US2676007A - Heat-treating apparatus - Google Patents

Description

A. W. DAVIS HEAT-TREATING APPARATUS April 20, 1954 Filed March ze, 1951 2 Sheets-Sheet. 2

FIG. 3

INVENTOR.- ALVIN W DAVIS ATT'Ys Patented Apr. 20, 1954 UNITED STATES PATENT GFFICE 9 Claims.

This invention relates to heat treating apparatus of the rccirculating type and particularly to improvements in method and means for treating metals and other materials through the application of heat. More particularly this invention relates to an improved heat treating apparatus construction, for the bulk treatment of metallic objects; and to an improved method for controlling the amount and distribution of heat imparted to the Work load through agency of a circulated gaseous heat-carrier medium.

The main objects of this invention are to provide an improved method of balancing the heat transfer effect obtained by radiation with that obtained by convection, to obtain a more uniform initial heating of the work load and maintenance of uniform temperature throughout the load when the desired control temperature has been reached, in a heating apparatus of the recirculating type; to provide an improved method of obtaining uniformity of temperature throughout the load during both the initial heating up and soaking phases of a heat treating process; to provide an improved method of heat treating bulk loaded metal objects with a two pass system utilizing a recirculated gaseous heat carrier medium to provide such a method wherein maximum heat transfer efiiciency is obtained from the heat carrier medium while maintaining temperature uniformity in all parts of the load; t provide such a method wherein the amount of heat transferred throughout the load and the temperature of the treated material can be quickly and accurately controlled; and to provide an improved heat treating apparatus for performing the herein described method.

Other objects are to provide an improved heat treating apparatus of the recirculating type; to provide such an apparatus which more eilciently utilizes the two pass system of heat distribution over and through bulk loads; to provide an improved combustion and mixing chamber construction for mixing products of combustion with a recirculated air stream in a heat treating apparatus; to provide an improved means for automatically metering the volume of gases vented from heat treating apparatus of the recirculating type; to provide such a means in which the coolest gases are vented automatically in direct proportion to the amount of gases introduced by combustion; to provide such a means which obviates reverse stack action in the flue and which without mechanical means controls the vent volume automatically in direct relation with any variation in pressure within the system; to provide a venting method which eliminates airv infiltration through the vent during cycling periods and which prevents excessive draft when heating up.

Specific embodiments of my improved heat treating apparatus construction, by which my improved temperature control process may be performed, are shown in the accompanying drawings in which:

Figure 1 is a section View, in side elevation, showing an improved heat treating apparatus of the vertically loaded pit type in which combustion is employed as a heat source.

Fig. 2 is a sectional plan view of the same as taken on line 2--2 of Figure l, and

Fig. 3 is a sectional View in side elevation, similar to Fig. 1, showing an improved pit type of structure in which electrical resistance heaters are employed as a heat source.

In the form shown in Figs. 1 and 2 the heat treating apparatus, or furnace, I is a hollow structure built up of re brick, slab insulation, and other suitable insulating materials to form an upright cylindrical Work chamber 2, a fan chamber 3, and a recirculation chamber orreturn passage il, which provides communication between the bottom end of the work chamber and the fan chamber. The Work chamber 2 is preferably lined with a steel shell 5 and opens at its upper end for the introduction and removal of the material to be treated. This top opening is normally closed by an insulated cover 6, which has a peripheral angle flange l, thedownward legs of which is spaced from the cover and fits into a channel 8 around the upper margin of the opening. Usually the channel 8 is partially lled with said or other suitable material to provide a seal when the cover 6 is in place.

As shown, the main part of the furnace structure is set in a pit, below the level of the iioor 9, so that the cover can be raised from the floor level when work is to be put into or removedv from the chamber 2, and only the upper end of the work chamber extends above the iioor line. Thus the fan chamber 3 and the burner, which are located to deliver directly into the bottom portion of the work chamber 2, are below the iioor and out or" the way of any activity about the work chamber opening.

The fan chamber 3 is included in the main furnace structure as a lateral extension from the Work chamber 2 and its general level is belowl that of the work chamber substantially in line with the return or recirculation chamber 4, which opens into the bottom end of the work chamber and runs horizontally outward therefrom. In the construction shown the fan IS is mounted with its drive shaft Ii in horizontal position so that that shaft extends through the insulated walls of the furnace structure and is journaled in outboard bearings I2 and i3 which are thus in a relatively cool location. The fan is preferably arranged with double inlets, each in direct communication with the recirculation chamber and the fan or blower discharge is horizontal into a passage Id which leads directly into the work chamber 2, at the bottom of its side wall. The passage I4 enters the work chamber 2 tangentally and delivers air along the side wall of the chamber, in a horizontal direction, so that the air in traveling from the bottom to the top of the chamber, will follow a generally spiral circular path.

