Question 10.9: Use Xist to obtain a final design for the vertical thermosyp...
Use Xist to obtain a final design for the vertical thermosyphon reboiler of Example 10.4.
Starting from the 15.25-in. unit rated in the previous example, the shell size is gradually increased until a suitable configuration is obtained. The following additional changes are made to the input data:
- The tube count is left unspecified so that it will be determined by the program based on the detailed tube layout.
- The central baffle spacing is adjusted to maintain B/d_{s} in the range 0.35 to 0.40, and the baffle cut is set accordingly at 35%.
- A fouling factor of 0.0005 h · ft² · ºF/Btu is included for steam to provide an added safety margin.
- The steam pressure is increased to 20 psia as suggested in Example 10.4.
- The steam flow rate is increased to 2450 lb/h to ensure a vapor generation rate of at least 15,000 lb/h. With lower steam rates, the simulations may converge to solutions having vapor rates slightly below the required value.
With these settings, the smallest viable unit is found to be an 18-in. exchanger with an over-design of approximately 7.9%. The Xist Output Summary for this case is given below. Notice that the baffle cut has increased to 37.97%; Xist adjusts this value for conformity with the tube layout, placing the baffle edge at the center of the nearest tube row. Re-running the case using the tube layout as input yields essentially the same results
The detailed output files from Xist were used to compile the design summary shown in the following table. Running a mechanical design program showed that 2-in. schedule 40 pipe is inadequate for the condensate nozzle; schedule 80 pipe is required for this nozzle. No other design modifications were indicated.
Design Summary for Example 10.9: Vertical Thermosyphon Reboiler
| Item | Value |
| Steam design pressure (psia) | 20 |
| Exchanger type | AEL |
| Shell size (in.) | 18 |
| Surface area (ft²) | 322 |
| Number of tubes | 157 |
| Tube OD (in.) | 1 |
| Tube length (ft) | 8 |
| Tube BWG | 14 |
| Tube passes | 1.0 |
| Tube pitch (in.) | 1.25 |
| Tube layout | Triangular |
| Number of baffles | 12 |
| Baffle cut (%) | 37.97 |
| Baffle thickness (in.) | 0.1875 |
| Central baffle spacing (in.) | 6.3 |
| Inlet baffle spacing (in.) | 14.72 |
| Outlet baffle spacing (in.) | 10.05 |
| Sealing strip pairs | 0 |
| Tube-side inlet nozzle | 6-in. schedule 40 |
| Tube-side outlet nozzle | 10-in. schedule 40 |
| Shell-side inlet nozzle | 4-in. schedule 40 |
| Shell-side outlet nozzle | 2-in. schedule 40* |
| Δ P_{i} (psi) | 2.16 |
| Δ P_{o} (psi) | 0.49 |
| Circulation rate (lbm/h) | 106,320 |
| Exit vapor fraction | 0.1445 |
| Vapor generation rate (lbm/h) | 15,363 |
| Steam flow rate (lbm/h) | 2450 |
| \left(\hat{q} / \hat{q}_c\right)_{\max } | 0.20 |
| Flow stability assessment | Stable |
| Two-phase flow regime(s) | Bubble, slug, annular |
| Boiling mechanism(s) | Nucleate, convective |
| Over-design (%) | 7.88 |
| *Schedule 80 required per mechanical design run. | |
The exchanger drawing and tube layout diagram from Xist are shown below. Tie rods were added manually using the tube layout editor. Notice that an impingement plate is provided at the steam inlet nozzle, as required for a saturated vapor. In addition, a shellside vent should be included to purge any non-condensable gases that may enter with the steam. The vent nozzle, which is not shown on the exchanger drawing, should be located above and close to the condensate nozzle (see Section 11.2.3).
Xist Output Summary for Example 10.9
