Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to boilers and in particular to
fluidised bed boilers and burners, and to a method of
controlling such boilers. In ~luidised bed boilers the
bed is normally external to the water circuit when the
fluidised bed burner is used for steam raising. At most,
a shell boiler is used in which a recessed combustion chamber
is formed in the wall which receives the base plate by
means of high-temperature seals to prevent the flow of
air around the outside edge of the base plate. The plenum
is attached to the base plate by further high-temperature
seals. Whilst the base plate can be shielded from the
combustion temperature by a quiescent layer of the carrier
to some extent, the base plate does become hot since it
is only cooled by the flow of the primary combustion
lS air and tends to distort putting stress on the seals.
It is an object of this invention to overcome this problem
by providin~ a base plate which is cooled during operation.
It is a further object of this invention to provide a
method and apparatus for reconditioning the bed, thereby
reducing the necessity of topping up the bed with new
sand or a similar carrier material.
~n fluidised bed burners which use solid fuels such as
coal, a certain amount oE ash and other particles which
are larger than the carrier material particles are left
behind after combustion. The ash, of course, is largely
elutriated in the air stream, but i-t is necessary, from
1~6~3~
time to time, to remove the large particles from the bed in
order to improve its characteristics since too great an
amount of large particles will af~ect the heat transfer
characteristics of the bed~ In the past these particles
have been removed by means of sieving, but it will be
appreciated that the operation of sieving is accompanied
by difficulties caused by the temperature of the bed itself
and its propensity to flame during the sieving operation.
The use of a water-cooled base plate and the reconditioning
of the bed allows the use of an accurate system of controls
although it will be appreciated that the con-trol system is
not dependent on the use of either the cooled base plate or
the reconditioned bed and may find application in
conventional fluidised bed boilers or heaters.
It is a further ob~ect of this invention to provide a
vertical and a horizontal boiler incorporating the above
mentioned features.
It will be appreciated that, while this invention is
described with the reference to the boi.lers suitable for
producing steam, it will find equal application in heaters
or burners for the production of hot wa-ter or for incineration
and the provision of hot gas for drying purposes.
According to one aspect of the present i.nvention, a boiler
is provided with a combustion chamber base plate having
i5~S
an upper surface adapted to suppor~ a fuel bed and a lower surface spaced apart
from a wall of the housing to define a water jacket, and an air plenum including
one or more air passages extending ~hrough the base pla~e for connecting the
plenum with the interior of the combustion chamber.
The base plate can be a block containing a plurality of ducts through
which the cooling water can be pumped; if the water is taken from, and returned
to, the boiler no heat is wasted. However it is preferred that the surfaces
are constituted by two spaced apart members so that the space between the
members can form part of a water jacket completely surrounding the combustion
chamber excep~ for necessary openings such as those required for the intro-
duction of fuel for instance. The water can circulate under convection in
such a water jacket and in practice the water jacket could be the shell of a
fire-tube boiler. The combustion chamber may further be penetrated by a plural-
ity of thermosyphon tubes.
In fluidised bed burners, there is usually a quiescent layer of the
carrier created by introducing the primary combustion/fluidising air into the
b0d above this quiescent layer through sparge pipes or stand pipes and another
aspect of this invention provides a fluidised bed burner having a base plate
with upstanding combus~ion air stand pipes in which at least some of the stand
pipes include or have associated therewith air flow control devices, each
device
i5~
being individual to a stand pipe and at least some of the
devices having a common operating means.
In a further aspect of the pre ent inve~tion the fluidise~
bed burner has an auxiliary fuel introduction means leading
into the bed.
The auxiliary fuel introduction means may conveniently be
for fuels which, because of their lightness or size
would be rapidly elutriated by the combustion air flow, for
example, waste straw,sawdust, or coal dust and for this
.... .~ .. ~ "
purpose the auxiliary fuel introduction means may comprise
a pipe extending into the fire bed preferably at the level
of the upper surface of the quiescent layex, in which pipe
the auxiliary fuel would be at least partially burnt. The
normal fuel would be in the form of lumps of coal bu-t all
lS coals contain a certain amount of coal dust ancd this is
elutriated and separated out from the combustion gases in
exhaust cyclones along with the elutriated ash; the elutriated
ash can be recycled so that the one or two percent elutriated
coal dust therein can be reclaimed. The auxiliary fuel
should preferably be injected along with air. The auxiliary
fuel introduction means could alternatively be desi~necl for
the introduction of a liquid Euel which Ini~ht otherwise
be difficult to burn.
