US3517916A - Aluminum annealing furnace - Google Patents

Description

June 30, 1970 R. H. Rciss ET 3,517,916

I Awumuu ANNEALING FURNACE Original Filed Nov. 14. 1966 4 Shets-Sheet 1 R. H. ROSS ET ALUHINUM ANNEALING FURNACE Original Filed Nov. 14. 1966 June 30, 1970 NRKQ:

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Even/0,19: 76x20 0% oi o United States Patent Offlce 3,517,916 Patented June 30, 1970 3,517,916 ALUMINUM ANNEALING FURNACE Robert H. Ross, Willard Roth, Robert C. Larko, and Walter G. Swartzfager, Meadville, Pa., assrgnors to Sunbeam Equipment Corporation, Meadville, Pa., a corporation of Delaware Original application Nov. 14, 1966, Ser. No. 594,126, now Patent No. 3,447,790, dated June 3, 1969. Divided and this application Feb. 26, 1969, Ser. No. 802,476

Int. Cl. F27b 9/00 U.S. Cl. 263-40 9 Claims ABSTRACT OF THE DISCLOSURE An aluminum coil annealing furnace having a work chamber, within the furnace enclosure, formed by a horizontal bafile with an opening for a fan and verticle bafl les spaced from the sidewalls and floor of the furnace. Radiant tubes are positioned in the space between the enclosure and the work chamber. A fan draws heating and cooling gases upwardly through and across the aluminum coils at high velocities. A gas cooling chamber is positioned adjacent to and connected to the furnace by means of ducts for rapidly cooling the aluminum coils. Thermocouples for sensing the temperature of the gas andaluminum coils are mounted in the gas stream near the fan and in the aluminum coils, respectively. Electrical signals proportional to the temperatures sensed by the gas and work thermocouples are compared to an electrical signal proportional to a set annealing temperature and the signals resulting from the difference between the gas and set anneal temperature and the difference between the work and set anneal temperature are compared after the electrical signal resulting from the difference between the gas and set anneal temperature has been reduced, by the introduction of a resistance value, by a preselected ratio. Signals from the comparison of the differences of the gas and work temperature from the set anneal temperature control the increase or decrease of the temperature of the heating or cooling gas.

This is a division of application Ser. No. 594,126, filed Nov. 14, 1966, now U.S. Pat. No. 3,447,790; assigned to the same assignee as the instant application.

This invention relates generally to furnaces and particularly to furnaces for annealing coils of metal stock such as aluminum foil or the like.

There have in the past been serious problems encountered in the annealing of some types of coiled material such as light aluminum stock. Because of the requirements of the heat treating cycle and the characteristics of the material in the coil form, the conventional annealing fur- .naces which have been used for this task in the past have been found to be unsatisfactory. The heat treating cycle requires that the aluminum be raised to a fairly specific temperature such as 700 F. and soaked at that temperature for a substantial period of time. It is important that the material not be heated higher than the target temperature since various types of deterioration occur at these elevated temperatures. Accordingly, it is the objective in the annealing furnace to heat the material as quickly as possible to the target temperature, to maintain it at that temperature for the desired soaking period. and then to cool the material as quickly as possible.

Much of the aluminum sold today by mills is in the form of large coils of stock which are to be used by sheet material fabricators. While aluminum is essentially a good conductor of heat, it has been found that the coiled aluminum presents serious problems as far as conducting heat from the exterior to the interior portions of the coil. The adjacent layers of aluminum present obstacles to conduction of heat radially through the coil. In some instances, there will be minute air spaces 'Which effectively insulate the adjacent coils and in other cases the contact between the coils will be of such a limited nature as to inhibit the heat transfer by conduction. Because of the factjthat the interior of the aluminum coil is more or less insulated from the exterior, it has been found to be very difficult to raise the temperature of the entire coil equally to the desired target temperature. If the heating is performed too rapidly, the interior of the coil will lag far behind the exterior temperature.

It is an object of the present invention to provide an improved heat treating furnace in which work may be heat treated more rapidly and with better temperature control than in prior art furnaces.

It is a further object of the present invention to provide an aluminum annealing furnace in which temperature control is accomplished automatically so that coiled aluminum may be heated up to its annealing temperature very rapidly.

