Why Finned Tubes?

Why Finned Tubes?

First let us understand why finned tubes are used. Finned tubes make boiler components compact and light and reduce the gas pressure drop compared to a plain tube design for the same parameters. In addition, the space occupied by the finned bundle will be smaller compared to a plain tube bundle. Given in Table E.1 is a summary of design details for a boiler evaporator with plain and finned tubes for the same duty. The procedure for sizing with plain tubes is discussed in Appendix C. We will discuss the calculation procedure for finned tubes a little later.

A study was made for a boiler evaporator with plain tubes and finned tubes. The steam and gas parameters are shown in Table E.1. The first column shows a plain tube design with

a given cross section, while column 2 shows a finned tube bundle design using serrated fins with the same cross section. The third column shows a plain tube bundle designed to match the gas pressure drop of the finned tube bundle.

It may be seen that for the same cross section, the number of rows deep for the plain tube boiler is 57, while for the finned tube option, it is only 13. As a result, the length of the headers will be small. This means a large savings in space if finned tubes are used. In addition, the gas pressure drop and weight of the finned tube bundle are lower. However, the surface area is much more and so is the heat flux inside the tubes, which will increase the tube wall temperature. If the choice of fins is properly done, the boiler can be compact

Appendix E: Calculations with Finned Tubes 415

TABLE E.1

Plain Tube versus Finned Tube Evaporator

Boiler Design

Plain Tubes

Finned Tube

Plain Tubes

Tubes/row 21 22 21 Number of rows deep

57 13 52 Effective length, m

4 4 5 Fins/m

Fin height, mm 0 19 0 Fin thickness, mm

0 1.5 0 Serration, mm

0 4 0 Transverse pitch, mm

101 Longitudinal pitch, mm

Surface area, m 2 762

55.2 (64.1) Tube wall temperature, °C

U, kcal/m 2 h °C (W/m 2 K)

259 Gas pressure drop, mm wc

55 32 33 Weight of tubes, kg

Heat flux inside tubes, kW/m 2 26.3 110

26 Notes: Tubes: 50.8 × 44 mm at 101 mm square pitch. Steam pressure = 39 kg/cm 2 g,

steam generation = 3.39 kg/s. Feed water at 105°C. Gas flow = 27.8 kg/s at 560°C in and 302°C out. Duty = 8 MW. Gas analysis: % volume

CO 2 = 3, H 2 O = 7, N 2 = 75, O 2 = 15. Fouling factors on gas and steam

sides—0.0002 m 2 h °C/kcal (0.000172 m 2 K/W).

and operate well within its allowable temperature limits. That is why we see finned tube bundles used in all sections of a gas turbine HRSG or in any clean gas heat recovery appli- cation. Finned tubes have also been used in gas-fired package boilers for the superheater as well as the evaporator as discussed in Chapter 3. If one had used plain tubes, it would

be nearly impossible to build HRSGs with multiple pressure modules. Also one may note that the weight of the finned evaporator is lower due to the fewer number of rows deep. If we try to match the gas pressure drop of finned tubes by manipulating the tube spacing and geometry, the cross section of the plain tube bundle will be larger and also its weight. This is shown in the third column of Table E.1. In spite of a bigger cross section, the length occupied by the evaporator will be much more than the finned tube bundle. At 101 mm longitudinal spacing, the minimum length for plain tube bundle is 5.2 m, while for the finned bundle, it is only 1.3 m.