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In Industry, precise control of process temperature plays a crucial role in achieving final product quality and sustaining productivity. The methods most commonly used are

- Manual Temperature Control

- Automatic Temperature Control

    • On-off temperature control

    • PID temperature control

Any control system performs four essential functions - measurement, comparison, computation and correction. In a manual temperature control system all these functions need to be performed by the operator. However, manual operation is prone to human error. Lapses in accuracy and continuity can affect the overall efficiency of the process.

What is an automatic temperature control system?

In automatic temperature control systems, the four essential functions of the system, i.e. measurement, comparison, computation and correction are done with help of sensors, controllers and control valve with actuator and positioner .

Typical temperature control loop

temperature control loop

In a typical temperature control loop, the sensor sends a signal to the controller. Based on this signal, the controller, which may take signals from more than one sensor, determines whether a change is required in the manipulated variable. It then commands the actuator to change the position of the valve to either open more or close more - depending on the requirement.

Why automatic temperature control?

As explained above, manual operation can result in reduced overall efficiency of the temperature control system.

On/off temperature control: On/off control will always lead to temperature overshoot and higher fuel consumption; thereby higher utility cost

Typical On-Off temperature control loop

In this type of a control system the solenoid valve performs the key function of regulating the supply to the actuator. The air supply is either 100% ON or 100% OFF.

Application for On-Off temperature control

Applications On/Off
Fast response/ quick opening required
Constant heat load

PID temperature control: PID control is the most accurate control wherein the exact set process value (set temperature) is achieved, as opposed to on/off control where temperature fluctuation within a band occurs.

In this type of a control system, the positioner mounted on the control valve performs the key function of regulating the air supply to the actuator. The air supply is modulated as the per the current signal received from the PID controller. An inbuilt E/P converter ensures modulated air supply to the actuator.

Applications for PID temperature control

Applications PID
Where accurate temperature control is essential; overshoot is not tolerable
Variable set temperature
Variable heat load
When product quality is dependent on temperature accuracy
When safety of operation is temperature sensitive

Proportional Control

This is a most basic continuous control mode. This controls the opening & closing of control valve depending upon process requirement. But, this control will always come with an error/offset, because this logic acts only when it senses the deviation.

Integral Action

The function of the “I” action is to eliminate the offset (sustained deviation) by continuously moving the proportional band up and down as necessary (by altering the set point)

Integral action is also known as automatic reset.

Derivative Action

With a P+I control system, overshoot is likely to occur on start up.

Derivative action provides extra push for quick return to set value. Hence low overshoot during start up.

Thus, PID based automatic temperature control system improves

Energy Efficiency

⚬No steam wastage due to better control of process temperature

⚬ Reduced fuel consumption, thereby reduced operating cost

Product Quality

⚬ Accurate temperature control within desired limits ensures optimum product quality and reduction in rejections / rework for temperature sensitive processes

⚬ Enables consistency in production and product quality

⚬ Leads to savings in raw material cost involved

Operational Safety

⚬ Ensures safety and there by eliminates risk during processing

⚬ Overheating/ cooling due to inaccurate temperature control may lead to unwarranted / dangerous process conditions, this situation can be eliminated / controlled

Control Mode Typical System Responses Advantages/Disadvantages
On/Off

•Inexpensive

• Simple

• Operating differential can be outisde of process requirements

Propotional (P)

•Simple and stable

• Easy to set up

• Fairly high initial deviation(unless a large P band is chosen), then sustained offset

• Offset occurs

Propotional plus Integral (P+I)

• No sustained offset

•Increase in propotional band usually required to overcome instability

• Possible increased overshoot

Control Mode Typical System Responses Advantages/Disadvantages
On/Off

•Stable

• Rapid response to chnages

• Some offset

Propotional plus Integral (P+I)

• Will give best control,no offset & minimal overshoot

•More complex to set up manually but most electronic controllers have an 'auto tune' facility

• More expensive where pneumatic controllers are concerned