Heat Transfer Handbook part 92 pdf

BOOKCOMP, Inc. — John Wiley & Sons / Page 905 / 2nd Proofs / Heat Transfer Handbook / Bejan REFERENCES 905 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 [905], (109) Lines: 4550 to 4574 ——— 11.31355pt PgVar ——— Normal Page * PgEnds: PageBreak [905], (109) T temperature gradient total transitional correction factor t tube or number of tubes tube-to-baffle leakage path transverse ts tube sheet tube tube tw number of tubes in one window V volumetric volumetric equivalent diameter void void volume Wf wetted perimeter for friction Wh wetted perimeter for heat transfer W 1 wetted perimeter of one channel w window path wall window we end-space condition wg gross window area wt tubes in window y tube pitch factor 1 inlet 2 inlet or outlet 3 outlet 4 outlet Superscripts m exponent, dimensionless n exponent, dimensionless y exponent, dimensionless REFERENCES Baclic, B. S. (1978). A Simplified Formula for Cross-Flow Heat Exchanger Effectiveness, J. Heat Transfer, 100, 746–747. Baclic, B. S. (1989). 1–2N Shell-and-Tube Exchanger Effectiveness: A Simplified Kraus–Kern Equation, J. Heat Transfer, 111, 181–182. Baclic, B. S. (1990). –N tu Analysis of Complicated Flow Arrangements, in Compact Heat Exchangers, R. K. Shah, A. D. Kraus, and D. Metzger, eds., Hemisphere Publishing, New York, pp. 31–90. Baclic, B. S. (1997). 1–(2N −1) Shell-and-Tube Exchanger Effectiveness: Explicit Equations, Heat Mass Transfer, 33, 163–165. BOOKCOMP, Inc. — John Wiley & Sons / Page 906 / 2nd Proofs / Heat Transfer Handbook / Bejan 906 HEAT EXCHANGERS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 [906], (110) Lines: 4574 to 4620 ——— 4.0pt PgVar ——— Normal Page PgEnds: T E X [906], (110) Baclic, B. S., and Heggs, P. J. (1985). On the Search for New Solutions of the Single-Pass Crossflow Heat Exchanger Problem, Int. J. Heat Mass Transfer, 28, 1965–1976. Barnes, J. F., and Jackson, J. D. (1961). Heat Transfer to Air, Carbon Monoxide and Helium Flowing through Smooth Circular Tubes under Conditions of Large Surface/Gas Temper- ature Ratio, J. Mech. Eng. Sci., 3(4), 303–314. Bejan, A. (1995). Convection Heat Transfer, 2nd ed., Wiley, New York. Bejan, A. (1997). Advanced Engineering Thermodynamics, 2nd ed., Wiley, New York. Bejan, A. (2000). Shape and Structure, from Engineering to Nature, Cambridge University Press, Cambridge. Bejan, A., Tsatsaronis, G., and Moran, M. (1996). Thermal Design and Optimization, Wiley, New York. Bell, K. J. (1963). Final Report of the Cooperative Research Program on Shell-and-Tube Heat Exchangers, Bulletin 5, Engineering Experimental Station, University of Delaware, Newark, DE. Bell, K. J. (1988). Delaware Methodfor Shell SideDesign, in Heat Transfer Equipment Design, R. K. Shah, E. C. Subbarao and R. A. Mashelkar, eds., Hemisphere Publishing, New York, pp. 145–166. Biery, J. C. (1981). Prediction of Heat Transfer Coefficients in Gas Flow Normal to Finned and Smooth Tube Banks, J. Heat Transfer, 103, 705–710. Bowman, R. A., Mueller, A. C., and Nagle, W. M. (1940). Mean Temperature Difference in Design, Trans. ASME, 62, 283–294. Brandt, F., and Wehle, W. (1983). Eine zusammendfassende Darstellung des W ¨ arme ¨ ubergangs Rohrb ¨ undeln mit glatten Rohren und Rippenrohren, VGB-Konferenz. Brauer, H. (1964). Compact Heat Exchangers, Chem. Proc. Eng., 451–458. Briggs, D. E., and Young, E. H. (1963). Convective Heat Transfer and Pressure Drop of Air Flowing across Triangular Pitch Banks of Finned Tubes, Chem. Eng. Prog. Symp. Ser., 59(41), 1–8. Butterworth, D. (1979). The Correlation of Crossflow Pressure Drop Data by Means of the Permeability Concept, Report