US2214582A - Furnace heating control - Google Patents

Description

3. shuts-Sheet l.

EI.` K. HANSEN Filedl Aug. 1:5, i932 FURNACE HEATING CONTROL sept, lo; 1940..

Sept. 10, 1940. E. K. HANSEN FURNACE HEAT ING CONTROL Filed Aug'. 13, 193e :L Sheets-Sheet 2 /ZOO /400 /600 /500 000 Temperafare, "if

INVENTOR WITNESSES:

can -f 5Pf- 10. 1940- E. K. HANSEN FURNACE HEATING CONTROL Filed Aug. 13, ma

3 Sheets-Sheet 3 f'ig 5.

INVENTOR .Elmer/f Hanse/7.

WITNEssEs;

. 2M/WM f ATroRNEY Patented Sept. 1

UNITED STATES PATENT oFFicE Ehner K.

vWestinghouse `summon HEATING coN'rRoL Hansen, Edgewood, Pa., as signor to Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application August 1a,'19ss,"ser1a1 No. 224,113 9 claims. (c1. 26e-s) My invention relates broadly tothe control of the rate of circulation of an atmosphere about a charge being heated in a furnace, preferably,

although not necessarily,l an electric furnace.

Many furnaces employed for heat treatment,

circulate a gaseous medium of, known and desired composition about the' charge therein, primarily to prevent deleterious eiects upon the charge by ordinary productsof combustion or by `a more common surrounding atmosphere such as air The protective atmosphere is directed to the heating chamber let deviceA and is permitted chamber through some gas outlet by means `of some gas into leave the heating means.

One type of furnace usually employing a protective atmosphere is In this type of furnace a completely .encloses rests upon a base in a gas-tight manner.

the so-called bell furnace.

the charge and the shell When it is desired to heat the charge, the heating-bell is placed about the hood and heat isapplied to the exterior surface mosphere introduced inside will not react chemically with the charge constituent or impurity of the thereof. The gaseous atthe shell is one which or any charge. An example of a heating process'employing a protective atmosphere is the bright annealing of copper.

kThe bright annealing of copper is accomplished to 1200", F. The' in- `ert gaseous atmosphere'usually comprisesin the neighborhood of 14% CO2, 1% the remainder N2, the gaseous CO, 1% Hz and mixture,Y having a dew point ofy approximately 40 F.

In order to 'obtain anduniform temperature rcibly circulated about the This isA particularly the atmospherejs ,fo charge.

processes wherein the A not very high and th charge almost entirely extent by convection.

fast heating of the charge distribution through it,

advantageous in heating temperatures are heat is transferred to the or at least to a very large Usually thev protective gas is circulated about the charge by a fan within the heating chamber shell tothe charge to transfer the heat of the' by convection, while the movement of the .atmosphere creates the uniform temperature desired. ing of the charge the Conversely, during coolcirculated atmosphere may be employed to increase the rate ofy cooling and at the same time ,maintain a more even temperature distribution thr ough the charge. Because of the circulation ofthe atmosphere the .heat

treatment cycle can be better controlled, and a more uniformv product obtained than would be phere.

the case without a rapid circulation of the atmosmetallic shell or hood ing the circulation of a loperate a fan that lis vrapid rate when gaseous atmosphere `is particularly advantageous and vuniform temperature stress of only 760 pounds per square inch at 2,0m00"e F. At temperatures below 1800'F., the per ssible stress for a 1%' creep and 10,000 hours of operation increases quite rapidly whileat tem- 15 peratures above 2,000o F. the permissible stress fast approaches zero.

In order to employ fans infurnaces where their permissible stress is a serious factor, it has heretofore been necessary to construct a fan 20 with an exceptional amount of heat resistant metal or to run a smaller fan at a very low speed. rIfhe former instance, of course, results in an expensive fan construction, while the latter results in an'rinoreased time for'the heating cycle.

' It is, accordingly an object of my invention to obtain a faster heating cycle in furnaces employgaseous atmosphere around the charge.

It is a'further object of my invention to so 30 employed to circulate the gaseous atmosphere withina furnace at a more the fan can withstand larger stresses at the lower temperatures and at a smaller rate when the fan cannot withstand such 35 large stresses.

I iind that the control of the speed of the fan and, therefore, the rate of Acirculation vof the withinthe heating chamber applicable with o decided improvement to processes where the gaseous medium is circulated about a fluid metal as, for example, in brazing. In such'processes `the work at hand may require that the brazing substance, be exposed to the circulating atmosphere." So long as the brazing substance is solid a highrate of gaseous circulation may be employed with an attendant'high heating rate and f distribution. When, however, the brazing substance softens and becomes liquid,too high a rate of circulation of the gaseblow the molten fluid away from the adjoining surfaces which are to be brazed. By controlling the speed at which the atmosphere circulates so 'that therate is low just 55 before the melting point of tne brazing substance is reached, the heating cycle may be hastened up to that point, and, conversely, the cooling cycle may also be hastened below that point.

