July, 1914
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
t o t h e same source as d o t h e other water-soluble salts. Further, Stewart a n d Greaves claim t h a t in Headden’s work wherever there was a variation in nitric nitrogen there was a variation in chlorine in t h e same direction which would seem t o indicate a common origin of t h e nitrates a n d chlorides. T o t a k e a particular case, they point out t h a t where there was a n increaese in t h e surface soil of j61 pounds of nitrates per acre two inches of soil during t h e years from 1909-1911 there was a n increase of 10,430 pounds or over five tons of chlorine. I n another case, referred t o by Headden in Bulletin 155, t h e nitrate nitrogen increased from 1907 t o 1911 from a trace t o 621 pounds. I n t h e same interval t h e chlorine content increased 236,883 pounds. They come t o the inevitable conclusion t h a t there must be a n upward movement of t h e water-soluble salts, t h a t t h e chlorides must come from t h e ground water. Accordingly, t h e y ask t h e pertinent question, “ W h y may not t h e nitric nitrogen be accounted for in t h e same way?” Evaporation of t h e soil water would explain t h e deposit of nitrates since according t o their calculations, assuming t h e optimum amount of water, 18 per cent, t o be present, only one-half year of maxim u m evaporation would deposit t h e quantity of nitrogen actually deposited in two years. If t h e ground water contains only 74.48 parts per million of chlorine as computed b y Headden, t h e evaporation would account for only 203 pounds of chlorine, whereas t h e actual amount found in t h e samples mentioned was
I
533
m a n y times greater. So they conclude t h a t t h e ground water has a greater concentration in chlorine a n d nitrogen t h a n assumed a n d t h a t both accumulate in t h e surface soil by evaporation of t h e water. Stewart a n d Greaves do not deny t h a t nitrogen fixation m a y take place to a certain extent in the Colorado soil a n d in some places t o a n appreciable degree, b u t do hold t h a t whatever theory accounts for t h e accumulation of chlorides in t h e Colorado soils must account also for t h e greater portion of t h e nitrates present. T o these arguments of Stewart and Greaves, Headden has opposed numerous objections which cannot be considered here. Suffice i t t o say, t h a t while there can be no possible question of t h e occasional occurrence of abnormal quantities of nitrates in t h e “alkali” soils of Colorado, t h e origin of these excessive nitrate accumulations is not settled. I n their endeavor t o explain t h e origin of the nitrate a n d t o remedy t h e conditions as t h e y arise, t h e various investigators should meet with general encouragement. I t is greatly t o be desired t h a t t h e nitrate formation should be considered from all viewpoints t o t h e end t h a t accumulating d a t a a n d suggestions may t h e more quickly develop a n harmonious conclusion, t o t h e advantage of practical agriculture a n d t o t h e increase of t h e sum total of human knowledge. h l . X . SULLIVAN BUREAUO F SOILS DEPARTMENTOF AGRICULTURE WASHINGTON
1
ORIGINAL PAPERS ELECTRIC FURNACES FOR HEATING STEEL’ B y ALCANHIRSCH Received M a y 25, 1914
T h e field of usefulness of t h e electric furnace for metallurgical purposes is so extensive t h a t i t is deemed advisable t o limit t h e scope of this paper t o a discussion of electric furnaces used for heating steel for t h e various kinds of heat treatment, forging a n d enameling. A broad view of t h e development of electric furnaces b y t h e writer a n d his associates during t h e past year, together with details of design, construction a n d operation, as determined b y t h e m are presented herein. T h e essential d a t a only are given as i t is believed t h a t extensive details are likely t o lead t o confusion. I t is thought such a presentation of basic principles will make t h e paper of more value t o users of electric furnaces t h a n an extended report of all t h e d a t a collected. Prior t o 1913 a t t e m p t s were made t o p u t forth furnaces for metallurgical purposes, b u t except for t h e very small furnaces, these cannot be considered as having h a d commercial success. T h e facts which form t h e basis of this paper occurred under t h e writer’s observation a n d are practically exclusively gathered from his experience of t h e past year. By reason of industrial practice a n d certain other 1 Author’s abstract of report on research carried out under a Carnegie Fellowship granted by the Iron and Steel Institute of Great Britain. The complete report of this work was presented a t the Annual Meeting of the Institute, May 7 , 1914.
limitations, both fuel a n d electric furnaces’ can be divided into two classes: I-Furnaces operating above 1800’ F. Forge furnaces are t h e main a n d most important division of this class. 11-Furnaces operating below 1800’ F. This class comprises furnaces for practically all heat treating as well as enameling. Although furnaces operating a t t h e lower temperatures will be considered first i t must be borne in mind t h a t t h e greater p a r t of t h e principle a n d theory underlying t h e construction and operation of moderate a n d low temperature furnaces also applies to t h e higher temperature furnaces. TRANSFEREKCE
OF
HEAT
PROM
HEATIIiG
XEDILbI
TO
METAL
The metal resting on t h e hearth of t h e furnace receives its heat in several different ways: ( I ) From t h e brickwork in t h e furnace in contact with t h e metal; (2) by conduction from t h e products of combustion; (3) b y radiation from t h e hot walls, roof a n d incandescent particles in t h e burning gases. Generally speaking, in t h e fuel-fired furnaces, each of these paths delivers heat of t h e same order of magnitude, b u t usually t h e amount of heat passing by means of brick a n d metal in contact is less t h a n t h a t by a n y other p a t h . If only a small portion of t h e heat passes into t h e m t t a l b y direct contact with t h e brick, t h e rate of heating in all except thin pieces is quite slow^. LIore frequcntly t h a n is generally supposed this p a t h of heat trans-
. 534
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
fer is t h e determining factor in t h e rate of heating. An excellent example of this kind was brought t o t h e writer's attention where die blocks were being heated. I n this instance t h e furnace was heated t o a sufficiently high temperature so t h a t t h e heat content of t h e brick work was sufficient t o supply t h e necessary heat t o raise t h e blocks t o t h e desired temperature. Sometimes t h e fuel was allowed t o r u n sparingly througho u t t h e operation, while a t other times i t was shut off entirely after t h e block was placed in t h e furnace. A T M 0S P H E R E I N F U R N A C E S
