Similar reasoning selects stage 19 as the maximum throughput stage in the bottom section. Note that on this stage, the volumetric vapor flow rate is slightly lower than that of tray 18; however, the difference is about 0.4 percent, while the mass flow rate of stage 19 is 1.9 percent higher than that of stage 18, and therefore tray 19 is more appropriate. If in doubt, the C-factor [Eq. (6.4)] serves as the best indicator; here it shows that stage 19 has the greater vapor load.
minimum loads Table 6.10 shows that for the bottom section, stage 9 is the apparent choice for the minimum throughput calculation. Stage 20 is ignored since it simulates the reboiler. For the top section, either stage 1 or stage 8 has the lowest vapor and liquid loads. If turndown is critical, a separate hydraulic calculation is required at each of these two stages. However, the loadings are within 2 to 3 percent of each other, and minimum throughput calculation is seldom critical. The objective of the turndown calculation is to define where efficiency begins to fall off. In stage 1, the loadings are low because of the cold entering liquid; this is a local phenomenon that affects the efficiency of one, or at most a handful, of trays. Stage 8, on the other hand, is more characteristic of the stages above it. Therefore, the minimum throughput calculation here is only performed for stage 8. summary Stages selected for hydraulic calculations are
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