IE913925A1 - Measuring the flow rate of a thin stream of molten material - Google Patents

Description

IE 913925 2 MEASURING THE FLOW RATE OF A THIN STREAM OF MOLTEN MATERIAL 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.

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 prior art technique consists in measuring the time separating the appearance of signals exceeding the threshold on each of the sensors, this measurement translating the flow velocity of the material. Measurement of the diameter of the thin stream completes the IE 913925 3 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, the number of sensitive elements receiving sufficient radiation corresponding to the width of the thin stream in question.

The arrangements provided in the prior art technique only fulfil the intended aims to a limited extent. In practice, the flow rate measurements are principally used as means supplying a regulating 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 permanently otherwise the system would be totally disorganised.

It is evidently possible to avoid deviant measurements affecting the regulation process by excluding any measurement which would differ from a "probable" variation range defined experimentally. This is not entirely IE 913925 4 satisfactory. It results in a systematic loss of the data used for this regulating process.

The presence of deviant measurements is inherent in the system previously selected 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. The system is found wanting in different cases, even if other "safety devices" in particular regarding the time which has to separate two signals, enable certain risks of errors to be eliminated.

The aim 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 minimising, and practically 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 IE 913925 5 the flow. The difficulties noted previously are avoided not by 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 signals from the two sensors enables it to be established which are the most similar or the best correlated sequences and, once these have been identified, to deduce therefrom the time elapsed 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 seen above that only some of the cross IE 913925 6 section of the thin stream of molten material was used as the emission source. One reason for this choice was connected with the necessity of minimising the causes of variations in the radiation observed and consequently of the signals analysed. By centring 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 material are advantageously used.

The remainder of the description describes the invention in a more detailed manner with reference to the sheets of drawings in which: - Figure 1 is a skeleton diagram of the technique used; - Figures 2a and 2b show schematically the parts of the thin stream of material observed IE 913925 7 according to the prior art and according to the invention respectively; - Figure 3 is a skeleton diagram of the measuring assembly; and - Figure 4 is a diagram showing a comparison of the signals from the means detecting emission irregularities.

In Figure 1 the thin stream of molten I 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 lead to the production of insulating mineral fibres comprising a flow of this type between the melting area and the conversion area proper, whether the technique used is centrifuging by means of a rotor which simultaneously acts as a die or whether it is the process of so-called 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 is the source of intense radiation. Still referring to these examples, the 8 1C 9109^9 thin stream of molten material comprises 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 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 sensors are respectively: - two photodetectors 4 and 5 which are used to analyse the radiation emitted from two zones at a distance from one another on the thin stream 1; - a detection system of the so-called "CCD" (Charged Coupled Device) linear camera type 15.

Figures 2a and 2b show comparatively the observations of the thin stream 1 of molten material in the prior art and according to the invention in a preferred manner.

IE 913925 9 In each of the two methods, the zones observed 6 and 7, 8 and 9 respectively are 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 emphasises the inaccuracy of the prior art ( techniques. It can be seen in Figure 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. It is even more obstructive that certain defects, located at the limit of the zones under observation, may be perceived as they pass out of one of the zones but not into the other, simply owing to even a very small variation in the relative position of the thin stream or the defect in this thin stream.

According to the invention, for the reasons indicated above, as shown in Figure 2b it is preferable to observe complete sections of the thin stream. In practice, for thin streams supplying insulating fibre production installations, the cross-section is of the order 10 It 913920 of 0.5 to 3 cm and observation of the entire section does not give rise to any particular problems. However, it is convenient to modify the image analysed by the sensor which is not generally oblong. This method of conversion is advantageously performed using an optical wave guide comprising a fibre bundle which, at the end turned towards the thin stream, has a highly elongate section 11, 12 and, at the end facing the light-sensitive sensor, a circular cross-section 13, 14.

Apart from the analysis of a 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 photodetectors to be located at a given distance from the exposed zones of the fibre 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. Being distant from the "electronic" section of the measuring device is also advantageous when the production installation IE 913925 11 comprises means heating by induction which generate great interference. A further advantage of using wave guides is, if necessary, being able to analyse the cross-sections of the thin stream located at points where the space available would not enable detectors to be installed in the immediate vicinity.

Figure 1 shows further the system for measuring the diameter of the thin stream. As indicated, a linear camera 15 is advantageously used which comprises a large number of sensitive cells or any other similar device. Accuracy of measurement evidently depends on the resolution capacity of the camera and thus on the number of aligned cells.

The data processing assembly is illustrated schematically in Figure 3.

On the lefthand side of the Figure there are illustrated the image 16 of the thin stream, the camera 15 and the two wave guides 17 and 18.

