Convection horizontal tubes

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). 

In the horizontal-tube box heater with side-mounted convection tube bank, the radiant-section tubes run horizontally along the walls and the flat roof of the box-shaped heater, but the convection section is placed in a box of its own beside the radiant sec tion. Firing is horizontal from the end walls. The design of this heater results in a relatively expensive unit justified mainly by its abihty to burn low-grade high-ash fuel oil. Duties are 53 to 210 GJ/h (50 to 200 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 3-21. Six bask designs used in horizontal-lube fired heaters. Radian section coil is horLmtai. (a) Cabin, (b) Two-cell box. (c) Cabin with dividing bridgewall, (d) End-fired box. e) End-fired box, with side-mounted convection section, (f) Horizontal-tube, single-row, double-h red. [From Chem. Eng., 102-103 (June 19, 1978).]...

For organic liquids, evaluate the natural convection film coefficient from Figure 10-103. Equation 10-29 may be used for the inside horizontal tube by multiplying the right side of the equation by 2.25 (1 + 0.010 Gr,i/")/logRe. 

Figure 9.31. Natural convection from horizontal tubes...

Yen T-H, Kasagi N, Suzuki Y (2003) Forced convective boiling heat transfer in micro-tubes at low mass and heat fluxes. Int J Multiphase Flow 29 1771-1792 Yu W, France DM, Wambsganss MW, Hull JR (2002) Two-phase pressure drop, boiling heat transfer, and critical heat flux to water in a small-diameter horizontal tube. Int J Multiphase Flow 28 927-941... 

Chen s method was developed from experimental data on forced convective boiling in vertical tubes. It can be applied, with caution, to forced convective boiling in horizontal tubes, and annular conduits (concentric pipes). Butterworth (1977) suggests that, in the absence of more reliable methods, it may be used to estimate the heat-transfer coefficient for forced convective boiling in cross-flow over tube bundles using a suitable cross-flow correlation to predict the forced-convection coefficient. Shah s method was based on data for flow in horizontal and vertical tubes and annuli. 

The boundary conditions for the stream-function-vorticity system requires specifying the stream function on all the boundaries. This is usually straightforward for known inflow and outflow conditions and solid walls. The vorticity boundary conditions comes from evaluating Eq. 3.281 on the boundary. Along the boundary, which usually corresponds with one of the coordinate directions, one of the terms in Eq. 3.281 (i.e., the one in which the derivatives align with the boundary) can be evaluated explicitly since the stream function is already specified. Thus the boundary conditions becomes a relationship between the boundary vorticity and a boundary-normal second derivative of stream function. For example, consider the natural convection in a long horizontal tube. Here, since there is no inflow or outflow, the stream function is simply zero all around the tube wall. Thus the vorticity boundary conditions are... 

Several steps can be taken to combat convection in a free electrolyte. Hjerten, for example, used a horizontal tube rotating around its own axis [7]. Any small convective displacement due to gravity is exactly reversed as the tube rotates 180°. Thus the gravitational convective displacements— while not eliminated—exactly cancel one another in the course of rotation. More recent experiments aboard earth satellites are carried out under near-zero gravity (nonconvective) conditions [8,9]. It has also been found useful to work at 4°C where water has its maximum density and where, consequently, density is least sensitive to temperature. Stabilizing density gradients have been introduced in some cases to counteract convection. 

Mori, Y. and Futagami, K., Forced Convective Heat Transfer in Uniformly Heated Horizontal Tubes (2nd Report, Theoretical Study) , Int. J. Heat Mass Transfer, Vol. 10, pp. 1801-1813, 1967. 

Choi, D.K. and Choi. D.H., Dual Solution for Mixed Convection in a Horizontal Tube under Circumferentially Non-Uniform Heating . Int. J. Heat and Mass Transfer. Vol. 35, pp. 2053-2056, 1992. 

Morcos, S.M. and Bergles, A.E.. Experimental Investigation of Combined Forced and Free Laminar Convection in Horizontal Tubes , J. Heat Transfer. Vol. 97. pp. 212-219. 1975. 

Figure 7-14 presents the regimes for combined convection in horizontal tubes. In this figure the Graetz number is defined as... 

Bergles, A. E., and R. R. Simonds Combined Forced and Free Convection for Laminar Flow in Horizontal Tubes with Uniform Heat Flux, Int. J. Heat Mass Transfer, vol. 14, p. 1989, 1971. 

To provide an example. Fig. Z16 shows a sin libed isometric view of a cell of a steam cracker with vertical tubes in the radiation zone and horizontal tubes in the convection zone. It also shows the positions of the adewaU burners, quench boilers, steam drum, and the injection device of the heavy cut which performs the supplementary direct quench. 

Heat-transfer coefficient in condensation Mean condensation heat-transfer coefficient for a single tube Heat-transfer coefficient for condensation on a horizontal tube bundle Mean condensation heat-transfer coefficient for a tube in a row of tubes Heat-transfer coefficient for condensation on a vertical tube Condensation coefficient from Boko-Kruzhilin correlation Condensation heat transfer coefficient for stratified flow in tubes Local condensing film coefficient, partial condenser Convective boiling-heat transfer coefficient... 

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