What is a Cartridge Heater and how does it work?
A cartridge heater is a tube-shaped, industrial heating element that can be inserted into drilled holes. Cartridge heaters provide localized and precise heating and are commonly used in the heating process industry. Typically, cartridge heaters are used to heat a metal block from the inside and can be custom manufactured to a specific watt density based on the requirements of the application.
Cartridge heaters are most frequently used for heating metal parts by insertion into drilled holes. For easy installation, the heaters are made slightly undersize relative to their nominal diameter.
A cartridge heater consists of resistance coil wound around a ceramic core that is surround by dielectric and encased in a metal sheath. Powered heat transferred through the coil to the sheath causes the sheath to heat up. This heat is then transferred to the inside metal part requiring heat.
To fit a cartridge heater in a low or medium temperature application (600°F or less), general purpose drills are usually adequate for drilling holes. Holes can be drilled .003” to .008” over the nominal size of the drill, resulting in fits of .009” to .014.” While this fit is slightly looser than would permit optimal heat transfer, it aids in the installation and removal of the cartridge heaters, especially those with long sheaths. At high watt densities, a close fit is much more important. The holes should be drilled and reamed rather than just drilled with a general purpose bit. With a tighter fit, the heater will run cooler and have a longer life expectancy.
Cartridge heaters can operate at low, medium, and high watt densities. They are designed to withstand a working temperature of up to 1400°F. However, the optimal operating temperature will depend on the application for which you are using the cartridge heater.
It’s also important to note that there are multiple factors, such as the cartridge heater watt density, the tightness of the cartridge inside the hole, and the thermal conductivity of the material being heated, that can impact the actual temperature of an industrial heater and the monitored temperature of a material during the heating cycle. For high temperature applications, such as those above 1000°F, incoloy sheathes are recommended for maximum heat transfer and durability.
It is also important to consider the electrical termination of a cartridge in regard to the operating temperature. When cartridge heaters are used at relatively high temperatures, the electric terminals should either be different than the common high temperature lead wires or the cartridge should be designed in a manner that the temperature around the lead wires is maintained at a lower temperature than the temperature limit of the lead wire.
Cartridge heaters are most frequently used for heating dies, platens, molds, and other metal parts by insertion into drilled holes. They can also be used in liquid immersion applications. Below are some examples of specific applications:
Heating gases and liquids
Laminating presses
Plastic molding
Hot runner molds
Medical equipment
Scientific equipment
Hot stamping
Semi-conductor
more
Temperature Controllers and Sensors for Cartridge Heaters
The sensor for the temperature control is also an important factor and should be placed between the working surface of the part and the heaters. The temperature of the part approximately 1/2" away from the heaters is used in selecting maximum allowable Watt density from the graph. Control of power is an important consideration in high Watt density applications. On-off control is frequently utilized, but it can cause wide excursions in the temperature of the heater and working parts.
Thyristor power controls are valuable in extending the life of high Watt density heaters, since they effectively eliminate on-off cycling. There are a variety of temperature controllers and sensors one can use depending on the application. One of the more popular sensor types for cartride heater applications are the surface mount temperature sensors. Thermocouple, RTD or Thermistors are available with an adhesive backing or the ability to be cemented to the surface being heated.
There also bolt on and magnetic surface mount type temperature sensors available. Digital temperature controllers come in many different sizes with many output and input choices. Thermcouple and RTD inputs are the most popular with a dc pulse output. DC pulse ouputs allow the user to go to a larger relay to switch the heater load and use proportional control versus on/off control which can shorten the heater life.