T h e atmosphere of fuel-fired furnaces is exceedingly uncertain. Slight variations in conditions have been found t o make marked variations in results, a n d as t h e atmosphere is capable of a great many variations, i t is, therefore, quite difficult t o maintain it a t a definite composition. With oxidizing conditions t h e formation of scale occurs, while in a reducing atmosphere local carburization results from t h e sooty flames. I n t h e production of high-grade steel t h e condition of hearth atmosphere is, of course, exceedingly important. T h e electric furnace provides in many respects just what t h e fuel furnace lacks; i. e., a means for t h e transference of heat in a very effective manner, a n d a furnace atmosphere which is not only of a very desirable composition, b u t which is absolutely dependable. This atmosphere is usually of a slightly reducing nature, caused b y t h e presence of carbon monoxide, due t o t h e combustion of t h e graphite or carbon resistor which liberates t h e electrical energy in t h e form of heat. I n some furnaces having more t h a n one door, or operated with doors open all t h e time, t h e atmosphere may be neutral. It is due t o these neutral or reducing conditions t h a t t h e formation of scale is greatly minimized. The writer has in mind a n electric furnace which was operated with a loss of scale amounting t o eighty or even ninety per cent less t h a n was occasioned by t h e employment of a n oil-fired furnace for t h e same work. For special work where a n oxidizing atmosphere is required, as for instance in enameling, this is easily accomplished in t h e electric furnace b y employing a muffle, t h e resistors being placed in a n y desired position on t h e outside of t h e muffle. ELECTRIC F U R N A C E S F O R T E M P E R A T U R E S B E L O W
1 8 0 0 F. ~
FURNACES W I T H METALLIC RESISTORS-The industrial electric furnaces of this t y p e which have obtained commercial success have employed a resistance wire or ribbon as t h e heating element. The limitations of these wire or ribbon-wound furnaces are quite marked, generally speaking, as regards both temperature a n d capacity. As will be shown subsequently, temperat u r e a n d capacity of a furnace are closely interrelated. This interrelation of temperature a n d capacity, however, is not of so much consequence in t h e small furnaces where t h e combined wall a n d door losses are considerably in excess of t h e heat actually utilized in raising t h e metal t o t h e desired temperature. T h e capacity of t h e metallic resistor furnace is at most b u t a very few kilowatts. Furnaces with a larger capacity would be quite out of t h e question because of t h e cost of t h e resistance element due t o t h e large amount of
Vol. 6, No. 7
wire required a n d t h e expense of winding. The furnaces are, therefore, limited t o productions of small size a n d also t o rather moderate temperatures as danger of burning out due t o overheating is quite imminent. For small furnaces, however, this type has proven quite satisfactory i n a large quantity of work of a n experiment31 nature. F U R N ACE s E MPL OYI K G N O E-MET ALLI c R E SI s T o R s co mprise two types: ( I ) Those where t h e metal t o be heated is in contact with t h e resistor; ( 2 ) those where t h e metal t o be heated is out of contact with t h e resistor. Furnaces of t h e first class have h a d b u t one commercial example, and t h a t has h a d varying success. This is t h e b a t h furnace' which employs a conducting b a t h of salt, usually barium chloride a n d potassium chloride, which is fused b y - t h e passage of t h e current through it. The steel t o be heated is immersed in this b a t h of fused salts. This t y p e of furnace appears t o t h e writer t o be too limited for extensive industrial application, and, therefore, will be given only this brief mention. Furnaces of t h e second class, those employing nonmetallic resistors, where t h e metal is heated o u t of contact with t h e resistor, hold forth much promise for future development, in t h e opinion of t h e writer. Recent experience with their operation has demonstrated their suitability t o many kinds of work. I n general, furnaces of this class appear t o t h e casual observer, very similar t o t h e fuel-fired furnaces, save for t h e fact t h a t instead of equipment for burning fuel, electrical equipment will be noted. The electric current is Srought t o t h e furnace by suitable cables which are connected t o electrodes projecting from t h e furnace. These electrodes r u n through t h e furnace wall a n d carry t h e current t o t h e resistor which liberates, in t h e form of heat, t h e electrical energy p u t into t h e furnace. The resistor is of a refractory conducting material, such as graphite, usually in granular form, a n d has a cross-section of 30 t o I O O square inches according t o t h e current desired. The resistors are usually placed beneath t h e hearth, t h e heat from t h e m being communicated through t h e hearth t o the metal. For t h e design of a heat-treating furnace t o operate a t a hearth temperature of 1800' F., or less, t h e following d a t a have been found necessary for t h e preliminary calculation of t h e major points of design: I-The hearth dimensions. 2-The production of metal per unit of time. 3-The maximum amount of metal on t h e hearth a t a n y time. 4-The desired temperature. 5-Time for charging a n d discharging. LOCATION OF RESISTOR-The first step in t h e design is t h e approximation of t h e location of t h e resistor, b u t this depends somewhat on t h e physical characteristics of t h e material employed for t h e resistor. Resistors placed in t h e furnace in granular or similar form have been much more extensively employed in t h e larger furnace t h a n a n y other kind. Rods of 1 An article on this furnace by I . Electrolech Zei!., Aug 2, 1906.
M
Cohn will be found in the
T H E J O C R N A L OF I N D C S T R I A L A N D ENGINEERING CHEMISTRY
July, 19I 4
graphite and also of other materials, metalloids as well a s t h e characteristic non-metallic materials, have been tried for use as resistors. Although some of these will undoubtedly find commercial fields, as yet nothing has proven satisfactory in this direction. Attention, therefore, will be confined t o granular or similar materials, of which granular graphite has served most satisfactorily. For the usual t y p e of electric furnace work of this class the location of t h e resistor is logically in t h e base of the hearth. For a small proportion of the furnaces, however, resistors can be placed elsewhere advisedly. These positions are along the side of the hearth and possibly even along the top. Furnaces requiring resistors in these latter locations are those taking piles of sheet metal or pots of materials, and the like. However, with one layer of pieces, which rests directly on t h e hearth, the location of the resistor had best be exclusively in the base. The reasons for this are: ( I ) heat has a tendency t o ascend rather t h a n t o descend; ( 2 ) contact between hot brick and the metal t o be heated facilitates heating; (3) t h e design is facilitated as will be subsequently developed. When the resistors are placed in t h e base of the furnace they are p u t in troughs of suitable refractory material and usually, b u t not always, covered partly or completely with brick or til?, which forms t h e hearth. S H A P E O F RESISTORS-The shape of t h e resistors can be exceedingly varied. They may be straight, TJ-, S-, T-, or Y-shaped. T h e y may be electrically connected in series or parallel, or some in series and others in parallel. They may be permanently electrically connected, or they may be capable of various electrical arrangements b y switching. P E R 3IA N E S T LY
E L E C T R I CA L LY
C 0 P;K E C T E D
R1: SI S T 0 RS
Furnaces with these resistors are constructed so t h a t they must be operated in one manner, a t all times from the point of view of electrical arrangement.
