This is done with a degree of safety by setting the control temperature to 1000°C and setting the maximum power range. Note that, as the thermal mass has not yet reached its required 700°C setpoint, the output from the thermal mass PID loop remains at 100 per cent.

While the oven temperature is being regulated to 1000°C, the thermal mass continues heating up. When the thermal mass temperature reaches the proportional band temperature set for the PID loop controlling the thermal mass, the output power from this loop also begins to reduce.

As the temperature of the aluminum mass approaches the required 700°C setpoint, the output power of this loop reduces to a point where it is less than the heater loop output power, and control switches over to the thermal mass PID control loop - overriding control from the heater loop.

The PID control loop with minimum output power is always in control (for Override in heating applications). Note that, as the oven temperature begins to reduce, the output power for the heater PID loop will increase to 100%. The thermal mass is then controlled as per a normal single loop control element.

Override control can be applied to any process where a vessel which can be heated or cooled to a level which may cause damage to the vessel or product.

If such a vessel is used to heat or cool an enclosing mass that is large enough to cause a significant lag in the heating or cooling time, a dual loop controller utilising Override control is the simple and easy solution, by providing all the required PID control functions and the override function in a single module.

The 2704 features a 120 x 160 pixel electro-luminescent display on which all the process information can thus be presented in a bright, easily understood graphical format, making programming and set-up and operation much simpler.

The instrument incorporates a self-correcting input circuit to maintain calibration accuracy. Configuration is achieved either via a front-panel interface or by using Eurotherm's "iTools" configuration package running under the Windows 95 or NT operating systems.

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