The CRC Handbook of Mechanical Engineering. Chapter 4. Heat and Mass Transfer (776127), страница 44
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(540 mm) and 191/4 in. (489mm) would appear to be likely choices.ReferencesAmerican Society of Mechanical Engineers. 1995. ASME Boiler and Pressure Vessel Code, Section VIII.New editions published every 3 years. ASME, New York.Gentry, C.C., Young, R.K., and Small, W.M. 1982. RODbaffle heat exchanger thermal-hydraulic predictive methods, in Proceedings of the Seventh International Heat Transfer Conference, Munich,Germany, 6, 197–202.Hewitt, G.F., Shires, G.L., and Bott, T.R.
1994. Process Heat Transfer, CRC/Begell House, Boca Raton,FL.Kern, D.Q. and Kraus, A.D. 1972. Extended Surface Heat Transfer, McGraw-Hill, New York.Kral, D., Stehlik, P., Van der Ploeg, H.J., and Master, B.I., 1996. Helical baffles in shell and tube heatexchangers. Part I: Experimental verification, Heat Transfer Eng., 17(1), 93–101.Palen, J.W. and Taborek, J.
1969. Solution of shell side flow pressure drop and heat transfer by streamanalysis method, Chem. Eng. Prog. Symp. Ser. No. 92, Heat Transfer-Philadelphia, 65, 53–63.Saunders, E.A.D. 1988. Heat Exchangers: Selection, Design, and Construction, Longman Scientific &Technical/John Wiley & Sons, New York.Schlünder, E.U., Ed. 1983.
Heat Exchanger Design Handbook, Begell House, New York.Singh, K.P. and Soler, A.I. 1984. Mechanical Design of Heat Exchangers and Pressure Vessel Components, Arcturus, Cherry Hill, NJ.TEMA. 1988. Standards, 7th ed., Tubular Exchanger Manufacturers Association, Tarrytown, NY.Yokell, S. 1990. A Working Guide to Shell and Tube Heat Exchangers, McGraw-Hill, New York.© 1999 by CRC Press LLC4-182Section 44.6 Temperature and Heat Transfer MeasurementsRobert J.
MoffatThere are two different kinds of material to consider with respect to experimental methods: the unitoperations of measurement (transducers and their environmental errors) and the strategy of experimentation. This section deals only with the unit operations: transducers, their calibrations, and correctionsfor environmental errors.Temperature MeasurementAn International Practical Temperature Scale (IPTS) has been defined in terms of a set of fixed points(melting points of pure substances) along with a method for interpolating between the fixed points. TheIPTS agrees with the thermodynamic temperature scale within a few degrees Kelvin over most of itsrange.
The IPTS is the basis for all commerce and science, and all calibrations are made with respectto the IPTS temperature. The scale is revised periodically.Accurate calibrations are not enough to ensure accurate data, however. If a sensor has been installedto measure a gas temperature or a surface temperature, any difference between the sensor temperatureand the measurement objective due to heat transfer with the environment of the sensor is an “error.” Inmost temperature-measuring applications, the environmental errors are far larger than the calibrationtolerance on the sensor and must be dealt with just as carefully as the calibration.ThermocouplesAny pair of thermoelectrically dissimilar materials can be used as a thermocouple.
The pair need onlybe joined together at one end and connected to a voltage-measuring instrument at the other to form ausable system. A thermocouple develops its signal in response to the temperature difference from oneend of the pair to the other. The temperature at one end, known as the reference junction end, must beknown accurately before the temperature at the other end can be deduced from the voltage.Thermocouples are the most commonly used electrical output sensors for temperature measurement.With different materials for different ranges, thermocouples have been used from cryogenic temperatures(a few Kelvin) to over 3000 K. In the moderate temperature range, ambient to 1200°C, manufacturer’squoted calibration accuracy can be as good as ±3/8% of reading (referred to 0°C) for precision-gradebase metal thermocouples.
Broader tolerances apply at very high temperature and very low temperatures.Thermocouple signals are DC voltages in the range from a few microvolts to a few tens of microvoltsper degree C. Because of their low signal levels, thermocouple circuits must be protected from groundloops, galvanic effects, and from pickup due to electrostatic or electromagnetic interactions with theirsurroundings. Thermocouples are low-impedance devices. Multiple channels of thermocouples can befed to a single voltage reader using low-noise-level scanners or preamplifiers and electronic multiplexers.The alloys most frequently used for temperature measurement are listed in Table 4.6.1.