FIG. I-SERPENTINE
RESISTOR FOR MODERATELY LARGE FURNACES
I n practically all furnaces of this class either a serpentine or U-shaped resistor niay be employed, but two or more straight resistors connected either in series or in parallel m a y be used instead. Fig. I shows t h e serpentine resistor which has been employed in moderately large furnaces only, i t being impossible t o adapt this t y p e t o t h e smaller furnaces. T h e U-shaped
53 5
resistor shown in Fig. I1 hasrbeen employed in furnaces of any size b u t has some limitations m-hich will be considered later. E L E C T R I C A L LOAD-There are several ways t o determine the electrical load for a given furnace, b u t they all resolve themselves into one method, which is the only one deemed sufficiently practical t o be given consideration in this paper. Only furnaces of zoo kilowatts capacity or less will be considered and it may be said t h a t furnaces for heat treating larger t h a n this are exceedingly rare. The electrical load is the sum of three factors: ( I ) t h e electrical equivalent of the amount of heat necessary t o raise t h e metal t o t h e required temperature; ( 2 ) the electrical equivalent of t h e loss of heat through t h e walls; (3) t h e electrical equivalent of the loss of heat through the door, in consideration of t h e fact t h a t this is alternately opened and closed. Since t h e power factors of furnaces of this size are from 97 t o 99 per cent, they can be neg-
FIG.11-U-SHAPEDRESISTOR
lected in t h e calculation of t h e necessary wattage. From the d a t a in Table I the watts necessary t o operate any particular furnace may be approximated quite closely. The door loss shows the watts passing through the door opening if t h e door is open all t h e time. If the door is open only half the time, only half the amount given in the table should be taken, and so on in proportion. The wall upon which the wall loss figures are based is 1 2 inches thick, consisting of 9 inches of silica brick and 3 inches of kieselguhr. By actual practice this has been found t o be a convenient standard. As a n example of the method of calculation, the electrical load for a particular furnace will be determined. Assume the following d a t a for this illustrative case: I-Outside dimensions-4 ft. X 4 ft. X 6 ft. long. a-Production--j o o pounds of steel per hour. 3-Temperature-I 700 O F. 4-Door-2 sq. it., open 40 per cent of the time. This furnace had a total outside area of 1 2 8 sq. ft. The table shows a wall loss of 0.060 watt per sq. f t . , making a total loss of 7.68 k.w. for the walls, top a n d base. The door loss would be 10.0 k. w. if t h e door were open all the time, b u t it being open b u t 40 per cent of t h e time, the loss is 4.0 k. w. As 0.0881 k. JV. is required t o raise I lb. of steel t o I . ~ O O " F. from 6 0 " F. in I hr., j o o lbs. per hr. would require 44.05 k. W.
T H E JCLURNAL O F I N D C S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
536
TABLEI
Wall loss(o) Door Kilowatts i n k . w. loss(b) t o raise per sq. ft. i n k . w. 1 lb. per hr. outside F. C. from 60' F. surface per sq. f t . 0.6 538 0.0366 0.035 594 0.9 0,0460 0.039 649 0.0527 0.042 1.3 704 1.7 0.0597 0.046 2.3 760 0.0668 0,049 815 3.0 0.0733 0.053 843 3.5 0.0765 0.055 870 0.0796 0.056 3.9 4.5 898 0.0840 0.058 926 5.0 0.0881 0.060 5.6 954 0.0926 0,062 982 6.3 0.0970 0.064 1010 7.1 0.0994 0.065 20.0 1316 0.122 0.085 ( a ) Based on wall 12 inches thick (9 inches of silica 3 inches kieselguhr). (b) Door open all t h e time.
---
Temperature of operation
.
+
Of
T h u s i t will be seen t h a t this furnace will require 55.7 k. w. for operation. No factor of safety need be applied t o this figure if t h e conditions selected are a t t h e maximum. On the contrary, if these are normal operating conditions a factor of safety should be applied according t o t h e possibilities of greater demands being made on t h e furnace. After calculating t h e number of kilowatts necessary for operation, t h e length of the resistor is approximated in order t o determine t h e voltage. With granular s a p h i t e l experience has shown t h a t the most satisfactory voltage is equivalent t o 1 l / 2 volts per inch length of the resistor. From this voltage and the wattage as computed above, the number of amperes may be easily determined. E. Y. F. REQUIREMENTS-The electrical resistance of a resistor in a furnace cannot be sufficiently closely predicted t o warrant calculating t h e size of the resistor with a very great degree of certainty. Of necessity, therefore, t h e exact voltage which will be required for a furnace t o take a certain number of kilowatts can be determined only approximately. A provision for obtaining various voltages is, therefore, necessary, a n d a transformer with several taps is ordinarily employed for this purpose. However, it is perfectly possible t o provide a variable voltage generator for t h e same purpose. Since t h e precise production of steel for a given furnace cannot be very closely ascertained. and since, in most cases, different productions are desired a t different times, it is absolutely necessary t h a t provision be made for altering the kilowatt input a t will of the operator. The usual and satisfactory method of meeting these requirements appears t o be the employment of a transformer with I O t o I j taps. Usually 13 is a satisfactory number, having a range of voltages on t h e secondary from a minimum equivalent to one volt per inch length of t h e resistor t o a maximum equivalent to two volts per inch length of t h e resistor. The various t a p s on t h e transformers used in most instances have given voltages which are in arithmetical progression, but i t is the opinion of the writer t h a t a progression of voltages in unequal steps is best suited for the work. For t h e purpose of making provision for t h e uncertainty of t h e resistance of t h e resistor, t h e voltages would logically be chosen in arithmetical progression. For purposes of regulation, however, since t h e killowatt in-
'
Artificial graphite averaging '/*-inch mesh, but containing no fine powder.
