Thermodynamic wet bulb temperature

If, instead, the air is damped adiabatically with the wet cloth, so that the state of the air varies, the cloth will settle to a slightly different temperature. Each state of air (0, x) is represented by a certain wet bulb temperature 6, which can be calculated from Eq. (4.116) or its approximation (4.123), when the partial pressures of water vapor are low compared with the total pressure. When the state of air reaches the saturation curve, we have an interesting special case. Now the temperatures of the airflow and the cloth are identical. This equilibrium temperature is called the adiabatic cooling border or the thermodynamic wet bulb temperature (6 ). 

Equation (4.137) is almost exactly the same as the approximation equation (4.123) derived for wet bulb temperature. When the partial pressure of water vapor is low compared with the total pressure—in other words when the humidity x is low—the specific heat of humid air per kilogram of humid air, Cp, and the specific heat of humid air per kilogram of dry air, Cp, are al most the same Cp = Cp. Therefore, in a situation where the humidity is low and Le s 1, the thermodynamic wet bulb temperature is very nearly the same as the technical wet bulb temperature dy... 

Adiabatic saturation temperature Final temperature reached by a small quantity of vapor-gas mixture into which water is evaporating. It is sometimes called the thermodynamic wet-bulb temperature. 

TEMPERATURE, WET BULB - Thermodynamic wet bulb temperature is the temperature at which liquid or solid water, by evaporating into air, can bring the air to saturation adiabatically at the same temperature. Wet bulb temperature (without qualification) is the temperature indicated by a wet bulb psychrometer constructed and used according to specifications. 

Wet-bulb temperature (WBT) Temperature measured by passing air over the bulb of a thermometer covered by a wick saturated with water. Also known as the thermodynamic wet-bulb temperature or the temperature of adiabatic saturation. Units °F (°C). 

Figure 23.1 is a psychrometric chart for the air-water system. It shows the relationship between the temperature (abscissa) and absolute humidity (ordinale, in g water per kg dry air) of humid air from 0°C to 130°C at one atmosphere absolute pressure. Line as representing percent humidity and adiabatic saturation are drawn according to the thermodynamic definitions of these terms. Equations for the adiabatic saturation and wet-bulb temperature lines on the chart are as follows (Geankoplis 1983) ... 

The ratio (h/M Ay)> termed the psychrometric ratio, lies between 0.96 and 1.005 for air-water vapor mixtures thus it is nearly equal to the value of humid heat c,. If the effect of humidity is neglected, the adiabatic saturation and wet-bulb temperatures and T, respectively) are almost equal for the air-water system. Note, however, that and are conceptually quite different. The adiabatic saturation temperature is a gas temperature and a thermodynamic entity while the wet-bulb temperature is a heat and mass transfer rate-based entity and refers to the temperature of the liquid phase. Under constant drying conditions, the surface of the drying material attains the wet-bulb temperature if the heat transfer is by pure convection. The wet-bulb temperature is independent of surface geometry as a result of the analogy between heat and mass transfer. 

A nuclear power plant produces 1000 MW of electricity with a power cycle thermodynamic efficiency of 30%. The heat rejected is removed by cooling water that enters the condenser at 293 K and is heated to 313 K. The hot water flows to two identical natural-draft cooling towers, where it is recooled to 293 K, and makeup water is added as necessary. The available air is at 298 K with a wet-bulb temperature of 285 K, and it will flow at a rate 1.2 times the minimum. Specialized packing will be used for which Kya is expected to be 1.0 kg/m3-s if the liquid mass velocity is at least 3.4 kg/m2-s and the gas mass velocity is at least 2.75 kg/m2-s. Compute the dimensions of the packed sections of the cooling towers and the makeup water requirement due to evaporation. 

In Eq. 8.5, the vapor pressure at wet bulb temperature can be calculated from the Antoine equation, while the solvents concentration (Pout) in the drying gas can be determined in the thermodynamic step described previously. For example, fast evaporation and low drying times can be imposed by manipulating Pout and droplet size (tfo) and used to promote the production of smooth spherical particles. 

Air treatment, thermodynamic Relating to the various thermodynamic changes that occur in the specific volume, enthalpy, and wet and dry bulb temperatures of treated air. 

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