Heat treating furnace with graphite heating elements

A heat treating furnace having a work chamber of circular cross-section and having banks of electric resistance heating elements formed by rigid and elongated graphite bars. The graphite bars of each bank are interconnected so as to form an octagon located closely adjacent the circular wall of the work chamber and closely approximating the circular shape of the chamber. As a result of the octagonal arrangement, ring-like banks of rigid graphite elements may be space longitudinally along the circular work chamber to enable front-to-rear temperature trim zones to be established in the chamber.

BACKGROUND OF THE INVENTION 
This invention relates to a heat treating furnace of the type in which 
electric resistance heating elements are disposed inside of a work chamber 
to radiantly heat workpieces therein. More specifically, the invention 
relates to a furnace of the type in which the work chamber is of circular 
cross-section and in which the heating elements are made of rigid graphite 
bars. 
In most prior furnaces of this type, the graphite heating bars are arranged 
in a circle around the chamber and extend longitudinally of the furnace 
between the front and rear thereof. As a result, it is not feasible to 
easily arrange the elements in groups or banks which may be differentially 
located and/or energized in order to establish front-to-rear temperature 
trim zones in the furnace. 
SUMMARY OF THE INVENTION 
The general aim of the present invention is to provide an electric heat 
treating furnace having a chamber of circular cross-section and having 
rigid graphite heating bars uniquely arranged in longitudinally spaced, 
ring-like banks conforming substantially to the circular shape of the 
chamber and enabling front-to-rear temperature trim zones to be created in 
the chamber. 
A more detailed object is to achieve the foregoing by connecting the rigid 
graphite bars end-to-end to form a heating element bank in the shape of an 
octagon or the like, the bars extending chordwise of the circular chamber 
in close proximity to the wall thereof and obstructing relatively little 
space in the chamber. Several of the banks are spaced longitudinally 
within the chamber and may be either differentially spaced or 
differentially energized to enable front-to-rear temperature trim zones to 
be established. 
The invention also resides in the provision of unique brackets which 
suspend the graphite bars from the wall of the chamber and which also 
enable adjacent bars to be connected to one another to form an octagon or 
the like. 
These and other objects and advantages of the invention will become more 
apparent from the following detailed description when taken in conjunction 
with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
For purposes of illustration, the invention is shown in the drawings in 
conjunction with a vacuum furnace 10 for heat treating workpieces (not 
shown) in a chamber 11 (FIG. 3). In general, the furnace comprises a 
substantially cylindrical outer vessel or shell 14 (FIG. 1) having a 
circular internal cross-section and closed at its forward end by a 
releasable door 15. The heating chamber 11 is defined within an internal 
structure in the form of a walled enclosure 16 disposed inside of the 
shell and spaced inwardly from the walls thereof. Several groups of 
electric resistance heating elements 20 (FIG. 3) are located within the 
internal enclosure 16 to heat the workpieces. 
The internal enclosure 16 also is of circular cross-section and includes a 
flat rear wall and a circular side wall. Each wall is defined by a metal 
outer wall member or skin 21 (FIGS. 2 and 4) and by suitable insulation 
packs 22. 
As shown most clearly in FIG. 3, the wall of the work chamber 11 is defined 
by the innermost side of the insulation packs 22, the chamber being of 
circular cross-section and being closed at its rear end by a vertical 
insulation pack. The front of the chamber is adapted to be closed off by a 
door-like structure 25 (FIGS. 1 and 2) carried by the main door 15 of the 
shell 14 and having insulation packs 26 for blocking and insulating the 
front of the chamber. 
After the workpieces have been heated, a motor-driven blower 27 (FIG. 1) at 
one end of the shell 14 circulates an inert cooling gas such as argon or 
nitrogen through the chamber 11 in order to quench the workpieces. As 
shown schematically in FIG. 1, the gas is directed into the space 28 
between the shell 14 and the enclosure 16 and flows into the chamber 
through tubular nozzles 30 located in the wall of the enclosure 16. The 
gas discharged out of the chamber 11 flows past the insulation packs 26 on 
the door structure 25 and returns to the blower 27 via a plenum 31 defined 
between the shell 14 and a jacket 32 which encircles the enclosure 16. 
Banks of cooling coils 33 are located in the plenum and chill the gas 
during its return flow. 
The heating elements 20 are made from rigid and elongated bars of graphite, 
the bars 20 herein being straight and having a rectangular cross-section. 
Graphite bars previously have been used as the heating elements for heat 
treating furnaces and the advantages of graphite bars themselves are well 
known. 
