Document YrZmLJykJ9Ra2Q8ryM2pRL9wD

28 CHAPTER 2 1965 Guide Arid Data Book also necessary to obtain the voltage rating at the design operating point, as selected from data similar to that shown in fig. 12. This is necessary in order to develop a power supply. In addition to the heat transfer and the thermoelectric as pects of device design, there are mechanical problems to' be considered. Thermoelectric material and joints between it and tiie copper straps are relatively fragile. Since a-module is strongest in compression, it is common to mechanically dump the module between heat transfer surfaces. Stresses developed as a result ofthermal expansion and contraction in a module must be minimized by proper design.- This'is specially! true if the heat pumping cycle is reversed, resulting in a reversal of stresses. Most thermoelectric devices operate with relatively high current and low voltage. For those designed to be powered from a 120 volt alternating-current source, a rectifier.unit is necessary. The rectifier unit can consist of a transformer, two silicon diodes for'full wave rectification, and a choke to filter the'output. Ripple in the direct-current output should be 10 percent or less, to minimize loss'in performance which1results if pure direct current is not used. A summary of this section shows that a complete thermo-: electric system is usually composed of three major areas: heat transfer, thermoelectric modules^ and power supply.' In order to design a device of optimum performance and minimum cost, it is necessary to develop ail three of they areas con currently. For example, a high current, design .will reduce the number of couples, but will probably.-increase the.power supply cost. Large heat transfer surfaces will reduce the couple temperature difference and, therefore, the number, of couples, but will increase the cost and weight of the surfaces. Q. " rate of heat emission at hot junction, watts.' - " maximum rate of beat absorption at cold junction watts. .>j R " electrical resistance of couple legs, ohms. . AT1___h muTimnm a T. AT m temperature difference between the hot and cold junctions of materials n and p, (7\ -- 71), C deg. T " absolute temperature, Kelvin. Tt = hot junction temperature, Kelvin. T. " cold junction temperature, Kelvin. V volts generated by couple. ! -, IF " total electrical power supplied to couple, watts. Z - figure of merit tA/WK^ + VKpO* '- = o*/0iK). ' *' : : REFERENCES ' 1 E. Altenkirch: PkysBudiache Zeitschrifl, It, 920, 1911/ :. ' A. F. Ioffe: Semiconductor thermoelements end thermoelectric cooling (Infoseorch Ltd., London 1957)., * R. L. Richhorn: Thermoelectric refrigeration (Refrigerating Engineering, June 1958,'p. 31). 4 L- A. Staebler: A primer; of thermoelectric refrigeration (ASHRAEJournal August 1959, p. 60). * A. F. Phillips: Thermoelectric air conditioning-aod refrigera tion for submarines {International Institute, of Refrigeration, August, 1962, Meeting in Washington, D. C.).1 * C. Terbush and J. Meess: Thermoelectric -Water Coolers (IEEE Appliance: Technical Conference, May. 1963,- Chicago' Illinois). ( ' D. W. Scofield, P. F. Taylor, and L. A. Staebler:. A.com parative study of the manufacturing costs of thermoelectric and mechanical refrigerating systems (ASHRAE Journal; July lOAO -- VTk LETTBJ SYMBOLS USED IN CHAPTER 2 a " absolute Seebeck coefficient, volts per C deg.. as " relative Seebeck coefficient (the difference between'the absolute Seebeck coefficients for materials A and 'B, volts per C deg). 1 :. 1 <u " absolute Seebeck coefficient of material in which con1 ` duction occurs principally due to electrons. ' a/ = absolute Seebeck coefficient of material in which con duction occurs principally due to holes. =* relative Seebeck coefficients, the difference' between materials of n and p type. '. r p = resistivity, ohm centimeter. -U *. Or' " resistivities of n and p, ohms eeotimeta-.' fit * contact resistance, ohms square centimeter. > " total electrical conduction, reciprocal ohms centimeter. rAB = relative Peltier coefficient for materials'A and B, volts. d TM coefficient of performance.' t; " maximum coefficient of performance.' r- TM coefficient of performance under conditions of maxi mum heat pumping. A, Ap TM cross sectional area of couple l*p, square centimeters. C -- thermal conductance of couple legs, watts per C deg. C, Cp " thermal conductance of couple l*g, watts per C deg. - Aopen circuit voltage developed, volts.