Convection coiled tubes

Janssen, L.A.M. and Hoogendoom, C.J., Laminar Convective Heat Transfer in Helically Coiled Tubes , Int. J. Heat Mass Transfer, Vol. 21, pp. 1197-1206,1978. 

Coiled-tube heat exchangers frequently have flow distribution problems that include (1) tube distribution (2) two-phase tube distribution and (3) two-phase shell distribution. Good flow distribution within the tubes can be obtained by designing the headers in such a way that their pressure drop is considerably less than that for the frictional pressure drop in the tubes. To obtain good shell-side distribution one must use symmetric bundles and separately introduce the vapor and liquid phases to the bundles. It is also advisable to arrange for downflow of the shell-side fluid. For two-phase annular flow, the vapor will flow mostly in the space between the tube layers while the liquid needs to be carefully distributed in the radial direction for proportionate vapor-liquid flow normal to each tube layer. To avoid convection on the shell side due to density gradients, it is normal practice to use sufficiently large pressure drops on the shell side. 

Honzontal-tube cabin heaters position the tubes of the radiant-section-coil horizontally along the walls and the slanting roof for the length of the cabin-shaped enclosure. The convection tube bank is placed horizontally above the combustion chamber. It may be fired From the floor, the side walls, or the end walls. As in the case of its vertical cylindrical counterpart, its economical design and high efficiency make it the most popular horizontal-tube heater. Duties are 11 to 105 GJ/h (10 to 100 10 Btu). 

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.)... 

The radiant section tube coils of horizontal fired heaters are arranged horizontally so as to line the sidewalls and the roof of the combustion chamber. In addition, tliere is a convection section of tube coils, winch are positioned as a horizontal bank of tubes above the combustion cham her. Nonnally the tubes are fired vertically from the floor, but they can also be fired horizontally by side wall mounted burners located below the tube coil. Tins economical, high dficiency design currently represents the majority of new horizontal-tube-t1icd heater installations. Duties run from 5 to 250 MMBtu/hr. Six types o) horizontal-tube-fired heaters arc-shown in Figure 3-21. 

Figure 3-20. Vertical-tube-fired heaters con be identified by the vertical arrangement of the radiant-section coil, (a) Vertical- lindrical all radiant, (b) Vertical-cylindrical helical coil, (c) Vertical-cylindrical, with cross-flow-convection section. d) Vertical-cylindrical, with integral-convection section, (e) Arbor or wicket type, (f) Vertical-tube, single-row, double-fired. [From Chem. Eng, 100-101 (June 19, 1978).]...

This design is not well adapted to free-convection heat transfer outside a tube or coil therefore, for this discussion only agitation is considered using a submerged helical coil, Oldshue and Kern . 

The dispersion in geometrically deformed tubes (squeezed, twisted and coiled ) has been extensively studied by Halasz (6, 7 and 8), and the effect of radial convection (secondary flow) on the dispersion introduced in tightly... 

At high temperature, the EDC decomposes into VCM and HC1 by a complex reaction mechanism discussed further in this section. The endothermic reaction takes place at temperatures between 480-550 °C and pressures from 3 to 30 bar. The reaction device consists of a long tubular coil placed in a furnace (Figure 7.4). The first part, hosted in the convection zone, preheats the reactant up to the temperature where the pyrolysis reaction rate becomes significant The second part, the reaction zone, is placed in the radiation chamber. The tube diameter is selected so as to give a superficial gas velocity between 10-20 m/s. The coil length should ensure a space-time of 5 to 30 s. 

Figure 1 illustrates conventional CVD reactors. These reactors may be classified according to the wall temperature and the deposition pressure. The horizontal, pancake, and barrel reactors are usually cold-wall reactors where the wall temperatures are considerably cooler than the deposition surfaces. This is accomplished by heating the susceptor by external rf induction coils or quartz radiant heaters. The horizontal multiple-wafer-in-tube (or boat) reactor is a hot-wall reactor in which the wall temperature is the same as that of the deposition surface. Therefore, in this type of reactor, the deposition also occurs on the reactor walls which presents a potential problem since flakes from the wall deposit cause defects in the films grown on the wafers. This is avoided in the cold-wall reactors, but the large temperature gradients in those reactors may induce convection cells with associated problems in maintaining uniform film thickness and composition. 

The convection heat transfer coefficient can also be increased by inducing pulsating flow by pulse generators, by inducing swirl by inserting a twisted tape into the tube, or by inducing secondary flows by coiling the tube. 

In forced convection, the fluid is moved over the surface by a pump or blower neglecting natural convection are usually neglected. The study of forced convection is of great practical importance and vast amount of data have been amassed for streamline and turbulent flow in pipes, across and parallel to tubes, across plane surfaces, and in other important configurations such as jackets and coils. 

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