AERE-R9435, UKAEA, Harwell, Berkshire, England. Chenoweth, J. M. (1990). Final Report of the HTRI/TEMA Joint Committee to Review the Fouling Section of the TEMA Standards, Heat Transfer Eng., 11(1), 73–107. Coppage, J. E.,and London, A. L. (1953). The PeriodFlow Regenerator: A Summary of Design Theory, Trans. ASME, 75, 779–785. Crozier, R., Jr., and Samuels, M. (1977). Mean Temperature Difference in Old Tube Pass Heat Exchangers, J. Heat Transfer, 99, 487–489. Dalle Donne, M., and Bowditch, P. W. (1963). Experimental Local Heat Transfer and Friction Coefficients for Subsonic Laminar, Transitional and Turbulent Flow of Air or Helium in a Tube at High Temperatures, Dragon Project Report 184, Winfirth, Dorchester, Dorset, England. Deissler, R. G. (1951). Analytical Investigation of Fully Developed Laminar Flow in Tubes with Heat Transfer and Fluid Properties Variable along the Radius, NACA-TN-2410. DeLorenzo, B., and Anderson, R. B. (1945). Heat Transfer and Pressure Drop of Liquids in Double Pipe Exchangers, Trans. ASME, 67, 697–703. Devore, A. (1962). Use Nomograms to Speed Exchanger Calculations, Hydrocarbon Process. Pet. Refiner, 41(12), 101–106. BOOKCOMP, Inc. — John Wiley & Sons / Page 907 / 2nd Proofs / Heat Transfer Handbook / Bejan REFERENCES 907 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 [907], (111) Lines: 4620 to 4666 ——— 11.0pt PgVar ——— Normal Page PgEnds: T E X [907], (111) Dittus, F. W., and Boelter, L. M. K. (1930). Heat Transfer in Automobile Radiators of the Tubular Type, Univ. Calif. (Berkeley) Publ. Eng., 2 (13), 443–461; Int. Comm. Heat Mass Transfer, 12 (1985), 3–22. Donohue, D. A. (1949). Heat Transfer and Pressure Drop inHeat Exchangers, Ind. Eng. Chem., 41(11), 2499–2511. Ehlmady, A. H., and Biggs, R. C. (1979). Finned Tube Heat Exchanger: Correlation of Dry Surface Heat Transfer Data, Trans. ASHRAE, 85, 117–123. Fischer, F. K. (1938). Mean Temperature Difference Correction in Multipass Exchangers, Ind. Eng. Chem., 30(4), 377–383. Fraas, A. P. (1989). Heat Exchanger Design, Wiley, New York. Ganguli, A., Tung, S. S., and Taborek, J. (1985). Parametric Study of Air Cooled Heat Ex- changer Finned Tube Geometry. AIChE Symp. Ser., 81(245), 122–128. Gardner, K. A. (1941). Mean Temperature Difference in Multipass Exchangers: Correction Factors with Shell Fluid Unmixed, Ind. Eng. Chem., 33, 1495–1500. Gardner, K. A. (1942). Mean Temperature Difference in an Array of Identical Exchangers, Ind. Eng. Chem., 34, 1083–1087. Gardner, K. A., and Carnavos, T. C. (1960). Thermal Contact Resistance in Finned Tubing, J. Heat Transfer, 82, 279–287. Gianolio, E., and Cuti, F. (1981). Heat Transfer Coefficients and Pressure Drops for Air Coolers with Different Numbers of Rows under Induced and Forced Draft, Heat Transfer Eng., 3(1), 38–46. Gilmour, C. (1952–54). Short Cut to Heat Exchanger Design, Chem. Eng., Parts I–VII. Gnielinski, V. (1976). New Equations for Heat and Mass Transfer in Turbulent Pipe and Channel Flow, Int. Chem. Eng., 16, 359–366. Grant, I. D. R. (1980). Shell-and-Tube Exchangers for Single-Phase Application, in Develop- ments in Heat Exchanger Technology, Vol. 1, Applied Science Publishers, London. Gunter, A. Y., and Shaw, W. A. (1945). A General Correlation of Friction Factors for Various Types of Cross Flow, Trans. ASME, 67, 643. Hausen, H. (1943). Darstellung des W ¨ armeauberganges in R ¨ ohren durch verallgemeinerte Potenzbeziehungen, Z. VDI, 4, 91–95. Hausen, H. (1959). Neue Gleichungen f ¨ ur W ¨ armeaubertragung bei Frier oder erzwungener Str ¨ omung, Allg. Waermetech., 9, 75–79. Hausen, H. (1974). Extended Equation for Heat Transfer in Tubes at Turbulent