Simulation of Fire Tube Boiler Performance

One may simulate the fire tube boiler performance at any gas inlet condition using the concept that U is proportional to W g 0.8 . Effect of gas properties are neglected in the first iteration. The effect of nonluminous heat transfer coefficient is also presumed to be small. The advantage of this method is that U need not be evaluated in detail.

Using the equation ln T T  ( 1 − s )( / T 2 − T s )

 0.8 = UA W C / ( g pg ) and substituting KW

g for U

and simplifying, we have ln T T − / T − T = KW / 02  . ( 1 s )( 2 s )  ( g ) .

One can estimate the constant K for the boiler using known conditions and then predict the exit gas temperature and duty at any other condition of gas flow or inlet gas temperature. The effect of flue gas properties is neglected, and hence, this is an approximate method, but for quick estimates, it is reasonable.

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

TABLE 4.23

Performance of Fire Tube Boiler with Economizer Gas Data

Gas flow 52,000

kg/h

Gas inlet 850

°C

Gas press.

0 kg/cm 2 g

Fouling ftr. 0.0002

m 2 h °C/kcal

CO 2 12 %

12 % N 2 70 % O 2 6 %

Heat loss

Steam data Fw. temp.

°C

Stm press.

12 kg/cm 2 g

Foul ftr. 0.0002

m 2 h °C/kcal

Blowdown

Mechanical Data—Boiler Sort

1 Pass

1 Tube OD

50.8 mm

Tube ID

45 mm

Number 600 Length

Ferrule ID

45 mm

Correc. factor 1 Economizer Mech. Data Tube OD

50.80 mm

Number of rows deep

Tube ID

4.00 m Fin density

44.00 mm

Eff. length

76.00 mm Fin height

0.00 fins/m

Tr pitch

76.00 mm Fin thickness

0 mm

Long pitch

0 mm

Streams

inline Fin ther. con.

counter flow Tubes/row

0 kcal/m h °C

Configuration

20 Evaporator Performance Pass

Gas inlet 850

°C

Gas outlet 273

±5°C

Gas sp ht. 0.2883

kcal/kg °C

Duty

8.52 MM kcal/h

Gas press. drop 114.00

mm wc

Max. ht. flux 39,589

kcal/m 2 h

Max. wall temp. 208

°C

overall U

61.19 kcal/m 2 h °C

Surf. area 509

Delt. 278

°C

Max. gas vel.

48 m/s

Gas vel.-ferrule

45 m/s

(Continued)

Waste Heat Boilers 241

TABLE 4.23 (Continued)

Performance of Fire Tube Boiler with Economizer Economizer Performance

Gas temp. in 274

24.00 mm wc Gas temp. out

±5°C

Gas press. drop

0.53 kg/cm 2 Duty

±5°C

Water press. drop

°C Wat. temp. in

1.28 MM kcal/h

Min. Wal. Temp.

58.92 kcal/m 2 h °C Wat. temp. out

°C

Overall U

4557 kcal/m 2 h °C Surf. area

±5°C

Tube-side htc

inline Max. gas vel.

m 2 Arrangement

Counter-flow Fw. temp. in

11 m/s

Flow direction

°C

Stm. Temp. 191

°C

Steam flow 17,472

kg/h

Gas flow 52,000

kg/h

Tot gas press. 137.00

Let us solve the earlier example for evaporator performance using the simulation approach.

First, the constant K is evaluated using known conditions. W g = 45,370 kg/h, T 1 = 815°C, T 2 = 260°C, t s = 185°C

 ( 815 185 − )  K ln = 

 = . 2 13 = KW / g 02 . = K / 45370 02 . . He n nce, K = 18 17 .

New conditions: W g = 52,000 kg/h. T 1 = 850°C, t s = 191°C.

ln [(850 − 191)/(T 2 − 191)] = 18.17/52,000 0.2 = 2.07 or (850 − 191)/7.93 = (T 2 − 191) or T 2 =

274°C. Very close to that obtained in the previous example. Thus, this is yet another useful method for plant engineers for obtaining quick estimates of fire tube boiler performance.

Simulation of Fire Tube Boiler Performance