A R N O L D REED
SIMPLIFIED TUBE CQUNTS For the practical man, here is a shortcut to accurate heat exchanger rating
t some time in the course of determining thermal
A rating of a heat exchanger, it is necessary to have an exact tube count. Actual number of tubes will be less than the theoretical, because of pass partitions, U-bends, tie rods, impingement baffle, and possibly a longitudinal baffle. Sormallg, an exact count is obtained by the drafting room after the engineer has optimized the design around an assumed count. This procedure produces less than optimum design or requires at least one more rating. Described here is a method enabling the rater to determine exact counts with no more than a compass, a small straight edge, graph paper, and special scales. Instead of using a standard scale-e.g. scale or scale-and drawing pitch lines with the required spacing, the tube pitch is maintained constant and the scale is varied as required. I find it easiest to let the tube pitch equal l j 4 inch on the drawing, and adjust the scale accordingly. The figure shows four such scales, for common tube I1j+ four tube spaces of pitches. Thus for the scale 1 1 / 4 inches each would measure an actual one inch. Rectilinear or triangular ruled graph paper with inch spacing is readily available (rectilinear : Dietzgen
AUTHOR Arnold Reed is an Encironmental Control Engineer f o r Grzimman Aircraft Engineering Corp., Bethpage, :V. Y . T h i s method was developed from the author‘s experience in rating heat exchangers for the Lummus Co., Hydrocarbon Research, Inc., and Fluid Controls Co. 44
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
340-4 or Keuffel and Esser 358-1 ; triangular: Dietzgen 340-1 or K Sr E 358-29B isometric). The figures on the next page show the scales and show how a pass partition and an impingement baffle can be taken into account. The first step after assuming sliell size and tube pitch is the determination of the “safe circle”, rather than the “outer tube limit” (the usual defining limit). The safe circle encompasses the center lines of all the usable tubes. The diameter of this circle is the inside diameter of the shell, minus twice the ckarance between the shell and the baffle, minus twice the ligament, minus the diameter of tlie tube clearance hole. Starting with tube zero in the first row, the center of the safe circle was determined by the displacement which in the figure is the pass partition allowance. A 23-inch diameter safe circle was then drawn using the = l6Il6 scale. The location of the impingement baffle was determined, in this instance, by measuring down ‘I4of the nozzle inside diameter from the centroid of the segmental area overlapped by the shell inside diameter and the nozzle. Tie rods, longitudinal baffles, pass partitions (if used) would be located where required. The tabulation given of the number of intersections of the triangular lines is the number of usable tubes in the given geometry. In the example 454 tubes can be used, less of course any other deductions. This method can be applied to the design of other equipment such as bubble cap trays in distillation towers or even rivet spacing in abnormal envelopes. For such use the designer can make the appropriate scales.
