Superheater Design and Off-Design Calculation

Superheater Design and Off-Design Calculation

We will now illustrate the design and off-design calculations for a finned tube super- heater (Figure A.2). One may refer to Appendix E, where the U value is evaluated for this superheater.

Example A.1

A 50.8 × 44 mm (2 × 1.732 in.) solid finned superheater with 78 fins/m (2 fins/in.), 12.7 mm (0.5 in.) high fins, and 1.52 mm (0.06 in.) thick is arranged in inline fashion with 101.8 mm (4 in.) square pitch: 18 tubes/row, 6 rows deep, 3.1 m (10.2 ft) long tubes with 9 streams. Gas flow is 100,000 kg/h (220,400 lb/h) with the following exhaust gas analysis by vol-

ume% CO 2 = 3, H 2 O = 7, N 2 = 75, O 2 = 15. The bundle is designed with inlet and exit exhaust

gas temperatures of 550°C (1022°F) and 479°C (892°F) and with 25,000 kg/h (55,100 lb/h)

of saturated steam at 51 kg/cm 2 g (725 psig) raised from saturation temperature of 265°C (509°F) to 369°C (696°F). Surface area is 188 m 2 (2030 ft 2 ). Check if the design is reasonable.

Flue gas data from Appendix F at the average gas temperature of 514°C have been esti-

mated as follows: C p = 0.2755 kcal/kg °C (0.2755 Btu/lb °F), µ g = 0.127 kg/m h (0.085 Btu/ ft h), k g = 0.0468 kcal/m h °C (0.0314 Btu/ft h °F). Assume 1% heat loss.

Appendix A: Boiler Design and Performance Calculations 365

Gas in

Steam out

Steam in

Gas out

FIGURE A.2

Typical convective superheater.

Solution

First let us check the design. The exit gas temperature is 479°C. The duty is then

Q =W g ×C pg × (T g1 −T g2 ) × (1 − h l ) = 100,000 × 0.2755 × (550 − 479) × .99 = 1.936 × 10 6 kcal/h (7.68 MM Btu/h) (2.25 MW).

Enthalpy absorbed by steam = 1.936 × 10 6 /25,000 = 77.4 kcal/kg (139.3 Btu/lb). From steam tables at 51.5 kg/cm 2 of steam pressure, saturated steam enthalpy from steam tables is 667.3 kcal/kg (steam-side pressure drop of 1.5 kg/cm 2 was assumed); exit enthalpy = 667 + 77.4 = 744.7 kcal/kg. Hence, exit steam temperature = 368°C (694°F). For counter-flow arrangement,

Δ T = [(479 − 265) − (550 − 368)]/ln [(479 − 265)/(550 − 368)] = 197.6°C (355°F).

U o from Appendix E is 51.9 kcal/m 2 h °C (10.6 Btu/ft 2 h °F).

Surface area required A = 1.936 × 10 6 /197.6/51.9 = 189 m 2 (2033 ft 2 ). Surface area pro- vided is 188 m 2 (2023 ft 2 ). Hence, the duty is close to that assumed, and the design is

reasonable. If U is not given, then one may use the methods discussed in Appendices B through E to arrive at U.

What if the plant engineer is asked to design a superheater from scratch for a specific duty or parameters? In such cases, the following points may be kept in mind.

1. Calculate the duty of the superheater as we know the mass flow of steam and its inlet and desired exit temperatures; evaluate the flue gas exit temperature. The flue gas mass flow should be known or will be available. Then, the LMTD is computed as all the four temperatures are known.

2. The cross section of the superheater should be selected such that the gas veloc- ity is in the range of 20–35 m/s (for dust-laden gases, the velocity will be much lower, say, around 10–20 m/s). Values beyond the range may sometimes be

366 Appendix A: Boiler Design and Performance Calculations

acceptable due to layout and shipping considerations. The gas pressure drop will have to be checked later and should be within allowable values; if not, the cross section is revised.

3. Streams should be selected on the tube side such that the steam-side velocity is in the range of 15–30 m/s depending on steam pressure and pressure drop. At lower pressure, the specific volume of steam is higher and so the velocity. For

a discussion on streams, refer to Appendix B. 3. Calculate h c ,h n ,h i , and U using the methods described in the appendices. 4. Using the equation Q = UAΔT, determine A and the number of rows deep as before. 5. Compute the gas- and steam-side pressure drops as shown in Appendices B and C and see if they are reasonable. If not, repeat from step 2 by manipulating the tube length, size, or length or tube spacing.