Understanding Baffle Types and Design Guidelines in Heat Exchangers

Explore different types of baffles and their design guidelines in shell-and-tube heat exchangers to optimize flow, support, and heat transfer efficiency.

Baffle types and their design guidelines are described in this blog.

 

As presented in earlier blog on baffles in shell and tube heat exchanger, baffles may be classified as either longitudinal or transverse type. Longitudinal baffles are used to control the overall flow direction of the shell fluid. Transverse baffles may be classified as plate baffles or grid baffles. Plate baffles are used to support the tubes, to direct the fluid in the tube bundle at approximately right angles to the tubes, and to increase the turbulence and hence the heat transfer coefficient of the shell fluid. However, the window section created by the plate baffles results in excessive pressure drop with insignificant contribution to heat transfer; flow normal to the tubes in crossflow section may create flow induced vibration problems. The rod baffles, a most common type of grid baffles, shown in Fig. 1.11, are used to support the tubes and to increase the turbulence. 

Flow in a rod baffle heat exchanger is parallel to the tubes, and hence flow-induced vibration is virtually eliminated by the baffle support of the tubes. The choice of baffle type, spacing, and cut are determined largely by the flow rate, required heat transfer, allowable pressure drop, tube support, and flow-induced vibration. The specific arrangements of baffles in various TEMA shells are shown in Fig. 10.3. Plate Baffles.

Two types of plate baffles, shown in Fig. 1.10 are segmental, and disk and doughnut. Single and double segmental baffles are used most frequently.

The single segmental baffle is generally referred to simply as a segmental baffle. The practical range of single segmental baffle spacing is 1/5 to 1 shell diameter, although optimum could be 2/5 to 1/2.  The minimum baffle spacing for cleaning the bundle is 50.8 mm (2 in.) or 1/5 shell diameter, whichever is larger. Spacings closer than 1/5 shell diameter provide added leakage. These are tube to baffle hole, baffle to shell, bundle to shell, and the tube pass partition leakages or bypasses described as – Shell-side Flow Patterns. Even though one of the major functions of the plate baffle is to induce crossflow (flow normal to the tubes) for higher heat transfer coefficients and hence improved heat transfer performance, this objective is not quite achieved in conventional shell-and-tube heat exchangers. This is because various clearances are required for the construction of the exchanger and the shell fluid leaks or bypasses through these clearances with or without flowing past the tubes (heat transfer surface). Three clearances associated with a plate baffle are tube-to-baffle hole clearance, bundle-to-shell clearance, and baffle-to-shell clearance. In a multipass unit, the tube layout partitions may create open lanes for bypass of the crossflow stream that nullifies the heat transfer advantage of closer spacings. If the foregoing limits on the baffle spacing do not satisfy other design constraints, such as p-max or tube vibration, no-tubes-in-window or pure crossflow design should be tried. The segmental baffle is a circular disk (with baffle holes) with one disk segment removed. 

The baffle cut varies from 20 to 49% (the height lc in Fig. 8.9 given as a percentage of the shell inside diameter), with the most common being 20 to 25%. At larger spacings, it is 45 to 50%, to avoid excessive pressure drop across the windows as compared to the bundle. Large or small spacings coupled with large baffle cuts are undesirable because of the increased potential of fouling associated with stagnant flow areas.

If fouling is a primary concern, the baffle cut should be kept below 25%. The baffle cut and spacing should be designed such that the flow velocity has approximately the same magnitude for the cross flow and window flow sections. Alternate segmental baffles are arranged 180 degree to each other, which cause shell-side flow to approach crossflow in the central bundle region y and axial flow in the window zone. All segmental baffles shown in Fig. 1.10 have horizontal baffle cuts. The direction of the baffle cut is selected as follows for shell-side fluids: Either horizontal or vertical for a single-phase fluid (liquid or gas), horizontal for better mixing for very viscous liquids, and vertical for the following shell side applications: condensation (for better drainage),  evaporation/boiling (for no stratification and for providing disengagement room), entrained particulates in liquid (to provide least interference for solids to fall out), and multishell pass exchanger, such as those in Fig. 1.62 and the F shell.

