In this article, you will learn about the different styles of band heaters and their advantages and disadvantages.
Types of Band Heaters
|Ceramic Band Heaters|
|Tubuler and Cast-In Heaters|
Band heaters are used in processes to melt or maintain process temperature. The style of band heater and the power output (wattage) used depends on:
• The material being processed.
• The cycle time expected.
• The amount of material being processed.
This article will outline the types of heaters available and provide guidance for selecting the correct heater.
All band heaters are constructed from resistance wire, an electric insulating material, and steel sheath. Insulating materials include ceramic, mica and magnesium oxide (MgO). A critical design specification is the watt density of the heater, which is measured in watts per square inch. For example, a mica band heater that measures 3" inner diameter (ID) by 2" wide, at 300 W, is rated at: 300/(3 X 3.14 X 2) or 16 W/in2.
Mica-insulated heaters are constructed from 20 to 24 gauge rust-resistant steel, Nichrome resistance wire and plate mica. The resistance wire is wound onto the plate mica and supported by the metal sheath.
With new grades of mica, this style of heaters is capable of a maximum operating temperature of 1,200°F (649°C) and up to 90 W/in2. Because these heaters transfer energy through conduction, a tight fit between the heater and the barrel or nozzle is critical. The higher the watt density, the more critical the fit becomes. Any voids, holes or grooves in the surface being heated can cause hot spots and burn out the element.
The advantages to this type of heater are its lower cost, durability, quick response times and design flexibility. Mica heaters can be made in many shapes, including round, rectangular, cone and plate, all with different electrical connections and lockups. The disadvantage is its lower energy efficiency: Without an insulating shroud or blanket, as much as 40 percent of the wattage input into the heater is lost to the surrounding atmosphere.
Standard mica bands are flexible in design but stiff in construction. New, thinner-style mica bands are more flexible but do not have the design flexibility. Flexing standard mica bands can damage the mica sheet and cause premature failure.
Mica bands can be made in two-, three- and four-piece segments. All mica bands larger than 20" dia. should include compression springs to help take up the expansion of the element under heat. Mica heaters should be slightly undersized to ensure a tight fit on the surface being heated, and they should be retightened after the first heatup. Lastly, mica bands can be made up to 24" wide, but they tend to work better and last longer when the width is held to 3" or less.
Ceramic heaters are constructed from 24 to 26 gauge stainless or aluminized steel. The element is a helically wound Nichrome resistance wire that is supported by interlocking ceramic blocks. Depending on the application, the ceramic blocks are made from cordierite, steatite or silicon carbide material. A ceramic fiber insulating material is sandwiched between the ceramic pieces and the outer steel shell to insulate against heat loss. The insulating blanket typically is 0.25" thick but can be manufactured up to 1" thick to provide additional energy efficiency and conservation.
Ceramic band heaters have a high maximum operating temperature of 1,400°F (760°C).
Besides being more energy efficient, ceramic band heaters have a higher maximum operating temperature of 1,400°F (760°C) and typically are rated at 35 to 50 W/in2. Ceramic heaters heat through both conduction and radiation and therefore do not need as integral a fit as mica bands. They are flexible in construction and can be opened up to wrap around a barrel or die. The disadvantages are that ceramic heaters are prone to contamination and have a higher cost. Ceramic heaters are best suited for applications requiring high process temperatures and faster cycle times.
Ceramic heaters can be made in two-, three-, and four-piece segments, but given the flexibility of the band, this usually is unnecessary. Ceramic bands greater than 20" ID should include compression springs in the lockup. Ceramic heaters commonly are manufactured in widths greater than 3" to take advantage of the integral insulating blanket.
Mineral-insulated heaters are manufactured using a stainless steel sheath with a sinuated Nichrome element sandwiched between thin layers of mineral insulation. The mineral insulation is a magnesium oxide material pressed into sheet form. Once the heater is assembled, it is pressed to compact the sinuated element into the sheet, then shaped into a cylinder and finally baked to remove any organic binders from the mineral insulation. This process creates excellent heat transfer properties allowing for high operating temperatures and high watt densities.
Mineral-insulated heaters can operate up to 1,400°F (760°C) and are rated as high as 230 W/in2. With low mass and high watt densities, mineral-insulated heaters heat up and cool down very quickly. The disadvantages are that magnesium oxide is a hydroscopic material, which can reduce dielectric strength. Further, mineral-insulated heaters are stiff and limited in design variations. Lastly, mineral-insulated heaters have energy efficiencies similar to mica heaters.
Tubular band heaters are constructed using either stainless steel or Inconel tubing. A helically wound element is stretched the length of the tube. The heater is filled with magnesium oxide for insulation and compressed. The heaters then are shaped into a cylinder. Lastly, a stainless steel strap is used to tighten the heater onto the die or barrel.
Similar to mineral-insulated heaters, tubular heaters have good heat transfer characteristics and, because of the tubing, they are highly resistant to contamination. Tubular heaters can reach operating temperatures of 1,200°F (649°C) and are rated up to 100 W/in2. Watt density is limited by application and heat transfer.
The disadvantage to tubular heaters is that while the heat will radiate from the element efficiently, it radiates 360° from the surface of the element. The heater only makes tangential contact with the surface being heated. This creates a very inefficient heat transfer from the element to the surface; most of the energy is lost to the open air. To overcome this problem, the tubular heaters either can be set into an aluminum extrusion, which is formed into a cylinder, or it can be cast directly into molten aluminum. Using either of these variations creates a heater with an excellent heat profile.
The heat will be transferred uniformly across the width of the heater. Also, these heaters are rugged and have quick heatup and cooldown times. However, they are the most expensive heaters on the market and have a maximum operating temperature of 650°F (343°C). At this temperature, the aluminum starts to get soft and it will melt at 750 to 800°F (399 to 427°C). PH