Importance of Shell-Side Cross-Flow

Shell-side flow should be mostly at right angles or perpendicular to the tubes. Unavoidably, as the fluid flows from the inlet nozzle to the outlet nozzle, there is some component of flow parallel to the tubes. The bigger the tube support baffle spacing, the greater the component of the flow parallel to the tubes. A larger baffle cut also increases the component of the flow parallel to the tubes. Thus, to promote perpendicular flow to the tubes or cross-flow, velocity baffle spacing should be about 20 to 30 percent of the bundle diameter. The baffle cut should be about 25 to 35 percent of the bundle diameter. By baffle cut, I mean the cutout section of the round baffle. 

Reducing the baffle spacing and the baffle cut increases the shell-side pressure drop. But that s the price we must pay for improved heat-transfer rates. 

I ve read about the importance of shell-side cross-flow in heat-exchanger design books. Higher cross-flow velocities reduce film resistance and promote increased heat-transfer rates. Recently, I had a dramatic personal experience to support this engineering principle. 

A refinery in Durbin, South Africa, had a debutanizer reboiler with an observed heat-transfer coefficient of 25 Btu/hr/ff/°F. The reboiler was a horizontal, thermos) hon, shell-and-tube exchanger. 

Any time 1 find that basic engineering principles actually apply in practice, 1 am always pleasantly surprised. 1 hope for the best, but plan for the worst. My fear of failure is a basic ingredient in good engineering design. 

---