Areas can be misleading..
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=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.
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.
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.
refers to the overall heat transfer coefficient on tube ID basis and DPg
is the gas pressure drop)
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.
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.
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.
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.
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
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.
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
Transfer with Finned Tubes
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.
above is in MM Btu/h. surface in ft2 and Heat Release rates based on HHV)
proj area (duty)
furnace exit gas temp,F
exit gas temp,F
exit gas temp,F
Waste Heat Boiler Surface
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.
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
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
of rows deep
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.
on Boilers,HRSGs(home page)