The Thermal Control Process Apparatus a common industrial process, including an air-conditioning plant, where a combination of adjustments can control temperature. These can be:

• Varying the heat energy input to the system.
• Varying the speed of a circulating fan.
• Using a variable vane to restrict the flow.
• Two temperature sensors measure the temperature of the block. 
• One sensor is in direct thermal contact with the block. 
• The apparatus has a variable-speed fan that forces air through a duct. 
• In the duct is an electrically-heated process block. 
• A balance of the heat gained from electrical heating and heat lost by convection and conduction gives a steady temperature at the block. 
• The other sensor mounts on an insulating spacer to introduce thermal inertia and variable-time constants into the control loop. The apparatus has scaled down time constants for shorter laboratory time.
• A servo-driven vane, mounted after the fan and the process block, creates a variable restriction downstream for more advanced experiments. 
• The control problem is to keep the process temperature within acceptable limits while it works under various conditions. 
• A combination of regulating the electrical energy to the heater, varying the air flow rate and rotating the vane gives the heat control.