Cartridge heaters — single-end immersion

What is a cartridge heater

A cartridge heater (also called an insertion heater or rod heater) is a relatively recent product in the tubular electric heater family. Its construction uses the same principles as standard tubular heaters, but with two key differences:

• All leads exit from one end only — hence the name "single-ended";
• Higher specific power density — the special internal design achieves 25–50 W/cm², compared to 10–12 W/cm² for standard tubular heaters.

The name says it all: a cartridge heater is inserted into a blind bore from one side, like a cartridge into a chamber. The colloquial term "pencil heater" is also common, referring to its slender shape and proportions.

Cartridge heater construction

A cartridge heater consists of several parts assembled inside a sealed cylindrical sheath:

• Resistance wire — Nichrome (NiCr), Kanthal (FeCrAl), or another high-resistivity alloy;
• Ceramic core — acts as a winding support and provides electrical insulation from the sheath;
• Powdered MgO (periclase) — fills the space between the wire and sheath, providing both heat transfer and electrical insulation.

The key to cartridge heaters is geometry. It is critical that the heater is fitted precisely to the bore diameter and has a perfectly straight cylindrical form. To achieve this, calibration is performed after assembly to H7 tolerance per ISO 286 / GOST 25347-82. Calibration is done by swaging or pressing on dedicated equipment; some heaters are additionally ground. The result is a heater with ideal dimensional accuracy in diameter and straightness — ensuring reliable installation with minimal thermal resistance.

Two designs: low-watt and high-watt

Depending on the required specific power, a cartridge heater is manufactured in one of two ways — differing in how the Nichrome or Kanthal wire is arranged.

Low-watt cartridge heaters are made by threading a coil through axial channels in the ceramic core. This is an older design. It does not provide efficient heat transfer from the coil to the heated part: the ceramic layer between the wire and the sheath acts as a thermal barrier. As a result, these heaters cannot achieve high specific power — typically up to 10–15 W/cm².

High-watt cartridge heaters have the coil uniformly wound on the outer surface of the ceramic core — as close to the sheath as possible. This design transfers heat efficiently and operates reliably at high power densities, enabling specific power values of 25–50 W/cm².

This same design also allows for uniform power distribution along the length. Uniform winding naturally leads to temperature variation along the heater, with "cold zones" at the ends. High-watt cartridge heater compensates by varying the winding pitch: more turns near the ends, fewer in the middle — producing even heating across the full active length.

In some applications, the opposite is needed — extended cold zones — for example, when the heater does not fully seat in the bore, or when only a specific section of the equipment needs to be heated. This is also achieved through winding configuration.

Cartridge heater with built-in thermocouple

For temperature control, a cartridge heater can be manufactured with a built-in thermocouple. During operation, the thermocouple keeps the heater temperature within safe limits, preventing overheating. Such heaters are used together with an external temperature controller.

The most common thermocouple types used in cartridge heater:

• Type J (iron-constantan) — up to 750 °C;
• Type K (chromel-alumel) — up to 1100 °C, the most universal;
• Type L (chromel-copel) — up to 600 °C, high sensitivity;
• RTD PT50, PT100 — for precision measurement from –200 to +600 °C.

Most modern temperature controllers are compatible with the listed thermocouple types. The specific type is selected based on operating temperature and required accuracy.

Cartridge heater dimensions

Two parameters matter most: diameter and length.

Standard diameter series from most manufacturers: 6.5 · 8 · 10 · 12.5 · 16 · 20 · 25 mm. Standard lengths: 35–300 mm in 10 mm steps.

Custom sizes are available: diameter from 3 mm to 32 mm in 0.1 mm steps, length up to 2 metres (subject to diameter — very thin long heaters are prone to deformation).

In the cartridge heater marking, size is given by the first two numbers: first — length in centimetres, second — diameter in millimetres. Example: cartridge heater 21.10.0.6N220 — cartridge heater 21 cm long, 10 mm diameter.

Power and specific power density

As with standard tubular heaters, the power of a cartridge heater primarily depends on the operating medium. Three main environments are considered for cartridge heater: air, water/oil, and metal (solid contact). The same principles as IEC 60335 / GOST 13268-88 apply.

Power is directly related to size — this is the specific power density (W/cm²). A larger heater can handle higher power. Important note: specific power is inversely proportional to service life. A cartridge heater at 50 W/cm² will last significantly less than the same heater at 15 W/cm².

In some cases, specific power can exceed standard limits thanks to excellent heat dissipation — particularly when heating massive metal parts. Under such conditions, some manufacturers produce cartridge heater with specific power up to 50 W/cm². However, such high-watt heaters require three mandatory conditions:

1. The bore must be machined to H7 tolerance (ISO 286) — minimum clearance;
2. A thermal compound must be applied between surfaces for good thermal contact;
3. Operation only with a temperature controller — the heater must never run without temperature monitoring.

In the marking, power is the third number in kilowatts. Example: cartridge heater 33.10.0.85N48 — cartridge heater rated at 0.85 kW.

Voltage

Standard heaters are manufactured for 230 V. Custom production is available for any voltage from 6 V to 600 V, AC or DC.

Important note: low voltages require high currents. At values above 20 A, lead wire overheating is possible — use a heavier gauge wire and verify the operating mode. In the marking, voltage is the fourth number in volts: cartridge heater 10.15.0.4N230 — 230 V cartridge heater.

Sheath materials

The vast majority of cartridge heaters use a stainless steel sheath in various grades — AISI 304, AISI 316, 321 (titanium-stabilised). The specific grade depends on the heating medium.

For applications above 500–600 °C, sheaths made from high-temperature alloys such as Incoloy 800/825 are used — nickel alloys with excellent corrosion resistance and thermal stability up to 1000 °C. These cartridge heater types are used in aluminium die casting and high-temperature steel processing.

Connection methods

Standard connection to the power supply uses heat-resistant insulated leads, 250 mm long. Lead insulation is selected based on temperature:

• Copper wire in silicone jacket — up to 200 °C (most industrial and commercial applications);
• Copper-nickel wire in fibreglass jacket — up to 350 °C (high-temperature work close to the heater).

The junction between the leads and the heater's internal conductor can be located outside or inside the cartridge body — as specified in the drawing. The connection is crimped with a metal ferrule.

For additional mechanical protection, the lead exit can be enclosed in a metal braid or corrugated metal conduit (D4/D6).

In environments with constant vibration or movement, additional lead clamping above the joint is recommended — using a metal bracket welded to the heater along the axis or at 90°.

Order worldwide

Custom-manufactured to drawing — from 3 days, from 1 piece. Worldwide shipping via DHL, FedEx, SDEK. 1-year warranty on all products. In business since 2010. To order, Send us a sketch, a photo of an existing heater, or describe your requirements — we will work out the specifications and provide a price within 15 minutes during business hours.

Certification: GOST 13268-88, on request — DIN, IEC, ASTM. TEN-Miass, ul. Shota Rustaveli 82a, Tashkent, Uzbekistan 100000. Phone: +998 50 090 19 24.