It is another object of the present invention to provide an aluminum coil annealing furnace having very high speed gas circulation for heating and cooling purposes.

It is still another object of the present invention to provide an aluminum annealing furnace utilizing high speed gas circulation as a heat transfer media and employing an automatic control means which correlates the heating head with the work temperature in order to heat the work to the final annealing temperature as quickly as possible.

It is still another object of the present invention to provide an improved annealing furnace having means for automatically controlling the rapid cooling of the work contained in the furnace.

Further objects and advantages of the present invention will become apparent as the following description proceeds, and the features of novelty which characterize the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

For a better understanding of the present invention, refierlelnce may be had to the accompanying drawings in W 1c FIG. 1 is a sectional view from the side of an improved aluminum coil annealing furnace embodying our invention;

FIG. 2 is a front elevational view of the furnace of FIG. 1 with portions cut away to expose the interior thereof;

FIG. 3 is a schematic diagram of the control means for the furnace;

FIG. 4 is a schematic diagram of the means for automatically regulating the cooling in the furnace; and

FIG. 5 is a diagram showing the work and the air temperature within the furnace plotted against time.

The present invention is directed to a furnace for annealing coiled aluminum in order to bring the large coils of aluminum stock up to the desired target temperature as quickly and accurately as possible. The furnace is provided with means for circulating heated gas across the coils of aluminum at very high velocities. By employing velocities higher than those heretofore known in the art, it has been possible to heat the coiis more quickly and thereby increase the output of a given size furnace considerably. In order to avoid the possibility of overshooting the target temperature for 'any portion of the coil, an improved control means is employed in which the heating head of the high velocity circulating gas is maintained at a maximum and is decreased in a proportional manner only as the work temperature approaches closely the target temperature for the work. The heating head is then decreased continually as the work temperature ap- 3 proaches the target temperature. This improved control means is particularly necessary with the increased heating rate since greater temperature differentials are present in the aluminum coils thereby requiring maximum heating heads and close control to assure maximum speed with absence of overshoot.

In order to complete the overall cycle as quickly as possible, the furnace is provided with improved cooling means whereby circulating cold gas is bled into the high speed circulating gas stream in a controlled manner to lower the temperature quickly at a uniform rate.

The means for producing the cooling gas is arranged so that the cooled gas is recirculated within the compartment containing the cooling coils and the bleeding of cool gas from this compartment and the introduction of high temperature air from the furnace is performed in a controlled manner so that the high temperature gas does not impinge on the cooling coils thereby damaging them or fouling them with deposits of lubricants and dirt which lower the efficiency of heat transfer. These deposits become baked on at elevated temperatures and are almost impossible to remove.

Referring now to the drawings, there is shown a furnace which is designated generally by reference numeral 11. From a structural standpoint, the furnace 11 is of conventional design being of sheet metal construction with a layer of insulating refractory material on the interior to form an insulated enclosure 12 having generally vertical side walls 13 and 14, a rear wall 15, and a front wall 16. The front wall 16 is formed with a large entrance opening 17 which is adapted to be closed by a vertically slidable door 18.

The top of the furnace is closed by a horizontally extending wall 19 which also serves as a support for a large gas circulating fan 20. The circulating fan 20 includes a vertically extending supporting shaft 21 which is journalled in a mounting frame 22 carried by the roof 19. Carried by the lower end of shaft 21 is a large axial flow fan member 23. To rotate the shaft 21 there is provided a reversible motor 24 which is drivingly connected by means of belts 25 to the shaft 21. The motor 24 is reversible so that the shaft 21 may be rotated in either direction to cause the fan member 23 to either move air upwardly or downwardly as will be explained in greater detail below.

Within the enclosure 12, there are baffles and 31 which form a work chamber 32 within the enclosure 12. As is evident from FIGS. 1 and 2, the baflles 30 extend vertically and are in parallel spaced relation to the side walls 13 and 14. The baffie 31 is generally horizontal and is positioned at the level of the fan member 23 extending across the tops of the bafiles 30. In addition, the bafile 31 extends from the rear wall 15 to the front wall 16. The work chamber is thus defined by the vertical bafiles 30, the rear wall 15, the door 18, a portion of the front wall 16, and the horizontal baffie 31.

