Pyrometric Temperature Measurement in the Glass Industry

Transcription

1 Pyrometric Temperature Measurement in the Glass Industry MESSEN STEUERN REGELN

2 Contents Introduction Typical measuring points Introduction Typical locations for measuring temperature Glass as a transparent volume radiator Glass as a grey body Temperature measurement of glass, whatever the thickness Choice of pyrometer type dependent on glass thickness KELLER HCW pyrometers for various applications within the glass production process Stationary pyrometers Portable pyrometers Complete KELLER HCW systems for the glass industry Overview of all pyrometers with technical data Temperature is one of the most significant variables in glass manufacture and processing. It has become imperative to not only monitor temperatures during the energy-intensive melting process to optimise efficiency. Temperature detection and control is vital to the forming process as well. Glass manufacturers have come to rely on the accuracy of non-contact pyrometry as the measuring technique of choice. Thermocouples, protected by ceramic sheathing, are sometimes still used to monitor arch temperatures during the melting process. However, because of their rapid deterioration and limited service life, accuracy checks must be performed periodically using portable pyrometers. More and more, thermocouples are being replaced by wearfree pyrometers. During the glass forming process, only a non-contact and thus nonwearing measuring method can be employed. Whether hand-held or stationary, the use of pyrometers helps streamline various processes within glassmaking. Especially the following processes will benefit from pyrometer usage: when starting up or running a production line when changing over to a different line of products for quality control of manufactured products when manufacturing glass laboratory equipment such as test tubes and beakers when conducting industrial research for the flat glass, container glass or household glass sectors for subsequent finishing and processing of glass products, such as ampoules or in metallic glass brazing In the glass industry, the locations for pyrometer temperature measurements can be categorized according to thermal radiation characteristics: Black bodies are measured by pyrometers which see through a small eyehole into a uniformly heated cavity (such as the tank furnace). A thermocouple with a protection tube can be installed at the crown or bridge wall of the tank furnace. Alternatively, a pyrometer can detect the temperature at the bottom of a closed ceramic sighting tube. Opaque emitter or grey body. In this case, the pyrometer is aimed at metallic surfaces, i. e. the open inside surface of a mould. The thermal energy is only emitted from the mould. Transparent volume emitter. The pyrometer not only detects the thermal radiation at the surface, but the energy from depths below the surface as well. The depth at which the pyrometer can detect the radiation depends on the spectral absorption coefficient of the hot glass and the pyrometer s spectral range. Ultimately, one must distinguish between temperature measurements performed at continuous or discontinuous production processes.

3 Glass as a transparent volume radiator When measuring black or grey bodies, the primary criteria for selecting a pyrometer are temperature range, target spot diameter, measuring distance and response time. For transparent glass applications, the thickness of the glass and the pyrometer s sighting depth must also be considered. This depends on the pyrometer s spectral range and the spectral absorption coefficient of the particular type of glass. ε(λ),ρ(λ) ε(λ) ρ(λ) λ µm λ1 = µm ; ε( λ1) = 0.95 λ1 = µm ; ε( λ) = 0.95 λ1 = µm ; ε( λ3) = 0.96 λ1 = µm ; ε( λ4) = 0.91 Fig. 1: Reflectivity ρ (λ) and emissivity ε (λ) for borosilicate glass, as well as the wavelength ranges of KELLER HCW pyrometers and the corresponding emissivity correction values to be set. Glass as a grey body In general, the relationship between reflectance, emittance and transmittance can be expressed in the following equation: ρ (λ, T) + α (λ, T) + T (λ, T) = 1. (1) If a glass medium, due to its thickness, is opaque for certain parts of the spectrum, in other words, if the transmissivity is negligibly low (T < 0.01), then the following equation based on Kirchhoff s law of thermal radiation will be true: ε (λ, T) = 1 - ρ (λ, T). () In this case, the measured glass object is a grey body. Equation () shows that the sum of emissivity and reflectivity is equal to one and that emissivity is a function of wavelength λ. Fig. 1 shows the measured reflectivity ρ (λ) of borosilicate glass and the emissivity ε (λ) as calculated using equation () and as a function of wavelength at T (λ) < In addition, the chart shows various KELLER HCW pyrometers, each sensitive to a different band of wavelengths in the spectrum, with the appropriate emissivity setting ε ( λ) for borosilicate glass. One can see that wavelengths between 0.5 and 7.8 mm exhibit a nearly constant emissivity between 0.95 and For the spectral range 8 14 µm an average emissivity value ε = 0.91 should be selected. Glass surfaces with negligible transmissivity can be considered as grey body emitters in the spectral range between 0.5 and 7.8 µm. Unlike metal surfaces which might undergo oxidation, a glass surface is not subject to variations. The necessary product thickness, for which the transmissivity will be < 0.01, can be calculated. The depth of temperature measurement X99 represents the glass thickness at which from the surface on down to the depth X99 a pyrometer can detect 99 % of the total emitted thermal energy. This thickness as well as the absorption coefficient are dependent on wavelength λ and the object temperature T. 3