The work to be treated in the apparatus, or furnace, is loaded into a metal basket I'5 which has an open top and a grate-like bottom S6, capable of supporting the Work and yet allowing air or gas to pass axially through the basket. The height of the basket I5 is less than that of the work chamber 2, to clear the cover 6, and the diameter of the basket is considerably less than the diameter of the work chamber to permit the free flow of gas or air, which is the heat carrier medium, around the outside of the basket and into the top end of the same, as indicated by the arrows in Fig. l. The basket I5 rests directly onthe bottom of the work chamber 2, when in use, and completely covers the entrance .I from the Work chamber to the recirculation chamber il. Thus the air or gas introduced into the chamber 2, from the passage I4, must travel around and to the top of the basket and then. through the work load in the basket in order to reach the recirculation chamber or return passage to the fan or blower IIJ.

In the form shown the fan outlet I1 is rectangular and the passage I4 is a direct continuation of the fan outlet leading into the treating chamber 2. The bottom wall I8 of the passage Id is, however, inclined away from the fan outlet so as to diverge from the center-line of the passage and approximately follow the orice angle of the air stream discharged from the fan, and an opening I9 is formed in this wall for a flue or vent 2D which leads to the outside of the furnace structure. The orifice angle is determined by the boundary of the air stream as it would normally expand against atmospheric pressure, outwardly from the air stream axis, upon discharge from the fan outlet, and the wall I8 is formed to lie just inside of the expanding air stream at an angle of about 7 from the discharge axis. Thus there is a slight pressure on the flue opening I9, created by the fan blast, which is generally suiiicient for average pilot fiame operating conditions, and which pressure is sufciently small that substantially only such air or gas that is in excess of the normal volume within the furnace structure will enter the flue opening.

In the combustion type of furnace, as shown in Fig. 1, the burner 2l, is located in a lateral passage 2I.I, in the wall opposite the diverging wall I8, which passage enters the main air passage I4 adjacent the entrance to the work chamber 2 and at an angle that is less than 90 from the orifice angle of the fan discharge, and so as to be at an acute angle, preferably about 30, with the center line of the fan discharge in the rearward direction. The passage 2I.I is positioned so that its axis intersects the axis of the main passage I4 substantially at the entrance thereof into the work or treating chamber 2 and the stream of combustion products thus meets the air stream from the fan and mixes therewith, while traveling in the same general direction and immediately prior to entering the treating chamber.

Combustion occurs within the passage 2 I .i and the angular disposition of this passage, relative to the air stream from the fan, results in a negative pressure at the burner so that the products of combustion are aspirated into the main air stream, mixing immediately with the cooler recirculated air and immediately entering the treating chamber 2. The result is cooler operating conditions at the point of confluence, or mixing, of the combustion products with the relatively cool discharge of the fan and a maximum delivery of the heat resulting from combustion to the work chamber 2. Also combustion at the burner is more stable, over the full range of burner capacity, and since the point of confluence of the hot and cool gases is substantially at the entrance to.

the work chamber, beyond the flue opening I9, the back pressure caused by the introduction of the products of combustion into the system results in the coolest air, or air and gas mixture, being deiiected by the combustion products stream and forced into the nue opening in direct volumetric proportion to the volume of gases introduced by the burner. This flue system is therefore self-metering, according to the burner load, and obviates the necessity of dampers and mechanical smoke exhausting devices, or other means, to remove fumes and prevent unnecessary heat losses; and when the burner is turned down, or orf, as in the cycling phase of the heat treatment operation, the recirculating gases move past the flue opening with only a minimum of heat loss therethrough.

