US2789808A

US2789808A - Method of and apparatus for controlling circulation of furnace atmosphere

Info

Publication number: US2789808A
Application number: US467022A
Authority: US
Inventors: Calvin C Blackman
Original assignee: Individual
Current assignee: Individual
Priority date: 1954-11-05
Filing date: 1954-11-05
Publication date: 1957-04-23
Anticipated expiration: 1974-04-23
Also published as: DE1125461B

Description

April 23, 1957 c. c. BLACKMAN 2,789,808

' METHOD OF AND APPARATUS FOR CONTROLLING CIRCULATION QF FURNACE ATMOSPHERE 4 Sheets-Sheet 1 Filed Nov. 5, 1 954 IN VENTOR. CA L Vl/V C BZAC/f/VA/V A Trot/V575 April 23, 1957 Filed Nov. 5, 1954 C. C. BLACKMAN METHOD OF AND APPARATUS FOR CONTROLLING CIRCULATION 0F FURNACE ATMOSPHERE 4 Sheets-Sheet 2 INVENTOR. CALV/N C'BZACK/VA/V M 9- ofM/ v ATTOIP/VE'YS c. c. BLACKMAN 1 METHOD OF AND APPARATUS FOR CONTROLLING CIRCULATION OF FURNACE ATMOSPHERE April 23, 1957 4 Sheets-Sheet 3 Filed Nov. 5, 1954 INVENTOR.

CAL V/N c- BLACK/VAN April 23, 1957 c. c. BLACKMAN METHOD OF AND APPARATUS FOR CONTROLLING cmcumnou OF FURNACE ATMOSPHERE 4 Sheefis-Sheet 4 Filed NOV. 5, 1954 IN V EN TOR.

Usuallyan inner cover is placed over the coils within METHOD OF AND APPARATUSFQR CONTIQOL- ...LING. CIRCULATION F. FURNACE- ATMOS- IPHERE v.uCalvin C..Blackman, Lakewood, Ghio, nssignonto' .Lee WilsomRocky River, @hio Application November 5, 1954, Serial No. 467.5022

- .18 Claims. (Cl.'263--f 40) .I his'invention relates to the heating of a charge. in a furnace or the like and more particularly to heat 'treating of metals such as the annealing of steel, copper, copper alloys, aluminum, .etc., in the formof coiled strip or wire.

' in carrying out annealing or other-heat treatingmpera 'tions'on large coils of strip steel bell-type furnacesuare commonly employed.v In this procedure the coils are stacked one uponthe other with spacers therebetween and the furnace bell placed over the stack and heated;

the furnace bell and an atmosphere of non-oxidizing'gas maintained within the .inner cover.- In order to improve the efficiency of the convection transfer of heat to the coils the atmosphere in" the enclosure surrounding the coils may be circulated Pans or blowers are commonly disposed in the'furnace base to efiect' this circulation. Coils of strip of the non ferrous metals are heat-treated in the same manner, also coiled bundles of wire,*-disposed on a supporting structure. In-some instances an inner" cover is not required; the bell furnace being the sole container of the atmosphere surroundin g-the -charge being heat treated. The chargemavalsb be' composed of axsinglestack of coils or single standof -wire=or it may becomposed of multiple stacks of coils orstands of wire. Inthe normal operationaof such a furnace' the fanbr blower is started when the furnace-bell is placed-in posi tion and heat. supplied thereto. When the-furnace cliarge is cold andtheiurnace atmosphere is' at'arelatively 'low temperature and high density a certain amount of po'wer 'isitnecessary. to drive the fan motor at -its rated speed-2 However; when thefur-nace atmospherebecomeslieated to :annealingtemperature (fonextmple 1200 F density is materially reduced and,--as-the volume constant and thepressure of the atmospherearou' the charge remains substantially atmospheric; the-power re: quiredtto drive the blowerat itsrated speed is ver y s'ubstnti'ally lower than that required when the operation is started :and the atmosphere is -relatively denser at -ioom temperature. r -:Priorcto thepresent invention it has-been necessary thatmheiblowerrnotor (assuming this'motor to 'b'eof the constant speed type commonly used) have sufficient capacity to; take 'care .ofxthehigh power required 'du ring the initial warming up period of. the heatingt-cyeleaaalthough, -during the annealing period fwhichtis; normally considerably ,longer than. the warming. up period, .;.the motor. operates at only a. fraction of its-rated, capacity. For. example it might becnecessary. in. the ;operation of this type-ofifurnaea-as commonly carried ou t -prior;to myv invention; ,to install a '15 H. 1 fan motor; in 'orderzto furnish the power necessary to circulate thecold: and relativley heavy or dense atmosphere.at'the-start of. the operation, although this .15 H. P. motonwouldneed; to ,delivertanoutput of only 7 /2. HFP. vto.effectthe sarne rate a of .fiow...or..circulation-when ther atmospherelis heated to .annealinghtemperature and is irclatively; light.

