CA1194693A

CA1194693A - Vacuum seal for continuous heat treating furnaces

Info

Publication number: CA1194693A
Application number: CA000425170A
Authority: CA
Inventors: John A. Owen
Original assignee: Stelco Inc
Current assignee: Stelco Inc
Priority date: 1983-04-05
Filing date: 1983-04-05
Publication date: 1985-10-08

Abstract

ABSTRACT
There is provided a method and apparatus for counteracting the tendency for outside air to be drawn into a continuous heat treating furnace in which a feeder pan extends through the charge end opening. A
problem commonly encountered is that the hot gaseous atmosphere within the furnace tends to escape through the charge end opening above the feeder pan, while outside air tends to be drawn into the furnace below the feeder pan. This invention provides a vacuum seal which includes air-withdrawing means located adjacently outside the opening and under the feeder pan, adapted to withdraw sufficient air to counter-balance the tendency for outside air to be drawn into the furnace below the feeder pan.

Description

VACUUM SEAL FOR_CONTINUOUS HEAT TREATING FURNACES
This invention relates generally to continuous heat treating furnaces, and has to do particularly with the prevention of the ingress of detrimental gaseous elements into the working chamber of such a furnace.
BACKGROUND OF T~IS INVENTIO~
Steel products are heat treated in furnaces to ensure uniformity of hardness and tensile strength as outlined in the specification of the product.
Heat treating furnaces are classified as either batch furnaces or continuous furnaces. Batch furnaces normally have the produc-t loaded and unloaded manually, whereas con-tinuous furnaces have an automatic conveying system to move the materials through tne furnace. Continuous furnaces are often described according to the way material is moved through the furnace, for example rotary hearth, roller hearth, pusher, conveyor, walking beam, or tunnel.
Continuous conveyor furnaces usually comprise a single, long, straight working chamber through which the product is conveyed. One end of the furnace is open to the air to allow constant entry of product, while -the o-ther end is sealed by a chute submerged in a quenching medium such as oil or water. The atmosphere within the working chamber of the furnace is controlled to obtain the desired surface conditions on the product.
The exclusion of air from the furnace working chamber represents a major problem. Air consists of approximately 79~ nitrogen and 21% oxygen, with trace amounts of carbon dioxide. Air in a heat treating furnace behaves like an oxygen atmosphere since oxygen is the most reactive constituent in the air. Oxygen reacts with the iron of the steel to produce iron oxide which takes the form o-f a detrimental surface scale.
It also reacts with the carbon in the steel product to lower the carbon con-tent of the surface. In other words, it decarburizes the steel.