The furnace 11 is designed to handle large coils of aluminum sheet material. To transport these coils into and out of the furnace, there is provided a supporting truck or car 33. The details of the work supporting car 33 are disclosed and claimed in Larko Pat. No. 3,125,328 which is assigned to the same assignee as the instant application. Briefly, the car 33 is provided with chassis 34 which carries a work supporting platform 35. The platform 35 is fabricated of a foraminous material such as expanded metal or some type of grill work so that the heated gases within the furnace may pass upwardly through the work being treated. The car 33 is supported on roller chains 36 which travel on tracks 37 which extend from within the furnace enclosure 12 to a suitable loading position outside of the furnace.

In order to heat the gases within the furnace which act as the heat transfer media, there are provided a plurality of U-shaped radiant tube heaters 40. Suitable gas burners are positioned in the end of each tube 40 in the conventional manner to heat the tubes 40 which in turn heat the gases within the furnace 11. The heaters or tubes 40 are positioned between the baffles 30 and the adjacent side walls 13 and 14 of the enclosure 12. The radiant tubes 40 may alternatively be heated by electric means. In the (115- closed embodiment, however, the burners for tubes 40 are gas fired and are provided with a suitable exhaust duct system 41. Under normal operating conditions, the fan member 23 is rotated in such a direction as to draw gases upwardly through the work chamber 32 discharglng them toward the roof 19 of the furnace enclosure. The gases thus discharged move outwardly toward the walls 13 and 14 and then circulate downwardly between the bafiles 30 and the side walls. The radiant tubes 40 are positioned within this space between the work chamber and the side of the enclosure 12. Thus, as the gases move downwardly through this space, they are heated by the radiant tubes. At the bottom of the enclosure 12, the gases are restricted and, therefore, move inwardly toward the work supporting car 33. Since the car 33 is merely an open framework with the platform 35 being of foraminous matenal, the gases circulate upwardly past and through the coils of aluminum positioned on the platform 35.

Aluminum coils of sheet material are particularly diflicult to heat treat properly. The coils of light sheet material have a tendency to insulate the interior portions, therefore, making it difficult to bring the entire volume of material up to the desired annealing temperature at which it should be soaked for a selected period of time. Because of this thermal lag between the interior and the exterior of the coil, there is a danger that the interior portions may not be completely annealed. In instances where high temperatures are employed in an effort to heat the interior of the coil, there is a danger that the exterior portions of the work will be overheated thereby damaging the grain structure of the aluminum. In the instant invention, these problems of temperature lag and flow heating to the annealed temperature are overcome by utilization of improved control means and high velocity gas circulation.

It has been common in prior art furnaces of this type to recirculate gases in the furnace at a rate of five to six hundred feet per minute. The furnace 11 is provided with means for circulating the gas at velocities from one thousand to five thousand feet per minute. This tremendously increased rate of gas circulation improves the heat transfer between the circulating gases and the coils of aluminum. Because of the increased velocities of the circulating gas, it is important to have the fan member 23 provided with a proper shroud. As is evident in FIGS. 1 and 2, there is a shroud 42 extending around the periphery of the fan member 23. The shroud 42 is carried by the horizontal baffle 31 and extends downwardly therefrom. The lower end of the shroud 42 is formed with an outwardly flared conical section 43. The flared portion 43 improves the flow conditions as the circulating gases move upwardly into the fan member 23.

For the purposes of control as will be explained in greater detail below, there is a thermocouple 45 which may be positioned immediately below the conical section 43 of the shroud. The thermocouple 45 thus measures the temperature of the circulating gas downstream of the load being heat treated. By measuring the downstream temperature of the gas, better and more rapid heating is achieved during the early stages of the heat treating process. The downstream temperature will be lower than the upstream temperature since the load itself has extracted a considerable amount of heat from the gas leaving it at a lower temperature in the downstream position. By positioning the control on the lower air temperature further downstream, the heat will be applied to the load more rapidly as will be more evident as a description of the control system proceeds.

