Chapter 8 - Heat Transfer and Heat Exchangers

Abstract

This chapter reviews the basic concepts of heat transfer, which are applied primarily to heat exchangers. Heat exchangers are equipment for the transfer of heat between two fluids through a separating wall, and the three recognized modes of heat transfer are by conduction, convection, and radiation. The thermal conductivity is a fundamental property of substances that basically is obtained experimentally, although some estimation methods also are available. It varies somewhat with temperature, and in various heat transfer situations an average value over the prevailing temperature range often is adequate. Heat transfer between a fluid and a solid wall can be represented by conduction equations. It is assumed that the difference in temperature between fluid and wall is entirely due to a stagnant film of liquid adhering to the wall and in which the temperature profile is linear. In a heat exchanger, heat is transferred between hot and cold fluids through a solid wall. The fluids may be process streams or independent sources of heat such as the fluids or sources of refrigeration. Heat transfer may be degraded in time by corrosion, deposits of reaction products, or organic growths. These effects are accounted for quantitatively by fouling resistances. Combining individual film coefficients into an overall coefficient of heat transfer allows one to take into account a greater variety and range of conditions, and should provide a better estimate. Transfer of heat by direct contact is accomplished in spray towers, in towers with a multiplicity of segmented baffles or plates, and in various packed towers. In some processes, heat and mass transfer occur simultaneously between phases in water cooling towers, in gas quenching with water, and in spray or rotary dryers.