It is an additional object of my invention to control the speed of rotation of a fan circulating a gaseous medium in a heating chamber by factors which involve allowable stresses in the fan or by factors which involve the allowable rate of circulation of the medium about a charge at the instant furnace temperature, or a combination oi both.

Other objects and features of my invention will be apparent from the -following description thereof taken in conjunction with the drawings in which:

Figure l is a cross section oi a bell type iurnace employing the principles of my invention;

2 is a part top View of the fan;

Fig. 3 is a part top view of the charging support plate;

Fig. l is a schematic electrical wiring diagram for a two-speed control o1 the fan motor;

Fig. 5 is a diagram similar to Fig. li, but showing a multi-speed control of the motor; and

Fig. 6 is a curve showing permissible working stress against temperature of two heat resistant iron alloys.

In Figure. l, I haveshown my invention applied to a bell type furnace, but it is to be distinctly understood that I do not desire to be limited to this type of furnace, and that the bell iurnace has been chosen merely as a vehicle by which to illustrate my invention. In this figure, the bell l removably rests upon a base 2, and is constructed heating units iat the sides thereof and heating units li centrally therein.

A charge supporting plate il is supported by the base 2 and is annular in general shape. The charge such as i@ in this instance comprises coils o1 wire resting upon the top of the charge supporting member 3. il hood l2 is placed about the charge and has a turned flange member ill ei:- tending into a liquid seal it, so that a gas-tight enclosure is provided within the shell l2.

The furnace may be oi any desired cross section, but the one being described is or circular cross section. The hood l2, therefore, comprises an inner cylindrical member lt and an outer cylindrical member 2li, the two forming an enclosed space 22 comprising the heating chamber. While I have shown my invention applied to a furnace having a central heating unit and hood shaped to correspond, I do not wish tdbe limitedv to any particular type of furnace. As a matter of fact, my invention is admirably suited to a ell furnace with the central heating units omitted and the hood shaped accordingly.

An annular-plate 24 seals the chamber at the top thereof and a dish-shaped member 26 closes the bottom of the cylindrical shell member I8. The latter element is spaced above the furnace base 2 for a purpose which will later become apparent. Suitable enpansionand contraction expedients (not shown) sary, as is well known to the art.

In this furnace the base 2 has a central aperture through which extends a. shaft 28 having attached thereto at its end within the interior of the furnace a fan 30 of the centrifugal type. The shaft may be guided and supported in lubri- Y cated bearings in a supporting structure generally indicated by the reference character 32 which maybe in any appropriate form forming a gastight t with the A preferred construc-v l a top annular plate may be employed, if necestion is disclosed in the application of F. Davis, Serial No. 221,719, filed July 28, 1938, assigned to the Westinghouse Electric & Manufacturing Company.' The shaft 28, and therefore the fan 30, are driven by a motor 34 through the medium of pulleys 36 and belts 38. I show the motor 3l supported by the furnace base at the exterior thereof, as, for example, by I-beams 4D, but it is obvious that the fan and shaft may be driven by any appropriate means. However, I employ a motor because oi the flexibility of the control thereof.

The fan which I employ, as aforementioned, is of the centrifugal type and has a central opening 42 which serves as an entrance for gas, and blades M which serve to discharge the gas radially in all directions. A bottom plate 46 covers the bottom of the ian in its entirety. The blades #lil are formed integrally with this bottom 46 and 48, the open central portion of which comprises the inlet 42.

The fan discharges in the general direction of the charge supporting member 8, the latter being webbed to provide passages through which the gaseous medium discharged from the fan may pass.

As shown more pmticularly in lilg. 3, the charge supporting plate comprises an annular' member till integral with I-shaped ribs 52 protruding radlally. The ribs are circumferentially spaced to permit passage of the atmosphere upwardly therethrough or into contact with the bottom of the charge if the latter completely covers the tops of spaces till beween the ribs.

A gas inlet 55 directs the protective atmosphere into the central aperture of the base and a gas outlet 58 permits the escape of the gaseous atmosphere. Valves (not shown) may control the now through the inlet and outlet pipes. Normally only an amount of gas is permitted to escape from the outlet pipe suliicient to keep the protective atmosphere clean and pure.

The path of the gaseous medium circulated by the fan is indicated by the arrows 6U. The 'gaseous atmosphere discharged from the fan passes through the charge supporting member 3 and then rises along the outside of the charge lo, passes over the top thereof, and down the inner side of the charge to the fan inlet opening 42.

