Helsinki University of Technology Department of Mechanical Engineering
Energy Engineering and Environmental Protection Publications
Steam Boiler Technology eBook
Espoo 2002
Steam/Water Circulation Design
Sebastian Teir, Antto Kulla
Helsinki University of Technology
Department of Mechanical Engineering
Energy Engineering and Environmental Protection
Table of contents
Introduction..........................................................................................................................................3
Large volume boilers ...........................................................................................................................3
Shell type boilers..............................................................................................................................3
Fire tube boilers ...............................................................................................................................4
Water tube boilers ................................................................................................................................6
Introduction......................................................................................................................................6
Natural circulation boilers................................................................................................................6
General .........................................................................................................................................6
Natural circulation principle ........................................................................................................6
Advantages and disadvantages.....................................................................................................7
Natural circulation design ............................................................................................................8
Introduction..............................................................................................................................8
Circulation ratio .......................................................................................................................8
Driving force of natural circulation .........................................................................................9
Downcomers ..........................................................................................................................10
Wall tubes ..............................................................................................................................11
Headers...................................................................................................................................12
Boiling within vertical evaporator tubes ................................................................................12
Heat transfer crisis .................................................................................................................12
Optimization of natural circulation design.............................................................................13
Special designs .......................................................................................................................13
Assisted or forced circulation boilers.............................................................................................14
General .......................................................................................................................................14
Principle of forced circulation....................................................................................................14
Flow distribution between parallel riser tubes ...........................................................................15
Boilers types...............................................................................................................................15
Lamont boilers .......................................................................................................................15
Controlled circulation boilers.................................................................................................16
Advantages and disadvantages...................................................................................................16
Once-through boilers......................................................................................................................17
General .......................................................................................................................................17
Once-through boiler types..........................................................................................................17
General ...................................................................................................................................17
Benson design ........................................................................................................................17
Sulzer design ..........................................................................................................................18
Ramzin design........................................................................................................................18
Spiral wall tubes.........................................................................................................................19
Multiple pass design...................................................................................................................19
Advantages and disadvantages...................................................................................................19
Operation....................................................................................................................................20
Manufacture and use of once-though boilers .............................................................................20
Internet links ..............................................................................................................................21
Combined circulation boilers .........................................................................................................21
General .......................................................................................................................................21
References ..........................................................................................................................................22
ii
Introduction
As presented in the previous chapter, boilers can be classified by their combustion method, by
their application or by their type of steam/water circulation.
This chapter will describe the different types of steam/water circulation in boilers. It will not
discuss steam/water circulation for the applications listed in Figure 1 under “Others” (i.e.
nuclear, solar, and electric). [1]
Steam boilers
Large volume
Water tube
Others
Fire tube
Natural
circulation
Solar
Gas tube
Assisted/forced
circulation
Electric
Shell
Once-through
Nuclear
Combined
circulation
Figure 1: Steam boiler types according to steam/water circulation.
Large volume boilers
Shell type boilers
A steam boiler can be either a large volume
(shell) type boiler or a water tube boiler. Shell
type boilers are boiler that are built similarly to
a shell and tube heat exchanger (Figure 2). In
large volume (shell) type boilers a burner or a
grate is situated inside a big tube, called
chamber. The chamber is surrounded by water
in a pressure vessel, that functions as the outer
boiler wall. Thus, the water absorbs the heat
and some of the water is converted to saturated
steam. Flue gases continue from the chamber
to the stack so that they are whole the time
situated inside the tubes. Nowadays fire-tube
Figure 2: Shell type boiler: Höyrytys TTKVfire tube boiler [Hoyrytys].
3
boilers are the most used type of large volume boilers. Also electric boilers where water is heated
with an electrode source can be considered large volume boilers. However, large volume boilers
are today used for small-scale steam and hot-water production only and, overall, they are not
common in large-scale industrial use anymore. [1]
Fire tube boilers
Modern fire tube boilers are used in
applications that require moderate pressures
and moderate demand. As the name implies,
the basic structure of a fire tube boiler consists
of tubes, where fuel is burned and flue gas is
transported, located in a pressurized vessel
containing water. Usually boilers of this type
are customized for liquid or gaseous fuels, like
oil, natural gas and biogases. Fire tube boilers
are used for supplying steam or warm water in
small-scale applications. [2]
Usually fire tube boilers consist of cylindrical
chambers (1-3) where the main part of
combustion takes place, and of fire tubes. In
most of the cases, fire tubes are situated
horizontally (fire tubes placed above
chambers).