Patented Apr. 7 23, 1957:

ice

2 a It is among the objects of myinvention,therefore, to provide a method of and apparatusfor controlling the circulation of atmosphere about a charge being heated whereby the-fan or blower'motor'will operate at sub? 'stantially itsfull rated power output both when the atmosphere beingci'rculated is coldand heavy during the warmup period: and thereafter when the atmosphere is high temperature and low density during theperiod. Iv Other objects of my invention include the provision of meansfor automatically limiting the load on, and output of, an atmosphere circulating blower inotor to a value not greaterthan normalor rated value at low furnace atmosphere temperatures when the density is relatively h-igh,--andautomatically permitting'theblower motor to I operaterat substantially its normal or rated load and-outl heating put .at highwtemperature low density furnace atmosphere conditionsathe provision of effective means for increasing the efiiciency of a heating, installation of the type dcscribed and reducing its first cost; and the'provision of .efiectivepruggemand trouble-free means for controlling I the output'of an atmosphere circulating fan-orblowei automatically and in relation tothe temperature of the atmosphere-being circulated, whereby-arelatively low rateiiof-fiow will bemaintained at low temperatures a 'relativelyhigh rate at high. temperatures. 7 .;=The above and. other objects of my invention will appearsfrorn. thev following description, referencetbein g' had.;to the accompanyingdrawings in which: 3 Figure 1 is .a vertical cross-sectional view through 5. conventional bell-type annealing furnace for coils'of strip steel havingh the usual motor driven fan or blower mountectinthe base thereof for circulating the atmos phere: within the inner cover. Z i-imFi gure 2-is a fragmentary. .plan view; taken substan tiallywon line. 2--'2 of Figure 1, illustrating the furnace baseor:change-support, a portion of the top'plate thereof being "broken away to show one form of apparatus for automatically;controlling. the .output of .theblower and the fatlOCf circulation throught the inner cover in accord ance with the temperature of the atmosphere within the inner cover. a 2 :.'F.igure 3 is. an enlarged illustrative view of two adjacent vanes of thefchargezsupport shown'in Figure 1 with the .bi-metal control-strips intheir coldyonful l' throttling .positions. 3 Figure 4-is' anillustrative fragmentary perspective View:

offithefurnace base and circulation control means showtiin--' Figures 2 and 3; the bi-metal elements being'showri in colder-throttling positions. Figure 5 is 'a vertical cross-sectional view of an-innei coverand the upper-coils. in a furnace of the type shown to %perform the same'function and disposed in the same I location as the device of Figure 5, the parts however beingin their open or high temperature positions. Figure 7 is a detached perspective view of 'the' upper endof the'top coilof astacl: with still another form of circulation control apparatus.v disposed therein,..the.part being. shown-in their cold or full-throttling positions. 13

Figure'S is a view generally similar to Figure 7 "and showingthe sameapparatuswith certain-elements omitted for clearness of illustration, the partsbeing. seen in theifi highrtemperature or full-flow positions. Atypical-bell type furnace for annealing-coils of'stri metaltis illustratedin Figure. 1. This -includes.' the fur nace bell F which is adapted to be'positioned overth'? charge on thezfurnace base B. Asillustrated;thefurnace tion.

bell F consists of a cylindrical, open bottomed structure which carries a series of combustion heating tubes 1 disposed around its inner wall. These tubes are suitably supplied with fuel and air from circular manifolds 3 and 4 respectively and the products of combustion are discharged through exhaust pipes 4. As is well understood, the bell B is provided with a downwardly projecting sealing flange at its lower edge which projects into a groove or trough 6 in the base B when the bell is in heating posi- Sand or other suitable sealing material in the trough 6 provides a seal to prevent undesired entry of room atmosphere into the bell.