When the charge is just being heated and the temperatures low, it may be desirable to circulate the medium at as rapid a rate as possible in order to shorten this part of the heat treatment cycle. For a fan of approximately 30 inches diameter and a discharge of 60 cubic feet per second, a speed of rotation of 1500'R. P. M.

may be employed initially. As the temperature rises, the fan loses its strength and, in Fig. 6, I show a typical curve of a heat resistance metal of a nickel-chrome-iron alloy showing the permissible Working stress for temperature ranges between 1200 and 2000 F. It may be observed that'startlng with exceptionally highstresses vat low temperatures, the allowable stress drops rapidly to about 1700 F. Thereafter the rate of clecrease of permissible working stress is considerably slower, but nevertheless the values thereof are now so low as to prevent the application of any appreciable load to the fan. The curves are typical of practically all heat resistant metals, and by my invention I seekto employ the fan at its utmost lefficiency, taking into consideration the fact that at the higher temperatures the fan.

may be so weak that it can safely-impart only low velocities to the circulating gaseous`atmosphere. I accomplish this by running the fan at higher speedslwhen the temperatures to which it is submitted are relatively low and its permissible working stress high, and at low speedsV connecting to a pyrometer` 64. 'I'his pyrometer has a single pair of contacts 66'which close when a predetermined temperature is reached dependent upon the adjustment of the pyrometer'.4

When the predetermined temperature is reached the contacts 66 close. to energize a relay 68 having a normally open contactor 1,2.

At,low temperatures the motor 34 has lits armature 14 directly connected between the power leads and its field 16 connected between the power leads through a resistance 18. When therelay 68 is energized it closes its normally opened contactor 12 to parallel a second resistance, 80 across the resi-stance 18 and thereby decrease the speed of the motor 34 by increasing the strength of its iield. The resistance 18 is made adjustable so ythat the initial speed of the motor may be v controlled, and if desired, th'e secondresistor 80 may also be adjustableto control the speed at the lower limit.

The maximum or minimum values of speed will necessarilyv depend upon the particular heating process employed. If the process is suchv as to permit of the higher speed of rotation of the fan throughout the cycle, then obviously my invention need not be employed, but if the process is such as to require a lower speed of rotation of the fan at some point in `the cycle and for some extent within it, then my invention provides a means wherein the cycle "is hastened. Thus, during the early stages of the cyclewwhen the charge is being heated or in` the later stages when the charge is being cooled, the fan may 4previously adjustedy to energize the relay 66 at this temperature, and the resistance 88 will be inserted in the motor ield circuit in parallel with the resistance 18 and thereby decrease the speed speed when the temperature has dropped to the ing substance melts atv approximately 1550 F1? At this temperature the fan has appreciable strength if it is formed of an iron alloy comprising 35% nickel and `15% chromium, so that orn dinarily We may suppose that the maximum rate `of circulation of the gaseous atmosphere may be employed throughout the process.` However, where the brazing is rto be accomplished between objects of such conformation that the brazing substance is exposed to the gaseous atmosz phere', then too high al speed of gaseous circulation may blow-the brazing substance away from the desired joint when the brazing substance melts. Accordingly, it may be desired to adjust the pyrometer to close the contacts 66 at al temperature somewhat below the melting vpoint of vthe brazing compound, "in this instance, say,

1500* F. 'Myinventiom therefore, has application to processes in which the permissible working stress of the fan is not involved.,

In steel to steel brazing, temperatures in the 'neighborhood of 2050o F. are common. In this instance, of course, the permissible working stress of the fan is of greater importance than the prevention of the blowing of the molten brazing copper away from the joint surfaces it is desired to braze. therefore, 64.

In Fig. 5,*I show a modification of the motor control in which the speed of the motor is controlled in a greater number of steps than that shown in Fig. 4. The contacts 82, 84, 86 and 88 are engaged by a contact 89 at different temperatures, in turn,'to control relays 98, 92, 94 and 96,

respectively. f During the increase in temperature part vof the heating cycle, elongated contact 89 will iirst engage contac't82 to cause insertion of resistance 98 in the armature circuit of the motor 34.y Subsequently, at some higher temperature, the contact 84 will be engaged to cause insertion of an additional resistance |88 in the armature circuit of the motor 34. With further increase in temperature, engagements of contact 86 and thenfcontact 88 by contact 89 energize relays 94 and 96, respectively, to further decrease the speedfof rotation of the fan motor 34 by the insertion of resistance in the armature-circuit.

From the diagram of Fig. 5 vit is obvious that Fig. 5 in the heating cycles in which the tem- I peratures are relatively high, as, for example, temperatures up to approximately 2000 F.. l.

