Document JMz3L4jzGZz4kKx1zqj1nJYe

Heating Ventilating Air Conditioning Guide 1938 and pi, pi, pi, etc. = the pressure of the gases or vapors corresponding to the observed temperature, then p = pi + pi + Pi, etc. (1) TEMPERATURES Air is said to be saturated at a given temperature when the water vapor mixed with the air is in the dry saturated condition or, what is the equiva lent, when the space occupied by the mixture holds the maximum pos sible weight of water vapor at that temperature. If the water vapor mixed with the dry air is superheated, i.e., if its temperature is above the temperature of saturation for the actual water vapor partial pressure, the air is .not saturated. 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 dean 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 from the air to the water on the bulb and the rate at which this heat is utilized in evapora ting 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 (t -- /'), 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 (e1 -- e). Carrier's equation for this dynamic equilibrium is e' -- e t - t< B -- e' 2800 - 1.31' (2a) In the form commonly used, , (B - ') - <) 2800 - 1.3? where 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 watervapor 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 Chapter 1. Air. Water and Steam 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. AIR PROPERTIES 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 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 (29.921 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 0.0742 lb. In the mixture the density of the dry air is 0.07307 and that of the vapor is 0.00115 lb (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 29.921 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 tem perature of 1 lb of air 1 F. Distinction should always be made between the instantaneous specific heat at any existent temperature and the mean specific heat, which is the average specific heat through a given tempera ture range. The. mean specific heat is the value required in most calcu lations. The specific heats at constant pressure, Cp, and the specific heats, Cv, at constant volume are different. The specific heat at constant pressure is commonly used and it varies, under a pressure of one atmos phere,from a minimum at 32 F from which it increases with either increase or decrease of temperature. The value of 0.24, as the mean specific heat at constant pressure, is sufficiently accurate for use at ordinary tem peratures. Values for instantaneous and mean specific heats are given in Table 3. The mean specific heat of water vapor at constant pressure is taken as 0.45 for all general engineering computations. 3