Flow, Waerme- Stoffuebertrag., 7, 222–225. Hofmann, H. (1976). M ¨ oglichkeiten zur Berechnung des W ¨ arme ¨ ubergangs und des Druckver- lustes vin Rippenrohrb ¨ unden, Luft- Kaeltetech., 12, 136–141. Jameson, S. L. (1945). Tube Spacing in Finned Tube Banks, Trans. ASME, 67, 633–640. Jaw, L. (1964). Temperature Relations in Shell-and-Tube Exchangers Having One-Pass-Split- Flow Shells, J. Heat Transfer, 86C, 408–416. Kakac¸, S. (1991). Boilers, Evaporators and Condensers, Wiley, New York. Kakac¸, S., and Yener, Y. (1994). Convective Heat and Mass Transfer, 2nd ed., CRC Press, Boca Raton, FL. Kakac¸, S., Aung, W., and Viskanta, R. (1985). Natural Convection: Fundamentals and Appli- cations, Hemisphere Publishing, New York. BOOKCOMP, Inc. — John Wiley & Sons / Page 908 / 2nd Proofs / Heat Transfer Handbook / Bejan 908 HEAT EXCHANGERS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 [908], (112) Lines: 4666 to 4710 ——— 2.0pt PgVar ——— Normal Page PgEnds: T E X [908], (112) Kakac¸, S., Shah, R. K., and Aung, W. (1987). Handbook of Single-Phase Convective Heat Transfer, Wiley, New York. Kays, W. M., and Crawford, M. E. (1993). Convective Heat Transfer, 3rd ed., McGraw-Hill, New York. Kays, W. M., and London, A. L. (1984). Compact Heat Exchangers, 3rd ed., McGraw-Hill, New York. Kern, D. Q. (1950). Process Heat Transfer, McGraw-Hill, New York. Kern, D. Q., and Kraus, A. D. (1972). Extended Surface Heat Transfer, McGraw-Hill, New York. Kraus, A. D., and Kern, D. Q. (1965). The Effectiveness of Heat Exchangers with One Shell Pass and Even Numbers of Tube Passes, ASME-65-HT-18, ASME, New York. Kraus, A. D., Aziz, A., and Welty, J. R. (2001). Extended Surface Heat Transfer, Wiley, New York. Kr ¨ oger, D. (1998). Air Cooled Heat Exchangers and Cooling Towers, Tecpress, Uniedal, South Africa. Kumar, H. (1984).The Plate Heat Exchanger, Construction andDesign, Inst. Chem. Eng. Symp. Ser., 86, 1275–1282. Kuntyish, V. B., and Iokhvedor, F. M. (1971). Effect of Relative Interfin Distance on the Thermal Convective Heat Transfer in Finned Tube Bundles and on Augmenting Heat Transfer, Heat Transfer Sov. Res., 3(2), 50–55. Kutateladze, S. S., and Borishanskii, V. M. (1958). A Concise Encyclopedia of Heat Transfer, Oxford; Pergamon Press, Oxford, UK. Li, C Ha. (1987). New Simplified Formula for Crossflow Heat Exchanger Effectiveness, J. Heat Transfer, 109, 521–522. London, A. L., and Seban, R. A. (1942). A Generalization of the Methods of Heat Exchanger Analysis, TR No N tu −1, Mechanical Engineering Department, Stanford University, Stan- ford, CA. London, A. L., and Seban, R. A. (1980). A Generalization of the Methods of Heat Exchanger Analysis, TR No N tu − 1, Int. J. Heat Mass Transfer, 23, 5–16. Mason, J. (1955). Heat Transfer in Cross Flow, Proc. 2nd U.S. National Congress on Applied Mechanics, ASME, New York. Mayinger, F. (1988). Classification and Applications of Two-Phase Flow Heat Exchangers, in Two-Phase Flow Heat Exchangers, S. Kakac¸, A. E. Bergles, and E. O. Fernandes, eds., Kluwer Academic, Dordrecht, The Netherlands. McAdams, W. H. (1954). Heat Transmission, 3rd ed., McGraw-Hill, New York. McQuiston, F. C., and Tree, D. R. (1972). Optimum Space Envelopes of the Finned Tube Heat Transfer Surface, Trans. ASHRAE, 78, 144–148. Mirkovic, Z. (1974). Heat Transfer and Flow Resistance Correlations for Helically Finned and Staggered Tube Banks in Cross Flow Heat Exchangers, in Design and Theory Source Book, N. H. Afgan and E. U. Schl ¨ under, eds., McGraw-Hill, New York. Moody, L. F. (1944). Friction Factors for Pipe Flow, Trans. ASME, 66, 671–678. Mueller, A. C. (1967). New Charts for True Mean Temperature