Note: Descriptions are shown in the official language in which they were submitted.
WO ~/087~3 2 V -- ` PCT/GBgo/o~lo5
Fee~er for P3rticulate Mate~lal
The present inven.ion relates tO a feeder for
~articulats m2 ~rial, fcr instance material in the form
of a powder or fibres, filaments or whiskers. More
particularly, it relates to a feeder for connecting a
mechanical feeding device to a pneumatic conveying
line. Feeders of this type, generally known as
"par~iculate feeders", are used when it is desired to
change the means of transporting a particulate material
in a conveying line from a mechanical means (e.g. a
screwfeeder) tO a gas driven means (e.g. fluidised
bed). Many known particulate feeders are effective
only when using large volumes (high velocities) of
conveying gas and!or connecting feeder outlet and
conveying line diameters of essentially similar size.
; 15 The feeder of the invention is particularly
useful as part of a controllable feeder system for
conveying particulate material from a hopper to a
pneumatic conveying line for controllably supplying
said particulate material for incorporation into metals
by spray co-deposition in the production of metal
matrix composites.
A typical spray co-deposition method of making
metal matrix composites comprises the steps of atomising
; a stream of molten metal to form a spray of hot metal
particles by subjecting the metal stream to relatively
cold gas directed at the stream, feeding a stream of
; particulate ceramic material in a fluidising gas to the
atomising zone where said particulate material becomes
;~ incorporated into the metal particles and co-depositing
the metal and the particulate material onto a
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W090/0~723 PCT/GB90/0010~
collecting surface. Co~ventionally, the parti~ulate
ceramic material i, c~nveyed pneumatically from 2 hop~er
tO the atomising z~ne. However, the am~ien- Dressure
conditions at the dis.harge point of the par~:cula;e
feed tube are variable because of the highly turbu~ent
gas jet flows present in the atomising region. Because
of this variable pressure at discharge, any powder
feeding device for use in this method which r lies on a
gas stream to control particulate feed rate tends to
be unreliable, particularly in view of the low
conveying line pressures (e.g. 0.3. bar 9) ar,d
conveying line flowrates (e.g. 80 dm3/min) used. A
conventional feeding system for transporting a
particulate material from bulk storage in a hopper to
the atomising zone uses two gas streams: one for
introducing the particulate material into the conveying
line from bulk storage and one for conveying the
particulate material to the atomising zone. Such a
system dictates that the two gas streams meet at equal
pressure at some point. A change in conditions at this
point will obviously cause changes to occur in all gas
flows including the feedrate of the particulate
material. The use of higher gas flows which might
otherwise overcome such a problem is not desirable for
a number of reasons. For instance, because of the
nature of the spray co-deposition method, it is
~ desirable to keep the flow rate of the gas used to
; convey the particulate material to the atomising zone
as low as possible in order that the conveying gas
stream does not affect the gas stream used to atomise
the molten metal at the atomising zone. Furthermore,
high flow rates for the ¢onveying gas are not desirable
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WO90/08723 ~ /G89~0/Q010
since equipment life can be greatly reduced because of
the effects of abrasiDn caused by high-speed -~-
p2rticulate material which abrasicn could 21s0 result
in the degrada~i~n cf the partlcu~2te material i~se'~
5 by size reduct!on and/or contamination. If higner
conveying gas flows and larger pipe bore sizes could be
used then it might be possible to maintain the
conveying velocity at its original levei. However, the
geometry and dimensions of the atomising zone are such
that the particulate feed entry has to be relatively
small (e.g. 7mm diameter) to ensure maximum delivery of
the particulate material to the atomising region
accurately.
In view of the above, it is clear that there is
lS a need for an improved system of feeding particulate
material from a hopper to a pneumatic conveyer for use
in metal matrix composite production by the spray co-
deposition process which system allows for greater
control over the feed rate of the particulate material
20 than achieved previou 5 Iy . - .
According to one aspect, the present invention
provides a particulate feeder for connecting a ,~
mechanical' feeding device to a pneumatic conveying line
;~ which comprises a funnel formed of a gas
pervious material mounted in a closed outer housing
formed of an impervious material, the walls of the
funnel dnd the outer housing together defining
a plenum chamber which is provided with an inlet for
connection to a supply of conveying gas under pressure,
wherein the housing at or towards the wide end of the
funnel is adapted to form a sealing engagement
with' t'he outlet of`the mechanical'feeding device and
, wherein the funnel at or towards its narrow end
'l ~ communicates with the pneumatic conveying line.
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W090/08723 PCT/GB90/0010
-- 4
3~' - According to another aspect, the pre,ent
invention prcvides a ree~er system for conveyins
particulate material from a hopper to a pneumatic
conveying line for use in a spray co-de~osition proc s,
of ~roa~ing me;al matrix composites which feeder
system comprises
(1 ) d mechdnical feeder device for moving
particulate mdterial from a bu]k stor2ge hopper to an
- outlet and
(2) a funnel formed of a gas pervious
material mounted in a closed outer housing formed of an
impervious mdterial, the walls of the funnel
and the outer housing together defining a plenum
chamber which is provided with an inlet for connection
to a supply of conveying gas under pressure, said
housing at or towards the wide end of the
; funnel being engaged with the outlet of the mechanical
feeder device to form a gas-tight seal therewith and
said funnel at or towards its narrow end being
in communication with the pneumatic conveying line.
