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Unformatted text preview: Chapter 7 External Forced Convection Special Topic: Thermal Insulation 7-73C Thermal insulation is a material that is used primarily to provide resistance to heat flow. It differs from other kinds of insulators in that the purpose of an electrical insulator is to halt the flow of electric current, and the purpose of a sound insulator is to slow down the propagation of sound waves. 7-74C In cold surfaces such as chilled water lines, refrigerated trucks, and air conditioning ducts, insulation saves energy since the source of coldness is refrigeration that requires energy input. In this case heat is transferred from the surroundings to the cold surfaces, and the refrigeration unit must now work harder and longer to make up for this heat gain and thus it must consume more electrical energy. 7-75C The R-value of insulation is the thermal resistance of the insulating material per unit surface area . For flat insulation the R- value is obtained by simply dividing the thickness of the insulation by its thermal conductivity. That is, R-value = L / k . Doubling the thickness L doubles the R-value of flat insulation. 7-76C The R-value of an insulation represents the thermal resistance of insulation per unit surface area (or per unit length in the case of pipe insulation). 7-77C Superinsulations are obtained by using layers of highly reflective sheets separated by glass fibers in an evacuated space. Radiation between two surfaces is inversely proportional to the number of sheets used and thus heat loss by radiation will be very low by using this highly reflective sheets. Evacuating the space between the layers forms a vacuum which minimize conduction or convection through the air space. 7-78C Yes, hair or any other cover reduces heat loss from the head, and thus serves as insulation for the head. The insulating ability of hair or feathers is most visible in birds and hairy animals. 7-79C The primary reasons for insulating are energy conservation, personnel protection and comfort, maintaining process temperature, reducing temperature variation and fluctuations, condensation and corrosion prevention, fire protection, freezing protection, and reducing noise and vibration. 7-80C The optimum thickness of insulation is the thickness that corresponds to a minimum combined cost of insulation and heat lost. The cost of insulation increases roughly linearly with thickness while the cost of heat lost decreases exponentially. The total cost, which is the sum of the two, decreases first, reaches a minimum, and then increases. The thickness that corresponds to the minimum total cost is the optimum thickness of insulation, and this is the recommended thickness of insulation to be installed....
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