Vol. 6, No. 7
p u t increases as the square of t h e voltage, it would appear, from this point of view, t h a t the voltage steps should be graduated to best meet this condition. The two conditions must be met, and t h e most satisfactory arrangement is t o make t h e steps in such progression t h a t the difference of t h e kilowatt input on adjacent taps in the higher voltages will not be so very much larger t h a n on t h e adjacent t a p s on t h e lower voltages. Accordingly, a satisfactory range of potentials on a transformer with 13 taps would have voltages equivalent to the following, per inch length of t h e resistor: 1.00,1.10, 1.20, 1.29, 1.38, 1.47, 1.56, 1.64, 1.72, 1.80,1.87,1.94,2.00. E L E C T R I C A L R E G U L A TI0 N
W I T HO C T T R A N S F 0 R bf E R
A methodl for obtaining this regulation and adjustment without the use of a transformer has been devised in t h e writer's laboratory very largely through the work of Mr. Richard S. Bicknell. I n this type of furnace several resistors are employed, which are not permanently electrically connected, and which by means of suitable switches may be connected in various ways while the furnace is in operation. They may be arranged in series, in parallel or in any combinations necessary t o effect t h e desired regulation. I n other words, this is regulation by altering t h e resistance of t h e resistor as contrasted with t h e aforementioned method where regulation was effected by altering t h e voltage impressed upon t h e resistor. As will be shown subsequently this type of regulation is particularly adapted t o furnaces having I O sq. ft. of hearth area or over. An example of a furnace capable of such regulation is shown in Fig. 111. With these four resistors in this particular furnace i t is possible to obtain I I O inches in length of resistor, or equivalent of same, in t h e circuit a t one time, and 220 inches in length of resistor a t another. A4large number of intermediate lengths of resistor between this maximum and minimum figure may also be placed in operation. This particular furnace is designed t o operate on 2 2 0 volts and i t will be readily seen t h a t t h e maximum voltage obtainable per inch of resistor is two volts a n d t h e minimum is one volt. A quite surprisingly large number of intermediate lengths of resistor are obtained by employing the four T-shaped resistors, as shown. The length of t h e resistor is, of eourse, merely another way of stating t h e resistance of the furnace. These T-shaped resistors have each three unequal legs. Resistors A and D are similar a n d B and C are similar, b u t A a n d B. have corresponding legs of different lengths. The resistance of the furnace resistors for a number of intermediate steps is made by connecting two legs in parallel in instances when a low resistance is desired. When a high resistance is wanted t h e resistors are run in series the current passing through the longest legs only. By properly proportioning t h e legs, i t will be seen t h a t t h e number of intermediate steps for purpose of regulation may be made as large as desired. It is, of course, possible t o combine these two methods of regulation, namely by voltage and resistance, having a few steps on the transformer and having one or a 1
Patented
July, 1914
T H E JOURiVAL O F I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
few resistors capable of being arranged either in series or parallel. A furnace so regulated is shown in plan in Fig. I V and t h e method is quite suitable for small furnaces of from 4 t o I O sq. it. hear h area. The construction of the resistor as shown in vertical section would be quite similar t o t h a t in Fig. 111. W I D T H A N D D E P T H O F RESISTOR-This discussion applies t o both types of furnaces where the two methods of regulation are employed, i. e . , either altering the voltage or t h e resistance. I t has been found t h a t a resistor placed beneath t h e hearth can be composed of two layers of materials t o advantage, the upper of granular graphite and t h e lower of some material
been computed for the normal running condition of 1l/2 volts per inch length of resistor. S H A P E O F RESISTOR-The width and depth of the resistor should be such t h a t as much heat as possible is liberated in the desired direction. For resistors in the hearth this direction is, of course, upward. According t o the theory, therefore, the logical shape of resistors of this sort would be as wide as possible and quite shallow. This section, however, is not a t all feasible for several reasons. The resistor burns away more rapidly when it is wide and it is more difficult t o spread the graphite on a wide resistor when it is replenished. Wide resistors require more lining and the expense of the lining is a relatively important item in t h e cost. When wide resistors are made t o run a t right angles the current has a tendency t o flow across t h e interior corner in much higher intensity t h a n a t the exterior corner. Sometimes carbon or graphite blocks have been placed in the resistor a t the corners for the purpose of reducingIthis local effect. This, however, is not a very good remedy, as the heat
FIG.
