Gas burner forced convection heating of glass sheets

A furnace (12) including a roller conveyor (18) and gas burners (20) distinct from the conveyor for supplying forced convection that is the dominant mode of heat transfer to glass sheets during a heating process. The gas burners (20) have general utility but have particular utility when utilized in a closely spaced relationship to the roller conveyed glass sheet to provide the forced convection heating. Each burner (20) includes a combustion member (38) defining an elongated combustion chamber (42) extending transversely to the direction of conveyance. Gas and air are introduced into the combustion chamber (42) in a tangential relationship with respect to its inner surface to provide a swirling motion that mixes the gas with the air for combustion prior to discharge through outlets (46) to provide the forced convection heating. A glass sheet heated by this gas burner forced convection on the roller conveyor and subsequently cooled has reduced roll-wave distortion and reduced edge distortion as compared to radiantly heated glass sheets.

TECHNICAL FIELD 
This invention relates to: a gas burner heated and process for heating 
glass sheets by forced convection; a gas burner of a novel construction 
that has general usage but has particular utility when used to provide 
forced convection heating of glass sheets; and a glass sheet that has 
improved characteristics as a result of having been heated by the gas 
burner heater and process. 
BACKGROUND ART 
Heating of flat glass sheets is performed to provide bending, tempering, 
bending and tempering, heat strengthening, and pyrolytic filming, etc. 
Usually the glass is heated above its strain point which is the 
temperature at which the glass acts as a viscous fluid rather than an 
elastic solid. The heated glass sheets are thus easily subjected to 
unintended deformation when heated to the viscous condition, and great 
care must be taken if the glass sheets are to have the required optical 
quality after subsequent cooling. 
Historically, most commercial heating of glass sheets until the early 1960s 
was performed by the vertical process where tongs are utilized to suspend 
the upper edge of the glass sheet which hangs downwardly and is conveyed 
through a heating chamber for the heating. One problem with this vertical 
process is that the entire weight of the glass sheet is supported by the 
tongs and, upon being heated sufficiently hot to become viscous, the glass 
sheets tend to deform at the tongs which leaves "tong marks" upon 
subsequent cooling. Also, furnace capacity can be wasted in the vertical 
process since short glass sheets require the same conveyor usage as long 
glass sheets. 
U.S. Pat. No. 2,841,925 of McMaster and U.S. Pat. No. 3,402,038 of Hordis 
disclose glass tempering furnaces of the vertical type described above. 
Each of these vertical furnaces is disclosed as including fans for 
circulating gas to provide uniformity in the heating. However, in 
commercial units manufactured in accordance with these patents, the gas 
pressures utilized have been relatively low, only on the order of about 
one-half inch water column at the output of the fans. As such, the amount 
of forced convection is not particularly great and the dominant mode of 
heating is by radiation from the furance walls and other components of the 
furnace such as the fan blowers and associated baffle plates, etc. 
In an attempt to overcome problems associated with vertical type furnaces 
for heating glass sheets, gas hearth furnaces were developed during the 
early 1960s. This gas hearth type of furnace includes a generally 
horizontal but slightly inclined hearth through which gas is supplied to 
provide a thin layer of gas on which the glass sheets are supported during 
heating. A pressurized plenum below the hearth supplies the gas through 
openings in the heart to support the glass sheets for conveyance. 
Recirculation of the gas between the furnace and the plenum provides the 
glass sheet support without heat loss that would result if the gas were 
allowed to escape to the atmosphere. At the lower edge of the tilted 
hearth, a movable frame is provided to provide movement of the glass 
sheets along the hearth on the thin layer of gas provided by the 
pressurized plenum. Heating of the glass sheet is thus performed by gas 
supplied by the hearth which constitutes a part of the conveyor of this 
type of furnace. Also, substantial radiant heat transfer takes place 
between the hearth and the lower surfaces of the glass sheets conveyed on 
the hearth. Substantial radiation also takes place between the furnace 
walls and the upper surfaces of the conveyed glass sheets. In addition, as 
disclosed by the U.S. Pat. No. 4,059,426 of Starr, gas heaters have 
previously been utilized to provide forced convection heating of the upper 
surfaces of the glass sheets. 
