Radiation heat transfer energy density

However there is a major difference between the two forms of trays and heating systems. As shown in Fig. 1.57, approx. 1.3 kg ice/h m2 can be sublimated by radiation heat, if the shelves have a temperature of + 100 °C and the product temperature is -20 °C. The main difference is the method of heat transfer With a flat tray and mostly radiation energy, the density of the heat flow is limited, and it can be substantially larger with ribbed trays standing on the heated shelf. Using the temperatures as above and an average value Klol = 100 kJ/h m2 °C from Tables 1.9 and 1.10, approx. 4.3 kg ice/h m2 can be sublimated. 

In conduction, heat is conducted by the transfer of energy of motion between adjacent molecules in a liquid, gas, or solid. In a gas, atoms transfer energy to one another through molecular collisions. In metallic solids, the process of energy transfer via free electrons is also important. In convection, heat is transferred by bulk transport and mixing of macroscopic fluid elements. Recall that there can be forced convection, where the fluid is forced to flow via mechanical means, or natural (free) convection, where density differences cause fluid elements to flow. Since convection is found only in fluids, we will deal with it on only a limited basis. Radiation differs from conduction and convection in that no medium is needed for its propagation. As a result, the form of Eq. (4.1) is inappropriate for describing radiative heat transfer. Radiation is... 

Natural convection heat transfer occurs when a solid surface is in contact with a gas or liquid which is at a different temperature from the surface. Density differences in the ffuid arising from the heating process provide the buoyancy force required to move the ffuid. Free or natural convection is observed as a result of the motion of the fluid. An example of heat transfer by natural convection is a hot radiator used for heating a room. Cold air encountering the radiator is heated and rises in natural convection because of buoyancy forces. The theoretical derivation of equations for natural convection heat-transfer coefficients requires the solution of motion and energy equations. 

Convection involves the transfer of heat by mixing one parcel of fluid with another. The motion of the fluid may be entirely the result of differences of density caused by temperature differences, as in natural convection, or it may be produced by mechanical means, as in forced convection. Energy is also transferred simultaneously by molecular conduction and, in transparent media, by radiation. 

Unlike the influence of the fluidization behaviour the heat and mass transfer between the process air and the product is highly dependent upon the solvent used. Certain physical properties are the density of the gas at particular pressures and specific heat capacity as well as the essential gas volume. The behaviours of the individual solvent vapours do not differ from one another until below 100 mbar. It is therefore barely possible for acetone vapours to transfer heat energy into the particles at 40 mbar system pressure, no matter which temperatures are obtained in the air heater. Even the radiation of the inlet air duct can hardly be compensated. Then again xylene vapours can be heated under similar circumstances without problems. 

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