A charge support and diffuser unit, generally indicated at S, is carried by the base'B which also supports the fan motor 7. The vertical shaft 8 of motor 7 extends upwardly through the base B and carries the fan or blower 9 at its upper end. This fan is of the usual centrifugal type having a plurality of blades or vanes 10, a bottom .plate 11 and a top plate 12. An inlet opening 13 is formed in the top plate and when the fan is operating gas is drawn inwardly through the opening 13 and discharged outwardly from the periphery of the fan blades.

As is clearly seen in Figures 1 and 2, the charge support and diffuser S includes a base plate 14, a plurality of spirally extending diffuser vanes 15 and a top plate 16. An outer rim 17 extends around the support S at the outer ends of the vanes 15 and it will be seen from Figure 2 that the diameter of the top plate 16 is smaller than the inside diameterof the rim 17, thus providing an annular discharge aperture 18 for the gas which is forced outwardly through the diffuser vanes 15 by the fan 9. A central opening 19 in the top plate 16 is provided to permitgas to enter the fan 9.

The coils to be heat treated are illustrated in phantom lines in Figure 1. Three coils are shown, the bottom coil 19 being supported on the top plate 16 of the charge support S and a spacer or Convector 20 being preferably interposed between the bottom of coil 19 and the plate 16.

Coils

21 and 22 are superimposed on coil 19 and are spaced from the adjacent coils by convectors or separators which are indicated at 23 and 24. These separators may take any desired form and are such as to provide a passage for the, flow of gas across the end faces of the coils from the outside into the inner opening in the coils, thus greatly facilitating the transfer of heat to the coils.

Disposed over the entire charge is the usual relatively thin sheet metal inner cover 25 which is closed at its upper end and provided with a flange portion 26 at its open lower end. When in operating position this flange 26 extends into a body of sand in an annular trough 27 in the base B. Means (not shown) are provided for maintaining the desired atmosphere around the charge by directing gas (usually a non-oxidizing gas) into the space within the inner cover 25 and an outlet for such gas is also provided. As is well understood, the space within the inner cover 25 is purged of air before the heating operation is started and a non-oxiding atmosphere is main tained during the annealing cycle.

As has been noted above, the atmosphere within the inner cover 25 is relatively dense and heavy when the apparatus is at room temperature before heat is applied by the combustion tubes 1. In order to facilitate the transfer of heat to the

coils

19, 21, and 22 the fan 9 is operated throughout the entire heating cycle. The circulation of atmosphere within the inner cover 25 is downwardly through the central openings in the coils into the fan 9, outwardly through the diffuser unit S, and upwardly through the annular discharge 18 into the space between the outside of the coils and the inside of the inner cover 25; Part of the gas discharged through the opening 18 passes through the

convectors

20, 23 and 24 into the central opening in the coils while another part moves up and, across the top of top coil 22 and downwardly into the central opening thereof.

' The fan motor 7 is of the constant speed type (for example an alternating current induction motor). It operates substantially at rated speed at all times and when the unit is cold it circulates the atmosphere at a certain rate and handles a certain number of cubic feet of atmos-- phere per minute. This requires an amount of electrical. energy which depends upon the density or weight of the gas being handled. As the furnace approaches annealing temperature the atmosphere within the inner cover 25 is similarly heated and, as the annealing temperature may be in the range of 1200-1400 F., the density and weight per cubic foot of the atmosphere being circulated becomes substantially less. The volume and pressure of the gas within the inner cover 25 however remain substantially constant during heating as the sand seals and the gas outlet opening are such as to prevent any build-up of pressure within the cover 25.

