Case Study #075: How to Improve the Life of a Mica Band Heater

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Case Study #075

A customer phoned our engineering department here at Thermal Corporation complaining that he was experiencing very poor life with a mica band heater– a life of only one week! He explained that the application was to heat a flange that was attached to a gate at the end of an extruder. The extruder was adding a layer of nylon to wire-in cross-head die. The flange sat between the gate and the cross-head die. (The cross-head die had its own separate heater.) The flange also had a thermocouple, extending radially, out of it. The application looked something like the image you see below.

The flange had a width of 1″ or so and a diameter of 5″. The mica band heater was our configuration 514 with a hose at 90° from the gap. This gap measured 1/2″ to accommodate the thermocouple. The heater the customer had been using had 675 watts at 240 volts. The watt density of this heater was 66.5 watt/in² and the operating temperature was 520°F.

What questions should we ask? What could we recommend?

Based on the diameter and operating temperature, the maximum watt density for this heater should be in the mid-30’s. The watt density the customer was getting was twice that (at 66.5 watts/in²). Earlier, Thermal had supplied the heater with 500 watts (49.3 watts/in²), but this heater did not last either.
By looking carefully at the flange width, we noticed that the width of the flange was about 1^1/4“. The mica band heater we had been supplying had only a 1” width. Next, we looked at the configuration of the mica band. A configuration 500, while not providing much more heated area, did provide a lower ribbon, or wire watt density. Then, we examined how much heat was required to control the process. Looking at the ammeter (a meter which displays the current being drawn by the heater), we saw that the heater was staying on for a total of 8 seconds. Then, it was turning off for only 6 seconds before turning on again. This means that the heater was on for 8/(6+8)%, or 57%, of the time. Thus, the heat required to control the process was 57% of the 675 watts, or 385 watts. To provide a safety margin, we would want to add 15% or so more heat to this minimum in the heater.

Therefore, with the wider width and the change in configuration from 514 to configuration 500, we could get 450 watts in the heater with a watt density of 33.25 watts/in². We made a sample of this configuration for the customer to try.

Additional Notes

We also noticed that in the process, following the cross-head die, was a laser sensor used to measure the diameter of the nylon coating. Because the smoke from the extrusion process got on the laser sensor lens, the customer had installed a fan. The purpose of this fan was to blow the smoke away from the lens. As it turns out, the fan was blowing directly on the heater and the gate it was attempting to heat. This increased the heat loss and the heat required from the heater. The customer agreed to place a shield, if needed, between the fan, and the heater and gate. This would most likely permit the wattage and watt density to be further reduced.

Written by Jim Dixon and Shelby Reece
Edited by Kyle Otte
Date Published: 07.19.19
Last Updated: 09.03.19