Measuring the flow rate of a thin stream of molten material

For measuring the flow rate of a thin stream of molten materials such as that of glass, the diameter of the thin stream is measured, as is the velocity. The velocity is measured on the basis of the measurement of the time separating the successive appearance of an emission sequence emitted at first and second points on the path of the molten material. A correlation is then established between the sequences and the time interval corresponding to the passage of the same irregularities at the two selected points identified by this correlation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to the improvement of techniques for measuring the 
flow rate of a thin stream of molten materials such as that of glass, 
basalts, slag, ceramics and the like, which materials in the molten state 
are the source of the emission of large-scale radiation. 
2. Description of the Related Art 
It is known to measure these flow rates by means described in particular in 
patent SE 82 03650. According to this document, the measuring principle is 
as follows: two sensors, sensitive to the radiation emitted, are disposed 
on the path followed by the molten material at a distance from one 
another. There are irregularities in the emission of the molten material. 
The sensors are disposed so as to receive the emission from a limited 
portion of the section of the thin stream in question. The signals 
received are selected so as to retain only those signals which exceed a 
given threshold. The conventional technique consists in measuring the time 
separating the appearance of signals exceeding he threshold on each of the 
sensors, the measurement being translated into the flow velocity of the 
material. Measurement of the diameter of the thin stream completes the 
determination process enabling the flow rate to be attained. The diameter 
is measured by forming the image of the cross-section of the thin stream 
on a linear camera and determining the number of radiation sensitive 
elements of the camera receiving sufficient radiation as corresponding to 
the width of the thin stream in question. 
The arrangements provided in the prior art technique only fulfills the 
intended aim to a limited extent. In practice, the flow rate measurements 
are principally used in the control of a regulating die or orifice. The 
measurements are compared with reference values selected by the operator 
and any difference relative to these reference values triggers an 
adjustment of the parameters such as the electrical power supply and 
consequently the temperature of the die from which the material flows 
freely. In other words, the flow rate in this type of application has to 
be measured precisely and continuously since otherwise the system would be 
totally disorganized. 
It is possible to avoid incorrect measurements affecting the regulation 
process by excluding any measurement which would differ from a high 
probability variation range defined experimentally. However, this 
technique is not entirely satisfactory since it results in a systematic 
loss of data. 
The presence of deviant measurements is inherent in the system previously 
described which is based on the consecutive recognition of two signals 
exceeding a given threshold by the two sensors. In a system of this type, 
the identification of the signals cannot be perfect, even if other "safety 
devices" (in particular those regarding the time separating two signals) 
enable certain risks of errors to be eliminated. 
SUMMARY OF THE INVENTION 
An object of the present invention is to improve the techniques used for 
measuring the flow rates of molten material of the types indicated above, 
in particular by minimizing or eliminating the risks of errors in the 
identification of the signals used to determine the rate of flow. 
In accordance with the invention and as stated above, the measurement of 
the flow is based on the variations in radiation from the thin stream of 
molten material, which variations are followed by two sensors disposed 
along the path of the flow. The difficulties noted previously are avoided 
by not plotting the "peaks" corresponding to irregularities of a given 
size--which moreover constitutes a limit of use if the thin stream in 
question does not have any irregularities or has insufficient 
irregularities--but by comparing all the signals received by the two 
successive sensors. In accordance with the invention it is no longer a 
matter, as it were, of comparing a momentary peak but a complete sequence 
corresponding to a certain lapse in flow time. The comparison of the 
complete signals from the two sensors enables a determination of the most 
similar or the best correlated sequences and, once these have been 
identified, a deduction therefrom of the time lapsed between the two 
sensing processes. 
Tests have shown that even taking into account inevitable modifications in 
the physiognomy of emission sequences over time, a practically certain 
correlation could be established via these means, thus avoiding any 
erroneous measurement. 
Likewise, the method of detection used according to the invention has been 
improved. It has been noted above that only some of the cross-section of 
the thin stream of molten material was conventionally used as the emission 
source. One reason for this choice was the necessity of minimizing the 
causes of variations in the radiation observed and consequently of the 
signals analyzed. By centering the observation on the median 
cross-section, the risks of large variations which show up at the edges in 
relation to the surrounding area are avoided. This selection is 
nevertheless manifested by a depletion of the available information. 
Conversely, in accordance with the invention, it is possible and even 
preferable to process a signal which is as "rich" as possible. The more 
complex the signal, the more definite the correlation. For this reason, 
sensors enabling the radiation emitted by a complete section of the thin 
stream of molten materials are advantageously used.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1 the thin stream of molten material is represented by the cylinder 
segment 1. A flow of this type is found in numerous applications, in 
particular in the glass making and ceramics industries. By way of example, 
the various methods of conversion leading to the production of insulating 
mineral fibers use a flow of this type between the melting area and the 
conversion area, whether the conversion technique used is centrifuging by 
means of a rotor which simultaneously acts as a die or external 
centrifuging from the periphery of a series of wheels. 
In the aforementioned examples, the molten material, glass, basalt, slag 
etc., flows freely over a given distance in the form of a thin stream of 
cylindrical cross-section. It is at a high temperature and so is the 
source of intense radiation. Still referring to these examples, the thin 
stream of molten material has a relatively large number of irregularities 
consisting almost exclusively of gas bubbles. Other irregularities may 
result from unmelted particles or particles which are insufficiently 
melted. In all cases, these irregularities give rise to variations in the 
radiation which may be detected. 
The radiation emitted by the thin stream of molten material 1 is passed 
through an optical system represented symbolically at 2. In the image 
plane 3 there are located the detectors which are used to measure the flow 
rate. These detectors are respectively: 
--two photodetectors 4 and 5 which are used to analyze the radiation 
emitted from two zones. The photodetectors 4 and 5 are at first and second 
points spaced at a distance from one another along the length of the thin 
stream 1; 
--a detection system of the so-called "CCD" (Charged Coupled Device) linear 
camera type having an aligned row 15 of light sensitive cells. Accuracy of 
measurement depends on the resolution capacity of the camera and thus on 
the number of aligned cells. 
FIGS. 2a and 2b show comparatively the observation ranges of the thin 
stream 1 of molten material in the conventional art and according to an 
embodiment of the invention. 
In each of the two methods, the zones observed (6 and 7, 8 and 9) are 
respectively spaced along the path of the stream. The defects which give 
rise to irregularities are represented by bubbles 10. 
The defects 10 are distributed in a random manner through the thin stream. 
This special feature alone explains one difficulty which illustrates the 
inaccuracy of the conventional techniques. It can be seen in FIG. 2a that 
some of the defects are not detected since they do not fall within 
observed zones 6 and 7. The richness of the signals is thus reduced 
thereby. Moreover, certain defects located at the lateral limits of the 
zones under observation may be perceived as they pass out of one of the 
zones but not into the other, owing to even a very small variation in the 
relative lateral position of the thin stream or the defect in this thin 
stream. 
According to the invention, for the reasons indicated above, as shown in 
FIG. 2b it is preferable to observe complete sections of the thin stream. 
In practice, for thin streams supplying insulating fiber production 
installations, the cross-section is of the order of 0.5 to 3 cm and 
observation of the entire section does not give rise to any particular 
problems. 
Sections 8 and 9 are elongate or oblong but conventional sensors are not. 
It is therefore convenient to modify or convert the image to be analyzed. 
This method of conversion is advantageously performed using an optical 
wave guide comprising a fiber bundle which, at the end turned towards the 
thin stream, has a highly elongate section 11, 12 and, at the opposite end 
a circular cross-section 13, 14. Thus, each of the photodetectors 4 and 5 
shown in FIG. 1, and the individual cells 15 of the CCD is formed by the 
elongate section 11, 12 of a fiber bundle which transmits light to a 
remote electronic sensor portion connected to the circular section end 13, 
14. 
Apart from the analysis of the cross-section with a geometrical shape 
better adapted to the requirements of the measuring process in question, 
the use of an optical wave guide also has the advantage of enabling the 
electronic portion of the photodetectors to be located at a given distance 
from the exposed zones of the fiber production installation. Even if 
certain precautions are taken, it is in effect difficult to avoid an 
increase in the temperature of the device when it is located in the 
vicinity of the means distributing the molten material in industrial 
installations. It is thus advantageous to be able to locate the fragile 
instruments at a given distance. Maintaining the "electronic" section of 
the measuring device distance from the hot stream is also advantageous 
when the production installation comprises means for heating by induction, 
which generates electrical interference. A further advantage of using wave 
guides is, if necessary, being able to analyze the cross-sections of the 
thin stream located at points where the space available would not enable 
electronic detectors to be installed in the immediate vicinity. 
The data processing assembly is illustrated schematically in FIG. 3. 
On the left hand side of the Figure there are illustrated the image 16 of 
the thin stream, the camera cells 15 and wave guides 17 and 18. 
The radiation intensity received by these wave guides is led to photodiodes 
19, 20. The signals are subsequently amplified and guided, after passing 
through an analog/digital convertor 21, 22 to a central processing unit 
23. Filters 24, 25 may be introduced in a conventional manner to eliminate 
interfering frequencies. 
The diameter is measured by means of the cells 15 of the linear camera and 
the signal is also converted and sent to the central processing unit (CPU) 
23. 
FIG. 4, shows the type of analog signals corresponding to a measuring 
process. The two diagrams I and II respectively originate from the two 
photodiodes. By use of these diagrams the positions of identical recorded 
profiles can be correlated in the CPU 23 by offsetting the curve II 
relative to the curve I at an offset interval determined by analysis of 
the entire signal profile, and not merely peaks. For example, matching 
portions of the sequences I and II are identified and the fine interval 
separating the matching portions is determined. A very accurate 
correlation is thus possible. The offset interval corresponds to the time 
t.sub.1 separating the passage of a given stream segment in front of the 
two sensors. 
Such correlation by the CPU 23 yields an automatic determination of the 
time interval t.sub.1 separating the two analog sequences observed by the 
two sensors. Knowledge of this time, together with that of the distance 
separating the two zones observation, enables a determination by the CPU 
23 of the flow velocity of the thin stream of molten material to be 
established. The CPU 23 can then use the stream velocity and the measured 
diameter of the stream to determine the rate of flow. 
The determination of the diameter of the flow should take account of the 
variation in luminance of the molten material. The width of the signal 
from the camera depends on the luminance. If a characteristic threshold of 
an "illuminated" pixel is determined, thin streams of the same diameter 
and different degrees of luminance will appear to have different 
diameters. 
In order to avoid this systematic error, efforts are made to operate at a 
constant signal amplitude. In accordance with the invention, this is 
achieved by making the camera exposure time dependent on the average 
luminance of the thin stream. This dependency is achieved by means of 
camera management software having an algorithm which causes a signal to be 
obtained of which the amplitude is just below the maximum output level of 
the camera and the benefit of all its dynamics to be gained. The signal 
amplitude may vary owing to the fact that the exposure time progresses in 
a step-wise manner. The quality of the measurement is likewise improved by 
rendering the threshold values used to measure the width of the signal 
dependent on the maximum amplitude of the signal. The measurement is taken 
half-way up the signal. 
Apart from the accuracy of measurement in dependence on the luminance, the 
process should also be performed such that differences between the 
position of the thin stream or its image opposite the camera does not 
interfere with the measuring procedure. The sensor should be sufficiently 
large in order to take account of lateral changes of limited size. In 
practice, when used in machines producing mineral fibers, it is chosen for 
example to proceed such that the image of the thin stream does not cover 
more than 60% of the width of the sensor. Nevertheless, for a given 
sensor, it is preferable if the image measured is a large part of the 
sensitive width so as to maintain satisfactory resolution and consequently 
a good degree of accuracy. 
Examples 
Tests using these measuring techniques have been carried out in insulating 
fiber production installations. Two series of tests have been carried out: 
the first test was performed on a glass wool production installation and 
the second on a rock wool installation (basalt or blast furnace slag). 
In the production of glass wool, the molten material comes from a 
continuously operating melting oven. After being routed through a 
fore-hearth the material is delivered to the centrifuger by a die of which 
the temperature (and consequently the flow rate) is adjustable. In an 
installation of this type, the molten glass has an emission spectrum which 
is generally very rich owing to a limited refining process, resulting in 
the presence of a large quantity of bubbles. 
In the tests conducted at a flow rate varying between 10 and 30 tonnes per 
day, i.e., 400 to 12,000 kg/h, the degree of accuracy achieved according 
to the invention is of the order of 0.3% or less in the arrangement 
indicated above. 
It should be stressed that in this calculation the relative degree of 
accuracy of the measurements of the flow velocity and of the diameter of 
the thin stream are of the same order of magnitude. In addition, an 
additional gain in accuracy in measuring would be of limited consequence 
for the regulating capacities of the installation. 
In the installation, the sections observed by the sensors for measuring the 
velocities are 50 mm apart in the flow direction and the velocity of the 
thin stream is of the order of 2 m/s. The time between each measuring 
cycle is of the order of a few seconds but may be reduced if necessary. 
Experience has shown, however, that at steady state operation the 
variations in flow rate occur very slowly and that a shorted measurement 
interval would have not effect on the regulating capacities in view of the 
thermal inertia of the system. 
In the tests carried out, the complete sampling time corresponding to each 
sampling was approximately 2 seconds. The average was then calculated over 
approximately 10 measurements, further reducing the risk of errors. 
The diameter was measured by means of a linear camera comprising 1728 
pixels for stream image diameters which usually do not exceed 10 mm, 
enabling the diameter to be measured with a degree of accuracy of the 
order of 1 micrometer by calculating the average of the successive 
measurements. The degree of accuracy may be increased by increasing the 
number of pixels of the camera. In practice, however, this is not 
necessary when the measurements are not averaged, especially since the 
speed of acquisition can be increased to enable a very high number of 
measurements in a very short amount of time (of the order of approximately 
100 per second) to be achieved. Consequently, for this measurement the 
interval of time between two successive measurements is also maintained at 
less than 5 seconds. 
In the rock wool production installation the measurement is performed under 
similar conditions. The advantage of this measurement is all the more 
marked in that the method of supplying the molten material is usually much 
less stable than in the previous case owing to the fact that "cupola 
furnaces" are used to melt the raw materials. 
Furthermore, a particular difficulty of molten slag and rocks is due to the 
fact that, unlike glass, which at melting temperature remains 
semi-transparent, these materials are opaque. In other words, although 
with glass it is possible to detect irregularities in the emission coming 
from the interior of the flowing thin stream, this is not possible in the 
case of rocks and slag. The only emission which can be analyzed is that 
which comes from the surface of the stream. For this reason it is also 
advantageous to proceed according to the method proposed by the invention 
which consists in analyzing a complete section of the thin stream of 
material. 
For the measurement carried out on the rock wool production installation, 
the distance separating the two detection points was reduced to 
approximately 25 mm for practical reasons connected with the geometry of 
the assembly. The degree of precision obtained with the flow velocity 
(which remained at the order of 1 to 2 m/s) is nevertheless approximately 
of the same order as that of the measurement carried out on molten glass. 
Despite the lateral stability of the thin stream of molten material being 
very approximate, it was possible to measure the diameter with the same 
degree of accuracy as before in the case of glass. Overall, the flow rate 
is obtained with a relative error which does not exceed 0.5% for flow 
rates ranging from 5 to 25 tonnes per day. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practices otherwise than as specifically described herein.