This fuel would be in~ected into the quiescent layer and
wet it (the ash compoent in particular, would soak it up
and act as a wick) and on reaching the upper surEace of the
--4--
j
i5~35
qui.escent layer some of the oil wetted ash would break
away and enter the active fire region. The quiescent
layer, although not agitated by the air flow, i5 in
contact with the seething mass of the active fire and this,
and a bombardment by falling fuel, ensures that the ~-
quiescent layer is not static. If the fuel is injected
under pressure and is such as to degrade, forming a skin,
the fuel pressure will break up the skin as it forms.
Quiescent layers are created in most fluidised bed burners
by introducing the combustion air some way above the true
base plate by means of stand pipes or sparge pipes.
It is poss:ible to in-troduce the air through a plane
perforated plate but this loses the insulating effect of
the quiescent carrier layer.
In yet a further aspect of the invention a fluidised bed
burner comprises a combus-tion chamber and an air plenum
separated by a plane perforated plate and a layer of
coarse bodies resting on the plate.
The coarse bodies would be resistant to elutriation and
could be dropped onto the plane plate along with an
easi.ly elutriated carri.er such as sand and alumina and
then separated into a lower layer o the coarse bod.ies
supporting the finer carr:ier by blasting air through it.
If -the bodies are c~raded in size, possibly progressively,
it should be possible to grade the layers of bodies and
-~ ~l65~S
carrier b~ blasting air therethrough at an abnormally
high rate so that as to get gradi~g by elutriation.
According to another aspect of the invention a method of
conditioning a fluidised bed includes the steps of
transferrin~ the bed to a crusher adapted to reduce the
particle size of material other than carrier material
(i.e. inert incombustible material) to a size approximating
the particle size of the carrier material, and returning
the crushed material to the bed.
The crushed material may be returned by any one of a
number of known transport methods, but in a preferred form of
the invention the crushed material is collected by way o:E
a venturi and pneumaticall~ transported to an inlet in the
burner above tlle plate supporting the bed.
The invention includes a control method for fluidised bed
heaters in which method at least a first parameter, being
the pressure of the steam raised or the temperature o~
the water being heated, and a second parameter, being the
bed temperature, are sensed and applied to regulate the fuel
feed, the ~irst parameter being applied in steps to limit
the maximum range over which fuel ma~ be fed and the second
parameter being app:Lied to regulate the feed in that range.
~lso according to the invention a method of controlling
combustion in a fluidised bed heater comprises the steps of:
.
~i5~
sensing the first and second parameters and a third
parameter, being the free-board gas pressure in the
fluidised bed heater, at chosen number of stepped ~evels
of the first paramete~, allowing the fuel feed to the
bed to operate over given ranges in steps with the first
parameter steps;
at the chosen steps of the first parameter dampening the
flow of exhaust gases in the same number of steps;
controlling the inlet for the fluidising air or gas to the
bed also in steps which result from a pneumatic connection
derived from the above bed combustion chamber gas pressure
in response to the same number of chosen steps in the
third parame-ter;
and controlling the fuel feed in the chosen range allowed
at any time in response to the second parameter.
The invention also provides that on st:art-up a special
start-up control circuit should override the control system
outlined above until a predetermined bed temperature has
been reached.
Embodiments of the invention are now described, by way
of example, with reference to the accompanying drawings
in which:
Figure 1 is a schematic vertical section of a horizontal
boiler according to the invention;
Figure 2 is a section taken on line II-II of Figure l;
~ -7-~
Figure 3 is an enlarged detail showing a combined stay
and primary air supply pipe;
Figure 4 is a similar detail showing a simple primary
air supply pipe;
Figure 5 is a similar detail showing an air supply pipe
which includes a flow control device;
Figures 6 and 7 illustrate auxiliary fuel introduction means;
Figure 8 illustrates a base plate structure;
Figure 9 is a flow diagram of an apparatus for a fluidised
bed burner incorporating a crusher;
Figuré 10 is a section in side elevation of a vertical
boiler;
Figure 11 is a section taken on line II-II in Figure 10;
and
Figure 12 is a diagrammatic illustration of a fluidised
bed heater and its associated control circuits.