The radiation 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 IE 913925 12 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 linear camera 15, the signal is also converted and sent to the central processing unit 23.

Figure 4 shows the type of analog signals corresponding to a measuring process. The two distinct diagrams I and II originate from the two photodiodes. In this diagram, by means of limited modifications, the identity of the recorded profiles can be observed if the curve II is offset relative to the curve I by an interval corresponding to the time separating the passage in front of the two superimposed sensors.

The main correlation corresponds to the automatic determination of the time interval separating two analog sequences observed by the two sensors. Knowledge of this time, associated with that of the distance separating the two zones under observation, enables the rate of flow of the thin stream of molten material to be established.

The determination of the diameter of the flow should take account of the variations in IE 913925 13 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 rendering the camera exposure time dependent on the average luminance of the thin stream. This dependency is achieved by means of the camera management software. The algorithm used enables 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 value 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 connected with the luminance, the process should also be performed such that differences between IE 913925 14 the position of the thin stream or its image opposite the camera do 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 fibres, 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 l measured is a large part of the sensitive width so as to maintain satisfactory resolution and consequently a good degree of accuracy.

Tests using these measuring techniques have been carried out in insulating fibre 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 IE 913925 15 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, ie. 400 to 12 000 kg/h, the degree of accuracy achieved according to the invention is of the order of 0.3% i 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 for measuring the velocities are 50 mm away and the velocity of the thin stream is of the order of 2 m/s. The acquisition time between each measuring step is of the order of a few seconds but may be reduced if necessary. Experience has shown, however, that at stationary operation the IE 913925 16 variations in flow rate are very slow and that a more rapid measurement would have no effect on the regulating capacities in view of the thermal inertia of the system.

In the tests carried out, the complete time corresponding to a sampling process was approximately 2 seconds. During this time the average was calculated over approximately 10 measurements further reducing the risk of errors.

The diameter is 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 micrometre by calculating the average of the successive measurements. Evidently 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 enables 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 too the interval of time between two successive measurements is maintained at less than 5 seconds.

IE 913925 η In the rock wool production installation the measurement is performed in similar conditions. The advantage of this measurement is all the more marked in that the method of supplying the molten material is usually much |£s>i> stable than in the previous case owing to the fact that^cupola furnaces^are used to melt the raw materials. ς 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 analysed 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 analysing 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 IE 913925 18 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.

Claims (10)

1. Process for measuring the flow rate of a thin stream of material emitting radiation comprising measuring the diameter of the stream and measuring the velocity, the velocity being measured on the basis of the measurement of the time separating the successive appearance at two points remote from one another over the path of the thin stream of identifiable emission i irregularities, this process being characterised in that the time interval measurement involves the determination of an emission sequence at a first point on the path of the molten material, the determination of an emission sequence at a second point on the path, the processing of this data enabling a correlation between the sequences and the time interval corresponding to the passage of the same irregularities at two selected points ! identified by this correlation to be established.

2. Process according to Claim 1, in which, in the measurement of the flow velocity of the emitting material stream, the points observed on the path of this flow correspond to complete cross-sect ions of this stream. IE 913925 20

3. Process according to Claim 1 or Claim 2, wherein the diameter of the thin stream is measured by means of a linear camera receiving the image of the stream, the operation of the camera being controlled so as to render the sequenced exposure time dependent on the average luminance of the thin material stream.

4. Process according to Claim 3, in which - the threshold used to determine the width of the i signal is dependent on the maximum amplitude of the signal.

5. Device for performing the process for measuring the flow rate of a thin material stream according to any one of the preceding claims, characterised in that it comprises two photodetectors disposed so as to receive the emission from the thin stream corresponding to two t different points on the path of the flow of the stream, an assembly for amplifying the signal emitted by each of the photodetectors, and an assembly for processing the amplified signals and measuring the time separating the appearance of signals identified by the photodetectors, which device further comprises a linear camera for determining the diameter of the thin stream, means for processing the signals emitted by the IE 913925 21 photodetectors, means establishing correlations between the signal sequences emitted by the photodetectors and determining the time interval separating two correlated sequences.

6. Device according to Claim 5, in which the image of the thin stream is led to the photodetectors by means of optical wave guides.

7. Device according to Claim 6, in which the optical wave guides have, on the thin material stream side, an elongate cross-section covering the entire cross-section of the thin stream. - 22 - IE 913925

8. Process for measuring the flow rate of a thin stream of material emitting radiation substantially as herein described with reference to the accompanying drawings. 5

9. Device for performing the process for measuring the flow rate of a thin material stream substantially as herein described with reference to and as shown in Figures 1 to 3 of the accompanying drawings.

10. 10. The features described in the foregoing specification, or any obvious equivalent thereof, in any novel selection.