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.
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: