Document 3JGJDk433MMKMgGevNOkNrz16

HEATING VENTILATING AIR CONDITIONING GUIDE 1943 where Z (1 + W) v778. PE = average potential energy, Btu per pound dry air. Z = average elevation, feet. W = humidity ratio, pound water per pound dry air. (30) Enthalpy No further discussion of enthalpy is required. It may be well to emphasize, however, that enthalpies have been figured on the basis of one pound of dry air. Heat and Shaft Work Between any two sections 1 and 2 in an apparatus through which steady flow occurs, there may be heat absorbed from outside, ,qt, Btu per pound of dry air, and.shaft work removed to outside, ih, Btu per pound of dry air. If heat is actually rejected to outside, ,p is intrinsically negative; and if shaft work is actually put in from outside ,h, is intrinsi cally negative. Steady-flow Energy Equation A complete energy accounting takes the form of Equation 31 which is usually referred to as the steady-flow energy equation. ,q, = (A, + KEt + PE,) - (fe + KE,+ PE,) + iU (31) where ,q, = heat added from outside between sections 1 and 2, Btu per pound dry air. hi = enthalpy of the mixture at section 2, Btu per pound dry air. KEt = average kinetic energy at section 2, Btu per pound dry air. PE, = average potential energy at section 2, Btu per pound dry air. fti = 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. . ifj = shaft work withdrawn between sections 1 and 2, Btu per pound dry air. In Equation 31 all quantities are per pound of dry air. If Equation 28 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 29 is used, multiplication by (1 + W) as in Equation 30 is required though this is a refinement seldom justified. Properties of saturated steam are given in Table 8 and for* additional definitions refer to Chapter 47. 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- 38 CHAPTER. I. THERMODYNAMICS OF AIR AND WATER MIXTURES 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 = To - 0.0019812 Z (32) or AT' = --0.0019812 (degree Centigrade per foot) (33) The second assumption is the validity of the perfect gas laws, namely, Pv -- BT (34) An horizontal disk 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 (lb per sq ft) between the upper and lower faces of the disk; hence, using Equation 34 dZ = BTdP P (35) Equations 33 and 35 can be combined to eliminate Z and then in tegrated to obtain the relation between pressure and temperature, namely; T ( P \0.1W3 To , \ Po / (36) 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 10 for altitudes in the standard atmosphere from -- 1000 to 50,000 ft above sea level. Values for altitudes below the lower limit of the isothermal layer conform to Equations 32 arid 36. For further explanation, reference (11) should be consulted. Table 10. 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 39 p S'