When an electric heat source is employed as in the case of the structure shown in Fig. 3, the burner passage is either eliminated, as shown, or blocked orf and resistance heaters are installed in the main air passage I4 and around the wall of the work chamber 2. Otherwise the furnace structure is similar to the combustion type, as shown in Fig. l. heating elements 22, installed in the work, or treating chamber 2, are ribbon, coiled wire, or rod type heaters arranged in three banks, one above the other, and each bank extends circumferentially around the inside of the chamber wall be- Ixzeen the Wall and the outside of the Work basket Each bank of heaters may be individually controlled by suitable electric control devices, not shown, for independent operation and the heaters are so arranged that the air circulated around the basket I5, under the action of the fan Iii, must pass over and through the heaters so as to be directly heated by convection when the heaters are energized. These heaters serve the dual function of transferring heat to the Work basket I5 by radiation, and thence by conduction to the material being treated, and of heating and circulated air or gas, by convection, which in turn transmits the heat to the material in the :basket as it passes downwardly through the load.

The second group of heaters 23, sometimes installed in the main air passage I4, serve to heat the circulated air as it enters the work chamber, and when radiant heat from the heaters 22 is not required, as in the case of cycling (for holding As shown in Fig. 3, the electric the desired heat balance) when the load has been brought up to the desired temperature.

It will now be seen that in the improved heat treating apparatus, whether heated electrically or .by suitable gas or oil fired combustion means, the iiuid heat conveying medium is delivered into the work chamber at one end and tangentially with respect to the load carrying basket and the chamber side wall. The heat carrier medium is thus caused to travel a rotary path which spirals around the basket progressively until it reaches the opposite end of the chamber Where its general direction is reversed and the carrier is both pushed and drawn, through the basket and its load, to the recirculation chamber t from Where it is returned to the fan or blower IS.

This may be termed a two-pass system and it will be apparent that, in ordinary operation and since the heat carrier medium has its maximum temperature while spiraling around the lower or near end of the basket, a large part of the heat transfer to the load in this area will be by radiation and conduction until the carrier medium reaches the top or far end of the basket and enters the load at which time heat transfer to the load is by convection.

Due to radiant transmission at the lower or near end of the basket the temperature and heat of the carrier medium available for convection transfer at the top or far end of the basket is, ordinarily, materially reduced so that the rate of heating of various parts of the total load is considerably uneven. This uneven heating not only requires a greater heating up period to bring the entire load up to proper temperature, but also results in an uneven soaking period for the inaterial in the various parts `of the basket. Such time and temperature differentials are often of serious consequence and heretofore attempts have been made to cure the diinculty by periodically reversing the flow of the heat conveying medium, through the load, during the treating time. This procedure is costly and often not satisfactory particularly because control is attempted through heat input and direction of circulation alone, and Without regard to the relative effects of radiation and convection in the heat transfer process and at various operating temperatures.

The improved method, hereafter described, utilizes combined control of volume, or velocity, and pressure of the heat carrier medium as well as control of the total heat input and the area and intensity of the radiant heat transfer phase of the heating process. W ith such a method I have not only attained uniformity or temperature throughout the load during the entire treating period, but have also obtained a much greater thermal efficiency and economy than was heretofore possible.

In a furnace or treating apparatus, of the type herein described heat is transferred from the vehicle or carrier, such as gas or air, to the work by both radiation and convection. The initial transfer of heat, as the carrier medium irst engages the work. or work basket, see Fig. 1, is by radiation through the basket walls and then as the carrier passes along its path of circulation and contacts the work within the basket, the heat transfer is mainly by convection.

Thus, referring to Fig. l, in the Zone T1 the initial heat transfer to the Work through the basket, at the bottom of near side, is mainly b'y radiation plus conduction. At the zone T2 the carrier shifts to the convection phase. In the zones T3 and T4 the heat transfer by convection progressively di- Y volume and at any part of the system, vary according to the actual temperature of the carrier. For example, in a heating apparatus employing forced convection at 200 F. and with a constant predetermined volume the heat transfer is 50% fby radiation and 50% by convection; at l800 F. the heat transfer (with the same carrier volume) will be approximately 89% by radiation and only 11% by convection (see The Heating of Steel, by M. H.. Mawhinney, published 1938 by the Reinhold Publishing Corp., New York). Thus, as the initial temperature of the heating source is in,- creased, the volume of the carrier must be increased, accordingly, to increase the amount of heat transferred by convection to avoid overheating of the lower or near end of the basket and possible overheating of the heat source itself.

Thus, since the temperature of the carrier is determined by the nature of the treatment desired for the work, a balance of the heat transferred to the various portions of the work load, by both radiation and convection, must be obtained through control and variation of the carrier volume and pressure during the treatment process. This is accomplished by means of a variable speed drive for the fan or blower le, which may be controlled automatically, or manually, according to the temperature differential between the zones T3 and T4.