These alloyshave been developed, over the years, for the linearity, stability, and reproducibility of their EMF vs.temperature characteristics and for their high-temperature capability.Calibration data for thermocouples are periodically reviewed by the National Institutes of Scienceand Technology based on the then-current IPTS.
Values in Table 4.6.1 illustrate the approximate levelswhich can be expected, and are from the National Bureau of Standards Monograph 125. Maximumtemperatures listed in this table are estimates consistent with a reasonable service lifetime. Allowableatmosphere refers to the composition in contact with the thermoelements themselves. Accuracy estimatesare provided for two levels of precision: standard grade and precision grade where these data are available.Noble metal and refractory metal thermocouples are often used with substitute lead wires, as a costsaving measure. These lead wires, described in Table 4.6.2 are cheaper and easier to handle than thehigh temperature thermocouples. They have the same temperature–EMF characteristics as their primarythermoelements, but only over the range of temperatures the lead wires will usually encounter (up to afew hundred degrees C).
Except for the substitute alloys, thermocouple extension wires have the same© 1999 by CRC Press LLCApplication Characteristics of Some Common Thermocouple AlloysAccuracy, %Max T°FMax T°C507228005000276040003720290028002372230022101800160015401300126018001600750980875400abcdefgAllowableAtmos. (Hot)Inert, H2,vacuumInert, H2,vacuumInert, H2OxidizingbOxidizingbOxidizingbOxidizingb,cOxidizingReducingaReducingReducingANSITypeaColorCodeOutputmV/100°FStandardaPrecisionaTungsten/tungsten 26% rhenium——0.86——Tungsten 5% rhenium/tungsten 26% rhenium——0.76——Tungsten 3% rhenium/tungsten 35% rheniumPlatinum 30% rhodium/platinum 6% rhodiumPlatinum 13% rhodium/platinumPlatinum 10% rhodium/platinumPlatinel II (5355)/Platinel II (7674)Chromel/Alumel,d Tophel/Nial,e Advance T1/T2,fThermo-Kanathal P/NgChromel/constantanIron/constantanCopper/constantan—BRS—K—————Yellow red0.740.430.640.572.202.20—1/21/41/45/84°F, or 3/4%—1/41/41/4—2°F, or 3/8%EJTPurple redWhite redBlue red4.203.002.501/24°F, or 3/4%3/43/82°F, or 3/8%3/8Material NamesHeat and Mass TransferTABLE 4.6.1Per ANSI C96.1 Standard.Avoid contact with carbon, hydrogen, metallic vapors, silica, reducing atmosphere.@ Engelhard Corp.@ Hoskins Mfg.
Co.Wilber B. Driver Co.Driver-Harris Co.The Kanthal Corp.4-183© 1999 by CRC Press LLC4-184TABLE 4.6.2Section 4Substitute Material Extension Wires for ThermocouplesThermocouple MaterialTungsten/tungsten 26%rheniumTungsten 5% rhenium/tungsten26% rheniumTungsten 3% rhenium/tungsten25% rheniumPlatinum/platinum rhodiumPlatinel II-5355/PlatinelII-7674Chromel/Alumel, Tophel/Nial,Advance, ThermokanthalcChromel/constantanIron/constantanCopper/constantanabcdThermocoupleTypeaExtension Wire,TypeaColor for(+) WireColor for(–) WireOverallColor—Alloys 200/226b————Alloys (405/426)bWhiteRedRedb—Alloys (203/225)bWhite/yellowWhite/redYellow/redbS, R—SX, SRP2XdBlackYellowRedRedGreenBlackdKKXYellowRedYellowEJTEXJXTXPurpleWhiteBlueRedRedRedPurpleBlackBlueANSI, except where noted otherwise.Designations affixed by Hoskins Mfg.
Co.Registered trade mark names.Engelhard Mfg. Co.composition as thermocouple wires, differing only in the type of insulation and the accuracy of calibration, which is not held as closely for extension wire as for thermocouple-grade wire.Any instrument capable of reading low DC voltages (on the order of millivolts) with 5 to 10 mVresolution will suffice for temperature measurements. Galvanometric measuring instruments can be used,but, since they draw current, the voltage available at the terminals of the instrument depends not onlyon the voltage output of the thermocouple loop but also on the resistance of the instrument and the looptogether.
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