Construction Of Cartridge Heaters and Features

Construction of Cartridge Heaters-Metric and Imperial Insertion Heaters:

The cartridge heater may be divided in 7 main parts:

Heating Coil
Insulation
Sheath
Sealing
Termination
Lead Wire Type
Watt Density

Heating Coil- The heating coil is the actual resistance which is where the electrical load occurs. The most common type of metal alloy used for this purpose is a Nickel-Chromium mixture, also known as Nichrome the Nichrome wire is wound around a ceramic core, and the number of spirals per inch vary according to the requested watt density. Potential from an alternating current source, which can either be 2 phase or 3 phase, flows through the coiled Nichrome wire, heating up the wire, which in turn, heats the cartridge heater sheath.

Insulation- is used to prevent the Nichrome coil to contact the sheath. If such event happened, it would ground the resistance and produce a catastrophic short-circuit, resulting in a melted sheath and a major equipment failure. To prevent the coil from touching the sheath, the coil is inserted into the sheath, and immediately filled with Magnesium Oxide (MgO). To ensure the MgO fills the empty space between the sheath and the coil, the cartridge heater is vibratory filled.

Sheath- The sheath is the part of the cartridge heater which makes contact with the material or substance to be heated. Several metal alloys are used, depending on the type of application, such as highly acidic or corrosive environments. The most common types of sheaths are 304 Stainless Steel, 316 Stainless Steel, and Incoloy 800. Incoloy has the highest temperature rating, it is considered a Superalloy.

Sealing- After the cartridge heater has been filled with MgO, a seal is applied to the open end of the cartridge heater (where the Nicrome coil was introduced), this prevents the coil and the MgO from coming out. Also, it prevents contamination, such as plastic debris, air, or moisture, to go inside the heater.

Termination- Since cartridge heaters are installed in a wide variety of machines, manufacturers must design the heaters to meet certain clearances. The cartridge heaters might be terminated with the leads coming out straight, or in a right angle. Also, manufacturer must be careful that the leads are not exposed to temperatures higher than the maximum rating for the lead wire. In order to prevent lead wire damage from temperature, movement or contamination, the lead wire can be protected with metal conduit or braided metal.

Lead Wire Type- Depending on the clearance and the design of the machine where the cartridge heater will be inserted, the type of wire used will vary. Fiberglass is the commonly used for cartridge heaters and other high temperature applications, such as automotive wiring harnesses and industrial equipment. Other variants used are silicone impregnated fiberglass and silicone rubber.

Construction and Features:

Up to 200 w/in2
Durable construction
Efficient heat transfer
Various special features
CSA UL and CE certified

In high-density cartridge heaters the resistance wire loops are positioned as close as possible to the outside shell. Because the MgO powder insulation round these loops is compacted by swaging and transformed into a very hard medium, heat transmission is very efficient. NPH cartridges can have up to 200 w/in2 watt densities. Cartridges are usually supplied with lead wires. The attachment of the leads to the central pins is done internally, in a 3/8" cold section. In excessively hot applications the length of this cold section could be increased. To facilitate installation and avoid excessive air pockets, cartridges are made 0.004" less than the nominal size of the receptacle hole with a tolerance of +/- 0.002". NPH cartridges can be dual-voltage, three phase, and/or be supplied with a ground terminal. With ten different termination styles, mounting attachments and various optional features, NPH high-density cartridges are widely used in numerous high temperature applications.

Cartridge Heaters Metric–Description

A cartridge heater is a device that is usually tubular or square and is inserted into drilled holes of metal blocks for heating. Metric & Imperial high density cartridge heaters are round tubular heaters with electrical terminations on one side. These reliable heaters are made to withstand tough industrial usage. With a tolerance of +/-0.002" on its outside diameter to secure a tight fit inside receptacle holes and rock hard compaction of MgO insulation through swaging, these heaters can attain 1500ºF sheath temperature. High density cartridge heaters are available with various termination styles and mounting attachments. In high-density cartridge heaters the resistance wire loops are positioned as close as possible to the outside shell. Because the MgO powder insulation around these loops is compacted by swaging and transformed into a very hard medium, heat transmission is very efficient. Metric and Imperial cartridge heaters can have up to 200 w/in watt densities. Cartridge heaters are usually supplied with lead wires. The attachment of the leads to the central pins is done internally, in a 3/8” cold section. In excessively hot applications the length of this cold section could be increased. To facilitate installation and avoid excessive air pockets, cartridge heaters are made 0.004” less than the nominal size of the receptacle hole with a tolerance of +/- 0.002”. Metric and Imperial cartridges can be dual-voltage, three-phase, and/or be supplied with a ground terminal. With ten different termination styles, mounting attachments and various optional feature, our high density cartridge heaters are widely used in numerous high temperature applications.

Cartridge Heaters Selection Guide

The key questions which need to be answered before selecting a cartridge heater for your application are:

How much wattage is required?
What voltage will be used?
What is the heated length? What is the overall length?
How long are the heater leads or what is the lead length required?
What is the outside diameter of the cartridge heater?
What is the ambient temperature the cartridge heater will see?
What is the maximum temperature required from the heater?
How quickly do you want to reach your operating temperature or set point in hour/minutes?
What is the type and weight (in lbs.) of the material you want to heat?
Special Modifications