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Cartridge heaters have insulated resistive heating elements that are encapsulated by a metal sheath. Insulation helps maximize the heat transfer rate as well as isolate the heating element from the conductive sheath. Cylindrical cartridge heaters are most common, but the cross-section of the heater may also be square, rectangular or other custom shapes.

Cartridge heaters provide localized heat to restricted work areas that require close thermal control. Typically, the heater is installed in a die, platen or other machined part that transfers the heat. In other applications, the heater is directly immersed in a fluid medium. When utilized as an immersion heater the heater’s sheath must be compatible with the fluid medium it contacts and it is advised to seal the end of the heater.

Backer Hotwatt Inc. manufactures low-, medium- and high-wattage cartridge heaters from 0.125 to 2.375 in. diameter in standard and custom shapes. The cartridge heaters are constructed either with standard heating coils that are inserted into ceramic tubes, or for high watt densities swaged cartridge heaters are available. While these look identical, the internal construction is quite different.

Standard Cartridge Heaters

Figure 1: A standard cartridge heater. Source: Backer Hotwatt Inc.

Standard cartridge heaters incorporate nickel-chrome heating coils, which are inserted into pre-formed holes within a ceramic tube. A pure magnesium oxide (MgO) filler is added to surround and support the heating elements within the holes and aid in heat transfer. These heaters also feature a heliarc-welded end cap and insulated lead wires.

Swaged Cartridge Heaters

Figure 2: Swaged cartridge heater. Source: Backer Hotwatt

Swaged cartridge heaters have a wire-wound nickel-chrome heating element wound externally around a ceramic core. This design puts the heating element in closer proximity to the heater’s sheath. Pure MgO is added to isolate the element from the sheath. After assembly, the heater is then swaged and compressed to a specific diameter. These heaters offer an improved heat transfer rate, which allows for high watt densities. They can also operate at higher temperatures and endure greater shock and vibration forces without failure than standard types.

Causes of Premature Device Failure

Premature failure of cartridge heaters occurs either when the heat generated in the internal resistance wire is not efficiently dissipated or when moisture or a foreign substance seeps inside the protective sheath, creating a short circuit. Inadequate heat dissipation results in an elevated internal temperature, which can rapidly deteriorate the heating element. This can occur when machined tolerances are outside of an accepted range, if the watt density is too high or when powered by an incorrect supply voltage.

Improper Fit

Improper fit is the most common cause of premature cartridge heater failure. The bore hole within which they are inserted must be held to tight tolerances. High watt density cartridge heaters are even more sensitive as the internal temperature of the heater can rise rapidly and jeopardize the life of the heating element. To ensure adequate thermal dissipation, the recommended hole diameter is no more than 0.002 in. greater than the nominal diameter of the heater.

Typical allowable watt densities for swaged cartridge heaters are based on fit and operating temperature.

Figure 3: Maximum watt density for Superwatt swaged cartridge heaters with various increasing temperatures and hole tolerances. Source: Backer Hotwatt

Moisture or Ingress

Even when cartridge heaters feature heliarc-welded end caps, they are prone to failure when the air surrounding the heater contains impurities or has a high moisture content and the heater’s leads are not adequately sealed. This is due to the nature of MgO insulation: it is a highly hydroscopic white powdered mineral, and when the heater undergoes thermal cycling a vacuum is created, drawing in moisture or other contaminants such as oil, which can result in internal shorting.

Incorrect Watt Density

The watt density of the heater is vital to its performance. This is a measure of thermal power density and the higher the watt density, the greater the needs are for thermal dissipation. High watt densities can lead to premature failure when thermal dissipation needs are not met, as the internal temperature of the heater will exceed the limits of the resistive heating element.

Incorrect Supply Voltage

In a resistive circuit, since the resistance is fixed, when the voltage is doubled, the current doubles as well as quadrupling the wattage output. Incorrectly specifying the supply voltage can lead to premature heater failure, as voltage has a dramatic effect on the wattage and the amount of heat generated as can be seen in the following formula.

Backer Hotwatt has over 65 years of experience designing resistive heating elements. They provide a wide range of termination options and sheath materials with the ability to supply sealed units. They have extensive knowledge in designing heaters as an original equipment manufacturer (OEM) and can design a thermal solution that best suits the application.

