Comparing Surface Areas

Example 2

This example illustrates the point that surface areas can be misleading. A boiler generates 100,000 lb/h of saturated steam at 300 psig. Feedwater is at 230°F, and blowdown is 2%. Standard natural gas at 10% excess air is fired. Boiler duty = 100.8 MM Btu/h, efficiency = 84.3% HHV, furnace backpressure = 7 in. WC

It is seen from Table 3.2 that boiler 2 has about 10% more surface area than boiler 1 but the overall performance is the same for both boilers in terms of operating costs such as fuel consumption and fan power consumption. Also the

Table 3.2 Comparison of Boilers with Same Efficiency and Backpressure

Itema

Boiler 1

Boiler 2

Heat release rate, Btu/ft3 h

90,500

68,700

Heat release rate, Btu/ft2 h

148,900

116,500

Furnace length, ft

22

29

Furnace width, ft

6

6

Furnace height, ft

10

10

Furnace exit gas temp, °F

2364

2255

Evaporator exit gas temp, °F

683

611

Economizer exit gas temp, °F

315

315

Furnace proj area, ft2 (duty)

802 (36.6)

1026 (40.4)

Evaporator surface, ft2

3972 (53.7)

4760 (52.1)

Economizer surface, ft2

8384 (10.5)

8550 (8.3)

Geometry

Evaporator

Economizer

Evaporator Economizer

Tubes/row

11

15

10

15

Number deep

66

14

87

10

Length, ft

9.5

11

9.5

10

Economizer, fins/in. x ht

X

5

.7

0.

X

3

7

5

0.

X

5

.0

0.

5 x 0.75 x 0.05

X 0.157

X thickness x (serration)

Transverse pitch, in.

4

4

4.375

4

Overall heat transfer coeff

18

7.35

17.0

6.25

ADuty is in MM Btu/h, fin dimensions in inches, heat transfer coefficient in Btu/ft2 h °F.

Energy absorbed in different sections is different, hence comparing surface areas is difficult unless one can do the heat transfer calculations for each surface.

It has become a common practice (with the plethora of spreadsheet users) to compare surface areas of boilers and generally select the design that has the higher surface area. Surface areas should not be used for comparing two boiler designs for the following reasons:

1. Surface area is only a part of the simple equation Q = UA AT, where U = overall heat transfer coefficient, A = surface area, AT = log-mean temperature difference, and Q = energy transferred. However, the Q and AT could be different for the two designs at different sections as shown in the above example. Hence unless one knows how to compute U, A values should not be compared.

2. Even if AT remains the same for a surface, U is a function of several variables such as the tube size, spacing, and gas velocity. With finned tubes, the heat transfer coefficient decreases as fin surface area increases, as discussed in Q8.19. Hence unless one is familiar with

All these issues, a simplistic tabulation of surface areas can be misleading.

Posted in Industrial Boilers and Heat Recovery Steam


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