Thus when the furnace reaches operating temperature the fan 9 is handling a much lighter gas and accordingly it is working under only a fraction of the load that it is subjected to when the atmosphere is cold.

In order to prevent overloading the fan motor during the warm-up period of operation (or to eliminate the necessity for a motor of much larger capacity than is required for the heating portion of the cycle) I propose to provide means for automatically throttling or reducing the volumetric output of the fan 9 when the furnace is in a cold state and automatically increasing the output as the temperature increases. The result of such automatic atmosphere flow control is that the load on the fan motor when the atmosphere is relatively cold, and during the heating-up period, may be held down to a value not substantially greater than the load required to give the desired circulation of hot gas at annealing temperature.

In Figures 2, 3, and 4 I have illustrated an atmosphere flow control apparatus which consists of a plurality of bi-rnetal flexible auxiliary vane ends 28 which are secured, as by welding adjacent their outer ends, to all or part of the diffuser vanes 1.5. As seen in Figure 2 the bi-metal elements 28 are in their cold or throttling positions. These elements are preferably of the same vertical width as the vanes 15 and when they are in their extended or cold positions (Figure 2) they project out into the space between the inner ends of adjacent diffuser vanes 15 and thus reduce the effective discharge outlet area of the fan 9. When the bi-metal auxiliary vane ends 28 are heated they bend inwardly (due to their bi-metallic construction and in well known manner) about their points of attachment to the vanes 15 until, when annealing temperature has been reached they are close to the vanes 15 as is indicated in plantom lines in Figure 3. When this occurs the maximum discharge opening for the fan 9 is provided and maximum rate of flow or circulation of atmosphere within the inner cover 25 will be secured. This occurs without undesirable change in the power output of the fan motor 7 as, with the same power consumption, the motor can handle a greater volume of hot relatively light atmosphere than cold relatively dense atmosphere.

As noted above the bi-metal automatic atmosphere flow control members 28 are illustrated as being mounted on each of the vanes 15 of the diffuser unit. However in some cases it may be necessary to employ the bi-metal elements only on part of the vanes.

The apparatus illustrated in Figures 2, 3, and 4 con trols the atmosphere circulation by throttling the flow at the throat section of the diffuser vanes 15 where a minimum movement of the throttling members 28 will produce the greatest throttling of the atmosphere circulating within the system. However, the same results in increased fan efficiency and reduction of required fan capacity may be obtained by locating throttling means at other points in the flow of atmosphere within the inner cover an angle somewhat greater than 90 to each other.

ways through the

convector plates

20, 23 and 24, or in the central passageway through the stack of coils. V As the latter location is particularly convenient and practical I have illustrated several embodiments of apparatus for automatically controlling the circulation of atmosphere by controlling the flow at the upper end of the central opening in the coils being heat treated. In Figure this apparatus includes a metallic disc 30 slightly larger in diameter than the central opening in top coil 22 and resting thereon and a movable cover plate 31 of slightly larger diameter than an opening 32 in the disc 30. Webs or flanges 33 are secured to the disc 30 and extend inwardly of the central opening 32 therein and carry at their inner ends vertical guide members in the form of bolts 34. The bottom heads of bolts 34 support a plate 35 which carries a bellows type expansible thermo-responsive unit 36 to the upper end of which the cover plate 31 is secured. When the apparatus is cold the bellows unit 36 is contracted and the cover plate-31 seats on the ,disc 30, thus completely closing the upper end of the central passage through the coils. However, when the temperature reaches a certain pre-determined degree the bellows 36 expands and lifts the cover plate 31 up into the phantom line position of Figure 5 which opens the passage through the coils and permits the desired increased flow of low-density hot atmosphere. Cover plate 31 is provided with holes loosely fitting the bolts 34 to permit vertical movement of plate 31 and stop nuts 37 are preferably provided on the upper ends of bolts 34 to limit the opening movement of cover plate 31. It will be observed that although the flow of atmosphere within the inner cover 35 is throttled at a difierent point than that shown in Figures 2, 3, and 4, the same advantages are obtained. v v Figure 6 illustrates a somewhat different means .for throttling the flow of atmosphere within the inner cover of a bell type furnace while the temperature is low and the density high and automatically opening up the passageways to permit increased fiow when the temperature becomes higher and the density less. Thisconsists of a plate or disc 40 disposed over and of somewhat larger diameter than the inner opening in the top coil 22 This disc is provided with a pair of rectangular openings 41 to the inner edges of which angle throttle members 42 are hinged. These throttle members 42 each consist of an upper section 42* and a lower section 42? disposed at The bottom portion 42 may be weighted as at 43 so that the bottom portion normally hangs vertically downwardly thus holding the upper portion 42 in an upwardly sloping position whereby the holes 41 are open. This is illustrated in Figure 6 and is the position of the parts when the apparatus is cold and the fan 9 is .not operating. As soon as the fan 9 is started, the apparatus being cold and the atmosphere relatively dense, the pressure differential between the top of the disc 40 and the inside of the passage in the coils will be such as to cause the angle throttle members 42 to swing on their hinges until the upper portions 42 thereof seat upon the disc 40 and close the holes 41, thus throttling the flow of atmosphere and reducing the load on the fan motor 7. When the atmosphere within the inner cover 25 reaches annealing temperature and its density is substantially diminished the pressure differential between the inside of the coils: and the outside thereof becomes less, permitting the weights 43 to swing the throttle members 42 into open position thus permitting an increased circulation of atmosphere but without increased load on the motor because of the reduced density of the hot atmosphere.

