A NEW TYPE OF ELECTRICALLY HEATED MUFFLE FURNACE* T.R. BALL,WASAINOTON UNIVERSITY, ST. LOUIS, MISS OUR^
A top-opening electric mufie furnace has been built holding 22 size 0 crucibles, any one of which may be removed without disturbing the others. Its chief characteristics are large hearth area and easy access to crucibles. A time-temperature curwe, cam@red with a furnace of conventional design but of 33 per cent less capacity, shows a higher rate of heating u p to 600°C.,but a lower rate thereafter. The two curves cross at 85OoC. The p m e r consumption per cruciblefor the hvo furnaces is practically the same.
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The stock type of end-opening muffle furnace universally used is poorly adapted to use in college laboratories where several students are required to use the same muffle simultaneously. The first crucibles inserted are certain to be the first removed and it is difficult to accomplish this without danger of overturning the crucibles near the front. An attempt has been made to design a top-opening muffle of considerable hearth area in which this source of trouble is obviated. Following our design, the Hevi Duty Electric Comlsl,oi~li!~mnr, pany, Milwaukee, WisFIGUREI.-CLOSED ~ ~ N A C L < consin, has built such a furnace, now in use in the chemical laboratory of Washington University. The two photographs, Figures 1 and 2, show views of the furnace closed and opened. The furnace shell is a sheet iron box 74 cm. long, 18.5 cm. deep, and 34.5 cm. wide, with ends of heavy transite board. The muffle is made of refractory, grooved muffle plate and is in the form of a rectangular box 51 cm. long, 7.6 cm. deep, and 11 cm. wide. The hearth area is 561 ~ m . ~ and is sufficient to carry twenty-two size 0 crucibles, which are each 41 mm. in diameter. Any crucible may be removed without disturbing the others. The capacity of the muffle is 3758 cu1.3 Electrical heating units are installed in grooves of the muffle plates which form the walls and the
* Presented before the Division of Chemical Education at the 80th meeting of the American Chemical Society, Cincinnati, Ohio, Sept. 8-12, 1930. 355
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FEBRUARY, 1931
hearth. These units are of an 80% nickel, 20% chromium alloy. They terminate in a ventilated sheet iron box shown a t the left end. The furnace is insulated with two courses of C 22 brick in the hotter zone next to the heating units, and with Johns-Manville Snperex slabs next to the shell. The top of the mufflebox is faced with strips of Enduro KA2 \!,, I : ! , steel, 6 cm. wide, placed FIGURE OFEN EN l i i i ~ ~ ~ c l . : with their edges flush with the inside of the muffleplates. At intervals a portion of the steel facing plate is bent downward about 1 cm. to aid in supporting the side walls of the muffle. (See Figure 2.) At 5-cm. intervals, a 2.5-cm. slot has been sawed into the plates to reduce the tendency to buckle when hot. A '/,-inch hole in the back wall of the muflle permits the insertion of a pyrometer point a t the geometrical center of the heating chamber. The lid of the furnace is an inverted box of sheet iron 74 cm. long, 23 cm.wide, and 6.8 cm. deep, hinged at the back on the projecting face of rethe furnace shell. The lower side of the lid consists of a grooved fractory plate carrying beating loo0 units. This is held in place by a facing plate of Enduro KA2 steel. (See Figure 2.) The total 800 weight of the lid is counterpoised by two adjustable iron weights. :600 (See Figure 1.) A long bar of '/p-inch square iron, supported on brackets, serves as a handle h for raising the lid. The furnace operates on 110 200 volts, A. c., through a line resistance rheostat and carries from 30 60 90 120 1: 24 to 25 amperes a t full load. Minutes It is designed to operate safely 3.-HEAT,NG RATE CURVES OF a t 980°C. and its maximum temMUFFLEFURNACES peratnre is 1040°C. A , top-opening; B, end-opening.
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Curve A, Figure 3, shows the heating characteristics of the furnace up to 960°C., when operating at full load. Curve B is the heating curve of a popular make of end-opening muffle of the conventional type, but having a much smaller cubical capacity and hearth area. It will be noted that the rate of heating for the new furnace is higher than that of the old up to about 600°C., but is slower thereafter. Both furnaces reached 850°C. in 77 minutes. Table I shows some comparative data on the two types of furnace. It will be noted that the power consumption, per crucible, is practically the same for the two furnaces. TABLE I Comparison of Top- and End-Opening Mume Furnaces Top opening
Cubical capacity Hearth area Ratio of hearth area to cubical capacity Cmcible capacity, size 0 Amperes at 110 volts Amperes per crucible Time (minutes) to reach 900°C.
End opening
3122 3759 cm.a 561 cm.% 318 cm.l 0.149 0.101 22 15 25 16.4 1.14 1.09 98 86
Because of the large area exposed when the furnace lid is opened for the removal of crucibles, it was feared that there would be a serious loss of temperature due to radiation. On opening the lid for the removal of two crucibles, the pyrometer temperature dropped from 960°C. to 790°C. but was back to 960°C. again in about four minutes. The end-opening muffle lost only 15°C. in the same operation. The pyrometer temperature loss is, of course, greater than the loss on the crucibles.
