WALLACE M . HAZEL AND WM. J. O’LEARY, N orton Com pany, Chippawa, O ntario, Canada
V ISITORS to the Research and Chemical Laboratories of this company have displayed so much interest in the practical operation of some small electric furnaces th a t it seems worth while to describe these in some detail; their simplicity and efficiency will undoubtedly appeal to chemists and engi
neers who are not fully satisfied with the performance of the conventional types of laboratory furnaces.
Nearly everybody who has used an electric furnace is familiar with the usual method of external winding. A re
fractory core is wound on the outside with a length of resist
ance wire calculated to raise the furnace to a predetermined tem perature; this winding is held in place by a refractory cement which in turn is surrounded by insulation. However, because of the thermal insulating properties of such refractory cores there is an appreciable drop in tem perature between the resistance wire and the center of the furnace formed by the core; it is not uncommon with this type of winding to find the center of the furnace 200° to 300° C. cooler than the resistance wire, across an air gap of only 3 inches. Shorten
ing the winding in order to raise the furnace tem perature may result in burning out the wire; if the wire does not m elt it is liable to fuse some ingredient in the retaining cement, in the insulation, or in the core, which ultimately fluxes the wire and causes a burnout. Consequently, unless operated at moderate temperatures (700° to 900° C.), externally wound furnaces may have poor service records.
On the other hand there is a definite need in some labora
tories for furnaces th a t will operate serviceably a t more elevated temperatures, from 1100° to 1500° C. and higher.
For instance, the ignition of precipitated alumina or silica to constant weight cannot ordinarily be accomplished under 1250° C. (1, 6,11); in the better texts on analytical chemistry a blast lamp is usually recommended for this purpose instead of standard types of electric furnaces. The inconvenience and disadvantages of blast lamps are too well known, particularly
Fig u r e 1. In t e r n a l l y Wo u n d Al u n d u m Co r e
in laboratories where speed and volume of work are prime factors, to require further mention.
An im portant reason for using electric furnaces in these laboratories is to minimize deterioration of platinumware at elevated temperatures. The authors are constantly fusing refractory materials th a t contain small amounts of iron, fer- rosilicon, carbides, free carbon, and iron oxides; in their hands, when such fusions are made with alkaline fluxes over gas-fired burners, the atmosphere surrounding the crucibles is often sufficiently reducing to cause alloying of p art of the iron with the platinum (6), with consequent erratic results in the iron determination and damage to the crucibles. This situation can be improved by prelining the crucibles with flux, but it is completely eliminated when fusions are conducted in a furnace whose atmosphere is subject to accurate control.
A loosely covered furnace like the types described is fully oxidizing.
The conventional externally wound furnace for carbon combustions has also been subjected to criticism since the advent of the more refractory high-carbon alloys and ce
mented carbides. Some analysts (9, 12) dispense entirely with the core, and wind their combustion tube directly with re
sistance wire in order to obtain sufficient tem perature to burn such alloys; to hold the element firmly in place, this practice requires th a t the combustion tube and winding be coated with a refractory cement. This procedure is satisfac
tory a t intermediate temperatures, provided the resistance wire does not melt any constituent in the cement and cause fluxing; the best grades of cements, however, contain very little low-melting constituents. When silica, glazed porce
lain, or siliceous clay tubes are wound directly with Kan- thal wire, failure is likely to occur above 1100° C.; the manufacturers of this resistance wire advise against having it in contact with free silica, low-melting silicates, phosphates, or ferric oxide, because these react with and flux the wire (10).
The analysis of refractory materials in these laboratories re
quires high-temperature fusions, ignitions, and combustions;
in order to make more efficient use of the various resistance wires available on the m arket the authors have adopted the furnace construction described below as applied to a crucible furnace.
Core
The use of cement to hold elements in place is usually disad
vantageous; the cores, shown in Figure 1, were therefore made to support the element a t all points to prevent sagging. The resistance wire is wound on the inside of the core and is left ex
posed, so as to obtain maximum heat transfer to the m aterial th a t is being heated. Laboratory muffle and tube furnaces of various manufacturers embodying this construction have been on the m arket for some time, but in the authors’ hands they have needed frequent repair because of the necessarily high operating temperatures. The core is made of Alundum: this contains no bond constituents th a t might melt, react with the heating ele
ment, or distill out and contaminate crucibles or samples.
This core is also made in two pieces; both pieces can be wound continuously w ith a single length of wire, the two halves can be wound separately and then connected in series, or with suitable current adjustm ents the two halves can be connected in parallel.
Separate winding of both halves has the doubtful advantage th a t a single defective unit can be removed for repair; th e authors have, however, preferred to wind both halves together w ith a single length of wire, because failure of one unit in a series after normal service is usually followed rapidly by failure of th e others due to progressive oxidation, recrystallization, and the like.