For the purposes of providing a rapid controlled cooling of the enclosure 12, the furnace 11 is provided with a cooler 85. The cooler consists of an elongated sheet metal compartment which extends vertically adjacent the rear wall of the .enclosure 12 as shown in FIG. 1. The cooler 85 is of substantially square cross section in the horizontal plane and is interconnected with the enclosure 12 by means of an upper inlet passageway 86 and a lower discharge passageway 87. The cooler 85 isdivided by a vertically extending wall 88 as is best shown in FIG. 1. The wall 88 extends completely across the cooler dividing it into an enlarged passageway 89 and a reduced passageway or conduit 90. Within the passageway 89 there are mounted in horizontally extending position a pair of cooling coils 91 which are cooled by means of water circulating through heat transfer tubes included therein. Immediately below the cooling coils 91 is a circulating fan 92. The fan 92 is a motor drive unit which circulates gas downwardly through the passageway 89 and discharges gas upwardly through the reduced passageway 90.

To regulate and control the flow of gas through the passageways 8'6, 87, and 90, there are provided movable dampers 93, 94, and 95, respectively. These dampers are all mounted for slidable movement in unison and are driven by means of a motor 96. In FIG. 1, the dampers 93 and 94 are shown in their closed position while the damper 95 is shown in the open position. When the motor 96 is energized to raise the dampers 93 and 94 to their open position, the damper 95 is moved to the closed position. The purpose of the dampers 93, 94, and 95 is to regulate the proportion of the gas from the passageway 89 which is to be bypassed through the reduced passageway or conduit 90. With the dampers 93 and 94 closed as indicated in FIG. 1, the entire output of the circulating fan 92 passes through the conduit or bypass 90 and recirculates through the passageway 89, through the cooling coils 91 and into the circulating fan 92 again.

When it is desired to cool the furnace enclosure 12, the dampers 93 and 94 are opened and the damper 95 is closed. In such a position, the discharge of the circulating fan 92 passes through the discharge passageway 87 into the enclosure 12. The discharge passageway 87 is connected to a cooling duct 98 which is best shown in FIG. 2. The duct 98 runs lengthwise of the furnace from the rear wall 15 to the front door 18 of the furnace. Suitable discharge openings 99 are provided in the duct so that the cooling gas may be circulated upwardly through the platform into contact with the work being cooled.

Referring now to the schematic diagram of FIG. 3, there is shown the location of the thermocouple within the work chamber 32. Supported on the car 33 is a single coil of aluminum 47. Inserted within the coils of aluminum is a second thermocouple 48 which is designed to measure a representative high temperature point within the coil 47. The selected point is usually one-half inch inward from the end of the coil and about one-half inch radially inwardly from the outer cylindrical surface of the coil. The thermocouple 48 is positioned in the outer coils of the aluminum coil to sense the hottest temperature of the work even though the sensing is done slightly inwardly from the outermost coil. It should be appreciated that in instances in which a number of coils 47 are being heat treated within the furnace, it may be necessary to utilize a plurality of work temperature sensing thermocouples such as 48. The sensing and controlling mechanism would then be arranged to select the thermocouple giving the highest work temperature reading and use the reading from this thermocouple for control purposes.

The thermocouples 48 and 45 are connected to a selector switch 58 which sequentially switches from one thermocouple to the other to provide a signal in accordance with the work temperature or the downstream gas temperature. The output of the selector switch 50 is connected to a bridge circuit 51. One of the arms of the bridge includes a first potentiometer 52 having a tap 52a carried by a movable carriage 53. As is conventional in sensing and recording instruments of this type, the carriage 53 is positioned to give an indication of the temperature sensed by the thermocouples 45 and 48. This positioning is accomplished by means of an amplifier 54 and motor 55 which is connected to rotate the screw 56 responsive to any unbalance in the bridge 51. Also connected to the carriage 53 is a second tap 58a for a second potentiometer 58. The tap 58a for the potentiometer 58 will be positioned in accordance with the temperature sensed at the thermocouples 45 or 48 depending on the position of the selector switch 50. The positioning of tap 58a will vary the resistance in potentiometer 58 which forms a portion of a second bridge circuit de scribed in greater detail below. The actual temperatures may be read or recorded on a scale 59 and an output is fed to the lead 60 which is proportional to the indicated temperature. The lead 60 interconnects the tap 58a on the second potentiometer 58 to a second selector switch 61. The selector switches 50 and 61 are interconnected so that they switch simultaneously from one portion of the circuit to the other in sequence as will be explained below.