Still another apparatus for automatically throttling the flow of atmosphere through the interior of the coils is shown in Figures 7 and 8. In this embodiment of my invention an annular disc member 45 is disposed on the .top of the upper coil 2 2 over the central opening therein. Disc 45 is provided witha central opening and has mounted thereon a vertical double wall member con sisting of a stationary outer wall 47 and a'movable inner wall 48. A top or cover member 49 (Figure 7) is secured at its edge to the top edge of outer vertical. wall .47.

Supported by a suitable bracket 50, which is mounted on plate 45, is a vertical shaft 51 which extends. downwardly through the top of cover 49 and has a transverse support bar 52 secured at its lower end. The outer'ends of this transverse bar 52 are secured to the inner movable vertical wall member 48 and provide a support for the wall member 48 during rotary movement withinthe stationary outer wall 47. In each of the

walls

47 and 48 are a plurality of circumferentially spaced openings 53. The spaces between these openings are slightlyv greater than the width of the openings so that when the wall portions, 47 and 48 are in closed position (seen in Figure 7) the openings 53 in one wall'portion will be out of register with those in the other wall portion'and there will be no passageway through the vertical wall. This is the position the parts assume when the apparatus is cold and thus the maximum throttling of the flow of atmosphere is obtained. a

, As seen in Figure 8 a fixed bar 54 extends diametricall across and is secured at its ends to the lower portion of the outer fixed vertical wall 47. Secured near one end of bar 54 is a bi-metal thermostatic element or arm 55 having its other end attached to a flange 56 projecting inwardly from the inner vertical movable wall 48. The bi-metal element 55 is so designed that when it is cold it will lie relatively close to the stationary bar 54 and the inner wall 48 will be disposed in the position'seen Figure 7 in which its holes 53 are out of register with the corresponding holes in the outer wall. As the temperature of the atmosphere is increased however the bi-metal element 55 bends away from bar 54 under-the action of heat and rotates the pivotally mounted inner wall member 48 into position wherein the

holes 53in members

47 and 48 are in-register (see Figure 8 thus opening the atmosphere passage and permitting the desired increased rate of flow of the lighter heated atmosphere.

