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Why do Cartridge heaters fail?
Dec 06, 2017


Why do Cartridge heaters fail?

 

1.IMPROPER FIT!

The most common cause of cartridge heater failure is an improper fit in the hole into which it is inserted. If the heater cannot dissipate the heat being generated and produced by contact with the sheath, the temperature inside the heater will continue to rise until the heater fails.

 

2. MOISTURE and/or IMPURITIES ARE PRESENT!

The MGO used in cartridge heaters is hydroscopic. Every time power to electric heaters is eliminated, an internal vacuum occurs which draws in air from the surrounding area. If moisture or impurities (oil, gas, etc.) are present, they can be drawn in to the heater, and cause a short circuit resulting in a heater failure.

 

3. WATT DENSITY IS TOO HIGH!

If the watts/square inch is excessive, the heater will not be able to dissipate the heat and the heater will fail.

 

4. INCORRECT VOLTAGE!

The wattage of any electric heater varies as the square of the voltage. If a 120 Volt heater is powered on 240 Volts, the wattage will be 4 times greater than that for the same 120 Volt heater. Under normal conditions, this may result in heater failure.

 

We have cartridge heaters, round, square , and rectangular from 1/8” to 2 3/8” diameter and metric sizes from 6mm to 20mm. We have special features for lead configuration and protection, end seals for adverse environmental situations, and sheath materials from stainless steel to titanium.

 

Obtaining maximum heat transfer and long life for high watt density heaters.

 

Fit

High watt density heaters require careful fit to insure optimum performance and long life. Hotwatt recommends that installation holes not be drilled and reamed over .002 larger than the nominal hole size required. The heaters are sized so that they never exceed .005 less than the nominal diameter and always at least .001 under the nominal diameter for a slide fit. These close fits insure rapid heat transfer from the heater and also help keep the unit as cool as possible, which contributes to long life.

 

Cycling

 

Rapid cycling of heaters from very low to very high temperatures shortens their life considerably. It is recommended therefore, that care be taken to compute the correct wattage for any given installation. Optimum wattage should result in a 50/50 off/on cycle. For very high temperature operation (over 750), off/on control might well be replaced by input voltage regulation through variable transformers or silicon rectifiers so that great temperature fluctuations in the heater wire are minimized.

 

Location of temperature control point

 

When thermostats are used, the sensing element ought not to be placed further than 1/2" away from the heater wherever possible. Location further away could conceivably cause the unit to run too hot and thereby shorten life.

 

Wattage

Minimum wattage is based on 60 watts per square inch. Units with lower watt densities may be manufactured for special conditions such as high temperature or vibration. Minimum wattage available can be determined using the following formula and the values in Table 1:

 

History of cartridge heaters

 

Cartridge Types – Usually made inside a metal tube that looks like a bullet Cartridge; a possible source of its name.  Various types listed below.

* Standard Cartridges – Original old standby made with ceramic supported wire insertion into a metal tube filled with loose fill inorganic ceramic powder (usually magnesium oxide).  Cross sections can be round from 1/8 inch diameter to 2 inches diameter; square, hexagon, triangular, etc. Cross sections are also available; length can be from < 1 inch to > 4 feet.This is usually the first choice consideration for OEM’s and accounts for an estimated 20% of total electric heaters made.  Power density is < 100 watts per square inch and temperature up to 1200 degrees F are possible.


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