Dr. Albrecht Kaupp
Page 1
The soot and scale
problems
Issue
Soot and scale do not only increase energy
consumption but are as well a major cause of
tube failure.
Learning
Objectives
 Understanding the implications of soot at
the fire side
 Understanding the implications of scale at
the water side
 Estimating the energy losses through soot
and scale
 Knowing additional negative side effects
of scale and soot build up
 Relating boiler performance parameters to
soot and scale build up
The soot and scale problems
Page 2
 NOTES
1. Introduction
Boilers are delivered clean with no soot, slag and scale.
Consequently a soot and scale problem is a classic management
and operational problem that has very little to do with boiler
design.
Soot and slag is a mixture of solid carbon, ash, and molten ash that
sticks to the fire side of the tube and prevents heat transfer. Slag
will also cause corrosion.
Scale is a hard coating or layer of chemical materials on internal
surfaces of the boiler exposed to the water side. Scale mitigates
heat transfer and may lead to corrosion as well.
Scale and soot prevention is one of the most important task of a
boiler operator besides reducing stack gas losses.
The cause of scale will be extensively discussed in lecture 15. Soot
and slag deposits at boiler tube surfaces are mainly a firing
problem and/or mismatch of the fuel and burners.
2. Soot and slag generation
In any combustion process of a fuel there will be always some
unburned carbon (soot) generated and some ash carried with the
stack gas stream. Soot, ash, and molten ash (slag) will accumulate
at the tube banks of the heat exchangers. Some ash will even melt
down at the tube surface. The final result is a layer insulating the
tubes against the hot combustion gases.
Soot generation has various causes such as
a)
Freezing the chemical reaction
The flame comes too close to the boiler walls, “freezing” the
kinetic reaction. This effect is best demonstrated with a knife
one passes through a candle flame. The flame will blacken the
knife.
Major causes are overfiring of the boiler, too much excess air,
and worn out burner nozzles.
The soot and scale problems
Page 3
 NOTES
b) Insufficient atomization of fuel oil
We talk about the three “T’s” in combustion, turbulence,
temperature and time. Low oil line pressure and lack of
turbulence will result in poor atomization of fuel oil, resulting
in larger droplets leaving the flame envelope partially
unburned.
Major causes are low oil line pressure, lack of primary
combustion air, or low fuel oil preheating temperature.
c)
High moisture content in the fuel
Too much water in a fuel leads to a “cold” fire causing
excessive smoke generation because there is not enough fuel
energy available to generate a sufficiently high flame
temperature. However some water helps to speed up kinetic
reaction. It also shortens the flame length.
Major cause of this type of soot generation is a combination of
high moisture content and too much excess air.
d) Erratic feeding of solid fuels
Solid fuel firing is not as “smooth” as liquid or gaseous fuel
firing. Excess air is changing constantly, causing cold pockets
in the furnace, where too much fuel and not enough “fire” and
air are present. The result is smoke generation.
Major causes are bad combustion air distribution and
malfunctioning or badly designed feeding mechanism.
e)
Dripping burner
Occasionally liquid oil drips directly from the burner down
into the fire tube and forms a pile of soot. This soot burns up
and generates smoke.
In particular in boilers with superheaters, soot and slag
accumulates at the superheater tube banks first because the
superheater is the first heat exchanger passed by the products of
combustion. All superheaters have “soot blowers” that are
activated periodically to blow off the soot with steam.
The soot and scale problems
Page 4
 NOTES
3. Fire tube and water tube differences
There is a basic difference between fire tube and water tube
arrangements, with respect to the location of the soot and scale.
Figure 1 shows a water or superheater tube with soot at the outside
of the tube and scale at the inside. In a water tube boiler the hot
combustion gases pass a bank of tubes at the outside and release
their energy to the water or steam flowing inside the tube.
Figure 1: Water tube with soot outside and scale inside
The soot and scale problems
Page 5
 NOTES
Figure 2: Fire tube with soot inside and scale outside
