Pyrometer assembly having a heated viewing tube

A viewing tube extension for an infrared radiation pyrometer includes a tube having a heater therein. The heater has a passageway so that the pyrometer sights a glass ribbon moving through a forming chamber of a flat glass making apparatus. Vapors from the heating chamber moving into the tube are maintained above their dew point as a purge gas moves the vapors out of the tube.

1. Field of the Invention 
This invention relates to a pyrometer assembly and method of using same to 
monitor the temperature of a substrate in a heated environment, e.g., a 
glass ribbon moving through a forming chamber of a glass making apparatus. 
2. Discussion of the Prior Art and Technical Problems 
Problems are associated with using radiation pyrometers to measure the 
temperature of a substrate in a heated vapor atmosphere. For example, 
radiation pyrometers are normally maintained below a temperature of about 
300.degree. F. (148.degree. C.) and even as low as 105.degree. F. 
(41.degree. C.) to prevent thermal damage to the pyrometers. Maintaining 
the pyrometers at the required temperature may be accomplished by mounting 
the pyrometer in a water cooled jacket. In the instance where the 
pyrometer is used to measure the temperature of a material in a heated 
vapor atmosphere, and the pyrometer is at a temperature approximately at 
or below the dew point of the vapors, the vapors condense on the 
pyrometer, e.g., on the lens of the pyrometer or sight tube of the 
pyrometer. Condensate on the pyrometer lens reduces the accuracy of the 
pyrometer. Accumulated condensate on the tube or on the lens may drop on 
the article whose temperature is being measured. In the instance where the 
article is a glass ribbon in a glass forming chamber, the condensate e.g. 
tin sulfide poses an additional problem. Namely, the accumulated 
condensate may fall on the glass ribbon or the condensate may be reduced 
by the atmosphere of the glass forming chamber to liquid tin which may 
drop on the ribbon. In either instance the result is glass that is not 
useable. 
The prior art has solutions to the above problems, however there are 
limitations to these prior art solutions. For example, U.S. Pat. No. 
4,034,780 teaches that the temperature profile of a glass sheet moving 
through a bending furnace is made by a pyrometer mounted outside the 
furnace and moving in alignment with a plurality of normally closed 
viewing tubes. As the pyrometer moves over a viewing tube, the end of the 
tube opens and the pyrometer sights through the tube to measure the 
temperature of the glass sheet. Continual movement of the pyrometer 
actuates the mechanism for each tube to open or close the tube end as 
required. With this arrangement the atmosphere within the furnace can be 
maintained and the pyrometer protected from thermal damage. Limitations of 
the above techniques are the need (1) for an operating mechanism to open 
and close each tube and (2) to maintain alignment between the tube and the 
pyrometer. 
In view of the above, it would be advantageous to provide a pyrometer 
assembly that can be used to measure the temperature of a substrate in a 
heated vapor atmosphere that does not have limitations or drawbacks of the 
prior art. 
SUMMARY OF THE INVENTION 
This invention relates to a viewing tube for use in combination with a 
pyrometer e.g., radiation pyrometer which is used for monitoring the 
temperature of a substrate, e.g. a glass ribbon in a heated vapor 
atmosphere such as a forming chamber of a glass making apparatus. The 
viewing tube has heating facilities having a viewing passageway 
therethrough to provide a field of view for the pyrometer and facilities 
for connecting the pyrometer and the heater to prevent uncontrolled 
movement of atmosphere between the pyrometer and the heater. 
This invention also relates to a method of monitoring the temperature of a 
substrate using the combination viewing tube and pyrometer. In one 
embodiment vapors moving into the heater toward the pyrometer are 
maintained above their dew point as a purging gas moves through the heater 
to move the vapors out of the heater. In another embodiment condensates of 
tin sulfide form on the unenergized heater. At a preselected time, the 
heater is energized as undiluted nitrogen moves over the condensate to 
sublime same and move the vapors out of the heater.

DESCRIPTION OF THE INVENTION 
This invention relates to a pyrometer viewing tube that prevents vapors 
from condensing in the tube and/or removal of condensate from within the 
tube. In the following discussion, the viewing tube and radiation 
pyrometer i.e., temperature monitoring device are used in the forming 
chamber of a glass making apparatus. However, as will be appreciated, the 
invention is not limited thereto. 
