Sheathed Heating Elements
Sheathed heaters (Sheathed Heater / Metal-Clad Heater), also known as jacketed heaters or mineral-insulated heating elements (MI Heater), are high-performance electric heating elements in which a resistive heating core wire is completely enclosed within a metal sheath and isolated from the sheath by a high-temperature inorganic insulating material. Their core feature is the "core wire–insulation–sheath" three-layer coaxial encapsulation structure, drawing on the manufacturing technology of sheathed thermocouples. This represents a major advancement in electric heating element manufacturing technology, greatly improving the element's temperature resistance, pressure resistance, corrosion resistance, and vacuum compatibility.
Based on the Joule-Lenz law: electric current passing through the internal resistive heating core wire generates Joule heat (Q=I²Rt). The heat is conducted radially through the highly thermally conductive insulating layer (magnesium oxide powder, etc.) to the inner wall of the metal sheath, and then transferred from the outer surface of the sheath to the object being heated through convection, radiation, or contact conduction. Since the core wire is positioned at the center of the sleeve, the insulation distance is uniform and reliable. The heating resistance wire is distributed in a straight line, with minimal mechanical damage during processing, uniform heat transfer, and essentially eliminating factors causing uneven heating.
Sheathed heaters have a precision three-layer coaxial structure, from inside to outside as follows:
Heating core wire: Made of nickel-chromium alloy (Cr20Ni80, suitable for ≤1000℃), iron-chromium-aluminum alloy (FeCr25Al5, suitable for ≤1200℃), or special alloys such as molybdenum disilicide (MoSi₂) and tungsten alloys (suitable for 1200–1800℃), usually wound into a spiral shape to ensure uniform heat generation
Insulating layer: High-purity magnesium oxide (MgO) or aluminum oxide (Al₂O₃) powder, compacted to a density of over 95% after repeated drawing, diameter reduction, and annealing, combining high dielectric strength and high thermal conductivity
Metal sheath: 304/316L stainless steel, Incoloy 800/840, Inconel 600/625, titanium alloy, Hastelloy, copper and copper-nickel alloys, etc., providing mechanical protection and a corrosion-resistant barrier
Sealing structures include tube reduction sealing, resistance welding, argon arc welding, active metal brazing, etc., ensuring reliable sealing.
Type Features Typical Applications
Sheathed heating rod (rigid) Diameter 3–20mm, fixed length rigid, high mechanical strength Reactor jackets, mold heating, tank and vessel heating
Sheathed heating cable (flexible MI) Diameter 1.0–6.0mm, continuously bendable, length up to 60 meters Pipe tracing, complex shape heating, long-distance pipelines
Sheathed heating plate Sheathed elements embedded or welded onto metal panels Semiconductor wafer heating plates, vacuum furnace heating panels
Precision formed parts Customized U-shape, spiral, ring shape, etc. according to customer drawings Semiconductor CVD/PVD chambers, lithography machine heating plates
Classified by sheath material and temperature resistance grade
Sheath Material Maximum Withstand Temperature Typical Scenarios
Copper sheath ≤300℃ Civil tracing, low-temperature pipe freeze protection
Copper-nickel alloy sheath ≤450℃ Long-distance pipeline tracing
304 stainless steel sheath ≤650℃ Industrial pipeline tracing, general corrosive environments
316L stainless steel sheath ≤650℃ Corrosive medium heating
Incoloy 800/825 sheath ≤800℃ High-temperature industrial pipelines, molten salt heating
Inconel 600/625 sheath 800–1300℃ High-temperature strong corrosion, vacuum environments
Special alloys (MoSi₂, etc.) 1300–2000℃+ High-temperature sintering furnaces, special metallurgy
Key Technical Parameters Executive Standards JB/T 12233-2015 "Sheathed Electric Heaters"
Diameter Range φ1mm–φ20mm (cable type), φ3mm–φ20mm (rod type)
Length Range 2–60 meters (cable type)
Operating Temperature Range 50℃–1500℃ (special up to over 2000℃)