1. Turning chamber
2. Flue gas collection
chamber
3. Open furnace
4. Fire tube
5. Burner seat
6.
7.
8.
9.
10.
11.
Figure 3: Höyrytys TTK fire tube steam boiler
[Hoyrytys].
Fire tubes
Manhole
Hatch
Cleaning hatch
Steam outlet
Water inlet
12.
13.
14.
15.
16.
Flue gas out
Blow-out hatch
Outlet and circulation
Feet
Insulation
Figure 4: Schematic of the Höyrytys TTKV-fire tube hot-water boiler from Figure 2 [Hoyrytys].
4
Fire tube boilers generally have tubes with a diameter of 5 cm or larger. They are usually straight
and relatively short so that the hot gases of combustion experience a relatively low pressure drop
while passing through them. The path of the flue gases goes from burners/grate, through one of
the chambers, to the other end of the chamber. There the flue gases turn to reverse direction and
return through the fire tubes and continue then to the stack (Figure 4).
1. Turning chamber
2. Flue
gas
collection
chamber
3. Open furnace
4. Flame tube
5. Burner seat
6. Manhole
7. Fire tubes
8.
9.
10.
11.
12.
13.
14.
15.
Water space
Steam space
Outlet and circulation
Flue gas out
Blow-out hatch
Main hatch
Cleaning hatch
Main steam outlet
16.
17.
18.
19.
20.
21.
Level control assembly
Feedwater inlet
Utility steam outlet
Safety valve assembly
Feet
Inslulation
Figure 5: Schematic of the Höyrytys TTK fire tube steam boiler from Figure 3 [Hoyrytys].
Fire tube boilers have a fairly large amount of contained water so that there is a considerable
amount of stored heat energy in the boiler. This also allows for load swings where large amounts
of steam or hot water are required in a relatively short period of time, as often happens in process
applications. Fire tube boilers can take a great deal of abuse and inattention and still function at
competent levels. Fire tube boilers have a life expectancy of 25 years or more. Boilers that are
older than 75 years are still known to be in operation. Consistent maintenance and careful water
treatment go a long way towards insuring the long life of these boilers.
Nowadays fire tube boilers are mostly used as district heating boilers, industrial heating boilers
and other small steam generators. Fire-tube boilers are not anymore used for electricity
production because of their upper limits (4 MPa steam pressure and about 50 kg/s steam mass
flow). The steam pressure limit is based on the fact that when the steam pressure in the boiler
rises, thicker fire tubes and chambers are needed – thus the price of the boiler rises. As a result of
this, boiler types where water/steam mixture is inside the tubes have lower prices for the same
steam capacity and pressure. Fire-tube boilers can reach thermal efficiencies of about 70 percent.
There are also special types of fire-tube boilers such as scotch marine boilers and firebox boilers,
but they will not, however, be discussed further here. The rest of this chapter concentrates on the
main types of water tube boilers.
5
Water tube boilers
Introduction
As contrast to large volume boilers, in water tube boilers water/steam mixture is inside the tubes
and is heated by external combustion flames and flue gases. The water tube boilers are classified
by the way of the water/steam circulation: natural circulation, forced or assisted circulation,
once-through and combined circulation type boilers. All boilers for power generation are
nowadays water tube boilers.
Natural circulation boilers
General
The natural circulation is one of the oldest principles for steam/water circulation in boilers. Its
use has decreased during the last decades due to technology advances in other circulation types.
Natural circulation principle is usually implemented on small and medium sized boilers.
Typically the pressure drop for a natural circulation boiler is about 5-10 % of the steam pressure
in the steam drum and the maximum steam temperature varies from 540 to 560 °C.
Natural circulation principle
The water/steam circulation begins from
the feed water tank, from where feed
water is pumped. The feedwater pump
(Figure 6) raises the pressure of the
feedwater to the wanted boiler pressure.
In practice, the final steam pressure
must be under 170 bar in order for the
natural circulation to work properly.
The feed water is then preheated in the
economizer almost up to the boiling
point of the water at the current
pressure. To prevent the feed water from
boiling in the economizer pipes the
economizer temperature is on purpose
kept about 10 degrees under the boiling
temperature.
From the economizer the feed water
flows to the steam drum of the boiler. In
the steam drum the water is well mixed
with the existing water in the steam
drum. This reduces thermal stresses
within the steam drum.
