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JOHN L. LOCKARD and JAMES H. WEBER
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Department of Chemical Engineering, University of Nebraska, Lincoln 8, Neb.
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pined ubes..
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in Cross-Flow Exchangers
Friction factors and heat transfer coefficients can be predicted from relationships based on experimental results
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A PREVIOUS study Hobson and Weber (,i)obtained performance data on concentric pipe heat exchangers in which the inner elements were spined tubes. In the study three different types of spined surfaces were used in a total of eight double pipe exchangers. Heat and momentum transfer data were obtained and the performance characteristics of these exchangers were compared with those of exchangers in which either a smooth pipe or another type of extended surface equipment was used as the inner element. These comparisons were made in the original study (5)and in a subsequent one ( 6 ) . The results of these studies show that the use of spined tubes in concentric pipe exchangers is, in general, not to be recommended because of the excessive pressure drop of the annular fluid. The loss of pressure and, in turn, increased fluid pumping costs would tend to outweigh any advantages which may be obtained from the better heat transfer characteristics. As a further test of spined tubes in heat exchangers, a study was undertaken to determine the heat and momentum transfer characteristics of this type of extended surface, when it is used in a
cross-flow exchanger. I n undertaking a study of this type it is necessary to limit the number of variables to be investigated. First, the spined tubes were arranged on centers of equilateral triangles. A “staggered” arrangement tends to give substantially higher heat transfer coefficients than does an “in-line” arrangement (74). The choice of equilateral triangles was arbitrary but this arrangement has been used previously by many investigators. Second, the tubes were placed in five rows. I n the first, third, and fifth rows there were three tubes per row, while in the second and fourth rows, two tubes per row. In order to keep a constant cross section normal to the direction of air flow, two baffles were , placed in each of the second and fourth rows. This helped to prevent the air from by-passing the heating surfaces in these rows. The number of rows was set at five, partially for practical reasons and partially because previous investigations showed that results obtained under these circumstances were representative. The greater the number of rows of tubes used in the tube bank, the greater the pressure drop. Because pressure drop of the fluid flowing across the tube bank
probably would be the most important drawback to the practical use of spined tubes, the reason for not using a large number of rows of tubes is apparent. Further, a number of studies (73-76) reviewed by McAdams (72) show that the mean values of the heat transfer coefficient for banks five rows deep were over 90% of the mean values for banks 10 rows deep. These results, of course, were obtained on equipment using banks of plain tubes, but it would appear that the percentage would be even higher for spined tubes, because the spines would increase the degree of turbulence in the fluid, Jameson ( 8 ) investigated the use of finned tubes in banks and found that the number of rows had only a slight effect on the heat transfer coefficient. Katz and others ( 9 ) made an extensive study of the use of extended surface heat transfer equipment in cross-flow exchangers and concluded that the value of the heat transfer coefficient was not affected by varying the number of rows of tubes in the exchanger. Further, they concluded that if the blower was located upstream, as was the case in the present investigation, there was little increase in heat transfer from the first to the second, or later rows. In a recent investigation Lapin and Schurig ( 7 7 ) obtained performance data on cross-flow exchangers in which finned tubes were used. These authors found that a t the higher Reynolds numbers (10,000 to 15,000) the heat transfer VOL. 52, NO. 1 1
NOVEMBER 1960
925