With reference to FIG. 1, there is shown a forming chamber 20 of the type 
taught in U.S. Pat. No. 3,976,460, which teachings are hereby incorporated 
by reference. In general, glass making ingredients are fed into and melted 
in a melter (not shown) to make molten glass 22. The molten glass 22 flows 
downstream into a refiner 24 where the molten glass is fined and 
conditioned. Thereafter the molten glass is controllably flowed past a 
tweel 26 onto a pool 28 of molten tin contained in the glass forming 
chamber 20. As the molten glass flows along the molten tin it is 
selectively and controllably cooled and sized to a dimensionally stable 
glass ribbon 30, e.g. as taught in U.S. Pat. No. 3,998,616, which 
teachings are hereby incorporated by reference. The glass ribbon 30 exits 
the chamber 20 by way of lift off roll 32. 
Mounted at a preselected position through metal casing 34 and suspended 
roof 35 of the chamber 20 is a pyrometer assembly 36 incorporating 
features of the invention to monitor ribbon temperature to control its 
viscosity as it moves through the forming regions of the chamber 20. With 
reference to FIG. 2, the suspended roof 35, in general, includes a 
plurality of spaced refractory supports 38 conveniently mounted on ends of 
suspension rods 39 to form a grid or generally rectangular sections (only 
3 supports 38 show in FIG. 2). Selected ones of the rectangular sections 
support plug block assemblies 40 and others support element block 
assemblies 41. The plug block assemblies 40 and element block assemblies 
41 are identical in construction except that the element block assemblies 
41 (only one shown in FIG. 2) have a passageway 42 therein for receiving 
an element which in the instant invention is an end of the temperature 
monitoring device 36. The plug block assemblies 40 and element block 
assemblies 41 each include a mullite refractory slab 43; a high 
temperature insulating layer 44 e.g., an insulating firebrick capable of 
withstanding temperatures of up to about 2600.degree. F. (1427.degree. 
C.); another layer 45 of insulating firebrick capable of withstanding 
temperatures of up to about 1800.degree. F., (982.degree. C.); and a layer 
46 of a hard faced insulating board to provide structural stability for 
rods 48 which hold the layers 43-46 together. As will be appreciated, the 
invention is not limited to the construction of the suspended roof 35 and 
any convenient type of suspended roof may be used in the practice of the 
invention. Further, in certain instances the depth of the plenum area 47, 
i.e. the area between the metal casing 34 and the suspended roof 35 varies 
and the pyrometer assembly 36 may be short of the suspended roof 35. When 
this occurs element block assemblies 41 may be piled one on top of the 
other. 
With reference to FIG. 6, pyrometer 60 (shown in phantom) is mounted in 
housing 62. The housing 62 includes an end cap 64 secured to a water 
cooled jacket 66 which in turn is secured to a gas purging section 68. The 
pyrometer 60 and housing 62 are not limiting to the invention and any 
convenient type known in the art may be used in the practice of the 
invention. The housing 62 is mounted on end 70 of viewing tube 72. The 
opposite end 76 of the viewing tube 72 is mounted in the hole 42 of 
element block assembly 41 as shown in FIG. 2. It is recommended that the 
tube end 76 extend below at least the plate 46 of the element block 
assembly 41 to prevent or minimize hostile atmosphere of the forming 
chamber from moving into plenum area 47 shown in FIG. 1. If the vapors of 
the forming chamber move into the plenum area they may chemically and/or 
thermally damage piping and wiring contained therein. 
With reference to FIGS. 2 and 4, heating element 80 having passageway 81 
therethrough is mounted in the tube 72 to maintain the temperature within 
the tube, i.e. the passageway 81 above the dew point of the forming 
chamber vapors to prevent condensation of the vapors in the viewing tube. 
Although not limiting to the invention, it is recommended that the heater 
80 terminate short of the pyrometer 60 (see FIG. 6) to prevent thermal 
damage thereto. To minimize or eliminate thermal conduction from the 
heater 80 to the plenum area 47 it is recommended that a layer 82 of 
thermal insulating material be provided around the heater 80. The 
temperature of the heater 80 may be monitored in any convenient manner, 
e.g. by thermocouple 84, to monitor the temperature of the heater 
passageway 81 to make certain the temperature of the passageway 81 is 
above the dew point of the vapors. Although not limiting to the invention, 
if a pyrometer sight tube, for example shown as numeral 86 in FIGS. 2-6 is 
used, it is recommended that its end, for example end 88, extend into the 
heater 80 as shown in FIG. 2. In this manner (1) vapors are prevented from 
moving between the walls of the viewing tube and sight tube and (2) 
turbulence of the flow of the purge gas is minimized to prevent 
uncontrolled movement of vapors upward toward the pyrometer. 