Difference in Heat Exchangers, AIChE Paper 10, 9th National Heat Transfer Conference. Nagle, W. M. (1933). Mean Temperature Difference in Multipass Heat Exchangers, Ind. Eng. Chem., 25, 604–609. BOOKCOMP, Inc. — John Wiley & Sons / Page 909 / 2nd Proofs / Heat Transfer Handbook / Bejan REFERENCES 909 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 [909], (113) Lines: 4710 to 4751 ——— 6.0pt PgVar ——— Normal Page PgEnds: T E X [909], (113) Nir, A. (1991). Heat Transfer and Friction Correlations for Crossflow over Staggered Finned Tube Banks, Heat Transfer Eng., 12(1), 43–50. Nusselt, W. (1911). Der W ¨ arme ¨ ubergang in Kreuzstrom, Verh. Ver. Dtsch. Ing., 55, 2021–2024. Nusselt, W. (1930). The Condensation of Steam on Cooled Surfaces, Verh. Ver. Dtsch. Ing., 60, 541–546. Translated into English by D. Fullerton, Chem. Eng. Fundam., 1(2), 6–9. Oskay, R., and Kakac¸, S. (1973). Effect of Viscosity Variations on Turbulent and Laminar Forced Convection in Pipes, METU J. Pure Appl. Sci., 6, 211–230. Perkins, H. C., and Warsœ-Schmidt, P. (1965). Turbulent Heat and Momentum Transfer for Gases in a Circular Tube at Wall to Bulk Temperature Ratios of Seven, Int. J. Heat Mass Transfer, 8, 1011–1031. Petukhov, B. S. (1970). Heat Transfer and Friction in Turbulent Pipe Flow with Variable Physical Properties, in Advances in Heat Transfer, J. P. Hartnett and T. F. Irvine, eds., Academic Press, New York, pp. 504–514. Petukhov, B. S., and Popov, V. N. (1963). Theoretical Calculation of Heat Exchange and Frictional Resistance in Turbulent Flow in Tubes of Incompressible Fluid with Variable Physical Properties, High Temp., 1(1), 69–73. Pohlhausen, E. (1921). Der W ¨ armeaustausch Zwischen Festen K ¨ orpen and Flo ¨ ssigkeiten mit kleiner Reibung und kleiner W ¨ armeleitung, Z. Angew. Math. Mech., 1, 115–121. Robinson, K. K., and Briggs, D. E. (1966). Pressure Drop of Air Flowing across Triangular Pitch Banks of Finned Tubes, Chem. Eng. Prog. Symp. Ser., 62(64), 177–183. Roetzel, W., and Spang, B. (1987). Analytisches Verfahren zur thermischen Berechnungen mehrg ¨ angiger Rohrb ¨ undel-w ¨ arme ¨ ubertrager, Fortschr. Ber. VDI, 19(18). Rogers, D. G. (1980). Forced Convection Heat Transfer in a Single Phase Flow of Newtonian Fluid in a Circular Pipe, CSIR Rep. CENG 322, CSIR, Pretoria, South Africa. Saunders, E. A. D. (1988). Heat Exchangers: Selection, Design and Construction, Longman Scientific and Technical, Harlow, Essex, England. Schindler, D. L., and Bates, H. T. (1960). True Temperature Difference in a 1–2 Divided Flow Heat Exchanger, Chem. Eng. Prog. Symp. Ser., 30, 56, 203–206. Schmidt, T. E. (1963). W ¨ arme ¨ ubergang an Rippenrohe and Berechnung von Rohrb ¨ undelnev ¨ ar- meanstauschenern, Kaeltetechnik, 15(4), 370–376. Schulenberg, F. J. (1965). Wahl der Bezugsl ¨ ange zur DarstellengunW ¨ arm ¨ ubergang und Druck- verlust in W ¨ armetauschern, Chem. Ing. Tech., 37, 431. Sekulic, D. P., Shah, R. K., and Pignotti, A. (1999). A Review of Solution Methods for Deter- mining Effectiveness–N tu Relationships of Heat Exchangers with Complex Flow Arrange- ments, Appl. Mech. Rev., 52(3), 97–117. Shah, R. K. (1981). Classification of Heat Exchangers, in Heat Exchangers: Thermal- Hydraulic Fundamentals and Design, S. Kakac¸, A. E. Bergles, and F. Mayinger, eds., Hemi- sphere Publishing, New York. Shah, R. K., and Bhatti, M.S. (1987). LaminarConvective Heat Transfer in Ducts, in Handbook of Single Phase Convective Heat Transfer, Wiley, New York, Chap. 3. Shah, R. K., and Focke, W. W. (1988). Plate Heat Exchangers and Their Design Theory, in Heat Transfer Equipment Design, R. K. Shah, E. C. Subbarao, and