Small Changes, Big Benefits

Custom designing process involves teamwork and understanding among design and fab- rication groups in a boiler company. While many boiler suppliers like to keep on doing

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

Steam in

Gas in

(a)

Steam out

Steam in

Gas in

(b)

Steam out

FIGURE 3.15

Superheater arrangement for (a) low-pressure and (b) high-pressure operations. what they have been doing for decades, many small boiler companies have found ways to

offer better value to the end user if they modified the existing designs whenever neces- sary. In the following example, a standard boiler design for an application involving FGR was found to have a very high back pressure, though the initial cost was attractive. The back pressure was reduced by making a minor change to the convection bank as a result of which the operating cost of the boiler was significantly reduced. The boiler cost did not increase, while the fan size and power consumption were drastically reduced. A strong thermal engineering team is required in a boiler company to perform this type of analysis. Plant engineers should review the thermal performance and question the boiler suppliers regarding operating costs and back pressure. Since many boiler companies do not want to spend time reengineering their boiler and want to push existing designs, plant engineers will not get a satisfactory answer! However, by becoming more knowledgeable, they can easily see how a particular boiler design can be improved. The following example illus- trates the point.

Example 3.1

A boiler supplier proposed a steam generator firing distillate oil with a capacity of 90,750 kg/h (200,000 lb/h) of saturated steam at 27.6 barg (400 psig) using 110°C (230°F) feed water to a chemical plant; to meet low emissions of NO x , the boiler supplier sug- gested the need for FGR at 17% and an excess air of 15%. On reviewing the boiler performance of the standard boiler, the plant engineer felt that the back pressure or flue gas pressure drop in the convection bank was high. The author also felt that the operating cost could be significantly reduced by making minor changes to the convection bank design without impacting the performance. The results are shown in Table 3.2 for the standard and custom designs showing how a small change can bring in so much relief in operating cost! The economizer exit gas temperature is the same in both cases,

Steam Generators 101

TABLE 3.2

Comparison of Standard and Custom-Designed Boiler

Item

Standard Boiler

Custom Boiler

Flue gas flow, kg/h

110,950 Furnace exit temperature, °C

1,130 Leaving evaporator, °C

422 Leaving economizer, °C

Evaporator surface, m 2 552

Economizer surface, m 2 2,370

Evaporator Economizer

Tubes/row 12 20 12 20 Number of rows deep

20 Tube length, m

3.66 2.743 3.66 Transverse pitch, mm

114 Gas press drop, mm wc

36 Notes: Gas analysis: % volume CO 2 = 11.57, H 2 O = 12.29, N 2 = 73.63, O 2 = 2.51. Fouling factor gas =

0.0006, and tube side = 0.0002 m 2 h °C/kcal. Tube size = 50.8 × 44.7 mm. Fins configuration of economizer: 197 fins/m, 19 mm high, 1.27 mm thick, 4.36 mm serration.

and hence, the efficiency is the same. The boiler furnace and convection bank surface area remained the same. However, the evaporator tube spacing was manipulated in the custom boiler to lower the gas pressure drop. To compensate for the slightly higher gas temperature entering the economizer, one more row was added to the economizer. Hence, boiler cost did not see any increase! (A large company in my personal opin- ion may not be amenable to such design modifications as it involves several layers of bureaucracy to make these small changes or they may say that they have thousands of such standard boilers in operation worldwide, and if the plant engineers are not process-savvy, they may go along with the poor design losing money in operating costs year after year!)

The boiler practically is the same in both cases except for the tube spacing of the evaporator section and an additional row of tube in the economizer. However, the operating cost of the custom-designed boiler reduces by over 130 mm wc. The fan power saved is

2.7 × 10 −6 × 90,000 × 130/(1.16 × 0.7) = 39 kW (1.16 kg/m 3 is the density of air and 0.7 is the

efficiency of fan motor combination and air flow = 90,000 kg/h). At 15 cents/kWh, the annual savings in operating cost due to this slight modification is = 39 × 0.15 × 8,000 = $46,800. Hence, the custom design pays for itself in a short period. It is not sufficient if the plant engineer reviews the painting or insulation or the welding aspects or weight of the boiler. The objective of the book is to make plant engineers process-savvy so that they can review boiler performance from boiler suppliers and suggest improvements or question their data!

