Abstract: A heat exchanger tube having an integral restricting and turbulating structure consisting of dimples formed by confronting indentations pressed into the sides of the heat exchanger tube. The dimples are comprised of indentations disposed in pairs which extend into the tube to such a depth as is necessary to significantly reduce the cross sectional area of the heat exchanger tube. The dimples of a pair are staggered or offset, longitudinally with respect to each other such that a restrictive passage is defined between each pair of offset dimples. The turbulence characteristics of the tube can be controlled by varying the depth to which the dimples project into the tube and the longitudinal spacing between the dimples that comprise the pair. Adjacent pairs of dimples may be rotated 90° with respect to each other or alternately can be arranged in a helix pattern.

Abstract: A gas fired furnace capable of operating with a 16:1 turndown ratio or greater. The furnace includes a plurality of burners (10) grouped into at least (14a) first and second (14b) groups, each group connected to a source of combustible gas through a control valve (30a, 30b, 30c). The control valve (30c) controlling at least one group of burners is of a modulating type having an output proportional to a control signal applied to the valve. The burners fire into associated heat exchange tubes (20a), each tube having an inlet (24) and an outlet. The tube outlets are connected to a collector chamber (44) that includes a baffle plate (60) that divides the collector into two sections, one of the sections communicating with the outlets of the tubes associated with the first group of burners, the other section communicating with the outlets of the heat exchanger tubes associated with the other group of burners.