R. A. Mashelkar, eds., Hemisphere Publishing, New York, pp. 145–166. Shah, R. K., and London, A. L. (1978). Laminar Flow Forced Connection in Ducts, Academic Press, New York. BOOKCOMP, Inc. — John Wiley & Sons / Page 910 / 2nd Proofs / Heat Transfer Handbook / Bejan 910 HEAT EXCHANGERS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 [910], (114) Lines: 4751 to 4796 ——— 0.0pt PgVar ——— Custom Page (5.0pt) PgEnds: T E X [910], (114) Shlykov, Y. P., and Ganin, Y. E. (1964). Thermal Resistance of Metallic Contacts, Int. J. Heat Mass Transfer, 7, 921–926. Sieder, E. N., and Tate, G. E. (1936). Heat Transfer and Pressure Drop of Liquids in Tubes, Ind. Eng. Chem., 28, 1429–1436. Sleicher, C. A., and Rouse, M. W. (1975). A Convenient Correlation for Heat Transfer to Constant and Variable Property Fluids in Turbulent Pipe Flow, Int. J. Heat Mass Transfer, 18, 677–684. Somerscales, E. F. C., and Knudsen, J. G. (1981). Fouling of Heat Transfer Equipment, Hemi- sphere Publishing, New York. Spang, B. 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Heat Transfer and Friction for Laminar Flow of Helium and Carbon Dioxide in a Circular Tube at High Heating Rate, Int. J. Heat Mass Transfer, 9, 1291–1295. Yang, K. T. (1962). Laminar Forced Convection of Liquids in Tubes with Variable Viscosity, J. Heat Transfer, 8, 353–362. Yovanovich, M. M. (1981). New Contact and Gap Conductance Correlations for Conforming Rough Surfaces, AIAA Paper 1164, 16th AIAA Thermophysics Conference, Palo Alto, CA. Zhukauskas, A. A. (1974). Investigation of Heat Transfer in Different Arrangements of Heat Exchanger Surfaces, Teploenergetika, 21(5), 24. Zhukauskas, A. A. (1987). Convective Heat Transfer in Cross Flow, in Handbook of Single- Phase Convective Heat Transfer, S. Kakac¸, R. K. Shah, and W. Aung, eds., Wiley, New York. BOOKCOMP, Inc. — John Wiley & Sons / Page 913 / 2nd Proofs / Heat Transfer Handbook / Bejan 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 [First Page] [913], (1) Lines: 0 to 78 ——— 8.29108pt PgVar ——— Normal Page PgEnds: T E X [913], (1) CHAPTER 12 Experimental Methods JOS ´ EL.LAGE Mechanical Engineering Department Southern Methodist University Dallas, Texas 12.1 Fundamentals 12.1.1 Measurement 12.1.2 Sensing 12.1.3 Calibration 12.1.4 Readability 12.2 Measurement error 12.2.1 Uncertainty: bias and precision errors 12.2.2 Mean and deviation 12.2.3 Error distribution 12.2.4 Chauvenet’s criterion and the chi-square test 12.3 Calculation error 12.4 Curve fitting 12.5 Equipment 12.5.1 Glass thermometers 12.5.2 Thermocouples 12.5.3 Resistance temperature detectors 12.5.4 Liquid crystals 12.5.5 Pyrometers 12.5.6 Heat flow meters Nomenclature References 12.1 FUNDAMENTALS 12.1.1 Measurement Measurement is one of the most important activities in science and engineering. Validationofnew theories, determination of material property values, classification of new materials, performance evaluation of new and existing devices, and monitoring and control ofexisting and new processes are activities that depend on measurements. 913 BOOKCOMP, Inc. — John Wiley & Sons / Page 914 / 2nd Proofs / Heat Transfer Handbook / Bejan 914 EXPERIMENTAL METHODS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 [914], (2) Lines: 78 to 93 ——— 0.0pt PgVar ——— Normal Page PgEnds: T E X [914], (2) Measurement, or measuring, is also the most important part of an experiment. Measuring is not absolute, as it does not define a quantity (standard) to be measured. Measuring is a relative effort and is made to compare and to evaluate. To be indepen- dent, a comparison requires a measure, a standard unit. The art of measuring is atleast as old as