Glass Melting Technology: A Technical and Economic ... - OSTI

elative to a defined zero with precision to the nearest one-hundredth inch, or the nearest one-thousandth of
an inch on a new system of larger furnaces.
The measurement system used is determined by the type of glass being melted and atmospheric conditions
above the melt.
• Probe type (physical contact with glass surface to measure elevation or noncontact using back
pressure sensor with low-pressure air flow);
• Laser type (reflecting laser beam across span to calibrated detector);
• Bubble generator (immersed tube to determine the pressure required to cause bubble release);
• Nuclear (beta radiation measurement with glass varying absorption between source and detector).
Furnace pressure
Furnace pressure and differential pressure relative to atmosphere are measured by piping from taps in the
glass melter’s superstructure to pressure transmitters measure. Results are reported as positive or negative
inches of water column, with precision to the nearest 0.0005 inch and a typical reading of about +0.070
inches of water column. Position of the pressure tap is important because the chimney effect of the hot
furnace atmosphere is relative to the outside ambient atmosphere. Pressure increases 0.01 inch for every
one-foot increase in tap elevation. For a large 1000 sq.ft. melt surface furnace, the pressure probe would be
mounted about six feet above the melt surface, which would read from 0.065 to 0.070 inches of water
column. Ideal melter pressure is the lowest pressure that prevents outside air (parasitic air) from entering the
furnace at glass-melt surface.
Oxygen measurement
Oxygen, an important variable in monitoring the combustion and melting process, is measured
by a probe mounted in the hot exhaust stream. Located under positive pressure leaving the melter, the probe
determines percentage and temperature of excess oxygen. Fresh reference air supplied
to the internal reference side of a zirconia oxygen sensor will prevent depletion of oxygen from the interior,
reference side of the sensor. If reference is less than 21 percent oxygen, the sensor output will indicate
higher excess oxygen than actual. The furnace is at a lower-oxygen, partial pressure as compared to the
reference, so the oxygen molecules pick up electrons and transfer them through the electrolyte to the furnace
to satisfy the oxygen imbalance. As oxygen is depleted inside
the sensor, voltage drops and provides a higher oxygen reading in the furnace than is actual
or accurate.
Oxygen depletion is common to values of 18 percent or lower. This oxygen depletion can cause errors of
two-plus percent excess oxygen, compared to the expected reference of 20.95 percent used in calculating of
excess oxygen. If furnace combustion air or oxygen is lowered, the readout will indicate the targeted value
but will drive the furnace into reducing conditions. By supplying 250 cc/min., or 0.5 CFH reference air,
more than enough oxygen will be present to overcome the flow of oxygen ions through the electrolyte and
cause error.
Batch moisture
For proper furnace operation, batch moisture must be controlled accurately. Batch samples are routinely
measured by an operator or technician. A 100-gram sample, excluding cullet, is dried at ~250°F and the
weight reported as percentage moisture. To maintain the typical 3 to 4 percent moisture level, adjustments
are made by flow control valves, scales, proportioning pumps or rotometers. Totalizing flow meters are
often used to calculate gallons per batch or ton. Batch moisture is checked by its appearance at the charger
and adjusted if necessary.
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