Document r747Zw5ErmMdXjGm83YeB7L7

American Society of Heating and Ventilating Engineers Guide, 1935 The starting point of most applications of thermodynamic principles to air-conditioning problems is the experimental determination of the dry- bulb and wet-bulb temperatures, and sometimes the barometric pressure. The dry-bulb temperature of the air is the temperature indicated by any type of thermometer not affected by the water vapor content or relative humidity of the air. The wet-bulb temperature is determined by a thermo meter with its bulb encased in a fine mesh fabric bag moistened with clean water and whirled through the air until the thermometer assumes a steady^ temperature. This steady temperature is the result of a dynamic equilibrium between the rate at which heat is transferred!rom the air to the water on the bulb and the rate at which this heat is utilized in evapor ating moisture from the bulb. The rate at which heat is transferred from the air to the water is substantially proportional to the wet-bulb depres sion (1 -- {'), while the rate of heat utilization in evaporation is propor tional to the difference between the saturation pressure of the water at the wet-bulb temperature and the actual partial pressure of the water vapor in the air (e' -- e). Carrier's equation for this dynamic equilibrium is --e t - t1 B 2800 - 1.31' (2a) In the form commonly used, where (B - e') (t - <') = e' -- 2800 - 1.3f e = actual partial pressure of water vapor in the air, inches of mercury. ' = saturation pressure at wet-bulb temperature, inches of mercury. B = barometric pressure, inches of mercury. t = dry-bulb temperature, degrees Fahrenheit. t' = wet-bulb temperature, degrees Fahrenheit. (2b) Formula 2b may be used to determine the actual partial pressure of the water vapor in a dry air-water vapor mixture. Then, from Dalton's Law of Partial Pressures, Equation 1, it follows that the partial pressure of the dry air is (B -- e). If a mixture of dry air and water vapor, initially unsaturated, be cooled at constant pressure, the temperature at which condensation of the water vapor begins is called the dew-point temperature. Clearly the dew-point is the saturation temperature corresponding to the actual partial pressure, e, of the water vapor in the mixture. PROPERTIES OF AIR Density is variously defined as the mass per unit of volume, the weight per unit of volume, or the ratio of the mass, or weight, of a given volume of a substance to the mass, or weight, of an equal volume of some other substance such as water or air under standard conditions of temperature and pressure. The term specific gravity is more commonly used to express, the latter relation but, when the gram is taken as the unit of mass and the cubic centimeter as the unit of volume, density and specific gravity have Chapter 1--Fundamentals of Heating and Air Conditioning the same meaning. The term specific density is sometimes used to dis tinguish the weight in pounds per cubic foot; and as here used, density is the weight in pounds of one cubic foot of a substance. The density of air decreases with increase in temperature when under constant pressure. The density of dry air at 70 F and under standard atmospheric pressure (29.92 in. of Hg) is approximately 0.075 lb (see Table 1), while that of a mixture of air and saturated water vapor at the same temperature and barometric pressure is only about 0.0743 lb. In the mixture the density of the dry air is 0.0731 and that of the vapor is 0:001151b (see Table 2). - ------ --------- -------- - In order to make comparisons of air volumes or velocities it is necessary to reduce the observations to a common pressure and temperature basis. The basic pressure is usually taken as 29.92 in. of Hg, but no basic tem perature is universally recognized. Common temperatures for this purpose are 32 F, 60 F, 68 F, and 70 F. Since 70 F is the most commonly specified temperature to which rooms for human occupancy must be heated, it is usually understood, when no other temperature is specified, that 70 F is the basic temperature for measuring the volume or the velocity of air in heating and ventilating work. The specific volume of air is the volume in cubic feet occupied by one pound of the air. Under constant pressure the specific volume varies inversely as the density and directly as the absolute temperature. The specific heat of air is the number of Btu required to raise the temperature of 1 lb of air 1 F. The specific heat at constant pressure, Cp, and that at constant volume, Cv, are different. The specific heat at constant pressure is commonly used and it varies, under a pressure of one atmosphere, from a minimum at about 32 F from which1 it increases with either increase or decrease of temperature. The value 0.24 is suf ficiently accurate for use at ordinary temperatures, but the values range1 from 0.2399 at 32 F to 0.2404 at 212 F, 0.2413 at 392 F, 0.243 !at -108 F, and 0.252 at -301 F. |, The mean specific heat of water vapor at constant pressure;is taken as 0.45 for all general engineering computations. Table 3 is intended to aid in determining the density of moist air, taking into account its temperature, pressure, and moisture content. Example 1. To'show the use of Table 3: Given air at 83 F dry-bulb and 68. F. wet- bulb (or a depression of 15 deg) with a barometric pressure of 29.40 in. of mercury. What will be the weight of this air in pounds per cubic foot? Solution. From Table 3 the weight of saturated air at 80 F and 29.00 in. barometer is found to be 0.07034 lb per cubic foot. There is a decrease of 0.00015 lb per degree dry- bulb temperature above 80 F. There is an increase of 0.00025 lb for each 0.1 in. above 29.00 in. From the last column of Table 3 it is found that there is an increase of approxi mately 0.000035 lb per degree wet-bulb depression when the dry-bulb is 83 F. Tabu lating the items: 0.07034 = weight of saturated air at 80 F and 29.00 bar. -- 0.00045 = decrement for 3 deg dry-bulb, 3 X 0.00015. + 0.00100 = increment for 0.4 in. bar., 4 X 0.00025. ; + 0.00053 = increment for 15 deg wet-bulb depression, 15 X 0.000035. 0.07142 = weight in pounds per cubic foot of air at 83 F dry-bulb, 68 F wet-bulb, 29.40 in. bar. 3