July 15, 1929
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Inductor Coils for the High-Frequency Furnace’ C. N. Schuette PACIFICEXPERIMENT STATION, U. S. BUREAUOII MINES, BERKELEY, CALIF.
NUMBER of high-frequency induction furnaces have been used for experimental work by the Pacific Experiment Station of the U. S. Bureau of Mines. Despite the qualities making these furnaces especially useful-such as ease and reproducibility of operation, rapidity of heating, higher temperatures, carbon-free melts, suitability for work in vacuum or inert gases or for the preparation of pure materials, and economic use of power-it was learned that each specific operation required care in bringing about the proper relationships between inductor and contents. The equipment at the station consists of an 18-kilovoltampere high-frequency converter, including a 220- to 6600-volt reactive transformer, a bank of twelve condensers, and a mercury-to-copper electrode discharge gap, operating in hydrogen. The power input to the furnace is regulated through variation of the length of the discharge gap, accomplished by raising or lowering the mercury level with an outside plunger. The furnace inductor is a coil of water-cooled copper tubing through whjch the high-frequency current is passed. Heating is accomplished by eddy currents induced in any conductor that is placed within this coil.
A
Coil Design
Experimental requirements, such as the size or shape of the container in which the heating is t o take place, or especially high temperatures, may necessitate the use of furnace coils of different dimensions and characteristics. Commercial coils are not always immediately available or especially suitable, but it was found that expeditious production of suitable furnace coils was possible, and the methods successfully used a t this station are the subject of this report. When the necessary materials and tools are available, a complete coil furnace can be built and assembled in 3 or 4 days by one man having occasional help from another. The tools developed and the method used in making such furnace coils which are described were selected as the best from a number having varying degrees of efficiency. The principles underlying the design of a furnace coil have been described by Northrup ( I ) . * Disregarding the more exact mathematical phases of this subject, the following considerations are important in the design and construction of furnace coils: First, the strength of the magnetic field is proportional to the ampere turns. It is desirable to have the number of ampere turns a maximum for good furnace performance, and this requires the coils to be spaced as closely together as possible. Second, the inductor must be cooled by a flow of water through it, and in practice as high as 50 per cent of the applied energy has been carried off by this water. This limits the flux density that can be used, and the maximum number of ampere turns; obviously the coils must not be flattened to such an extent that they will no longer pass sufficient cooling water a t the pressure available. Third, the voltage applied a t the coil terminals determines the insulation that must be supplied between the individual turns of the coil. Fourth, the coil diameter governs the maximum temperature that can be reached by a fixed maximum power input. Thus, if very high temperatures are wanted, a smaller coil diameter will concentrate the magnetic field in a IReceived November 26, 1928. Published b y permission of the Director, U. S. Bureau of Mines. (Not subject to copyright.) Italic numbers in parenthesis refer to literature cited a t end of article.
*
smaller area, and so induce a higher temperature in it for the same power input. Fifth, a certain amount of heat insulation is needed between the object to be heated and the watercooled copper coil. The effectiveness of this insulation is not accurately calculable, and the experimenter may find after trial that his coil design does not meet his experimental requirements. Another coil modified in design on the basis of actual coil performance must be used. This eventuality is a t times urgent, and makes it desirable that the experimenter be able to make his own coils by a simple and rapid method. Winding Coils