Connected across the potentiometer 58 to form a second bridge circuit is a power supply 62 and a third potentiometer 63. The potentiometer 63 is provided with a tap 64 supported on a carriage 65 which may be adjustably positioned by a rotatable screw 66. The tap 64 and carriage 65 are arranged to provide a visual indication of temperature on a scale 67. The purpose of the potentiometer 63 and scale 67 is to provide means for setting the control means to the anneal temperature to which the work will be heated. This is performed manually at the start of the heating cycle. The positioning of tap 64 at the anneal temperature will establish a set resistance value in potentiometer 63. As is evident from FIG. 3, when either the gas temperature, sensed by thermocouple 45, or the work temperature, sensed by thermocouple 48, is different from the anneal temperature, the second bridge circuit formed by potentiometer 58, power supply 62 and potentiometer 63 will be unbalanced. The tap 64 of the potentiometer 63 is connected by means of a lead 68 to a null detector '69. Connected across between the leads '60 and 68 is a potentiometer or ratio resistance 70. The tap 71 of the potentiometer is connected to the second selector switch 61. The interconnection of selector switch 61 and selector switch 50 referred to above, results (as is shown, for example, in FIG. 3 for the signal resulting from the tem perature of the gas as sensed by thermocouple 45) in the signals from the unbalanced second bridge circuit proportional to the difference between the gas temperature and the set anneal temperature being fed through ratio resistance 70 into null detector 69 while the signals from the unbalanced second bridge circuit proportional to the difference between the work temperature and the set anneal temperature bypass the ratio resistance 70 directly into null detector 69. A suitable scale 72 is provided to indicate the proportion of the ratio resistance connected between the tap 71 and the lead 60.

The output of the null detector is connected to a heat control relay 74 which in turn controls the energization of the radiant burners for it. A suitable switch 75 is provided with a circuit between the null detector and heat control relay 74 to provide a safety measure whereby the heating elements will be shut off when the furnace temperature raises above a certain predetermined maximum. The normally closed switch 75 is mechanically connected so that the carriage 53 of the first potentiometer 52 operates the switch 75 when it reaches the maximum safe temperature at the upper end of the tempera ture range.

To best understand the manner in which the control circuitry of FIG. 3 operates, reference should be made 7 to the temperature diagram of FIG. 5. In the selected example, the maximum temperature of the circulating gas within the furnace is 1,000 F. at annealing temperature or target temperature to 700 F. When the coil 47 is initially placed within the furnace, the entire heating head of the furnace causes the coil 47 to heat up very rapidly. The third potentiometer 63 is adjusted so that tap 64 is positioned to indicate the anneal temperature of 700 F. on the scale 67. Under normal conditions, the heaters 40 will maintain the maximum gas temperature of 1,000 F. as measured by the thermocouple 45 until the work reaches the so-called control band at which time the heating head must be reduced to prevent the load from being heated to a temperature above the anneal temperature of 700 F.

The control is established by the maximum furnace gas temperature, the setting of the anneal temperature, and the setting of the ratio resistance or potentiometer 70. Essentially, the purpose of the control circuitry of FIG. 3 is to compare (1) the increment of temperature of the gas above the anneal temperature (to be referred to as AG) against (2) the increment of temperature of the work below the anneal temperature (to be referred to as AW). The circuit automatically controls the gas temperature to maintain a selected ratio between these two temperature increments. The setting of potentiometer 70 determines what this ratio shall be. In the example graphed in FIG. 5, the maximum temperature is 1,000 F., the anneal temperature is 700 F. and the selected ratio between increment of gas temperature above set point to the increment of work temperature below set point is 3. Thus, as the work temperature approaches the anneal point and enters the control band, the control circuit will maintain the relationship of AG.=3-AW.

