Tube banks cross flow

Figure 12.31. Heat-transfer factor for cross-flow tube banks...

The heat transfer coefficient for an ideal cross-flow tube bank can be calculated using the heat transfer factors /f, given in Figure 12.33. Figure 12.33 has been adapted from a similar figure given by Mueller (1973). Mueller includes values for more tube... 

The limited experimental results of Williams and Katz [25] and Briggs et al. [26] can be used to modify the results for plain tubes to apply to cross flow in banks of low-finned tubes commonly used in shell-and-tube heat exchangers. 

APm.AP,., Pressure drop for ideal-tube-bank cross-flow and ideal window respectively AP for shell side of baffled exchanger kPa Itf ft ... 

It is shown in Section 9.9.5 that, with the existence of various bypass and leakage streams in practical heat exchangers, the flow patterns of the shell-side fluid, as shown in Figure 9.79, are complex in the extreme and far removed from the idealised cross-flow situation discussed in Section 9.4.4. One simple way of using the equations for cross-flow presented in Section 9.4.4, however, is to multiply the shell-side coefficient obtained from these equations by the factor 0.6 in order to obtain at least an estimate of the shell-side coefficient in a practical situation. The pioneering work of Kern(28) and DoNOHUE(lll who used correlations based on the total stream flow and empirical methods to allow for the performance of real exchangers compared with that for cross-flow over ideal tube banks, went much further and. 

Grimison. E.D. Trans. Am. Soc. Mech. Eng. 59 (1937) 583 and ibid. 60 (1938) 381. Correlation and utilization of new data on flow resistance and heat transfer for cross flow of gases over tube banks. 

The complex flow pattern on the shell-side, and the great number of variables involved, make it difficult to predict the shell-side coefficient and pressure drop with complete assurance. In methods used for the design of exchangers prior to about 1960 no attempt was made to account for the leakage and bypass streams. Correlations were based on the total stream flow, and empirical methods were used to account for the performance of real exchangers compared with that for cross flow over ideal tube banks. Typical of these bulk-flow methods are those of Kern (1950) and Donohue (1955). Reliable predictions can only be achieved by comprehensive analysis of the contribution to heat transfer and pressure drop made by the individual streams shown in Figure 12.26. Tinker (1951, 1958) published the first detailed stream-analysis method for predicting shell-side heat-transfer coefficients and pressure drop, and the methods subsequently developed... 

The Reynolds number for cross-flow through a tube bank is given by ... 

The pressure drop in the cross-flow zones between the baffle tips is calculated from correlations for ideal tube banks, and corrected for leakage and bypassing. 

Any suitable correlation for the cross-flow friction factor can be used for that given in Figure 12.36, the pressure drop across the ideal tube bank is given by ... 

The combustion gases flow across the tube banks in the convection section and the correlations for cross-flow in tube banks can be used to estimate the heat transfer coefficient. The gas side coefficient will be low, and where extended surfaces are used an allowance must be made for the fin efficiency. Procedures are given in the tube vendors literature, and in handbooks, see Section 12.14, and Bergman (1978b). 

For widely spaced tubes, the target efficiency Tp, can be calculated from Fig. 17-39 or from the impaction data of Golovin and Putnam [Ind. Eng. Chem. Fundam., 1,264 (1962)]. The efficiency of the overall tube banks for a specific particle size can then be calculated from the equation T = 1 — (1 — la/A) where a is the cross-sectional area of all tubes in one row, A is the total flow area, and n is the number of rows of tubes. 

The onset of liquid reentrainment from tube banks can be predicted from Fig. 14-116. Reentrainment occurred at much lower velocities in vertical upflow than in horizontal gas flow through vertical tube banks. While the top of the cross-hatched line of Fig. 14-116a predicts reentrainment above gas velocities of 3 m/s (9.8 ft/s) at high liquid loading, most of the entrainment settled to the bottom of the duct in 1 to 2 m (3.3 to 6.6 ft), and entrainment did not carry significant distances until the gas velocity exceeded 7 m/s (23 ft/s). 

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

Condensing steam at 150°C is used on the inside of a bank of tubes to heat a cross-flow stream of C02 which enters at 3 atm, 35°C, and 5 m/s. The tube bank consists of 100 tubes of 1.25-cmOD in a square in-line array with S = Sp = 1.875 cm. The tubes are 60 cm long. Assuming the outside tube wall temperature is constant at 150°C, calculate the overall heat transfer to the C02 and its exit temperature. 

Grimson, E. D. Correlation and Utilization of New Data on Flow Resistance and Heat Transfer for Cross Flow of Gases over Tube Banks, Trans. ASME, vol. 59, pp. 583-594, 1937. 

Saturated steam at 100 lb/in2 abs is to be used to heat carbon dioxide in a cross-flow heat exchanger consisting of four hundred 1-in-OD brass tubes in a square in-line array. The distance between tube centers is j in, in both the normal- and parallel-flow directions. The carbon dioxide flows across the tube bank, while the steam is condensed on the inside of the tubes. A flow rate of I lb ,/s of CO at 15 lb/in2 abs and 70°F is to be heated to 200°F. Estimate the length of the tubes to accomplish this heating. Assume that the steam-side heat-transfer coefficient is 1000 Btu/h ft2 °F, and neglect the thermal resistance of the tube wall. 

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