Tests of Turbulators for Fire-Tube Boilers

Heat transfer coefficients and friction factors were obtained for three turbulator inserts used in fire tube boilers. An electrically heated flow facility was developed to deliver hot air to a water-cooled steel tube of 2.675 in. i.d. and 71 3/4 in. length. Tube wall temperatures, fluid bulk temperatures, and flow rates were measured. The cooling coil was segmented so that sectional average heat transfer coefficients could be derived for four regions of the tube. Reference data for the empty tube are in excellent agreement with the usual correlations. Two commerical turbulators, consisting of narrow, thin metal strips bent and twisted in zig-zag fashion to allow periodic contact with the tube wall, displayed 125 and 157% increases in heat transfer coefficients at a Reynolds number of 10,700. Insert orientation had at most a 10% effect on the heat transfer coefficient. A third commercial turbulator, consisting of a twisted strip with width slightly less than tube diameter, provided a 60% increase in heat transfer coefficient. The friction factor increases accompanying these heat transfer coefficient increases were 1110%, 1000%, and 122%, respectively. These data should be helpful in assessing the overall performance gains to be expected when the turbulators are used in actual boilers. Originator-supplied keywords include: Turbulators, Heat transfer augmentation, Enhancement, Fire tube boilers. (Author)