4 Fig. demonstrates the spectral measuring depth X99 for borosilicate glass at product temperatures of 600, 1000 and 100 C. The chart also illustrates the maximum measuring depth for borosilicate glass at some typical wavelengths. For the calculation of the spectral measuring depth X99, glass with homogenous temperature distribution was assumed. In the spectral region 0.5 to.0 µm (which is the range of free transmission for uncoloured glass) the measuring depth X99 will be between 90 and 400 mm, depending on temperature. 300 mm 00 X In Ga As Si µm λ 1300 C Sheet glass 0 C 150 C Green glass 0 C Fig. 3 Measurement depth X99 of sheet glass at 0 and 1300 C and X99 of green glass at 0 and 150 C subject to wavelength λ At wavelengths >.0 µm, these temperature gradients will decrease at longer wavelengths. When a pyrometer with a Si sensor ( µm) is employed, 99 % of the radiance (X99) will be transmitted to the glass surface at the following temperatures and depths: at 600 C merely 90 mm deep; the glass will appear dark red, at 1000 C to a depth of 170 mm, and at 100 C approx. 300 mm deep. Pyrometers with a InGaAs-sensor (1.1 to 1.7 µm) will measure at a somewhat greater depth than pyrometers with Si-sensor. The measurement depth X99 for pyrometers with a spectral range from 4.46 to 4.8 µm is a maximum of 0.7 mm. Pyrometers with a wavelength sensitivity >8 µm will only reach X99 at a depth of 0.04 mm. Fig. 3 shows variations for the X99 depth of spectral transmittance for sheet glass and green glass at wavelengths from 0.5 to 3 µm and at temperatures of 0 C, 150 C and 1300 C. Whereas the X99 depth of measurement for sheet glass at 1300 C is approximately that of borosilicate glass, X99 for green glass will only be 6 mm below the surface at 0 C or 1 mm at 150 C due to the higher absorption coefficient for green glass mm ,1 5 0,01 Si In Ga As 400 mm 300 mm µm λ µm µm 0,7 mm 0,04 mm µm µm 100 C 1000 C 600 C Fig. : Measuring depth X99 for borosilicate glass as a function of wavelength and glass product temperature. This means that even pyrometers which measure at short wavelengths and have a Si or an InGaAs sensor cannot see very deep into a coloured glass object. As a rule, glass objects are nontransparent (opaque) at wavelengths > 4 µm, thus at this spectral region, the shallow measuring depth X99 for borosilicate glass (as shown in Fig. ) will also apply to sheet glass and coloured glass. 4