-7a-
s~
Figures 1 and 2 show a horizontal shell boiler enclosed by an
outer shell 11 within which a fluidised bed reactor chamber 12
is wholly contained. The base plate 14 of the reactor chamber,
on the upper surface of which rests the bed, is spaced apart from
the lower part of the shell 11 by stays 15 leaving the space be-
tween the plates 14, and the lower part of the shell 11, open to
the water jacket surrounding the reactor chamber 12. Further
stays 16 are used to locate the chamber 12 which is of consider-
able height so that the carrier or bed material of sand, alumina
or the like is not over-prone to elutriation (entrainment in the
air stream). To avoid the large size of the chamber blanking off
water circulation~ a plurality of water tubes 17 cross through the
chamber at an inclination to the horizontal and at various bearings
to induce convective flows of water. If the boiler is used with
a chimney generating an induced draught, an exhaust gas turbine
can be installed. Banks of fire or smoke tubes 18 lead off from
the combustion chamber to deliver heat from the combustion gases to
water in the boiler. The banks can form a single or a multiple
pass as shown in the drawings.
It will be seen that the chamber is surrounded by
water. The base plate 14 is shaped so that any steam bubbles form-
ed on its water side will float away and will not impair the heat
transfer characteristics, and can be contoured for strength or
other purposesO A series of primary air tubes 20 extend between
the two members to conduct primary air from a forced draught plen-
um 21 attached on the outside of the shell into
~6~
the combustion chamber 12, some of these air pipes can
serve as stays 15, which pipes 2Oa are shown in Figure 3.
Each air pipe has a bore 22 and receives a standpipe 23
which in Figure 3 is screwed directly into the air pipe and
S in Figure 4 which a non-structural standpipe 20b is shown,
- provided with a threaded collar which screws onto the air
pipe. The standpipes are of heat-resistant material and
varying lengths of standpipes can be fitted to suit the
~uel to be burnt and other relevant factors. Each standpipe
has its upper end blanked olf and has holes in the sides.
In Figures 3 and 4 the upper ends are blanked off by an
umbrella plate 25 so that when the bed .is slumped, without
an air flow preventincJ the carricr entering the holes, the
umbrella plate creates a clear space with the holes being in
the clear space. The size of the umbrella plate will,
of course, depend on the angle of repose of the particular
carri.er material used so that the clear space is in fact large
enough to leave the holes ree.
Fi~ure 5 shows another design o~ standpipe which does not
require the top closure -to be in the form of an umbrella
plate. This has a succession oE air outlet holes 27 in an
outer tube 28 which hole~ can be pro~ressively blocked
off by a valve device which comprises a tube 29 slidably
located within the bore of the air pipe 21 associated with
the standpipe. Vpward movement of the inner tube 29 will
block off the holes 27 which may be staggered -to give an
infinitely variable blocklng off action. ~he end o~ the tube
-
3~ s
29 may be cut on an incline to give the same effect. The
lower end of the tube 29 is blanked off by a plate 30
above which there is a plurality of air inlet apertures
31. When the tube 29 is fully inserted, a seal on the
plate 30 sealingly engages a seat on the lower end of the
air pipe 28, thereby sealing off the aperture 31 within
the air pipe 28. A common activating means for the
tubes 29 comprises a regulator plate 32 within the plenum
which can be displaced up and down by suitable means
sealingly entering the plenum. This regulator plate can
be perforated or otherwise adapted to balance out
any unevenness of air pressure within the plenum or be
connected adjustably to the various devices by flexible
couplings 33 so as to allow each standpipe to ta~e equal
lS amounts of air.
It will be appreciated that use of the cooled base plate
results in a number of advantages. Meat is delivered
through the base plate itself from the fluidised bed
so increasin~ the heat transfer 5urface area and owing to
the cooling, the base plate is less liable -to distort
so that a relatively inexpensive material can be used. The
shell is also water cooled so that the problems associa-ted
with the seal between the plenum and the prior art base
plate no longer arises, an~ this enables the use of a
higher forced draught pressure. In fact a hiyher pressure
throughout the air flow path can be used. A higher
combustion pressure with the resultant greater oxygen content
concentration, leads to more intense cornbustion and thus a
--10--
smaller boiler. The advantages are more pronounced in
the so-called shallow bed versions of fluidised bed
combustion.
