Ducts with Twisted Tape

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. 

For forced flow in ducts, a spiralator has been proposed. This consists of a loose-fitting twisted tape secured at the downstream end in a bearing. Penney [254] found that with heating of corn syrup, the heat transfer coefficient increased by 95 percent at 100 rev/min. A surface scraper based on this principle (see next subsection) has been patented [384],... 

Glass wool and aluminum foil (shiny side in) are often used for insulation. You can wrap the column with glass wool and then use a wrapping of the aluminum foil to keep it in place. An especially effective method is to make an insulation blanket by placing a layer of glass wool or cotton between two rectangles of aluminum foil, placed shiny side in. The sandwich is bound together with duct tape. This blanket, which is reusable, can be wrapped around the column and held in place with twist ties or tape. 

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