, .~,,~
!3 Nitrogen in the molecular state is passive to iron. However, the nitrogen must be completely dry to be passive to high carbon steels~ The presence of even slight traces of moisture will cause decarburization.
Carbon dioxide at austenitizing temperatures reacts with surface carbon in the steel surface to produce carbon monoxide. This reaction continues until there is no carbon dioxide available or until the steel surface is completely depleted of carbon, at which point, if there is a continuing supply of carbon dioxide, iron and ferrous oxide will be oxidized.
In a continuous heat treating furnace, the opening through which product enters the furnace is often referred to as the charge end opening, or the product entry opening. Typically, the feeder pan extends substantially horizontally through the charge end opening and "feeds" product into the furnace and onto the endless conveyor located within the furnace.
Most of the air that does contaminate the protective atmosphere within the furnace enters through this charge end opening. More specifically, because of the high temperatures within the furnace, there tends to be a positive gas pressure in the top half of the furnace working chamber which prevents air from entering the furnace in the part of the charge end opening which is above the feeder pan. In fact, flames tend to exit the furnace in this region and are captured in an exhaust hood. However, the region of the charge end opening which is below the feeder pan has a negative gas pressure (compared to atmospheric), and this results in air being drawn into the furnace belQw the feeder pan.
~ost of these furnaces use a horizontal feeder pan which vibrates product onto the conveyor. Furnace manufacturers, in order to reduce air entry into the furnace, have generally changed from the earlier design of a sloping gravity-fed entry chute to that of a horizontal vibrating chute.
In the past, many attempts have been made to overcome the problem of air contamination. In one such attempt, mechanical seals made of asbestos or similar material were held against the bottom of the feeder pan by a spring mechanism to maintain constant contact in an attempt to exclude air from the furnace. These seals required constant maintenance and were only partially successful. In other attempts, gas seals and flame curtains were also tried, but were generally unsuccessful at solving the problem. Recognizing these limitations, furnace manufacturers have provided the user with a source of additional natural gas which is introduced into the protective atmosphere to overcome the infiltration of oxygen. The amount of natural gas added varies with the condition of the furnace and in particular the charge end seal. The amount can vary from 150 cubic feet per hour to 500 cubic feet per hour. However, adding excess natural gas can be both costly and deleterious~ The excess gas imparts a carburizing tendency to the furnace atmosphere.
Furthermore, it is subject to thermal decomposition at furnace temperatures, resulting in the formation of soot. The carbon component of the excess gas combines with the chrome in the alloy parts inside the furnace, such as the radiant heating tubes, to form chrome carbides which precipitate out of the alloy and cause it to become brittle. The cost of alloy components is high, and therefore excess carbon in the furnace should be kept to a minimum to prolong component service life.
G_NERAL DESCRIPTION OF THIS INVENTION
In view of the foregoing problems associated with air contamination of the atmosphere within a furnace, it is an aspect of the present invention to provide a method and apparatus by which such air contamination can be very significantly reduced.
Accordingly, this invention provides the combination of a continuous heat treating furnace with a vacuum seal. The heat treating furnace has a feeder pan extending through the charge end opening, the furnace containing a hot, gaseous atmosphereO The feeder pan is spaced from the top and from the bottom of the opening whereby the hot~ gaseous atmosphere tends to escape through the opening above the feeder pan while outside air tends to be drawn into the furnace below the feeder pan. The vacuum seal includes air-withdrawing means located adjacently outside the opening and under the feeder pan, such means withdrawing sufficient air to counter-balance the tendency for outside air to be drawn into the furnace below the feeder pan.
This invention also provides a method improvement in the operation of a continuous heat -treating furnace which has a feeder pan extending through a charge end opening. The furnace contains a hot, gaseous atmosphere, and the feeder pan is spaced from the top and from the bottom of the opening so that the hot, gaseous atrnosphere tends to escape through the opening above the feeder pan while outside air tends to be drawn into the furnace below the feeder pan. The method irnprovement consists of restraining gaseous flow through the opening both above and below the feeder pan by withdrawing air from a location adjacent the opening and below the feeder pan so as to create at that location a vacuum sufficient to counter-balance the tendency for ou-tside air to be drawn into the furnace below the feeder pan.
GENERAL DESCRIPTION OF THE DRAWINGS
Two embodiments of this invention are illustrated in the accompanying drawings, in which like numerals denote like parts throughout the several views, and in which:
Figure 1 is an end elevational view of a continuous heat treating furnace to which this inventionl in a first embodiment, is applied;
Figure 2 is a partial vertical sectional view taken along the line 2-2 in Figure l;
Figure 3 is a perspective view of an air-evacuating member forming part of this invention;
Figure 4 is a view similar to Figure 2, showing a variant thereof; and 9~

