Abstract
The transfer functions of heat exchangers to changes in flow rate, have been derived in most papers assuming that the wall has an infinite thermal conductivity in the radial direction. However, poor heat conductors such as glass and stainless steel, are sometimes used as shell wall and/or tube wall.
In the present paper, the transfer functions, relating the outlet temperature to changes in flow rate, for parallel-flow and counterflow heat exchangers, are obtained by taking into account the finite values of thermal conductivities of walls in the radial direction. Numerical examples of frequency and step responses are given, and the effects of the finite thermal conductivities on the dynamic response are discussed. The theoretical response derived in this paper is compared with the one computed under the assumption that the radial thermal resistance of the tube wall is included in the film coefficients of heat-transfer. As the result, it is shown that there is, in some cases, a considerable difference between the two responses stated above, for the heat exchangers with relatively thick walls made of bad heat conductors. The experiments are carried out based on the frequency response method. A good agreement between the experimental and theoretical results is shown.
