Document p2MeEBd8Y4qJJ6zoK3R8e1M7d

HEATINC VENTILATING AIR CONDITIONING GUIDE 1942 F t -96 -64 -32 0 Table 2. Specific Volume of Dry Air3 at 29.921 In. Hg Cu Ft per Lb Pq 9.1488 9.9597 10.7699 11.5796 1 | | I Temp F i 32 64 96 128 Cu Ft per Lb Pa 12.3888 13.1977 14.0063 14.8147 | U 1 I 1 Temp F t 160 192 224 256 Cu FT PER Lb Pa 15.6229 16.4310 17.2389 19.0467 ^Prepared by John A. Goff. energy of the molecules. In ordinary calculations not requiring too great accuracy, the effect of intermolecular forces may be ignored and Equation 2 simplified to pv = BT (1) Example 2. Calculate an approximate value for the specific volume of dry air at 64 F, 29.921 in. Hg. ,,. . 53.351 X 523.70 Solution, v = 29.921 x o.ifllTTx 144 = 13200 CU ft ** PU"d- Note: This answer may be compared with the value in Table 2. The difference is due to intermolecular forces. It should not be concluded, however, that because the effect of intermolecular forces on the volume is so small these forces can be ignored entirely. The relationship of Equation 1 expresses certain familiar laws approxi mately true for gases at not too high pressures. Thus, with temperature constant, the volume of a given weight of gas is inversely proportional to its absolute pressure is a statement of Boyle's Law. If Vi denotes the specific volume at absolute pressure pi, then at the same temperature, the specific volume v2 at absolute pressure pi is approximately, *= * () Also, with pressure constant the volume of a given weight of gas is directly proportional to its absolute temperature is a statement-of Charles' Law. If Vi denotes the specific volume at absolute temperature Ti, then at the same pressure, the specific volume vt at absolute temperature Ti is approxi mately, Absolute Temperature For the range 0 to 660 C, the standard temperature scale is the Inter national Centigrade Scale, namely, the readings of a platinum resistance thermometer standardized at the ice-point (0 C), the steam-point (100 C) and the sulphur-point (444.60 C). The corresponding Fahrenheit scale t used in scientific work is derived from the International Centigrade Scale by means of the relation, ( = 1.8 (Int. Cent. Temp.) + 32 (3) Temperatures on the absolute Fahrenheit scale are then obtained by adding 459.70 according to the equation T = t + 459.70 (4) 4 CHAPTER 1. THERMODYNAMICS OF AIR AND WATER MIXTURES Absolute temperatures computed from Equations 3 and 4 are practically identical with the fundamental' thermodynamic temperatures to which the zero-pressure values of the product pv for gases are proportional in accordance with Equation 2. Specific Enthalpy Most air conditioning processes are of the steady-flow type. In steady flow the energy convected with the fluid crossing a given section is the sum of (a) kinetic energy due to velocity, (b) gravitational energy due to elevation, (c) enthalpy due to the condition of temperature, pressure and composition at a given section. It is clear, therefore, that in order to apply the Law of Conservation of Energy to steady-flow processes, information regarding the enthalpy is needed. Recent developments in quantum mechanics have made it possible to calculate the zero-pressure specific enthalpy of a gas from spectroscopic measurements, and with a degree of accuracy exceeding that with which this property can be inferred from direct calorimetric measurements. Available data for each gas listed in Table 1 have been assembled and critically examined; and from them have been calculated best values for the.specific enthalpy of dry air at zero pressure. These are listed in Table 3'. The unit of energy is the Btu which is related to the foot pound as follows: 1 Btu = 778.18 ft-lb1 (5> In Table 3 are also listed values of mean zero-pressure specific heat for the range 0 to t F. This is simply the increase of specific enthalpy from 0 to / F, divided by the increase of temperature or, with 0 F as the reference point, by the temperature itself. The numerical values indicate that a rounded figure of 0.24 Btu per pound can be used in ordinary calculations.. Applying well known identical relations of thermodynamics to Equation 2, the following expression for specific enthalpy, valid at not too high pressures,- is obtained (6> where h0 denotes specific enthalpy at zero pressure. This equation emphasizes that the effect of pressure on specific enthalpy is not so much due to the second virial coefficient A itself as to its variation with tempera- Table 3. Specific Enthalpy of Dry Air at Zero Pressure3 Temp F t Specific Enthalpy Btu per Lb Mean Specific Heat ki: Temp F t Specific Enthalpy Btu per Lb Mean Specific Heat ta Temp F t Specific Enthalpy Btu per Lb -I Mean Specific Heat tei; -96 -64 -32 0 -22.839 -15.186 -7.529 +0.131 0.2393 0.2393 0.2394 0.2394 32 64 96 128 7.796 15.466 23.145 30.831 0.2395 0.2396 0.2397 0.2398 160 192 224 256 38.529 46.238 53.962 61.702 0.2400 0.2401 0.2403 0.2405 ^Prepared by John A. Goff from published data computed from spectroscopic measurements. `This conversion factor is not exact by definition, but involves an experimental determination of the relation between the absolute and the standard electrical units of energy. The value 1 int. joule = 1.00019' abs. joule, recommended by Osborne. Stimson and Ginnings (8) was used. 5