Seamless soft-drawn copper tubing is used to make the furnace coil; usually tubing of 3/8 inch diameter was used for the coils made a t the Pacific Experiment Station. This size of tubing gives a coil section inch thick by ’/2 inch wide, after being flattened and wound. The coil should be rolled from a single length of tubing, if such a length can be obtained; otherwise i t should be wound before joining the sections. The winding of the coil is done in a lathe. A mandrel may be made of iron pipe or of hard wood. Soft-wood mandrels have been used, but the tubing is forced into the surface to some extent and it is difficult to remove the finished coil. The mandrel should be 2l/2 or 3 feet long for a coil containing forty or fifty turns, and preferably should have a very slight taper toward the tail stock of the lathe. A small hole just large enough to pass the copper tubing should be drilled into the mandrel near the head stock end. The winding and flattening are done a t a single operation, and a special roller tool was developed for this purpose. This tool is illustrated in Figures 1, 2A, and 3. A ‘/zby 2 inch steel bar, 16 inches long, is bent a t right angles about 4 inches from one end. The longer end is twisted through a right angle. The short end is drilled and fitted with a heavy bolt and plate washer suitable for clamping this “tool post” in the tool-post slot on the carriage of the lathe. Two roller by 1inch pieces of cold-rolled steel about arms are made of 10 inches long for coils up to 8 inches in diameter. On one end l/r-inch holes are drilled perpendicular to the narrow edge of these arms, and Stubb’s steel pins 11/2 inches long are driven into them. These pins serve as shafts of the rollers, and rollers are made of I/2-inch round Stubb’s steel and are 9/16 inch long for S/s-inch tubing. One-half inch of the surface of the rollers is ground slightly concave, the smallest diameter being 0.475 inch. A concave roller face prevents the tubing from becoming lemniscate or indented in the center, as happens when flat-faced rollers are used. A 3 / ~ a inch bolt, 2 inches long, is passed through the two roller arms an inch or so back of the rollers. The roller arms are bolted to the tool post by two ‘/4 by 2 inch bolts, one arm being on each side of the tool post. The position of these arms on the tool post varies with the size of the mandrel used, but is such that the roller axis is radial to the mandrel a little past the top element, and touches lightly upon the surface. After the mandrel and roller tool have been properly mounted in the lathe, the coil of copper tubing is unwound and laid out approximately straight on the floor, or it may be mounted on a reel for feeding to the lathe. The roller tool is moved to the head stock end of the lathe
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and opened to prevent flattening the tubing. One end of the copper tubing is passed between the rollers; the end is now bent over a t right angles and thrust into the hole drilled into the head end of the mandrel. A wooden peg is driven in beside it to secure this end firmly to the mandrel. One or two turns of the tubing are now wound up on the mandrel to make the lead-in tubes shown in Figure 2, C and D. Then the rollers are brought together by tightening the 3/16inch bolt through the roller arms. The copper tubing is rolled to its finished dimensions by one pass throughtherolls. The roller spacing desired, inch, is set by say placing a shim of this
VOl. 1, KO.3
coil for an inch a t one end is fitted to the V cut with a slight overlap. Care must be taken to smooth all burrs and to prevent filings from getting into the coil. The inlet pipe or pipes after careful fitting are silver-soldered. Silver solder gives a strong reliable joint. The three or more water tubes are now run out straight from the coil about 4 to 6 inches, where a neat right-angled bend is made, as shown in Figure 2, C and D. The coil should now be wired or tied in compressed form and tested for leakage and water-carrying capacity. A leakage test should be made by water of somewhat greater than operating pressure. A capacity test is made by timing the discharge into measuring containers, and with a coil of one inlet and two outlets the discharge of the two outlet pipes should be equal. Insulation of Coils
Figure 1
thickness between the ends of the roller arms. As the rollers have a tendency to spread a t the bottom, this shim is placed above the bolt center on the arm and serves to keep the rollers parallel as well as the proper distance apart. The mandrel is now rotated by hand by a bar placed through the jaws of the chuck, but if a powerful lathe is available the back gear may be used provided that some means is used to prevent the chuck from coming off. A little oil is needed on the rollers, and they should be watched for any tendency to lift. Rotation should be steady and even. Some coils were made using as many as four passes through the rolls, but this was' found to be unnecessary as one pass gives an equally good result, though the rotation requires the application of a greater force. The coils should be mated evenly during the winding. The lathe carriage is set ioose and wiIl generally feed itself along as the lathe is turned and the successive coils are spaced the width of the roller. With one man t o rotate the lathe and another to oil and watch the roller operation a fiftyturn coil can be