Abstract
The heat transmitting tubes used in multitube heat exchangers are desired to have such characteristics as superior heat transmission capacity and minor draft resistance loss. However, heat transmission capacity and draft resistance loss are generally in a contradicting relation, so that the overall performance including the effect of these characteristics should be taken into account in designing heat exchangers. Hitherto, circular tubes and flat tubes have been used as heat transmitting tubes, and many fins have been attached on the gas flow side of tubes to enlarge their heat transmission area. While many reports have been published on the studies of such finned heat transmitting tubes, there are few reports systematically studied on bare or finned tubes with various cross-sectional shapes. In this paper, the effects of cross-sectional shapes of the tubes on the heat transmission capacity and draft resistance loss were experimentally investigated. Further, heat transfer coefficients of the tubes were estimated under the constant resistance horse power to drive the outer air flow, and their overall performance including two characteristics above mentioned was also estimated. It will be preferable to discuss the characteristics of the elliptical tubes in relation to those of a circular tube which is considered as the fundamental shape of heat transmitting tubes. In this experiment, six kinds of the elliptical tubes were prepared, in which a circular tube as an elliptical tube with the maximum axial ratio (ratio of minor and major axes) and an elliptical tube with the minimum axial ratio similar to that of a flat tube were included. Each of the heat transmitting tubes is of copper and has the same outside perimeter of 79.8mm, thickness of 0.8mm, and effective length of 400mm. The heat transmitting tubes were arranged in a single row with the tube pitch of 80mm. Heat was transferred in the cross flow from air of a constant temperature of 80℃ to the tube, with the air velocity ranging from 3.5 to 12m/s, and the flow rate of the water in the tubes was ranged from 3 to 20kg/min. All temperatures were measured with thermocouples and all pressures were measured by pitot tubes. The main results obtained were as follows. When the resistance horse power was low (the air velocity was low), the elliptical tube with the cross section similar to that of a flat tube was superior in the overall performance. However, when the resistance horse power was large (the air velocity was large), the excellent overall performance was obtained with the circular tube. Some problems may occur from a point of the structural strength for elliptical tube, when the pressure difference between the fluids flowing outside and inside of the tube is large. These problems, however, may be relieved by attaching many fins on the outside surfaces of the heat transmitting tube, and therefore, in case of the low air velocity the use of the elliptical tube with cross section similar to flat tube will reduce the drafting power for given heat transmission capacity, and the economical operation may be realized.
