Graduate Thesis Or Dissertation
 

An investigation of heat transfer and pressure drop across a frosted finned coil

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  • The formation of frost on a forced convection refrigeration coil, operating below the freezing temperature is known to have the effect of imparing the efficiency of a refrigeration system, This investigation was directed toward the study of the behavior of the overall coefficient of heat transfer and the air static pressure drop for a frosted coil. The experiment was conducted with a finned tube coil having staggered tube arrangement, The experimental equipment was so designed that the frost accumulation on the test coil could be measured with a platform scale. A chilled solution of 30 percent by weight of ethylene glycol and water supplied the cooling for the test coil. Tests were run at constant air flow rates of 585, 685 and 775 CFM. In each test the inlet air was maintained approximately at 32° F with a specific humidity of 0.003 lbs of moisture per lb of dry air; the supplied antifreeze solution was kept at about 14° F with a flow rate of 1,853 lbs per hr. Sufficient data were recorded, during each test, to determine the overall coefficient of heat transfer and the air static pressure drop through the test coil at various frost accumulations. It was observed that the overall coefficient of heat transfer at constant air flow rate increased slightly as frost first accumulated on the test coil and then decreased after the peak had been reached as the frost continued to increase. The air pressure drop increased continuously with increasing frost accumulation on the test coil. For each increment of frost weight per square foot of coil surface the magnitude of the air pressure drop was found to be greater at the higher air flow rates. The agreement between the results of the present investigation and the one previously made on the coils having in-line tubes arrangement suggested that the behavior of the overall coefficient of heat transfer and the air pressure drop were independent of coil geometry. For a system with a fixed capacity and design conditions the decrease in overall coefficient of heat transfer, during the heavy frosting, would lower the system efficiency by the reduction in the coefficient of performance of the refrigeration system. The increase in pressure drop at increasing frost accumulation on the coil surface would have a direct effect on lowering the system capacity by reducing the air flow in the system. The limited information made available by this investigation was not enough to cover the wide range of operating conditions and heat exchangers encountered in practice. More extensive research would be necessary to provide sufficient information for a better design of the refrigeration systems when the formation of frost on the heat exchanger surface could not be avoided.
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