wound in a quarter hour. When the requisite number of turns have been wound, rollers are loosened. The tubing is cut a foot or more beyond the flattened part, thus furnishing the other unflattened lead-in tube. The tool is removed, the pegged end of the coil is taken out of the mandrel, the tail stock of the lathe is run back, and the coil is readily slipped off the mandrel. For purposes of rotation, as well as to support the head end of the mandrel during removal of the coil, the use of a large scroll chuck is advantageous. The tail stock end of the mandrel does not drop when the tail stock is removed, and the removal of the coil is much easier, 'especially when a heavy iron-pipe mandrel is used. T h e next step is to provide the inlet pipe or pipes in the body of the coil. The coil is now laid out on a bench on two wooden rails to hold it in line. At the point desired a V-shaped cut is made, extending to the center and nearly as wide as the flattened tubing of the coil, and a piece of copper tubing flattened to the same shape as the
The thickness of insulation used has a direct bearing on the possible number of ampere turns. The insulating material should not be bulky. Asbestos or mica sheet and an insulating varnish have both been found satisfactory. For purposes of illustration assume that the potential between individual turns of the coil has been calculated to be not more than 100 volts and that two thicknesses of 0.015-inch asbestos paper and a phenol-resin varnish will serve for insulation. Assume that the coil in question is 4 inches in internal and 5 inches in external diameter. Rings of asbestos paper that will extend inch beyond the edges of the coil are required; that is, they should be of 37/s inch internal and 5l/8 inch external diameter. If the coil has 45 turns, about one hundred of these rings of asbestos paper should be made. To make them rapidly one hundred 6-inch squares of asbestos paper are stacked and bolted together between two boards. Circles of the required diameters are scribed on the top board and the circles are cut on a band saw. A radial cut should be used in running the saw into the inner circle, For support during insulation three picces of 1-inch doweling as long as the extended coil are drilled a t 11/2-inch dis-
n
Figure 2
tances with holes the size of a large finishing nail. The supports are mounted vertically on the edge of the bench 120 degrees apart in a 37/&ch circle, as shown in Figure 2, B. The coil is lowered over them and is held in the open coil
July 15, 1929
INDUSTRIAL A N D ENGINEERING CHEMISTRY
position by finishing nails inserted into the guide rods under every third or fourth coil. Starting a t the bottom, from three to five coils are first coated with insulating varnish. Then a single thickness of the asbestos-paper rings is laid on the coil spiral as far as it has been varnished. This paper is now in turn coated with varnish and then the second layer of asbestos paper rings is laid on top of the first one, the joints of the second layer being laid 180 degrees from those of the first layer. In other words, care is taken to lap the joints. The second layer of asbestos paper is now coated with varnish: then the insulated coils are pushed down firmly on each other and are held in this position by finishing nails inserted in the guide rods above them. This process is repeated successively until the entire coil has been insulated. The coil is then removed from the guide rods and is held in the contracted position by two wooden cross arms over the top and bottom, fastened by a bolt. A wooden clamp is made to hold the inlet and outlet tubes in their correct position, as shown in Figure 2, C. The coil is now given another coat of varnish inside and out and is allowed to dry. If phenol-resin varnish is used, it must be baked after drying. Baking is best accomplished in a controlled-temperature oven. If no oven is available an iron pipe can be set through the coil after it has been mounted, and a MBker burner inside this iron pipe will supply enough heat t o bake the varnish. The furnace coil must now be mounted. A very satisfactory mounting for general experimental work is that illustrated in Figure 2, D. A composition board of asphaltic asbestos, about 15 by 22 inches and ‘/z inch thick, is used as a base. This material is a fair insulator and does not absorb water. It can be drilled, cut by a saw, and large circles can be cut in it by the usual fly-cutter such as is used in a drill press. The asphalt will boil out of it when it becomes too hot, but this is a minor source of annoyance compared with the advantages of good insulation in combination with ready workability. Asbestos board with a cement binder only was tried, but is more difficult to work and is unsatisfactory as electrical insulator. A hole equal in diameter t o the inside