IV-FURNACE REOULATED BY ABLE FOR 4 TO
FIG. 111-FURNACE C A P A B L E FORMER-ADAPTED
OF REGULATION WITHOUT A T R A N S FOR 10 sQ. F T . OR O V E R HEARTHAREA
of lower electrical conductivity t h a n graphite, such as charcoal. The lower layer takes a smaller part of t@e current than does t h e upper layer of graphite, thus placing the major part of t h e heat liberated quite near t o the top of the resistor. This method seems t o protect the part of the lining upon which t h e resistor rests. It protects i t sufficiently well not only t o warrant its use, but t o make its use absolutely necessary in the case of furnaces operating close t o or above 1800' F. A resistor consisting of half charcoal moderately tamped by hand and half graphite gently tamped in is very satisfactory. T h e resistance of a n inch cube consisting in the upper half of granular in.) and the lower half of hard graphite (pieces 3/32 t o wood charcoal put in according t o the method described above is approximately 0.125 ohm a t 1700' F. T h u s the area of the cross-section of t h e resistor may be easily determined after the current necessary has
53 7
V O L T l C E AND RESISTAXCE-SUIT-
10 SQ. F T . H E A R T H AREA
liberated in any event is not so great per unit area of t h e resistor in the corner as in other parts of the furnace. The cross-section of the resistors had, therefore, better be made about square, or wider t h a n the depth by a small amount. If possible t h e resistors should not be narrower than their depth, b u t it is impossible t o observe this requirement in all cases. Resistors less than 2 ' / 2 in. wide should not be used. They should be not less t h a n 6 in. deep and preferably about 7 in., except for resistors over I O inches wide which can be made 8 in. deep, though more t h a n this is likely t o cause excessive heating in the base of the furnace. MAXIMIUM L I M I T A T I O N I K S I Z E O F RESISTORSThe designer frequently has the opportunity of employing one large resistor or two smaller ones t o do the same work. Small resistors less t h a n 3 in. wide should be avoided because slight variations in shape have a more marked effect on their resistance t h a n in the case of the large resistors. It is much better as a rule t o employ a short resistor 4 or 5 in. wide than a correspondingly longer one 2'/2 or 3 in. wide. On the other hand, large resistors are also t o be avoided. Since the
53 8
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
thermal conductivity of graphite is rather low it is evident t h a t large resistors are much more likely t o become excessively heated in their centers t h a n small ones. Resistors 6 t o g in. wide are t o be used wherever possible, and for resistors of this type 1 2 inches wide a n d 8 inches deep are about the maximum dimensions for furnaces used for heating steel. This is quite too large, however, for a n y heat-treating work, b u t i t is mentioned merely t o give an approximation of the maximum limitation of this type of furnace. A 1 2 b y 8 in. resistor would carry 1000 amperes and a number of them could be arranged in a furnace so as t o liberate 16 k. w. per sq. ft. of hearth. This is equivalent t o a production of about I I O lbs. of steel heated t o 1700' F. per hr. per sq. ft. of hearth under the usual conditions. This production is not only more t h a n is usually desired, b u t is far too much for good work. Experience has shown t h a t a maximum production of about 50 lbs. per hr. per sq. ft. of hearth a t 1700' F. is all t h a t can be expected in electric heat-treating furnaces. P R O D U C T I O X OF S T E E L - T h e temperature and t h e production of steel from a n electric furnace are mutually dependent. T h e heat liberated in the resistor, if not taken up b y t h e metal, will occasion a rise in temperature of the furnace. T h e larger t h e electrical capacity per unit area of hearth, the greater the effect on t h e temperature b y alteration in the production. It is for this reason t h a t productions over 50 lbs. per hr. per sq. f t . of hearth should be avoided. With moderate capacities of 4 or 5 k. w. per sq. f t . of hearth area (equivalent t o a production of 3 0 t o 35 lbs. of steel per hr. t o 1700' F.) variations in production have b u t very little effect on t h e temperature. T h e heat capacity of the resistor lining and brickwork in furnaces of this size is amply able t o compensate for changes in production, so t h a t the temperature remains practically t h e same. A furnace designed for a normal running load of 4 k. w. per sq. ft. of hearth (with 1'/2 volts per inch length of the resistor) will prove very satisfactory. The uniformity of temperature on t h e hearth in furnaces employing the T-shaped resistors shown in Fig. I11 is quite remarkable, b u t even with the U-shaped or serpentine resistor a temperature variation of less t h a n IO' F., in any point of the hearth from t h e desired temperature, is t o be expected. ENAMELING F U R N A C E S fall quite in the same category with t h e heat-treating furnaces. Although they are larger in size they are not correspondingly large in electrical capacity. For enameling furnaces resistors should be placed on the sides, but about three-quarters of the kilowatt input should be liberated in the base. Resistors when placed along the sides of a muffle of a n enameling furnace should be small, I O t o 20 sq. in. in section and should consist entirely of graphite. T h e lining of such a resistor is usually designed so a s t o form a part of t h e interior wall of the muffle. E L E C T R I C F U R N A C E S FOR T E M P E R A T U R E S A B O V E 1800' E.
With respect t o t h e class of furnaces operating over 1800' F., t h e writer knows of no example in industrial work, except on a small scale, which has
Vol. 6 , No. 7
proven satisfactory. 4 . number have been constructed and tried for various lengths of time, b u t a durable furnace, certain of operation, is yet t o be produced. RIost of t h e experiments which have been conducted have employed furnaces with a single resistor about half graphite and half charcoal, as mentioned above. These resistors have been made about a foot and a half wide and placed in a trough of a mixture of refractories, the basis of which is firesand. The metal t o be heated was placed directly above the resistor, but not touching it. The metal was steel bars for forging and heated t o about 2400' F. I n order t o effect a production similar t o t h a t of an oil-fired forge furnace of the same size, the temperature of the resistor had t o be above 2900' F. For this temperature i t seems impossible t o construct a furnace which will have a very long life. I n the course of a few weeks the lining or the bricks will have fluxed t o some degree and rebuilding will be found necessary. A lining of substantially pure silicon carbide brick might stand up under these conditions, but it is questionable if a refractory any poorer t h a n this would be satisfactory. The electrodes, too, ea: difficult t o hold in place without costly supports which might have t o be water-cooled. These furnaces have been used for heating metals for forging and have shown in some instances good 4 , current consumption of 370 k. w. hrs. economy. per 2 2 4 0 lbs. of metal on a I O O k. w. furnace was noted. I n general, t h e type of construction on these furnaces was similar t o t h a t shown in Fig. 11, except t h a t the U-shaped resistor was substituted b y a single straight one running from one end of t h e hearth t o the other. For work a t forging temperatures a furnace employing a graphite resistor does not seem capable of becoming a commercial reality unless a very unique lining can be developed. E L E C T R I C F O R G E F U R N A C E O F T H E ARC T Y P E
