Band heaters, as offered by Tempco and Backer-Marathon, are electric heaters that are installed around the circumference or perimeter of an object to elevate the temperature of that object through conduction (conduction and radiation for ceramic heaters). Conduction is ensured via various clamping options, which ensure that the heater maintains a uniform physical contact with the object it heats.
The construction materials of that band heater can typically be either insulated tubular, mineral insulated, mica insulated, or ceramic insulated. The construction type primarily impacts performance. The following is intended to be a generalization, not a specification or limitation of any of the manufacturers. The manufacturer’s actual specifications and installation guidelines should always be reviewed and followed.
- Tubular heaters: Typical maximum temperatures for tubular band heaters is 650 degrees F, with watt densities to approximately 35 watts/square inch, and available up to 277 volts. Construction of the tubular band heaters is significantly different from all other band heater types, with Incoloy sheath tubing protecting it from contamination and allowing it to be applied in some of the most adverse conditions. The aluminum channel that the heating tubes reside in provide excellent heat sinking for rapid heat transfer. Tubular band heaters are also available with added cooling coils, placed adjacent to the tubular heating elements. Tubular band heaters are frequently used as nozzle heaters on injection mold machinery.
- Mineral Insulated heaters: Typical maximum temperatures for mineral insulated band heaters are rated up to 1400 degrees F, with high maximum watt densities up to 20-100 watts/square inch, and a up to 480 volts. The mineral, e.g. magnesium oxide, provides both electrical insulation for the nichrome resistance heating wire and thermal conduction for maximum heat transfer. The mineral insulated thin cross-section provides faster heating with reduced cycle times, and longer rated life.
- Mica insulated heaters: Mica band heaters are up to 900-1000 degrees F with a maximum watt density of approximately 20-54 watts/square inch, and available up to 480 volts. Mica is ideal as it provides excellent electrical insulation and thermal conduction.
- Ceramic insulated heaters: Ceramic band heaters are generally rated up to 1400 degrees F, with a maximum watt density of around 20-45 watts/square inch, and available up to 480 volts. The nickel-chrome resistance heating wire is run through ceramic bricks, which provide both electrical insulation as well as thermal conduction to the interior walls of the band heater. The inthermal insulation characteristic of ceramic fibers used inbetween the ceramic bricks and the exterior band heater wall minimizes heat loss to the ambient. For example, a band heater heating an extruder barrel to 1200 degrees F, only produces a band heater external surface temperature of 400 degrees F. Ceramic band heaters are easily recognized by their discrete, multiple segments on the inside walls of the band.
Watt density (watts/square inch) is a calculation of the consumed power in watts, divided by the inner surface area of the band heater. Maximum watt density defines the maximum that a given construction of band heater should not exceed. Failure to adhere to this maximum allowable value can result in poor operating life. Maximum watt density is dependent on heater size, operating temperature, and terminations. When a customer requests a new or replacement band heater, the following considerations should be addressed:
- What is the intended application? What type of machine is the band heater for, e.g., Is it a band heater for a extruder barrel or for an extruder nozzle?
- What is the maximum operating temperature?
- What mains voltage will this band heater be powered from?
- Is it single or three-phase?
- Does the customer need single or dual voltage rating for the band heater?
- If dual voltage, what are the specified dual voltages?
- What is the specified wattage rating of the band heater?
- What is the band heater physical size? Two alternatives exist for circular band heaters:
- For absolute certainty, measure the barrel, nozzle, or object’s o.d. (or circumference, where diameter=circumference/3.14159), and measure its width.
- The other alternative, when the o.d. of the object being heated is unknown, and the band heater is not installed in its application, close the band heater to the specified gap below, and with that gap maintained, measure its i.d and its width. This latter method assumes the band heater which the customer may provide is in fact correctly sized for the application by whoever ordered it.
Band Width Gap Width
1.00” – 5.99” .250”
6.00” – 11.99” .350”
12.00” Plus .450”
Note that the “gap” specification of a band heater once fully installed is to allow for both the thermal expansion during heating (e.g., as in the expansion of the aluminum channel in a tubular band heater), as well as to ensure sufficient adjustment range to fit securely around the object to be heated. Keep in mind that existing band heaters may have been ordered intentionally with an “oversized gap” to allow room for probes or thermocouples, so the above table should be used cautiously.
- Terminations
- If wire leads, specifics on where leads exit, how exit, lead length, and protection for wire leads (e.g., fiberglass insulation, strain relief, stainless steel armored cable)
- If nuts/screw terminals; where & how located in the band area; are protective caps or a terminal box being used to prevent direct exposure to applied voltages?
- Connectors are also offered on some band heaters, depending on maximum current and temperature specification.
- Clamping;
- What type of clamping mechanism is preferred by customer to ensure band heater maintains physical contact with object to be heated during power application cycles with no air gaps? The type of clamping mechanism may be defined and limited by the physical constraints of the application.
- Since ceramic band heaters heat by both conduction and (IR) radiation, physical fit with respect to the barrel, nozzle, or object is not as critical.
- Holes & Cut-outs:
- Does the band heater require one or more holes in it to allow for passage through it by thermocouples or existing structures?
- Either a drawing should be provided with details, or thorough definition of hole and/or cut-out locations in degrees and hole diameter.
- Options:
- Depending on the application specifics, there may be additional options that should be considered, such as 1-piece, 2-piece, 4-piece, or 1-piece expandable,
- Does the customer need a thermocouple?
- Does the band heater require one or more holes in it to allow for passage through it by thermocouples or existing structures?
Excel Automation/Eagle Sensors & Controls are Authorized Distributors of several manufacturers of band heaters, including Tempco and Backer-Marathon. Contact us for technical, application, or sales support.