Reduce Weight of Tube Bundles Using Smaller Tubes

Reduce Weight of Tube Bundles Using Smaller Tubes

It is possible to reduce the weight of a boiler using smaller tubes for the same duty and gas pressure drop values. This is true for plain as well as finned tubes. The reason is the higher heat transfer coefficient with smaller-diameter tube. The weight of the tube

434 Appendix E: Calculations with Finned Tubes

bundle will also be less. However, the number of tubes used will be more with smaller tube option, which may increase the labor cost or cost of fabrication of the boiler as the number of tubes to be welded increases as the tube diameter decreases. Hence, the deci- sion must be made based on not only the cost of material and weight of boiler but also the cost of fabrication.

Example E.6

Here is an example of an HRSG with an evaporator and economizer generating satu-

rated steam at 40 kg/cm 2 g using feed water at 111°C. Exhaust gas flow = 150,000 kg/h at 580°C and analysis is % volume CO 2 = 3, H 2 O = 7, N 2 = 75, O 2 = 15. Two options were

considered in the design of the HRSG. One uses 50.8 tubes for the evaporator and econ- omizer, while the other uses 38 mm tubes for evaporator and economizer. Tubes are all in staggered arrangement. Tables E.9 and E.10 show the geometry for both options, while Table E.10 shows the performance results.

1. Total gas pressure drop and duty are same for both the options. Steam produc- tion is the same. 2. It is seen that the 38 mm tube option has higher heat transfer coefficients about 3%–8%. 3. The 38 mm tube option has lower weight; about 60,000 kg versus 67,500 kg for the 51 mm tube option. With large boilers, this may be a deciding factor as shipping costs can be lowered.

4. The evaporator has 720 tubes and economizer has 561 tubes to be welded in the 38 mm option, while the 51 mm tube option has only 540 tubes in the evapo- rator and 420 in the economizer. In countries where labor costs are high, this may be a deciding factor.

Hence, based on local conditions, shipping, cost of labor or materials, and cost of fabri- cation, one may make suitable decisions. This exercise shows that with smaller tubes, it is possible to lower the weight of boilers and perhaps the shipping and manufacturing costs also.

TABLE E.9

Geometrical Data for 38 and 50.8 mm OD Tubes

Tube OD, mm 38 38 50.8 50.8 Tube ID, mm

31.8 31.8 43 43 Fins/m

197 Fins height, mm

19 19 19 19 Fin thickness, mm

1.5 1.5 1.5 1.5 Serration, mm

4 4 4 4 Tubes/row

36 33 30 30 Number of rows deep

20 17 18 14 Length, m

5 5 5.3 5.3 Transverse pitch, mm

101 Longitudinal pitch

Staggered Flow direction

Staggered

Staggered

Staggered

Counter-flow

Counter-flow

Appendix E: Calculations with Finned Tubes 435

TABLE E.10

Performance Results for 38 and 50.8 mm OD Tubes

Evaporator-51 Economizer-51

Gas temp. in, ±5°C

253 Gas temp. out, ±5°C

150 Gas spec. heat, kcal/kg °C

0.2584 Duty, MM kcal/h

4,036 Gas pressure drop, mm wc

Surface area, m 2 5,023

65 46 73 40 Fouling factor—gas

0.0002 U, kcal/m 2 h °C

39.51 36.64 38.01 33.80 LMTD, °C

67 28 67 29 Gas velocity, m/s

15 10 16 10 Weight, kg

29,526 Fluid temp. in, ±5°C

111 Fluid temp. out, ±5°C

1.47 Fouling factor—fluid

Pressure drop, kg/cm 2 —

0.0002 Fluid velocity, m/s

1.1 Steam flow, kg/h