Roller conveyor furnaces for heating glass sheets did not receive any 
widespread commercial acceptance until introduction of the frictionally 
driven roller conveyor furnace for heating glass sheets as disclosed by 
the U.S. Pat. No. 3,806,312 of McMaster and Nitschke. Thereafter, further 
commercial acceptance of roller conveyor furnaces for heating glass sheets 
was achieved upon introduction of the furnaces disclosed by the U.S. Pat. 
Nos. 3,934,970 and 3,947,242 of McMaster and Nitschke. Subsequently, the 
oscillating type of roller conveyor furnace for heating glass sheets, as 
disclosed by the U.S. Pat. No. 3,994,711 of McMaster, received further 
commercial acceptance. All of these roller conveyor furnaces utilized 
electric resistance heaters for providing radiant heat transfer as the 
dominant mode of heating the glass sheets. 
U.S. Pat. Nos. 4,505,671 and 4,529,580 of McMaster disclose glass sheet 
heating by the use of forced convection. In U.S. Pat. No. 4,505,671 to 
McMaster, the forced convection heating is disclosed as providing glass 
temperature control that maintains planarity of glass sheets during 
tempering. In U.S. Pat. No. 4,529,580 to McMaster, the forced convection 
heating is disclosed as providing the primary source for heating the 
furnace in which glass sheets are heated prior to quenching that tempers 
the glass sheets. 
In glass sheet radiant heating, radiant energy emitted from electric 
resistive elements operating in the 650 to 750 degree Centigrade 
temperature range is primarily absorbed by a thin layer of the glass 
surfaces. Therefore, the edges which are heated by three surfaces will 
become hotter than the central areas which are heated by only two 
surfaces. During subsequent cooling, the hotter edges will cool faster 
than the center since the cooling rate is proportional to the temeperature 
differential between the glass and the ambient air or the quenching air if 
the glass is to be tempered. The faster cooling tensions the glass edges 
relative to the center such that tension cracks tend to result. When glass 
is being quenched for tempering, almost all quench breakage starts at the 
glass edges. Reducing the probability of edge breakage by forced 
convection heating, which does not overheat the edges, allows tempering to 
be accomplished at a lower overall temperature. Lowering the temperature 
by only about 10 degrees Centigrade during tempering doubles the stiffness 
of the glass and thereby reduces distortion of the tempered glass. 
Also, roller conveyor heating of glass sheets necessarily involves a 
certain amount of increased lower surface heating due to radiation and 
condition from the rolls. Upon subsequent quenching to provide tempering 
of the heated glass sheets, the hotter bottom surface will shrink more 
than the top surface if the heat transfer rates on the two surfaces are 
identical. This hotter bottom surface causes the glass to arch upwardly in 
the center if equal pressure of quenching gas is supplied from both above 
and below. As such, increased pressure must be utilized at the bottom 
surface, which results in the glass sheet curling down around its edges 
and thereby distorting planarity. While this "edge" distortion problem is 
present at all edges of the glass sheet, it is a particular problem at the 
leading edge that initially enters the quench station before the rest of 
the glass sheet. 
DISCLOSURE OF INVENTION 
One object of the present invention is to provide an improved gas burner 
heater and process for heating glass sheets by forced convection as the 
dominant mode of heat transfer. 
Another object of the invention is to provide an improved gas burner that 
has general usage but has particular utility when utilized to heat glass 
sheets, especially when the heating is performed on a roller conveyor. 
A further object of the invention is to provide an improved glass sheet 
that has reduced distortion as a result of the manner in which the glass 
sheet is processed. 