Plant engineers can also verify the performance data for the standard and custom boilers as follows. Appendix A shows the procedure.

Solution

Calculations for Evaporator Performance in Standard Boiler Gas mass velocity G = 110,940/[12 × 2.743 × (0.133 – 0.0508) = 41,000 kg/m 2 h = 11.39 kg/m 2 s.

Average gas temperature in the bank tubes = (1130 + 386)/2 = 758°C = 1031 K.

A good estimate of gas film temperature = 0.5 × (758 + 231) = 494°C or close to 500°C. Gas properties for heat transfer are evaluated at this temperature. From Appendix F,

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

C p = 0.2863 kcal/kg °C, µ = 0.1209 kg/m h, k = 0.046 kcal/m h °C. Tube wall temperature may be assumed to be 237°C, a little more than saturation due to the high

tube-side boiling heat transfer coefficient of about 10,000 kcal/m 2 h °C (assumed). As discussed in Appendix A, the tube-side coefficient will not impact U. Let us use Grimson’s correlations for h c . For S T /d = 133/50.8 = 2.62 and S L /d = 101.6/50.8 = 2, from Table C.2, B = 0.213, N = 0.64. Reynolds number Re = 41,000 × 0.0508/0.1209 = 17,227. Nu = 0.213 × 17,227 0.64 = 109.5 = h c × 0.0508/0.046 or h c = 99.15 kcal/m 2 h °C. A correction factor of 10% is used with Grimson’s

correlations for convection bank with membrane panel end walls (based on the author’s experience). Hence, use h c = 109 kcal/m 2 h °C. Calculation of nonluminous heat transfer coefficient h n . Using methods discussed in Appendix D, beam length L = 1.08 × (0.133 × 0.1016 – 0.785 × 0.05058 × 0.0508)/0.0508 =

0.244 m. K = (0.8 + 0.12 × 1.6) × (1 − 0.00038 × 1031) × 0.238/(0.238 × .244) 0.5 = 0.596.

∈ g = 0.9 × [1-exp(.244 × 0.5960] = 0.122

h n = 5.67 × 10 −8 × 0.122 × [1031 4 − 510 4 ]/(1031 − 510) = 14.1 W/m 2 K = 12.12 kcal/m 2 h°C (1031°K is the average gas temperature and 510 K is the average tube wall temperature.)

1/U o = 1/(109 + 12.12) + 0.0006 + 0.0505 × ln(50.8/44.7)/(2 × 35) + 50.8/(44.7 × 9800) + 0.0002 × 50.8/44.7 or U o = 107.6 kcal/m 2 h °C (125.1 W/m 2 K)

Let us check the evaporator exit gas temperature. One may use the following equation for evaporator performance as shown in Appendix A.

Ln[(1130−231)/(T – 231)] = 107.6 × 552/(110,941 × 0.303) or T = 385°C agrees with the data given by the boiler supplier (0.303 is the specific heat of flue gas at the average gas temperature).

Flue Gas Pressure Drop MW of flue gas = 44 × 0.1157 + 18 × 0.1229 + 28 × 0.7363 + 32 × 0.0251 = 28.72. S T /d = 2.625.

S L /d = 2. Density at average gas temperature of 758°C = 12.17 × 28.72/(273 + 758) = 0.339 kg/m 3 .

Reynolds number at 758°C = 41,000 × 0.0508/0.1469 = 14,178. See Appendix C for flue gas pressure drop calculations. Friction factor f = 14,178 −0.15 [0.044 + 0.08×2/{1.625 (0.43 + 1.13/2) }] = 0.034 Δ P = 0.204fG 2 N d /ρ g = 0.204 × 11.39 2 × 0.034 × 105/0.339 = 279 mm wc, which is close to the boiler supplier’s data.