5 Fig. 4 illustrates the standardised radiant energy E(D/X99)/ E100 % received by a spectral pyrometer with a Si-sensor as a function of the standardised glass depth D/X99 for coloured glass, for sheet glass and for borosilicate glass at homogeneous and inhomogeneous temperature gradients within the glass object. The pyrometer detects 50 % of the energy from the glass surface and from layers below the surface down to a depth which is commensurate to one sixth of the measuring depth X99. When the temperature distribution is inhomogeneous (i. e. surface is hotter than layers below the surface), this 50 % will be will even shallower (indicated by the dot-dash line. Selecting a pyrometer according to glass thickness Spectral pyrometer In order to obtain a representative temperature for a glass product, one must employ a spectral pyrometer which can measure or see at least halfway to all the way down into the glass thickness at the targeted location. If the object thickness is less than the instrument s measuring depth, the pyrometer will see through the glass object and pick up radiant energy from the background. Variations in object thickness will distort the signal and thus the temperature reading, for example when a spectral pyrometer with a Si-sensor is aimed horizontally through a gob. Miniature pyrometer PS 41 with its spectral sensitivity from 4.46 to 4.8 µm, measures temperatures just below the glass surface. Because a gob of colourless glass with a diameter between 0 and 80 mm will constitute a measuring depth of 300 mm for Si-sensor pyrometers, the temperature reading will depend on the object thickness. Tglass 1300 C E (D/X99)/E 100 % D/X % Nonetheless, an accurate temperature reading can be obtained if the pyrometer is aimed diagonally at the gob of molten glass as it is dispensed from the gob feeder, measuring into the orifice where there is sufficient depth. Alternatively, a two-colour pyrometer with a Si-sensor can be employed. If, however, the measured glass object is substantially thicker than the measuring depth, the pyrometer will predominantly pick up thermal energy at the uppermost layers just below the surface. This near-surface radiation might be subject to irregular convection currents which will greatly influence the temperature reading D/X mm sheet glass at 1300 C mm sheet glass at 1300 C mm borosilicate glass at 100 C Therefore, a pyrometer which measures close to the surface at a depth of 0.7 mm and at wavelengths between 4.46 and 4.8 µm is not recommendable for detecting the temperature of molten glass at lower depths. Fig. 4 Standardised radiant energy E(D/X99)/ E100 % received by a spectral pyrometer with a Si-sensor as a function of the standardised glass depth D/X99 for various types of glass at homogeneous and inhomogeneous temperature gradients within the glass object. 5

6 Fig. 5 demonstrates how considerable convection interference at the glass surface can occur due to the large size of the extraction opening. Glass extracted from lower depths will not be affected by convection. The automatic temperature control system of the overhead heater is extremely sensitive to this convection interference to the extent that the heater will shut off when the ball gatherer, coated with molten glass, enters the gathering bay, and turn on again when the ball gatherer is withdrawn. This will produce misleading temperature data. This source of error can be effectively eliminated by employing a pyrometer which, due to the wavelength at which it operates, will measure the temperature from below the glass surface. A pyrometer with a Si sensor, for instance, has a measuring depth of approx. 300 mm. Even when tilted at an angle of 30, this pyrometer will see 150 mm into the glass target. Spectral Type of glass Meas. Locations/applications for range depth pyrometer usage in µm X99 Temperature in mm D > X99 1/6 Xpp<D< X99 range in C Spectral Two-colour pyrometer pyrometer 0.8 to 1.1 Green glass 1 Gob Si Sheet glass 190 Furnace 500 to 3000 Borosilicate glass 300 Feeder 1.1 to 1.7 Green glass 4 Gob InGaAs Sheet glass 90 Mould 50 to 500 Borosilicate glass 400 Melting tank 4.46 to 5.8 Green glass Flat glass 300 to 500 Sheet glass 0.7 Cooling zone Borosilicate glass 8.0 to 14.0 Green glass Thin-walled glass -30 to 1000 Sheet glass 0.04 Cooling zone Borosilicate glass Gob ca When measuring glass temperature, always make sure to select a pyrometer whose measuring depth X99 is less than the glass thickness. This will prevent the sensor from looking through the glass und thus picking up energy from beyond the target which would otherwise result in an error. Two-colour pyrometers Pyrometers which measure at two channels, or two wavelengths, should be employed when the thickness of the measured glass product is less than the depth X99, but greater than one sixth of that value. At that thickness, both channels will still receive 50 % of the thermal radiation (Fig. 4). ca. 100 mm The two-colour pyrometer will determine the true temperature value, because signal attenuation will be the same at both wavelengths (numerator and denominator). Thus the quotient or ratio will remain constant. ca. 70 mm 1 Fig. 5 Arrangement for measuring temperature at the ball gatherer within a glass melting tank furnace. (1 glass-coated ball gatherer is plunged into the tank of molten glass, measured target spot, 3 Pyrometer). Table 1 on page 8 lists various KELLER HCW pyrometers with their respective spectral and temperature ranges. The table shows the depth X99 for different kinds of glass and some typical measuring locations/applications for spectral and two-colour pyrometers. 6

