Split-Sheath Cartridge Heaters

The efficiency and life expectancy of a cartridge heater depends mainly on the fit tolerance. Cartridge heaters are typically manufactured .003" to .005" smaller than the nominal diameter. Drilled holes are typically .001 to .003 larger than the nominal diameter. Inserting a cartridge heater with a nominal minus of .005" into a drilled hole with a nominal plus of .003" results in a fit tolerance of .008". Although a tighter fit is desirable to efficiently transfer heat and to get long heater life, a looser fit will help in installing and removing long heaters. We therefore recommend split-sheath cartridge heaters(.pdf).

A "loose" fit condition of .008" may be acceptable for:

1. low watt density at higher temperatures (example: 30 WSI @ 1400°F)
2. higher watt density at lower temperatures (example: 200 WSI @ 200°F)

 Besides the heater, you need to consider also:

If the process requires higher temperature and/or higher watt densities , consider this:

1. Instead of drilling the hole, ream it to a tighter tolerance
2. Use split sheath cartridge heaters (see below)
3. Add more heaters to reduce watt density per cartridge heater

General considerations when ordering cartridge heaters:

1. Application
2. Total watts required
3. Lead configuration

Common Issues with Cartridge Heaters

1. Heater removal: a cartridge heater will oxidize after many hours of operation and maybe difficult to remove. Consider drilling a through hole so the heater can be hammered out or choose a split sheath cartridge heater.
2. Lubricants like graphite or silicone facilitate the insertion of the cartridge heater but can carbonize at higher temperature and then the heater will be really stuck.
3. For process temperatures above 450 deg F, use high- temp lead wire and ceramic bead insulation. Or order the heater with an additional length of cold section.
4. The location of the temperature sensor is very critical. If the sensor is too far from the heater, then the surrounding area's ability to dissipate the heat generated by the heater may be exceeded.

Split Sheath Cartridge Heaters

The split sheath design allows the independent, bilateral expansion of each half of the heater outward against the walls of the surrounding bore. This maximized metal to metal contact results in greatly improved heat transfer under normal fit conditions of .005" to .007". This design assures superior heat transfer, more uniform process heat, greater efficiency and a reduced consumption of electricity (.pdf). It is also a cost effective solution for repair works to avoid re-machining an existing bore.

Split-sheath cartridge heater with hot tip from Dalton Electric

superior MgO compaction with split sheath cartridge heaters

different lead terminations offered by Dalton Electric

clip support for lead wires