Since the set point in the example of FIG. 5 is 700 F. and the maximum gas temperature is 1,000 F., the initial AG is 300 F. With a value of AG initially of 300 F. and a selected ratio of AG/AW equal to 3, the control band will extend 100 below the set point as shown in FIG. 5. Therefore, as discussed above, in order to maintain AG/AW equal to 3 ratio resistance 70 is set at 3 so that only one-third of the signal generated by the unbalanced second bridge circuit proportional to the difference of the gas temperature as sensed by thermocouple 45 and the set anneal temperature is fed into null detector 69. As the work temperature enters the control band, AW will become less than 100 and the gas temperature is automatically reduced to maintain the selected ratio between the temperature increments because the difference between /2.AG and AW, as detected by null detector 69, is greater than one (1). By the proper selection of the ratio, it is possible to obtain maximum rate of heating of any particular type of material in a particular form While avoiding any danger of exceeding the anneal temperature and thereby damaging the work.

During the cooling of the work within the enclosure 12, the main gas circulating fan 20 continues to operate, drawing gases upwardly across the work within the work chamber 32 and circulating the gas outwardly and downwardly between the side walls of the work chamber 32 and the side walls 13 and 14 of the enclosure 12. In view of the position of the duct 98 with its discharge openings 99, it should be obvious that the cooled gas delivered from the cooler 8-5 will be introduced uniformly into the circulating gas stream moving upwardly through the work chamber 32.

The intake passageway 86 for the cooler 85 is positioned within the work chamber 32 where it draws ofi heated gas in a manner that leaves undisturbed the fiow of cooling gas across the work.

The cooler 85 provides a distinct advantage over coolers heretofore known in the art since it includes means for cooling the temperature of the gas circulated through the cooling coils 91. By providing the bypass passageway 90 within the cooler and dampers 93, 94, 95 which regulate the amount of cooling gas circulated through the bypass, it is possible to mix the desired amount of heated gas with the recirculating cool gas so as to maintain the temperature of the gas passing through the cooling coils below a certain maximum which would be undesirable. In most instances, therefore, the dampers 93, 94, and '95 will be in some intermediate position where they partially close the passageways 86, 87, and 90, respectively.

Referring now to the circuit diagram of FIG. 4, there is shown the means for automatically controlling the cooling of work within the furnace 11. Because of the fact that there is no danger of damaging the aluminum coils in cooling as there is in heating, there is no necessity to have the cooling rate controlled as precisely as is necessary during heating. Because of the dangers of damaging the work by heating it past the annealing temperature, it was necessary to relate the heat control to both the circulating gas temperature as well as the work temperature. In connection with the cooling, however, only the thermocouple 48 is employed. During the cooling operation, the selector switch 50 is set to continuously connect the thermocouple 48 to the bridge 51. As explained above, the control means including the amplifier 54 and motor 55 position the carriage 53 in accordance with the work temperature sensed by the thermocouple 48.

As is shown in FIG. 4, the tap of the second potentiometer is connected by lead 101 to the null detector 69. The tap of the third potentiometer 63 is connected by lead 102 to the null detector 69. The potentiometers 58 and 63 form the legs of a bridge circuit including the power supply 62. The output of the bridge circuit is delivered to the null detector 69 by means of the leads 101 and 102. When the control circuit was used during the heat-up period, the tap of the potentiometer 63 was set for the preselected anneal or target temperature. In the use of the circuit for control of the cooling cycle, the tap on the potentiometer 63 is moved downwardly from the anneal temperature position to a reduced temperature at a. controlled rate. This controlled rate is obtained by a motor 103 which is drivingly connected to the screw 66 which supports the carriage 65 and moves the potentiometer tap downwardly as the motor 103 is driven in the desired direction. Suitable means may be provided to vary the rate at which the motor 103 rotates the screw 66. Either a variable speed drive may be used or conventional means for intermittently energizing the motor 103.

