Helical fins

Circular or oval finned-tube banks utilize a variety of enhanced surfaces, as illustrated in Fig. 11.14. With the exception of material pertaining to smooth helical fins, data are rather limited and no generalized correlations presently exist. Webb [108] provides a guide to the literature. 

Helically finned tubes coiled around mandrels have been used for the compact Joule-Thomson heat exchangers. The high-pressure gas flows inside the tubes and the low pressure gas flows outside the tubes, in combined counter- and cross-flow. Heat exchanger designs were based on test data, some of which have been previously reported [6]. 

Answer by author Yes, we tested heat exchangers with many different designs, including pin fins, coiled fins, tubes coiled in spirals, etc. It is our opinion that the present helically finned and coiled exchanger design is the most useful design for the small compact exchangers. 

These have helical fins fastened on a shaft that rotates in a trough. The material gets conveyed by continuous pushing action of the fins. However, it can get crashed and thoroughly mixed as it passes through the conveyor. 

Comparisons between the datawhichwere obtained in this work and previously published data for compact exchangers are presented in Fig. 10. The over-all J factor for heat transfer for the helically finned and coiled heat exchanger is only slightly less than the individual j factor reported for the Collins-t3rpe ribbon-packed heat exchanger [6]. The approximate j factor reported in this work is substantially higher than the j factor reported above. 

It is to be noted that the ratio of the friction factor / to the heat transfer factor J for compact ribbon-packed heat exchangers is approximately equal to the ratio of / to j reported in the present work. F rom this it may be concluded that more compact heat exchangers can be designed with helically finned and coiled tubing, which also represents a Collins type of heat exchanger, without any sacrifice in shell-side pressure drop. 

For the preliminary design of helically finned and coiled heat exchangers, operating at shell-side Reynolds numbers of approximately 1000, the(NTU) per coil is about 1/3 and the NVH per coil is approximately equal to 4. 

Mechanically bonded tubes may be obtained by mechanically stressing the fin material and/or the tube material to hold the two elements in pressure contact with one another. So called tension wound fins are formed by winding the fin material under tension in a helical manner along the length of the tube. 

Helically wrapped fins are fabricated such that the fm height can be between about 3/8 to 3/4 of the tube diameter, but limited because of fabrication requirements to a maximum of about 2.54 cm (1.0 in.) in height. Fin spacings vary between about 275 and 450 fins per meter of tube length, while fin thicknesses range from 0.025 to 0.075 cm. For particular cases these parameters may be varied further. 

The work by Cronet et al [136] made dear that general roles for residue exchanges in membrane helices are hard to derive because residue preferences are very position specific. A good Day half-type matrix can therefore not be derived fin- the transmembrane helices, which makes it difficult to me standard sequence-alignment techniques ... 

The spiral baffles are 1.5" OD x 2" long in the M-10 models and 1.5" OD x 1" long for the M-11. Each suppressor uses one left and one right hand twist "spiral. These units are actually helical,as there is no taper along their length.The fins are cut to a depth of. 4375" leaving a. 625" OD core section. There are four fins per inch, with a. 125" space between fins. Four rows of 1/8" diameter holes are drilled in the spaces between the fins. M-10/9mm and M-11 spirals are bored. 4" ID. The M-10/45 units are bored to. 5" ID. 

The extension of these PECs to two-phase heat transfer is complicated by the dependence of the local heat transfer coefficient on the local temperature difference and/or quality. Heat transfer and pressure drop have been considered in the evaluation of internally finned tubes for refrigerant evaporators [14] and for internally finned tubes, helically ribbed tubes, and spirally fluted tubes for refrigerant condensers [15]. Pumping power has been incorporated into the evaluation of inserts used to elevate subcooled boiling critical heat flux (CHF) [16, 17]. A discussion of the application of enhancement to two-phase systems is given by Webb [373],... 

Formed Webb [26] Standard low-fin tubing with fins bent to reduce gap Helical circumferential reentrant cavities... 

J. H. Masliyah and K. Nandakumar, Fluid Flow and Heat Transfer in Internally Finned Helical... 

The use of ordinary surface roughness elements tends to reduce the critical heat flux. However, by introducing helical and fin-form deformations on the tube surface, it is possible to induce swirl and turbulence in the flow, which promotes transfer of the liquid phase to the surface and thus gives enhancement. An example of work of this kind is illustrated in Fig. 15.135, which shows results obtained by Nishikawa et al. [329] using rifled and ribbed tubes. 

Individually Finned Tubes. In this fin geometry, helically wrapped (or extruded) circular fins on a circular tube as shown in Fig. 17.14a, is commonly used in process and waste heat recovery industries. The following correlation for j factors is recommended by Briggs and Young (see Webb [47]) for individually finned tubes on staggered tubebanks. 

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). 

Exchanger Type Plate-Fin Plate-Fin Plate-Fin Helical Coil Tubular... 

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