Figure 2 shows a fire tube with soot inside the tube. In a fire tube
boiler the hot combustion gases pass through a bundle of tubes and
release part of their energy to the water at the outside.
Fire tube boilers are usually smaller (1 to 25 t/h) and mostly in the
5 to 20 bar range. Soot cleaning is simple, requiring only to open
the back and front door of the boiler to expose the horizontal fire
tubes. Soot cleaning of water tube boilers is much more
complicated, since they are larger and more complicated built.
4. Stack gas temperatures versus
soot and scale deposits
The rated heat output of a boiler is based on firing a specific fuel at
a specific excess air factor. In a clean boiler this will result in a
well defined stack gas temperature for a specific fuel firing rate.
Stack gas temperatures depend very much on the firing rates. The
stack gas temperature is usually lower at “low fire”, while it
reaches its peak at “high fire”.
Most manufacturers specify what stack gas temperature is to be
expected at “low fire”, “high fire” or rated heat output. The stack
gas temperature always refers to a point in the system where the
combustion gases do not release heat any longer to the water-steam
The soot and scale problems
Page 6
 NOTES
circuit. This boiler specific stack gas temperature is our yardstick
and the best we can achieve in the field.
High stack gas temperatures are therefore a sure sign that boiler
heat exchanger surfaces have accumulated soot and/or scale.
A highly recommended method of reducing fuel consumption is to
set a so called best stack gas temperature at “high fire” and to
instruct operators to clean the boiler if stack gas temperatures
exceed this value by 10 to 20 oC.
This practice is in particular easy to implement with fire tube
boilers, where cleaning of the fire side can be accomplished in half
a day. Descaling of the water side is usually complicated and done
yearly if necessary.
Implementation of this practice would require to record stack gas
temperatures about three times a day at the high fire setting. Note,
that the stack gas temperature at a lower firing rate will be
considerable lower. Also, increasing the excess air lowers stack gas
temperature. Furthermore most in line temperature sensors get
easily fouled, with soot deposits at the stem. Fouled temperature
sensors show a lower temperature because the soot layer insulates
the stem against the hot stack gas.
As discussed, any 20 oC decrease of stack gas temperature could
easily save 1 % fuel and is therefore worthwhile to consider.
5. A crash course in applied heat transfer
In a fire tube boiler, the energy of hot combustion gases are
transferred through the soot layer, the tube wall and the scale layer
to the water.
Both, the soot as well as the scale act as insulators. The effect is a
hotter than usual stack gas temperature because the combustion
gases were prevented from efficiently transferring their energy to
the water side.
Depending on the water chemistry, different types of scale exist.
 High density scale
( = 2,000-2,700,  = 0.7-2.33)
The soot and scale problems
Page 7
 NOTES
 Medium density scale
 Low density scale
( = 1,000-2,500,  = 0.15-1.16)
( = 300-1,200,  = 0.08-0.23)
Roh, (), is the scale density in kg/m3, while Lambda, (), stands
for the thermal conductivity in W/moC.
The thermal conductivity of soot is between 0.03 and 1 W/moC,
depending on how much slag has been accumulated at the surface.
For comparison, good insulation of steam distributions line have 
= 0.05 to 0.3 W/moC. Boiler tube walls have a thermal conductivity
of   50 to 60 W/moC and are very good heat conductors.
An insulator causes a high temperature drop across its thickness,
while a very good conductor causes a very small temperature drop
across its thickness.
Since on both sides of the tube we have a fluid flowing (stack gas,
water, or steam) there are also two so called film heat transfer
coefficients, hH2O, for the water or steam side and hgas for the gas
side. The film heat transfer coefficient, hgas, describes how easily
the gas transfers its energy to the metal or soot surface, while hH2O
describes how easily the water or steam picks up the heat that
penetrates through the metal tube or the scale surface.
The combined effect is expressed in a well known equation
describing heat transfer through tubes with three layers (scale,
metal and soot).
Q =
Ta
Tb
r0
r1
r2
r3
ki
L
h0
h3
=
=
=
=
=
=
=
=
=
=
2 L Ta  Tb 
 1
ln r r ln r2 r1 ln r3 r2
1 
 1 0 