To allow a higher combustion pressure to be used)
a fuel inlet hatch-37 in the outer wall of the shell
and the combustion chamber is fitted with a fuel feed
device which limits blow-back of fuel on failure of
combustion as well as limiting ingreSS of air additional to that
required as secondary combustion air.(see Figure 1)
The use of flow controlling standpipes also gives many
advantages. One advantage is the elimination of the need
to have an umbrella plate on each s-tandpipe which permits
closer spacing of the standpipes for intensely active shallow
bed burners. However the main advantage of regulating
each standpipe instead of, or even as well as, using a
common damper i5 that the air flow is more evenly shared
between standpipes and regulating down t~e air flow does
not change the balance of the air flow between standpipes.
It is thought that the life of the base plate 14 will be
extremely long not only because of :its cooling but also
because the carrier or hed material will after initially
polishing the upper face of the member tend to plate out
giviny a wear and heat resistant surface.
In Figure 6, a series of separators 39 are provided in the
exhaust circuit for separating out any solids elutriated
out of the reactor chamber 12, which solids will be either
ash or unburnt fuel dust. The use o~ multiple separators
such as cyclones would permit the various fractions such as
the heavier fuel particles from the cyclone 39a, ash from
the cyclone 39b and light fuel particles from the cyclone
39c, to be individually separated or at least for some
~ractions to be richer in unburnt fuel than others. At
least the fuel rich fractions are fed back into a hopper
40 into which can also or alternatively be fed any liyht
fuels li.kely to be elutriated such as straw, waste and
sawdust. The contents of the hopper are allowed to fall or
are fed into a stream of air which can be derived from the
forced draught fan (not shown) for feeding the plenum
21. A tube or retort 41 leads into the active fire region
preferably just above the quiescent layer ~nd the stream of
air with entrained fuel particles is directed into and
through this tube which is of course hot,so that the
particles are heated whilst they pass along the tube and
are at leas-t partially burnt therein. This fuel return
system can be incorporated into the reconditioning system
described below. (shown in dotted out].ine in Figure 9).
The auxi].iary fuel i.nt.roduction means of Fi.yure 6 is suitable
for burning solid liyht fuels and Fiyure 7 shows another
auxi.liary fuel introduction means suitable for liquid fuels,
especially those which may be difficul-t to burn because
-12-
a65~5
of a high flash point or a tendency to clog normal
nozzles. The fuel is introduced under pressure by means o~
a pump 42 with an outlet 43 leading into the quiescent
layer of the fluidised bed that is below the level of
the air out7ets from the standpipes or sparge pipes.
The ash component of the quiescent layer soaks up the
liquid fuel acting similarly to a wick in distributing the
liquid fuel throughout the quiescent layer. On reaching
the turbulent upper surface of the quiescent layer some of
the wet ash is broken ofE and burnt in the active fire
region. If any skin tends to form due to degradation of
the liquid fuel, the skin will be bro~en away by the fuel
pressure whilst the skin is still forming.
Figure 8 shows an alternative base plate structure. In
thi.s structure, a quiesc~nt layer is not formed by means
of stand or sparge pipes but th~ base plate proper 4'l
is a plain perEorated plate which is thermally insulated
from the active fi.re re~ion 45 by a layer of coarse
xefractory bodies 46 resistant to elutriation. Above this
layer of bodies 46 -there is the~usual active fire region
consisting of :Euel and carrier~ The carrier can be charged
into the chambe.r along with the heavier bodies and the
graded layers can be Eormed by elutriation hy a
high pressure air stream. Alternatively, the heavi.er hodies
can be introduced firs-t and then the carrier added with or
without an air current.
-13-
~:~65~
The gaps between the coarser heavy bodies must be sufficiently
small forthe carrier to be unable to seep down through
the gaps and through the perforations in the base plate
proper. This can be done by judicious grading.
As is shown in Figure 8, the base plate 44 can be spaced
from the shell 11 by the air pipes 20 which would be welded
to the shell and the base plate 44, or c~lternatively, a
unitary, prior art hase plate may be used which relies solely
on the refractory material 46 for cooling.
A bed reconditionin~ sys-tem is shown in Figure 9 to include
a fuel inlet 60 to -the reaction chamber 12. When the
average pa.rticl.e size or density of the bed material is
such as to interf~re with tlle efficiency of the reaction,
the bed material 54 is removed through the drop tube 62
into a crushing unit 64 where the final par-ticle si~.e
o~ the material can be reduced to a predetermined value.
