Question 11.10: Use Xist to obtain a final design for the C4–C5 condenser of...

Using Xist to rate the 39-in. J-shell condenser in the previous example, the following two problems were identified:

• The condenser is over-sized.
• The vibration analysis predicts tube vibration problems.

The tubes in the vicinity of the inlet nozzles are the most susceptible to vibration problems. Hence, the following changes are made to mitigate vibration in the inlet regions:

• A full support plate is specified at the U-bend to keep velocities negligible in the U-bend region. This specification is made on the Geometry/Baffles/Supports panel.
• The circular impingement plates are replaced with impingement rods to reduce the bundle and shell entrance velocities. The rod diameter is set at 0.75 in. to match the tube size, and the rod pitch is set at 1.125 in. (1.5 times the rod diameter). These changes are made on the Geometry/Nozzles/Impingement panel.
• A vibration support is installed under the impingement rods in each inlet region. These support plates encompass only the first few tube rows below the rods. This specification is made on the Geometry/Baffles/Supports panel.

The over-sizing is addressed by reducing the shell ID until an acceptably small over-design is achieved. In this process, the number of tubes is left unspecified, allowing Xist to determine the tube count from the tube layout. As the shell size is reduced, it is also necessary to reduce the number of tube passes from four to two in order to keep the tube-side pressure drop within the prescribed limits. In this manner, a 35-in. or 34-in. shell is found to be marginally acceptable with a small positive or small negative overdesign, respectively. In either case, an acceptable margin can be achieved by increasing the tube length. For the purpose of this example, the 34-in. unit is selected and the design changes are summarized as follows:

• The shell ID is reduced from 39 in. to 34 in.
• The number of tube passes is reduced from 4 to 2.
• The tube length is increased from 16 ft to 18 ft.
• The baffle spacing is rounded to 14 in., giving B/d_{s} = 0.41.
• The baffle cut is automatically adjusted by Xist to 35.67% for compatibility with the tube layout.

The Output Summary for this case is shown below, from which it is seen that the over-design is about 10%. In addition, the baffle cut and baffle spacing fall within the recommended region for condensing vapors in Figure 5.3. The maximum unsupported tube length is 36.7 in., which is safely within the TEMA limit from Table 5.C.1 (material group B).
Despite the measures taken to minimize tube vibration, Xist still flags a potential vibration problem in the inlet regions. However, Xist performs a screening analysis only; a more precise analysis can be performed with Xvib, which is a software tool to perform finite element analysis of a single tube. Using Xvib to analyze the tubes in the vicinity of the nozzles confirms that vibration levels are satisfactory in the inlet regions and, therefore, should not be a cause of tube damage. Hence, the design is considered to be acceptable. However, acoustic vibrations are still a potential problem in this unit, and deresonating baffles may be needed for noise control.

Due to the challenges of a J-shell design with respect to vibration, an X-shell design should be considered for this application. Vibration is mitigated in the X-shell condenser by implementing the following features:

• Eight support plates spaced uniformly along the tube bundle. Support plates are specified on the Geometry/Baffles/Supports panel.
• A full support plate at the U-bend, as in the J-shell design.
• Three 10-in. inlet nozzles to better distribute the vapor along the length of the tube bundle and to reduce the inlet velocity.

The following additional specifications are made:

• Three 8-in. schedule 40 outlet nozzles. These nozzles will be self-venting.
• Circular impingement plates, which are selected on the Geometry/Nozzles/Impingement panel.
• The number of sealing strip pairs is set to be chosen by the program on the Geometry/Tube Layout/Clearances panel.

Starting with a 35-in. shell and a maximum tube length of 20 ft, an acceptable design is readily found consisting of a 33-in. shell containing 878 tubes. The over-design is 10% and Xist generates no warning messages concerning potential vibration problems. Using the tube-layout editor, 8 tubes (4 U-tubes) are replaced by tie rods, leaving a bundle with 870 tubes. Re-running the program with this tube bundle as input produces the Output Summary shown below.

Design details for both the J-shell and X-shell units are summarized in the table below. Tube layouts and exchanger drawings from Xist are also shown. Notice that the horizontal pass-partition lane, in combination with the side-to-side flow pattern, results in an F-stream bypass flow in the J-shell condenser. Four seal rods with diameters of 0.75 in. (as specified by Xist) are employed to partially block the bypass-flow area.

Design Summaries for Example 11.10