k1
k2
k3
r3 h3 
 r0 h0
Watt
temperature of the fluid (steam, water, stack gas) inside the tube, oC
temperature of the fluid (gas, water) outside the tube, oC
inner radius of the free cross section of the pipe, m
outer radius of the first layer, m
outer radius of the second layer, m
outer radius of the third layer, m
thermal conductivity of layer i, W/moC
tube length, m
inner film heat coefficient, W/m2 oC
outer film heat coefficient, W/m2 oC
To demonstrate some of the temperature and heat loss effects it is
sufficient to assume the following:
The soot and scale problems
Page 8
 NOTES






Film heat transfer liquid water side, hH2O
Film heat transfer steam side, hH2O
Film heat transfer gas side, hgas
 of scale
 of soot
 of metal tube
= 10,000 W/m2 oC
= 1,000 W/m2 oC
=
100 W/m2 oC
=
1 W/m oC
=
1 W/m oC
=
50 W/m oC
The above numbers are used in the exercise section.
6. Heat exchanger tube failures
There is a specific set of circumstances, where you as a consultant,
trying to reduce fuel consumption, may unintentionally contribute
to premature tube failure in heat exchanger tubes.
Some boiler operators neither care about soot nor scale and operate
their boiler inefficiently. Recall that scale as well as soot are
barriers to heat transfer. Your recommendation is to clean the fire
side of the boiler and remove the soot regularly, while descaling of
the water side can wait until the next major shutdown for
overhauling of the boiler.
Consequently one barrier for heat transfer is removed, but the other
remains. Note that the barrier on the very hot combustion gas side
was removed, and subsequently the metal surface temperature will
go up significantly (thermal stress), depending on the scale
thickness on the water or steam side. This phenomena would not
happen to this extend if both barriers, scale and soot, are removed.
Keep in mind that under certain circumstances cleaning the fire
side without cleaning the water or steam side will increase the
danger of thermal stress of boiler tubes. This happens in particular
at the tube seats of the first pass of fire tube boilers.
It is in general bad practice to frequently clean the fire side and
totally ignore scale built up at the water side. This will reduce fuel
consumption at the expense of more frequent retubing of boiler
tubes. Tube corrosion is also accelerated at high temperatures. The
costs of retubing and repairing water walls damaged by
overheating are much higher than any additional fuel costs due to
The soot and scale problems
Page 9
 NOTES
reduced efficiencies. One should therefore always inquire about
descaling practice and feedwater treatment.
7. Fuel reduction potential
Only “rules of thumb” figures for scale and soot can be given as
shown in Figure 3 and Figure 4. Losses are expressed in percent of
fuel input as a function of the scale or soot thickness. Inaccuracies
are not caused by rough estimates of film heat transfer coefficients.
Their value has no significant impact on the result. More critical is
the thermal conductivity of scale and soot layers that may vary
widely.
Nevertheless even very thin layers (0.5 to 1 mm) of soot and scale
cause significant fuel losses. In addition scale built up causes
thermal stress to the tubes and increases repair and maintenance
costs.
Effect of Soot on Fuel Consumption
% Fuel Energy Loss
10
8
6
4
2
4
3.5
3
2.5
2
1.5
1
0.5
0
Thickness of Soot Layer, mm
Figure 3: Fuel energy loss due to soot
The soot and scale problems
Page 10
 NOTES
Effect of Scale on Fuel Energy Losses
12
% Fuel Loss
10
8
High density
6
Medium Density
Low Density
4
2
2
1.75
1.5
1.25
1
0.75
0.5
0.25
0
0
Scale Thickness,mm
Figure 4: Fuel energy loss due to scale
8. Measurement techniques
The stack gas temperature of a boiler, if continuously recorded
over longer periods (weeks, months), gives very important
information about the efficiency of a boiler and its operating hours.
Temperature recording is accurate, inexpensive and easy to
perform.
In case a boiler requires closer observation one may first
install automatic single channel data loggers that measure the
temperature in 1 to 10 minute intervals over days and weeks.
Equipment costs are US$ 350 only.
A long term temperature profile of a boiler is like a
electrocardiogram. It can show on a time scale the following:

