Diameter, Tolerances, Actual vs Nominal Diameter-Cartridge Heaters

If you measure the actual diameter of a cartridge heater, it won't be the same as its specified diameter (i.e., a 3/4" diameter cartridge heater won't measure 0.750"...ever). The reason for that is simple; the heater must fit a (carefully) bored hole.

The typical design process goes something like this (when heating 'blocks' of steel, aluminum or other materials):

1. Determine the total power required-Use Calculator below: Calculator for heating solids fluids and gases

2. Figure out how to distribute the power

3. Determine the optimal length and diameter to achieve even heating

Once the diameter of the cartridge heaters has been determined, the holes for the heaters should be bored to the exact diameter. For example, if you decided that 3/4" cartridge heaters are what you need, then you should use a 3/4" drill to bore the receptacle for the heaters. It is important that the drill or boring tool doesn't wander during the process. We have seen cases where heaters that should have given several years of service life lasted mere hours in holes of non-constant diameter.

As always, if you have an industrial process heating task and you're not quite sure about the best heater or how to size them, you can always give us a call at (1-877-674-9744), email us at sales@nphheaters.com , or fill out one of our contact forms.

Cartridge Heaters - Operating Temperature vs Actual Temperature of a Cartridge Heater

Cartridge heaters are designed to withstand a sheath temperature of up to 1500ºF. The recommended maximum operating temperatures for different applications are much less than that. There are many factors that have a direct effect on the lag between the actual sheath temperature of a cartridge heater and the monitored temperature of a material during the heat-up cycle. In some cases, this temperature lag is so significant that the cartridge will reach its elevated critical temperature even when the surrounding material is monitored to have a relatively lower temperature level. The most common factors that contribute to the degree of temperature difference are the following:

Thermal conductivity of the material being heated

The cartridge heater sheath watt density

The tightness of the cartridge inside the hole

The location of the temperature sensor

The allow of the cartridge sheath material

Contamination around the cartridge heater

These factors should be taken into consideration while selecting a cartridge for a specific application. One common practice is to use stainless steel cartridge sheathes for temperatures up to 1000ºF and incoloy sheathes for temperatures up to 1400ºF.

Another design consideration related to the operating temperature is the electrical termination of a cartridge. Teflon and TGGT leads have 480ºF rating while MGT wires can withstand up to 840ºF. When cartridges are used at relatively high temperatures, the terminals selected should be either different than the common high temperature lead wires or the design should be done such that the temperature around the lead wires (whether the leads are connected internally or externally to the cartridge) is maintained at a temperature level lower than the critical temperature limit of the lead wire.

Cartridge Heater Diameter Tolerance and Other Tolerances

The tables below give the maximum and minimum actual diameter of cartridge heaters both Imperial and metric sizes available with National Plastic Heater. If you're looking for something specific that isn't in our list below, you can call us and tell us what you need or you can send your cartridge heater specifications to us at sales@nphheaters.com and we will quote you within several hours.
 

Cartridge Heater Tolerance of Power, Length, and Camber:

Power tolerance, or how close the actual dissipated power at the specified voltage is to the specified power, is typically +5%/-10%. It is possible to have cartridge heaters built with tighter tolerances, but it will drive the price up. For instance, we have one customer who uses cartridge heaters as damping resistors in an RF application. For that guy, we specify the resistance and the resistance tolerance of the cartridge heater rather than its power.

Length tolerance, is a measurement of what the actual length is relative to the specified length. For our stock list cartridge heaters, length tolerance is ±0.005" per six inches of length for units up to 12" long and ±0.020" per foot for units over 12" long. For quick turn Build-To-Order cartridge heaters, length tolerance is ±2%.

Camber tolerance, is a measure of straightness of the cartridge heater. Cartridge heaters may vary from a straight line by as much as 0.010" per foot for lengths up to 12", and by as much as 0.018" per foot for lengths greater than 12".

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