diameter of the coil to be mounted is cut in the center of the base. At the proper distance toward one end, holes are drilled to pass the copper inlet and outlet tubes. The coil itself is held in a frame made of three red fiber rods j / s inch in diameter, which are set in two rings made of the asphaltic asbestos board. These rings have an inside diameter equal to that of the coil and may be about 2 inches wide. They are drilled a t 120 degrees to take the three fiber rods which project through the lower ring into the base. The rings are held to the fiber rods by fiber pins through the edge of the ring. The pitch of the coil is compensated by fitting blocks of the asphaltic asbestos board between the coil and the ring just inside of the rods. The copper tube terminals are insulated above the base by slipping lengths of ordinary laboratory rubber tubing over them. Below the base the copper tubes are held by split brass or copper blocks, 1 inch square and 31/2inches long, bolted to the under side of the base. These bolts are countersunk and the screw head is then covered with sealing wax to give absolute insulation above the base. This is an important item, because in operation the operator may manipulate a metal stirring rod, and with dark glasses to hinder his vision may accidentally touch the terminals. Since this design of furnace coil has no uninsulated parts above the base, the protecting “squirrel cage” furnished with commercial units may be eliminated. The power connection is made to the blocks under the base, which are drilled and provided with bolts for this purpose. For general experimental work the base is conveniently
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provided with a hinged door. When closed, a protection tube of fused silica or other suitable material can be set on it. When left open, long tubes such as those used for melting in uacuo, as described by Schuette and Maier ( 2 ) , can be set through the opening. The base itself is mounted on a suitable frame about 18 inches high, and can be hinged to the frame on one side, so that, by tilting the entire base, melts can be poured from crucibles fixed in the coil. This device works well in practice and combines the advantages of a stationary and tilting furnace in one mounting. When tools and accessory appliances have been made, and all material is ready, a coil can be wound, soldered, and tested in a day. The insulation will take a half day. Then while the coil is drying, the base and frame may be made, and on the fourth day the new furnace can be in full operation.
I
--COIL ROLL&? DETAILS
Figure 3
The maintenance of the furnace coil requires little if any care. The insulation may be impaired if the coil is run too hot, or the insertion and removal or the occasional breakage of a protection tube may damage the insulation on the inside of the coil. The coil should be inspected and revarnished as occasion offers. In operation the cooling water outlets are best connected to separate hoses, and the discharge from these should be visible t o the operator. If the greatest heat is developed in the upper part of the coil, the water through this part of the coil may be dangerously hot, while the lower half is cool. The operator may detect this condition by placing his hand on each individual discharge hose. If the cooling water is discharged through a hose common to all outlets, such check is impossible. If the discharge streams become smaller owing to a drop in line pressure, the operator is warned immediately if he can see the discharge plainly. Sometimes the discharge streams become less or unequal because the source of the cooling water is not clean and sediment lodges in the coil. In this event the coil should be cleaned immediately. In severe cases hydrochloric acid can be run through the coil to dissolve or loosen foreign material, but generally a reversal of the current will remove sediment or obstructing pieces of rubber torn from the hose connections. The water flow should be tested for volume a t intervals. Conclusion
I t is hoped that this article will aid those who may wish to construct high-frequency induction furnace coils that are exactly suited to the requirements of experimental conditions.
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This type of furnace is so eminently suited to experimental work and the furnace are so made that lack Of the correct size of coil should not be allowed to prevent or delay the use of this valuable new aid to Scientific investigation.
VOl. 1, KO.3 Literature Cited
(1)Northrup, Trans. A m . Eleclrochem. SOL.,311, 69 (1919); Gen. Elec. Rev., 656 (1922), (2) Schuette and Maier, Trans. A m . Electrochem. SOL.,54, 155 (1928).
Determination of Total Replaceable Bases in Soils' R. H. Bray and F. M. Willhite DEPARTMENT OF AGRONOMY, UNIVERSITY OF ILLINOIS, URBANA, ILL.
