ELECTRICALLY HEATED SAND-BATH HOT PLATES FOR ANALYTICAL LABORATORIES* G. FRE~ERICK S m m , THEUNIVERSITY OR ILLINOIS, URBANA, ILLINOIS
The problem of providing for extensive analytical evaporations, particularly in the case of large-scale operations in which hundreds of evaporations are conducted simultaneously, offers difficulties. Such operations as the quantitative concentration of beaker filtrates, the digestion of precipitates and particularly evaporations to dryness with subsequent digestion for dehydration processes call for special designs of hot-plate construction. The object of the present work is to provide data to cover the construction of various sizes of laboratory hot plates which will cover the various working conditions outlined and be made from standard stock electrical heating units and otherwise easily cut, machined, and assembled supporting parts. Provision is made for easy replacement of "burned out" heating units which, under the principles of assembly, becomes infrequent if a t all necessary. As far as is known there is not obtainable from the usual laboratory supply houses such equipment or a satisfactory substitute. The designs about to be described have been in use in the laboratories of the University of Illinois for an extended period of time and have proved entirely satisfactory and the first heating unit has yet to be replaced. Heating Elements Selected Standardized electrical space heaters are obtainable from various sources, advertised and advocated for a wide range of heating problems such as vacuum driers, annealing ovens, air-drying operations, and oiland paraffin-bath heaters, etc., for which high-pressure steam is either not sriited or of not sufficient heat capacity and temperatures up to and somewhat under the first visible red heat are desired. Such space heaters are designed to function indefinitely if not too severely insulated. The type selected for the problem a t hand was the G. E. X. 1183 space heater, 500 watts capacity, working under a potential of 110 volts. The heaters are provided with contacts a t each end insulated from the remaining metal parts of the heater so that they may be mounted on a conducting support without further insulation. They are not expensive and replacements are always accessible but seldom required. Mounting the Space Heater The material employed in the construction of such an electrical hot plate consists of the following list in addition to 4 space heaters just described:
* Presented before the Division of Chemical Education of the American Chemical Society at the Atlanta meeting. April 7-11, 1930. 1915
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22-gage galvanized sheet iron (2 pcs., 20L/4 X 26'/dV) (2 pa., 7 X 26'/,") (2 pcs., 7 X 20'/*") I-inch angle iron (4 pcs.. 26'/dV) (4 pcs., 20'/2") (4 p a . , 12") 1-inch X l/s-inch strap iron (4 pcs., 24l/,') (4 pa., 207 square iron (2 pcs., 24') (2 . .Dcs... 21"). "Flat" and "bent" corner braces 2'/.-inch and 2-inch, respectively (4 pieces each). 6/ls-inch X ' / h c h cap saews (8pieces) "4 lb." tinned rivets (100 pieces) =/&ch round head machine screws 10-24 thread (24) Round head stove bolts 1 X a/ls" (24) x 2'/1 X 20" (1 piece) Transite board Black enamel M asphaltum paint (1 pint)
The amounts listed in parenthesis following each item represent the amount and size used in each of the hot plates employing four heating units and illustrated in Figure 2. Three different hot plates are pictured in the partially assembled condition in Figures 1, 2, and 3. The body of the arrangement (selecting Figure 2 again for illustration) is a shell seven inches deep with an area at the top 201/2 X 26l/%inches with the space heaters extending across the 20'/z-inch dimension. The heaters are mounted with their flat side
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1917
in a plane with the top and bottom of the bath and are placed two inches from the top edge of the hot plate. The four legs are of one-inch angle iron framed a t the top with oneinch strap iron, and a t the bottom with one-inch angle iron so placed that the bottom of the bath is one inch elevated from the outside bottom edge of the shell. The angle iron frame supporting the bottom of the bath is fastened to the legs by the use of cap screws. The strap iron frame for the support of the top tray is fastened to the upper part of the four legs using machine screws. The galvanized sheet iron sides and bottom are riveted into place on their frame work support and a clearance of 6 inches underneath is provided by the length of the legs. The top tray is framed in a similar fashion using angle irons, sheet iron, and square iron. I t is one inch deep and the square iron outside border acts as its support when in position in a fashion clearly shown in Figure 1. The top tray is held into position against any tendency to warp, using a small screw and clamp shown in Figure 2 lying a t the bottom of each leg. The space heaters are inserted through a slot made in the front and back of the side walls 2 X '/2 inches in size and are supported against contact with the remaining metal of the bath by two pairs of transite stirrups fastened within and without the bath by machine bolts. A further description of these need not be made since the illustrations are sufficiently