Calculations for Evaporator Performance in Custom Boiler Now let us look at the option of S T = 159 mm. Average gas temperature is (1130 + 422)/2 =

776°C. G = 110,940/[12 × 2.743 × (0.159 – 0.0508) = 31,150 kg/m 2 h = 8.65 kg/m 2 s.

Approximate gas film temperature = 0.5 × (776 + 231) = 503°C, which is not much dif- ferent from the earlier case. So let us use the same flue gas properties.

For S T /d = 3.125, S L /d = 2, B = 0.198, and N = 0.648 from Appendix C. Re = 31,150 ×

0.0508/0.1209 = 13,088. Nu = 0.198 × 130,880.648 = 92.1 or h c = 92.1 × 0.046/0.0508 = 84.3

kcal/m 2 h °C. Using 10% margin as before, h c = 91.8 kcal/m 2 h °C. Beam length L = 1.08 × (0.159 × 0.1016 – 0.785 × 0.05058 × 0.0508)/0.0508 = 0.300 m

K = (0.8 + 0.12 × 1.6) × (1−0.00038 × 1049) × 0.238/(0.238×.300) 0.5 = 0.531

∈ g = 0.9 × [1 – exp(–.0300x0.531] = 0.132

Steam Generators 103

h n = 5.67 × 10 −8 × 0.132 × [1049 4 −510 4 ]/(1049−510) = 15.87 W/m 2 K = 13.65 kcal/m 2 h °C 1/U o = 1/(91.8 + 13.65) + 0.0006 + 0.0505 × ln(50.8/44.7)/(2 × 35) + 50.8/(44.7 × 9800)

+ 0.0002 × 50.8/44.7 or U o = 95 kcal/m 2 h °C (110.5 W/m 2 K). Let us compute the exit gas temperature T. Ln[(1130 − 231)/(T – 231)] = 95 × 552/(110,941 ×

0.303) or T = 420°C agrees with the data given by the boiler supplier. Flue Gas Pressure Drop

Reynolds number at average gas temperature of 776°C = 31,150 × 0.0508/0.1486 = 10,649. Friction factor f = 10,649 −0.15 [0.044 + 0.08 × 2/{2.125 (0.43 + 1.13/2) }] = 0.0297.

Δ P = 0.204fG 2 N d /ρ g = 0.204 × 8.65 2 × 0.0297 × 105/0.333 = 143 mm wc, which is close to the value shown. Density of flue gas at average gas temperature is 0.333 kg/m 3 .

Economizer Performance A simple way of checking the economizer duty will be explained as the economizer

design or cross section is unchanged. Calculate the LMTDs for both cases and the duty that is proportional to the temperature rise of the water. The ratio of the duty

for custom to standard economizer is Q c /Q s = (U c × ΔT c × 2495)/(U s × ΔT s × 2370) =

87/76 (87°C and 76°C refer to water temperature rise, which is a proxy for the duty as water flow is the same in both cases and the surface areas are, respectively, 2495

and 2370 m 2 ). Δ T c = [(386 − 186) – (148 − 110)]/ln[(386 − 186)/(148 − 110)] = 97.5°C.

Δ T s = [(422 − 197) – (149 − 110)]/ln[(422 − 197)/(149 − 110)] = 106°C

Hence, (U c × 106 × 2495)/(U s × 97.5 × 2370) = 87/76 or U c /U s > 1. That is, it is sufficient if U c is equal to U s to transfer the desired duty in the economizer. However, note that U c will be higher than U s as the average gas temperature in the economizer is now higher. Hence, the duty Q c can be easily achieved. Hence, the custom design is a far better

offering than the initial standard design. Only by challenging the boiler supplier, a better design emerged. Thus, plant engineers should have process-oriented background, and this book will help them become a little more process-savvy.

Small Changes, Big Benefits