The contact 89 is of a length suiiicient to span.

rising temperature portion of the heating cycle, 'each contact 82, 84, 86 and 88 is engaged by the contact 89 at the adjusted temperature for which engagement is to take place, and remains engaged show wherein the motor speed is lowered at the Y515 appropriate value or values of high temperature, it is obvious that at any value the motor circuit can be entirely opened to stoprotation ofthe fan entirelyi The manner of accomplishing this is at once obvious and can be accomplished with a relay such as 96 opening the motor circuit insteadof varying the speed of the .moton Or in -brazing an' embodiment such as that of Fig. 4 may be4 employed with the controlling relay opening the motor energizing circuit instead-of increasing the fleld current.

While Figs. 4 and 5 show a direct current motor whose speed is controlled in the one instance by the resistance in the field `andin the second by the resistance in the armature, it is obvious that any type of speed control may be employed. Moreover, the method of speed control will depend largely on the type of fan drive being used and I do not desire to be limited in any way to lall four contacts 82, 84, 86 and 88. During the the rate of circulation of the gaseous atmosphere within the furnace.

While I have illustrated my invention in the form which I believe to be the best mode of application thereof, it is obvious that many changes may be made within the spirit and scope of the novel system which I have introduced. It is desired, therefore, that the appended claims be given the broadest construction and limited only by the prior art.

I claim as my invention:

1. A heat treatment furnace having a heating chamber characterized by an atmosphere about a charge being heat treated, a ian for circulating said atmosphere, and means for varying the speed of said ian from one speed to another responsive to temperature conditions within said heating chamber.

2. A heat treatment furnace having a heating chamber characterized by an atmosphere about a charge being heat heated therein, a fan subjected to the heat in said chamber for circulating said atmosphere, and means to vary the speed of said fan from one speed to another responsive to temperature conditions within said heating chamber, reversely as compared to temperature variations, within said heating chamber.

3. A furnace includingr heating means, an enclosed charge heating chamber, means for pervadingsaid chamber with a gaseous atmosphere, a metal :ian in said heating chamber for circulating said atmosphere, a thermocouple subjected to the heat of said atmosphere, means including a motor outside said chamber for driving said fan, and means operable by said thermocouple for varying thev speed ci said motor from one speed to another reversely to temperature changes or" said thermo'couple so that said ian is rotated at higher speeds when its strength is greater. v

4. The method of employing a metallic ian in a heating chamber for circulating an atmosphere about a charge in said chamber, which comprises rapidly rotating said fan While the temperatures of said atmosphere are relatively low and the strength ci the fan relatively large, and decreasing the speed of rotation of said ian when the temperatures have become higher and the strength of the fan relatively lower.

' 5. The method of employing a metallic ian for circulating an atmosphere in a heat treating furnace chamber which comprises rotating Said ian while the temperatures are higher at a speed commensurate with the lower strength of said ian at the said high temperatures, and increasing the speed of the fan when the temperatures have become relatively lower and the strength of the fan greater.

6. A heat treating cycle involving clrculation of a protective atmosphere about a charge which comprises confining. said charge in a chamber, applying heat to said chamber, circulating the' protective atmosphere'about the charge in said chamber at a rapid rate when the temperature of said charge is to be changed rapidly and at a slower rate thereafter.

7. In the method of brazing two metallic substances with a brazing metallic substance which comprises heating said substances in bracing relation in a gaseous atmosphere, circulating said atmosphere about said substances at a rapid rate while the temperature is below the melting point of said brazing substance and decreasing said rate when said melting point is approached whereby the circulating atmosphere will not blow the melted braz'ing substance when the last melts.

8. A heat-treatment furnace having a chamber in which a charge is to be heat-treated in accordance with a cycle involving increasing or decreasing the temperature oi the charge, the furnace having provisions so that the charge may be enveloped by a protective atmosphere, said furnace being provided with energy-changing means for changing or maintaining the temperature of said charge and atmosphere, means to circulate said atmosphere, and means for controlling the rate of said circulation responsive to temperature in said furnace so that a relatively higher rate obtains at relatively lower temperatures, and a relatively lower rate at higher temperatures which are in proximity to that at which the charge is to be or is maintained whereby that part of the cycle involving change of temperature of the charge is increased, especially where a cold charge is first brought into a hot furnace.

9. In a device of the class described, a 'furnace base, a hood on said base forming therewith an enclosed heating chamber, a'heating bell about said hood, means for discharging a protective atmosphere to permeate said chamber, e. metal ian in said chamber for circulating said atmosphere in said chamber, driving means for said fan comprising a motor heat-insulated from said chamber, and temperature responsive means in said chamber for controlling said motor to control the speed of said fan.

ELMER K. HANSEN.

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