From the above description of my metho d and several embodiments ofapparatus adapted to carry same'it will be understood that substantial savings in the first cost of the fan motor, increased heating efiiciency, and automatic variations and modifications may be made in the specific apparatus employed to carry out my method Qfauto; matic throttling in accordance with the temperature of the atmosphere being circulated and accordinglyl. do not wish 'to be limited to the particular arran gementls herein illustrated but claim as my invention a ll ernbodi ments thereof coming within the scope of'the appended claims. i 7

I claim: a k

1. The method of heating a charge in an enclosure which includes the steps of applying heat to the atmosphere and charge in said enclosure, forcefully circulating the atmosphere in the enclosure to facilitate'transfer of heat to said charge, maintaining said atmosphere under substantially constant volume and pressure'conditions during said heat application, and increasing the rate. of

flow of said'atmosphere as the temperature of said atmosphere and charge is increased.

of heat to said charge, maintaining said atmosphereunder substantially constant volume and pressure conditions during said heat application, and varying the cross-sectional area of the path of said atmosphere in relation to the temperature thereof whereby the rate of flow of said atmosphere will 'be increased upon a predetermined increase of atmosphere temperature without increase in the force applied to cause said circulation.

'3. The method of heat treating coils of strip metal or the like which includes the steps of enclosing a stack of coils and maintaining a non-oxidizing atmosphere therearound, circulating said atmosphere around said coils, applying heat to said atmosphere and said coils, maintaining said atmosphere under substantially constant volume and pressure'conditions while applying said heat, and increasing the rate of flow of said atmosphere around said coils as the temperature of said atmosphere and coils 'is increased by said application of heat.

4. The method of heat treating coils of strip metal or the like which includes the steps of enclosing a stack of coils and maintaining a non-oxidizing atmosphere therearound, forcefully circulating said atmosphere around said' coils, applying heat to said atmosphere and said coils, maintaining said atmosphere under substantially "constant volume and pressure conditions while applying said heat, and varying the cross-sectional area of the path of said atmosphere in relation to the temperature thereof whereby the rate of how of said atmosphere will be increased upon a predetermined increase of atmosphere temperature without increase in the force applied to cause said circulation.

5. In heat treating apparatus the combination of a charge support, a cover for a charge disposed on said charge support, power operated means for circulating atmosphere within said cover around said charge, and 'means' responsive to the temperature of the atmosphere being circulated within said cover for throttling the flow passage of said atmosphere whereby maximum throttling 'is obtained under cold conditions and minimum throttling under hot conditions.

" 6.-In heat treating apparatus the combination of a charge support, a cover for a charge disposed on said charge support, fan means for circulating atmosphere :within said cover around said charge, substantially constant Speed electric motor means for driving said fan means, and means responsive to the temperature of the atmosphere being circulated within said cover for throttling the flow passage of said atmosphere whereby maximum throttling is obtained under cold conditions and minimum throttling under hot conditions.

7'. In heat treating apparatus the combination of a charge support, a cover for a charge disposed on said charge support, power operated means for circulating atmosphere within said cover around said charge, means for maintaining the atmosphere within said cover at substantially constant pressure, and means responsive to the temperature of the atmosphere being circulated within said cover for trottling the flow passage of said atmosphere'whereby maximum throttling is obtained under cold conditions and minimum throttling under hot conditions.

8. In heat treating apparatus the combination of a charge support, a cover for a charge disposed on said charge support, fan means for circulating atmosphere within said cover around said charge, substantially constant speed electric motor means for driving said fan means, means'for maintaining the atmosphere within said cover at substantially constant pressure, and means responsive to the temperature of the atmosphere being circulated within said cover for throttling the flow passage of said atmosphere whereby maximum throttling is ob tained under cold conditions and minimum throttling under hot conditions.

9. In heat treating apparatus, a charge support, a cover for a charge disposed on said support, fan means for circulatingatmosphere within said cover around said charge, means for heating said charge and atmosphere, means for varying the cross-sectional area of the flow path of said" circulating atmosphere, and thermoresponsive means for actuating vsaid cross-sectionvarying means to increase 'said cross-sectional area when the atmosphere temperature is increased.

10. In heat treating apparatus, a charge support, a cover for a charge disposed on said support, fan means for circulating atmosphere within said cover around said charge, means for heating said charge and atmosphere, means for maintaining the atmosphere within said cover at substantially constant pressure, means for varying the cross-sectional area of the flow path of said circulating atmosphere, thermo-responsive means for actuating said cross-section varying means to increase said cross-sectional area when the atmosphere temperature is increased, and substantially constant speed electric motor means for driving said fan means.