Figure 5 is a partial vertical sectional view of a heat treating furnace to which this invention, in a second embodiment, is applied.
DETAILED DESCRIPTION OF l'HE DRAWINGS
Attention is first directed to Figure 1, which shows a continuous heat treating furna~e 10 in end ele~ation, the furnace 10 having a charge end opening 12 and a feeder pan 14 extending through the opening 12. More particularly, the feeder pan 14 includes a bottom panel 16 and two side panels 18 and 19. As can be seen in Figure 2, the feeder pan 14 includes a number of sections, and Figure 2 shows an end section 20 which projects through the charge end opening 12, connected through flange connection 22 to an ad~acent section 24. Neither the support means nor the vibration means for the feeder pan 14 are illustrated. In Figure 1, the feeder pan 14 is understood to be seen in section, although the remainder of the furnace ls seen in true elevational vieW.
In Figure 2, the internal volume 26 of the furnace 10 is seen to contain an endless conveyor 28 which is entrained around a suitable roller 30. It is to be understood that additional rollers would be provided for the conveyor 28, along with a drive means which has not been shown.
In the normal operation of the furnace 10, the volume 2~ contains a hot, gaseous atmosphere.
Because the feeder pan 14 substantially divides the charge end opening 12 into an upper portion and a lower portion, the natural tendency for hot gases to rise will result in a tendency for the atmosphere within the furnace 10 to escape through the charge end opening 12 above the feeder pan 14, while outside air tends to be drawn into the furnace 10 through the charge end opening 12 below the feeder pan 14. Multiple head arrows in Figure 2 illustrate this convective tendency.
In order to counter balance this tendency for gas interchange between the surrounding air and the volume within the furnace 10, this invention pro~ides a vacuum seal which includes air-withdrawing means located adjacently outside the charge end opening 12 and under the feeder pan 14. More particularly, and with reference to the drawings, there is provlded a substantially horizontal, elongate tube 32 closed at the end 34 which extends under the feeder pan 14 and which has a plurality of suction apertures 36 (see Figure 3) which are spaced apart along the tube, i.e.
along the breadth of the charge end opening 12. The tube 32 is supported on a pair of brackets 39, and air is withdrawn from the tube 32 by virtue of its connection to the suction port 41 of an eductor 43 having an entry tube 45 through which pressurized air is forced, and having a diverging downstream portion 47 through which the pressurized air is exhausted along with the air or other gas withdrawn from the tube 42.
Such eductors 43 are well known, and need not be described in greater detail here. In a particular installation which provided an effective vacuum seal against entry of outside air through the charge end opening 12 under the feeder pan 14, the tube 32 was provided with a series of holes 36 ranging in diameter from 3/8 inch to ~ inch, and spaced 1~ inch apart. The holes 36 were located above the mid-horizontal plane of the tube, and were angled toward the charge end opening 12 by approximately 10 from the vertical axis. The eductor 43 drew a suction of approximately ~ inch water co.lumn on the tube 32, and the tube 32 was placed under the feeder pan 14 with a 1/8 inch gap.
In certain installations it may be desirable to graduate the diameters of the suction apertures 36 so that the largest diameter is furthest from the eductor 43, thereby compensating for the greater pressure drop experienced by air being drawn into the more remote suction apertures.
Attention is now directed to Figure 4 which, like Figure 2, includes a feeder pan 14 having a bottom panel 16 and two side panels 18 and 19 (side panel 19 is not visible in Figure 4). The feeder pan 14 extends through the charge end opening 12 of the furnace 10 which, like the embodiment of Figure 2, includes an endless conveyor 28 which is entrained around a suitable roller 30.
The embodiment shown in Figure 4 again provides a vacuum seal which includes air-withdrawing means located adjacently outside the charge opening 12 and under the feeder pan 14. With reference to Figure 4, this air-withdrawing means comprises a substantially horizontal, elongate tube 32 which is closed at one end and which extends under the feeder pan 14. The tube 32 has a plurality of suction apertures 36 lonly one seen in section in Figure 4), which are spaced apart along the tube, l.e~ along the breadth of the charge end opening 12. The tube 32 is supported on a pair of brackets 39, and air is withdrawn from the tube 32 by virtue of a connection to the suction port of an eductor, as in the embodiment previously described with reference to Figure 2. This particular structure need not be repeated here.
The embodiment of Figure 4 includes a displaceable sealing means across the opening 12 and above the feeder pan 14, so that items to be treated in the furnace can enter the furnace on the feeder pan 14 by displacing the sealing means. More particularly, the sealing means includes a plurality of L-shaped baffles 53, in side-by-side juxtaposition and pivoted about a lateral, horizontal axis located above the feeder pan 14 and defined by a stationary shaft 55.
Only one baffle 53 is visible in Figure 4 due to the sectional illustration, but it is to be understood that a plurali~y of such baffles, which each may conveniently measure approximately 2 inches in width, are provided. The plurality of baffles are independently swingable about the axis defined by the shaft 5~. As particularly seen in Figure 4, each baffle 53 includes a loop portion 56, a first flat portion 58, and a second flat portion 60 at right ~ a angles or substantially so to the flat portion 580 This particular configuration is not to be considered limiting in terms of the scope of this invention.
A primary effect of the sealing means is to direct the outgoing furnace atmosphere gas, represented by the arrows 63, to a location as close to the feeder pan 14 as possible, while still allowing products with different heights to be fed along the feeder pan 140 The effect of the plurality of baffles 53 is to cause the furnace atmosphere gas to ignite at the location shown at A in Figure 4, where the gases meet incoming outside air moving in the direction represented by the arrows 65. The products of combustion are lighter than the air, and rise up along a path shown by the arrows 67.
While the provision of the tube 32 by itself definitely represents a improvement over a conventional heat treating furnace which does not have such a provision, the presence of the baffles 53 further enhances the operation of the furnace, by ensuring that combustion of the furnace atmosphere with air will take place at a location down as close to the feeder pan bottom as possible, and will not allow a "raised"
combustion 20ne to be established, under which outside air can pass to the inside of the furnace.
The shaft 55 is supported between side walls 61 which depend from opposite edges of a mounting bracket 62 which is affixed to the furnace 10.
Attention is now directed to Figure 5 in which the furnace 10' has a slightly different construction at the charge end than the furnace 10 shown in Figure ~. In Figure 5~ the furnace 10' has a stepped configuration at 71, which defines a charge end opening 73 through which a gravity-feed product chute 76 extendsO The chute 76 differs from conventional chutes in that it incorporates a vacuum tube 78 having a plurality of spaced-apart openings 80 directed rightwardly through the bottom of the chute. In effect, looking at Figure 5, the chute bottom is defined by an upper wall portion 82 which is welded to the tube 78 at the location 83, and a downward]y displaced wall portion 85 which is welded to the tube 78 at the location 86. Thus, the bottom of the chute is stepped, and the tube 78 is located at and defines the step.
As in the previously described embodiment, the furnace 10' i~cludes a conveyor 28 entrained about a roller 30, the conveyor 28 being adapted to xeceive product as it passes into the furnace 10' along the chute 76O
In this embodiment, sealing means are provided as in the embodiment of Figure 4, the sealing means consisting of a plurality of baffles 89 in side-by-side juxtaposition and pivotally mounted about a lateral, horizontal axis defined by a shaft 9~ located above the chute 76. The shaft 30 is supported from side plates 92 depending from the lateral edges of a bracket 94 secured to a furnace 10' by conventional means.
In Figure S, the arrows 96 show the flow direction of furnace atmosphere approaching the charge end opening 73~ Arrows 97 show the direction of flow of furnace gases in the "trap" defined in part by the baffles 89. Eventually, the furnace gases meet up with the outside air flowing in along the arrows 100, and give rise to combustion at about the location shown by B. From that location the products of combustion pass upwardly along the arrows 102. Because the products of combustion are so much lighter than the air, there is a tendency for a good deal of air to be drawn in along the bottom of -the chute 75 (arxows 100). The provision of the tube 78 acts to prevent thls air from moving any further than the location of the openings 80, by drawing in either the air itself, ox the products of combustion, or a combination of the air and some of the products of combustion. As in the embodiment shown in Figure ~, the baffles 89 have the effect of directing 1~

