Temperature radiation pyrometers

Equipment Description INDICATORS - TEMPERATURE-RADIATION PYROMETER... 

Control Devices. Control devices have advanced from manual control to sophisticated computet-assisted operation. Radiation pyrometers in conjunction with thermocouples monitor furnace temperatures at several locations (see Temperature measurement). Batch tilting is usually automatically controlled. Combustion air and fuel are metered and controlled for optimum efficiency. For regeneration-type units, furnace reversal also operates on a timed program. Data acquisition and digital display of operating parameters are part of a supervisory control system. The grouping of display information at the control center is typical of modem furnaces. 

The filter and screen of the pyrometer shown ia Figure 9 require specific mention. From equation 21 it is evident that the observed radiation must be limited to a narrow bandwidth. Also, peak intensity does not occur at the same wavelength at different temperatures. The pyrometer is fitted with a filter (usually red) having a sharp cut-off, usually at 620 nm. The human eye is insensitive to fight of wavelength longer than 720 nm. The effective pyrometer wavelength is 655 nm. 

The temperature in the hottest part of the kiln is closely controlled using automatic equipment and a radiation pyrometer and generally is kept at about 1100—1150°C (see Temperature measurement). Time of passage is about four hours, varying with the kiln mix being used. The rate of oxidation increases with temperature. However, the maximum temperature is limited by the tendency of the calcine to become sticky and form rings or balls in the kiln, by... 

Total Radiation Pyrometers In total radiation pyrometers, the thermal radiation is detec ted over a large range of wavelengths from the objec t at high temperature. The detector is normally a thermopile, which is built by connec ting several thermocouples in series to increase the temperature measurement range. The pyrometer is calibrated for black bodies, so the indicated temperature Tp should be converted for non-black body temperature. 

Because indirect-heat calciners frequently require close-fitting gas seals, it is customaiy to support aU parts on a selFcontained frame, for sizes up to approximately 2 m in diameter. The furnace can employ elec tric heating elements or oil and/or gas burners as the heat source for the process. The hardware would be zoned down the length of the furnace to match the heat requirements of the process. Process control is normaUy by shell temperature, measured by thermocouples or radiation pyrometers. When a special gas atmosphere must be maintained inside the cyhnder, positive rotaiy gas se s, with one or more pressurized and purged annular chambers, are employed. The diaphragm-type seal ABB Raymond (Bartlett-Snow TM) is suitable for pressures up to 5 cm of water, with no detectable leakage. 

Boslough, M.B., and Ahrens, T.J. (1989), A Sensitive Time-Resolved Radiation Pyrometer for Shock-Temperature Measurements above 1500 K, Rev. Sci. Instrum. 60,3711-3716. 

The temperature measurement devices which do not contact the hot surfaces, for example, optical -, radiation pyrometers, and infrared techniques, are not typical for high-pressure application. 

The Czochralski Technique. Pulling from the melt is known as the Czochralski technique. Purified material is held just above the melting point in a cmcible, usually of Pt or Ir, most often powered by radio-frequency induction heating coupled into the wall of the crucible. The temperature is controlled by a thermocouple or a radiation pyrometer. A rotating seed crystal is touched to the melt surface and is slowly withdrawn as the molten material solidifies onto the seed. Temperature control is used to widen the crystal to the desired diameter. A typical rotation rate is 30 rpm and a typical withdrawal rate, 1—3 cm/h. Very large, eg, kilogram-sized crystals can be grown. 

In several renewable energy processes, including the concentrating solar collectors, boilers, and combustion systems, the accurate measurement and control of high temperatures are required. These (over 1,000°C) temperatures are most often detected by thermocouples (types B, C, R, and S) and by optical and IR-radiation pyrometers. These devices are only briefly mentioned here, because they will be discussed in detail later. Here, the emphasis will be on some of the other high-temperature detectors such as sonic and ultrasonic sensors. 

IR pyrometers offer a very viable noncontact method to measure temperatures all the way up to 3,600°C (6,500°F) and can be the best choice for most applications. When high temperature is to be detected in hard-to-reach locations, IR radiation pyrometers are combined with the use of optical fibers. These applications will also be discussed later. 

One advantage of a spectral radiation pyrometer is that the emissivity or emittance at only a specific wavelength (e.g. 0.653 pm) is of importance. A non-blackbody source will be less luminescent than a blackbody source at the same temperature. Thus, a falsely low temperature will be determined by sighting a calibrated disappearing filament pyrometer on the non-blackbody. This temperature has been referred to as the brightness temperature . 

Another area of research that could be profitably explored is the use of remote sensing instruments to measure surface temperatures of textile assemblies. Infrared thermovision cameras have been used to visualize temperature distributions over clothed and nude persons in order to study the transport of microorganisms by convective heat flow (112). A variety of less expensive radiometers and radiation pyrometers that are used to measure and automatically control the temperature of textiles during drying and texturing (113, llU, 115) could also assess the thermal behavior of apparel and clothing assemblies and thus elucidate their contribution to thermal comfort indoors. 

Pyrometers. A radiation pyrometer can estimate very high temperatures by using the Planck equation, Eq. (5.6.4), and detecting by optical means the maximum radiated heat. 

The temperature of a substance in a particular state of aggregation (solid, liquid, or gas) is a measure of the average kinetic energy possessed by the substance molecules. Since this energy cannot be measured directly, the temperature must be determined indirectly by measuring some physical property of the substance whose value depends on temperature in a known manner. Such properties and the temperature-measuring devices based on them include electrical resistance of a conductor (resistance thermometer), voltage at the junction of two dissimilar metals (thermocouple), spectra of emitted radiation (pyrometer), and volume of a fixed mass of fluid (thermometer). 

Figure 1. Schematic diagram of furnace used to pull lithium niobate crystals 1, alumina pull rod 2, after heater 3, temperature control signal to auxiliary controller 4, Pt seed holder 5, LiNbOj crystal 6, LiNbOs melt 7, Pt crucible 8, alumina plate 9, rf coil 10, alumina tubes 11, firebricks 12, sapphire rod-radiation pyrometer 13, temperature signal to main controller 14, Fiberfrax insulation. (Courtesy W. L. Kway.)...

Radiation Pyrometer Temperature Scale.—The temperature scale for the radiation pyrometer is based upon the Stefan-Boltaimann law expressing the relation between the total energy J radiated per unit time per unit area by a black body and its absolute temperature, absolute, as follows ... 

Thus knowing the total emissivity E of any material it is possible to obtain the true temperature d from the apparent temperature E as measured by a radiation pyrometer. 

The following table shows the true temperatures corresponding to the apparent or measured temperatures observed with a radiation pyrometer when sighted upon various materials in the open. This table must not be confused with Table 12 for the optical pyrometer. It will be noticed that the corrections are entirely different for the two types of pyrometer. 

Table 18.—Tree vs. Apparent Temperatures Measured by Radiation Pyrometers WHEN Sighted upom Materials in the Open...

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