11. In heat treating apparatus, a charge support, a cover for a charge disposed on said support, fan means for circulating atmosphere within said cover around said charge, means for heating said charge and atmosphere, means for maintaining the atmosphere within said cover at substantially constant pressure, means for varying the cross-sectional area of the flow path of said circulating atmosphere, and thermo-responsive means for actuating said cross-section varying means to increase said crosssectional areawhen the atmosphere temperature is increased.

12. In heat treating apparatus, a charge support, a cover for a charge disposed on said support, fan means for circulating atmosphere within said cover around said charge, means for heating said charge and atmosphere, means for maintaining the atmosphere within said cover at substantially constant pressure, means for varying the cross-sectional area of the flow path of said circulating atmosphere, thermo-responsive. means for actuating said cross-section varying means to increase said cross-sectional area when the atmosphere temperature is increased, and substantially constant speed electric motor means for driving said fan means.

13. In heat treating apparatus, a charge support, a cover for a charge disposed on said support, fan means for circulating atmosphere within said cover around said charge, substantially constant speed electric motor means for driving said fan, means for heating said charge and atmosphere, a diffuser for said fan having a plurality of outwardly diverging vanes, and bi-metal thermo-responsive throttling members disposed in the paths between at least some adjacent pairs of said vanes, said throttling members being adapted to extend into said paths and when at relatively low temperature to offer maximum resistance to flow through said paths and when at relatively high temperature to offer less resistance to said flow.

14. In heat treating apparatus including a charge support, a cover for a charge disposed on said support, fan means for circulating atmosphere within said cover around said charge, substantially constant speed electric motor means for driving said fan and means for heating said charge and atmosphere, a diffuser for said fan comprising a'base plate, a plurality of outwardly diverging vanes on said base plate, a top plate supported on the top edges of said vanes, and bi-metal thermo-respo'nsive throttling members secured to at least some of said vanes, said throttling members extending into the paths between adjacent pairs of vanes when the atmosphere is at relatively low temperature to offer maximum resistance to flow through said paths and being retracted when the atmosphere is at relatively high temperature to otter less resistance to said flow.

15. Apparatus for controlling the flow of atmosphere around a centrally apertured coil of strip metal being heat treated within a cover comprising, a plate member adapted to be disposed on the top face of said coil over the central aperture therein, said plate member having an opening, and thermo-responsive means for closing said opening 'when the temperature of the surrounding atmosphere is below a predetermined value and opening said opening as said temperature rises.

16. Apparatus for controlling the flow of atmosphere through the central opening in a coil of strip metal or the like while being heat treated within a cover member comprising, a plate member adapted to be disposed on the top of said coil over the central opening therein, said plate member having an opening therethrough, means carried by said plate member for throttling the flow of atmosphere through said opening in said plate member, and thermo-responsive means for closing said throttling means when the atmosphere temperature is low and opening said throttling means when the atmosphere temperature is increased.

17. Apparatus for controlling the flow of atmosphere through the central opening in a coil of strip metal or the like within a cover member during heat treating comprising, a plate member adapted to be positioned on top of said coil over said central opening therein, an opening in said plate member, pivotally mounted closure means for said opening in said plate member, said pivotally mounted closure means including a weighted portion adapted to hold said closure means open when the pressure differential between opposite sides of said plate member is relatively small and to permit said closure means to be closed by a predetermined increase in said pressure diiferential.

18. In heat treating apparatus the combination of a charge support, a cover for a charge disposed on said charge support, power operated means for circulating atmosphere within said cover around said charge, and throttling means responsive to the pressure differential on opposite sides of said throttling means for controlling the flow passage of said atmosphere whereby maximum throttling is obtained when said pressure difierential is maximum and minimum throttling is obtained when said pressure differential is minimum.

References Cited in the file of this patent UNITED STATES PATENTS 2,153,604 Wheller Apr. 11, 1939 2,277,592 Keener et a1. Mar. 24, 1942 2,278,581 Dexter Apr. 7, 1942 2,303,901 Baker Dec. 1, 1942 2,603,411 Trumpa July 15, 1952