Low finned tubes

The low-finned tubes are made by machining grooves into the exterior surface of the tubes, which increases the surface area by 2.5 times. 

The use of low-finned tubes is not a new development. It is an idea that was in common use in the 1960s with mixed results. A successful application is the use of low-finned tubes in clean propane refrigerant condensing service, with clean cooling water on the tube side. Observed heat-transfer rates increased by about 70 percent. 

However, if the controlling resistance to heat transfer is on the tube side, the increased outside surface area of the tubes does not improve heat-transfer rates. 

Caution If the shell side is subject to fouling, then dirt gets between the fins, in which case the heat transfer will now be worse than with smooth (unfinned) tubes. 

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Low-fin tubes (Mfi-in-high fins) provide 2.5 times the surface per lineal foot. Surface required should be divided by 2.5 then use Fig. 11-41 to determine basic cost of the heat exchanger. Actual surface times extra costs (from Table 11-14) should then be added to determine cost of fin-tube exchanger. 

Approximate Estimating Physical Data for Low-Finned Tubing for Use in Design Calculations... 

Figure 10-143 is useful in roughly predicting the relative economic picture for adapting low finned tubes to the heat or cooling of oil on the shell side of conventional shell and tube units. This is not a design chart. 

Figure 10-145. Generalized design evaluation of low-finned tubes and fluid heat exchangers. (Used by permission An Opportunity. Wolverine Tube, Inc.)...

Design for Heat Transfer Coefficients by Forced Convection Using Radial Low-Fin tubes in Heat Exchanger Bundles... 

Figure 10-148. Shell-side friction factors for bundles with 20%-cut segmental baffles, one seal strip per 10 rows of tubes, and TEMA clearances. These factors can be used for plain or low-finned tubes with the appropriate values of D s or d s. (Source Engineering Data Book, 1960. Wolverine Tube, Inc. Used by permission Kern, D. Q., and Kraus, A. D. External Surface Heat Transfer, p. 511, 1972. McGraw-Hill Book Co., Inc. All rights reserved.)...

Figure 10-148 presents a recommended pressure drop correlation for low-fin tubes in shells and is based on clean tube pressure drop with no dirt sealing the leakage clearances between tubes and baffle holes or baffle-to-shell clearances. A fouled condition pressure drop may be an indeterminate amount greater. The authors state that this University of Delaware correlation has some lactors built in that limit the deviations to a relatively small range. Figure 10-148 has... 

Figure 10-149. Exchanger rating for intercooler, using low-fin tubes. Note specifications here are for illustration purposes. The design as developed represents more conservative surface area than substantiated by current data.

Figure 10-150. Boiling coefficients for low-finned tubes. (Used by permission Katz, D. L, Meyers, J. E., Young, E. H., and Balekjian, G. Petroleum Refiner, V. 34, No. 2, 1955. Gulf Publishing Company. All rights reserved.)...

Gentry, G. G., R. K. Young, and W. M. Small, RODbaffle Heat Exchanger Thermal-Hydraulic Predictive Methods for Bare and Low-Finned Tubes, National Heat Transfer Gonference, Niagara Falls, NY, Aug. 5-8, (1984). 

WlMPRESS, N. (1978) Chem. Eng., NY 85 (May 22nd) 95. Generalized method predicts fired-heater performance. WOLVERINE (1984) Wolverine Tube Heat Transfer Data Book—Low Fin Tubes (Wolverine Division of UOP Inc.). 

You may have seen bundles constructed with serrated, or very small, fins covering the exterior of the tubes. These are called low-fin-tube... 

Actually, retrofitting a tube bundle with low fin tubes often reduces heat-transfer capacity. This happens when the controlling resistance to heat transfer is shell-side fouling. The fouling deposits get trapped between the tiny fins. This acts as an insulator between the shell-side fluid and the surface of the tubes. In severe shell-side fouling services, I have replaced fin tubes with bare tubes, and doubled the heat-transfer duty on the exchanger. 

In summary, low fin tubes may be used to advantage only when the controlling resistance to heat transfer is the shell-side fluid itself. 

Figure 8.6. Examples of extended surfaces on one or both sides, (a) Radial fins, (b) Serrated radial fins, (c) Studded surface, (d) Joint between tubesheet and low fin tube with three times bare surface, (e) External axial fins, (f) Internal axial fins, (g) Finned surface with internal spiral to promote turbulence, (h) Plate fins on both sides, (i) Tubes and plate fins.

Condensation Outside Horizontal Tubes. Figure 8.14(d) shows a condenser with two tube passes and a shell side provided with vertically cut baffles that promote side to side flow of vapor. The tubes may be controlled partially flooded to ensure desired subcooling of the condensate or for control of upstream pressure by regulating the rate of condensation. Low-fin tubes often are advantageous, except when the surface tension of the condensates... 

For low-fin tubes, the laminar condensing coefficient can be calculated by applying an appropriate correction factor F to the value calculated using the preceding equation for laminar-film condensation. The factor F is defined thus ... 

Low-fin tubes, often used for horizontal-pool boiling, reduce the minimum temperature difference required to achieve nucleate boihng. In addition, the boihng coefficients for low-fin tubes are higher than those for bare tubes at a given temperature difference. 

Calculate h for low-fin tubes. For low-fin tubes, the shell-side mass velocity is reduced because of the space between the fins. This reduction can be closely approximated with the expression (s — D0)/(s — D0 + 0.09), each term being expressed in inches. 

Determine weighted fin efficiency. Weighted fin efficiencies for low-fin tubes are functions of the outside heat-transfer coefficient. Weighted fin efficiencies t) can be determined from curves provided by various manufacturers. Table 7.4 permits approximation of weighted fin efficiencies. For this problem, the weighted efficiency rj is 0.94. 

Calculate water savings using low-fin tubes. The tube-side fluid must be further cooled the water savings is represented by the difference in heat recovery between bare and low-fin tubes. Thus,... 

Related Calculations. Low-fin tubes are tubes with extended surfaces that have the same outside diameter as bare tubes. They can therefore be used interchangeably with bare tubes in tubular exchangers. Various geometries and materials of construction are available from several manufacturers. 

The procedure outlined in this example can also be used when designing equipment using low-fin tubes. The same approach is used when condensing or boiling on the outside of low-fin tubes. 

Low-fin tubes. The method presented above can be used to predict pressure drop for banks of low-fin tubes. For low-fin tubes, the pressure drop is calculated assuming that the tubes are bare. The mass velocity and tube diameter used for calculation are those for a bare tube with the same diameter as the fins of the low-fin tube. 

Banks of finned tubes. For fin tubes other than low-fin tubes, the pressure drop for flowing across banks of transverse fin tubes can be calculated from... 

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