In accordance with the present invention, the graphite heating bars 20 are 
connected end-to-end with one another to form ring-like banks 35 spaced 
longitudinally within the circular work chamber 11 and adapted to be 
differentially located and/or energized in order to enable front-to-rear 
temperature trim zones to be established within the chamber. As shown in 
FIG. 3, the graphite bars are arranged so as to define a substantially 
regular polygon, preferably an octagon, within the chamber so that the 
bars may be located in close proximity to the circular wall 22 of the 
chamber. In this way, the elongated and rigid graphite bars may be formed 
into a ring-like heating element bank in the circular chamber to enable 
temperature trim control and yet, being closely adjacent the wall of the 
chamber, the bars obstruct relatively little work space in the chamber. 
More specifically, the present furnace 10 has been shown as including six 
longitudinally spaced banks 35 of graphite heating elements or bars 20, 
each bank herein being formed by eight separate bars although as few as 
six bars could be used with a sacrifice of usable work space. Adjacent 
ends of adjacent bars in a bank are electrically and mechanically 
connected in a manner to be explained except that the left end of the bar 
20A (FIG. 3) immediately to the right of the twelve o'clock position is 
not mechanically connected to the right end of the bar 20B immediately to 
the left of the twelve o'clock position. Instead, the left end of the bar 
20A is connected to a power feed through conductor 36 (shown schematically 
in the drawings) which extends through the internal enclosure and which 
connects the bank of heating elements to a voltage source. A graphite 
cross connector 37 (FIGS. 2, 3 and 4) is attached to the right end of the 
bar 20B by a graphite screw 38 and a graphite nut 39 and connects the bar 
20B electrically with the corresponding bar 20B of a longitudinally paired 
bank 35 having a bar 20A connected to another power feed through conductor 
36. The cross connector is supported on a hanger rod 40 (FIG. 4) attached 
to and extending inwardly from the wall 21 of the internal enclosure 16. 
An insulating sleeve 41 and an insulating washer 42 prevent the connector 
37 from shorting through the hanger rod 40. 
The bars 20 all are of equal length except that the bar 20B is just 
somewhat shorter than the other seven bars. Also, adjacent bars are 
inclined relative to one another at an included angle of roughly 135 
degrees. Accordingly, the bars define a substantially regular octagon 
around the outer portion of the chamber. Because of the octagonal shape, 
the bars lie along relatively short chords which closely approximate a 
circle and thus the shape of the overall heating element bank conforms 
closely to the circular shape of the chamber 11. As a result, the bars 20 
do not occupy any significant area in the center portion of the chamber 
and do not significantly reduce the work-holding capacity of the chamber. 
Moreover, the octagonal arrangement of the bars causes heat from the bars 
to be radiated toward the center of the chamber in a substantially uniform 
manner approximating that effected by truly circular metal heating 
elements. 
Because the heating elements 20 are arranged in longitudinally spaced banks 
35, various banks may be differentially energized in order to establish 
front-to-rear temperature trim zones. For example, more power may be 
applied to the front two banks to compensate for heat loss through the 
doors 15 and 25 and to enable a substantially uniform temperature to be 
maintained along the length of the furnace in spite of such heat loss. 
Alternatively, equal power may be applied to all of the banks and a 
greater number of banks may be concentrated adjacent the forward end 
portion of the chamber 11 to compensate for heat loss through the doors. 
The present invention also contemplates the provision of novel mounting 
brackets 50 (FIGS. 3 and 5) for electrically and mechanically 
interconnecting the bars 20 and for suspending the bars from the internal 
enclosure 16. Herein, each bracket is made of a single piece of graphite 
and includes two oppositely extending wings 51 inclined relative to one 
another at the same obtuse included angle as the bars. Adjacent ends of 
adjacent bars are conductively connected to the wings by graphite screws 
52 and graphite nuts 53. 
Each bracket 50 also includes a plate-like mounting section 55 (FIG. 5) 
formed integrally with and extending between the two wings 51. The 
mounting section of each bracket is disposed substantially perpendicular 
to a radius of the chamber 11. Extending inwardly along such radius is a 
hanger rod 56 whose outer end is attached to the outer wall 21 of the 
enclosure 16. The inner end portion of the hanger rod extends through a 
hole 57 in the mounting section 55 and thus suspends the bracket 50 and 
the bars 20 from the wall 21. The bracket 50 is captivated on the rod 56 
by a pin 58 and a washer 59 and is electrically insulated by a ceramic 
sleeve 60 and ceramic washers 61. 
The brackets 50 are of relatively simple construction and yet they serve 
the dual purpose of connecting the bars 20 to one another in an octagonal 
bank and of suspending the bars within the chamber 11.