The writer has given considerable thought t o this important field of electric furnaces for forging and has developed a furnace which appears t o eliminate most of the difficulties encountered. This furnace is as yet only in the experimental stage, although i t appears t o offer attractive commercial possibilities. I t is of the arc type and thus immediately many of the difficulties inherent t o the resistor furnace disappear. ,4s there is no resistor there is, of course, no resistor lining. The metal is placed on the hearth and is heated directly b y the arcs, the bases of which play a few inches above the metal t o be heated, t h u s obtaining a high thermal efficiency. The arcs are deflected b y means of a n auxiliary electrode which spreads the flame of the arcs so as t o distribute the heat comparatively evenly and also serves t o protect the roof of t h e furnace. The roof, if built of silicon carbide brick, will have a long life. As t h e electrical equipment is placed above the hearth i t is easily accessible and may be removed b y a crane so t h a t a new top can be placed on the furnace in a very few minutes. As arcs of a few kilowatts are difficult t o operate, it would probably be necessar.y t o build a furnace capable of a very substantial production. The writer hopes
J u l y , 1914
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
t h a t definite commercial d a t a regarding this furnace c a n be secured very shortly. DESIGh-
OF FURNACES
FROM
STRUCTCRAL STAiXDPOINT
Fig. I11 shows a heat-treating furnace of 1 2 j k. w. capacity under t h e normal running load. It has a maximum capacity of 1 7 j k . w. The hearth is j f t . wide a n d 9 i t . long inside. A very satisfactory furnace wall has been Found t o consist of two bricks laid so as t o make 9 in. and 7%-ith 3 in. of kieselguhr. The kieselguhr m a y be placed between t h e bricks forming a vertical channel, or m a y be placed outside t h e bricks, in which case, sheet metal is employed for t h e outside of t h e furnace t o hold t h e kieselguhr in place. Asbestos mill board m a y be used in place of sheet metal. This does not lower t h e heat losses, a n d , of course, t h e mill board is not quite so durable as t h e sheet metal. Furnaces may be well insulated on t h e t o p a n d sides of t h e hearth, b u t care m u s t be observed not t o insulate t h e base too well. It m u s t be recognized t h a t in electric furnaces t h e heat is evolved within t h e brickwork a n d is, consequently, somewhat different from t h e fuel-fired furnaces. Hence, t h e furnace should be set clear of t h e floor with only about I j in. of brick allowed up t o t h e lining o n t h e base of t h e resistor. Ten or twelve in. are quite sufficient for this dimension for furnaces working above 1700' F. T h e brick employed should be good fire brick. Silica brick serve excellently for this purpose. These brick r u n about 9 j per cent silica a n d contain a little lime. Good masonry work a n d particularly well constructed arches will be much t h e cheapest in t h e long run. Throughout t h e furnace construction t h e same principles as are observed in oil furnace design are, of course, applied t o electric furnace construction. R E F R A C T O R I E S F O R LININGs-It is most important t o select t h e proper refractory for t h e lining of t h e resistor. The lining is usually in t h e shape of a trough, t h e resistor being placed in it. The lining material must have a high melting point; i t must not have a high vapor pressure a t i t s operating temperature; it must not react chemically with t h e hot resistor on one side or t h e brick on t h e other; a n d i t must not become soft or "mushy" a t t h e operating temperature. I t s electrical conductivity a t t h e operating temperature must be considerably less t h a n t h a t of t h e resistor a n d i t must be relatively cheap. T h e operating temperature of t h e lining is rather high, normally a few hundred degrees higher t h a n t h e hearth temperature, b u t for various reasons t h e maxim u m temperature obtained in a lining may be a thousand or more degrees higher t h a n t h e normal operating temperature. T h e reason for obtaining these high temperatures in t h e lining m a y be due t o neglectful operation or a n a t t e m p t t o get a n extraordinarily large production at a particular time. It is, therefore, necessary t o employ a lining having a high factor of safety as regards t h e temperature. It is exceedingly difficult t o find a material which meets these requirements a t a temperature of 2000' C. (3632' P.). It must be remembered t h a t although refractories oE high melting point are available, t h e addition of t h e
53 9
necessary binder, even though in small a m o u n t , m a y lower t h e fusing point of t h e lining very materially. Either of t h e following substances' having high melting points might be employed as t h e principal constituent of a refractory for use in furnaces operating above
F.
2 0 0 0 ~
Melting point
c.
Refractory Calcium carbide (boils a t 2015' C . ) . . . . . . . . 1995 Aluminum o x i d e . . . . . . , , . , . . , . . . . . . . . . . . 2020 Uranium oxide (CzOa). . . . . , , , . . . . . . . . . . , 2176 Uranium carbide. . . , . . . . , . . . . , , , . . , . , , 2425 Zirconium oxide.. , . . , . . . . . , , . , , , , , , , . . , 2500 Vanadium carbide. . . . . . . . . . . . 2i50
.
.
.
.
F. 3623 3668 3946 4396 4532 4982
I n addition, t w o other substances should bc mentioned. Glucinum oxide has a high melting point, a n d boron nitride s h o w no sign of sintering a t 3000' C. (j432' F.). Magnesium oxide begins t o vaporize under atmospheric pressure at 2 0 0 9 ~ C. ( 3 6 j o " F . ) . All metallic oxides are somewhat acted upon by hot carbon, a n d as t h e speed of reaction approximately doubles for every IO' C. rise in temperature, a t electric furnace temperatures this chemical action is often sufficient t o deteriorate considerably if not destroy t h e lining entirely. At about 2 0 0 0 ' C . (3632' F.),magnesia is rapidly attacked b y c a r b o n 2 The electrical conductivity of lining materials, especially metallic oxides, increases very rapidly with t h e temperature. Materials which are excellent insulators a t room temperatures become fine conductors a t electric furnace temperatures a n d this should be borne in mind in connection with t h e linings. For example, a slab of alundum (fused alumina) h a d a specific conductivity3 a t 1600' C. (2912' F.) about 50,ooo times as great as a t 20' C. (68' F.). I n t h e writer's experience silicon carbide firesand offers advantages over other refractory materials for t h e linings of furnaces considered in this paper. A mixture of silicon carbide firesand 8 5 parts, a n d m-ater glass (38' B.) ~j parts, forms a good lining when well baked in t h e furnace. 4 slightly better lining can be made b y using t h e above mixture for t h e outside of t h e trough a n d pure silicon carbide with t h e water glass in t h e same proportions for t h e inside next t o t h e resistor. T h e lining is p u t in place with t h e assistance of wooden forms, a n d allowed t o d r y ; when t h e furnace is started i t becomes well baked. A t operating temperatures such a lining is a much poorer conductor t h a n graphite, a n d there is practically no chemical action between it a n d either t h e carbon or silica brick with which i t is in contact. Silicon carbide begins t o decompose a t about 2300' C. (4172' F.) into carbon and silicon, which vaporizes, b u t it has given excellent service over a considerable period of time. A silicon carbide lining, properly built in a heat-treating furnace designed t o give a hearth temperat u r e of 18jo' F., should not need renewal more often t h a n once in six months a n d frequently such linings last much longer. It will be seen, however, t h a t linings are relatively expensive items in furnace construction so t h a t t h e firesand is probably t h e most 1 "Fusion and Volatilization of Highly Refractory Materials," Ruff and Goecke, 2.uzgew. Chem., 24 (1911). 14.59. 2 0. P. Watts, "Action of Carbon on Magnesia," Trans. Amcr. Eieclrochem. Soc.. 11 (1907). 279. 8 E. F. Northrup, M e l u l . Chem. Eng.. 12 (1914). 125.