Superheaters
Steam drum
Economizer
Downcomers
Mud drum
Evaporator
(riser tubes)
Feedwater
pump
Figure 6: Natural circulation principle
The saturated water flows next from the steam drum through downcomer tubes to a mud drum
(header). There are usually a couple of downcomer tubes, which are unheated and situated
outside the boiler.
6
The name "mud drum" is based on the fact that a part of the impurities in the water will settle
and this 'mud' can then be collected and removed from the drum.
The saturated water continues from the header to the riser tubes and partially evaporates. The
riser tubes are situated on the walls of the boiler for efficient furnace wall cooling. The rises
tubes are sometimes also called generating tubes because they absorb heat efficiently to the
water/steam mixture. The riser tubes forms the evaporator unit in the boiler.
After risers, the water/steam mixture goes back to the steam drum. In the steam drum water and
steam are separated: the saturated water will return to the downcomer tubes and the saturated
steam will continue to the superheater tubes. The purpose of this separation is to protect the
inside of the superheater tubes and turbine for impurity deposition.
The steam from the steam drum continues to the superheater, where it is heated beyond its
saturation point. After the last superheater stage the steam exits the boiler.
This type of circulation is called natural circulation, since there is no water circulation pump in
the circuit. The circulation happens by itself due to the water/steam density differences between
the downcomers and risers. [4]
Advantages and disadvantages
Natural circulation (NC) boilers have the following advantages compared to other circulation
types:
•
•
•
•
•
•
NC boilers are more tolerant on feed water impurities than other types of water tube
boilers
NC boilers have lower internal consumption of electricity than other water tube boiler
types.
NC boilers have a simple construction. Therefore the investment cost is low and the
reliability of the boiler high.
NC boilers have a wide partial load range, practically even 0-100 % have the feature to be
held in a stand-by state, which means "warm at full pressure".
NC boilers have constant heat transfer areas independent of boiler load, since the drum
separates the three heat exchangers - economizer, evaporator and superheater - from each
other.
NC boilers have simpler process control, due to the big volume of water/steam side,
which behaves as a "buffer" during small load rate changes.
Natural circulation boilers have the following disadvantages compared to other circulation types:
•
NC boilers have a high circulation ratio (between 5 and 100), which leads up to massive
dimensions of the evaporator as the amount of water circulating in wall tubes can be up to
100 times of the mass flow of steam generated. This increases the requirement for space
and steel.
• NC boilers need large diameters (large volume) of all tubes where the water/steam
mixture flows. This is because smaller diameters in tubes would cause pressure drop and
thus higher boilers would be needed for adequate pressure difference.
• NC boilers need more accurate dimensioning as compared to other boiler types.
7
•
•
•
•
•
NC boilers are quite slow in start-up and "stop" situations (also when the load rate
changes a lot) because of the large water/steam tube volume (about 5 times the
water/steam volume of a once through boiler).
NC boilers are only suitable for subcritical pressure levels (practically for steam pressures
under 180 bar in the steam drum). This is due to the lack of density difference in
supercritical steam, and thus the lack of a driving force.
NC boilers have problems with more frequently occurring tube damages, due to the
relative large diameter of the boiler tubes.
NC boilers are sensitive to pressure variations. Sudden pressure drops or build-ups causes
increased rate of evaporation and thus the steam drum water level will also rise. This can
lead to water passing into the superheater tubes and water circulation problems that lead
to tube damages.
NC boilers require a steam drum, which is a very expensive part of the boiler.
Natural circulation design
Introduction
The following chapters concentrate on some
design issues in natural circulation boilers:
This chapter will use graphics and photos of an
Andritz
recovery
boiler
(Figure
7,
manufactured by Foster Wheeler), which is the
same boiler that was presented in the chapter
on recovery boilers. [3]
Circulation ratio
The circulation ratio is one important variable
when designing new boiler. It is defined as the
mass rate of water fed to the steam-generating
tubes (raisers) divided by the mass rate of
generated steam. Thus, it is meaningful to
define the circulation ratio only for water tube
boilers:
U=
m& raisers
m& feedwater
(1)
Figure 7: The feedwater circulation
construction of the recovery boiler using
natural circulation drum [3].
The variations in circulation ratio result from
the pressure level of the boiler, therefore highpressure boilers have low ratios and lowpressure boilers have high ratios, respectively. Other parameters that affect the circulation ratio
are the height of the boiler, heating capacity of the boiler and tube dimension differences
between riser and downcomer tubes.
For certain natural circulation applications dimensioning the circulation ratio is very difficult.
