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Surface Areas   can be misleading..
  V.Ganapathy
Some engineers have the notion that more the surface area better the boiler design.This is incorrect. Comparing boiler designs  based on surface areas alone can be misleading.The basic equation for energy transfer is:
Q=USDT,where Q=energy transferred,U=overall heat transfer coefficient, S=surface area and DT=log-mean temperature difference. For a given DT and Q ,S is a function of U.Higher the U,lesser the S and vice versa. Several variables affect U in water tube boilers such as gas analysis,temperature,gas velocity,fluid velocity inside tubes,tube size,tube spacings and fin configuration. In the case of fire tube boilers also tube sizes play a great role.
The example below shows a fire tube waste heat boiler design for the following parameters using different tube sizes.The example is completely worked out in my book,"Waste Heat Boiler Deskbook".Only the results are presented here.Remember the duty is the same in all the cases.
gas flow=100,000 lb/h colled from 1300 to 474 F generating 150 psig saturated steam.Gas analysis:% volume co2=12,h2o=12,n2=70 and o2=6. Fouling factors on gas side=0.002 and 0.001 on steam side.
 
tube size,in  1.75x1.521 1.75x1.521 1.75x1.521 2.5x2.238 2.5x2.238 2.5x2.238
velocity,fps 98 123 163 98 123 162
tubes 1000 800 600 470 375 280
length,ft 15.75 16.75 18 24.75 26.0 28.5
surface,ft2 6269 5333 4298 6812 6710 4673
Ui 9.47 11.08 13.70 8.73 10.29 12.72
DPg,in wc 2.05 3.34 6.23 1.95 3.16 6.00
(Ui refers to the overall heat transfer coefficient on tube ID basis and DPg is the gas pressure drop)
Conclusions
1.As the tube size reduces,the surface area required is lower due to higher U.Heat transfer coefficient is a function of tube size  and increases as the diameter decreases. For correlations,calculation procedures, see the author's books.
2.Tube length required is reduced as tube size is reduced for the same gas pressure drop and duty.It is about 17 ft for 1.75 in tubes versus 26 ft for 2.5 in tubes for a gas pressure drop of about 3.4 in wc.
3.As gas velocity increases,the surface area reduces and gas pressure drop increases.A 50 % variation is seen between the maximum and minimum gas velocity cases.
Hence simply looking at surface areas without evaluating the other variables can be misleading.Purchasing engineers often commit this sin using spread sheets when they present data to the management!! Henceforth please avoid this! A table showing surface areas is meaningless without associated heat transfer coefficients,pressure drop data.
Packaged Steam generators
In the case of packaged water tube boilers,there are more variables to confuse the engineer.In a packaged steam generator,for example,the superheater can be located in different regions. We have for example  the convective and radiant superheaters.  Features of radiant and Convective superheaters
With the location of superheater being different in different designs,the log-mean temperature is also different.Furnace areas can be different leading to a different gas temperature entering the convection bank.Gas temperature affects not only the convective but also the non-luminous radiation heat transfer coefficient. Hence U will be different,resulting in significantly different surface areas. Also,tube spacings can be different,affecting the heat transfer in convective sections.
Presence of fins can also complicate things as discussed in another article.With finned tubes you can even have more surface area and yet transfer less duty!  Heat Transfer with Finned Tubes
My suggestion is: Look at the overall performance,gas pressure drop,fan power consumpton,fuel consumption ,emissions and then decide. Don't conclude that because some supplier shows more surface area,their boiler is better. Also,surface areas have to be checked.Don't accept values shown in tables published years ago as completely correct. Some suppliers include even the refractory surface partly covered by tubes or use tube circumferential surface area in radiant sections  instead of projected area. Given below is an example of a packaged water tube  boiler with the same overall performance but with different surface areas. The idea is to bring out out the point that surface areas for the same overall performance can be different. The boilers generate 100,000 lb/h of saturated steam at 300 psig using 230 F feed water at 2 % blow down. natural gas is the fuel at 10 % excess air. Furnace back pressure is 7 in wc in ,efficiency is 84.3 % HHV and boiler duty=100.8 MM Btu/h in both cases.
 
  item  boiler 1    boiler 2
 furnace widthxheightxlength    6x10x22  6x10x29
 furnace proj area (duty)   802(36.6)   1026(40.4)
 volumetric HRR(Btu/ft3h)   90,500    68,700
 area  HRR(Btu/ft2h)   149,000   116,600
  furnace exit gas temp,F    2364  2255
 boiler exit gas temp,F    680   610
 economizer exit gas temp,F    315   315
 evaporator surface(duty)    3972(53.7)   4760(52.1)
 economizer surface(duty)   8384(10.5)   8550(8.3)
  geometry   evap/econ   evap/econ
   tubes/row    11/15   10/15
  number deep    66/14    87/10
  length,ft    9.5/11    9.5/10
  economizer fins     3x.75x.05x.157   5x.75x.05x.157
  transverse  pitch,in    4/4   4.375/4
 (duty above is in MM Btu/h. surface in ft2 and Heat Release rates based on HHV)
Waste Heat Boiler Surface Areas
With waste heat boilers using finned surfaces and varying fin densties,one has to be extremely careful evaluating surface areas. As illustrated in the article on finned tube heat transfer,the surface areas can vary by 50-200 % for the same duty.
Example: A gas turbine HRSG generates 200 psig saturated steam from feed water at 230 F.An Economizer is not used.The gas flow=150,000 lb/h at 1000 F. Exit gas temperature=423 F.Duty=22.74 MM Btu/h. gas pressure drop= 2 in wc. Steam flow=22,600 lb/h. For these SAME parameters,let us design 2 evaporators with different fin configurations as shown below and compare the surface areas.Tube size=2x.105 in ,fouling factors=0.001 on both sides.
 
 item option 1  option 2
 tubes/row 30 27
no of rows deep 14 24
effective length,ft 7 7
fins 4.5x.875x.05 2x.625x.05
transverse pitch 4 3.875
overall U 6.46 11.04
surface areas,ft2 17,523 10,204
Conclusion:It may be seen from the above,that using a lower fin size results in higher overall heat transfer coefficient and hence lower surface area.This point was also explained in my article  Heat Transfer with Finned Tubes
The difference is very significant,nearly 65 %. Hence going by some ageold norm such as 5 sq.ft per Boiler Horse Power or some other norm invented decades ago should not be applied to water tube steam generators,particularly those with finned tubes. It is unfortunate that some engineers still have those misleading concepts and myths about surface areas.I hope that some of my articles and my books would have dispelled these notions.

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