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FOULING IN WASTE HEAT BOILERS by V.Ganapathy
This article describes how one may evaluate fire tube and water tube boilers for fouling.Fouling on water,steam side can affect the boiler in several ways.The exit gas temperature will increase,thus resulting in loss of energy recovery and reduced steam production. Tube wall temperature can increase signifcantly leading to tube failures due to scale formation. Fouling on gas side leads to loss of steam output and can increase the gas side pressure drop.However it does not increase the tube wall temperatures. The simple procedure developed here explains how the evaporator performance may be evaluated for possible fouling.One may also compute the exit gas temperature and compare it against operating data to see if fouling has occurred.
 
Performance of Evaporators  
The performance of an evaporator(fire tube or water tube,bare or finned) may be obtained by solving the following equations:(The performance evaluation of superheater,economizers are more involved and readers may see my books,"Waste Heat Boiler Deskbook" or "Steam Plant Calculations Manual").  
 Q=WgCpg(tg1-tg2)hl =USDT=US[(tg1-ts)-(tg2-ts)]/ln[(tg1-ts)/(tg2-ts)] 
Wg=gas flow,lb/h  
tg1,tg2,ts=gas inlet and exit temperatures and saturation steam temperature,F  
Cpg=gas specific heat,btu/lbF  
hl=heat loss,factor.1 % heat loss means hl=0.99  
U=overall heat transfer coefficient,Btu/ft2hF  
S=surface area,ft2  
DT=log-mean-temperature difference,F  
Simplifying the above,we have:  

ln[(tg1-ts)/(tg2-ts)]=US/(WgCpgxhl) 

 
 

Example of a Fire Tube Boiler
Let us assume that a fire tube boiler is operating as shown below in clean conditions:
gas inlet temperature=1500 F
gas flow=100,000 lb/h
gas exit temperature in clean conditions=500 F
steam pressure=150 psig (ts=366 F)
feed water temperature=230 F
blow down=5 %
tubes: 2x1.77in,600 tubes,20 ft long,surface area=6280 ft2(OD basis)
Duty=100,000x.98x.287x1000=28.1 Mm Btu/h and steam=27,710 lb/h
Let us compute U.
ln[(1500-366)/(500-366)]=Ux6280/(100,000x0.287x0.98) or U=9.56 Btu/ft2hF
Let us assume that a scale has formed and the equivalent additional fouling=0.05 ft2hF/Btu. What happens to the performance?
This will decrease U to: 1/(1/9.56)+.05)=6.5 Btu/ft2hF
This in turn will affect the exit gas temperature as follows:
ln[(1500-366)/(tg2-366)]=6.5x6280/(100,000x.98x0.287) or tg2=630 F
The duty=100,000x0.98x0.287x(1500-630)=24.4 Mm Btu/h and steam generation=24,100 lb/h
Table below summarises the results.(For more detailed calculations of tube wall temperatures,heat flux,see my books) Note that even though the heat flux has reduced due to lower U in fouled condition,the tube wall temperature is significantly higher!
 
item clean fouled
fouling factor,ft2hF/Btu  .001 .05
U,Btu/ft2hF 9.6 6.5
heat flux,Btu/ft2h 6086 4133
drop across scale,F 6 207
tube wall temp,F 377 577
duty,MM Btu/h 28.13 24.47
exit gas temperature,F 500 630
steam flow,lb/h 27,710 24,100
The difference in energy loss at 3$/MM Btu=3.66x8000x3=$ 87,800 per year
Calculations for a water tube boiler are similar.One can also use the above equation to adjust for variations in gas flow by correcting U by (gas flow)0.6 for water tube boilers and (gas flow)0.8 for fire tube boilers inorder to check if surfaces have fouled. See my books for more examples.
The type of scale formed also affects the fouling resistance and the increase in tube wall temperatures as shown below.

Effect of scale conductivity on tube wall temperatures
scale ther cond,Btu in/ft2h fouling resistance temperature rise,F
calcium phospate 25 0.0008 8
calcium sulfate 16 0.00125 13
magnesium phospate 15 0.00133 14
silicate 0.6 0.0333 333
boiler steel 310 0.00006 0.6
magnetic iron oxide 20 0.001 10
[ for a scale thickness=0.02 in,heat flux=10,000 Btu/ft2h;  fouling resistance=thickness/th cond]
It is seen that even a small layer of silicate can increase the tube wall temperatures significantly.So one has to monitor the boiler performance,watch for increases in exit gas temperatures,tube wall temperatures or loss of steam production for the same gas inlet conditions.In the case of finned tube evaporators,the heat flux inside tubes will be very high compared to a bare tube boiler and hence even a small  fouling layer can increase the tube wall temperature significantly.
 
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