The circulation ratio varies between 5 and 100 for natural circulation boilers. The circulation
ratio of forced circulation boilers is normally between 3 and 10. For La Mont type of boilers the
8
normal values are between 6 and 10, for controlled circulation boiler between 4 and 5,
respectively. Once through boilers generate the same mass rate of steam as has been fed to
boiler, thus their circulation ratio is 1.
Driving force of natural circulation
The driving force of the natural circulation is
based on the density difference between
water/steam mixture in riser and downcomer
tubes, of which the riser tubes represent the
lower density mixture and downcomer tubes
the higher density mixture. The driving
pressure can be defined as following:
∆pd = g ⋅ (H evaporator − H boiling )⋅ (ρ dc − ρ r )
(2)
where g is the gravitational acceleration (9,81
m/s2), the heights are according to Figure 8
[m], and ρ dc − ρ r the difference in the average
density between the downcomers (dc) and
raiser (r) tubes [kg/m3], which is the most
difficult parameter to determine.
The conditions in the steam drum are such that
H2O is there as saturated water. There will be a
slight increase in water pressure because of the
hydrostatic pressure when the water travels
Figure 8: A representation of the height
down in downcomer tubes. Thus, the water is
parameters of the driving force.
subcooled in the header (mud drum) after
downcomer tubes. Hence, in riser tubes the
water has first to be heated up till the water has
reached the evaporation (boiling) temperature
before it can evaporate. The boiling height, i.e. the height where water has high enough
temperature to boil, can be calculated using the circulation ratio and water/steam enthalpies:
H boiling =
h ′′ − h ′
⋅ H evaporator
∆h ⋅ U
(3)
where h” is the enthalpy [kJ/kg] of saturated steam, and h’ enthalpy of saturated water (at the
pressure of the steam drum), U is the circulation ratio, and ∆h is the enthalpy change caused by
the rise in evaporation pressure (because of the subcooling of water in downcomer tubes).
9
Downcomers
Downcomer tubes have a relatively large
diameter because the entire water amount for
the evaporator flows through the downcomer
tubes before it is lead to wall tubes (riser
tubes). Normally the amount of downcomer
tubes is between one and six.
Downcomer tubes are placed outside the boiler
to prevent the water from evaporating, which
could decrease the driving force of natural
circulation (decrease average density in
downcomer tube). If downcomer tubes have to
be placed inside boiler construction, heat load
to downcomers has to be strongly restricted to
prevent downcomer tubes from water boiling.
Possible boiling in downcomer tubes
complicates circulation because the steam
bubbles travel upwards and thus increase
pressure loss.
Figure 9: Photograph of downcomers from the
steam drum [3].
An ideal downcomer tube is as short as
possible and the flow velocity of the water
transported is as high as possible.
Figure 9 and Figure 10 show examples of
downcomers in the recovery boiler.
Figure 10: Photograph of downcomers from
the steam drum [3].
10
Wall tubes
Pressure loss caused by wall tubes (or risers,
evaporator tubes) of a natural circulation boiler
should be at low level because of the natural
circulation principle. Thus, vertically installed
riser tubes in natural circulation boilers have a
larger diameter than riser tubes in forced
circulation boilers.
All natural circulation boilers must have an
upwards-rising arrangement of wall tubes
because of the circulation principle. There are
variations on how sharp the rise is:
In conservative vertical furnace boilers the wall
tubes are placed in a straight vertical direction Figure 12: Photograph of the furnace wall [3].
(Figure 11 and Figure 13). In corner tube
(Eckrohr) boilers the wall tubes are arranged as
slightly rising or horizontal wall-tube banks.
This particular boiler has a furnace height of 40
m. The diameters of the water tubes are about
60 mm. The riser tubes are all welded together,
and form a gas-tight panel construction, a tube
wall. Since the boiler is a recovery boiler, the
floor barely slopes (Figure 12 and Figure 14),
in order to support the smelt, and is therefore a
different structure than coal-fired boilers
(which have a wedge-shaped floor for
collecting ash).
Figure 13: Photograph of the front furnace
wall being installed [3].
Figure 14: Photograph of the furnace wall [3].
Figure 11: Photograph of water tubes [3].
11
Headers
The word "header" (Figure 15) is used in boiler
technology for all collector and distributor
pipes, including the mud drum (Figure 16).
The most important design parameter for
headers is diameter. It is defined by the flow
rate and the number of tubes connected to the
header (here the number of riser tubes).
Header construction is basically a miniature
version of a simple steam drum (diameters are
smaller than the ones of steam drums).
However, in headers there are usually no
internals except the orifices in forced
circulation and once-through principle boilers.