In carrying out the above objects, an elongated glass sheet heater 
constructed in accordance with the present invention includes a roller 
conveyor for engaging and conveying flat glass sheets along a plane of 
conveyance. A plurality of gas burners are spaced from each other along 
the length of the conveyor in upper and lower sets on both sides of the 
plane of conveyance. Each gas burner includes a combustion chamber that 
extends transversely to the direction of conveyance and in which 
combustion takes place. Each combustion chamber has outlets positioned 
relatively close to the conveyed glass sheets to permit the products of 
combustion to flow outwardly from the combustion chamber to supply a 
heated gas flow that impinges on the conveyed glass sheets from both sides 
thereof to provide forced convection that is the dominant mode of heat 
transfer to the conveyed glass sheets. 
Utilizing forced convection as the dominant mode of heat transfer permits 
lower energy cost as compared to conventional radiant glass sheet heating. 
In addition, the forced convection allows the glass sheets to be heated 
along a shorter length of conveyance since the heat transfer takes place 
faster than the radiant glass sheet heating. lower sets of gas burners 
include adjustable supplies of gas and air to independently control the 
extent the upper and lower glass surfaces are heated. Also, the upper and 
lower sets of burners are preferably aligned with each other at each 
spacing between the conveyor rolls. 
The construction of the gas burner disclosed has general usage but has 
particular utility when utilized to provide forced convection heating of 
roller conveyed glass sheets. This burner construction includes a 
combustion member having an inner surface that defines the combustion 
chamber. At least one inlet is provided through which gas and air are 
introduced into the combustion chamber in a tangential relationship with 
respect to the inner surface to provide a swirling motion that mixes the 
gas with the air within the chamber for smooth, quiet combustion. Without 
the swirling action, the burners pulsate and are difficult to keep lit. 
In the preferred construction, the combustion member of each burner 
comprises an elongated combustion tube whose inner surface has a round 
cross section. A plurality of the inlets are provided in the combustion 
tube spaced along its length with the inlets extending tangentially with 
respect to the round inner surface. The of outlets are provided in the 
combustion tube spaced along its length in a staggered relationship with 
respect to the inlets. The number of outlets provided is preferably 
greater than the number of inlets such that each inlet supplies gas and 
air to be burned into products of combustion for delivery through more 
than one outlet. There are most preferably at least twice as many outlets 
as inlets for the combustion chamber of each burner, and the inlets and 
outlets are positioned in the staggered relationship such that each inlet 
supplies gas and air for combustion and delivery through at least one 
associated outlet and also supplies gas and air for combustion and 
delivery through at least one other outlet along with the gas and air 
supplied therefor by an adjacent inlet. Best results are achieved in the 
burner disclosed when there are three times as many outlets as inlets for 
the combustion chamber of each burner. With this construction, each inlet 
supplies gas and air for combustion and delivery through two associated 
outlets and also supplies gas and air for combustion and delivery through 
two other outlets along with gas and air supply for the two other outlets 
by the two adjacent inlets. 
With the burner construction described above, the products of combustion 
delivered through each set of three outlets are of generally equal 
temperatures at each outlet. During experimentation, an attempt to supply 
more than three outlets per inlet resulted in the outlets most remote from 
the inlets supplying cooler products of combustion than the outlets closer 
to the inlets. Likewise, attempts to supply a long row of nozzles 
connected by a manifold to a remote heat source results in large 
variations in the temperature along the manifold. As such, the disclosed 
construction is preferred by virtue of the uniformity of temperature that 
results in supplying the forced convection by the burners. 
In the preferred construction, the burner also includes a plurality of 
supply tubes for respectively supplying gas and air to the inlets of the 
combustion chamber and further includes a manifold that feeds gas and air 
to the supply tubes for flow to the combustion chamber. 
In one preferred embodiment, the glass sheet heater includes a furnace 
defining a heating chamber through which the conveyor extends and in which 
the gas burners are received. Also, the manifold of each burner includes 
insulation for controlling heat flow to the gas and air supplied thereby 
for subsequent combustion in the combustion chamber. The combustion member 
preferably provided by the elongated tube is made of stainless steel in 
order to withstand the temperatures involved with heating of glass sheets 
to a sufficiently high temperature for bending or tempering. 
