The Pacific Thermo Coil Sag Detector System protects expensive Silicon Carbide process tubes from being damaged or destroyed when a heating element coil sags and touches the tube.
Occasionally heating element coils sag over a period of time due to repeated heating and cooling (temperature ramping). Each time the element is heated up to process temperature the coils grow longer due to thermal expansion and micro-cracks form in the oxide layer on the wire.
More oxide forms in these micro-cracks and when the element temperature is ramped back down the micro-cracks cannot close all the way which makes the heating coil wire grow slightly in length. Over a period of time the coil grows to the extent that it begins to expand into the I.D. of the element; it cannot expand outwards due to the element insulation.
Typically this expansion takes place in the top portion of the tube due to the effect of gravity on the heated coil and therefore it is referred to as â€œcoil sag.
This coil expansion (sag) takes place over a period of time and, when the sagging coil comes into contact with the process tube, if the tube is not removed from the furnace and the element replaced, the coil will embed itself into the tube, eliminating the possibility of removing the tube from the furnace. When this happens, the process tube is typically lost.
Since coil sag takes place over a period of time, when it does come into contact with the process tube there is still plenty of time (typically 2-3 process runs) to safely remove the process tube from the furnace if this contact is detected. When using a SiC process tube, when the sagging coil initially comes into contact with the tube a slight change in resistance within the heater takes place. The Pacific Thermo Coil Sag Detector System detects this change and alerts the user by audio alarm and visual light, allowing ample time to remove the process tube before any damage is done.
Each Coil Sag Detector System will monitor up to four elements and may be easily and quickly installed on any brand of horizontal diffusion furnace.