Muffle furnace for the school workshop. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Muffle furnace for the school workshop. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Ham Radio Technologies

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The main part of the furnace is the working chamber, or muffle, made of refractory clay. Its internal dimensions are 210x105x75 mm and the wall thickness is 10 mm. It is molded according to a pre-made form. It can be a box made of cardboard impregnated with stearin. Clay is applied to it from the inside. If you sculpt from the outside, a crack may form: after all, clay shrinks quite a lot during drying. The ceramic element of the door should also be molded from the same clay. After drying in air, both clay parts are placed in a conventional industrial school muffle furnace and dried for two hours at a temperature of about 100 ° C. Then they are fired, gradually raising the temperature to 900 ° C. Then the furnace is turned off and allowed to cool slowly along with the parts .

Muffle Furnace for School Workshop

The fired muffle and the door element are adjusted to each other, carefully filing with a file and cleaning with sandpaper. Then 18 m of nichrome wire 0,75 mm thick are wound on top of the muffle, maintaining the interval between turns as accurately as possible. To prevent the winding from untwisting, the first and last turns are short-circuited. And so that a short circuit does not occur, the gaps between the turns are carefully coated with clay. When it dries, a second coating is made with a thickness of 10-15 mm with clay mixed with asbestos chips.

The finished dried heating element of the furnace is placed in a metal case made of a steel sheet of millimeter thickness. Case size 270x180x180 mm. For ease of assembly, it is better to make it with removable front and rear covers that are fastened with screws. A door is attached to the front cover on a hinge, which should lean horizontally. A ceramic element is installed on the door with the help of two bolts through asbestos gaskets, and the gaps are sealed with clay. The ends of the nichrome wire are brought out to the back cover of the case. Ceramic insulating "beads" should be strung on both terminals. The wire is attached to the pin connector. Such connectors, as well as insulating ceramics, can be taken from old faulty electrical appliances. A standard cord with a plug will be attached to the connector to be connected to the mains. All free space between the heating elements and the housing should be tightly packed with asbestos chips. In addition, the design of the furnace must be provided with a housing ground terminal.

For convenience of work, two small holes can be made in the heating chamber: one on the back wall - for installing a thermocouple, the other on the door - for observing the interior of the furnace during operation. Both openings must be provided with closable metal shutters. At the bottom of the working chamber, a lining plate made of thin stainless steel should be placed.

Our oven is rated for 220V AC. It heats up to the maximum temperature in just an hour and is 1 kWh less energy-intensive compared to the standard school muffle furnace. Its weight is about 10 kg, so when it is not needed, it can be easily removed into the closet.

ATTENTION! Before operation, the device must be grounded and installed on an asbestos-cement slab away from flammable objects. It is possible to work with the furnace only in dry mittens, using special steel tongs with long handles.

Author: K. Skvortsov

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