Convection radiation, simultaneous loss

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. 

The third important loss is by air convection inside the distiller. This air circulation is a necessary accompaniment of water distillation. The higher the basin temperature, the higher is the water vapor pressure and the lower the ratio of air to water vapor in the atmosphere of the distillation unit. Factors tending to maximize basin temperatures will therefore reduce this heat loss because of the lower concentration of air in the atmosphere of the distiller enclosure. But as radiation loss increases with rise in basin temperature, these two losses cannot be simultaneously minimized by temperature change. Any effort toward reducing convection in the distiller would be undesirable, because this is the only significant mechanism for water distillation. 

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