- i / " direct current flowing through couple, amperes. R = thermal conductivity, watts per (square centimeter) (Centigrade degree per centimeter). ' A., Kp " thermal conductivities of materials n and p, watts per (square centimeter) (Centigrade degree per centi meter). Lb, L, - length of couple legs, centimetera. Q " rate of heat emission or absorption in unit time, watts. QrTM rate of heat absorption at cold junction/ iratta. BIBLIOGRAPHY. I. Cadoff and E. Miller (Editors): Thermoelectric Materials and Devices (Reinhold Publishing Corporation, New York, 1960). "< Egli (Editor): Thermoelectricity (John Wiley 4 Sons.Inc^ .ftew York, I960)-.'. R. Eichbom: A Review of Thermoelectric Refrigeration (Proceedings of the IEEE, May 1963, p. 721). _ T. .Harman: Special techniques for measurement of thermo electric properties (Journal of Applied Physics, Vol. 29, 4958, p. 1373). ^tj . R. Hetkes and R. Ure: Thermoelectricity: 'Science and Engineer~ ing (Interedence Publisher*, New York, 1961). A. Ioffe: The revival of.thermoelectricity (Scientific American. November 1958). - . , -. ; . . . F. Jaumot: Thermoelectric effects (Proceedings of'the Institute qf Radio Engineers, Vol. 46, March' 1958). J. Kaye, and J. Welsh (Editors): Direct Conversion of Heat to Electricity (John Wiley and Sons, New York I960). E. H. Lougher: Measurement of parameters in the thermo electric figure of merit (Electrical Engineering, VoL 79, 1960, p. 358). 'i- A. Newton: Thermoelectric systems of cooling and heating (Heating, Piping and Air Conditioning, November, 1963, p. 152). E. S. Rittner: On the theory of the Peltier heat pump (Journal of Applied Physics, VoV.30, 1959, p. 702). F. Roei, B. Abeles, and R. Jensen: Materialsfor thermoelectric refrigeration (Journal of Physics and Chemistry of SoLidr. Voi. 10. 1959, p. 191). ......... R. G. Sickert: A thermoelectric refrigerating'system for sub marines (ElectricalEngineering, Vol. 79,1960, p/364). W/Stoecker and J. B. Chaddock: Transient performance of a thermoelectric refrigerator under step-current control (ASHRAE Journal, September 1963, p. 61). .. R. Stratton: On the elementary theory of thermoelectric phenomena (British Journal of Applied Physics, VoL 8, 1957, p. 315). * D. Wright: Thermoelectric refrigeration (New Scientist, Feb ruary 12, 1959).^,- W. L. Wright: Thermoelectric refrigeration (Electrical Erim- neenng, Vol. 79, 1960,:p/ 380). CHAPTER 3 PSYCHROMETRICS Thermodynamic Properties of Moist Air, Thermodynamic Properties of Water at Saturation, Degree of Saturation, Thermodynamic Wet-Bulb Temperature, Perfect Gas Relations, U. S. Standard Atmosphere, Psychrometric Charts, Typical A/'r-Condrf/on/ng Processes HE earth's atmosphere is a mixture of several gases in 1(F) -- Fahrenheit temperature defined in terms of absolute Tcluding nitrogen, oxygen, argon, water vapor, and traces temperature T by the relation, of others. Atmospheric air generally-contains various kinds T - t + 459.67 of particulate matter. Often, additional vapors are present IF, -- humidity ratio at saturation. Saturation is the condi Before one may consider thermodynamic properties of aimospheric'air, the substance must be precisely defined. The working 6nbpt*nc< in air-conditioning problems is called moist qtrl Moist air is by definition a binary mixture of dry air tion at which the vapor phase (moist air) may exist in equi librium with a condensed phase (liquid or solid) at the given temperature and pressure (standard atmospheric pressure t in the of Table 1). At riven values of temperature and pres sure, the humidity ratio W can have any value from zero to W,.: and water vapor. . ** specific volume of dry air, cubic feet per pound. Dry dir has the following exact composition, adopted by the International Joint Committee on Psychrometric Data,* and expressed in mol fractions: oxygen, 0.2095; nitrogen, 0.7809; argon,.0.0093; carbon dioxide, 0.0003. Traces of other gases are neglected. The molecular weight of dry air is 28.966. Moist air may contain varying amounts of water vapor ranging from zero (dry air) to that for saturation. Moist air is saturated when it can coexist in neutral equilibrium with a p, -- r,, the difference