Duct heaters are used to heat moving gas streams. They are installed in the middle of a moving air / gas stream in order to heat the air as it moves through the heater. A duct heater can also be used to radiantly heat an object at the end of a moving gas stream. The heater transfers heat to the gas, which transfers heat to the object at the end of the stream. This method of indirect heat transfer is useful for heating objects in locations where a traditional heater would not fit. Features common to duct heaters include cooling options, finned heating elements, integrated thermostats, adjustable thermostats, thermocouple wells, and explosion proof housings. Some products are rated for cryogenic use or feature a custom heater design. Others are corrosion resistant or are approved by Underwriters Laboratories (UL). 
 
Important parameters to consider when specifying duct heaters are maximum operating or sheath temperature, AC voltage phase, AC voltage required watts and watt density. Maximum operating temperature is the maximum temperature that the sheath covering the heater may reach. The maximum sheath temperature does not represent the maximum temperature that a heated substance may reach.  AC voltage phase can be either single-phase or three-phase.  AC voltage required represents the minimum AC voltage required to operate the heater. Watts represents the maximum wattage available from the heater. Watt density is the amount of wattage, per square inch, that the duct heater is capable of delivering. Watt density is calculated by dividing the available wattage by the heated area. Watt density is a good measure of the duct heater's ability to quickly heat a substance. High watt density heaters should not be used with extremely viscous materials, materials that are not circulated well, or explosive/volatile materials due to risk of fire.  Important dimensions for duct heaters include duct depth required for heater installation, heater height, and heater width.  For duct heaters it is necessary to specify the number of heating elements the device contains.