Small diameter headers are constructed from a
tube with welded front and end plates, whereas
the big headers are made of bent steel plates in
the same way as steam drums.
Figure 15: Photograph of the economizer
header [3].
Boiling within vertical evaporator tubes
The boiling process in a vertical riser tube
begins with single-phase water flow in the
lowest part of the evaporator. Heat transfer
from the furnace produces initially some steam
bubbles.
Continuous heat transfer increases the steam
content in the mixture. In the annular boiling
state of the steam/water mixture the tube wall
is still covered by a water film, but as the
steam content increases water can be found in
the tube as mist only. This state is called the
misty/drop state (Figure 17).
Figure 16: Mud drum and collector headers
[3].
Heat transfer crisis
Boiling process can be considered also in heat
transfer terms. The heat flux in a furnace
generated by the combustion process is
extremely high. There is a critical value that
the heat flux can reach which results in a
sudden decrease of the heat transfer capacity of
the tube. This is called departure from nucleate
boiling (DNB), dryout, burn out, critical heat
flux or heat transfer crisis (Figure 18).
The phenomenon responsible for this problem
is the transition from annular boiling state to
Figure 17: Different types of water/steam flow
during the boiling process [1].
12
misty/drop state. In the misty/drop state, the
boiler wall is no longer covered with water.
This dryout causes the drastic fall in the
waterside heat transfer coefficient.
Critical heat flux is dependant on operating
pressure, steam quality, type of tube, tube
diameter, flux profiles and tube inclination. For
a boiler design to be acceptable the critical heat
flux for the furnace walls must always be
greater by a margin than the heat flux
generated in the combustion chamber.
Optimization of natural circulation design
The following are some of the main methods
used for natural circulation optimisation. All
methods lead to an increase in the driving
force:
Figure 18: Dryout occurring in an evaporator
tube.
1. Increase furnace height or elevate
steam drum at higher level.
2. Increase density in downcomer tubes
by increasing
steam
separation
efficiency in the steam drum, by
pumping feedwater to the steam drum
as sub-saturated liquid or by
minimizing the axial flow in the steam
drum.
3. Decrease density in riser tubes by
increasing temperature in lower
furnace.
Special designs
There are some special applications of natural
circulation principle that are not currently
covered by this eBook, but can be found
elsewhere on the net. These specific boiler
types are:
•
Natural circulation boilers with two
drums (Figure 19)
• Conservative vertical furnace boilers
• Corner tube or Eckrohr boilers
Figure 19: Recovery boiler utilizing two steam
drums [Andritz].
13
Assisted or forced circulation boilers
General
In contrast to natural circulation boilers, forced circulation is based on pump-assisted internal
water/steam circulation. The circulation pump is the main difference between natural and forced
circulation boilers. In the most common forced circulation boiler type, the Lamont boiler, the
principles of forced circulation is basically the same as for natural circulation, except for the
circulation pump.
Thanks to the circulation pump, the operation pressure level of forced circulation boiler can be
slightly higher than a natural circulation boiler, but since the steam/water separation in the steam
drum is based on the density difference between steam and water, these boilers are not either
suitable for supercritical pressures (>221 bar). Practically the maximum operation pressure for a
forced circulation boiler is 190 bar and the pressure drop in the boiler is about 2-3 bar.
Principle of forced circulation
The water/steam circulation begins
from the feed water tank, from where
feed water is pumped. The feedwater
pump raises the pressure of the
feedwater to the wanted boiler
pressure. In practice, the final steam
pressure is below 190 bar, in order to
keep the steam steadily in the
subcritical region.
The feed water is then preheated in
the economizer almost up to the
boiling point of the water at the
current pressure.
The steam drum is usually the same
kind as those used in natural
circulation boilers.
Figure 20: Principle of forced/assisted circulation.
Same symbols used as in Figure 6, except for the
circulation pump, marked with an arrow.
In a forced/assisted circulation boiler, the circulation pump (Figure 20) provides the driving force
for the steam/water circulation. Since the pump forces the circulation, the evaporator tubes can
be built in almost any position. Greater pressure losses can be tolerated and therefore the
evaporator tubes in a forced circulation boiler are cheaper and have a smaller diameter
(compared to natural circulation evaporator tubes).
The saturated water flows next from the steam drum through downcomer tubes to a mud drum
(header). There are usually a couple of downcomer tubes, which are unheated and situated
outside the boiler. The headers that distribute the water to the evaporator tubes are equipped with
chokers (flow limiters) for every wall tube in order to distribute the water as evenly as possible.
