Compact heat exchangers may be classified by the kinds of compact elements that they employ. The compact elements usually fall into five classes:
- Circular and flattened circular tubes. These are the simplest form of compact heat exchanger surface. The designation ST indicates flow inside straight tubes (example: ST-1), FT indicates flow inside straight flattened tubes (example: FT-1) and FTD indicates flow inside straight flattened dimpled tubes. Dimpling interrupts the boundary layer, which tends to increase the heat transfer coefficient without increasing the flow velocity.
- Tubular surfaces. These are arrays oftubes ofsmall diameter, from 0.9535 cm down to 0.635 cm, used in service where the ruggedness and cleanability ofthe conventional shell-and-tube exchanger are not required. Usually, tubesheets are comparatively thin, and soldering or brazing a tube to a tubesheet provides an adequate seal against interleakage and differential thermal expansion.
- Surfaces with flow normal to banks of smooth tubes. Unlike the radial low fin tubes, smooth round tubes are expanded into fins that can accept a number oftube rows, as shown in Fig. 11.16a. Holes may be stamped in the fin with a drawn hub or foot to improve contact resistance or as a spacer between successive fins, as shown, or brazed directly to the fin with or without a hub. Other types reduce the flow resistance outside the tubes by using flattened tubes and brazing, as indicated in Fig b and c below. Flat tubing is made from strips similar to the manufacture of welded circular tubing but is much thinner and is joined by soldering or brazing rather than welding. The designation considers staggered (S) and in-line (I) arrangements oftubes and identifies transverse and longitudinal pitch ratios. The suffix (s) indicates data correlation from steady-state tests. All other data were correlated from a transient technique. Examples include the surface S1.50-1.25(s), which is a staggered arrangement with data obtained via steady-state tests with transverse pitch-to-diameter ratio of1.50 and longitudinal pitch-to-diameter ratio of1.25. The surface I1.25-1.25 has an inline arrangement with data obtained from transient tests with both transverse and longitudinal pitch-to-diameter ratios of1.25.
- Plate fin surfaces. These are shown in Figs. d through i below.
- Finned-tube surfaces. Circular tubes with spiral radial fins are designated by the letters CF followed by one or two numerals. The first numeral designates the number offins per inch, and the second (ifone is used) refers to the nominal tube size. With circular tubes with continuous fins, no letter prefix is employed and the two numerals have the same meaning as those used for circular tubes with spiral radial ins. For finned flat tubes, no letter prefix is used; the first numeral indicates the fins per inch and the second numeral indicates the largest tube dimension. When CF does not appear in the designation ofthe circular tube with spiral radial fins, the surface may be presumed to have continuous fins.
- Matrix surfaces. These are surfaces that are used in rotating, regenerative equipment such as combustion flue gas–air preheaters for conventional fossil furnaces. In this application, metal is deployed for its ability to absorb heat with minimal fluid friction while exposed to hot flue gas and to give up this heat to incoming cold combustion air when it is rotated into the incoming cold airstream. No designation is employed.
Please click at picture to enlarge size
Some compact heat exchanger elements
