Cross-flow velocity shell-side

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. 

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 baffle used in the preceding equation has 20% segmental cuts. Shell-side cross-flow velocity ... 

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 ... 

The baffle cut, shown in Fig. 19.3, is usually about 20 to 30 percent of the diameter of the baffle. The smaller the baffle cut, the more perpendicular the flow across the tubes. Perpendicular flow encourages desirable cross-flow velocity and vortex shedding. But a smaller baffle cut will also increase the pressure drop on the shell side. 

Effect of shell-side pressure drop. Reducing the baffle spacing increases cross-flow velocity and improves heat transfer. But it also increases the shell-side pressure drop. Reducing the baffle cut also improves heat transfer, but increases AP. 

If the baffle spacing gets too small, the shell-side heat-transfer rate will actually worsen. This happens even though the cross-flow velocity increases. What can be done to correct this problem ... 

The function of seal strips is to interfere with, and hence reduce, the fluid flow through the bypass area. Often, one pair of seal strips is used for every 18 in of shell ID (inner diameter). These seal strips encourage good shell-side cross-flow velocity and also help reduce localized fouling, caused by low velocity. 

The shell-side cross-flow velocity may be altered in much smaller increments, by changing the tube support baffle spacing. This is one advantage of placing the fluid with the poorer heat-transfer properties on the shell side. But there is another, far more critical, advantage in placing the fluid with the poorer heat transfer properties on the shell side. 

Ti, 11 supply ternperamre of hot and cold streams, F 72, 2 target temperature of hot and cold streams, °F AT i hot end temperature approach, °F AT 2 cold end temperature approach, °F AT ijn logarithmic mean temperature difference (LMTD), °F u shell side cross-flow velocity or tube velocity, ft/h U overall heat transfer coefficient, Btu/(ft °F h)... 

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. 

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. 

When fluid flows at a 90° angle across a tube, it creates turbulence, which is called vortex shedding. The swirls created when a river flows across a tree stump is a common example. The swirls destroy the film resistance to heat transfer created by laminar layers of fluid surrounding the tube. My experience indicates that a shell-side cross-flow velocity of 3 to 5 ft/s is sufficient to obtain this benefit. 

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. 

Pj = shell-side pressure drop, psi friction factor, fP/ in. cross-flow mass velocity, lb/ (ft ) (hr) shell I.D., ft = number of baffles... 

The shell side is provided with a number of baffles to promote high velocities and largely more efficient cross flow on the outsides of the... 

The shell side is provided with a number of baffles to promote high velocities and largely more efficient cross fiow on the outsides of the tubes. Figure 8.4(c) shows a typical construction and flow paths. The versatility and widespread use of this equipment has given rise to the development of industrywide standards of which the most widely observed are the TEMA standards. Classifications of equipment and terminology of these standards are summarized on Figure 8.11. 

---