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clear on this point. The space heaters may be reversed in position in the bath from that given as illustrated in Figure 3 and stand with their flat side vertical to the plane of the top and bottom of the bath. With the top tray in position the space heaters are in the first case one inch below the bottom of the tray and in the latter case almost in contact with the bottom. To prevent most of the downward radiation from the space heaters from being dissipated on the bottom side of the hot plate the space within the heating chamber is filled with infusorial earth or a substitute to within '/%inch of the space heater if mounted with the flat side in the plane of the bottom and up to the space heater if mounted in t h e opposite position. I n Figure 1 a hot plate with one space heater and therefore consuming 500 watts of electrical power is shown. I n Figure 2 the four-unit type is shown. In this case the space heaters are connected in series by pairs of two and each of the two pairs thus formed wired in parallel with the main 110-volt lead wires. I n Figure 3 is pictured the nine-unit type. In this case the four coils of each end are wired exactly as in Figure 2 and the middle space heater is provided with a switch (not shown because on the bottom side) by which it can be heated or turned off as desired. In
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all cases the wires leading to the main conduit and switch box are asbestos insnlated and in all the hot plates a guard to protect the bare wires and contacts is mounted on the side of the bath as shown in the case of Figure 1. When the top trays are in position sand 3 / 8 to inch thick over the whole tray is provided. Upon calculation the power consumed with the four-unit hot plate of Figure 2, like Figure 1, is approximately 500 watts. Since the hot plate of Figure 3 is an assembly of the hot plate of Figure 1 as the middle portion and both ends the duplicate of that of Figure 2, with the switch of the middle coil in contact there is a consumption of 1500 watts or with the switch of the middle coil off the power consumption is 1000 watts. The Hot Plate Performance under Varying Conditions The performance of such a hot plate will cause the temperatures attainable to fluctuate with the room temperature, draft in the hood in which i t is installed, and fluctuations in line voltage. Otherwise, the evaporation vessels will vary in temperature depending upon their position on the various plates and the depth to which they are immersed in the sand of the top tray. Temperatures attainable in various .,. positions of the hot plate of Figure 1 were determined by placing a t various positions 125-cc. Erlenmeyer flasks containing 50 cc. of concentrated sulfuric acid provided with a suitable =. thermometer. The flasks were subFIGUW4 merged through the 3 / 8 inch of the sand of the tray and when temperature equilibrium was attained the results recorded. The diagram of:the results obtained is shown in Figure 4. The temperatures are in centigrade degrees for a room temperature of 20 degrees. It will be observed that the temperature graded dowm ward from the center position diago@ @ @ @ @ nally and radially in all directions. This hot plate proves admirable for quantitative evaporations just under FIGU~B5 the normal boiling point of water. Under the conditions as given the bottom of the bath will permit of contact with the back of one's hand without discomfort. Under the hot plate is an admirable storage space for beakers and contents undergoing yet
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milder evaporation or digestion or for beakers completely evaporated and awaiting an opportune time for the operator's attention. The operation of the single unit hot plate of Figure 1under the conditions stated is shown in Figure 5. The top row of figures are for theErlenmeyer flasks resting on top of a 8/8-inch thickness of sand, the middle row for the same flasks buried '/z inch in sand and the bottom row the same flask buried 8/8 inch in sand. This bath is admirable for evaporations to fumes of sulfuric acid, the dehydration of metasilicic acid for silica determination and similar operations. The legs of this hot plate elevate the bath proper 5 inches above the supporting bench instead of six inches as in the case of the other hot plates. This change is made to bring the leg length into better proportion with the remainder of the dimensions. The operation of the eight-unit hot plate is shown in Figure 6 with the test flasks buried 3 / 8 in the @ 8 @ sand. The upper portion of @ @ @ @ @ Figure 6 records tempera8 @ @ @ @ tures for one-half of the hot @ @ @ @ @ plate with the middle coil @ @ @ operating. The lower part of Figure 6 shows the same with the middle coil cut out of the electrical circuit. The nine-unit hot plate is seen to he the duplicate in heating @ 8 @ @ range of two four-unit plates, two single-unit plates, and a @ @ @ @ @ still higher heating portion in the middle area over the 500-watt space heater while 6 FIOURE the full power is on. With the middle 500-watt space heater cut out the nine-unit hot plate is practically the equal in heating range and area to three of the four-unit types. @
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Further Variations in the Operations of the Hot Plates Described Many other desirable combinations of wiring can be made besides those mentioned. By varying the thickness of the sand layer in the top tray or by its complete omission further temperature graduations may he obtained. By providing an air insulation a t the bottom of the bath by closing up the one-inch recession of the bottom above the outside bottom edge, a further increase in radiation upward could easily be provided. The construction of these hot plates is greatly facilitated by the operation of making a number a t one time through the use of templets, multiple drilling and fitting, tandem sawing, etc. It is hoped that this description
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is sufficientlyclear that prospective users of this equipment can have the same prepared in their own machine shops. The author in any event will be glad to supply additional information if desired and other ways assist in making this apparatus available at the request of those interested.
