Water phase changes, heat

Fig. 17. Heat-transfer coefficient comparisons for the same volumetric flow rates for (A) water, 6.29 kW, and a phase-change-material slurry (O), 10% mixture, 12.30 kW and ( ), 10% mixture, 6.21 kW. The Reynolds number was 13,225 to 17,493 for the case of water.

The plate dryer is limited in its scope of apphcations only in the consistency of the feed material (the products must be friable, free flowing, and not undergo phase changes) and diying temperatures up to 320°C. Applications include speci ty chemicals, pharmaceuticals, foods, polymers, pigments, etc. Initial moisture or volatile level can be as high as 65 percent and the unit is often used as a final dryer to take materials to a bone-dry state, if necessary. The plate dryer can also be used for heat treatment, removal of waters of hydration (bound moisture), solvent removal, and as a product cooler. 

Another technique that can be used to account for the presence of liquids is to assume that the water and oil in the stream pass through the choke with no phase change or loss of temperature. The gas is assumed to cool to a temperature given in Figure 4-8. The heat capacity of the liquids is then used to heat the gas to determine a new equilibrium temperature. 

The heat accompanying the phase change (2) is 1.44 kcal/mole. This is much less than the molar heat of vaporization of water, 10 kcal/mole. Table 5-II contrasts the melting points and the heats of melting per mole (the molar heat of melting, or the molar heat of fusion) of the same pure substances listed in Table 5-1. 

Show that the ratio of the molar heat of formation of gaseous water from the elements (a chemical reaction) to the molar heat of the fusion of water (a phase change) is of the order of 50. 

Carey van P (1992) Liquid-vapor phase-change phenomena. An introduction to the thermophysics of vaporization and condensation processes in heat transfer equipment. Hemisphere, New York Celata GP, Cumo M, Mariani A (1997) Experimental evaluation of the onset of subcooled flow boiling at high liquid velocity and subcoohng. Int J Heat Mass Transfer 40 2979-2885 Celata GP, Cumo M, Mariani A (1993) Burnout in highly subcooled water flow boiling in small diameter tubes. Int J Heat Mass Transfer 36 1269-1285 Chen JC (1966) Correlation for boiling heat transfer to saturated fluids in convective flow. Ind Eng Chem Process Des Develop 5 322-329... 

Latent heat is the energy associated with phase changes. Evaporation of water requires an energy input of 2.5 x 10 J per kilogram of water at 0°C, almost 600 times the specific heat. When water vapor is transported via atmospheric circulation and recondensed, latent heat energy is released at the new location. Atmospheric transport of water vapor thus transfers both latent and sensible heat from low to high latitudes. 

Phase changes can go in either direction Steam condenses upon cooling, and liquid water freezes at low temperature. Each of these is exothermic because each is the reverse of an endothermic phase change. That is, heat is released as a gas condenses to a liquid and as a liquid freezes to a solid. To make ice cubes, for instance, water... 

This example is based on the model description of Sec. 3.3.4, and involves a multicomponent, semi-batch system, with both heating and boiling periods. The compositions and boiling point temperatures will change with time. The water phase will accumulate in the boiler. The system simulated is based on a mixture of n-octane and n-decane, which for simplicity will be assumed to be ideal but which has been simulated using detailed activity coefficient relations by Prenosil (1976). 

What is unique about these three properties of water boiling point, specific heat capacity, and density change over phase change ... 

The variation of enthalpy for binary mixtures is conveniently represented on a diagram. An example is shown in Figure 3.3. The diagram shows the enthalpy of mixtures of ammonia and water versus concentration with pressure and temperature as parameters. It covers the phase changes from solid to liquid to vapour, and the enthalpy values given include the latent heats for the phase transitions. 

Applications of PCM cover many diverse fields. As mentioned before, the most important selection criterion is the phase change temperature. Only an appropriate selection ensures repeated melting and solidification. Connected to the melting and solidification process is the heat flux. The range of heat flux in different applications covers a wide range from several kW for space heating with water or air, domestic hot water and power plants to the order of several W for temperature protection and transport boxes (Figure 124). 

The basic idea of such systems is that snow/ice is stored in a more or less water tight pond where a cold carrier is cooled by the snow, to utilize the large latent heat of fusion. For comfort cooling about 90% of the extracted energy is in the phase change, i.e., the melting. The cold carrier is either circulated between the load and the snow or rejected after it has been used for cooling. 

In pharmaceutical systems, both heat and mass transfer are involved whenever a phase change occurs. Lyophilization (freeze-drying) depends on the solid-vapor phase transition of water induced by the addition of thermal energy to a frozen sample in a controlled manner. Lyophilization is described in detail in Chapter 16. Similarly, the adsorption of water vapor by pharmaceutical solids liberates the heat of condensation, as discussed in Chapter 17. 

Measurements based on the law of conservation of energy are of two main types. In phase change calorimetry the enthalpy of the reaction is exactly balanced by the enthalpy of a phase change of a contained compound surrounded by a larger reservoir of the same compound used to maintain isothermal conditions in the calorimeter. The latter enthalpy, the measurand, is often displayed indirectly through the change in the volumetric properties of the heat reservoir compound, e.g. ice/water. 

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