As before mentioned the path of the heat carrier medium, in relation to the work, begins with the tangential introduction of the fluid stream at the point T1 Where initial heat transfer to the Work occurs by radiation and conduction, the radiation eect varying according to the initiai temperature of the carrier. Immediately the carrier'medium begins to travel lengthwise of the treating chamber, around the work basket or container, in a flat spiral path and the rate of heat transfer to the work ordinarily becomes slightly reduced, because of the progressively lowering temperature of the carrier, until it reaches T2. At T2 the now is reversed and the carrier travels into the basket at T3 and deposits its heat directly upon the surface of the work by convection. As the carrier passes through the work load, progressively releasing its heat to the Work according to the surface area of the work, its temperature is reduced until at T4 the minimum conditions prevail and the heat transfer rate is at its lowest.

The object of the system and its method of operation is to balance the heat transfer at T1 and T4 with that occurring at T3 and T2, by controlling the radiation effect at T1 and the convectionl effect at T3. The radiation eect at T1, and hence the heat transferred to the work through radiation and conduction from the basket or work container, is governed by the amount of heat introduced into the system and the initial carrier temperature, i. e. radiant heat area and radiant heat intensity. The convection effect at T3 is controlled by the volume and pressure of l carrier medium passing through the system, i. e. convection heat carrier volume and convection carrier heat content. Thus by increasing the fan speed, and hence the volume and pressure of the carrier medium circulated, according to any differential between T3 and Tl, and gradually reducing the fan speed as the temperatures T3 and Tl approach equilibrium, the rate of heat transfer from the carrier to the work by both radiation and convection may be accurately controlled throughout the work load so that the entire work load will be heated at a uniform rate.

Standard temperature responsive control equipment is available for automatic regulation of fan speed, whether by regulation of the motor speed or through operation of a variable speed drive placed between a constant speed motor and the fan or blower, and therefore such control equipment has not been shown or described in detail.

In the electrically heated type of apparatus, such as that shown in Fig. 3, total radiation is controlled by turning the heater banks 22, or part of the heater banks, on or off, selectively, and convection is controlled by variation of fan speed, and hence the carrier volume and pressure, to maintain equilibrium between T1-Tl and 'T2-T3. Also during a holding or soaking period, where a constant temperature is maintained after the load has been heated to the desired point, the heater banks 23 may be used alone and equilibrium maintained solely by a selected, constant, blower or fan speed suitable to provide the proper carrier volume and pressure.

The main advantages of this invention reside in the ability of the apparatus and the method of its operation to maintain substantially constant uniformity of temperature in all parts of the work load both during heating up periods and holdingT or soaking periods; and in the improved economy of time, fuel, and elimination of rejected work, that is achieved through uniformity of rate of heat transfer throughout the Work load and the avoidance of overheating portions of the work load. Soaking time is reduced, further improving production and decreasing energy requirements.

Other advantages reside in the improved form and arrangement of the heat treating apparatus and particularly the combustion-mixing chamber and self metering flue whereby heat loss through venting excess gases is kept to a minimum; in the improved recirculation arrangement whereby variation of fan speed does no-t affect burner combustion conditions; and in the improved combustion chamber construction whereby the products of combustion are mixed immediately with the recirculated air, on the dis' charge side of the blower and at the entrance to the treating chamber, thereby minimizing high temperature construction problems.

Although two speciiic embodiments of this invention are herein shown and described it will be understood that details of the construction shown and of the method described may be altered or omitted without departing from the spirit of this invention as dei-ined by the following claims.

I claim:

l. In a heat treating apparatus of the class described comprising an enclosed treating chamber having an air inlet passage in one wall thereof, a centrifugal blower having its discharge outlet aligned with and connected to said inlet pas-` sage and in direct communication with said chamber theiethrough, and said chamber having an outlet passage leading to the inlet opening of said blower, said inlet passage having one wall substantially following the boundary of the normal orifice angle of expansion of the blower discharge, and said inlet passage having a lateral flue opening in said one wall.

2. In a heat treating apparatus comprising an enclosed treating chamber having an air inlet passage and an air outlet passage in its walls, a centrifugal blower having a discharge opening aligned with and connected to said inlet passage and communicating with said chamber therethrough, and an inlet opening in said blower communicating with the outlet passage of said chamber, said blower discharge opening being spaced from said chamber, one wall of said air air inlet passage between said blower and chamber being formed to lie along and adjacent the normal orice angle boundary line of the blower discharge, and said inlet passage having a lateral iiue opening in said one wall betweenthe blower discharge opening and said chamber.

