Sizing and Performance Calculations

The procedure for package fire tube boilers is similar to that of water tube boilers described in Figure 3.38. The chart shown in Chapter 2 for furnace exit gas temperature may be used as a good starting point to obtain the furnace duty. The convection section calculations are illustrated in Chapter 4. Appendix B may be used to determine the tube-side heat transfer coefficients and gas pressure drop. Any superheater or economizer may be evaluated using the methods discussed in Chapter 4 and Appendices

C and E. Chapter 4 illustrates an example of a fire tube boiler performance with an economizer. The same methodology may be applied to the fired boiler except that it has

a furnace section. All the comments and discussions in Chapter 4 on fire tube waste heat boilers will

apply to package fire tube boilers also. Using smaller-diameter tubes will reduce the boiler weight and length. Tube thickness required for a given steam pressure is nearly twice that required compared to when pressure acts inside the tubes. The simplified pro- cedures discussed in Chapter 4 may also be used for the performance evaluation of the convection passes.

Computer programs may also be developed for evaluating the performance of the fire tube boiler given the mechanical data. Tables 3.16 and 3.17 show the input data and performance results for a typical fire tube boiler with three passes and an economizer fir- ing natural gas and generating 40,000 lb/h. The efficiency on LHV basis is 94%. The back pressure in the tubes is about 5.3 in wc. The tube geometry may be changed as desired to decrease operating costs or to improve efficiency; performance without the economizer may also be obtained. Part load performance may also be obtained.

Steam Generators 161

TABLE 3.16

Input Steam, Fuel, and Tube Details

Steam Data

Steam flow, lb/h

Steam temp., °F Fw temp., °F

Blowdown, %

Sh press. drop, psi Stm press., psig

Ambient temp., °F

Stack gas temp., °F

Excess air, %

FGR, % Rel. hum., %

Casing loss, %

Unacc. loss, %

Fuel: 2—gas, 1—oil, 3—wood

Foul factor, gas

Foul factor, steam

Fuel temp., °F

Makeup air, °F

Fuel gas analysis, % volume Methane

Mechanical data–boiler Wet back len., in.

80 Number of passes

24 Dia., in.

Pass #

Tube OD, in.

Tube ID, in.

2.5 Number

2 95 110 Length, in.

Economizer: 1—yes, 2—no

Tube OD, in.

2.00 Number of

rows deep

6.00 Fins/in.

Tube ID, in.

1.77 Eff. len., ft

4.00 Fin ht., in.

5.00 St, in.

4.00 Fin thk., in.

0.75 Sl, in.

Streams

Serr., in.

Arrgt-in./st

Fin K, Btu/ft h °F

25 Confg-cf/pf

Tubes/row

Note: Arrgt 1 means staggered and 0 is inline. Configuration 1 is counter-flow, 2 parallel-flow.

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

TABLE 3.17

Typical Performance Results (Predicted Performance)

Boiler efficiency—per ASME

Excess air

Amb. Temp.

80 °F

Air temp.

75 °F

Fuel temp.

75 °F

Exit gas temp.

±10°F

Dry gas loss

Air moist. loss

Fuel moist. loss

Casing loss

Unacc. and margin

Efficiency, HHV

Efficiency, LHV

Fuel HHV

Btu/lb

Fuel LHV

Btu/lb

Boiler duty 40.33 MM Btu/h Burner duty, HHV

47.56 MM Btu/h Actual vol. HRR

Btu/ft 3 h Area HRR, HHV

Btu/ft 2 h

Air and flue gas

% Volume

Wet

Dry CO 2 8.29 10.13

18.17 0.00 N 2 71.07 86.86 O 2 2.46 3.01 SO 2 0.00 0.00

Evaporator performance Pass

±10°F Gas outlet

±10°F Gas sp. ht.

0 2822 Btu/lb °F Duty

15.79 20.25 2.45 MM Btu/h Gas press. drop

0.09 0.89 3.57 in wc Man. ht. flux

Btu/ft 2 h Max. wall temp.

°F Overall U

7.64 7.67 10.69 Btu/ft 2 h °F Surf. area

ft 2 Delt

91 °F Avg. gas vel.

37 57 81 ft/s (Continued)

Steam Generators 163

TABLE 3.17 (Continued)

Typical Performance Results Economizer performance

Gas temp. in 430

Gas pr. Drop, in wc 0.72 Out

±10°F

Wart pr. drop, psi 4.86 Duty

270

±10°F

Tube wall temp, °F 233 T w1

1.84 MM Btu/h

6.24 T w2

230

±10°F

U o , Btu/ft 2 h °F

275

±10°F

Tube-side htc,

1309

Btu/ft 2 h °F

Surface area 3497

Stagg Max. gas vel.

ft 2 Arrgt.

Counter flow Air flow

29 ft/s

Flue gas-stack 42,025

lb/h

Steam flow 40,000

lb/h

Fluid temp. in 230

°F

Fluid temp. out 406

°F

Fluid press. 250

psig

Foul ftr., gas 0.0010

ft 2 °F h/Btu

Foul ftr., stm. 0.0010

ft 2 °F h/Btu

Excess air

15 %

Total duty

40.33 MM Btu/h

References

1. V. Ganapathy, Specify packaged steam generators properly, Chemical Engineering Progress, Sept. 1993, p62. 2. V. Ganapathy et al., Designing large package boilers, Power, Feb. 2011, p80. 3. V. Ganapathy et al., Trends in package boiler design, Power Engineering, Nov 2011, p100.

Waste Heat Boilers

Sizing and Performance Calculations