The crushed materlal is then conveyed by a conduit 66 to
be drawn off and entrained in an air stream by a venturi
68 which is fed with air from -the forced draught fan in
the direction of the arrow 70. The reconditione~ carrier
material is fed back through the fuel inlet 60 to the reaction
chamber 12. Thus, par-t~ lly burnt coal .is returned to the
bed and burnt to ash which is elutriated, and ground inert,
` incombustible ma-teri,al is introduced to the bed 54, which
further reduces the necessity to top the bed up with fresh
sand.
-14-
This embodiment permits substantially continuous operation
of the fluidised bed burner and ensures that substantially
all of the ash will be collected in the cyclones in
the exhaust of the system.
In Figures 10 and 11 the plenum 110 of a vertical fluidised
bed boiler is fed with air under pressure in the direction
of arrow 112. The air rises through the base plate 114,
on which rests a fLuidised bed 113. The heat generated
from the fluidised bed flows upwardly throu~h the reaction
chamber which rises high enou~h to constitute a vertical
flue 115, as a first pass. The side walls 116 of the
reaction chamber 115 are therefore heated by direct
contact.
The heated gas then flows through a series 118 of smoke
tubes which constitute a second pass, and then into a
smoke box 120, from which the hot gases pass upwardly
through a series 122 of smoke tubes constituting a third
pass, terminatincJ in a manifold 124, which is connected to
suitable cyclones and chimneys ~not shown).
A series of thermic siphon tubes 126 is provided, leading
from the jacket 128, throllgh the bed 113 and the vertical
Elue 115 in order to produce steam. 'rhis s-team rises and
is then allowed to enter the steam space 130 above the
level 132 of water above the flue 115. Baffles 132 are
provided to avoid instability of the s-team space 130.
-15-
~.~6~
The working pressure inside the steam space in the boiler
shown is 860 KPa~
Coal or other fuel is introducPd through the inlet
orifice 134 and water enters through inlet 136a. An access
door 138 is provided as well as a manhole ring 140~ A
æeries o~ stays 142 serve to ensure constructional strength
and the smoke pipe 118 and 122 also serve as constructional
units.
A microprocessor may be used which constantaly monitors
the various parameters and all of some mechanical factors.
This microprocessor may be used with a numbex of boilers
in conjunction with a modulator/de-modulator (MODEM) unit,
which allows the microprocessor to receive and send
message~ along telephone wires to a central computer and/
lr~ or to individual computers. Thus, the central cont.rol
can send warnin~ signals to customers' boilers in advance
of catastrophe or damage or malfunction, thereby providing
a preventative maintainance feature.
In Figure 12 a heater 210 is formed with an air dis~ributor
and plenum chamber 211 havirlg an air inlet controlled by
a damper 212. The i.nlet leads from the forced draught fan
(not shown). Above t:he chamber 211 i.s a space occupied
by the fluidised bed 213 of conventional design with its
associated free~board 214. The s-team space is indicated by
-16-
5~1l9~i
the reference numeral 215. There is a gas outlet controlled
by a damper 216. In this case a suction fan (not shown)
is connected to the outlet.
The heater 210 is fitted with four transducers. There are
two ther-mocouples 217 and 218 in the bed 213 serving as
temperature transducers. There are two pressure transducers
219 and 220 serving to sense the steam pressure and free-
board pressure respectively.
The heater 210 is also fitted with a bed preheating unit
221 of any conventional construction which is controlled
by an ignition control uni-t 222 which in turn is controlled
by a start up control unit 223. Once activated the Ullit
223 is controlled by a signal from the thermocouple 217.
~t a predetermined bed temperature, the unit 223 causes
the ignition control unit 222 to s~itch off the preheating
unit 221. At another and lower temperature the unit 223 is
once more activated and causes the unit 222 to operate
once more.
The other thermocouple 218 signals a temperature controller
224 to control tlle fuel feed in a manner described later
on. The controller 224 functions through the unit 223 and
as long as the Ullit 223 is activated, signals from the uni-t
224 are blocked and only signals from the unit 223 pass
along the line 225 to a speed con-troller 226 which regulates
the coal feed to the bed 213 of a coal feeder (not shown) so
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~ ~ 6 ~ ~ ~t3
that only the quantity of coal required at start-up is fed
to the bed 213.
