Differential Performance of Seven Compact Heat Exchangers for Aviation Thermal Management

The characteristics of seven heat exchangers (HEXs) (shell and tubes, concentric tubes, plate-fin, plate-frame, manifold-microchannel, cold plate, and cross flow (radiator) ) when used to remove approximately 35,320 W of heat are compared in this study in terms of the process conditions and geometry. The results compared include the heat flow rate, tube or passage counts, outlet temperatures, pressure losses in the hot and cold streams, log-mean temperature difference (LMTD) or mean temperature difference (MTD), overall heat transfer coefficient (U-value), heat transfer area, empty weight, total weight, effectiveness, number of transfer units (NTU), coefficient of performance (COP), and the F-factor. The approach used is that of sizing, in which we ask the thermal analysis software package, which is INSTED in this study, to size a HEX for the required duty. The realization from the sizing is task is selected and put through a normal rating analysis to determine the performance. The agreement for some metrics between a few of the devices is quite surprising but pleasing. The shell and tubes HEX, as expected, shows up very badly, in terms of the size and the weight, even when the same material is assumed as used for the Plate-fin HEX, and the fin thicknesses in plate-fin are used for the thicknesses of the shell, tubes, and baffles in the shell and tubes device. Default software enforcement of TEMA standards for the equipment – other than the thicknesses, which we have relaxed-and particularly, the lack of compactness, are responsible for the relatively huge size and weight of the shell and tubes HEX. The results for five of the heat exchangers are tabulated in this paper. Besides the need to satisfy the energy, mass, and momentum conservation equations across the two streams of a regular HEX, there is also the implicit requirement to match the heat flow rate obtained from an overall U-value with that obtained from heat capacity, which is a requirement that is not explicitly built into the conservation equations. Accordingly, for the plate-frame and shell and tubes HEXs, the introduction of a fouling factor was necessary in order to obtain the specified thermal duty.