Cross-flow velocity exchangers

The baffle cut determines the fluid velocity between the baffle and the shell wall, and the baffle spacing determines the parallel and cross-flow velocities that affect heat transfer and pressure drop. Often the shell side of an exchanger is subject to low-pressure drop limitations, and the baffle patterns must be arranged to meet these specified conditions and at the same time provide maximum effectiveness for heat transfer. The plate material used for these supports and baffles should not be too thin and is usually minimum thick-... 

The concept of cross-flow velocity is quite important in understanding how heat exchangers work. This concept is related to a flow phenomenon called vortex shedding. Perhaps you have seen a wire quivering in the wind. What causes the wire to vibrate with such energy ... 

To encourage vortex shedding and turbulence on the shell side of a heat exchanger, we must increase the cross-flow velocity. To calculate the cross-flow velocity, we proceed as follows ... 

G = cross - flow velocity between tubes at the centerline of the exchanger... 

A cross-flow heat exchanger uses a bundle of 132 tubes perpendicular to the flow in a 0.6-m square duct. Water at 150°C, 0.5 m/sec velocity enters the tubes (inside and outside diameters of 10.2 and 12.5 mm). Air enters at 10°C with a volumetric flow rate of 1 m /sec. The heat transfer coefficient on the outside of the tubes is 400 W/m °K. What are the fluid outlet temperatures ... 

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. 

Also, AP, as discussed above for the shell-side flow (see preceding section on cross-flow velocity), varies with the number of passes, cubed (not squared). That is, flow path length doubles when going from two to four tube-side passes. Meaning, if I convert a two-pass exchanger to four-pass, the fluid has to flow twice as fast and twice as far. 

Favored locations for erosion-corrosion are areas exposed to high-flow velocities or turbulence. Tees, bends, elbows (Fig. 11.5), pumps, valves (Fig. 11.6), and inlet and outlet tube ends of heat exchangers (Fig. 11.7) can be affected. Turbulence may be created downstream of crevices, ledges (Fig. 11.8), abrupt cross-section changes, deposits, corrosion products, and other obstructions that change laminar flow to turbulent flow. 

Gj = mass velocity, lb/hr(fti) or, mass rate of flow on shell side of exchanger, lb/ (hr) (fti of flow area) ako, cross-flow on shell side. 

Increasing fluid velocity also reduces fouling tendency. Bennett et al. (2007) provided design guidelines for heavy fouling services with fluid velocity for shell-and-tube exchangers tube side velocity >2 m/s (6.5 ft/s) and shell side B-stream (the main cross-flow stream through the bundle) >0.6 m/s (2 ft/s). 

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