In an alternate embodiment, the furnace defines a heating chamber through 
which the conveyor extends and in which the combustion tube of each burner 
is received. The supply tubes of each gas burner extend from the 
combustion tube thereof out of the furnace, and the manifold of each 
burner is located externally of the furnace to thereby control heat flow 
to the gas and air being supplied thereby for subsequent combustion and 
delivery to provide the forced convection heating of glass sheets conveyed 
on the conveyor within the furnace. 
In the preferred construction of both embodiments, the manifold of each 
burner includes an outer member into which the supply tubes extend. An 
inner member of the manifold is received within the outer member and has 
openings located in proximity to the supply tubes. An inlet is provided 
for supplying gas to one of the members of the manifold, and another inlet 
is provided for supplying air to another member of the manifold such that 
the supply tubes feed both gas and air to the combustion chamber. As 
disclosed, the inner member of the manifold includes the gas inlet and the 
outer member thereof includes the air inlet. This construction permits the 
gas inlet of the inner member to feed gas to each supply tube within a 
concentric blanket of air fed thereto by the air inlet of the outer 
member. Such flow of the gas within the concentric blanket of air within 
each supply tube prevents premature ignition before reaching the 
combustion chamber defined by the combustion tube. 
Both the outer and inner members of the manifold preferably comprise tubes 
that extend parallel to the combustion tube. The supply tubes project into 
the outer tube into proximity with the opening in the inner tube. Opposite 
ends of the supply tubes are suitably secured to the manifold and the 
combustion tube with the supply tubes extending parallel to each other 
between the manifold and the combustion tube. 
A process for heating a glass sheet in accordance with the invention 
involves conveying the glass sheet by a roller conveyor along a plane of 
conveyance between upper and lower sets of gas burners spaced in a close 
relationship to the plane of conveyance and spaced from each other along 
the length of the conveyor on both sides of the plane of conveyance. Gas 
is burned within a combustion chamber of each burner to supply heated gas 
flow through outlets of the combustion chamber toward the plane of 
conveyance in order to impinge on the glass sheets in order to provide 
forced convection that is the dominant mode of heat transfer to the 
conveyed glass sheet. 
In performing the glass sheet heating process, the upper and lower sets of 
burners have adjustable supplies of gas and air to permit independent 
adjustment of the extent of heating of the upper and lower glass surfaces. 
Thus, the upper and lower sets of burners can be adjusted to deliver more 
heat to the upper glass surface in order to balance the effect of 
conduction heating of the bottom glass surfaces from the conveyor rolls. 
Best results are achieved in the glass sheet heating process when gas and 
air are supplied to the combustion chamber of each burner in a tangential 
relationship to the combustion chamber. As previously discussed in 
connection with the description of the gas burner, supplying the gas and 
air in this tangential relationship provides a swirling motion that mixes 
the gas and air for smooth combustion over a wide range of pressures near 
each inlet. 
The invention also involves the resultant product which is a glass sheet 
that has been heated while conveyed on rolls of a roller conveyor and 
subsequently cooled. In accordance with the invention, the glass sheet has 
reduced roll-wave distortion and reduced edge distortion as compared to 
glass sheets heated primarily by radiant heat. This reduction in both the 
roll-wave and edge distortion of the glass sheet results from the manner 
in which the glass sheet is heated by forced convection from gas burners 
as the dominant mode of heat transfer to the glass sheet during conveyance 
thereof on the rolls of the roller conveyor. 
The objects, features, and advantages of the present invention are readily 
apparent by the following detailed description of the best modes for 
carrying out the invention when taken in connection with the accompanying 
drawings.

BEST MODES FOR CARRYING OUT THE INVENTION 
As illustrated in FIG. 1 of the drawings, a glass sheet processing system 
is generally indicated by reference numeral 10 and includes a glass sheet 
heating furnace 12 that is constructed in accordance with the present 
invention. Furnace 12 includes a heating chamber 14 in which heating is 
performed as is hereinafter more fully described. System 10 also includes 
a processing station 16 for processing heated glass sheets to provide 
tempering, heat strengthening, bending, bending and tempering, or 
pyrolytic filming, etc. 