humanity itself. The human body performs measurements all the time. One ofthemost basic quantities continuously measured by the human body is the environment temperature. Feeling hot or cold is a consequence of this measuring. Although not descriptive (not quantified with a parameter such as temperature), the natural measuring of the environmental temperature by the human body is nevertheless a relative process. This process is based on a comparison of the environmental temperature with a certain standard, in this case the temperature at which the body feels neither hot nor cold—the null point of human thermal control. In heat transfer, temperature and heat flow are unquestionably the most important quantities to be measured. Other quantities of interest to heat transfer include fluid speed, pressure (force), mechanical stress, electric current, voltage, length, surface area, volume, and displacement. In this chapter the focus is on temperature and heat flow measurements. General measuring concepts such as sensitivity, hysteresis, calibration, accuracy, and readability are presented first. Then the discussion turns to statistical concepts such as mean, deviation, standard deviation, normal distribution, Chauvenet’s crite- rion, and the chi-square test, related to the determination of precision, bias error, and measuring uncertainty. The final section of this chapteris devoted to a brief discussion of some common instruments for measuring temperature or heat flow. 12.1.2 Sensing Among the two possible alternatives for sensing devices, the most common are the contact sensing devices such as thermocouples that measure by physical contact. In general, contact sensing devices are rugged, economical, relatively accurate, and easy to use. Disadvantages commonly associated with contact sensing devices include susceptibility to wear (e.g., breaking of thermocouple junction). They also require accessibility for physical contact. Because of the contact nature of these devices, they tend to interfere with the medium where measurement is to be taken, frequently affecting the state and the value of the quantity to be measured. The last disadvan- tage can be a serious problem. For instance, the conductive wires of a thermocouple will always provide a heat path when in contact with the medium where tempera- ture is to be measured. This heat path can modify the state of the medium where temperature is to be measured by adding energy to, or extracting energy from, the medium. Another alternative is a non-contact sensing device such as an infrared sensor or pyrometer. This type of sensing device is advantageous because it does not require physical contact; that is, the measurement can be made remotely. They are more convenient than the contact sensing devices for measuring quantities from surfaces in movement without contact with the surface. In addition, they do not interfere with the quantities being measured (no heat sink/source), are generally faster in BOOKCOMP, Inc. — John Wiley & Sons / Page 915 / 2nd Proofs / Heat Transfer Handbook / Bejan FUNDAMENTALS 915 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 [915], (3) Lines: 93 to 113 ——— 0.0pt PgVar ——— Normal Page PgEnds: T E X [915], (3) obtaining the measurement (because of the smaller thermal inertia), andare lessprone to wear. Some disadvantages include cost (generally more expensive than contact sensing devices), difficult calibration, and the effect of the environment on the measurement (e.g., dust and smoke in the field of view will affect the measurement of surface temperature using an infrared detector). Most measuring devices do not measure