Heat transfer enhancement, twisted tape

On the basis of heat transfer enhancement and pressure drop considerations, Twisted Tape H/d/8.5 would be chosen. However, a detailed examination of capital cost might cause this to be revised. 

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). 

It is important to note that in most cases the value static mixing brings to heat transfer does not translate to turbulent flow, where cost and pressure drop cannot be justified by the heat transfer enhancement. Turbulent flow heat transfer processes are best handled by empty tubes and tubes with spiral wrapped cores or tubes containing twisted tapes. These other pipeline devices, though not discussed here, are nevertheless important in industry. The reader is encouraged to seek other literature if interested (see Burmeister, 1983a,b). 

The enhancement of heat transfer inside a circular duct is often achieved by inserting a thin, metal tape in such a way that the tape is twisted about its longitudinal axis, as indicated in Fig. 5.47. Swirl flow is created in this manner. The width of the tape is usually the same as the internal diameter of the duct. The tape twist ratio XL is defined as H/d. When XL approaches infinity, the circular duct with the twisted tape becomes two semicircular straight ducts separated by the tape. 

Data for uniform-wall-temperature heating are plotted in Fig. 11.32, and the isothermal friction factors are plotted in Fig. 11.33. Twisted tapes and propellers were used by Koch [4] to heat air (curves a-d). Propellers produce higher heat transfer coefficients than twisted tapes however, this enhancement is at the expense of a rather large increase in friction factor, as seen in Fig. 11.33. Up to Re = 200, the friction factor for the twisted tape is the same as that for the empty half-tube (y = °o). The twisted-tape data of Marner and Bergles [114] with ethylene glycol exhibit an enhancement of about 300 percent above the smooth-tube values. Swirl at the pipe inlet does not produce any effective enhancement [192],... 

Shiralkar and Griffith (1970) determined both theoretically (for supercritical water) and experimentally (for supercritical carbon dioxide) the limits for safe operation, in terms of the maximum heat flux for a particular mass flux. Their experiments with a twisted tape inserted inside a test section showed that heat transfer was enhanced by this method. Also, they found that at high heat fluxes, DHT occurred when the bulk fluid temperature was below and the wall temperature was above the pseudocritical temperature. 

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