Document 85K4q7zDGJZLe4ko4eweX1woo

26 CHAPTER 3 1957 Guide , Dry-Bulb Temperature. The temperature indicated. by any type of thermometer or thermocouple not affected by the water vapor content,of the air, or by radiation. . Thermodynamic Wet-Bulb Temperature. The temperature at which liquid or solid water, by evaporating into air, can bring the air t6*satura- tion adiabatically'at the same temperature. Consider an adiabatic system as shown in Fig. 2. - Unsaturated air at the state k,, Wi, enters the system at section 1, and saturated air at the state h*, W*, leaves the system at section 2. Liquid water at the state h,*t corresponding to the temperature of the saturated air leaving the system is supplied. Then, since no work is done and the system is strictly adiabatic, the energy equation becomes h,+ {W* - Wi)hS = h* (8) where * indicates condition at thermodynamic wet-bulb temperature. Table 1. Magnitudes of /, fob the Range 0 to 125 F (Standard Barometric Pressure, 29.921 in. Hg) fsTemp. F /a Temp. F 0 1.0048 70 1.0045 10 1.0046 80 1.0047 20 1.0046 90 1.0048 30 1.0045 100 1.0050 40 1.0044 110 1.0053 50 1.0044 120 1.0055 60 1.0044 125 1.0057 Wofa; The original source1 gives / to seven significant figures over the temperature range 208 F to + 202 F and over the pressure range 20 to 35 in. Hg. The temperature corresponding to h* for given values of hi and IF, is called the thermodynamic wet-bulb temperature, or the temperature of adiabatic saturation. The temperature indicated by an ordinary wet-bulb thermometer is affected by a number of factors not accounted for in Equation 8, and hence, may be quite different from the theoretical temperature obtained from its use. The measured wet-bulb temperature is influenced by (a) radia tion from the surroundings to the wick; (b) conduction of heat along the stem of the thermometer; and (c) impact of the air on the wick or bulb of the thermometer. Arnold* has developed a theory which makes pos sible the calculation of the true thermodynamic wet-bulb temperature from observed data through the use of suitable corrections to be applied to the readings of the wet-bulb thermometer. However, unless extreme precision is required, the observed temperature may be taken equal to the theoretical temperature for most engineering problems, if no attempt is made to shield the wick from radiation and the air velocity past the wick is about 1000 fpm. Dew-Point Temperature. The saturation temperature corresponding to a given combination of humidity ratio W and barometric pressure is called the dew-point temperature. It is the lowest temperature at which the Thermodynamics 27 given humidity ratio can exist at the corresponding barometric pressure. At this temperature condensation will first start to form when moist air is cooled. Perfect Gas Relationships Hypotheses, based on experimental observation of the physical behavior of gases, which were advanced by Boyle, Charles, Gay-Lussac, Dalton, Gibbs, Joule, Kelvin and others, were reduced to reasonably simple mathe matical expressions and came to be regarded as physical laws. However, as scientific knowledge increased, and as more precise methods of measure ment were developed, it became apparent that these simple equations did not describe the behavior of the mixtures of actual gases and vapors ac curately. The original statements have been found to be useful tools, nevertheless, in many cases. For example, the behavior of common diatomic and tria- Fio. 2. Illustration of Adiabatic Saturation tomic gases at low pressures follows these equations closely enough so that they may be used for some types of engineering problems. Many useful engineering works have been constructed through the use of approxi mations. The degree of approximation, however, which may be tolerated in engineering design must be decided by the engineer, based upon his study and experience in the field. Boyle's Law. One of the original observations of the physical behavior of gases was made by Robert Boyle who noted that, if a constant weight of gas is compressed with the temperature held constant, the volume V varied inversely as the absolute pressure P. Stated mathematically, PV -- constant (temperature constant) (9) Charles' Law. Experiments made independently by Charles and GayLussac led to the formulation of what is now known, as Charles' Law: If a constant weight of gas is heated or cooled at constant volume, the absolute pressure P varies as the absolute temperature T; if a constant weight of