With combined reference to FIGS. 1 and 2, the glass sheet heating furnace 
12 includes a conveyor 18 for conveying glass sheets G within the heating 
chamber 14 along a plane of conveyance that is schematically indicated by 
A. Furnace 12 includes a convection heater having a plurality of gas 
burners 20 that are distinct from the conveyor 16 and positioned within 
the heating chamber 14 along the length of the conveyor on both sides of 
the plane of conveyance A. As is hereinafter more fully described, the gas 
burners 20 are positioned relatively close to the conveyed glass sheet and 
supply heated gas flow toward the plane of conveyance A from both sides 
thereof to provide forced convection that is the dominant mode of heat 
transfer to the glass sheets. 
Distinct advantages are achieved by the furnace 12, by providing forced 
convection as the conveyed dominant mode of heat transfer for heating the 
glass sheets, i.e., providing at least 50% of the total heat supplied to 
the glass sheets and, more preferably, at least about 2/3 to 3/4 of the 
total heat supplied to the glass sheet. More efficient heating is involved 
with forced convection as the dominant mode of heat transfer such that 
lower energy cost goes into each processed glass sheet. Also, the heating 
takes place faster with forced convection as the dominant mode of heat 
transfer to the glass sheet in order to permit the furnace to be shorter 
and thereby reduce construction cost as well as the factory floor space 
necessary to perform the processing. 
The conveyor 18 is of the roller type including horizontally extending 
rolls 22 on which the glass sheets G are conveyed during the forced 
convection heating. As previously mentioned, the convection heater is 
embodied by gas burners 20 that burn combustible gas and air. These gas 
burners 20 are arranged in upper and lower sets 20a and 20b spaced from 
each other along the length of the conveyor above and below the conveyor 
rolls 22 as shown in FIG. 1. The upper and lower burners 20 are aligned 
with each other at the spacing between each adjacent pair of rolls 22 and 
respectively provide upward and downward gas flows that provide the forced 
convection heating as the dominant mode of heat transfer to the conveyed 
glass sheets and allow the balancing of top and bottom surface heat 
transfer from all sources. 
Particular advantages result, in addition to the efficiency and faster 
heating times mentioned above, by the manner in which the glass sheets are 
heated on the roller conveyor 18 by the forced convection provided by gas 
burners 20. Since radiation is not the dominant mode of heat transfer to 
the glass sheets, the edges do not overheat by radiation due to the 
increased surface area adjacent the edges as compared to the center of the 
glass sheet. This allows tempering to be performed at a lower overall 
temperature and there is thus less chance for roll-wave distortion of the 
heated glass sheet as compared to radiant heating where the overall 
temperature must be higher. Furthermore, the upper and lower surfaces of 
the glass sheet can be heated more equally throughout the cycle with 
forced convection as the dominant mode of heat transfer. When tempering is 
to be performed, the bottom surface which is usually heated higher than 
the top surface in radiant heating furnaces must necessarily be quenched 
to a greater extent than the top surface. Such differential quenching 
causes the glass to curl downwardly around its edges, particularly at the 
leading edge, and thereby distorts planarity. When the final temperatures 
of the upper and lower glass surfaces are equal, the upper and lower 
quench rates can also be equal. Thus, both roll-wave and edge distortion 
are substantially reduced by utilizing forced convection as the dominant 
mode of heat transfer in accordance with the present invention. 
Roll conveyor 18 illustrated in FIGS. 1 and 2 is preferably of the 
frictionally driven type disclosed by U.S. Pat. Nos. 3,806,312, 3,934,970, 
3,947,242, and 3,994,711. At each of its lateral sides, the furnace 12 
includes a side slot 24 (FIG. 2) through which the conveyor roll 22 
projects for frictional driving. Each lateral side of the furnace 12 
includes a continuous drive loop 26 and also includes an external support 
surface 28 that faces upwardly with an upper driving reach of the drive 
loop 26 supported thereon for movement along the length of the conveyor. 