The speed controller226 is arranged to be operative over
one or more ranges each from zero to a predetermined maximum.
This is done by means of a three step controller 227 which
enforces a maximum speed setting on the controller 226
in response to signals from the transducer 219. At a
predetermined maximum pressure the coal feed is stopped.
At a predetermined high pressure the lowest speed setting
becomes operative and at two lower pressures, high speed
settings become operative.
The three step controller 227 operates the damper 216
in a similar manner among one or more positions. At the
maximum pressure in the freeboard the damper 216 is at its
lS smallest opening ancl at two lower pressures it is at an
intermediate and at its fullest openincJ. The clamper 216
should never be closed.
The setting of the damper 216 affects the pressure in the
freeboard 214 so that the transducer 220 senses any changes
in pressure as the aperture of -the damper 216 is changed.
The transducer 220 then sicJnals a three term proportional
controller 228 which controls the damper 212 causing the
flu:idising and combustion air to increase or decrease thus
balancing the above bed pressure to a predetermined positive
or negative pressure level. This change in combustion air
-18-
~1 ~6~ ~t.;;~
also increasçs and decreases the heat transfer surfaces
covered by the fluid bed, i.e. increase in fluidising
and combustion air gives an increase in heat transfer
surface. A decrease in~ fluidising and combustion air gives
a decrease in heat transfer surface. In all uses of fluid
combustion the greater the quantity of combustion air the
greater the heat extraction rom the bed and the lower
the combustion ~ir the lower the heat extraction becomes
from the bed. This is because the air, as it passes through
the bed absorbs heat while being involved in the combustion
process. The air and combustion ~ases in the boiler
situation remove a proportion of the heat from the hed,
while in a fluidised bed incinerator and crop drier the
combustion air and gases, remove nearly all of the heat
from the combustion system. The rest of the heat becomes
dissipated through losses in the system which, however, is
a very small percentage~
The control therefore of damper 216 in relationship with
damper 212 and the correct delivery of coal to the combustion
system must be related correctly with the combustion air,
this being done by the three step ~ontroller 227. The
response time of the controller 228 and the damper 212 is
shorter than the response time oE the damper 216 to
prevent pressurization of the space 214.
Note that when the unit 223 is activated it overrides the
control by the transducer 214 to enforce the setting of the
--19--
~6~
damper 212 to that required at start up. As soon as the
unit 223 is deactivated, the transducer 214 takes over.
In addition to the above,conventional safety circuits may
also be provided, but these do not affect the operation
of the invention.
As an illustration take the case of a boiler designed to
deliver steam at a pressure of about 960 kPa with a
maximum allowable pressure of 1000 kPa. The controller
227 would then be set to select one of four boiler states:
1. Hiyh fire at 890 kPa
2. Medium fire at 930 kPa
3. Low fire at 960 kPa
9. Slump at 1000 kPa
Corresponding to the boiler states the controller 226 will
have one or more maximum speed settin~g:
1. High speed
2. Second speed
3. Third speed
9. Stop
The damper 216 will also have ~our settings:
1. Wide open
-20-
.' .
~6,5~
2. Partially open
3. Partially closed
4. Almost closed
In response to this the controller 228 would place the
S damper 12 into four settings resulting from a pneumatic
connection derived from the above bed combustion chamber
gas pressure:
1. Wide open
2. Partially open
3. Partially closed
4. Closed.
For a coal fired furnace the average bed temperature should
be about 950C and the maximum allowable about 1000C.
In such a case the start up control Ullit 23 should become
deactivated at AC and come into operation again if the bed
temperature drops to BC, which is a temperature below
that of AC. In effect the temperature controller 224 will
then send out signals over a range of AC to 1000C. At
the latter temperature i-t would order the speed controller
226 to stop the coal eecl while at AC it would order the
controller 226 to feed coal at the maximum rate permissible
in terms of the setting allowed by the controller 227.
The effect is that once the steam pressure is up and steam
is consumed, more air will pass through the bed tending to
-21-
iS~9S
lower the temperature, so that the controller 224 will
cause more fuel to be fed to the bed once more to raise
the temperature and also to provide more heat ~o raise
more steam. As the demand drops, air flow will first
drop while the controller 226 will operate in a range which
will cause the fuel feed to be diminshed in step with the
decline in demand.
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