Y3. In a heat treating apparatus structure comprising an enclosed treatment chamber and a. separate enclosed blower chamber, said structure having an enclosed air delivery passage leading directly from the blower chamber into the treatment chamber and a return air passage leading from the treatment chamber to the blower chamber, a blower disposed in said blower chamber with its discharge outlet aligned with and fitting the adjacent end of said air delivery passage, one wall of said air delivery passage diverging outwardly from the axis of the blower discharge outlet toward the treatment chamber and lying outside of the orifice angle boundary line from said blower outlet, said one wall having a lateral flue opening therein.

4. In a heat treating apparatus structure comprising an enclosed treatment chamber and a. separate enclosed blower chamber, said structure having an enclosed air delivery passage leading directly from the blower chamber into the treatment chamber and a return air passage leading from the treatment chamber to the blower chamber, a blower disposed in said blower chamber with its discharge outlet aligned with and iitting the adjacent end of said air delivery passage, a burner passage leading into said air delivery passage and intersecting the same between the blower and treatment chamber at an acute angle toward and adjacent said treatment chamber, and a fuel burner directed into said burner passage along the axis thereof.

5. In a heat treating apparatus structure comprising an enclosed treatment chamber and a. separate enclosed blower chamber, said structure having an enclosed air delivery passage leading directly from the blower chamber into the treatment chamber and a return air passage -leading from the treatment chamber to thev blower chamber, a blower disposed in said blower chamber with its discharge outlet aligned with and tting the adjacent end of said air delivery passage, a burner passage leading into said air delivery passage and intersecting the same between the blower and treatment chamber at an acute angle toward and adjacent said treatment chamber, and a fuel burner directed into said burner passage along the axis thereof, one wall of said air delivery passage diverging outwardly fromthe axis of the blower discharge outlet to ward the treatment chamber and lying outsidey of the orifice angle boundary line of air dis-g charged from said blower. outlet, and said one Wall having a lateral nue opening therein located between the blower discharge outlet and the point of intersection of said burner passage with the air delivery passage.

6. The method of ejecting excess gases from a closed recirculatory heat treating system having a circulating means discharging an expanding gaseous stream from a restricted outlet, which consists in introducing a second stream of gaseous combustion products into the expanding rst stream at an acute angle relative to the axis thereof and in the direction of its flow, and bleeding excess gases from the expanding rst stream substantially at the boundary of the oriiice angle thereof and prior to its conuence with the second stream.

7. The method of Venting excess gases from a closed recirculatory heat treating system having a circulating means discharging an expanding gaseous stream from a restricted outlet and wherein extraneous gases are introduced into the system in the form of combustion products, which consists in introducing a second stream of gaseous combustion products into the expanding first stream at a predetermined distance beyond the restricted outlet in the direction oi? rst stream ow, and bleeding excess gases from the expanding rst stream substantially at the boundary of the orifice angle of expansion thereof and prior to its confluence with the second stream.

8. The method of venting excess gases from a closed recirculatory heat treating system having a circulating means discharging an expanding gaseous stream from a restricted outlet and wherein extraneous gases are introduced into the system during the heat treating process, which consists in bleeding the excess gases from the expanding gas stream at substantially the boundary of the orifice angle of said expanding gas stream.

9. The method of venting excess gases from a recirculatory heat treating system having a circulating means discharging an expanding gaseous stream from a restricted outlet and wherein extraneous gases are introduced into the system during the heat treating process,

which consists in bleeding the volumetric excess of gases from the expanding gas stream at substantiaily the boundary of the orice angle of said expanding gas stream and under the pressure derived solely from the volumetric excess resuiting from the extraneous gases.

References Cited in the file 0f this patent UNITED STATES PATENTS Number Name Date 1,719,684 Besta July 2, 1929 1,750,924 Darrah Mar. 18, 1930 2,277,592 Kenner et a1. Mar. 24, 1942 2,434,491 Elder et al. Jan. 13, 1948 2,463,222 Vaughan Mar. 1, 1949 2,485,453 Munford Oct. 18, 1949 2,543,388 Urquhart Feb. 27, 1951 FOREIGN PATENTS Number Country Date 18,473 Great Britain s 1914

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