Experimental Investigation of Effects of Equipment Size on Convection Heat Transfer and Flow Resistance in Cross Flow of Gases Over Tube Banks

Abstract

Abstract To establish the effect of tube size as a factor in heat transfer and pressure drop for gases flowing transversely over tube banks of a given tube arrangement, new experimental determinations have been made for several tube sizes, and results compared with those of Pierson who used very small tubes of 0.31 in. diameter. The arrangements used included some of those studied by Pierson and these tests on a larger scale have served as a check on the accuracy of his small-scale results with a different method and different imposed conditions, and the applicability of model results to full-scale equipment, the two series forming parts of a research program of the Babcock & Wilcox Company. Nine tube arrangements as shown in Fig. 1, defined by center spacings in the direction of flow and transversely, were used with tubes of both ½ in. diameter and 11/12 in. diameter, the spacing ranging from a minimum of 1¼ × 1¼ diameters, to a maximum of 2 × 3 diameters with ten rows in the direction of flow both in-line and staggered, and ten tubes wide except for one case of eight tubes wide. In addition, three arrangements of tubes of 2-in. diameter were used in banks of ten rows deep and from nine to fifteen tubes wide. For the banks of smaller tubes, ½ in. and 11/12 in. diameter, heat was transferred from hot gases outside to cold water inside, while for the banks of 2 in. tubes, heat was transferred from condensing steam inside to air outside. Values of gas-boundary conductance and pressure drop were found for several identical tube arrangements and spacing and three tube sizes of ½ in., 11/12 in., and 2 in., with heat flow inward and outward; these are found to be consistent with those for the Pierson model tubes of 0.31 in. diameter. Heat-transfer rates are reported in terms of the Nusselt number, and pressure drop in terms of the Fanning equation for friction factor, both of them in relation to the Reynolds number. For values of the Reynolds number in the range of commercial practice, the Nusselt number varies as the 0.61 power of the Reynolds number, and the friction factor is not so regularly related to the Reynolds number but varies with the tube arrangement. Values of both gas-boundary conductance and friction factor for a given tube arrangement are consistent for all tube sizes from the full-scale 2-in. tubes down to the Pierson model tubes of 0.31 in., including the intermediate sizes of ½-in. and 11/12-in. tubes, thus confirming the validity of the principle of similarity applied to tube banks in spite of some departure from true geometric similarity in the ratio of length to diameter or to intertube space for the range of Reynolds’ numbers tested, 2000 to 70,000.