In one embodiment of the invention, the heater 80 is energized as a purge 
gas moves through the viewing tube 72. Vapors in the chamber 20 which move 
upward in the heater passageway 81 are maintained above their dew point by 
the heater to prevent condensation. The vapors are prevented from 
continued upward movement and urged out of the tube by the purge gas. The 
type, pressure and temperature of the purge gas are not limiting to the 
invention. However, it is recommended that the gas be inert relative to 
the vapors to prevent chemical reactions therewith. Further, the 
temperature and flow of the purged gas out of the tube should not upset 
thermal equilibrium of the chamber containing the article, for example 
glass ribbon 30. The temperature of the gas can be elevated by preheating 
before moving the purged gas through the heater passageway 81 or the gas 
can be heated as it moves through the heater passageway 81. Another 
embodiment of the invention which is used to remove condensate from the 
tube 72 is discussed below. 
DETAILED DESCRIPTION OF THE INVENTION 
The instant invention is practiced to monitor the temperature of a glass 
ribbon 30 as it is supported on the molten tin bath 28 and moves through 
the forming chamber 20 to determine the viscosity of the glass as it moves 
through the sizing regions of the forming chamber. If the temperature as 
determined by the pyrometer indicates that the viscosity is too high, in 
other words the temperature is too low, heat input to the chamber 20 is 
increased. Conversely, if the temperature of the glass indicates that the 
viscosity is too low, i.e. the temperature of the glass is too high, heat 
input to the chamber 20 is decreased. In this manner the temperature of 
the glass and its viscosity can be controlled as it moves through the 
sizing regions to provide controlled attenuation of the ribbon which 
results in uniform thickness between the edges of the ribbon. Pyrometer 
assembly 36 incorporating features of the invention is mounted in metal 
casing 34 and suspended roof 35 of the forming chamber 20 in a manner to 
be discussed below and spaced about 50 feet (15 meters) downstream of the 
tweel 26. 
With reference to FIGS. 2 and 6, the pyrometer assembly 36 includes a 
viewing tube 72, a pyrometer 60 and housing 62. The viewing tube 72 
includes a first steel tube 90 having an end 91 overlapping end 92 of a 
second steel tube 93 and welded together to provide the viewing tube with 
an overall length of about 41/2 feet (1.35 meters). With reference to FIG. 
6, the first tube 90 has a length of about 5 inches (12.5 centimeters), a 
wall thickness of about 1/4 inch (0.64 centimeter) and an inside diameter 
of about 5 inches (12.5 centimeters). A first flange 70 having a thickness 
of about 0.37 inch (0.97 centimeter) and an outside diameter of about 63/4 
inches (17.14 centimeters) is mounted adjacent tube end 94 for securing 
the housing 62 to the tube 72 in any convenient manner. A second flange 96 
spaced about 2.35 inches (5.97 centimeters) from the first flange 94 has 
an outside diameter of about 81/2 inches (21.6 centimeters) and a 
thickness of about 0.38 inch (0.97 centimeter) for supporting and securing 
the viewing tube 72 in position through the roof casing 34 by bolts 97. 
Mounted through the wall of the tube 90 between the flanges 94 and 96 are 
a first and second pair of posts 98 and 100, to provide external 
electrical access to a split heater 80 to control its heat output. Also 
mounted between the flanges 94 and 96 in the wall of the tube 72 is a 1/8 
inch (0.32 centimeter) compression fitting 101 through which thermocouple 
wires 102 and 103 pass to provide external electrical access to the 
thermocouple 84 mounted in the heater 80. The second tube 93 has a length 
of about 47 inches (1.1 meters) and a wall thickness of about 1/8 inch 
(0.32 centimeter) and an outside diameter of about 5 inches (12.7 
centimeters). With reference to FIGS. 2 and 5, pins 104 spaced 120.degree. 
apart and having a diameter of about 0.187 inch (0.47 centimeter) and a 
length of about 0.25 (0.64 centimeter) are mounted inside the tube 
adjacent the end 76 to assist in maintaining the heater 80 in the tube in 
a manner to be discussed below. 