The null detector 69 is connected to a polarized relay 104 which in turn is connected to the reversible motor 96 which operates the dampers 93, 94, and 95. The null detector is adapted to sense the unbalance in the bridge including the potentiometers 58 and 63 and, depending on the direction of the unbalance, will drive the motor 96 to either open or close the dampers 93 and 94 so as to return the bridge to a balanced position. As a consequence, the work temperature sensed by the thermocouple 48 will decrease at a constant rate corresponding to the rate at which the temperature indicated at the potentiometer 63 decreases. The desired rate of cooling is determined empirically for particular loads and types of material being heat treated within the furnace 11. The circuit disclosed in FIG. 4 provides a simple and effective means for cooling the furnace 11 at a uniform and rapid rate. The adjustment of this rate by merely varying the speed of rotation of the screw 66 driven by the motor 103 provides a simple means of obtaining any selected rate of cooling. Further, the use of the recirculating means within the cooler permits a simple variation in the amount of cooling delivered to the furnace at any one time while providing a means for limiting the temperature of heated gases entering the cooler 85. It should be understood that a safety circuit may be provided to limit the introduction of gases above a certain maximum into the cooling coils if too rapid a rate of cooling is selected. This safety circuit in its most simple form may involve only a thermostatic sensing means at the coils 91 which will shut off the circulating fan 9 2 when the maximum temperature is exceeded. Accordingly, applicants have provided a simple and effective means for automatically heating aluminum coils quickly to a desired annealing temperature and cooling them rapidly to the removal temperature of about 350 after the soaking has been completed.

The arrangement of the fan 23 and the baflles 30 and 31 which form the enclosure 32 provide a significant advantage over the furnaces heretofore known in the art. It has been conventional in these furnaces to rotate the gas circulating fan in a direction so that the gases are circulated downwardly toward the work being heat treated. The fan 23 is rotated to circulate gas upwardly through the enclosure 32 and downwardly through the space between the walls 30 and the furnace walls 13 and 14.

This arrangeemnt of the baflles and the fan flow permits an improvement in the gas leakage which is always present to some extent in a controlled atmosphere furnace. Other than in a vacuum furnace, the parts includ ing the doors, walls, fan seals, etc. are constructed of sheet metal and refractory material and are not expected to be completely gas tight. The disclosed arrangement minimizes considerably the harmful leakage which is present in most furnaces of this type. The area within the work enclosure 32 under normal operating conditions is maintained at atmospheric pressure by control of the gas input. The upward discharge of the fan 23 causes the portion of the furnace above the wall 31 and the areas between the enclosure baflles 30 and the outer furnace walls 13 and 14 to be at a pressure-above atmospheric pressure. Accordingly, any cracks or-openings in the furnace walls 13, 14, 15 and the roof 19 will permit gas to leak outwardly rather than air passing inwardly. It is advantageous to have the gas leakage outwardly since the reverse situation in which air passes inwardly causes contamination of the atmosphere within the furnace. In the area within the enclosure 32 the pressure is atmospheric and there is, therefore, little tendency for the gas or air to pass through the cracks or openings between the door 18 and the opening 17. By having atmospheric pressure within the enclosure 32, there is little tendency for hot gases to escape through the seal between the door 18 and the front wall 16. As a consequence, there is little deterioration of the seal from the heat of the furnace.

In the conventional arrangement in which the fan forces gas downwardly through a work enclosure, a pressure condition exists along the outer walls of the furnace which is less than the pressure within the work enclosure, thereby causing a situation in which air tends to leak in and contaminates the furnace atmosphere. The only way to counteract this leakage is to raise the overall pressure within the furnace so that the spaces adjacent the outer walls are equal to or slightly above atmospheric pressure. This increase in pressure results in the pressure within the work enclosure and adjacent the door being considerably higher than atmospheric pressure as a result of the direction of gas flow caused by the fan. This super-atmospheric pressure adjacent the door causes the hot gases to pass outwardly through the door seal thereby deteriorating them rapidly. I

Another advantage resulting from the arrangement of the fan 23 and the baffles 30 is that a far more uniform flow of gas through the work enclosure is obtained. It is desirable to have the gas velocities across the work and through the work enclosure be as uniform as possible. By having the gas forced into the work chamber by the fan 23 which is located remote from the gas entering the work chamber 32, the velocity of the gas is lower and more uniform than if the fan were positioned at the gas intake to the work chamber. In addition, the fan efiiciency is increased by having the output of the fan adjacent the furnace roofv'rather than the input. The gas approaches the fan 23' in essentially an axial direction thereby providing the less turbulence and more efficient gas flow. The output of the fan is directed outwardly by the surface on the roof 19 of the furnace. There is no necessity to utilize the vanes and patlles which are characteristic of fans which are positioned to draw gas across the roof and expel it axially downwardly into a work chamber.