the quantity of interest directly. In the case of thermocouples, for instance, what is measured is a voltage across the open- circuit terminals of the thermocouple wires. This voltage emanates from the electrical effect that temperature has on the electrical potential (EMF) along two distinct but connected conductive wires. By knowing how the voltage read by the electrical instrument relates to the temperature at the connectionof thetwo thermocouple wires, the voltage can be translated into temperature. The same is true in relation to the more common mercury thermometers. The mercury thermometer does not measure the temperature directly but the variation caused by the temperature on the volume of mercury inside the thermometer. The scale printed on the thermometer glass, translating the mercury volume variation into temperature, allows the measurement of volume to be translated into temperature. These are types of indirect measurement. The sensitivity of measuring devices is a very important characteristic for measur- ing. Sensitivity can be understood as the relation between cause and consequence. Thermocouples have sensitivities listed in mV/°C because the consequence of mea- suring an increase in temperature is a change in voltage across the thermocouple terminals. Suppose that a thermocouple for measuring a temperature variation of 60°C is needed, and a multimeter having a scale from 0 to 100 mV is available. Unless the thermocouple has a sensitivity S smaller than 100 mV per 60°C, or equivalently, S<1.67 mV/°C, measurement cannot be performed within the available voltage range. It should be kept in mind that small quantities are, in general, more difficult and expensive to be measured; that is, thermocouples with small sensitivities tend to be more costly. The sensitivity of an instrument might not be uniform along the entire range avail- able for measurement. In such cases, translation from the quantity actually measured to the quantity of interest is not linear. Moreover, the sensitivity of an instrument might depend on the direction of variation of the quantity being measured. For in- stance, when increasing the temperature, a certain thermocouple device might have a uniform sensitivity equal to 1.5 mV/°C. But when the temperature to be measured de- creases, the sensitivity might be nonuniform; for example S(T ) = 1.5α(T ) (mV/°C), where α(T ) is a certain function of temperature T . In this case, the temperature value corresponding to a certain voltage when the temperature increases will differ from the temperature value at the same measured voltage when the temperature decreases. The instrument is then said to exhibit hysteresis. Figure 12.1 demonstrates this phe- nomenon considering a thermocouple with uniform sensitivity equal to 1.5 mV/°C when measuring an increase in temperature (continuous line) from 0°C to 20°C, and nonuniform sensitivity equal to 1.5(2.33 −0.133T +0.0033T 2 ) mV/°C when mea- suring a decrease in temperature from 20°C to 0°C. . Convective Heat Transfer in Finned Tube Bundles and on Augmenting Heat Transfer, Heat Transfer Sov. Res., 3(2), 50–55. Kutateladze, S. S., and Borishanskii, V. M. (1958). A Concise Encyclopedia of Heat. (1930). Heat Transfer in Automobile Radiators of the Tubular Type, Univ. Calif. (Berkeley) Publ. Eng., 2 (13), 443–461; Int. Comm. Heat Mass Transfer, 12 (1985), 3–22. Donohue, D. A. (1949). Heat Transfer. Effectiveness: Explicit Equations, Heat Mass Transfer, 33, 163–165. BOOKCOMP, Inc. — John Wiley & Sons / Page 906 / 2nd Proofs / Heat Transfer Handbook / Bejan 906 HEAT EXCHANGERS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 [906],