Heat transfer coefficients, film convection and radiation

Heat transfer coefficients, film convection and radiation, 189 equations, 189-192 Heat transfer coefficients, overall, 170... 

Simultaneous Loss by Radiation The heat transferred by radiation is often of significant magnitude in the loss of heat from surfaces to the surroundings because of the diathermanous nature of atmospheric gases (air). It is convenient to represent radiant-heat transfer, for this case, as a radiation film coefficient which is added to the film coefficient for convection, giving the combined coefficient for convection and radiation (h + hf In Fig. 5-7 values of the film coefficient for radiation are plotted against the two surface temperatures for emissivity = 1.0. 

Specific correlations of individual film coefficients necessarily are restricted in scope. Among the distinctions that are made are those of geometry, whether inside or outside of tubes for instance, or the shapes of the heat transfer surfaces free or forced convection laminar or turbulent flow liquids, gases, liquid metals, non-Newtonian fluids pure substances or mixtures completely or partially condensable air, water, refrigerants, or other specific substances fluidized or fixed particles combined convection and radiation and others. In spite of such qualifications, it should be... 

There are upper and lower limits of applicability of the equation above. The lower limit results because natural-convection heat transfer governs at low temperature differences between the surface and the fluid. The upper limit results because a transition to film boiling occurs at high temperature differences. In film boiling, a layer of vapor blankets the heat-transfer surface and no liquid reaches the surface. Heat transfer occurs as a result of conduction across the vapor film as well as by radiation. Film-boiling heat-transfer coefficients are much less than those for nucleate boiling. For further discussion of boiling heat transfer, see Refs. 5 and 6. 

Select a convection tube arrangement that will give a maximum flue gas mass velocity of about 0.3 to 0.4 pounds per square foot per second. Calculate the mass velocity, G. For precise ratings, calculate the gas film coefficient using Figures 1-12, 1-13 and 1-14 and equations for f, the factor for wall radiation in the convection section and he, the convective heat transfer coefficient. Calculate the in tube coefficient using any applicable method. Then calculate the overall transfer coefficient from the equation for Ue, the convective heat transfer velocity. 

During the constant rate period, evaporation is taking place from the fabric surface. The rate of diying is essentially that of the evaporation of the liquid component under the conditions of temperature and airflow during the process. High air velocities will reduce the thickness of the stationary gas film on the surface of textile material and hence increase the heat and mass transfer coefficients. In commercial forced convection dryers the effects of heat transfer by conduction and radiation may be appreciable, due to the fact that the material surface temperatures are higher than the wet-bulb temperature of the diying air. 

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