No matter what the type, or use of the cartridge heaters may be, the major components are almost similar, but they are altered or changed to meet size and heat requirements. A crucial factor that the designers of cartridge heaters are the most considerate about is, watt density. Watt density is the rate of heat that gets transferred from the heater’s surface.
Watt density is a deciding factor that decides how long the cartridge heater can last. With an increase in the value of the watt density, there is a proportional increase in the temperature within the cartridge heater. This significant increase in temperature can be responsible for pushing the components of the cartridge heater to its breaking point. Operating in such circumstances can reduce the life of a cartridge heater considerably.
The making of cartridge heaters
The basic components of a cartridge heater include the cartridge heater’s sheath, its ceramic core, the lead wires, resistance wires, and the insulation. These core components are at times arranged in different ways by manufacturers in order to enhance the heating capabilities and quality of their products.
In comparison, cartridge heaters that have split sheath lack a core but do have the running wire that’s packed tight in an insulating material. Split sheath cartridge heaters are new arrivals by design and they are improvisations to existent design with intention of overcoming their flaws.
Despite ceramic being the more commonly used type of core, use of magnesium is also the norm. For cartridge heaters with a core, the core often has a resistance wire wound around it. The resistance wire is made of nickel chromium.
The heating coil acts as the resistance wire for the electrical load. There are many different options available for resistance wire. The most popular and common variety is nickel chromium (NiCr), or nichrome, a type of wire that is often used in heating elements like space heaters and toasters. When speaking about the other electrical heating elements, the number of turns per inch wound around the core decides the watt density. It is heated by the current flowing through the wire, which in turn heats the sheath of the cartridge heater.
The sheath is filled with insulation to avoid the resistance wire from touching the sheath, which is often magnesium oxide (MgO). If the resistance wire is permitted to touch the sheath, it would short circuit, ground, and melt the sheath resulting in heater failure. When the sheath is being filled with magnesium oxide, the sheath is vibrated to make sure it is tightly packed. The insulation is further tightened and packed when a cartridge heater is swagged.
The two important functions of the sheath are to transfer heat to the material that is being heated and also serve as the container for the cartridge heater elements. When inserted, the sheath is in continuous contact with the material and results in efficient transfer of heat.
Different forms of alloyed metals are utilised in producing the sheath and also include Incoloy 800 and stainless steels 316 and 304. For specific applications, the sheath is corrosion and acid resistant.
The sealing process is designed in order to seal and contain the contents of the cartridge heater. Once the MgO has been tightly packed around the core and coil this part of the process is completed. A commonly used type of sealing material called epoxy allows the heater to pass different electrical tests to prevent shorts and ensure dielectric material performance.
There are large varieties of termination types that vary depending on the type of manufacturer and cartridge heater. With straight being the standard method, the leads could exit the cartridge heater in many different ways. In applications where the leads might be exposed to extreme heat or harmful chemicals, they are shielded with silicone or metal.
The different types and kinds of lead wires differ based upon the conditions under which the cartridge heater would be used. The lead wire gives the electrical connection to the cartridge heater. Since the lead wire is necessary for the efficient and smooth operation of the cartridge heater, it is selected carefully in order to match the conditions and application. Fibreglass insulated wire is often used for high temperature cartridge heaters.
The seizing of cartridge heaters creates multiple problems for operation efficiency and production. In case a cartridge heater seizes, it has to be removed by drilling it out of the hole if it gets stuck in the application, a process that could cause major delays.
The problem of seizing has led to the development of the split sheath cartridge heaters that help in expanding the hole in order to make contact with the material that is being heated. As the cartridge heater starts to cool, it retracts and can be removed easily.
Another way to avoid seizing is using an anti-seize coating that is applied as the cartridge heater is inserted. Anti-seizing coatings are a thermally conductive, insulating, and high temperature coating that improves heat transfer and minimises oxidation. It can be sprayed or brushed to form a thin layer that makes it easy to insert the cartridge heater into the hole.
Heatcon Sensors is an ISO 9001:2015 certified company and has been in operation for 30 years. Heatcon has received accreditation from certifying bodies like TUV SUD, DRDO CEMILAC, NSIC, and NABL. Production of heat sensing devices like Thermocouples and RTDs has been our forte! We also produce special sensors which find application in numerous industries including the ones specializing in aeronautics, railways, turbines, nuclear plants, and research labs.
With over three decades of expertise in temperature sensor and heater manufacturing, Heatcon Sensors provides a high level of quality, accuracy, and an unbeatable commitment to customer support.