The water continues to the riser tubes, where it evaporates.
14
The steam is separated in the steam drum and continues through the superheaters, as in natural
circulation boilers.
This type of circulation is called forced circulation, due to the existence of a water circulation
pump in the circuit. The steam/water circulation is forced by the pump and does not rely on
density differences as in natural circulation.
Flow distribution between parallel riser
tubes
Smooth flow distribution from header to riser
tubes prevents riser tubes from overheating. In
forced circulation boilers (in this context oncethrough boilers and combined circulation
boilers belong to this group as well)
water/steam is pushed through evaporator
tubes with a pump. Pressure loss strongly
defines the water distribution between several
parallel-coupled tubes. The tubes with biggest
steam fraction (highest pressure loss) get thus
the least amount of water (i.e. not enough
cooling water).
It has been marked that a smooth water
Figure 21: Schematic of an orifice for water
distribution between tubes is easiest to practice
tubes
with orifices (chokes, flow limiters) situated in
inlet of each riser tube (Figure 21). They give
extra pressure loss in each tube and thus the proportional differences in flow losses between
parallel tubes become insignificant. Orifices are dimensioned separately for each riser tube to
provide a smooth distribution of flow between parallel riser tubes (evaporator tubes).
Another possibility is to place small diameter tubes as mouthpieces in each riser tube and thus
increase the pressure losses. However, tubes utilizing orifices is a more common practice.
Boilers types
Lamont boilers
The most usual type of forced circulation boilers is the Lamont type, named after an engineer
who developed this boiler type. In this type of boilers the pump forces the steam/water
circulation. The operational pressures remain below 190 bar because with higher pressures the
share of the heat of evaporation becomes too low. The wall tube direction arrangement is not
limited for the Lamont type. The pressure loss in wall tubes is 2-3 bar.
Applications for Lamont boilers:
•
•
Customized boilers, where the boiler dimensions are determined e.g. by the building where
the boiler will be placed.
Heat recovery steam generators (HRSGs) and boilers equipped with separate combustion
chambers
15
Controlled circulation boilers
The controlled circulation principle is also known as thermal, pump-assisted circulation. It has
been developed mainly in the USA and it is one kind of modification of Lamont boiler. In this
type of boilers the pump merely assists the steam/water circulation. The benefit of controlled
circulation boilers is the less need of pumping energy because natural circulation principle is
partially used for circulation. Controlled circulation boilers are used for high subcritical
pressures up to 200 bar and usually for relatively large boilers.
Advantages and disadvantages
The advantages of forced circulation (FC) boilers are:
•
•
•
•
•
FC boilers can use tubes with smaller diameter than boilers based on natural circulation
due to the more efficient (pump-assisted) circulation.
FC boilers have a wide suitability range of power plant sizes.
An FC boiler gives also more freedom for placement of heat transfer surfaces and can be
designed in almost in any kind of position (thus forced circulation is very common in
HRSG:s, boilers in gas turbine based combined-cycle power plants).
FC boilers have a low circulation ratio (3-10).
Water circulation not reliable on density differences because circulation pump is taking
care of the circulation whenever the boiler is operated.
Forced circulation boilers have the following disadvantages compared to other circulation types:
•
•
•
•
•
•
•
•
•
•
FC boilers have restrictions regarding the placement of the circulation pump, since it has
to be placed vertically below the steam drum. Otherwise the saturated water could boil
(cavitate) in the circulation pump.
FC boilers have a higher internal electrical consumption. The circulation pump consumes
typically about 0,5-1,0 % of the electricity produced by the controlled circulation unit in
question.
FC boilers need a higher level of water quality than boilers based on natural circulation.
FC boilers require a mass flow rate of 1000-2000 kg/(m2s) for maximum pressure levels.
FC boilers are only suitable for subcritical pressure levels (practically for operation
pressures under 190-200 bar). This is due to the lack of density difference in supercritical
steam, which is the principle for the operation of the steam/water seperation in the steam
drum.
FC boilers require a circulation pump and flow limiting orifices, which increase the
capital cost of the boiler.
FC boilers are sensitive to pressure variations. Sudden pressure drops or build-ups causes
increased rate of evaporation and thus the steam drum water level will also rise. This can
lead to water passing into the superheater tubes and water circulation problems that lead
to tube damages.
FC boilers require control and regulation of the co-operation between the feed water
pump and circulation pump, which is difficult in controlled circulation units.
A steam drum is required, which is a very expensive part of the boiler.