110 IN D U STR IA L AND E N G IN E E R IN G CH EM ISTR Y VOL. 12, NO. 2 H eating Elem ents
Any commercial resistance wire m ay be used, depending on the tem perature desired. Chromel and Nichrome, for instance, are satisfactory for tem peratures under 1100° C. with this type of core (IS); from 1100° to 1300° C. K anthal has been found very serviceable; from 1300° to 1500° C. a platinum or platinum- rhodium winding is m ost satisfactory; and molybdenum or tungsten (8, 16), operated in an atmosphere of hydrogen, m ay be used for still higher temperatures. In general, a resistance wire m ay be used serviceably a t a tem perature 150° C. below the softening or failure point of the wire; this information, together w ith d ata on specific resistance, calculations as to lengths re
quired for certain temperatures, etc., are obtainable from manu
facturers’ and distributors’ literature (8, 7, 10); good reviews of practical data on furnace construction have been published (14, 16). The proper length of wire, including leads th a t are left uncoiled, is then wound on an arbor of such size th a t the outside diameter of the coil will slide through the grooves in the core: a coil 0.1875 inch in outside diameter wound on a 0.125-inch arbor is used in the core shown in Figure 1. The wire should be wound with a light, even tension to avoid stretching and consequent hot spots. After winding, the coil is stretched to the proper length and threaded through the grooves.
As b e s t o s.Bo a r d
fore beginning to thread the wire: the tape or string will burn off when the furnace is heated, b u t when firmly tam ped the insulation will hold the core immovable.
I t is obvious th a t the better the insulation of the wound core, the better will be the performance of the furnace. The authors’ prac
tice has been to place the 4-inch outside diameter core assembly (Figure 1) on a standard 4-inch diameter Alundum disk to keep insulating m aterial from sifting into the bottom of the furnace;
both core and disk are then placed upon a Sil-O-Cel (8) No.
JMC-22 brick in a sheet-iron shell 8 inches in diameter by 6 inches high, and are surrounded with insulation. M ost of the authors’
furnaces are packed with firmly tamped, powdered Sil-O-Cel, surrounding the core to a thickness of 2 inches: the size of the shell and the am ount and type of insulation will depend largely
on convenience and the space available for the furnace. Asbestos board, or better still, a thin layer of refractory cement over the insulation prevents dusting.
The heat retentivity of this type of assembly, shown in Figure 2, can be appreciated after studying the data in Table I, as determined from typical furnaces.
All the tem peratures were measured with the furnaces op
erating in a hood with a good draft. The central tem peratures were measured w ith a potentiometer and thermocouple, b u t the tem perature of the shells, which were air-cooled under these conditions, was determined by means of a mercury thermometer attached to the shell w ith Plasticene or bound in place with asbestos paper. Such a shell can be painted successfully on the outside with “bronzing liquids” ; if linseed oil is present, however, the paint will soon scale off.
The top cover for such a crucible furnace should be selected for both its insulating value and durability. Molded clay and Alun
dum tops are the m ost durable but are th e poorest insulators; a furnace covered with a porous “H-W No. I l l refractory” top (4) runs from 75° to 100° C. hotter than when covered with a clay or Alundum top. One of the best furnace covers, although rather crumbly, is Sil-O-Cel No. JMC-22 brick (asbestos bonded): this where most steels require no accelerator or flux; a silica tube will ordinarily be expected to devitrify and crack before failure of the element occurs.
T a b l e I . T e m p e r a t u r e M e a s u r e m e n t s o n C r u c i b l e F u r n a c e s w i t h 3 - I n c h I n s i d e D i a m e t e r Outside
D iam eter R ating Looation T em perature
Inches Walts ° C.
As the electric current supplied to some localities is subject to fluctuation because of peak loads, line voltage drops, etc., actual measurements should be made on the power available before condemning a factory-built internally wound furnace for poor service, or before proceeding to build a home-made furnace with internal winding. Provided the specific resist
ance and tem perature coefficient of resistance of the wire element are known, a definite length of wire carrying a definite power input as an internal winding will carry a furnace a t a certain tem perature range w ith a given insulation; some typical d ata from the authors’ crucible furnaces are listed in Table II.
These furnaces are operated interm ittently, running 10 hours a day for 5 days a week; the life of a furnace in this tem perature range on this schedule can confidently be expected to be from 9 to 18 months. On the other hand
ex-FEB RU A RY 15, 1940 ANALYTICAL E D IT IO N 111 contamination of the furnace and winding, there is embedded in the center of the Sil-O-Cel brick a plate of a nonflaking molded refractory (Alundum or H-W 111) 4 inches in diameter to cover the core.
Sil-O-Cel shrinks and sinters markedly above 1350° C., so a layer of 200-mesh flint 0.5 inch thick is placed next to the core and is held in place by a rolled sheet of paper; powdered Sil-O-Cel is then used to fill the intervening space between the shell and the flint. The flint is the poorer insulator, b u t the tem perature drop across the 0.5-inch thickness is sufficient to pro
tect the Sil-O-Cel and preserve its insulat
ing value.
ternally wound furnaces of the same type, size, and power in
pu t seldom attained 1000° C., and seldom had a life exceeding 100 to 300 hours; a tem perature of 1250° C. could be reached by increasing the power, b u t the life of the element, even though embedded in specially prepared cements, was cor
respondingly shorter.