7 Typical applications for KELLER pyrometers in the glass industry The scope of KELLER HCW stationary and portable pyrometers covers the entire range of temperatures relevant to the glass industry, from 0 to 3000 C. The PS 36 with fibre optic cable is particularly suitable for temperature measurement at the feeder. The very small sensor head (ø 16 mm or ø 30 mm) is connected to the electronics by means of a fibre optic cable. This pyrometer can be used in ambient temperatures as high as 50 C without cooling. suitable location, into which the pyrometer can be aimed. When the cavity s drill depth is at least six times the diameter, the pyrometer will see into what can be considered a grey body with an emissivity of In this way, regardless of the cast iron mould's inner surface condition, a stationary pyrometer can obtain a reproducible and representative temperature value for the mould. CellaTemp PS 36 with fibre optic sensor head ø 16 mm. Stationary pyrometers Spectral pyrometers with Si or InGaAs sensors are especially suitable for measuring glass temperatures at the melting tank and feeder, due to their favourable measuring depth for colourless glass and their broad temperature ranges, from 50 C to 3000 C. The PZ 0/30 with throughthe-lens sighting is preferable for measuring temperatures at the ball gatherer. From a safe distance, this pyrometer sees through the orifice into the glass melt (Fig. 5). This pyrometer features a peak picker function to smooth any sporadic signal fluctuations caused by the movement of the ball gatherer in the pyrometer s optical path. Ceramic sighting tubes and air purging devices serve to keep the pyrometer optics clean and help extend maintenance intervals. For inspection purposes, the sensor head can be easily disconnected in seconds without the use of tools. With green glass, spectral pyrometers with a Si sensor can measure at a maximum depth of 1 mm. Thus the PZ 30 with through-the-lens sighting can be employed for measuring gob temperature when the gob diameter is 15 mm. The instrument s peak picker function can compensate for signal interruptions which occur during gob cutting operations (when the gob has fallen). The PZ 0/1 spectral pyrometers which have InGaAs sensors can measure deeper into the glass than pyrometers with Si sensors, when used for green and colourless glass. Due to its spectral range ( µm), the PZ 0/1 can even detect temperatures as low as 50 C. Therefore, these pyrometers are also suitable for measuring temperatures of cast iron glass pressing moulds. However, since the surface characteristics of the mould will vary over time the cast iron mould is extremely shiny when new and dulls with age and because these moulds are periodically lubricated, their surface emissivity will fluctuate greatly. To eliminate inaccuracies due to such fluctuations, the outside of the mould should have a cavity, drilled at a Of course the temperature of the mould s inner surface will be different than the temperature measured in the drilled cavity; this difference will remain a constant value. The closer the proximity of the bottom of the drilled cavity (down to a remaining thickness of 4 mm), the smaller the difference between these temperatures. The PZ 1 with a fibre optic cable and a laser spot light is also ideal for measuring the temperature of moulds. The small sensing head can be mounted to a convenient location near the press or press-and-blow machine and aimed into the drilled cavity. The spot light feature, when switched on, illuminates the target location and indicates the exact spot diameter in true size. Cellatemp PZ 1/41 with fibre optic sensor head ø 30 mm. 7