Reliability of FC boilers is lower than that of natural circulation boilers, due to possible
clogging of orifices and failures in circulation pump operation.
16
Once-through boilers
General
A once-through (or universal pressure) boiler
can be simplified as a long, externally heated
tube (Figure 22). There is no internal
circulation in the boiler, thus the circulation
ratio for once-through boilers is 1.
In contrast to other water tube boiler types
(natural and controlled circulation), oncethrough boilers do not have a steam drum.
Thus, the length of the evaporator part (where
saturated water boils into steam) is not fixed
for once through boilers.
Once-through boilers are also called universal
pressure boilers because they are applicable for
all pressures and temperatures. However, oncethrough boilers are usually large sized boilers
with high subcritical or supercritical steam
pressure. A large modern power plant unit
(about 900 MWth) based on the once-through
design can be over 160 m high with a furnace
height of 100 m.
Q
Figure 22: Simplified once-through boiler
principle
The once through boiler type is the only boiler type suited for supercritical pressures (nowadays
they can reach 250-300 bars). The available temperature range for once through type is currently
560-600 °C. Pressure losses can be as high as 40-50 bar.
Once-through
boilers
need
advanced
automation and control systems because of
their relatively small water/steam volume.
They do not either have a buffer for capacity
changes as other water tube boiler types do.
Once-through boiler types
General
There are three main types of once through
boilers: Benson, Sulzer and Ramzin design.
Benson design
The simplest and most common design is the
Benson design (UK, 1922). In Benson boilers,
the point of complete evaporation (where all
the water has turned into steam) varies with the Figure 23: Benson design once-through boiler.
capacity load of the boiler (Figure 23). The
temperature of the superheated steam is
regulated by the mass flow ratio of fuel and
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water. The Benson-design is used in the
biggest power plants in Finland, e.g. Meri-Pori,
Haapavesi and IVO Inkoo.
Sulzer design
Sulzer monotube boiler was invented in
Switzerland by Gebrüder Sulzer Gmbh. The
Sulzer boiler uses a special pressure vessel,
called Sulzer bottle, for separating water from
steam (Figure 24). The steam is free from
water after the bottle. Therefore the point of
evaporation in a Sulzer boiler is always at the
bottle, and thus constant. Originally the bottle
was
used
for
separating
impurities
(concentrated salts etc.) from the steam.
Another typical feature for Sulzer type boilers
is the controlling the water flow of each tube
outgoing from a certain header with separate Figure 24: Sulzer design once-through boiler.
The separation bottle is marked with an arrow.
orifices for each tube.
Ramzin design
The Ramzin boiler is a Russian design, which
is known for the coil-like formation of the
evaporator tubes surrounding the furnace
(Figure 25). Due to the tilted and bended water
tubes the construction of Ramzin boilers is
complicated and thus expensive.
The tilted design of the furnace is nowadays
also used occasionally in Sulzer and Benson
design.
Figure 25: Ramzin once-through boiler.
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Spiral wall tubes
Once-through boilers use a special design on
water tubes. These are called spiral or rifled
wall tubes (Figure 26). The rifles in the tube
increase the wall wetting, i.e. improve the
contact between the tube wall and steam/water
mixture and thus improves the internal heat
transfer coefficient. The rifled wall tube is also
more resistant against dryouts. Due to the more
complex manufacture process of spiral tubes,
the spiral wall tube is more expensive than
regular smooth wall tubes.
Smooth wall tubes are used in tilted wall tube
design (like in Ramzin boilers).
Figure 26: Sketch of a spiral wall tube
Multiple pass design
In order to obtain the high mass flux necessary
for efficient tube cooling, the lower part of the
furnace can be divided into two sequential
water flow paths. These two parallel paths are
formed by altering first and second pass tubes
around the furnace.
As illustrated in the picture (Figure 27), the
water from the economizer flows up the first
pass tubes to the outlet headers, where the
water is mixed and led to downcomers. From
the downcomers the water/steam mixture is led
to the second pass tubes, from where it is
collected and mixed in the second pass header.
The water/steam mixture then flows to the
headers for the 3rd pass tubes, which the rest of
the evaporator consists of.
Using two passes, the lower part of the furnace
has effectively twice the water mass flow of
the upper part. Thanks to the headers, the
temperature differences between individual
tubes are decreased.
Figure 27: Multiple pass furnace design
Advantages and disadvantages
Once-through (OT) boilers have the following advantages compared to other circulation types:
•
OT boilers can use tubes with smaller diameter than boilers based on a steam drum due to
their lack of internal circulation.
