Helical Baffless

Figure 10.14. Some kinds of in-line mixers and blenders, (a) Mixing and blending with a recirculating pump, (b) Injector mixer with a helical baffle, (c) Several perforated plates (orifices) supported on a rod. (d) Several perforated plates flanged in. (e) Hellical mixing elements with alternating directions (Kenics Corp.). (f) Showing progressive striations of the flow channels with Kenics mixing elements.

The preferred method for cutting the helical baffles is to use a milling machine with an indexing head and a 1/8" thick disc cutter. The tangent table angle is set to 87 for proper cutter clearance. Depth of cut is set to. 4375". Spiral pitch is. 25" (one turn in. 25"). Gear change (velocity ratio) is 1/40 for machines with a standard pitch of 10 inches. For other machines, use the fol-... 

Figure 8.19 shows the flux-time profiles obtained in filtration of 5% yeast cell suspension using a mbular membrane of 6 mm i.d. (inside diameter) and 0.14 pm pore size with a helical baffle (HB), a rod baffle (RB), and the mbular membrane without baffle (NB) [35]. The comparison has been made at the same hydraulic-dissipated power, which is defined as the product of the flow rate and the pressure drop along the mbular membrane, or the energy consumed to generate the crossflow through the mbular membrane. Using the hydraulic-dissipated power rather than the crossflow rate as a control parameter for the comparison of the mbular membrane with and without inserts eliminates the effect of the reduced crossflow section by... 

FIG. 11-40 Plate baffles, [a) Baffle cuts for single segmental baffles, h) Baffle cuts for double segmental baffles, (c) Baffle cuts for triple segmental baffles, d) Helical baffle construction. 

A great amount of proprietary research has been conducted by a few companies into the workings of helical baffled heat exchangers. The only known open literature method for estimating helical baffle performance has been Comparison of Correction Factors for Shell-and-Tube Heat Exchangers with Segmental or Helical Baffles by Stehlik, Nemcansky, Krai, and Swanson [Heat Transfer Engineering, 15(1), 55-65]. 

Unique design variables for helical baffles include the baffle angle, adjacent baffle contact diameter (which sets the baffle spacir and is usually about half of the shell I.D.), and the number of baffle starts (i.e., number of intermediate baffle starts). Of course, consideration is also given to the tube layout, tube pitch, use of seal strips, and all the other configuration characteristics common to any plate baffled bundle. 

A helical baffle bundle built in this w produces two distinct flow regions. The area outside of the adjacent baffle contact diameter tends to produce a stable helical cross flow. However, inside the diameter where adjacent baffles touch is a second region where vortical flow is induced but in which the intensity of the rotational component tends to decrease as one approaches the center of the bundle. For a fixed flow rate and helix angle, this tendency may be minimized by the proper selection of the baffle contact diameter. With the correct selection, stream temperatures may be made to be close to uniform across the bundle cross section through the shell. However, below a critical velocity (for the baffle configuration and fluid state), the tendency for nonuniformity of temperatures increases as velocity decreases until ever-increasing portions of the central core surface area pinch out with respect to temperature and become ineffective for further heat transfer. 

The potential for fouling on the shell side of shell and tube heat exchangers is well known. The problem arises largely because the fluid in the shell does not flow uniformly across the tubes. As a result, it is possible that deposits will form where the velocity is low. By the use of helical baffles, it is possible to redirect the fluid flow so that dead spaces are eliminated. ... 

A somewhat different methodology has also been used to enhance the mass transfer using Dean vortices to promote turbulence using helical baffles, and stamped... 

FIGURE 6.31 Schematic of shell-side flow in a helically baffled heat exchanger. (Courtesy, Lummus Technology Heat Transfer, Bloomfield, NJ, USA, with permission.)... 

Helically baffled heat exchangers, illustrated in Figure 6.31, are a recent development. The spiral path through the shell side eliminates or reduces eddy regions and bypass flows, resulting in improved heat transfer for a given pressure drop [61]. 

Krai, D., Stehfik, P., Van der Ploeg, H. J., and Master, B. I., Helical Baffles in Shell-and-Tube Heat Exchangers, Part I Experimental Verification, Heat Transfer Engineering 17 (1), 93-101 (1996). American Petroleum Instimte, API Standard 662, Plate Heat Exchangers for General Refinery Services, API, Washington, DC, 2002. 

It is important to ensure that the helical baffle spirals downward and is covered. The author is familiar with situations where failure to do this caused excessive entrainment to the trays above the feed. 

Tangential feeds, often with helical baffles, are commonly used with high-velocity feeds in which the vapor phase is continuous and the liquid is present in the form of a spray (143). These arrangements are discussed in detail in Sec. 2.2 (arrangement j). 

FIGURE 10.19 Schematics of an HB. (Reprinted from J. Membr. Sci., 102, Gupta, B.B., Howell, J.A., Wu, D., and Field, R.W., A helical baffle for cross-flow microfiltration, 31-42, Copyright 1995, with permission from Elsevier.)... 

Gupta BB, Howell JA, Wu D, Field RW. A helical baffle for cross-flow microfiltration. J. Membr. Sci. 1995 102 31-42. 

Tube-side improvement use tube inserts, twisted tubes and tubes with internal fins Shell-side improvement use helical baffles, EM baffles, externally finned tubes, twisted tubes and shell-side seal strips (Fieberman, 2010)... 

Heat Transfer Enhancements Heat transfer enhancements increase the film heat transfer coefficient, thus improving U and consequently heat tfansfer in the exchanger. In the case of the ubiquitous ST heat exchanger, heat transfer enhancement can be achieved on the shell and/or tube sides as required. Tube-side enhancements help in improving the film heat transfer coefficient on the tube side, and are useful if the exchanger involved has lower film heat transfer coefficient on the tube side. Tube-side enhancements include, but are not limited to, twisted-tape inserts, coiled-wire inserts and internal fins. Similarly, shell-side enhancements are used to improve the heat transfer coefficient on the shell side. They include helical baffles, external fins and Expanded Metal (EM) baffles. More details on heat transfer enhancements are available in Pan et al. (2013). 

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