5 40
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
economical, particularly if 1700' F. is t h e maximum hearth t e m p e r a t u r e desired. T h e writer begs t o suggest t h a t t h e question of linings f o r this t y p e of furnace offers a n attractive field for research a n d he sincerely hopes t h a t scientific work along these lines will be made b y investigators. E L E C T R O D E S are placed in t h e furnace imbedded in a mixture consisting of silicon carbide go parts, t a r (melting point 100' F.) I O parts. T h e y project from t h e furnace wall a b o u t 6 inches into a box without a t o p a n d made of asbestos mill board with a framework of small angles attached t o t h e furnace. The leads are r u n into the box from t h e t o p a n d are clamped t o t h e electrodes. T h e box is t h e n filled with kieselguhr. T h e electrodes may be of either graphite or carbon, preferably t h e former. T h e size of t h e electrodes m a y be calculated b y t h e well known methods,' a n d either t h e square or r o u n d form may be used. SWITCHBOARD-The switchboard has a main switch, a n d a radial switch with a number of points, if a transformer is used. T h e switching may be accomplished on t h e high tension or t h e low tension side of t h e transformer, as is desired. If regulation is effected b y altering t h e resistance with a furnace such as is shown in Fig. 111, knife switches alone are used, as no transformer is employed. T h e resistors are thrown in or cut out b y single bladed knife switches, as desired. An ammeter is employed in all cases unless t h e primary tension is too high t o warrant satisfactory operation of same. T h e ammeter is, of course, always placed on t h e primary side of t h e transformer. If a n ammeter cannot be used a wattmeter m a y be employed, or i t may be used if an ammeter is not desired b y t h e operator. WIRING should be quite carefully done a n d with s t u r d y construction. T h e chief precaution t o observe is t o insulate all conductors of current a n d have such insulation located so t h a t t h e furnace heat will not injure it. I n most cases it is preferable t o r u n t h e conductors down from t h e electrodes along or beneath t h e floor t o t h e switchboard. T h e switchboard, of course, must be accessible t o t h e operator of t h e furnace, a n d is preferably placed by the side of t h e furnace near t h e end having t h e door. O P E R A T I O N of a n electric furnace of t h e graphite resist'or t y p e is f o u n d t o be very simple. A new furnace is started u p slowly, t h e voltage equivalent t o one volt per inch length of t h e resistor being applied ' f o r a b o u t 1 2 hrs. At t h e end of this time i t may be brought up t o t e m p e r a t u r e b y gradually raising t h e voltage per unit length of t h e resistor. T h e metal is passed through t h e furnaces, t h e operation being quite t h e same as in t h e fuel-fired furnace, a n d with a very little experience t h e proper load for each particular job may be readily ascertained. If a change in t h e production of steel occurs in t h e furnace, or a change in temperature is desired, it is advisable t o raise or lower t h e kilowatt input, as t h e case m a y be, 1 The writer believes the best method for determining the proper crosssection of electrodes is contained in an article by Carl Hering, "Empirical Laws of Furnace Electrodes," Trans. Amer. Electrochem. Sac., 11 (19 IO). Table VI in this paper may be employed very satisfactorily for the ralculations.
Vol. 6 , No. 7
a b o u t I j or 2 0 minutes before t h e change occurs. Otherwise, failure t o get t h e production on t h e one hand, or failure t o get t h e t e m p e r a t u r e on t h e other, m a y be t h e result. When t h e furnace is t o be shut down t h e current m a y be t u r n e d off ~j or 2 0 minutes before t h e last piece of metal is removed. T h e furnace is t h e n closed a n d as i t is quite hot, will t a k e b u t a short while t o heat u p after t h e usual over-night shut down of 14 hrs. T h e t i m e t o heat u p after such a period is from 20 t o 3 0 min. T o heat t h e furnace u p from cold requires a b o u t I hr. A s pre vi0 us1y st a t e d R E P LE KIs HI x G o F R E SI s T o Rt h e resistor b u r n s away partially a n d must be replenishe'd. This should be done a t intervals of a b o u t 7 0 hrs. of operation on furnaces operating from 1700' F. t o 1800' F., b u t furnaces running a t lower temperatures will last for longer intervals without replenishing. At I ~ O O ' F. hearth t e m p e r a t u r e t h e consumption of graphite is approximately 0.01 lb. per k. w. hr. T h e graphite may be charged through small port-holes, located in the sides of t h e furnace, b y means of long handled scoops. It is t h e n raked down with a small rake a d a p t e d t o t h e particular resistor.' I n some instances t h e resistor runs uncovered or partly uncovered along t h e sides of t h e hearth in t h e interior of t h e muffle. I n this case t h e graphite is shoveled in through t h e furnace door. T h e proper a m o u n t of graphite t o be used will be found readily b y a few trials, noting t h e position of t h e switch before a n d after charging for t h e same load. As t h e regulation is accomplished b y simply throwing t h e switch, t h e furnace m a y be regulated b y t h e workmen. T h e wiring, of course, must be arranged so t h a t it is impossible t o short-circuit t h e line b y means of t h e switches. T h e workmen, however. must be instructed not t o run t h e furnace on a voltage equivalent t o 1.8 or 2 volts per inch length unless t h e production really warrants it. Otherwise, overheating m a y sometimes occur between charges while t h e furnace is e m p t y . T o provide against overheating in this manner, fuses should be carefully selected of low amperage, which is determined b y t h e maximum r a t e a t which t h e work should be done. This precaution is t o be observed particularly on furnaces where productions are varying widely from d a y t o day. For temperatures over ISOOOF., it might be advisable t o equip t h e furnace with a pyrometer constructed so as t o operate a circuit breaker when t h e maximum desired temperature is reached. EFFICIENCY
Although the furnace has been designed with t h e idea of supplying enough current t o heat t h e metal a n d t o make u p the losses of radiation, etc., t h e superintendent of t h e heat-treating d e p a r t m e n t is very likely t o consider t h e furnace from a standpoint of efficiency. This m a y be expressed in per cent or kilow a t t hours per unit of production. Table I1 shows t h e relation between these t w o methods of expressing efficiency. 1 The upper surface of the resistor should be raked as level as possible but i t is better t o have it slightly concave laterally than convex
T H E J O U R N A L O F I A V D K S T R I A L AAVD E L V G I N E E R I N G C H E ; I J I S T R Y
J u l y , 1914 TABLEII--SUMBER (2240 LBS.) O F
OF
Rise in temperature 7 -
' F.
