Question 11.11: 100,000 kg/hr of propane are condensed on the shell-side of ...

The overall attributes of a design with single segmental baffles are shown in the tube layout and exchanger setting plan given below. This design will work satisfactorily, but the baffles will cause a buildup of condensate. This buildup may interfere with the desired weir-type drainage in the outlet nozzle, and some tube rows may become submerged. Designers often cut the bottom of the baffles to facilitate drainage.

Another approach is to use double segmental baffles. The tube layout for a double segmental design with two tube rows of overlap at the baffle cuts is shown below. As can be seen, only the center baffle blocks condensate drainage and designers often cut the bottom of the center baffles. The shell-side pressure drop decreases from 12.52 kPa to 3.49 kPa. In some situations this pressure change can significantly increase the mean temperature difference, but in this case the improvement is negligible since the inlet pressure is high. The Rating Data Sheet generated by Xist for this design is also shown below:

From the Rating Data Sheet, the thermal resistance on the shell-side dominates (56.59% of the total resistance) and low-finned tubes are a suitable enhancement for condensation with low-surface-tension fluids. For this application with U-tubes, a tube supplier that can provide finning on the straight tube portion only would be needed to avoid wall-thinning under the fins. The tube velocity should remain greater than 1 m/s to reduce sedimentation fouling but less than 2.5 m/s to minimize the potential for erosion and avoid excessive pressure drop. Since fewer low-finned tubes are needed than plain tubes, lower cooling water flow rates can be applied, which reduces pumping costs but also lowers the EMTD. With enhanced surfaces, lower fouling margins should be considered to prevent fouling margin from becoming the dominant thermal resistance. In this case we consider zero fouling allowance on the shell side (since propane condensation is a clean service) and half the original fouling resistance on the tube side (since the velocity is sufficient to minimize sedimentation fouling).
Using the above guidelines, condenser designs were obtained with low-finned tubes and double-enhanced tubes in place of plain tubes. In the table below the results are compared with those for plain tubes obtained in part (a). Carbon steel tubes with a straight tube length of 6.096 m and double segmental baffles are employed in all three cases. Low-finned tubes provide a significant advantage over plain tubes with respect to shell size and associated tube count, which is reduced by more than 50%. Compared with low-finned tubes, the double enhanced tubes provide a relatively small advantage, which is not surprising since the tube-side thermal resistance was not very high to begin with. (For the low-finned design, the shell-side and tube-side resistances are nearly equal.) Low-finned tubes, and possibly double-enhanced tubes, can provide a cost-effective alternative to plain tubes, particularly for process upgrades when duty increases are desired.

*1-in. OD carbon steel tubes
**1-in. OD plain tube end, 1024 fins/meter, 1.24 mm fin height
***Wieland GEWA-KS tubes with 1-in. OD plain ends, low fins on the outside (748 fins/meter, 1.5 mm fin height) and helical fins on the inside surface