Document 5bk1GGDREQXgXmjk9EGxGXdXe

. TO CHAPTER 3 1946 Guide where = heat added from outside between sections 1 and 2, Btu per pound dry air. _lh = enthalpy of the mixture at section 2, 'Btu per pound dry air. KE, = average kinetic energy at section-2, Btu per pound dry air. PE, =.average potential energy at section 2, Btu per pound dry-air. _*i = enthalpy at section 1, Btu per pound dry air. KE, = average kinetic energy at section 1, Btu per pound dry air. PE, = average potential energy at section 1, Btu per pound dry air. ' . ,k = shaft work withdrawn between sections 1 and 2, Btu per pound dry air. In Equation 28 all quantities are per pound of dry air. If Equation 25 is used in computing average kinetic energy, the result will be in Btu per pound of dry air if v is taken as volume per pound of dry air. If Equation 26 is used,-multiplication by (1 + W)- a.s in Equation .27 is required though this is a refinement seldom justified; - Thermodynamic properties of water at saturation are given in Table 2 for the range --160 to +212 F. U. S. STANDARD ATMOSPHERE The so-called U. S. Standard Atmosphere is an essential standard of reference in aeronautics and as such has become important to the air conditioning engineer who frequently has to simulate atmospheric con ditions at high altitudes in connection with aeronautical research. In defining this standard it is first assumed that temperature T varies linearly with altitude Z above sea level, at any rate up to the lower limit of the isothermal layer at 35,332 ft. Thus, T = T0 - 0.0019812 Z (29) or -- = --0.0019812 (degree Centigrade per foot) Z (30) The second assumption is the validity of the perfect gas laws, namely, Pv = RT . . (31) A horizontal disc of air having unit cross-sectional area (1 sq ft) and vertical thickness dZ (ft) weighs dZ/v (lb); This accounts for the dif ference of pressure dP {\b per sq.ft) between:the upper and lower faces of the disc; -hence, using Equation 31. % , dZ = RT dP P (32) Equations 30 and 32 ;can be combined to eliminate Z and then in tegrated to obtain the relation between pressure and temperature, namely, p / p \o.woa To \ Po / (33) The values T0 = 288 K and P0 = 29.921 in..Hg are parts of the definition of. the standard atmosphere. Values of pressure and temperature are listed in Table 4 for altitudes in the standard atmosphere from --1,000 to 50,000 ft above sea level. Values for altitudes below the lower limit of the isothermal layer conform Thermodynamics v71 to Equations 29 and 33. For further explanation, reference (7) should be consulted. Table 4. Pressure and Temperature for Altitudes in U. S. Standard Atmosphere Altitude Feet Z - 1,000 - 500 0 + 500 + 1,000 . + 5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 . 45,000 50,000 Pressure In; of Hg P 31.02 30.47 29.921 29.38 28.86 24.89 20.58 16.88 13.75 11.10. 8.88 7.04 5.54 4.36 3.436 ... , Temp F t' +62.6 +60.8 +59.0 +57.2 +55.4 +41.2 . +23.4 + 5.5 -12.3 -30.1 --47.9 -65.8 -67.0 -67.0 -67.0 - LETTER SYMBOLS USED IN CHAPTER 3 ja = degree of saturation or per cent saturation, p = density of fluid, pounds per cubic foot. <? = relative humidity (decimal). a = ratio of apparent molecular weight of dry air (28.966) to the molecular weight of water (18.016) = 1.6078. . ; . A -- coefficient from Table 3`for use in Equation 4a (obtained from Table 3). B = Coefficient to be used in Equation 5a (obtained from Table 3). C = coefficient for use in Equation 6a (obtained from Table 3). h = enthalpy of moist air, Btu per pound of dry air. ` A -- enthalpy correction term to bemadded above 150 F, to enthalpy. . Aa = specific enthalpy of dry air, Btu per pound. AasAs ~ Aa = the difference between the enthalpy of moist air at saturation per pound of dry air, and the specific enthalpy of-the dry air itself, Btu per pound of dry air. hs* = enthalpy of moist air at saturation thermodynamic wet-bulb tempera ture, Btu per pound of dry air. " As = enthalpy of moist air at saturation per pound of dry air, Btu per pound of dry air. hv ^ velocity pressure, inches of water at 60 F.: ;\ Aw = specific enthalpy of condensed water (liquid or solid) at standard pres sure, Btu per pound water. Aw* = specific enthalpy of water as added-at * the thermodynamic wet-bulb temperature}*,.Btu per pound of dry air.,.. K Kelvin degrees. KE = kinetic energy, Btu.per pound. KE = average;kinetic energy, Btu per pound. , ............. I -- shaft work withdrawn, Btu per pound of air: 1 ilt = shaft work withdrawn between sections 1 and 2, Btu per pound of air..