= c.
950 1050 1150
528 584 639 695 i50 806 834 86 1 889 916 945 972 1000 1306
1800
2350
KILOWATTHOVRSREQUIREDTO RAISE
ONE
TON
TEMPERATURES AT SEI'ERAL EFFICIENCIES
STEEL TO VARIOUS
Percentage efficiency 7
100 89 103 11s ~~
~-
80 70 60 Kilowatt hours reauired 148 li2 196
50
134 150 164 lil 178 188 197 207
21i 222 272
345 362 3 70 453
I n general, t h e larger t h e production of metal in a particular furnace, t h e greater t h e electrical efficiency. Too high production, however, usually means difficulties in control a n d with very high production there is danger of overheating should there be slackening i n t h e production without a corresponding change in t h e kilowatt i n p u t being made. T h e aim t o make a production at a r a t e equivalent t o 4 or 5 k . w. per sq. f t . of hearth a n d also at 6 5 t o 7 5 per cent efficiency is a n excellent one. When t h e cost of current is rather high, work which necessitates a production a t a lower efficiency t h a n 60 per cent should be transferred t o a smaller furnace if t h e shape of t h e pieces permits. On furnaces under 30 k . w. capacity these figures d o not apply, a s t h e efficiency on such small furnaces is very much less t h a n on t h e moderate size ones of jo t o 1 2 j k . W. INSTALLATIOX O F FURNACES
I n order t o determine t h e n u m b e r a n d size of furnaces for a n installation a careful inquiry into t h e n a t u r e a n d q u a n t i t y of t h e production is necessary. T h e maximum a n d minimum productions must be met mith as high efficiency a s possible for t h e various productions. Frequently a moderately large furnace t o operate all t h e time, accompanied b y a smaller one t o be operated as needed, is much more economical from a point of view of current consumption in t h e long run, t h a n one furnace capable of t h e maximum production. Care m u s t be observed not t o make t h e furnace t o o large for t h e sake of being o n t h e safe side. A furnace so large t h a t i t t a k e s I O per cent more current t h a n a smaller one exactly suited t o t h e work, would waste enough current in t h e course of six months, or a year a t t h e outside, t o p a y for a complete new installation. Sometimes t h e labor requirements determine t h e size of t h e units. T w o men on a single furnace might not be able t o accomplish so much on t h e one furnace as t h e y would if operating t w o smaller size furnaces. Here t h e decreased efficiency in t w o furnaces m u s t be carefully compared with t h e saving in labor. SELECTIOX OF TYPE OF FURNACE
I n t h e selection of t h e t y p e of furnace for t h e particular work, if I O sq. f t . or more of hearth area are required, a furnace regulated b y t h e resistance method, Fig. 111, is certainly to be preferred. This furnace costs a b o u t half as much a s t h e furnace equipped with a transformer a n d has a much more uniform heat liberation in t h e hearth. Electric furnaces with re-
541
sistance regulation including all t h e electrical equipment, cost a b o u t t h e same for installation as a good oil furnace, a n d frequently are somewhat cheaper if blowing equipment installation charge for t h e oil furnace is included. As a rule these furnaces operate on a higher voltage t h a n do t h e furnaces having a special transformer so t h a t a saving in copper for t h e conductors is sometimes effected. T h e y can be designed t o operate on t h e s t a n d a r d voltages, 2 2 0 volts being very satisfactory, a n d , of course, can operate on either direct or alternating current. T h e larger furnaces can be built t o operate on two- or three-phase lines, b u t those taking 100 k. w. or less are best constructed for single-phase or direct current. For t h e small furnaces, such as t h e one shown in Fig. I\-, a regulation b y means of voltage is best with a transformer having a secondary with several taps, as mentioned above, either with or without t h e resistance regulation in conjunction with i t . USE
OF
ELECTRIC
FCRNACE
FOR
HE4T
TREATYENT
D E P E h - D E N T O K COST O F C U R R E K T
T h e extent of use of t h e electric furnace for heat treating depends quite largely on t h e cost of current. Fortunately, i n this connection, t h e resistance furnaces have a remarkably s t e a d y load. T h e starting load is somewhat less t h a n t h e running load. I n a large n u m b e r of cases t h e mean running load is found t o be between 80 a n d 9 0 per cent of t h e maximum demand. Under these circumstances, particularly with operation extended into or through t h e night, current can usually be furnished for a low figure. T h e price of one cent per k. w. hr. is frequently sufficient t o warrant t h e employment of t h e electric furnace in place of oil on t h e ground of cheaper cost of operation alone. Current for three-quarters of a cent per k . w. hr. frequently proves as cheap a s a n y method of firing when all factors are considered. Among these factors, most of which have been duly considered, might be mentioned t h e lower labor charge which is usually effected b y t h e introduction of t h e electric furnace. T h e furnace is conducive t o high production on account of t h e fact t h a t i t is not so uncomfortable for t h e workmen as t h e fuel-fired furnaces. Besides t h e fact t h a t i t is relatively cool a n d t h e elimination of all smoke a n d dirt, a n d t h e a t t e n d a n t difficulties with t h e products of combustion, t h e electric furnace is much more satisfactory from t h e workmen's standpoint t h a n a n y heretofore produced. ACKSOFVLEDGNEKT-The writer wishes t o express his appreciation of t h e assistance of his associate, M r . Richard S. Bicknell, both in t h e s t u d y a n d design of these electric furnaces, a n d in t h e preparation of this paper. 50 E A ~ 41ST T STREET. S E N
YORK
_ _ _ ~ ~~-
STUDY OF AUTHENTIC SAMPLES OF GUM TURPENTINE By A. U' SCIIORCER
Received March I t , 1914
VARIATIOS I S PROPERTIES
Turpentine, spirits of turpentine, or oil of turpentine, is t h e volatile oil obtained ordinarily b y t h e distillation of t h e oleoresin of various species of pines.
