Wednesday, December 20, 2017

Cartridge Heater Design

All cartridge heaters are not created equal. Some are designed to provide reasonable performance at a reasonable cost. Some cartridge heaters are designed for high performance and long life, but come at higher cost. Neither design is superior or inferior when applied properly. Both designs have their place in the world.

First, it's important to understand a very basic concept when applying electric heating elements. All electric heating elements need to move the heat away from the resistance wire into the surrounding media. This is done through conduction, convection, and radiation. Preventing resistance wires from getting too hot is the goal.

The difference comes down to internal design. 

Low watt density cartridge heater
Low watt density cartridge heater.
From the outside, all cartridge heaters may look the same, but internally they're worlds apart. Lower watt density (lower cost) heaters use a helically wound resistance wire threaded through holes a ceramic bobbin. The nichrome resistance wire and bobbin are then inserted in a metal tube with loose magnesium oxide (MgO) back-filled around the bobbin. The result is a cartridge heater designed for lower operating temperatures and lower watt densities. These limitations are caused the insulating affect of the air surrounding the magnesium oxide grains and placement of the helically wound wire. Loose magnesium oxide provides poor thermal conduction and therefore prevents the resistance wire from conducting it's heat outward. The inability for the wire to shed heat is further complicated by the placement, in relationship to the sheath, of the resistance wire.
High watt density cartridge heater
High watt density cartridge heater.

High watt density cartridge heaters, capable of higher operating temperatures and long life include a different internal design, incorporating a swaging process and great care to resistance wire placement. High watt density heaters have very precisely wound nichrome resistance wire around the ceramic bobbin, and then, with exact centering and very tight tolerances, vibrate MgO into the metal tube surrounding the bobbin/wire core. Once the MgO is added, the heater goes through a swaging process that reduces the inner diameter of the metal tube. The result is compacted magnesium oxide with rock-like properties and much better thermal conduction from resistance wire to external heater sheath.

Both styles of cartridge heaters have their value and proper application. Using low watt density cartridge heaters on a simple applications like glue pots is fine, and will yield good life and performance. On the other hand, high temperature applications such as plastic injection mold machines require the higher performance and extended life that only a swaged heater will provide.

Contact BCE for more information on electric heating elements. Visit https://www.belilove.com or call (510) 274-1990.