The opposite roll ends 30 are supported on the upper driving reach of the 
drive loop 26 and include central end pins 32 that are received by 
longitudinal positioners 34 projecting upwardly from the associated 
support surface 28 to prevent movement of the conveyor rolls 22 along the 
length of the conveyor during frictional driving of the rolls. Each drive 
loop 26 is received by an associate pair of drive sprockets 36 (FIG. 1) 
which are rotatable about associated axes B to move the drive loops 26 and 
thereby provide the frictional driving of the conveyor rolls. 
Counterclockwise driving of the left sprocket 36 pulls the upper driving 
reach of the drive loop 26 over the support surface 28 to thereby rotate 
the conveyor rolls 22 clockwise and effect conveyance of glass sheets from 
the left toward the right. During such driving, the lower reach of the 
drive loop 26 moves from the left sprocket 36 toward the right sprocket 
36. Similarly, right to left conveyance is effected by driving the right 
sprocket 36 in a clockwise direction if the conveyance is to be of the 
oscillating type as disclosed by the previously mentioned U.S. Pat. No. 
3,994,711. 
With reference to FIGS. 2, 3, and 4, the construction of the burner 20 
disclosed has general usage but has particular utility when utilized to 
provide heating of roller conveyed glass sheets as previously described, 
especially when used with the furnace 12 disclosed in FIGS. 1 and 2. This 
construction of the burner 20 includes a combustion member 38 having an 
inner surface 40 that defines an elongated combustion chamber 42 that 
extends transversely to the direction of conveyance of conveyor 18. 
Combustion member 38 is provided with at least one inlet 44 through which 
combustible gas and air are introduced into the combustion chamber 42 in a 
tangential relationship with respect to the inner surface 40. Such 
introduction of the gas and air in this tangential relationship provides a 
swirling motion that mixes the gas with the air within the chamber for 
pulse free combustion over wide ranges of pressures. Combustion member 38 
is also provided with at least one outlet 46 from the combustion chamber 
through which heated gas flow therefrom is discharged. A spark plug 48 is 
provided on one end of the combustion member 38 as shown in FIG. 3 to 
initially start the burning of gas and air mixed by the swirling motion. 
Thereafter, the flame of the mixed gas and air is self-sustaining to 
provide the heated gas flow through each outlet 46. On its other end, 
combustion member 38 has an end plate 49 suitably secured thereto such as 
by welding to close the chamber 42. Each plate 49 can also support a flame 
detector, such as a flame rod, for safe operation. 
As best illustrated in FIGS. 3 and 4, the combustion member 38 of the 
burner 20 preferably comprises an elongated combustion tube whose inner 
surface has a round cross section. Combustion tube 38 has a plurality of 
the inlets 44 spaced along its length and also has a plurality of the 
outlets 45 spaced along its length in a staggered relationship with 
respect to the inlets 44. There are preferably a greater number of outlets 
46 than inlets 44 such that each inlet supplies gas and air for more than 
one outlet. 
As best seen in FIG. 3, there are at least twice as many outlets 46 as 
inlets 44 for the combustion chamber 42 of the burner 20. Furthermore, the 
inlets 44 and outlets 46 are positioned in the staggered relationship such 
that each inlet burns gas and air for at least one associated outlet and 
also burns gas and air for at least one other outlet along with the gas 
and air burned therefor by an adjacent inlet. Best results are achieved 
when there are three times as many outlets 46 as there are inlets 44 for 
the combustion chamber 42 of each burner 20. Each inlet 44 burns gas and 
air for two associated outlets 46 and also burns gas and air for two other 
outlets along with gas and air burned for the two other outlets by the two 
adjacent inlets. 
As illustrated in FIGS. 2 and 3, the construction of burner 20 includes a 
plurality of supply tubes 50 for respectively supplying gas and air to the 
inlets of the combustion chamber in the tangential relationship previously 
described for combustion adjacent each inlet 44. A manifold generally 
indicated by 52 feeds gas and air to the supply tubes for flow to the 
combustion chamber for combustion. 