Watt Density Selection and Thermal Cycling

Suggested watt density is based on several factors including the fluid medium to be heated, the desired operating temperature and process variables such as flow rate. In general, operating temperature is inversely related to the suggested watt density. Additional considerations are taken when heating a fluid to a point near its boiling point, as phase changes drastically reduce its heat transfer capabilities. Highly viscous fluids or fluids that tend to coke or carbonize also require a low watt density. Highly corrosive solutions also need a low watt density, as the increased watt density increases the potential for corrosion, drastically reducing the life of the heater’s sheath.

Figure 4: Table of suggested watt density based on the material being heated and the maximum operating temperature. Source: Backer Hotwatt

Selecting an incorrect watt density can have adverse effects to the response of a thermal system, but it is not the only factor to consider. There are four basic elements to any thermal system, including the thermal load, the heat source, the heat transfer device and the temperature controller.

Thermal power delivered by a heating element is a function of wattage, and a correctly sized heating element will provide an ideal thermal response without rapid cycling of the element. The optimal wattage results in a 50/50 off/on cycle, which prevents or minimizes hunting or temperature overshooting. For more precise thermal control, variable voltage devices or solid-state controllers may be used.

Additional considerations should be given to sensing element placement. In applications where the thermal load is relatively steady, the sensing element should be located closer to the heat source, while highly variable thermal demands are best identified by placing the sensing element near the work area.

Device selection is equally as important as the physical make-up of the system. Proper heat and load configurations, heat transfer medium or devices and feedback loops are all essential in the design of a reliable and effective thermal system.

Close Fit Tolerances

Close fit tolerances are the single most important factor affecting the heat transfer rate and the life of the heating element. A close fit allows users to achieve high efficiencies while keeping the heating element as cool as possible. Additional care should be taken when finishing or reaming drilled or cast holes to ensure a smooth surface that increases contact surface area between the heat transfer device and the cartridge heater.

To calculate the expected heater life, the heat sink operating temperature, the temperature drop across hole fit and the internal temperature are summed in order to estimate the internal wire temperature.

Empirical Guideline for Cartridge Heater Life

Figure 5: Expected heater life as a function of internal wire temperature. Source: Backer Hotwatt

Figure 6: Temperature differential across fit as a function of watt density. Source: Backer Hotwatt

Figure 7: Internal temperature differential for standard and superwatt cartridge heaters as a function of watt density. Source: Backer Hotwatt

History of Backer Hotwatt Inc.

Established in 1952, Backer Hotwatt Inc. began manufacturing open coil heater elements for the appliance industry. The company has since evolved to address the needs of OEMs in industrial, medical, commercial, packaging, instrumentation, aviation, transportation, refrigeration and air conditioning and military fields.

Their product range has also expanded. While they specialize in low-, medium- and high-wattage cartridge heaters, the company also manufactures air process, immersion, strip, tubular, band, foil, flexible glasrope, crankcase and ceramic heaters and compatible accessory items with the ability to provide complete heater solutions.

As of December 2016, Backer Hotwatt Inc. belongs to the NIBE Element North American business group. Gerteric Lindquist, president and CEO of NIBE Industrier AB stated: “The acquisition of Hotwatt adds another well-known brand to our North American Element business providing cost synergies and commercial advantages to the NIBE Group. Their market position and notable customer base will provide yet another platform for profitable growth.”

Backer Hotwatt Inc. is a leading manufacturer of resistance heating elements with over 65 years of experience and is located in Danvers, Massachusetts.

Conclusion

As OEMs address critical design criteria, thermal demands have often been an afterthought. Ignoring thermal requirements can lead to limitations and obstacles that inhibit device functionality and manufacturability. Backer Hotwatt Inc. is an industry leader dedicated to the design and manufacture of resistive heating elements for OEMs. They have a wealth of industry knowledge and by engaging with Backer Hotwatt early on in the design phase, OEMs will benefit from the company’s wealth of industry knowledge, with well-defined thermal device requirements and a complete heater solution developed to best suit their needs.