HEN a soil sample is leached with a neutral normal ammonium acetate solution, the bases present in a replaceable form are replaced by the ammonium ion and are found in the filtrate. I n using this method, it was noticed that when this filtrate was taken to dryness and heated, carbonates of these bases were formed, as a result of heating the acetates. This suggested the idea that perhaps the total amount of replaceable bases could be determined by titrating the carbonates or oxides thus formed. Ignition and titration of different acetates confirmed this idea and showed in the procedure the possibility of a quantitative determination. Known amounts of sodium, potassium, calcium, magnesium, barium, and manganese acetates were ignited and titrated. All the titration results were within 1 per cent of the theoretical values. Iron and aluminum acetates gave no titration value. Procedure
A 10-gram sample of 20-mesh air-dried soil is leached with 500 cc. of neutral normal ammonium acetate solution. The filtrate is placed in a 600-cc. Pyrex beaker and evaporated to dryness on the steam bath. The residue is heated gently on a silica plate over a Bunsen burner for a few minutes, then a t full heat for 20 minutes. After cooling, an excess of standard acid is added. The solution is warmed and the bottom of the beaker is rubbed with a rubber policeman. The back titration is made with standard base using methyl red as an indicator. I n testing the method from 3 to 40 cc. of acid were neutralized by different samples and duplicates usually checked within 0.2 cc. The blank from the evaporation of 500 cc. of ammonium acetate was 0.4 cc. Results
A number of soils were analyzed for replaceable potassium, sodium, calcium, and magnesium by the usual methods. Table I gives the results compared with those obtained by the titration method. All results are given in milligram equivalents of replaceable base per 100 grams of soil. The blanks for the different determinations were approximately equal. The principaI replaceable bases found in mature soils of a humid region are calcium and magnesium with small amounts of potassium and sodium. All the soils listed in the table belong in this category and include samples from the various horizons and subhorixons. The sum of the four principal bases, as determined by the regular methods, is here compared with the total value as found by titration. Some samples show a wide variation, but in general the agreement is good. 1 Received February 28, 1929. Published with the approval of the Director of the Illinois Agricultural Experiment Station.
Table I-Total Replaceable Bases in Soils Determined b y Titration Compared w i t h t h e S u m of Individual Replaceable Bases Determined by Separate Analyses TOTAL REPLACEABLE BASES SOIL CALMAG- POTASDeterBy DIPFERNUMBERCIUM NESIUM SIUM SODIUM mined titration ENCE
Mg.
equzv.
13051 13052 13053 13054 13055 13056 13057 13080 13081 13082 13083 13084 13085 13086 11159 11215 11229 56102 56130 S6138 S6122 56123 S6124 S6125 56126 13063 13063 Bianks
Mg.
equzv.
Mg.
equzv.
Mp: equzv.
0.34 0.35 0: 39
0.33 0.24 0.19 0.17 0.18 0.14 0.13 0.14 0.25 0.44
.. .. .. ..
.. ..
.... ..
..
..
Mg.
Mg.
Mg.
equzv.
equzv.
equzv.
14.59 10.56 8.03 9.98 16.91 14.62 12.30 9.46 11.64 14.67 14.95 13.24 13.02 11.06 7.35 1.24 7.24 5.78 9.33 9.05 9.84 9.35 8.46 8.44 8.80
14.56 10.52 9.48 10.71 18.11 14.84 12.39 9.35 11.55 14.63 14.63 13.04 13.75 11.31 8.62 1.78 7.19 5.94 8.97 9.02 9.91 9,08 8.79 8.45 8.74 27.17 27.46 0.28
-0.03 -0.04 t-1.45 +0.73 +l.ZO
0.26
$0.22 +0.09
-0.11 -0.09 -0.04 -0.32 -0.20
$0.73 $0.25 +1.27 +O. 54 -0.05 +O. 16 -0.36 -0.03 $0.07 -0.27 $0.33 $0.01 -0.06
Value in Limestone Experiments
The soil numbers preceded by S represent soil from a limestone experiment in which an acid soil very low in replaceable bases was treated with calcium carbonate. Here the differences between the quantitative determinations and the titration values are small. A great deal of time can be saved by using this titration method on experiments of this kind, obtaining the value for the increase in replaceable calcium by subtracting the titration value of the untreated soil from that of the treated. The method is suitable, since the limestone additions cause no appreciable increase in replaceable bases other than in calcium. Use in Base-Exchange Studies
Another use for this method has been found in base-exchange studies in this laboratory. A soil leached with a sodium or potassium acetate solution retains in a replaceable form the base of the wash solution. The amount of base retained as replaceable base can easily be determined by leaching with ammonium acetate and proceeding as above, thus avoiding time-consuming sodium or potassium determinations. Sample 13,063 shows results obtained when the soil was thus saturated with sodium and potassium. Where total replaceable bases only are wanted, as may be the case in soil type horizon studies, acidity studies, and in determining the degree of saturation of the soil with bases, the titration method gives quick and accurate results.