Reference should be made to FIGS. 3 and 4 which illustrate the construction 
of the manifold 52. As will be noted, manifold 52 includes an outer member 
54 into which the supply tubes 50 extend. An inner member 56 of the 
manifold 52 is received within the outer member 54 and has openings 58 
located in proximity to the supply tubes which are slightly spaced from 
the inner member. End plates 60 are secured in a suitable manner such as 
by welding to the opposite ends of the outer manifold member 54. Likewise, 
an end plate 62 closes one of the ends of the inner member 56 while an 
inlet 64 is provided at its other end in order to permit the introduction 
of pressurized combustible gas into the inner member for flow through the 
openings 58 into the supply tubes 50. Outer member 54 of the manifold 52 
has an inlet 66 through which pressurized air is introduced into the 
manifold for flow into the supply tubes 50 along with the gas supplied 
through the openings 58 of the inner member 56. It is believed that the 
gas and air supplied in this manner flow in a generally segregated manner 
with the gas in a central flow and the air flowing in a blanket around the 
gas without substantial mixing to thereby prevent premature ignition until 
being tangentially introduced into the combustion chamber 42 where the 
swirling motion provides the mixing for combustion adjacent each inlet 44 
as previously described. 
In the preferred construction, the outer and inner members 54 and 56 of the 
burner manifold 52 comprise concentric tubes that extend parallel to the 
combustion tube 38. The supply tubes 50 are secured by suitable welds 68 
to the outer member 54 of the manifold with the adjacent tube end 70 
projecting into the outer member into proximity with the aligned opening 
58 in the inner member 56 of the manifold. Likewise, welds 72 secure the 
opposite ends 74 of supply tubes 50 to the combustion member 38 in order 
to provide the inlets 44 that introduce the gas and air into the 
combustion chamber 42 in the tangential relationship that provides the 
swirling motion for mixing the gas and air. Between the combustion member 
38 and the manifold 52, supply tubes 50 extend in the parallel 
relationship best illustrated in FIGS. 2 and 3. 
With reference to FIG. 2, a central control 76 provides control of the 
forced convection heating in the furnace 12 by the gas burners 20. A 
suitable source 78 of pressurized combustible gas is operated by the 
control 76 and feeds the gas through separate conduits 80 to the gas 
inlets 64 of the burner manifolds 52. The source 78 is adjustable to 
provide independently adjustable supplies of gas to the upper and lower 
sets of burners in order to permit adjustment of one burner set with 
respect to the other. Likewise, a suitable source 82 of pressurized air is 
controlled by the central control 76 and feeds the pressurized air through 
conduits 84 to the air inlets 66 of the burner manifolds 52. The source 82 
is also adjustable to provide independently adjustable supplies of air to 
the upper and lower sets of burners in order to permit adjustment of one 
burner set with respect to the other. Control of the gas and air supply by 
the central source 76 permits the proper proportioning of gas and air for 
most efficient operation and also permits adjusting of the extent of upper 
and lower air and gas flows with respect to each other to control the 
extent the upper and lower glass surfaces are heated such as to account 
for the conduction heating of the lower glass surface by the conveyor 
rolls. Also, the manifold 52 of each burner 20 is preferably enclosed 
within suitable insulation 86 to prevent premature heating of the gas and 
air prior to combustion. Suitable adjustable supports 88 position the 
burners within the burner heating chamber 14 so that the combustion tubes 
38 are equally spaced from the conveyed glass sheet G in order to provide 
uniform heating of both the top and bottom surfaces. Adjustment of the 
supports 88 in any suitable manner permits the equally spaced relationship 
to be maintained such as when the thickness of the glass being heated is 
changed and it is thus necessary to change the elevation of either the 
upper or lower sets of burners. 
With reference to FIGS. 6 and 7, the processing system 10 includes an 
alternate furnace embodiment 12' which is the same as the previously 
described embodiment except as will be noted such that like reference 
numerals are applied to like components thereof and the description 
thereof need not be repeated. In the alternate embodiment, the furnace 12' 
defines a heating chamber 14 through which the conveyor 18 extends and in 
which the combustion tube 38 of each gas burner 20 is received in the same 
manner as the previously described embodiment. However, the supply tubes 
50 of each gas burner 20 extend from the combustion tube 38 thereof out of 
the furnace 12' as best shown in FIG. 7. The manifold 52 of each burner 20 
is located externally of the furnace 12' to thereby control heat flow to 
the gas and air being supplied by the manifold for subsequent combustion. 
