Understanding Boiler Surface Areas

Many plant engineers and purchase managers make the mistake of evaluating or purchas- ing boilers solely based on surface areas. The misconception is that more surface area, better the boiler and more steel is offered! This is a wrong notion. In HRSGs, for example, we can show that based on fin geometry, one can have 50%–100% more surface area and yet have the same performance!! In steam generators, 15% difference in surface areas is not uncommon based on furnace sizing, location, and type of superheater whether convective or radiant and fin geometry used for the economizer.

Example 3.3

For a 60 t/h, 28 kg/cm 2 g, 400°C gas-fired steam generator with feed water at 110°C,

two boiler designs are being offered, one with a semi-radiant superheater and another with convective superheater. The tube geometry details of both designs are shown in Tables 3.5 and 3.6. (Incidentally, any steam generator or HRSG supplier should pro- vide data for his boiler in this format.) Tube material information may also be included for each section. If there are more heating surfaces, each of them should have the data shown. Streams are the number of tubes that carry the total flow; this has been explained in Appendix B.

TABLE 3.5

Geometric Data with Convective Superheater

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

44.7 44 44.7 44.7 Fins/m

197 Fin height, mm

19 Fin thk., mm

1.25 Fin width, mm

Fin conductivity

37 Tubes/row

12 12 12 16 Number of rows deep

Length, m

2.7 2.75 4.8 Tr pitch, mm

101 Long pitch, mm

8.000 Parl = 0, countr = 1

Note: Furnace length = 10.67 m, width = 2.14 m, height = 2.05 m.

114 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

TABLE 3.6

Geometric Data with Radiant Superheater

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

44.7 44 44.7 44.7 Fins/m

197 Fin height, mm

19 Fin thk., mm

1.25 Fin width, mm

4 Fin conductivity

37 Tubes/row

12 12 12 16 Number of rows deep

2 16 67 12 Length, m

2.7 2.75 4.8 Tr pitch, mm

101 Long pitch, mm

8.000 Parl = 0, countr = 1

Note: Furnace length = 9.14 m, width = 1.83 m, height = 2.05 m. One may see that the overall performance is nearly the same with both the options.

The total surface area of the radiant superheater design is about 12% lesser compared to the convective superheater option. The fan power consumption is the same as the back pressure. Efficiency also is close. The heat release rates are reasonable, and though the heat flux with the smaller furnace is more, it is far below the allowable limits for depar- ture from nucleate boiling (DNB) and hence acceptable. The radiant superheater has a higher tube wall temperature, but considering the low steam pressure, the thickness provided is adequate. Life of the radiant superheater may be shorter if we use the Larsen Miller chart for life estimation, but it is likely to exceed the expected 40 years at these temperatures. One has to check the part load performance to see if this is acceptable and make a decision. The convective superheater design, even though it may be more expen- sive, is preferred for the comfort factor it offers and its longer life. However, the purpose

TABLE 3.7

Summary of Performance for Radiant and Convective Superheaters

Item

Radiant SH

Convective SH

Volumetric heat release rate, kcal/m 3 h 928,500

389,352 Exit gas temperature, °C

Area heat release rate, kcal/m 2 h 482,400

Flue gas flow, kg/h

75,430 Efficiency, % LHV

Back pressure, mm wc

227 Furnace projected area, m 2 98 122 Superheater surface, m 2 64 97

Evaporator, m 2 399

1,221 Economizer fin geometry

Economizer, m 2 1,036

197 × 19 × 1.26 × 4 Furnace width × length × height, m

2.14 × 10.67 × 2.05 Type of superheater

Convective Superheater tube temperature, °C

Radiant

Steam Generators 115

of the exercise is to show that variations in surface areas can be there for the same overall performance, and operating cost and the design with lesser surface area cannot be ignored but properly evaluated (Table 3.7).

Understanding Boiler Surface Areas