The temperature built up due to scale and soot formation

The cycling behavior of the oil burner or solid fuel feeding
system

The low and high fire intervals

The boiler operating hours
Consultants are encouraged to continuously record boiler
temperature in case they intend to engage in a long term contract to
improve boiler efficiency.
The soot and scale problems
Page 11
 NOTES
EXERCISES
Task 1
Assume there is a scale with  = 1 W/moC at the inside of a boiler
water tube (OD = 38.1 mm, wall thickness =3.4 mm). The steam
and water mixture is at 40 bar and 250oC. The overall heat transfer
is 400 kW/m2 inner pipe surface. Follow the steps to calculate the
temperature increase across each of the surfaces from the inside to
the outside (Water film, scale, tube wall).
Step
Input Data or Equation
Result
250
250 °C
1) Steam temperature,TS, °C
2) Inner radius, r0, m
0.01905
2
3) Water film hWF, W/m °C
10,000
2
4) Heat input, qo, W/m
400,000
q0
h WF
5) Temperature difference across
water film,  Twater film, °C
6) Scale thickness, ts, m
0.001
7) Conductivity scale s, W/m °C
8) Temperature difference across
scale,  Tscale, °C
9) Tube wall thickness, tT, m
1
r0q0
r t 
ln 0 s 
 r0 

s
0.0034
10) Conductivity tube, T, W/m°C
50
11) Temperature difference across,
tube,  Ttube, °C
r t t 
r0  q 0  ln 0 s T 
 r0  t s 

12) Tube surface temperature, Tsurface
13) Soot thickness, tsoot, m
sum of rows 1, 5, 8, 11
0.001
14) Conductivity soot, soot, W/m°C
15) Temperature difference across
soot layer,  Tsoot, °C
1
r t t t 
r0  q 0  ln 0 s T soot 
 r0  t s  t T 

16) Soot surface temperature, °C
T
soot
sum of rows 12 and 15
The soot and scale problems
Page 12
 NOTES
SA-210 Carbon steel shall not exceed an oxidation temperature of
450°C. This exercise gives a very educational insight view on the
effect of scale and soot on wall temperatures of tubes. The case
refers to the water wall in a furnace that receives its energy input
mostly by radiation and not by convection. Note the rather high
heat transfer of 400 kW/m2 .
Task 2
Often an energy auditor is in no position to measure water or steam
temperatures. Assume a situation where the temperature gages are
missing or you don’t trust the reading, but you are able to measure
the surface temperature of the bare pipe correctly ( 2 oC).
Did you make a large error by assuming the fluid temperature is
2% higher than the measured surface temperature?
Repeat this exercise under the assumption that there was a 1 mm
scale inside the tube, you didn’t know about. Use the same
procedure as in task 1. Asssume steam at 250°C and a film heat
transfer coefficient of 1000 W/m2 °C. The ambient temperature is
35°C. Depending on the emissivity of the bare pipe surface (shiny
or black) we may have a heat transfer of between 4.4 kW /m2 and
7.5 kW/m2 from the pipe surface to the ambient.
Task 3
Measuring surface temperatures requires special surface
temperature sensors and some skills. It is always advisable to apply
enough pressure on the sensor and clean the surface, prior to
measuring. Test the effect of paint at the pipe surface. Even a very
thin layer of paint will change the surface temperature of a pipe
carrying hot feedwater at 95 C by about 3-7 C.
Does the temperature go up or down as compared to the surface
temperature of the clean pipe?