P latin u m -W o u n d Furnaces
Furnaces wound with pure platinum will probably not give long service a t high temperatures because of the volatility of the platinum. A furnace similar to the ones described, but wound with platinum and more heavily insulated, was op
erated for a time between 1350° and 1450° C. with a power input ranging from 550 to 650 w atts; its resistance, however, increased slowly and steadily a t these temperatures, indi
cating loss of platinum.
Alloys such as platinum-rhodium have better heat stability, and are recommended for this purpose by the manufacturers of precious metals. Platinum-wound furnaces should be op
erated with an external resistance (preferably variable) in order to avoid an overload of current a t the sta rt which might melt off the leads; platinum and its alloys have a rather high positive temperature coefficient of resistance, and the leads have less opportunity to dissipate their heat than has the winding. As an added precaution against overheating the authors use doubled leads as far as the core, made from leads twice the necessary length, doubled back on themselves and described, but are enclosed in a shell 10 inches in diameter direction of the incident and transm itted light is changed.
The question as to whether a platinum-wound furnace is practical m ust be decided by each individual, usually on the basis of cost of base metal windings, cost of replacements, and length of service. Against the disadvantage of the initial high cost of the platinum there are the advantages th a t the metal has intrinsic value which can be liquidated, th a t a burned-out winding is never a total loss, and th a t the wire can be mended if broken, or even lengthened if necessary, very simply and in a m atter of minutes by means of an underwater arc. M any base metals can, of course, also be welded in the same way, provided embrittlement with use does not make this impractical.
Operating Tem peratures
The maximum tem perature th a t a furnace will attain is not necessarily its practical operating temperature. The authors arbitrarily define the operating tem perature as th a t
112 IN D U STR IA L AND E N G IN E E R IN G CH EM ISTR Y VOL. 12, NO. 2 rule these operations involve from 10 to 30 seconds, and the
furnaces are not cooled much more than 50° to 100° C.; if, however, a bulky charge is placed in a furnace, or the top is left off for a longer time, a proportionately longer interval m ust be allowed for the return to operating temperature.
Furnaces are wound for operation within a given tem pera
ture range directly on the line current w ithout external resistances, except as noted in connection with platinum wind
ings. External resistances can be a nuisance in a busy labora
tory, because when a hot furnace is required in a hurry the tendency is to cut out all the resistance to ensure rapid heat- mg; if one forgets to adjust the resistance a t the proper time on a furnace th a t is overpowered for direct operation, some
thing is bound to melt, necessitating the rebuilding of the fur- nace. more cumbersome type of cover used on the platinum-wound furnaces. In addition, however, to the added inconvenience of such a bulky top, the practical operating tem perature of the furnaces is not increased sufficiently to w arrant such a cover when the furnaces are being constantly opened and closed.
Connectors
Ju st as im portant for the life of the furnace as the resistance winding is the method of connecting the ends of this winding to suitable terminals or binding posts; improperly made con
nections will cause an early failure of the wire near the point of contact because of a tendency to arc, because of embrittle
ment due to recrystallization of the wire, or because of a tendency toward accelerated oxidation a t this point. Con
nections are best made a t a point outside the furnace so as to afford maximum air-cooling of the terminals. For this pur
pose, the ends of the winding are led through Alundum tubes and brought through the shell a t some convenient point;
they are then attached to metal terminals rigidly fastened to a bracket supported by the shell. The m etal terminals, shown in Figure 3, should be large enough to dissipate heat rapidly from the ends of the winding; as set screws used to make con
nections often freeze upon being heated, and tend to oxidize rapidly, it has been found more convenient to om it screws, drill the terminals slightly larger than the resistance wire, crimp the wires, and force them into the holes. Suitable con
nector plugs are then attached to the terminals. A number of furnaces were built in which connections were made inside the shell and the terminals protruded through the shell; these were very compact and neat in appearance, b u t all failed within a short time a t the connections.
Acknow ledgm ent
The writers wish to express their appreciation to J. A.
Upper for all the electrical measurements and for his help and suggestions in improving these higher tem perature furnaces.
A small furnace embodying these ideas is now being tested with inorganic microcombustions.
All the construction is of glass, blown to form one piece. Tube A of 3-cm. diameter, is sealed a t the top to the egg-shaped bulb, C, and a t th e bottom fits into the conical flask, B. Several holes, about 2 mm. in diameter, are made about 5 mm. from the bottom of tube A . The bottom of C opens into tube D, which extends about 2 mm. below the bottom of A. A spiral of tube E, about 5 cm. in diameter and closely wound, is sealed to the side of C and to th e top of B. Tubes G and F make connection a t the top of A and C, respectively, and are extended to make convenient con
nections to the gas supply and to the apparatus requiring the gas.
A short tube, H, is sealed to