4-12 The criteria by which the concepts are to be evaluated are discussed below. The parametric charts used to select an optimized straight-tube and helical-tube bundle are included. The selected tube-bundle sizes are indicated on the concept sketches in section 4.3.3. 4.3.2 I '
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Selection Criteria
The selection criteria that were applied to various IHX concepts are primary salt inventory: thermal stresses; thermal-hydraulic performance; manufacturing; and inspection, maintenance, and repair. Primary Fuel Salt Inventory
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Designs with low primary salt volumes appear highly desirable due to the design requirement to minimize fuel inventory. In most conventional designs of heat exchangers the ratio of tube-side volume to shellside volume is low (Figure 4.11). When using these conventional designs, the arrangement of primary fuel salt on the tube side is mandatory. For example, the reference tube bundle desigi chosen has 3/8in.diameter tubes and 518 in. pitch (or 1/4in. between tubes). Figure 4.11 shows that the ratio of shell-side volume to tube-side volume is about 4.The chart is based on a straight-tube bundle, but the results can be applied to most tube bundle designs. Figure 4.12 shows a typical arrangement of parameters for a straight-tube-bundle IHX. This chart shows the tradeoff among length, bundle diameter, tube size, tube spacing, and tube-side and shell-side pressure drop. For easy comparison rough tubeside volume numbers, corresponding to each tube size, are given. The advantage of the smaller sized tubcs can be easily noted when envelope and primary volume are considered. For most tube designs, such as the sine-wave, the C tube, the hockey-stick, and the J tube, the envelope of the heat transferring zone is about the same as that for a straight bundle. There will be small differences, but most designs are assumed to have 3/8 111 outside-diameter tubes on a 5/8 in. pitch, and the salt volume penalty will depend on the bundle ends and not on the main heat transferring region.
The helical-tube bundle is different. It would be extremely difficult to use 3/8 in. diameter tubes, as the bundle heightldiameter ratio would be quite small. Accordingly, the helical-tube design is heavily penalized on a volume basis. Thermal Stress
The stress criterion, which is used as a measure of the adequacy of the various IHX designs, is the magnitude of the tube stresses caused by differential thermal expansion. It is not immediately obvious why this condition exists. When stress analysis is performed,