Controlling the Roasting (Burn-off) Oven

(Click for more...) Most electric motor service centers use a roasting or burn-off oven in the process of stripping laminations. I prefer the term “roasting” over “burn-off”, because we’re removing varnish and insulation by roasting these materials and eliminate or at least limit open flame. When winding-system combustibles (e.g. varnish, slot liners, top sticks, lead-wire insulation etc.) reach their ignition point and burst into flames, which they will, the water suppression system adds a mist of water to the combustion chamber, cooling and extinguishing open flames. How often the water is injected depends upon combustion material re-igniting. Limiting outside air from entering the combustion chamber limits re-ignition and excessive cooling of the oven.

Three ingredients are generally necessary to have a fire:

1. Heat;

2. Fuel and;

3. Air (oxygen).

While auditing between 45 and 60 motor service centers yearly, from small to large, the most common complaints are:

1. At 700° F the roasting time must be extended considerably, possibly impacting delivery times, and consuming more heating fuel;

2. After installing a chart recorder, folks notice wide swings in up and down roasting chamber temperatures throughout much of the roasting cycle.

By the way, excessive water suppression eventually causes steel walls and the floor of roasting ovens to rust. There have been several ovens I’ve looked at, that I could push my finger through outside wall rust holes and touch the inside insulation.

Here’s a suggestion; While the roasting oven is operating, check for outside air sources using smoke or a visible suspended particle (e.g. baby powder or baking soda) being sucked into the oven around.

1. Door gaskets;

2. Double door closing mechanisms (latches);

3. Doors closing over part-cart tracks, leaving open voids around the floor tracks and;

4. Check outside oven walls for rust through spots.

The great thing about oven temperature recording, you can see the rollercoaster of combustion chamber and part temperatures when outside air is allowed entry. On the flip side, after making necessary maintenance repairs, you should see a flatter or even flat part-temperature record and reduced processing time.