What's the Difference Between Cartridge Heaters and Tubular Heaters?

A cartridge heater is an industrial heating element. It consists of an outer metal enclosure called a sheath containing resistive wiring separated from the sheath by electrical insulation. Most cartridge heaters are straight cylindrical tubes and have diameters designed to slide into holes drilled with common English or metric drill sizes. (Some manufacturers advise reaming holes after drilling for high-watt density heaters.)

Typical cartridge heater diameters range from 1 in. (English) or 20 mm (metric) at most down to 1/8 in. (English) or 4 mm (metric).  Smaller cartridge diameters are possible but the wattages are limited by the small surface areas associated with these diameters. Lengths vary by diameter but can range from 8 ft down to less than an inch.

Both cartridge heaters and tubular heaters typically contain resistive Nichrome wire heating elements that are insulated from a metal sheath by MgO insulating material. The heating element terminates at the end of the tube for an external connection point. Cartridge heaters typically use flexible electrical leads which are attached to the cartridge heaters power pins through either an (external) insulated crimp connection or an internal mechanical connection via the swaging process. The electrical connections on tubular heaters are usually through threaded studs or terminals.

A cartridge heater consists of the metal sheath, power pins, resistive wire, insulation, sealing at one end of the sheath, an electrical connection between the resistive wire and power pins, a termination of the resistive wire into wire leads, and the leads themselves.

Tubular heaters

Cartridge heaters are sometimes confused with tubular heaters. Cartridge heaters are generally cylindrical or rectangular with no bends or other geometrical shaping or curves. They are basically designed to insert in drilled blind holes. Tubular heaters, on the other hand, may be formed into complex geometries with a variety of lengths, diameters, terminations, and sheath materials. Whereas cartridge heaters typically slide into drilled holes, tubular heaters may be welded to metal surfaces or cast into metals. They often are inserted into ovens, chambers, and vessels.

The typical connection on a tubular heater is a threaded stud terminal with either a ceramic or mica insulator.

Tubular heaters are used for immersion and air heating. They often get clamped to objects such as vessels and tanks, fitted into milled groove platens, immersed directly into a liquid, or even are mounted in ducts for heating air and gas. They may also be available in single and double-ended versions.

The internal make-up of tubular heaters resembles that of cartridge heaters. They generally contain a helical Nichrome resistive wire that serves as a heating element. It is welded on each end to terminal pins. The heating element is centered in the sheath which is filled with MgO. The MgO is compacted to stabilize the coil and promote heat transfer to the sheath.

Because cartridge heaters have a simpler geometry, they can sometimes provide a higher surface-watt density than tubular heaters. The reason is that their heating elements can lay closer to the sheath surface than in tubular heaters where the resistive wire may have to accommodate bends or changes in geometry. Also as a consequence of the simple geometry, cartridge heaters may be fabricated in diameters down to about 1/8 in. where the minimum diameter for tubular heaters is usually ¼ in.