Power Density Tracing type 5–50W/m, heating rod type 50–500W/m, high power density scenarios can reach over 20W/cm²
Insulation Resistance ≥1000MΩ at room temperature (20 times higher than conventional tubular elements), ≥10MΩ at high temperature
Pressure Resistance Can withstand external pressure up to 64MPa
Minimum Bending Radius Approximately 6 times the element diameter
Continuous Service Life 10,000–50,000 hours
Temperature Control Accuracy ±0.5℃–±1℃ achievable with PID
Fully Enclosed Outgassing Rate <10⁻⁹ Torr·L/s·cm² (meets ultra-clean process requirements)
Semiconductors and Microelectronics: Vacuum CVD/PVD reaction chamber heating, lithography machine heating plates, ion implantation post-annealing chambers, wafer heating plates
Petrochemical: Reactor jacket and coil heating, high-viscosity fluid and corrosive medium heating, pipeline tracing
New Energy: Lithium battery cathode material sintering furnaces, hydrogen electrolyzer preheating, photovoltaic cell production
Concentrated Solar Power: Molten salt constant-temperature transport heating, explosion-proof ICT4 certified, Incoloy 800 sheathed core stable operation in extreme environments
Aerospace and Nuclear Energy: Engine component heating, space simulation chamber thermal loads, nuclear fusion experimental devices, submarine nuclear power plant pressurizer
Medical and Food: High-pressure steam sterilizers, tunnel food drying, aseptic packaging heat sealing
Industrial Manufacturing: Vacuum electric furnaces, experimental electric furnaces, large drying rooms, localized high-temperature heating of tanks and vessels
Extremely wide temperature range: Can cover from -60℃ to over 2000℃, far exceeding conventional tubular heating elements
Fully enclosed structure: Extremely low outgassing rate, meeting ultra-clean and vacuum process requirements, no dust or gas generated in the furnace
High mechanical strength: Compact solid structure, can withstand 64MPa external pressure, shock-resistant and pressure-resistant
Excellent insulation performance: Room temperature insulation resistance up to 1000MΩ, 20 times higher than conventional tubular elements
Good flexibility: Cable type can be bent arbitrarily like a cable, meeting uniformity requirements in complex heating environments
Corrosion resistant: Multiple sheath materials available, suitable for acid-base, seawater, electroplating solution and other corrosive environments
Vacuum compatible: All-metal sealed structure, can be directly used in vacuum furnaces without in-furnace wiring, eliminating discharge risks
Uniform heating: Core wire distributed in a straight line, dense and uniform insulating layer, essentially eliminating localized overheating
JB/T 12233-2015: "Sheathed Electric Heaters" (core product standard, implemented October 1, 2015)
JB/T 2379-1993: Metal Tubular Heating Elements
JB/T 4088-1999: Daily-use Tubular Heating Elements
GB 4706.1-1998: General Requirements for Safety of Household and Similar Electrical Appliances
GJB 8792-2015: Specification for Sheathed Electric Heaters for Submarine Nuclear Power Plant Pressurizers (Military Standard)
🛒 Selection Key Points
Clarify operating temperature: Select the corresponding sheath material and core wire alloy based on actual working condition temperature. It is recommended to control the operating temperature within 80% of the rated temperature resistance
Determine product form: Choose flexible MI cable for straight pipeline tracing, rigid heating rods for mold/jacket heating, sheathed heating plates for panel heating
Match medium characteristics: Choose 316L/Hastelloy/titanium alloy sheath for corrosive media, 304 stainless steel is sufficient for general media
Power density control: 5–10W/cm² for liquid heating, 1–3W/cm² for oil media (to prevent coking), 1–2W/cm² for air heating (requires fins)
Vacuum/clean requirements: For ultra-clean scenarios such as semiconductors, confirm outgassing rate indicators and choose laser-welded sealed types
Installation space: Confirm bending radius, insertion length, and connection method (thread/flange/welding)
Explosion-proof requirements: For hazardous areas such as petrochemicals, confirm explosion-proof grade matching