With reference to FIGS. 2 and 4, a heater 80 of the type sold by 
Thermocraft, Inc., Model No. RL157-1910W-230 Volts, 3 inch (7.62 
centimeter) inside diameter (I.D.) and 30 inch (0.75 meter) long has a 
chromel alumel type thermocouple 84 inserted through the heater wall 
intermediate its ends. The heater 80 is wrapped with a layer 82 of 
Fiberfrax thermo insulating material. Thereafter, the wrapped heater is 
inserted through end 94 of the viewing tube 72 and urged downward toward 
the pins 104. With reference to FIG. 5 a washer 106 having three spaced 
cutout portions 108 capable of passing over the pins 104 is mounted in the 
end 76 of the tube 72 and pushed upward beyond the pins 104 and rotated. 
The wrapped heater is urged against the washer. The washer 106 has a 
thickness of about 1/8 inch (0.32 centimeter) and an inside diameter of 
3.25 inch (8.26 centimeter) and an outside diameter of 4.75 inches (12.04 
centimeters). After the heater is mounted in position, the pair of wires 
110 and 112 of the heater 80 (see FIG. 3) are connected to the posts 98 
and 100 respectively, and wires 102 and 103 of the thermocouple 84 are 
inserted through the fitting 101 shown in FIG. 6. A sight tube 86 shown in 
FIG. 2 of the type sold by Land Instrument, Ltd., Model No. STO-24 
Sillimanite is mounted through a steel tube adapter 116 (see also FIG. 6) 
and extends downward with the end 88 of the viewing tube extending into 
the heater 80 as shown in FIG. 2. The adapter 116 is secured to the end 94 
of the viewing tube 72 by bolts 117 as shown in FIG. 6. 
A pyrometer 60 of the type sold by Land Instruments, Model No. 
NCGV6/22F-42V 1800 with an integral preamplifier and a remote processing 
unit Model 2U-GV11/22F-A is mounted on the sight tube 86. The pyrometer 60 
has a spectral response of 4.8 to 5.2 microns to measure the temperature 
of the glass and not heat reflections of the tin bath. A housing 62 of the 
type sold by Land including an O/N/C backcap 64 is mounted on an N/WJP 
water-cooled jacket with integral air purge shown as numerals 66 and 68, 
respectively, is mounted about the pyrometer and secured in position by 
bolts 118 passing through the flange of the air purge section 68 and sight 
tube 86 as shown in FIG. 3. The tube 72 is passed through a hole in the 
casing 34 and has its end 76 mounted within the passageway 42 formed in 
the element block assembly 41 as shown in FIG. 2. With reference to FIG. 
6, the posts 98 and 100 are connected to an electrical potential 120 by 
cables 122 and 124, respectively to supply current to the heater 80. The 
thermocouple 84 is connected through wires 102 and 103 extending outside 
of the viewing tube 72 to a temperature recorder 126. The purge section 68 
is connected to a nitrogen supply 128 by pipe 130. The inlet pipe 132 and 
outlet pipe 134 of the water-cooled jacket 66 are conveniently connected 
to a water supply 136 to maintain the pyrometer at temperature of about 
100.degree. F. (38.degree. C.). 
The heater 80 is energized to heat the heater passageway 81 to a 
temperature of about 1500.degree. F. (817.degree. C.). The tin sulfide 
vapors in the forming chamber move upward into the heater passageway 81 as 
shown in FIG. 2. The vapors have a dew point of about 1400.degree. F. 
(760.degree. C.) and are normally at a temperature of about 1650.degree. 
F. (899.degree. C.). As vapors move upward through the tube toward the 
pyrometer, the heated passageway 81 maintains the vapors above their dew 
point. The purge gas moving through the sight tube 86 and the heater 
passageway 81 at a flow rate of 35-50 standard cubic feet per hour (SCFH) 
urges the vapors back into the forming chamber. 
When condensates such as tin sulfide sublime the following technique, which 
is another embodiment of the invention, may be used. The tin sulfide 
vapors are allowed to condense within the heater passageway 81 and at 
periodic intervals, e.g. once a day, the heater 80 is energized to heat 
the passageway 81 to a temperature of about 1500.degree. F. (817.degree. 
C.) as an inert gas e.g. nitrogen moves through the tube heater passageway 
81. In the presence of nitrogen the condensate sublimes at these 
temperatures after which the vapors are moved out of the tube. 
As can be appreciated, the invention is not limiting to the above examples 
which are presented for illustration purposes only.