Since one of the principal functions of the plate baffle is to support the tubes, the terms baffle and support plate are sometimes used interchangeably. However, a support plate does not direct the fluid normal to the tube bank, it may be thicker than a baffle, it has less tube-to-baffle hole clearance, and it provides greater stiffness to the bundle.  Support plates with single-segmental baffles are cut approximately at the centerline and spaced 0.76 m (30 in.) apart. This results in an unsupported tube span of 1.52 m (60 in.) because each plate supports half the number of tubes. The double-segmental baffle (Fig. 1.10), also referred to as a strip baffle, provides lower shell-side pressure drop (and allows larger fluid flows) than that for the single segmental baffle for the same unsupported tube span. The baffle spacing for this case should not be too small; otherwise, it results in a more parallel (longitudinal) flow (resulting in a lower heat transfer coefficient) with significant zones of flow stagnation. Triple-segmental baffles have flows with a strong parallel flow component, provide lower pressure drop, and permit closer tube support to prevent tube vibrations. The lower allowable pressure drop results in a large baffle spacing. Since the tubes in the window zone are supported at a distance of two or more times the baffle spacing, they are most susceptible to vibration. To eliminate the possibility of tube vibrations and to reduce the shell-side pressure drop, the tubes in the window zone are removed and support plates are used to reduce the unsupported span of the remaining tubes. The resulting design is referred to as the segmental baffle with no-tubes-in-window, shown in Fig. 1.10. The support plates in this case are circular and support all the tubes. The baffle cut and number of tubes removed varies from 15 to 25%. Notice that low-velocity regions in the baffle corners do not exist, resulting in good flow characteristics and less fouling. Thus the loss of heat transfer surface in the window section is partially compensated for. However, the shell size must be increased to compensate for the loss in the surface area in the window zone, which in turn may increase the cost of the exchanger. If the shell-side operating pressure is high, this no-tubes-in-window design is very expensive compared to a similar exchanger having tubes in the window zone. The disk-and-doughnut baffle is made up of alternate disks and doughnut-shaped baffles, as shown in Fig. 1.10. Generally, the disk diameter is somewhat greater than the half-shell diameter, and the diameter of the hole of the doughnut is somewhat smaller than the half-shell diameter. This baffle design provides a lower pressure drop compared to that in a single-segmental baffle for the same unsupported tube span and eliminates the tube bundle-to-shell bypass stream C. The disadvantages of this design are that (1) all the tie rods to hold baffles are within the tube bundle, and (2) the central tubes are supported by the disk baffles, which in turn are supported only by tubes in the overlap of the larger diameter disk over the doughnut hole. Rod Baffles. Rod baffles are used to eliminate flow-induced vibration problems.

For certain shell-and-tube exchanger applications, it is desirable to eliminate the cross flow and have pure axial (longitudinal) flow on the shell side. For the case of high shell-side flow rates and low-viscosity fluids, the rod baffle exchanger has several advantages over the segmental baffle exchanger: (1) It eliminates flow-induced tube vibrations since the tubes are rigidly supported at four points successively; (2) the pressure drop on the shell side is about one-half that with a double segmental baffle at the same flow rate and heat transfer rate. The shell-side heat transfer coefficient is also considerably lower than that for the segmental baffle exchanger. In general, the rod baffle exchanger will result in a smaller-shell-diameter longer-tube unit having more surface area for the same heat transfer and shell-side pressure drop; (3) there are no stagnant flow areas with the rod baffles, resulting in reduced fouling and corrosion and improved heat transfer over that for a plate baffle exchanger; (4) since the exchanger with a rod (grid) baffle design has a counterflow arrangement of the two fluids, it can be designed for higher exchanger effectiveness and lower mean (or inlet) temperature differences than those of an exchanger with a segmental baffle design; and (5) a rod baffle exchanger will generally be a lower-cost unit and has a higher exchanger heat transfer rate to pressure drop ratio overall than that of a segmental baffle exchanger. If the tube-side fluid is controlling and has a pressure drop limitation, a rod baffle exchanger may not be applicable. Refer to Gentry (1990) for further details on this exchanger.

Impingement Baffles. Impingement baffles or plates are generally used in the shell side just below the inlet nozzle. Their purpose is to protect the tubes in the top row near the inlet nozzle from erosion, cavitation, and/or vibration due to the impact of the high velocity fluid jet from the nozzle to the tubes. One of the most common forms of this baffle is a solid square plate located under the inlet nozzle just in front of the first tube row, as shown in Fig. 10.2.

FIGURE 10.2 Impingement baffles at the shell-side inlet nozzle. (From Bell, 1998.)

 

The location of this baffle is critical within the shell to minimize the associated pressure drop and high escape velocity of the shell fluid after the baffle. For this purpose, adequate areas should be provided both between the nozzle and plate and between the plate and tube bundle. This can be achieved either by omitting some tubes from the circular bundle as shown in Fig. 10.2 or by modifying the nozzle so that it has an expanded section (not shown in Fig. 10.2). Also, proper positioning of this plate in the first baffle space is important for efficient heat transfer.

 

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What challenges have you faced when designing or maintaining baffle systems in heat exchangers, and how did you overcome them? Share your experiences and tips in the comments!👇