Flow Baffles

This implies that the LMTD or M I D as computed in equations 20 through 26 may not be a representative temperature difference between the two heat-transferring fluids for aU tubes. The effective LMTD or M ID would be smaller than the value calculated, and consequentiy would require additional heat-transfer area. The tme value of the effective M I D may be determined by two- or three-dimensional thermal—hydrauUc analysis of the tube bundle. Baffle—Tube Support PlateXirea. The portion of a heat-transfer tube that passes through the flow baffle—tube support plates is usuaUy considered inactive from a heat-transfer standpoint. However, this inactive area must be included in the determination of the total length of the heat-transfer tube. 

Through-ckculation compartments employ perforated or screen bottom trays and suitable flow baffles so gas is forced through the material. If material is not inherently pervious to gas flow, it may be mechanically shaped iato noodles, pellets, or briquettes. These dryers are used ia small-scale operations to dry explosives, foods, and pigments. Dryer efficiency is 50—70%. Based on tray area, water vaporization rates are 1—10 kg/(h-m ). 

Baffle cuts are expressed as the ratio of segment opening height to shell inside diameter. Cross-flow baffles with horizontal cut are shown in Fig. 11-36 7, c, andf. This arrangement is not satisfactoiy for horizontal condensers, since the condensate can be trapped between baf-... 

Another type of classifier directs an air stream across a stream of the particles to be classified. An example is the radial-flow classifier Kennedy Van Saun Corp.), which Features adjustable elements to control the flow and classification. A further development on this principle is the Vari-Mesh classifier Kennedy Van Saun Corp.), which controls classification by adjustable flow baffles. A change in direction of air flow is the operating principle of the reverse-flow Superfine classifier Hosokawa Mineral Processing Systems). 

The scmbber is comprised of one or more trays. Each tray contains numerous venturi openings. Each of the MultiVenturi openings is surmounted by a spider cage holding a floating Flexicap (see insert), in addition, each tray is equipped with one or more downcomers" and weir flow baffles that control the scrubbing liquid as it flows across the tray and then to the tray below. 

Besides preventing a destruction of the tubes, impingement plates serve to spread out and distribute the incoming fluid into the tube bundle. If they are used in proper relation to the bundle cross-flow baffles, the fluid can be effectively spread across the bundle near the inlet end. If this is not accomplished, part of the tube area will be stagnant, and its heat transfer will be less than the other parts of the... 

Nominal pipe size 30, schedule number 80 —Shell thickness 3 1 mm —Constructed from carbon steel —6 internal cross-flow baffles... 

For shell-and-tube heat exchangers with cross-flow baffles, the preceding methods assume that an adequate number of baffles has been provided. If the shell-side fluid makes less than eight passes across the tube bundle, the mean temperature difference may need to be corrected for this cross-flow condition. Appropriate curves are presented in Caglayan and Buthod [20]. The curves in this reference may also be used to determine correction factors for cross-flow exchangers with one shell pass and more than two tube passes. 

Longitudinal Flow Baffles In fixed-tube-sheet construction... 

Turbine designs are intermediate between paddles and propellers. Turbines are effective mixers over a wide viscosity range and provide a very versatile mixing tool. The ratio of radial to tangential flow, the predominating parameters with this impeller, increases as the operating speed increases. Pitched-blade turbines are sometimes used to increase axial flow. Baffles must be used to limit swirling unless the turbine is shrouded. 

Baffles. Longitudinal baffles are used in the shell to control the overall flow direction of the shell fluid as in F, G, and H shells. The transverse baffles may be classified as plate baffles and axial-flow baffles (rod, NEST, etc.). The plate baffles (see Fig. 17.4) are used to support the tubes, to direct the fluid in the tube bundle at about 90° to the tubes, and to increase the turbulence of the shell fluid. The rod (and other axial-flow) baffles (see Fig. 17.5) are used to support the tubes, to have the fluid flowing axially over the tubes, and to increase the turbulence of the shell fluid. Flow-induced vibration is virtually eliminated in rod and other axial-flow baffles having axial shellside flows. 

FIGURE 17.8 (a) Axial flow baffle, courtesy of Brown Fintube Company, Houston, Texas, (b) a twisted tube exchanger, courtesy of ABB Lummus Heat Transfer, Bloomfield, New Jersey. 

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