Note: Descriptions are shown in the official language in which they were submitted.
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STEAM BOILERS
This inven~ion relates to steam boilers and in parti-
cular marine steam boilers used to supply steam to the steam
turbines powering a s'nip.
B~CKGRO~ND TO THE INVENTION
In marine steam boilers providing high temperature
and pressure superheat and reheat steam there is a danger of
slagging and/or corrosion o the tubes forming at least the
'nighest temperature superheater and reheater bank when burning
heavy bunker fuel oil which has a high ash and sulphur content.
It is therefore an object of this invention to reduce this
slagging and/or corrosion in such steam boilers.
BRIEF SUMMARY OF THE INVENTION
According to the invention there is provided a steam
boiler having a heavy oil burning furnace and a separate clean
fuel burning furnace, and a number of tube banks providing one
or more stages of superheated steam and optionally one or more
stages of reheated steam, the tube banks being arranged so that
the tubes of the highest temperature superheater tube
bank and of the highest temper~ture reheater tube bank
if provided are heated by the combustion p~oducts from the clean
fuel burning furnace.
In such a boiler the slagging on and/or corrosion of
the tubes of the highest temperature superheater t~e
bank and reheater tube bank is largely avoided because those
tubes are contacted solely by the clean combustion gases from
tlle clean fuel burning furnace. The other tubes of any lower
temperature superheater and reheater tube banks can be swept
by the combustion gases from the heavy fuel oil burning furnace
30 but for these tubes which operate at lower temperatures the
risks of slagging and/or corrosion are less.
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The clean fuel for the clean fuel burning furnace can
be provided by the pre-combustion of heavy fuel in a gasifier
which can provide a supply of clean product gas for burning
whilst retaining the impurities in the heavy fuel. These
gasifiers are known and can take the form of a fluidized bed
gasifier.
Under no-sleam flow conditions in the reheater, cooling
air must be passed over the tubes of the reheater tube bank or
banks. This can be achieved by positioning the bank or banks
in gas passes whose outlets under such conditions can be closed
by dampers and cooling air can then be fed in reverse flow
through the pass or passes directly from an air supply fan such
as the forced draught fan for the combustion air.
Separate forced draught fans and air pre-heaters can
be provided for each furnace if r~quired to provide air for the
furnaces and/or the gasifier. This may serve to avoid the back-
feeA of contaminants from the air preheating system for the
clean fuel burning furnace by the air supply to the heavy oil
burning furnace so as to ensure that the combustion products
from the clean fuel burning furnace remain clean.
BRIEF SUMMARY OF TH~ DRAWINGS
An example of a marine steam boiler according to the
invention will now be described with reference to the accom~
panying drawing which is a sectional diagram of the boiler.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The boiler 10 shown in the drawng has a heavy fuel
oil burning furnace 12 and a separate clean fuel gas burning
furnace 14. The combustion gases from the furnace 12 pass up
through two parallel passes 16 and 18 while the combustion
gases from the furnace 14 pass up through a pàss 20. The flow
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of gases through the passes. 16, 18, and 20 are controlled by
dampers 22, 24 and 26, respectively. The gases Erom all three
passes are thereafter combined in a common flue 28 containing
an economiser tube bank 30. The walls of the furnaces 12 and
14 and passes 16, 18 and 20 are defined by conventional water
tube walls which are not shown in detail.
The furnace 12 is f~red by heavy fuel oil from a
supply 32. The heavy fuel oil is burned by burners 34 and com-
bustion air is supplied from a windbox 36. Air for this wind-
box is supplied from a common duct 38 and its rate of supply is
controlled by a damper 40.
The furnace 14 is fired by clean gas from a gasifier
44. This gasi.fier receives heavy fuel oil from the supply 32
and air from the common duct 38 via a passage 45 at a rate
determined by a damper 46 and converts the oil to a clean com-
bustible gas which is fed to a burner 48 in the furnace 14
via a duct 50. Combustion air for the furnace 14 is supplied
to a windbox 52 from the common duct 38 and the rate of supply
is controlled by a damper 54.
The air supply to the common duc-t 38 is made from
a forced draught ~an 56, the air being preheated in a regenera-
tive air heater 58 heated by the combined combustion gases in ~.
the flue 28.
The pass 16 contains a first or low temperature
superheater 60 and a first or low temperature reheater 62 while
the pass 18 contains a second or intermediate superheater 64
serially connected with superheater 60 and a by-pass economiser
66. Dirty combustion gases from the furnace 12 flow over these
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items. The pass 20 contains a thi.rd or final superheater 68
: 30. serially connected with superheater 64 and a second or final
reheater 70 serially connected with reheater 62, this superheater.
~68 and rehea~er 70 being the hïghest temperature superheater and
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reheater passes, respect;vely. Clean combustion gases from the
furnace 14 pass over these items and to ensure this the damper
26 associa~ed with the pass 20 is controlled to maintain a
slight positive pressure differential between that pass and pass
18 at the level of intermediate open screens 80 and 82 provided
respectively between the passes 16 and 18 and passes 18 and 20.
- These screens 80 and 82 are provided so that the com-
bustion gases can deviate from one pass to another after flow
over the first, second or third superheater in dependence upon
the positioning of the dampers 22, 24 and 26, the damper 26
being arranged as noted above to ensure that dirty combustion
gases do not flow from the pass 18 to the pass 20.
Under no-steam flow conditions through the first and
second reheaters 62 and 70, these have to be cooled. There-
fore cooling air ducts 90 and 92 lead from the fan 56 to the
tops of the passes 16 and 20, respectively. The passage of the
' cooling air through its ducts 90 and 92 is controlled by dampers
; 94 and 96, respectively. Thus to prevent flow of hot combustion
gases over the reheaters 62 and 70 when they contain no steam,
the dampers 22 and 26 are closed, all of the combustion gases
then passing over the economiser 66, and cooling air is allowed
to flow down over the reheaters 62 and 70 by opening the dampers
94 and 96 and this cooling air then passes through the open
screens 80 and 82 to join the combustion products.
If desired a separate forced draught fan can be ~ro-
vided to supply air to the furnace 14 and gasifier 44 and dirt
- picked up by such air from any air preheater can be fed back
into the combustion air for the furnace 12 so as to keep the
combustion gases from the furnace 14 clean.
3~ The operation of the boiler 10 should be clear from
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the above description.
As will be appreciated, the final or third superheater
68 and the final or second reheater 70 both operate at the
highest temperatures and 50 are most liable to be affected by
slag and corrosion problems. However, both can only receive
clean combustion gases and so problems of slagging and corrosion
are reduced as compared with a situaton in which they receive
combustion gases from the furnace 12.
Although the words water and steam have been used
herein those woeds are to be construed as including any liquid
and its vapour unless the context specifically requires other-
wise.
A latitude of modification, change and substitution
is intended in the foregoing disclosure and in some instances
some features of the invention will be employed without a corres-
ponding ase of other features. Accordingly it is appropriate
~hat the appended claims be construed broadly and in a manner
consistent with the spirit and scope of the invention herein.
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