# 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.