UGH U N I V E R S I T YSTUDIED Science and technology Series, No. !TABLES OF THE P R O P E R T I E S OFAQUA-AMMONIA SOLUTIONSPart 1 of The Thermodynamics of Absorption RefrigerationBUIIOESS H. JENNINGS, . . , M.S.Associate Professor of Mechanical EngineeringandFRANCIS 1\ SHANNON, B.S. in M.S., M.S.Formerly 0. Kemble Baldwin Hcscanli Fellow'h \*1fe:.\ws ,«*t v*Reprinted from Refrigerating EngineeringLEHIGH UNIVERSITYBETHLEHEM, PENNSYLVANIA938

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231677The ThermodynamicsOf Absorption RefrigerationPart 1. Tablet of the Propertie*of Aqua-AmmoniaSolutiontBy Burgess H. Jennings1 andFrancis P. Shannon2HESE tables of aqua-ammonia make available comTpleteand accessible tabulations, in the English language and in customary engineering units, of the properties of aqua-ammonia solutions. The richest sources ofbasic data in this field are found in German literature,although some experimental work has also been done inthe United States, England and France. The problemfaced by the authors was one of first critically evaluatingthe various sources of data, then of correlating and preparing the best of these data for interpolation (and extrapolation), and finally of preparing the tables.Nobasic experimental work was done by the authors. Thepreviously published work of A. A. Berestneft* (1) andA. B. Stickney (2) in this country, who prepared chartsand tabulations, should be mentioned, as they were thefirst to make use of the recent German data in this field.The form selected for the tables gives as arguments;pressure in pounds per square inch absolute as ordinateand weight concentration of the ammonia in the liquid asabscissa. For each value of pressure and of weight concentration, there are tabulated:t temperature in *F., at which a solution of the concentration given is saturated, i.e., boils under thepressure in question.lit enthalpy (total-heat) Btu. per lb., of the saturatedliquid aqua.It. enthalpy (total-heat) Btu. per lb., of saturated vapor in equilibrium with above mentioned liquid.x. weightpercentageammonia concentrationofsaturated vapor in equilibrium with the above mentioned liquid. Thus, if the liquid were heated, vaporof this concentration (x.) would start to rise fromit and each lb. of vapor generated would have anenthalpy of magnitude h.The enthalpy (total-heat) function, of such wide utilitywith simple fluids, can also be applied for fluid mixturesas in this case of ammonia and water. Thermodynamically,enthalpy is what is known as a point function, i.e., themagnitude of the value for a given condition is independent of the path by which the fluid is brought to thatcondition. In the case of liquid, ammonia-water mixtures,the heat of solution has been considered in computingthe enthalpy values. In the case of the vapor mixturesthe heat of mixing is considered zero. This is very closelytrue with vapors free from liquid. The datum for enthalpy computations was taken as 32" F. Thus enthalpyreadings for anhydrous ammonia from the Bureau of* Presented at the JJrd Annual Meeting- of The American Society ofRefrigerating Engineer*. New York. N. V. Jan. 2e . 193S. Copyrightprivileges held by RirticrisTiNG KNCINBIIINC.1Associate Prof, of Mechanical Engineering. Lehigh University.* Refrigerating Engineer. I'ottsville Shops, Philadelphia sad ReadingCoal and Iron Company, formerly C. Kenible Baldwin Research Fellow at Lehigh University.Copyright. 193 , The American Society of Refrigerating Engineers.Standards Ammonia Tables (16), which use a datum of—40" F., must have 77.9 subtracted from them to permit,their use with these aqua tables. However, as the aquatables are rather complete in themselves the use of theBureau of Standard Tables in absorption refrigerationshould seldom be necessary. In deciding the question of adatum point temperature, both steam-water with its32" F. datum and ammonia with the customary —40 F.datum were considered for setting the temperature, as wasalso the possibility of using a —40* F. datum for theammonia phase and a separate 32" F. datum for thewater-steam phase. The single 32 F. datum appearedpreferable and was adopted. Thus the enthalpy of purewater at 32 F. and of anhydrous ammonia at 32 F.are each taken as essentially zero for these tables.Enthalpy values for sub-cooled liquid can also be readfrom these tables. Sub-cooled liquid is liquid at a temperature lower than that at which it is saturated under itssuper-imposed pressure. Thus liquid at 181.4* F. with aconcentration of 32 per cent has an h, of 63.8 Btu. per lb. atits saturation pressure of 100 lb. per sq. in. or in subcooled state at any higher pressure such as 150 lb. persq. in. Sub-cooled liquid frequently occurs in practice.This constancy of values for sub-cooled liquids disregardsthe slight magnitude of the so-called feed-pump work.A separate