Using Mica Band Heaters Effectively
February 1, 2007
With different electrical termination styles, clamping mechanisms and the ability to accommodate holes and cutouts, mica-insulated band heaters are used successfully in many applications, particularly plastics.
Loss Prevention for Band HeatersMost band heaters do not “burn out”; instead, environmental factors create a short, cause hot spots to develop, or simply push the heater beyond its normal operating temperature. All of these factors cause a heater to fail prematurely and require replacement. Obviously, minimizing these environmental factors can reduce the frequency of replacing band heaters in your operation.
Contamination. By far, the most frequent culprit of band heater failure is contamination. Liquid plastic, hydraulic oil and moisture (often from high ambient humidity) are three main causes of premature failure from contamination. Obviously, keeping the heaters free of contaminants will reduce failure and replacement rates. In applications where liquid plastic and oil exposure are frequent and difficult to manage, the best solution may be to select a low cost band -- the heater will be replaced often, but the financial loss will be minimized. Alternatively, a band heater designed to resist contamination can be used. Keep in mind, though, that most heaters fail from lead wire damage caused by contamination -- not from contaminants finding their way inside the band -- and contaminant-resistant heaters will not prevent lead wire damage.
Poor Contact Between Heater and Barrel. The second most common cause of premature failure is poor contact between the machine barrel and the band heater. A tight fit is critical. Without it, localized hot spots can form on the band and cause the resistance wire to fail.
There are two steps you can take to ensure good contact with the machine barrel. First, make sure the machine barrel outer diameter (OD) measurement is accurate; then, order band heaters with that same measurement. The heater manufacturer will factor in a 0.25" gap, so there is no need to undersize the band’s dimensions.
Second, follow a strict installation and tightening procedure for mica, high watt density or extruded aluminum band heaters. Before installation, clean and smooth the machine barrel surface, removing any plastic residue. To install, tighten the heater snugly to the barrel using a clamping bolt torque of 10 ft/lb. Next, apply power to the heaters and allow them to reach halfway to setpoint temperature or approximately 300°F (149°C). Once at this temperature, cut the power and retighten the bands at 10 ft/lb torque. Retightening the band at an elevated temperature will account for the heater’s thermal expansion. (Remember, ceramic bands are radiant heaters and should not be tighten in this manner.)
Handling Procedures. Often, failing to understand a heater’s internal design leads to heater losses. For instance, stretching a one-piece mica band over a barrel during installation can damage the internal mica, resulting in a short circuit. Some manufacturers offer band heaters similar to mica bands that can be stretched for installation, but using installation techniques inappropriate for a specific heater type will cause problems. So, in cases where one heater design will not work -- for example, a machine setup that requires stretching the band over the end of the machine barrel, which would damage a mica band heater -- use a heater that is better suited to application such as a two-piece mica or a one-piece expandable band. A two-piece design also is a good choice when a large diameter band is required as the design minimizes the chance that air gaps will develop. Two-piece ceramic bands also can be used on large diameter barrels.
Another simple handling tip is to use two wrenches to install the wiring onto the band’s post terminations. This practice can eliminate failures because the wrench on the post’s lower nut acts as a strain relief. If this procedure is not followed, the post’s internal connection to the resistance wire can be damaged and become a weak link within the heater.
Runaway Temperatures. Heaters are extremely obedient entities. If a controller tells them to produce temperatures beyond their limitations, they will do so -- until their demise. Runaway temperature commands often occur when the thermocouple or RTD does not make solid contact with the surface measured. If the sensor becomes loose or disconnected from the surface, its readings may be hundreds of degrees lower than the process or barrel’s actual temperature. This faulty input then is received by the control device, which calls for full output from the heaters when in fact the process is already up to appropriate temperature.
Given the range of products plant maintenance and engineering personnel encounter, is not realistic for them to become experts on every piece of equipment used. This article only touches the surface of band heater design, options, performance expectations and loss prevention. Users should link up with a qualified supplier that can help design a new system or perform a design analysis on an existing system, then make recommendations to ensure the best performance for the given application. Systems arising from a good supplier/user partnership will extend equipment life and allow critical production schedules to be reached.
Sidebar: Understanding Watt Density CalculationsWhen designing a system, it is a good idea to match the total wattage applied to the actual wattage required. This practice decreases cycling frequency and temperature overshoots while increasing the heater’s life span. When possible, it also is good practice to specify strap-style clamping devices to hold the heater in place. These devices have a lower thermal expansion rate than the heater, so they can help hold the heater tightly against the barrel during operation.
The durability and performance of a heater depends on selecting the appropriate wattage. Exceeding the maximum allowable watt density for the specified heater size will result in premature heater failure. While calculating watt densities, remove the area of the cold section form the overall calculated surface area of the heater.
Watt Density (W/in2) = Wattage / (3.14 x Inner Diameter x Width) - (Cold Section)
Other points to consider while selecting watt density include:
- The watt density should be selected in accordance with the operating temperature.
- To avoid short cycling and inefficient operation, select a wattage as close to the needed capacity as possible.
- The wattage should be in accordance with the voltage and current rating of the controls.
- The safe heating pattern of the material heated, thermal conductivity and coefficient of expansion of the cylinder are other factors that should be taken into consideration while deciding wattage.