8 3 1 air purge The pyrometer series CellaTemp PZ 10 (with through-the-lens sighting), PS 1x and PS 4x all measure at long wavelengths and feature state-of-theart microprocessor-based electronics. Fig. 6: Example of a pyrometer mounting assembly at a feeder comprising: PS 36 spectral pyrometer with 1 fibre optics head, air purge and 3 sighting tube. The PS 4x features a wavelength range of µm and a temperature range of C or C. Due to its relatively shallow measuring depth X99 < 0.7 mm, the PS 54x is ideal for measuring temperatures at the surface or just below the surface of the glass. This pyrometer is suitable for temperature monitoring of thin glass products (thickness 1mm) such as flat glass, containers, pressed or blown decorative glass, and laboratory ware during forming, finishing and cooling processes. The PS Series Infrared Temperature Switch, PS 1, can detect glass residue in the moulds within milliseconds, thereby eliminating production downtimes and expensive equipment repair caused by glass which has stuck to the mould. Within the measuring range C, the switching point can be configured. The two-colour pyrometers PZ 40/41 and PZ 50 can be employed for temperatures ranging from C in the following situations: when the glass thickness D at the measured spot is less than the pyrometer's measuring depth for glass (X99) but more than one sixth of that value, in other words: 1/6 X99 < D < X99 when measuring the temperature of inductively heated metal parts by seeing through thick (and often coloured) glass. The PZ 40/50 with through-thelens sighting is suitable for measuring the temperature of colourless gobs which have a diameter of > 40 mm and are aimed at horizontally. In case the gob diameter is between 30 and 40 mm, then the pyrometer should be aimed toward the gob at an angle of 30 from the horizontal position. This will provide more depth for the measurement. During gob cutting, the transmitted radiation will be periodically interrupted. This instrument s peak picker and smoothing function mode, however, can compensate for this signal attenuation. The PZ 41 with fibre optics and spotlight can be used to measure the temperature of metals by detecting their radiation through thick, tinted glass. Fluctuations in glass thickness will not affect the measurement. With the instrument s emissivity adjustment, possible signal attenuation at both wavelengths can be corrected. Pyrometer CellaTemp PS with accessories The PZ 10 and PS 1x Series, with their wavelength range of 8 14 µm and temperature ranges of C or C measure at a depth of X99 < 0.04 and are thus appropriate for measuring only the surface temperature of glass. These pyrometers should be selected for measuring very thin glass products (thickness 0.1 mm). Because of the PZ 10 s focusable, interchangeable optics, this instrument is especially suitable for use with very small target objects such as household and table glassware. The PZ 10 is commonly used to monitor temperatures of glass products after the cooling zone as well as before and after subsequent processing. 8

9 The stationary pyrometers of the PZ Series are characterized by their focusable, through-the-lens sighting systems. The marked target, as seen through the viewfinder, indicates the true size of the spot measured. This feature plays a significant role in facilitating pyrometer alignment, particularly when measuring small objects or when looking through narrow sight openings. In addition, the desired measuring range of the PZ pyrometer can be adjusted both at the instrument itself as well as via a standard RS 3 interface for maximum application flexibility. The temperature data can be transferred to a PC for further processing by means of a serial interface such as RS 3, RS 4 or RS 485 or to a PLC with the Profibus output. The instruments of the CellaTemp PS Series are merely 30 mm in diameter and are thus ideal when working in cramped quarters. Housed in a rugged, IP 65 stainless steel enclosure, the PS can be employed in the extremely harsh industrial conditions. The PZ Pyrometer Series features focusable optics, through-the-lens sighting and target marking to facilitate aiming. All KELLER HCW pyrometers operate without mechanical moving parts. These instruments are therefore maintenance-free and have a long service-life. The range of stationary pyrometers is enhanced by a comprehensive selection of mounting fittings and accessories. Portable pyrometers KELLER s HCW portable pyrometers correspond to the stationary instruments in terms of temperature and wavelength range. These handheld instruments are suitable for spot checking and can be employed for any temperature measuring tasks which previously called for comparable stationary pyrometers. Another application for portable pyrometers is temperature verification of thermocouples in the port arch or tank furnace. The thermoelectric voltage generated in a thermocouple will cause aging. Depending on the particular operating conditions, this can result in a temperature drift up to 30 K within four weeks at 1550 C. Fig. 7 Temperature measurement at a mould: a pyrometer with fibre optics and a spot light is aimed into a drilled cavity. The function of such thermocouples, for instance, can be spot-checked weekly by aiming the spectral pyrometer through a sight window or inspection hole and then comparing the pyrometer s temperature reading to that of the thermocouple. The thermocouple and its protection tube should be replaced regularly, subject to the degree of temperature deviation displayed. 9