the furnace atmosphere toward the tube 78, close to which combustion of the gaseous mixture takes place.
EXAMPLE
Two identical furnaces, each with a rated capacity of 4,000 pounds per hour and sharing the same atmosphere control instrument, were used to test the first embodiment of the charge end vacuum seal as described herein. The seal was installed on the first furnace, and the second furnace was operated without the seal. Reducing gas was lntroduced manually into both empty furnaces to bring them into working condition as quickly as possible. Normally, once the percentage of carbon dioxide in a measured sample of the furnace's atmosphere falls to 1% or below, the furnace can be put on automatic control. The sooner a furnace can be put onto automatic control, the better.
In the test, the furnace with the seal was able to go on automatic control after 4.25 hours, whereas the non-sealed furnace took 9.5 hours. Thus, 5.25 hours of productive time was gained by the sealed furnace.
Once on automatic control, the percentage of carbon dloxide in the furnace atmosphere is reduced until a desired set point is attained. The set point in this test was .375% carbon dioxide. The furnace with the seal reached the set point in 4 hours, whereas the second furnace took 6 hours to reach the set point.
The sealed furnace was more stable than the unsealed furnace and closely followed the desired set point of .375% carbon dioxide. The sealed furnace required an average of 50 cubic feet per hour of natural gas, whereas -the unsealed furnace required an average of 375 cubic feet per hour of natural gas.
ADVANT~GES
A number of advantages arise from the use of the method and apparatus set forth herein. Firstly, little or no enriching natural gas is necessary to run good quality hardening products~ Secondly, set point control is kept within a tighter tolerance. Because of the reduction in soot, breakdown of alloy components ll within the furnace is reduced. Further, maintenance of the charge end seal is virtually eliminated as there are no parts in contact with the feeder pan. A further advantage is that the start-up time from an idle condition may be substantially reduced, by as much as 4 hours. Finally, automatic contro] can be achieved from 4 to 36 hours faster, thus allowing high grade fasteners to be run sooner.
While a particular embodiment of this invention has been illustrated in the accompanying drawings and described in the foregoing disclosure, it will be evident to those skilled in the art that changes and modifications may be made therein without departing from the essence of this invention, as set forth in the appended claims.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS
FOLLOWS:

