Document RJ0O76J8Y2xM1Dg96pNG2bX8z

26 CHAPTER 3 '1958 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 to satura tion adiabatically at the same temperature. Consider an adiabatic system as shown in Fig. 2.. Unsaturated air at the state ft., Wx, enters the system at sectionT, and saturated air at the state h*, W*, leaves the system at section 2. Liquid water at the state h,*, 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* - WJhS = h* (8) where- ' indicates condition .a t thermodynamic wet-bulb temperature. Table L Magnitudes'of / fob the Hangs 0 to 125 F . . (Standard Barometric Pressure, 29.921 in. Hg) Temp. F /a Temp. F /a 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 1.0053l 50 1.0044 120 1.0055: 60 1.0044 125 1.0057. Note: The nriKimil source' gives /. to seven significant figures oyer 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 h, and Wx 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 (o) 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,Hhe'behavior of common diatomic and tria- Fig. 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 f 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