10 The present-day family of Optix pyrometers, which includes the G, S and Q Series, stems from the original Optix instrument which was based on intensity comparison and was successfully used for decades in the glass industry. Portable pyrometer series Portix with interface adapter. These pyrometers are characterised by the following features: microprocessor-controlled signal processing through-the-lens sighting is trueto-side; temperature reading is displayed in the field of view memory function to save up to 00 temperature readings, whether instantaneous, maximum or minimum values interchangeable, screw-on lenses (standard and telephoto) with continuously adjustable focus serial interface RS 3. Temperature values are stored as an ASCII data file to enable further processing, spreadsheet analysis etc. with common software such as Excel. housed in a rugged aluminium dustproof and waterproof enclosure Optix G and Optix S, which can measure temperatures ranging from C or C, are ideal for spot-checks in the furnace, melting tank or feeders as well as for verifying thermocouples in the port arch. The Optix Q is a portable two-colour pyrometer. It is able to produce reliable measurement data, even when there is a great amount of steam or dust in the atmosphere, or when the gob is very small. Keller s HCW Portix Series encompasses various models of a small-sized, handy instrument which can measure temperatures between C. Portix B (0 600 C) with a target spot of 5 mm is suitable for quick temperature checks of moulds and small glass products before and after the cooling process. A spot light indicates the true size of the measured spot. Portix D (0 600 C), when positioned at a distance of 1 m to the target, will have a spot diameter of 10 cm and is thus ideal for measurements of larger glass products, or at conveyor belts. Both models are available as combined instruments: for contact measurements using either a PT 1000 or a NiCr-Ni probe, as well as for noncontact infrared temperature detection. The Portix H, with its measuring range from 300 C to 1999 C, rounds off the Portix series. Applications include temperature measurements in the furnace tank and feeder, of gobs with a diameter > 5 mm and moulds. Optix portable pyrometer series All Portix pyrometers feature as a standard an integrated data storage for up to 64 temperature readings. The infrared interface module Adaptix C enables measurement data to be transferred to a PC for further analysis or graphical display. 10

11 Summary of pyrometers and technical specifications Pyrometer Spectral Temperature Distance Smallest Focus Response Output range range ratio ) possible [mm] time [mm] [ C] measuring [sec] spot ø [mm] Portable Pyrometers Portix Spectral Pyrometers Portix B 1) : infrared Portix D 1) : interface Portix H 1) : Optix Spectral Pyrometers Optix G : < = 1 Optix S : < = 1 Optix Two-colour Pyrometer Optix Q 0.95/ : < = : RS 3- interface Stationary Pyrometers PS Series Spectral Pyrometers PS : < = 0.1 PS : < = PS : < = (4) 0 ma PS 36 with fibre optics : < = 0.01 PS 41/ : < = : PS 1 with switching output : < = 0.01 PZ Series Spectral Pyrometers PZ 10 with through-the-lens sighting : < = 0.1 PZ 0 with through-the-lens sighting : < = PZ 1 with fibre optics and spot light : < = 0.00 PZ 30 with through-the-lens sighting : < = 0.00 PZ 31 with fibre optics and spot light : < = 0.00 PZ Series Two-colour Pyrometers PZ 40 with through-the-lens sighting 0.95/ : < = : PZ 41 with fibre optics and spot light 0.95/ : < = 0.0 PZ 50 with through-the-lens sighting 0.95/ : < = (4) 0 ma RS 3/4 interface or Profibus interface 1) Also available as a combined instrument for contact and non-contact temperature detection ) Standard distance ratio, other distance ratios available on request. 11

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