Document nrjDzZdrQb3mD2NXEOyvrX3z

14 CHAPTER 3 1960 Guide mixing or separating sir and water vapor at fixed constituent states, and neglecting the solubility of air in water and ice, the fluid enthalpy magnitude can be written as where Osh = Gi(K + Wh.) + Lhf + Shi (5) h, " enthalpy of dry air, Btu per pound. h, -- enthalpy of water vapor, Btu per pound. IF " humidity ratio, pounds of water vapor per pound of dry air. hf -- enthalpy of liquid water, Btu per poundhi -- enthalpy of ice, Btu per pound. l> a flow rate of liquid water in the mixture, pounds per hour. 8 = Sow rate of ice in the mixture, pounds per hour. Fig. 1.... Energy Change between Two Sections of a Steady-Flow System If there should be more than one inflow or outflow stream, Equations 3 or 4 are written as a summation over all streams involved. The corresponding steady-flow equation for the conserva tion of mass is G*(l + IF,) + Lx + Si = (&0 + IF.) + Lt + St (6) As before, it is essential that mass conservation be summed over all entering and leaving streams. The case of water being deposited or picked up of a constant rate within the external envelope of a system can be handled by adding such a mass-rate tom to Equation 6. The corre sponding change must be made in the mass rate factors of Equations 3, 4, and 5 for energy conservation; mid further in this case any heat or work exchanges with the surroundings associated with such storage already are included in the broad definitions of (Qt and ,W,. There are two generally significant observations pertaining to the preceding formulations, namely: 1. Enthalpy is the only fluid property entering steady-flow or constant-pressure nonflow processes. 2. The composition of the. fluid introduces the mixture ra tios, W, L/G, and 8/G. The vapor to-air mixture ratio W is called humidity ratio in air-conditioning problems. A process of some air-conditioning importance involves compression or expansion. In such a process entropy becomes a property of importance. THERMODYNAMIC PROPERTIES OF MOIST AIR Air and water are the working substances involved in the . usual air-conditioning processes. The thermodynamic proper ties of liquid and solid water are well known, and hence will not be discussed in detail except to point out that the solu bility of air in phases of liquid water and ice is small enough to be neglected for air-conditioning problems of toe type con sidered in this chapter. Dry Air has the following exact composition, adopted by the International Joint Committee on Psychrometric Data, and expressed in mo! fractions: oxygen, 0.2095; nitrogen, 0.7S09; argon, 0.0093; carbon dioxide, 0.0003. Traces of other rare gases are neglected. In all calculations to be discussed here, air is treated as a single gas having a molecular weight of 28.966. The common designation air generally means moist air; however, in air conditioning the term dry air is also used to indicate the water-free constituents of air having any degree of moisture. Terms used to describe the condition of mixtures of air and water vapor are defined in the following paragraphs. Saturation. Moist air is said to be saturated when its con dition is such that it can coexist in neutral equilibrium with an associated condensed moisture phase presenting a flat sur face to it. Hie word neutral is necessary to avoid metastable states. A fiat surface is necessary to avoid any influence of surface ten sion. Humidity Ratio is the mass of water vapor per unit mass of dry air in a vapor-air mixture. The units are pounds of water vapor per pound of dry air. Humidity ratio has been called specific humidity and this term still is occasionally used. Degree of Saturation ft is toe ratio of the prevailing humidity ratio to the saturation humidity ratio at the same dry-bulb temperature and pressure. Dew-Point Temperature is the saturation temperature cor responding to the existing humidity ratio and barometric pres sure. (In practice, dew-point temperature is the temperature at which condensation will just begin when the moist air mix ture under consideration is cooled at constant pressure.) Mol. One mol of any substance is toe number of pounds of that substance equal to its molecular weight. For example, the molecular weight of water is 18,016, and one mol of water is 18.016 pounds of water. Mol Fraction of s substance in a given amount of a homo geneous-phase mixture is the ratio of the number of mols of toe substance in the given amount of mixture to the total num- Table 1 ... .Magnitudes of f for the Range 0 to 125 F (Standard towwfrit freetan, 39.921 in. H9) ftap. F ft trap. F ft 0 1.0048 70 1.0045 10 1.0046 80 1.0047 1.0046 90 1.0048 30 1.00(5 100 1.0050 40 1.0044 110 1.0053 50 1.0044 120 1.0055 60 1.0044 125 1.0057 N**; Th* origiml source1 give* /, to w*ea aifaificust figures over the teapenUire T*s*e --*18 F to -fSB F tad over the pressure re&fe 20 to XS in. H*. Thermodynamics her of mob oi all substances present in the given amount of mixture. For example, if a particular amount of moist air con tains ft. mols of air and mols of water, the mol fraction of air in the mixture is n -* (n. + nj). Partial Pressure of any substance in a given vapor-phase mixture is defined as the product of the mol fraction of that substance and the total pressure of toe mixture. Relative Humidity, 4>, oi any mixture of air and water vapor is the ratio of the mol fraction of water vapor in toe mixture to the mol fraction of water vapor in saturated air at the same dry-bulb temperature and (barometric) pressure. Relative humidity and degree of saturation are related as follows: where f -* relative humidity, expressed as a decimal. ft -- degree ofsaturation, expressed as a decimal. Pt = observed (or barometric) pressure of the moist air. Pm <0 saturation pressure of pure water at the prevailing temperature, inches of mercury. /, * a dimensionless factor which may be regarded as ac counting for influences arising when air and water are intermixed. Magnitudes of / have been reported by Goff and Cratch' and by Goff.' Table 1 gives values of / for a limited range of conditions. Fig. 2 shown the relationship between d> and ft for the range of the ASHRAE Psychbometric Chart. 15 Dry-Bulb Temperature is the temperature of toe mixture of air and water vapor at rest; it is to be measured by an instru ment whose reading is not affected by the humidity ratio or by thermal radiation. Thermodynamic Wet-Bulb Temperature is toe temperature at which liquid or solid water, by evaporating into air, can bring the air to saturation adiabatically at the same tempera ture. Consider an adiabatic Bystem as shown in Fig. 3. Unsatu rated air at the state hy,Wi, enters toe system at Section 1, and saturated air at the state h*, W*, leaves the system at Section 2. Liquid waterat the state A,*, corresponding to toe temperature of the saturated air leaving the system is sup plied. Then, since no work is done and the system is strictly adiabatic, toe energy equation becomes hi + (IF* - WX)K* - A* (8) where * indicates condition at thermodynamic wet-bulb tempera ture. The temperature corresponding to A* for givenvalues of A, and IF, is called the thermodynamic wet-bulb temperature, or the temperature of adiabatic saturation. The temperature indicated by an ordinary wet-bulb ther mometer is affected by nonthermodynamic factors not ac counted for in Equation 8, and hence, may be quite different from toe true temperature obtained from Equation 8. The measured wet-bulb temperature is influenced by (a) radia tion from the surroundings to toe wick; (A) conduction of beat along the stem of the thermometer; (c) impurities in toe water or on the wick; and (d) impact of the air on the wick or bulb RELATIVE HUMIDITY, PERCENT PERCENT SATURATION Rg. 2 .... Comparison of Relative Humidity and Percent Saturation Within the Range of toe ASHRAE PsycHROMEtHC CHART.