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:
There are a number of types of closed-loop control, for temperature control the main types are: on-off control, proportional, and PID control.
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, are broadly the types of air heaters available:
In the situations where a thermocouple is used for feedback there are a number of rules of thumb that should be considered:
Read our article on Thermocouples in Hot Air Systems for more information.
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.
Originally published: 09/23/2014