Heater Element Failure

[Philrock]

I am working on a project where we are heating intermittently moving air to 450 deg F. The air passes directly over a nichrome wire heating element. When the air is not moving the nichrome wire glows dull red. Power to the element is controlled by chopping the 120 volt 60 Hz line wave form with a triac - no feedback - just a user controlled potentiometer.

We are experiencing a very high failure rate of the heating element.

I feel that the primary cause for the failures is overheating and excessive thermal cycling of the element due to lack of heat sinking. This will be addressed by embedding the heating element in a heat exchanger with lots of heat transfer surface area.

I am also concerned that chopping the line wave form with the triac may be introducing higher order harmonics that could be stressful to the heating element, and I'm thinking about controlling power by turning on and off only at zero crossings, and sending packets of complete sine waves to the element - say six or so packets per second - the power level will be proportional to the number of full sine waves sent every sixth of a second.

Does anyone know if chopping the line wave form with a triac at points other than zero crossing is stressful to the heating element?

 

[smah]

Chopping the wave form shouldn't be a problem. This is not an uncommon thing to do. Glowing "dull red" will limit the life, but generally shouldn't be too detrimental. The heat cycling and element design may be the greater problem that is limiting the life.

Every heating element will fail eventually. How much life are you getting and what do you expect?