• OT boilers have a secure external water circulation (relies on process feed water pump)
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•
Spiral (rifled) water wall tubes are more resistant against dryouts than smooth evaporator
tubes.
• OT boilers have a no internal circulation (circulation ratio = 1) and thus there are no
regulation or design needed for the internal circulation.
• The OT boiler is the only boiler able to operate at supercritical pressures, since there is no
density dependant steam separation needed (the Sulzer-bottle is not used for supercritical
steam values).
• OT boilers do not use a steam drum, which decreases boiler expenses.
Once-through (OT) boilers have the following advantages compared to other circulation types:
•
•
•
•
•
OT boilers require high level of water control, since the steam/water goes directly
through the boiler and into the turbine.
OT boilers require complicated regulation control, due to small water/steam volume (no
buffer for capacity changes), lack of steam drum, and the fact that the fuel,air and water
mass flows are directly proportional to the power output of the boiler.
OT boilers require a large mass flow rate of 2000-3000 kg/(m2s) in furnace wall tubes.
Spiral wall tubes are more expensive than smooth wall tubes due to a more complicated
manufacture process.
OT boilers have no capacity buffer, due to the lack of a steam drum and their oncethrough nature.
Operation
The basic difference between once through boiler types has traditionally been the point of total
evaporation in tubing. However, supercritical pressure range operation removes this clear
difference between water and steam states, and thus both Sulzer and Benson boilers are similarly
operated in supercritical pressures.
However, the development has led to constant point of evaporation also for Benson boilers
(thanks to improved process control) and nowadays the operational behaviour of once through
boiler is very similar. Today the biggest operational differences between Benson and Sulzer
types are the control system and heat-up procedures.
Overall, all once through boilers need certain special arrangements for heat-up procedure and
low capacity operation.
Manufacture and use of once-though boilers
Benson boilers are nowadays mostly manufactured by companies that belong to the Babcock
group (Deutsche Babcock, etc.). Sulzer boilers are mostly manufactured (by license) by ABB
Combustion Engineering, Mitsubishi, EVT, Andritz, etc. Ramzin boilers can be found in Russia.
Most of the new capacity of conventional steam power plants is based on once through principle,
because it allows higher steam pressures and thus higher electricity efficiency.
A Sulzer boiler can be found e.g. at Naantali power plant in southwestern Finland (also at
Mussalo power plant). The boiler of the Meri-Pori power plant, situated in western Finland, is
based on a Benson type. Also Inkoo and Haapavesi power plants use Benson design boilers.
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Internet links
Here's a list of links to interesting reading regarding once-through boilers:
•
•
•
•
•
•
Siemens-Westinghouse: BENSON publications and references
Siemens-Westinghouse: Benson boilers
BOOSTER Co.Ltd: About the Once-Through Boiler
Mitsubishi Once-through boilers
Babcock & Wilcox Company: Supercritical (once-through) boiler technology (PDF)
Foster-Wheeler: The once-through supercritical boiler (PDF)
Combined circulation boilers
General
This boiler type is a combination of controlled
circulation boilers and once-through boilers.
Combined circulation (once-through with
superimposed recirculation) boilers can be
used for both subcritical and supercritical
steam pressure operation. Figure 28 shows a
simplified principle of the combined
circulation.
When the firing rate is between 60 and 100 %,
the boiler operates as a once-through boiler. At
lower than 60 % capacity load, combined
circulation boilers operate as forced circulation
boilers in idea to maintain adequate
water/steam flow in wall tubes.
The biggest advantage of combined circulation
type boilers is reduced demand of pump energy
because the operation mode changes depending
on the capacity load. Main disadvantages are
the troublesome co-operation between feed
water pump and circulation pump and also the
high level needed for water treatment (as
needed for once through boilers).
Figure 28: Simplified principle of combined
circulation (Combustion)
The main manufacturer of this type of boilers is ABB Combustion Engineering and other
companies with a license from ABB CE. However, Mitsubishi is practically the only license user
company outside USA.
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References
1.
Esa Vakkilainen, lecture slides and material on steam boiler technology, 2001
2.
Ahonen, V. “Höyrytekniikka II”. Otakustantamo, Espoo. 1978
3.
Recovery Boiler Operation Manual, Ahlstrom Machinery Corporation 1999, CD-rom,
Andritz.
4.
Huhtinen, M., Kettunen, A., Nurmiainen, P., Pakkanen, H. ”Höyrykattilatekniikka”.
Painatuskeskus, Helsinki. 1994.
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