table (Table 2) for specific volume values ofsaturated or sub-cooled aqua is also presented. A chartshowing the freezing point temperatures of ammoniawater solutions appears as Fig. 1. For convenience inchanging from molal to weight concentration, or viceversa, Fig. 2 has been prepared. The molecular weight ofN'H. being 17 and of H.0 18, the data of Fig. 2 give forexample at .36 molal, .36 — .0131 — .3469 wt. and at.32 wt., .32 -f .0126 .3326 molal.The P, T, X relations of the ammonia-water system,under conditions prevalent in the absorption cycle, havebeen studied to some extent by several investigators (3)(4). However, the recent work of WUcherer (5) offersthe most extensive and precise set of data on these relations. The investigations of Zinner (6), corroborated bythose of Baud and Gay (7), provide quite complete datafor computing the enthalpy of the liquid phase. Hitherto this function had been derived entirely from the verylimited data of H. Mollier ( 8 ) . Because of the comprehensive range and the thermodynamic consistency of thedata by WUcherer and Zinner ( 9 ) , these sources wereused exclusively in these tabulations for the P, T, Xrelations of the liquid and vapor phases and for the enthalpies of the liquid phase. The enthalpy of the vaporphase was calculated on the accepted assumptions thatthe vapor phase follows Dalton's law of partial pressures, and that there is no heat of gaseou? solution (10).The properties of the pure components were taken fromthe Bureau of Standards Ammonia Tables (16) andKeenan's Steam Tables (15).Reprinted from the May, 1938, itsue of Refrigerating Engineering%

temperature for a given liquid concentration, the equilibrium vapor—concentration was calculated. The 100final constant pressure lines in English units were then readily obtainedby standard multiple difference interpolation formulae. 50The liquid phase enthalpies were—derived from the data of Zinner (6)by exactly the same method used for0the P, T, X relations, namely, suc\— — cessive interpolation with multiple\difference formulae.\-50The enthalpy of the vapor phase\SOLU T I O Nwas calculated in the following manI CE*.« »OLUT IONner. The partial pressure of each-I0Ocomponentis considered proportional\"" J v - -—, - - S O L I D N H s to the molal concentration of that4 SOL. H HsOisoc '/* 8. SOL.component in the vapor. The enthalpy, using the weight concentraNHyH,C2NH H 0 SOLID NHj1 tions, of these components, is then- -—1found from the appropriate anhydrous ammonia or steam tables asi0A0J3507060809J0(z0()superheated vapor existing at theWEIGHT «* NH.calculated partial pressures andFIG. 1. AQUA AMMONIA FREEZING POINT CURVE.equilibrium temperatures. The enthalpy of the aqua ammonia vaporis then considered the sum of that of the two comThe values for the specific volume of the liquid phaseponents. These calculations were made for all key isobarswere derived from several sources (7) (11) (12) (13)and subdivided into final pressure intervals by interby a method to be described later. The freezing for aqua ammonia were taken from the CriticalTables (14).To calculate values of specific volume of the liquidThe P, T, X relations of the liquid phase were taken phase under conditions found in the absorption cycle, ituncritically from the data by WUcherer and are presentedwas necessary to extrapolate considerably from knownessentially in the form used by him, namely with P and Xdata. Use was made of a basic thermodynamic relationas the arguments. This was done to minimize the labor developed by V. Fischer (9), which states that the molaland possible error of interpolation. Constant pressuremixture-contraction (the molal volume of the componentslines were selected at close intervalsfrom WUcherer data and interpolatedto smaller sub-intervals of concentration by standard multiple differenceinterpolation formulae. These formu014lae were carried to the point thatfurther computation had no effect onthe second decimal place. The "second\\/012differences" of the interpolation meth*ods were usually adequate for this/\degree of precision. The resulting isotbars, expressed in metric atmospheres,y OIO/\zwere then sub-divided into pressure/\a:intervals in lb. per sq. in. absolute byu.u.similar interpolation methods. From/\a.008the resulting data, many arbitrarilyzo\1selected values were checked againstithe original data by bivalent interpo\Z.006lation. The vapor concentrations inIdUzequilibrium with the liquid phase forothe above key isobars had to be calu.004culated by another method, becausethe rapid curvature of their path is/\too severe for usual interpolation/-A002methods. The method used was todeterriiine the deviation between thevapor concentration, as calculated byWEIGHT CONCEN r R A T I O NRaoult's Law, and the vapor concen1X1 «15J .000 *sV13ctration of WUcherer (5). This devia T**.MOLA