between the volume of moist air at saturation, per pound of dry air, and the specific volume of the dry air itself, cubic feet per pound of dry air. p, -- volume of moist air at saturation per pound of dry air, cubic feet per pouod of dry sir. A specific enthalpy of dry air, Btu per pound of dry air. The specific enthalpy of dry air has been assigned the value zero at 0 F, standard atmospheric pressure. The energy unit Btu is related to the foot-pound by definition, os follows: 1 condensed phase (liquid or solid) presenting a flat surface to it. THERMODYNAMIC PROPERTIES OF MOIST AIR Btu - 778.3 ft4b. Ab, = A, -- A., the difference between the enthalpy of moist' air at saturation, per pound of dry air, and the specific enthalpyof the dry air itself, Btu per pound of dry air. A. -- enthalpy of moist air at saturation per pouod of dry air, Various thermodynamic, properties are associated with moist air. The thermodynamic state of moist air is established by the pressure and two other independent properties. Pres sure may be taken as barometric pressure in practical calcu Btu per pound of dry air. Sm " specific entropy of dry air, Btu per (pound) (Fahren heit degree absolute). It will be noticed that the specific en tropy of dry air has been assigned the value zero at 0 F and1 standard atmospheric pressure. lations. Humidity ratio W is the mass of water associated with unit mass of dry air, pounds of water per pound of dry air. Degree 'of saturation p is the ratio of the humidity ratio of moist air to the humidity ratio at saturation for the same -- i,, the difference between the entropy of moist air at saturation per pound of dry air, and the spemfic entropy, of the dry air itself, Btu per (pound of dry air) (Fahrenheit; degree absolute). , -- entropy of moist air at saturation per pound of dry air,. Btu per (pound of dry air) (Fahrenheit degree, absolute). temperature and pressure. A -- specific enthalpy of condensed water (liquid or solid) Relative humidity tf> is the ratio of the mol-fraction of water vapor in moist air to the mol-fraction of water vapor in saturated moist air at the same temperature and pressure. Dry-bulb temperature t is the true temperature of moist air at rest. in equilibrium with saturated air at standard atmospheric Eressure, Btu per pound of water. The specific enthalpy of quid watei has been assigned the value zero at 32 F, satura tion pressure (0.088586 psia). Note that A. is greater than the steam-table enthalpy of. saturated pure condensed phase by the amount of the enthalpy' Dew-point temperature U is the saturation temperature cor responding to the humidity ratio and pressure of an existing moist air state. Thermodynamic wet-bulb temperature f* is the temperature at which water (liquid or solid), by evaporating into moist air, increase corresponding to the pressure increase from Batura-, tion pressure to 1 atmosphere, plus influences from the pres ence of air. Sw ~ specific entropy of condensed water (liquid or solid) in equilibrium with saturated air, Btu per (pound water) (Fahrenheit degree absolute), i. differs from the entropy of pure can bring the air to saturation adiabatically at the same water at saturation pressure, similarly to A*.. > temperature. In an ASHVE cooperative research project at the Towne Scientific School, University of Pennsylvania, Goff and Gratch* formulated accurate thermodynamic properties of p, - vapor pressure of water in saturated moist air, pounds per square inch or inches of mercury. This pressure, differs from the saturation pressure of pure water because of the pres ence of the air. moist air. Table 1 shows various thermodynamic properties for moist air as determined by Goff and Gratch for standard atmospheric pressure (29.921 in. Hg) and for temperatures THERMODYNAMIC PROPERTIES OF WATER AT SATURATION. from-160 F to 200 F. Table 2 shows thermodynamic properties of water at' Table 1 contains 15 columns of figures. Brief explanations of saturation for temperatures from --160 to 212 F. A detailed^ the data in each column arc given in the following paragraphs. explanation of methods employed in constructing Table 2 is Ttw cmerml tmpooftQuIity fa* Uiif chapter b --tinned to TC LS, Pryehio- given in a paper by Goff and Gratch.* Symbols used in Table 2n follow standard steam-table nomenclature.' 29