1. In combination with a continuous heat treating furnace having a substantially imperforate feeder pan extending through a charge end opening thereof, the furnace containing a hot, gaseous atmosphere, the feeder pan being spaced from the top and from the bottom of the opening whereby the said hot, gaseous atmosphere tends to escape through the opening above the feeder pan while outside air tends to be drawn into the furnace below the feeder pan:
a vacuum seal comprising air-withdrawing means located adjacently outside the opening and under the feeder pan, said means withdrawing sufficient air to counter-balance the tendency for outside air to be drawn into the furnace below the feeder pan.

2. The invention claimed in claim 1, in which the feeder pan is substantially horizontal and in which said means is a substantially horizontal, elongate tube extending under the feeder pan and having a plurality of suction apertures spaced apart along the breadth of said opening.

3. The invention claimed in claim 2, in which said tube is substantially cylindrical, and in which said suction apertures are located above the mid horizontal plane of the tube, angled toward the opening.

4. The invention claimed in claim 2, in which the tube is connected to the suction port of an air eductor through which pressurized air is forced.

5. The invention claimed in claim 3, in which the tube is spaced below the feeder pan by a distance smaller than or equal to 3/16 inch.

6. The invention claimed in claim 3, in which the suction apertures are located along a line inclined at substantially 10° from a vertical plane through the tube axis.

7. The invention claimed in claim 4, in which said suction port draws approximately ?" water column suction.

8. The invention claimed in claim 7, in which the apertures are between about 3/8" and about ?" in diameter.

9. The invention claimed in claim 8, in which said tube is substantially cylindrical and in which said suction apertures are located above the mid horizontal plane of the tube, angled toward the opening at substantially 10° from a vertical plane through the tube axis, the tube being spaced below the feeder pan by a distance of substantially 1/8".

10. The invention claimed in claim 1, which further includes displaceable sealing means across the opening above the feeder pan, whereby items can enter the furnace on said feeder pan by displacing the sealing means, the sealing means being adapted to direct internal furnace atmosphere down toward the feeder pan and any items thereon.