It will also be noted that this alternate embodiment of the furnace has a 
relatively shallow construction which is effective in operation with the 
forced convection heating described. 
As illustrated in FIG. 5, the heat transfer that takes place by forced 
convection heating in accordance with the invention provides the dominant 
mode of heat transfer to the glass sheet. In other words, the forced 
convection heating provides at least 50% of the heating of the glass 
sheet. Preferably, the amount of forced convection heating is much 
greater, at least 2/3 to 3/4 of the total heating. As illustrated by lines 
90 and 92, the extent of forced convection heat transfer in accordance 
with this invention is much, much greater by several orders of magnitude 
than the heat transfer involved with natural convection as illustrated by 
line 94. Likewise, the forced convection heat transfer is significantly 
greater than the radiant heat transfer as illustrated by line 96 until the 
glass temperature exceeds 1200 degrees Fahrenheit which is the upper 
temperature extreme to which glass sheet heating is normally performed 
during processing. The extent of the forced convection heating depends 
upon the temperature Tj of the products of combustion upon exiting through 
the burner outlets 46 (FIG. 4) as previously described. As illustrated by 
line 90 in FIG. 5, the extent of the heat transfer is greater when there 
is a higher temperature with little excess air supplied to the burners. A 
greater introduction of excess air as illustrated by line 92 decreases the 
temperature Tj of the products of combustion and thereby decreases the 
extent of the forced convection heating. In order to accomplish the 
dominant forced convection heating of glass sheets, the supply pressure of 
gas and air to the burner illustrated were between about five and twenty 
five psi which is almost an order of magnitude greater than pressures 
which have previously been utilized. Also, the outlet pressures can be 
varied considerably while still having smooth combustion in the burners as 
a result of the burner construction disclosed. 
As shown in FIG. 8, both embodiments have the combustion members of the 
upper and lower sets of burners positioned relatively close to the 
conveyed glass sheets G, i.e. spaced no more than about 3/8 inch or 1 cm. 
from the adjacent glass surface. More specifically, when the outlets 46 
are round, the spacing between the outlets and the adjacent glass surface 
should be no more than about six times the diameter of the outlets. This 
closely spaced relationship permits the forced convection heating to 
provide the dominant mode of heat transfer to the conveyed glass sheets. 
Also, the spaced relationship of the burners with respect to each other 
along the length of the conveyor allows the gas flows to impinge with the 
glass surfaces and then freely flow back away from the glass sheet without 
any back pressure inhibiting the gas flow and the effectiveness of the 
forced convection heating. 
As previously described in connection with the furnace 12 illustrated in 
FIGS. 1 and 2, the glass sheet which has been processed while conveyed on 
the rolls 22 of the roller conveyor 18 and subsequently cooled has 
improved characteristics as compared to glass sheets heated primarily by 
radiant heat. The glass sheet G will have reduced roll-wave distortion 
since the uniform heating achieved with gas burner supplied forced 
convection does not cause the edges of the glass sheet to be overheated 
relative to the center of the glass sheet. Likewise, there will be reduced 
edge distortion since the top and bottom surfaces will be heated to the 
same extent as each other and the edges will not curl downwardly upon 
cooling as previously took place with radiant heating. These advantages 
occur by virtue of the glass sheet having been heated by the gas burner 
supplied forced convection as the dominant mode of heat transfer to the 
glass sheet during the conveyance on the rolls of the conveyor. As such, 
while the forced convection has general utility regardless of the type of 
conveyor utilized, it will be appreciated that the forced convection 
heating as the dominant mode has particular utility when utilized with 
roller conveyors. 
While the best modes for carrying out the invention have been described in 
detail, those familiar with the art to which this invention relates will 
recognize various alternative ways of practicing the invention as defined 
by the following claims.