While there has been, shown and described a particular embodiment of the present invention, it will be apparent to those skilled in the art that various changes and modi-. fications maybe made without departing from the invention in its broaderg aspects, and it is, therefore, contemplated in theappended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed as new an desired to be secured by Letters Patent of the United States is:

1. An annealing furnace comprising an insulated furnace enclosure, bafilesyforming a work chamber within said enclosure, a first .gascirculating means comprising an axial flow fan positioned within said enclosure to circulate a gas through said work chamber, radiant heaters positioned outside of said chamber between said baffies and the walls of said enclosure to heat said gas circulating through said chamber, cooling means for said enclosure comprising a compartment equipped with a second gas circulating means, externally cooled heat exchange means withinlsaid compartment for cooling gas circulating therethrough, wall means within said compartment forming a Work chamber bypass conduit connecting the outputv and input of said second circulating means to continuously circulate gas across said heat exchange means within said compartment, spaced passageways extending between said compartment and said enclosure whereby cooled gas may be introduced into the gas circulated through said work chamber, controlled closure doors for at least one of said passageways, and closure means for said conduit operable with said controlled closure doors to obstruct gas circulation through said conduit when said closure doors open to permit circulation of gas through said passageways.

2. The annealing furnace of claim 1 having an automatic control means responsive to the temperature sensed by a thermocouple in said enclosure, said automatic control means providing a predetermined cooling rate by controlling said closure doors and closure means to regulate the cooling of said furnace.

3. The annealing furnace of claim 2 wherein said thermocouple is positioned in the work load in said enclosure.

4. The annealing furnace of claim 2 wherein said automatic control means comprise a null detector responsive to the temperature sensed by said thermocouple and responsive to means producing a signal directly proportional to said predetermined cooling rate, said null detector being operatively associated with said closure doors and said closure means whereby the circulation of the cooled gas within the compartment and to said.

enclosure is automatically regulated.

5. The annealing furnace of claim 2 wherein said automatic control means comprise first and second potentiometers responsive to said thermocouple and said means producing a signal directly proportional to said predetermined cooling rate respectively, said potentiometers being connected with a power supply to form a bridge circuit, a null detector connected to said first and second otentiometers for sensing the unbalance in said circuit, reversible driving means for said closure doors and said closure means operatively associated with said null detector to automatically control the circulation of the cooled gas within the compartment and to the enclosure for regulating the cooling by. the predetermined rate. 1 x 1 6. The annealing furnace of claim 5 wherein a polarized relay is connected between said null detector and said reversible driving means-to'sense the direction of unbalance of said circuit and-operate the driving means in the properdirection to control' -thecirculation-of the cooled gases to regulate the cooling by the predetermined rate. 1'

7. The annealing furnace of claim 1 having duct means adjacent the bottom of said'enclosure communicating with one of said spaced passageways whereby th'ecooled gas from said compartmentis introduced into the gas circulated through said work chamber below a work load.

8. The annealing furnace of claim 7 wherein said duct means extends the length of the furnace enclosure and is centrally positioned therein, the top of said duct means angling downwardly on each side of its center and having discharge openings in said top to permit the cooled gas to be introduced below the work load into the gas circulated through the work chamber by the axial flow fan.

9.'=The annealing furnace of claim 1 wherein the closure-means is connected with the. closure doors so that'boththe closure means and the closure doors oper-ate together but in such amanner that the closure means obstruct recirculation of the cooled gas within said-compartment when an upper closure door is open to permit heated gas from the work chamber to enter said compartment and a lower closure door is open to permit discharged the cooled gas into the Work chamber, said closure means and closure doors being positionable 'intermediately between a fully opened and fully closed position to effect a controlled rate ofcooling.

References Cited UNITED STATES PATENTS 2,228,088 1/1941 Roth.

2,477,796 8/1949 Germany.

JOHN J. CAMBY, Primary Examiner us. c1. X.R.

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