11. The invention claimed in claim 10, in which the sealing means comprises a plurality of L-shaped baffles in side-by-side juxtaposition and pivoted about a lateral, horizontal axis located above the feeder pan.

12. In the operation of a continuous heat treating furnace having a substantially imperforate feeder pan extending through a charge end opening thereof, the furnace containing a hot, gaseous atmosphere, the feeder pan being spaced from the top and from the bottom of the opening whereby the said hot, gaseous atmosphere tends to escape through the opening above the feeder pan while outside air tends to be drawn into the furnace below the feeder pan:
a method of restraining gaseous flow through said opening both above and below the feeder pan, comprising withdrawing air from a location adjacent the opening and below the feeder pan so as to create at said location a vacuum sufficient to counter-balance the tendency for outside air to be drawn into the furnace below the feeder pan.

13. The method claimed in claim 12, in which said withdrawing of air is accomplished by providing at said location a substantially horizontal, elongate tube having a plurality of suction apertures spaced apart along the breadth of said opening, and drawing a vacuum on said tube.

14. The method claimed in claim 13, in which the tube is substantially cylindrical, and in which said apertures are positioned above the mid horizontal plane of the tube, angled toward the opening.

15. The method claimed in claim 13, in which the drawing of the vacuum is accomplished by connecting the tube to the suction port of an air eductor, and by forcing pressurized gas through said air eductor.

16. The method claimed in claim 14, in which the tube is spaced below the feeder pan by a distance smaller than or equal to 3/16 inch.

17. The method claimed in claim 14, in which the apertures are located along a line inclined at substantially 10° from a vertical plane through the tube axis.

18. The method claimed in claim 15, in which said drawn vacuum is substantially ?" water column.

19. The method claimed in claim 18, in which the apertures are between about 3/8" and about ?" in diameter.

20. The method claimed in claim 19, in which the tube is substantially cylindrical and in which said suction apertures are located above the mid horizontal plane of the tube, angled toward the opening at substantially 10° from a vertical plane through the tube axis, and in which the tube is spaced below the feeder pan by a distance of substantially 1/8".

21. The method claimed in claim 12, further including the step of directing the internal furnace atmosphere down toward the feeder pan and any items thereon, by utilizing a displaceable sealing means across the opening above the feeder pan.

22. The method claimed in claim 21, in which a plurality of L-shaped baffles are utilized as the sealing means, the baffles being in side-by side juxtaposition and pivoted about a lateral, horizontal axis located above the feeder pan.

23. A continuous heat treating furnace having a substantially imperforate feeder pan extending substantially horizontally through a charge end opening thereof, the furnace containing a hot, gaseous atmosphere, the feeder pan being spaced from the top and from the bottom of the opening whereby the said hot gaseous atmosphere tends to escape through the opening above the feeder pan while outside air tends to be drawn into the furnace below the feeder pan, means for withdrawing air adjacently outside the opening and under the feeder pan at a sufficient rate to substantially counter-balance the tendency for outside air to be drawn into the furnace below the feeder pan.

24. In a continuous heat treating furnace having an inclined gravity chute extending obliquely downwardly through a charge end opening thereof, the furnace containing a hot, gaseous atmosphere, the improvement comprising:
an air seal means for withdrawing gaseous material from a location just above the chute and intermediate its ends, whereby air and/or products of combustion arising from the burning of the furnace atmosphere in air are withdrawn so as to prevent air from entering the furnace.

25. The invention claimed in claim 24, in which the air seal means comprises a substantially horizontal tube extending adjacent to the underside of the chute but having a plurality of spaced-apart suction apertures which open above the chute, and means for withdrawing gaseous material from said tube.

26. The invention claimed in claim 25, further including displaceable sealing means across the opening above the chute, whereby items can enter the furnace along said chute by displacing the sealing means, the sealing means being adapted to direct internal furnace atmosphere down toward the tube where combustion thereof can take place.

27. The invention claimed in claim 26, in which the sealing means comprises a plurality of L-shaped baffles in side-by-side juxtaposition and pivoted about a lateral, horizontal axis located above the chute.