Document qd98Ew3EL7qbr5rovJBRXYxzx

Heating Ventilating Air Conditioning Guide 1939 PROPERTIES OF STEAM Steam is water vapor which exists in the vaporous condition because sufficient heat has been added to the water to supply the latent heat of evaporation and change the liquid into vapor. This change in state takes place at a definite and constant temperature which is determined solely by the pressure of the steam. The volume of a pound of steam is the specific volume which decreases as the pressure increases. The reciprocal of this, or the weight of steam per cubic foot, is the density. (See Table 8). Steam which is in contact with the water from which it was generated is known as saturated steam. If it contains no actual water in the form of mist or priming, it is called dry saturated steam. If this be heated and the pressure maintained the same as when it was vaporized, its temperature will increase and it will become superheated, that is, its temperature will be higher than that of saturated steam at the same pressure. REFERENCES Temperature of Evaporation, by W. H. Carrier (A.S.H.V.E. Transactions, Vol. 24, 1918, p. 25). The Evaporation of a Liquid into a Gas, by W. K. Lewis {A.S.M.E. Transactions, Vol. 44, 1922). Temperature of Evaporation of Water into Air, by W. H. Carrier and D, C. Lindsay (A.S.M.E. Transactions, Vol. 46, 1924). A New Psychrometric or Humidity Chart, by C. A. Bulkeley (A.S.H.V.E. Trans actions, Vol. 32, 1926, p. 163). A Review of Psychrometric Charts, by C. 0. Mackey (Heating and Ventilating, June, July, 1931). Air Conditioning Applied to Cold Storage and a New Psychrometric Chart, by C. A. Bulkeley (Refrigerating Engineering, February, 1932). The Psychrometric Chart, by E. V. Hill (Aerologist, April, May, June, 1932). Air Conditioning Theory, by John A. Goff (Refrigerating Engineering, January, 1933). . Mixtures of Air and Water Vapor, by C. A. Bulkeley (Refrigerating Engineering, January, 1933). Basic Theory of Air Conditioning, by Lawrence Washington (Western Conference on Air Conditioning, San Francisco, Calif., February 9-10, 1933). Heat Transmission in Cooling Air with Extended Surfaces, by W. L. Knaus (Refrigera- ; ting Engineering, January, February, 1935). A New Psychrometric Chart, by F. O. Urban (Refrigerating Engineering, November 1935). % Psychrometric Charts, by Donald B. Brooks ( U. S. Bureau of Standards Miscellaneous Publication No. 143). PROBLEMS IN PRACTICE 1 A Given air at 70 F dry-bulb and 50 per cent relative humidity with a baro metric pressure of 29.00 in. Hg., find the weight of vapor per pound of dry air. Pressure of saturated vapor = et = 0.7387 in. Hg. (Table 6). From Equation 5a, ( 0 5 X 6.7387 \ w - u.ozz ^29 00 _ (0 5) x (0.7387) / ' .? W = 0.008024 lb of vapor per pound of dry air at 70 F dry-bulb and 50 per cent relative humidity. 32 Chapter 1. Air, Water and Steam Approximate Method: Weight of saturated vapor per pound of dry air = Wt - 0.01574 lb (Table 6). 0.01574 X 0.5 = 0.00787 lb of vapor per pound of dry air at 70 F dry-bulb and 50 per cent relative humidity. 2 Given air with a dry-bulb temperature of 80 F, relative humidity of 55 per cent, and a barometric pressure of 28.85 in. Hg., calculate the weight of a cubic foot of mixture. Pressure of saturated vapor at 80 F = ei = 1.0316 in. Hg. (Table 6). Pressure of the vapor in the mixture = 1.0316 X 0.55 = 0.5676 in. Hg. Pressure of the dry air in the mixture = 28.85 -- 0.5676 = 28.282 in. Hg. pV = wR (t + 460) (R = 0.753 when partial pressure of air U expressed in in. Hg.). 28.282 X 1 = da X 0.753 X (80 + 460) 28 282 da = 0 753 X 540 = 0-06955 lb = weight of dry air in 1 cu ft of the mixture. Likewise from Equation 4a, ^ T20l'5f540- = -000868 lb relative humidity. weight of vapor per cubic foot at 55 per cent Weight of 1 cu ft of the mixture = 0.06955 + 0.000868 = 0.070418 lb. 3 Given air with a dry-bulb temperature of 75 F, a relative humidity of 60 per cent, and a barometric pressure of 28.80 in. Hg., calculate the volume of 1 lb of the mixture. Pressure of saturated vapor at 75 F = et = 0.8745 in. Hg. Pressure of vapor in the mixture'= 0.8745 X 0.6 = 0.525 in. Hg. Pressure of dry air in the mixture = 28.80 -- 0.525 = 28.275 in. Hg. . = 28.275 ^ 0.753 X 535 = 0.07018 lb = weight of dry air in 1 cu ft of the mixture. From Equation 4a, , = 0.525 . 1.21 X 535 relative humidity. = 0.000811 lb = weight of vapor per cubic foot at 55 per cent Weight of 1 cu ft of the mixture = 0.07018 + 0.000811 = 0.070991 lb. Volume of 1 lb of the mixture = = 14.08 cu ft. 4 It is desired to maintain a temperature of 80 F and a relative humidity of 50 per cent in a factory where the equipment gives off 6000 Btu per hour. If the entering airisat70F with an average barometric pressure of 29.92 in. Hg.; determine the relative humidity, and the pounds of air required per hour if there is no heat interchange between the walls, windows, or floors of the building. Pressure of saturated vapor at 80 F = 1.0316 in. Hg. (Table 6). Pressure of vapor in the mixture = 1.0316 X 0.5 = 0.5158 in. Hg. s) = 0W = 0.622 0.5158 v 29.92 - 0.5158, = 0.01091 lb. Pressure of saturated vapor at 70 F = 0.7387 in. Hg. With the same specific humidity 0.01091 - 0.622 ^29.920-.73(807.73X847>X <F\)j ip = 69.8 per cent relative humidity at 70 F. 33