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AN EFFICIENT STUDENT DRYING OVEN F. C. HICKEY, O.P., J. W. HACKETT, O.P.,
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and P. A. CAUCHON Providence College, Providence, Rhode Island
A SOURCE of inefficiency and annoyance in the undergraduate analytical laboratory is the standard-type drying oven. Where many students use these ovens in common. it is difficultto maintain the tem~eraturerequired to satisfy procedural demands, especially if reasonable use is made of their capacity. These ovens are constructed with doors offering full access to the interior. For student use involving the drying of samples, crucibles, and other small apparatus, the opening is larger than necessary and results in a considerable drop in temperature with slow recovery each time the oven door is opened. If this is frequent and prolonged, as is the case when students are obliged to adjust the oven contents to place or remove apparatus, it becomes im~ossibleto remlate the time of drvine. ' h i s difficulty can be lessened by"li&ing access to the ovens to half-hour intervals; but such interference with individual freedom of operation is not desirable. A better solution is a better, more practical oven. Such an oven was constructed in our shop eighteen months ago and was found to be so satisfactory that another was built almost immediately. These have been in operation for a t least two full terms and have proved their usefulness. The design and construction details are offered as a ~ossibleanswer to similar difficulties met in other laboratories. A schematic diagram of the oven is shown in Figure 1, and has for outside dimensions 18 in. by 18 in. by 43 in. The frame is of welded la/&-in., 1a/4-in., 3/lein. angle iron with l/rin. transite panels. The adjustable shelves are '/kin. transite circles of 14 in. diameter. These are bolted to steel supports of spiderlike design-% collar to which six radial arms of I/&in. by 1-in. steel are welded. A set-screw through this collar holds the shelf a t any desired height on the rotating axial support. The shelves are divided into six pieshaped areas, each large enough to accommodate three 400-ml. beakers. The segments correspond to dial positions on a numbered knob attached externally to the supporting axis. The arrangement permits the student to select and bring m y of the 18 shelf areas to the front position before opening the 7-in. oven door. This reduces the need of a prol~ngedinspection or adjustment of the contents and eliminates the necessity for handling another's apparatus. The supporting axial rod is of 3/s-in. stainless steel set in a socket cut from a 2-in. stainless steel bar. The socket is bolted directly to the transite floor of the oven. A similar ring of stainless steel, bolted to the top of the oven, forms the upper bearing.
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The wiring diagram is given in Figure 3. The heating units are a series of four coiled nichrome wire U's each fastened to 6-in. panels of transite bolted diagonally across the inside corners of the oven (Figure 2 and A of Figure 3). The coils of 19 B. and S. nichrome wire (one-half ohm per foot) were wound on a 3/16-inmandrel.
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the four resistances A. This decreases the heating energy to 300 watts. At this point the pilot light C comes on, indicating that the oven has reached the predetermined temperature. The bimetallic element consists of a hack-ww blade I10 V6Ow to which a strip of copper sheet of approximately the same dimensions has been silver soldered. The contact on the strip is a patch of coinage silver on the copper side of the element a t its free end. Coinage silver is also used for the contact point on the micrometer screw. Fi""r. 3. Wlxins DimA five-microfarad condenser E is wired parallel with They extend from the top to the bottom of the oven. the thermostat contacts in order to reduce arcing. This The resistance of each coil is five ohms. An auxiliary condenser is fastened to the bottom of the oven. Uniform temperature distribution a t all times is aided heating coil of 20 ohms (D, Figure 3) is wound in the form of a spiral on a round piece of transite bolted to the by the arrangement of the resistance coils, which throw floor of the oven. A circular hole in the center allows the heat either upward or centerward, and is furthered by the perforations in the transite shelves. A Weston the axial rod to pass through it and rest on its socket. When the oven is cold, the bimetallic element makes Thermometer is passed through a front and diagonal contact a t B and short circuits coil D. Approximately panel (Figure 1)to indicate the oven temperature, with 600 watts of heating energy are thus provided. On a choice from approximately 80" to 200°C. With adequate shop facilities the oven can be built reaching the desired temperature, the bimetallic element breaks the contact a t B and places coil D in series with a t a nominal cost.
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