Drying adiabatic saturation temperature

Example 1 Compare Wet-Bulb and Adiabatic-Saturation Temperatures For tne air-water system at atmospheric pressure, the measured values of dry-bulh and wet-hulh temperatures are 85 and 72 F respectively. Determine the absolute humidity and compare the wet-bulb temperature and adiabatic-saturation temperature. Assume that h /k is given by Eq. (12-4). 

The temperature driving force for drying is the difference between the drying-gas outlet temperature and, in the case of pure water, the gas wet-bulb temperature. In the case of a solution, the adiabatic saturation temperature of the pure saturated solution is employed rather than the wet-bulb temperature. 

Making a heat balance over the column, it is seen that the heat of vaporisation of the liquid must come from the sensible heat in the gas. The temperature of the gas falls from 6 to the adiabatic saturation temperature 6S, and its humidity increases from to Jfv (the saturation value at 9S). Then working on the basis of unit mass of dry gas ... 

As shown in the results section, the relative humidity of the gaseous phase is the most important variable in the reaction of SO2 with dry Ca(0H)2 solids. These results are in agreement with results reported in the literature for SO2 removal in the bag filters of spray dryer pilot and commercial plants C3, 5-9). In the spray dryer plants, the moisture content of the gases is normally indicated as approach to the adiabatic saturation temperature (difference between the temperature of the gas and the adiabatic saturation temperature) rather than relative humidity. 

At adiabatic saturation temperature Ha, Ya lb/lb dry air Phase equilibrium ratio Relative distribution of two components K K = y x... 

Many wet- and dry-bulb hygrometers operate without any form of induced air velocity at the wet bulb. This may be explained by examining another air-water system. If a limited quantity of air and water is allowed to equilibrate under conditions in which heat is neither gained nor lost by the system, the air becomes saturated and the latent heat required for evaporation is drawn from both fluids which cool to the same temperature. This temperature is the adiabatic saturation temperature, T. It is a peculiarity of the air-water system that the adiabatic saturation temperature and the wet-bulb temperature are the same. If water at this temperature is recycled in a system through which air is passing, the incoming air is cooled till it reaches the adiabatic saturation temperature at which point it is saturated. The temperature of the water, on the other hand, remains constant and all the latent heat required for evaporation is drawn from the sensible heat of the air. Equilibrium is expressed by Eq. (17),... 

Estimate the water requirement of a spray dryer (dry SO2 scrubber) at a coal-fired incineration facility that treats 150,000 Ib/h of a flue gas at 2180°F. Assume an approach temperature to the adiabatic saturation temperature (AST) of 40°E The AST can be assumed to be 180°R... 

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. 

Air at 356 K, Y = 0.03 kg water/kg dry air, and 1 atm is contacted with water at the adiabatic saturation temperature until it becomes saturated. What are the final temperature and humidity of the air ... 

Hydrated lime is used to remove the sulfur dioxide, sulfur trioxide, and hydrogen chloride from the flue gas in this method. Water is injected into the bed to obtain an operation close to the adiabatic saturation temperature. More than 95% sulfur dioxide removal efficiency can be achieved by using this process. The final product is a dry powdered mixture of calcium compounds requiring disposal operations. 

Solution The wet bulb temperature of 29.5°C can be assumed to be the same as the adiabatic saturation temperature 7, as discussed. Following the adiabatic saturation curve of 29.5°C until it reaches the dry bulb temperature of 60°C, the humidity is H = 0.0135 kgHjO/kg dry air. 

In Fig. 9.10-5 typical temperature profiles of the gas Tq and the solid Tj are shown for a continuous countercurrent dryer. In the preheat zone, the solid is heated up to the wet bulb or adiabatic saturation temperature. Little evaporation occurs here, and for low-temperature drying this zone is usually ignored. In the constant-rate zone, I, unbound and surface moisture are evaporated and the temperature of the solid remains essentially constant at the adiabatic saturation temperature if heat is transferred by convection. The rate of drying would be constant here but the gas temperature is changing and also the humidity. The moisture content falls to the critical value at the end of this period. 

This quantity, which has already been encountered in cotmection with the adiabatic saturation temperature, is the specific heat of moist air expressed in units of kj/kg dry air. 

In heat transfer applications, either the liquid stream or the gas stream can be cooled. The most common application, in which the liquid stream is cooled, is a water cooling tower. Although there is sensible heat transfer between the warmer water and the cooler air, in this device the warm water primarily is cooled by evaporation of some of the water into the air stream. Both the heat and mass are transferred in the same direction, from the water to the air stream. Exit air from the water cooling tower may be assumed to be saturated with water vapor. Kern reports tests showing the exit air is 95% to 99% saturated [2]. Exit air enthalpy can be obtained from a psychrometric table. The inlet air condition usually is defined by the dry-bulb and wet-bulb temperatures. For practical design purposes the inlet air is assumed to be saturated with water at the wet-bulb temperature, because the enthalpy of this air is sufficiently close to the theoretically accurate adiabatic saturation temperature. 

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