Flow-through Tube Banks

For two-phase gas/liquid horizontal cross flow through tube banks, the method of Diehl and Unruh (Pet. Refiner, 37[10], 124-128... 

Forced convection heat transfer is probably the most common mode in the process industries. Forced flows may be internal or external. This subsection briefly introduces correlations for estimating heat-transfer coefficients for flows in tubes and ducts flows across plates, cylinders, and spheres flows through tube banks and packed beds heat transfer to nonevaporating falling films and rotating surfaces. Section 11 introduces several types of heat exchangers, design procedures, overall heat-transfer coefficients, and mean temperature differences. 

Flow-through Tube Banks Ahgned and staggered tube banks are sketched in Fig. 5-6. The tube diameter is D, and the transverse and longitudinal pitches are ST and SL, respectively. The fluid velocity upstream... 

Flow through tube banks is studied experimentally since it is loo complex to be treated analytically. We arc primarily interested in (be average heat transfer coefficient for the entire tube bank, which depends on the number of tube rows along (lie flow as well as the arrangement and the size of the tubes. 

T. Fujii, H. Uehara, K. Hirata, and K. Oda, Heat Transfer and Flow Resistance in Condensation of Low Pressure Steam Flowing through Tube Banks, Int. J. Heat Mass Transfer, 15, pp. 247-260,1972. 

Flow-induced tube vibration occurs at critical fluid velocities through tube banks, and is to be avoided because of the severe damage that can result. Methods to predict and correct vibration problems may be found in Eisinger Trans. ASME J. Pressure Vessel Tech., 102, 138-145 [May 1980]) and Chen J. Sound Vibration, 93, 439-455 [1984]). 

This example will use only SI units, except that pressure will be in atm, not kPa. Flue gas containing 6 percent CO2 and 11 percent H2O vapor, wet basis, flows through a bank of tubes of 0.1016 in (4-in) outside diameter on equilateral 0.2032 m (8-in) triangular centers. In a section in which the gas and tube surface temperatures are 691°C (964 K) and 413°C (686 K), what are the emissivity and absorptivity of the gas From Table 5-8, = (2.8)(0.01016) = 0.2845 m (only SI... 

Cross-flow over tube banks is commonly encountered in practice in heal transfer equipment such as the condensers and evaporators of power plants, refrigerators, and air conditioners. In such equipment, one fluid moves through the tubes while the other moves over the tubes in a perpendicular direction. 

S l 12 Design a heat exchanger to pasteurize mill by steam in a dairy plant. Milk is lo flow through a bank of 1.2-cni internal diameter lubes while. steam condenses outside the tubes at I aim. Milk is to enter the tubes at 4 C, and it is to be heated to 72°C at a rate of 15 L/s, Making reasonable assumptions, you are lo specify the tube length and the number of tubes, and the pump for the heat exchanger. 

The heat-transfer phenomena for forced convection over exterior surfaces are closely related to the nature of the flow. The heat transfer in flow over tube bundles depends largely on the flow pattern and the degree of turbulence, which in turn are functions of the velocity of the fluid and the size and arrangement of the tubes. The equations available for the calculation of heat transfer coefficients in flow over tube banks are based entirely on experimental data because the flow Is too complex to be treated analytically. Experiments have shown that, in flow over staggered tube banks, the transition from laminar to turbulent flow Is more gradual than in flow through a pipe, whereas for in-line tube bundles the transition phenomena resemble those observed in pipe flow. In either case the transition from laminar to turbulent flow begins at a Reynolds number based on the velocity in the minimum flow area of about 100, and the flow becomes fully turbulent at a Reynolds number of about 3,000. The equation below can be used to predict heat transfer for flow across ideal tube banks. 

Example 7 Radiation in Gases Flue gas containing 6 percent carbon dioxide and 11 percent water vapor by volume (wet basis) flows through the convection bank of an oil tube stiU consisting of rows of 0.102-m (4-in) tubes on 0.203-m (8-in) centers, nine 7.62-m (25-ft) tubes in a row, the rows staggered to put the tubes on equilateral triangular centers. The flue gas enters at 871°C (1144 K, 1600°F) and leaves at 538°C (811 K, 1000°F). The oil flows in a countercurrent direction to the gas and rises from 316 to 427°C (600 to 800°F). Tube surface emissivity is 0.8. What is the average heat-input rate, due to gas radiation alone, per square meter of external tube area ... 

For turbulent flow through shallow tube banks, the average friction factor per row will be somewhat greater than indicated by Figs. 6-42 and 6-43, which are based on 10 or more rows depth. A 30 percent increase per row for 2 rows, 15 percent per row lor 3 rows and 7 percent per row for 4 rows can be taken as the maximum likely to be encountered (Boucher and Lapple, Chem. Eng. Prog., 44, 117—134 [1948]). 

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