Tube shape selection for heat recovery from particle-laden exhaust gas streams

Heat recovery from exhaust gas streams is applicable to a wide variety of industries. Two problems encountered in exhaust gas heat recovery are: the high heat transfer resistance of gases and the presence of entrained particulate matter, which can limit the use of extended surface area. Standard heat exchangers use round tube. This study uses Computational Fluid Dynamics (CFD) to investigate whether round or another shape is the best tube selection for exhaust heat recovery. Tube shape rankings are based on taking into account heat transfer, gas flow resistance and foulability. Foulability is inferred from the average wall shear stress around the front or back of each shape. An estimated asymptotic fouling resistance is used to calculate an equivalent fouled j factor, jf. CFD results suggest the best tube for exhaust heat recovery is an elliptical tube. The ellipse shape produced j/f and jf/f ratios (where f is the tube bank friction factor) over 1.5 times larger than that of standard round tube. A flattened round tube is also promising and may be the practical and economic optimum

Numerical performance comparison of different tube cross-sections for heat recovery from particle-laden exhaust gas streams

Heat recovery from exhaust gas streams is applicable to a wide variety of industries. However, high heat transfer resistance of gases and the presence of entrained particulate matter that readily fouls limits industry uptake of current heat recovery technology. Improvements to standard heat exchanger designs are needed. In this study Computational Fluid Dynamics (CFD) is used to investigate the effect of ten different tube cross-sections on heat transfer resistance, gas flow resistance and foulability. The average wall shear stress around the shape is used to predict foulability and an estimated asymptotic fouling resistance is used to calculate an equivalent fouled Coulburn j factor, jf. CFD results show the best tube for exhaust heat recovery is an elliptical tube closely followed by the flattened round tube. The ellipse shape produced fouled Coulburn j factor, jf values, expressed as a ratio of tube bank friction factor f, over 100% higher than that of standard round tube. A flattened round tube is also promising, given enough spacing between the tubes, and may be the practical economic optimum

The effect of inlet conditions on the air side hydraulic resistance and flow maldistribution in industrial air heaters

Experimental system hydraulic resistance measurements on a scale air heater unit have highlighted the excessive hydraulic resistance of typical industry configurations. Both poor header inlet conditions and large header expansion angles are shown to contribute to system hydraulic resistance magnitudes 20–100% higher than suitable benchmark cases. Typical centrifugal fan system efficiencies well under 80% multiply the system resistance effects resulting in larger fan power penalties. Velocity profile measurements taken upstream and downstream of the test heat exchanger under flow maldistribution conditions provide insight into the flow maldistribution spreading caused by the heat exchanger resistance. The anisotropic resistance of the plate fin-and-tube heat exchanger is shown to result in resistance induced flow dispersion being concentrated in the axis parallel to the plate fins