Helical tubes

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],... 

Coils are pieces of tubing helically colled around a rigid cylinder of the desired diameter. 

Precooler and Intercooler Finned tube helical bundle... 

Tubes Helical coil tube bundle, primary outside the tubes... 

Smooth high alloy tubes Low-finned tubes Sintered metal tubes Spiral heat exchanger Tube inserts Twisted tubes Helical tube support baffles... 

Steam generator type Helical Straight tube Helical Helical Helical... 

Fig. 3. Bourdon pressure elements (a) C-Bourdon tube (b) spiral Bourdon tube (c) helical Bourdon tube.

Coils For flow inside helical coils, Reynolds number above 10,000, multiply the value of the film coefficient obtained from the apphcable equation for straight tubes by the term (1 + 3..5 D /DJ. 

For flow outside helical coils use the equation for flow normal to a bank of tubes, in-line flow. 

Rhodes, and Scott Can. j. Chem. Eng., 47,445 53 [1969]) and Aka-gawa, Sakaguchi, and Ueda Bull JSME, 14, 564-571 [1971]). Correlations for flow patterns in downflow in vertical pipe are given by Oshinowo and Charles Can. ]. Chem. Eng., 52, 25-35 [1974]) and Barnea, Shoham, and Taitel Chem. Eng. Sci, 37, 741-744 [1982]). Use of drift flux theoiy for void fraction modeling in downflow is presented by Clark anci Flemmer Chem. Eng. Set., 39, 170-173 [1984]). Downward inclined two-phase flow data and modeling are given by Barnea, Shoham, and Taitel Chem. Eng. Set., 37, 735-740 [1982]). Data for downflow in helically coiled tubes are presented by Casper Chem. Ins. Tech., 42, 349-354 [1970]). 

Description These exchangers are typically a series of stacked helical-coiled tubes connected to manifolds, then inserted into a casing or shell. They have many advantages hke spiral-plate designs, such as avoiding differential expansion problems, acceleration effects of the helical flow increasing the heat transfer coefficient, and compactness of plot area. They are typically selected because of their economical design. 

Satisfactory performance is obtained with tubes having helical ribs on the inside surface, which generate a swirling flow. The resulting centrifugal action forces the water droplets toward the inner tube surface and prevents the formation of a steam film. The internally rifled tube maintains nucleate boiling at much higher steam temperature and pressure and with much lower mass velocities than those needed in smooth tubes. In modern practice, the most important criterion in drum boilers is the prevention of conditions that lead to DNB. 

Vei tical cylindrical helical coil heaters are hybrid designs that are classified as vertical heaters, but their in-tube characteristics are like those of horizontal heaters. There is no convection section. In addition to the advantages of simple vertical cylindrical heaters, the helical coil heaters are easy to drain. They are limited to smaU-duty applications 5 to 21 Gl/h (5 to 20 10 Btu/h). 

Schematic elevation sec tions of a vertical cylindrical, cross-tube convection heater a horizontal-tube cabin heater and a vertical cylindrical, helical-coil heater are shown in Fig. 27-51. The seven basic designs and some variations of them are pictured and described in the reference cited above and by R. K. Johnson Combustion 50(5) 10-16, November 1978).

FIG. 27-51 Representative types of fired heaters a) vertical-tube cylindrical with cross-flow-convection section (h) horizontal-tube cabin (c) vertical cylindrical, helical coil, from Berman, Chem. Eng. 85 98-104, June 19, 1978.)... 

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. 

Symmetric helical tubes and cylinder with a different from 0° or 30°... 

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