Temperature Control of Air Heaters

​In an air heater, air is blown over a resistance coil which is heated by a voltage applied across it. The temperature control of the air at the outlet of an air heater depends on the level of sophistication of the heater and any control systems with which it is integrated. Before discussing air heater control, first we will take a very high level look at system control.

System Control 
Generally, there are two types of process control: open-loop and closed-loop. In an open-loop system, there is no feedback loop to tell the heater to increase or decrease output; temperature can vary depending on changes in the process environment such as ambient temperature or reduced airflow. In a closed-loop control system, a process measurement is used to correct the parameter you are trying to control. For example, when trying to control temperature, a closed-loop control system will use an output temperature reading taken with a thermocouple to tell whether the output temperature is too high or too low (this is called a feedback loop). The system will then make an adjustment to the power output of the heater to shift the output temperature closer to the set-point temperature. The official definitions are given below: 

1. Open-Loop: a control system in which an input alters the output, but the output has no feedback loop and therefore no effect on the input. Open loop is used when maintaining tight temperature tolerance is not required and the operating environment is stable.
2. Closed-Loop: a control system in which an input alters the output, but there is a feedback loop which adjusts the input to maintain a desired output value. Closed loop is desirable when temperature must be maintained within tight tolerances and it is necessary to compensate for environmental changes. See Figure 1 above. 
   
   There are a number of types of closed-loop control. For temperature control the main types are: on-off control, proportional, and PID control.
   1. On-off control: On-off is the simplest type of closed-loop control. The heater is turned 100% on when the temperature falls below a set point and 100% off when the temperature is above a set point.
   2. Proportional Control: This is a more advanced form of control. The percentage of power the heater uses depends on how near the actual temperature is to the set point. This can be done by changing the ratio of the time at 100% power to time at 0% power. With this type of control there are three states:
      1. If the temperature is significantly above or below the set point the heater will be fully off or fully on
      2. If the temperature is at the set point the heater will switch on and off at a rated designed to maintain a constant temperature
      3. In a band close to the set point the ratio of time on to time off changes with distance to the set point. This prevents the overshooting that happens in pure on-off control.
      If the heater has onboard power electronics and can be controlled by a continuous signal such as 4-20mA or 0-10V, proportional control can also be accomplished by adjusting the percentage output power level of the heater rather than the time ratio of full on and full off. See Figure 2 above. 
   3. PID control: A Proportional, Integral, Derivative controller is one step more advanced; with PID control the proportional controller above is augmented with two further adjustments. With a pure proportional controller there can be an offset between the set point and the actual temperature. The “I” part of a PID controller corrects any offset. The "D" accommodates disturbances or sudden changes but in practice is often not used because of its possible negative impact on stability.
      
      

Heater Type and Impact on Control 
Air heaters can be differentiated based on the level of temperature control possible; different levels of control are possible through their design and the electronics that are included in the heater. Outlined below, in increasing level of sophistication, is largely what you will find when you begin researching air heaters: 

1. Heaters with no onboard electronics: This type of heater has power connected directly to the heating coils. The heater is either fully on or fully off. The temperature at the outlet of the heater when it is fully on depends on the volume of air flow through the heater. The greater the air flow the lower the maximum temperature.
   
   These heaters use on-off temperature control or on-off proportional control. A temperature controller is connected to a Solid State Relay (SSR) or Silicon Controlled Rectifier (SCR). Both of these components control the large voltage supplied to the heaters by means of a much smaller control voltage. The control voltage from the temperature controller acts like a very fast switch, allowing power to the heater when output temperature is too low and stopping power to the heater when output temperature is too high. See Figure 3 above. 
   
   
2. Heaters with some power electronics: Heaters are available with power electronics on board to allow for adjustment of the voltage applied across the heating coils without the need for an external controller. Often this is accomplished by means of a potentiometer. This does not control the temperature per se, as there is no feedback loop, but it allows the users to select a percentage of maximum power output. The amount of temperature fluctuation will depend largely on the operating conditions.
   
   As an alternative to the potentiometer, an external controller can be used in an open-loop or closed-loop (with a thermocouple feedback) set-up. In an open loop set-up, the controller will set the percentage of power output much the way the potentiometer does. In a closed loop set-up, the temperature reading from a thermocouple is used by the controller to change the percentage of power output thereby changing the output temperature. Because of the power electronics, the temperature controller can also be used in PID control mode.
   
   
3. Heaters with a full onboard temperature controller: The most sophisticated heaters available have a thermocouple and a temperature controller built into the heater. This makes implementing the heater straightforward for the user. In some heaters, this gives the user the choice to use the heater in open- or closed-loop mode, with on board controller/thermocouple, or by external controller/thermocouple. With the added functionality comes increased design flexibility for the user.

Thermocouple Placement 
In the situations where a thermocouple is used for feedback there are a number of rules of thumb that should be considered: 

• The thermocouple should be located where the temperature is most important
• The further the thermocouple is placed from the heater, the less accuracy and greater time delay that will be associated with the temperature control
• The thermocouple should never be removed from the output air stream

Need help picking the best temperature control system for your application? Give your technical sales representative a call. They’ll ask questions about your application and then recommend the best solution.