Crossflow tube bundles

For the CHF condition for two-phase crossflow on the shell side of horizontal tube bundles, few investigations have been conducted. Katto et al. (1987) reported CHF data on a uniformly heated cylinder in a crossflow of saturated liquid over a wide range of vapor-to-liquid density ratios. Recently, Dykas and Jensen (1992) and Leroux and Jensen (1992) obtained the CHF condition on individual tubes in a 5 X 27 bundle with known mass flux and quality. At qualities greater than zero, they found that the CHF data are a complex function of mass flux, local quality, pressure level, and bundle geometry. 

In the following, the heat transfer and pressure drop in a tube bundle in crossflow will be investigated. The individual tubes in the bundle are either in alignment with each other or in a staggered arrangement, according to Fig. 3.25. 

Fig. 3.25 Tube bundle in crossflow, a aligned tube arrangement b staggered tube arrangement with the smallest cross section perpendicular to the initial flow direction c staggered tube arrangement with the smallest cross section in the diagonal...

Zukauskas, A. A. Makayawizus, V.I. Zlantzauskas, A. K. Heat transfer in tube bundles with crossflow of fluids (russ.). Vilnjus Mintis 1968... 

M. K. Jensen, R. R. Trewin, and A. E. Bergles, Crossflow Boiling in Enhanced Tube Bundles, Two-Phase Flow in Energy Systems, HTD vol. 220, pp. 11-17, ASME, New York, 1992. 

C. M. Chu and J. M. McNaught, Tube Bundle Effects in Crossflow Condensation on Low-Finned Tubes, Proc. 10th Int. Heat Transfer Conf, Brighton, 3, pp. 293-298,1994. 

External Flow (Shell Side). Two-phase flow patterns for flow normal to tube bundles (crossflow), such as on the shell side of a shell-and-tube heat exchanger, are much more complex than those inside a plain circular tube. Consequently, prediction of flow patterns in such situations is very difficult. It is important to note that two-phase shellside flow patterns are substantially less analyzed than those for internal flows. A review of the shellside flow pattern is presented by Jensen [68]. The dominant flow patterns (see Fig. 17.51 [69]) may be assessed... 

In Table 17.22, two correlations are presented for shellside two-phase flow pressure drop estimation, based on modifications of the internal flow correlations. The first correlation uses the modified Chrisholm correlation [69, 79], and the second one [80] employs the modified Lockhart-Martinelli correlation. The first correlation is for horizontal crossflow (crossflow in a baffled horizontal heat exchanger with horizontal or vertical baffle cuts). The second one is for vertical crossflow (upflow in a horizontal tube bundle). 

Bell-Delaware Method. Pressure drop and heat transfer calculations (the step 6 of the above thermal design procedure) constitute the key part of design. Tubeside calculations are straightforward and should be executed using available correlations for internal forced convection. The shellside calculations, however, must take into consideration the effect of various leakage streams (A and E streams in Fig. 17.30) and bypass streams (C and F streams in Fig. 17.30) in addition to the main crossflow stream B through the tube bundle. Several methods have been in use over the years, but the most accurate method in the open literature is the above mentioned Bell-Delaware method. This approach is based primarily on limited experimental data. The set of correlations discussed next constitutes the core of the Bell-Delaware method. 

The ideal pressure drop in the central section Apbi assumes pure crossflow of the fluid across the ideal tube bundle. This pressure drop should be corrected for (a) leakage streams (A and E, Fig. 17.30 correction factor Re), and (b) bypass flow (streams C and F, Fig. 17.30 ... 

FIGURE 17.55 Colburn factors and friction factors for ideal crossflow in tube bundles, 90° inline layout [106]. 

TABLE 17.36 Reference Crossflow Velocity in Tube Bundle Gaps [5] ... 

Fluid static pressure drop associated with the tube bundle central section (crossflow zone), Pa, lbf/ft2 (psi)... 

The height of liquid calculated using the correlations above is only approximate because it ignores any pressure drop effects and the fact that there are generally tubes in the flow erea calculated. However, it can be used to give a reasonable approximation. If the height of condensate calculated is appreciable, then more than one condensate connection may be desired. The flow pattern of the condenser can be changed so that vapor and condensate make only one crossflow path across the tube bundle. Other baffle types may also be used. 

The flow around and therefore the heat transfer around an individual tube within the bundle is influenced by the detachment of the boundary layer and the vortices from the previous tubes. The heat transfer on a tube in the first row is roughly the same as that on a single cylinder with a fluid in crossflow, provided the transverse pitch between the tubes is not too narrow. Further downstream the heat transfer coefficient increases because the previous tubes act as turbulence generators for those which follow. From the fourth or fifth row onwards the flow pattern hardly changes and the mean heat transfer coefficient of the tubes approach a constant end value. As a result of this the mean heat transfer coefficient over all the tubes reaches for an end value independent of the row number. It is roughly constant from about the tenth row onwards. This is illustrated in Fig. 3.26, in which the ratio F of the mean heat transfer coefficient Oim(zR) up to row zR with the end value am (zR —> oo) = amoo is plotted against the row number zR. 

Frequently, tubes are omitted from the baffle window areas. For this configuration, maldistribution of the fluid as it flows across the bank of tubes may occur as a result of the momentum of the fluid as it flows through the baffle window. For this reason baffle cuts less than 20% of the shell diameter should be used only with caution. Maldistribution will normally be minimized if the fluid velocity in the baffle window is equal or less than the fluid velocity in crossflow across the bundle. This frequently requires baffle cuts greater than 20% of the shell diameter. For such cases, the number of tube rows in crossflow will be less than that assumed in the methods above consequently a correction is required. In addition, the pressure drop for the first and last baffle sections assumes tubes in the baffle windows the factor 2.66 in Equation 8.19 should be reduced to 2.0 for the case of 20% baffle cuts. 

The correlation of transfer-rate studies - - with commercial practice has been difflcult because not only does the flow range from longitudinal flow along the tubes to crossflow across the bundle, but flow through the bundle varies because of its circular shape. Commercial tests conducted by Donohue led him to recommend the early equation of Bowman ... 

---