Thus, the feeder system of the invention makes
use of a mechanical feeder device to m,ove the
particulate material from a bulk storage hopper to a
particulate feeder wherein the particulate material is
fluidised for introduction to the pneumatic conveying
line. The use of the mechanical feeder device overcomes
the above-mentioned problems arising from the use of a
gas stream to move the particulate material from a
storage hopper. This is because, in the case of a
mechanical feeder device, the particulate solids
, feed rate from the hopper is independent of other process
, conditions and is essentially dependent only on the
speed of the motor driving the mechanical feeder
device.
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WO90/08723 5 PCT/GB90~0010~ ^
Mechanical feeder devices are, of course, well
known and include screwfeeders, vlbrating conveyor
feeders and rotary valve feeders. We haYe obtaine~
cood results using a screwfeeder in the present
invention. In ideal circumstances with a free flowing
powder, the solids feedrat~- delivered by a mechani~ ;
feeder device is proportional to the speed of the drive
motor. However, in normal circumstances, particulate
materials are far from ideal in their flow behaviour
and handeability, especially if the particulate
material is fine in size, irregular in shape, damp and
cohesive. Usually such factors are responsible for
variations in powder "bulk density". When a mechanical
feeder device operating at a fixed speed is used,
fluctuations in the mass flow rate of particulate
delivered by the mechanical feeder device may be
experienced. It is, therefore, preferable in the
present invention for the rate of delivery of
particulate by the mechanical feeder device be
controlled by a feedback system. To do this, the
mechanical feeder device is suspended or loaded on a
weighing device (for example, a load cell). The change
in weight of the feeder device under operation as a
f,unction of time is monitored and aut~matically
compared with the change in weight that would be
expected for a desired feedrate of particulate. If the
actual rate of weight decrease in the system being
monitored is greater than that expected, the system
compensates by reducing the speed of the feeder device
'~ 30 accordingly. Alternatively, if the actual rate of
weight decrease is less than expected for the
particulate feedrate desired, the system automatically
increases the speed of the mechanical feeder devic
accordingly. Such controlled feeder devices are known
generally as "Loss in Weight" feeders. The process o,~
j ~ sampling the feeders weight, calcuIating the result:ng
feedrate and effecting the appropriate motor speed
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WO9U/08723 ~ 3 ~ 6;~- -
control action is almost continuous during feeder
bperation and allowance is made f cr the fini~- ~i me
required for data samDling and microproressor progra~
calculation time (Iypically milliseconds per cycle3
5 whi.h is neglisible in terms of syste~ (m~ or) re,ponse
time. By using a "Loss in Weight" feeder of thls kind,
particulate materials of different kinds can easily
and accurately be fed using the same feeding
device and change from one particulate material to a
different particulate material to be fed to the feeding
device can be effected easily and rapidly.
The particulate feeder of the invention for
connecting the mechanical feeder device to the
pneumatic conveying line typically comprises a
fùnnel made of a porous material which is pervious to the
conveying gas which, when the feeder is in operation,
is supplied to the plenum chamber which lies between
the funnel and the external gas impervious
housing in which the funnel is mounted. In order
that the speeds of the mechanical feeder device
involved are acceptable to the particulate materials
involved, the discharge part of the mechanical feeder
is typically an order of magnitude larger than the
diameter of the pneumatic conveying tube. Thus, the
funnel in the particulate feeder has to achieve a
transition from'an inlet diameter of, for instance,
100 mm to an outlet diameter of, for instance, 10 mm
; with a minimum hold-up volume being created whilst, at
the same time, without the transition being made so
rapidly that blockages of particulate material are
caused in the pneumatic conveying line. The funnel in
the particulate feeder of the invention is preferably a
conical funnel. However, a non-conical funnel, such as
one having a bowl shape wherein the sides curve inward
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W090/08723 PCT/GB90/0010
towards the narrow end wnich communicates with the
pneum3tic conveying line, can also be used in the
present inventicn. In such d case, the inwardly
curving sides of the bowl-shape funnel will preferably
n^~ ~e so great as to present a sur'a e where any
build-up of particulate material could occur during
operation Ot the particulate feeder. We have found
that the transition is preferably effected by using a
conical funnel having a vertical axis and walls at an
angle of from 30to 60 to the vertical axis, and more
preferably between 30 and 45. The shape of the
external housing is not critical although, preferably,
it will be large enough to provide uniform filling of
the plenum chamber that surrounds the funnel. The
funnel used in the present invention is formed of a
material which is pervious to the conveying gas that
will be supplied under pressure to the plenum chamber
during operation of the particulate feeder. Gas-
pervious materials such as sintered plastics, filter
cloths and woven wire meshes have been used
successfully in the present invention for forming the
funnel. However, because the conveying gas will be
supplied unde'r pressure, during operation, to the
plenum chamber, the walls of the funnel should have
sufficient mechanical rigidity so that the pervious
funnel has sufficient dimensional stability to
withstand this pressure. Preferably then, we use
material such as sintered metal or perforated metal for
forming the funnel for use in the present invention.
In the case of a funnel formed from a perforated metal
sheet, the metal sheet imme~iately at the periphery of
the perforations may advantageously be deformed away
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WO90/08723 PCT/GB90/00105
-- 8
from the plane of the metal sheet so as to shield the
particu]ate material (when the feeder is in operation)
frcm the ~er-sr2t;0ns. Preferably, the internal
surface o- t~,e funnel should be sufficiently smootn so
5 2; not to ~IIG~ the bulld-u~ o' any ~ar.iculcte
materi21 the!eon. In operation, as the particulate
material free-falls into the funnel from the
mechanical feeder device, the conveying gas which
passes throu3h the bowl fro~ the plenum chamber flushes ~.
it towards and into the pneumatic conveying line.
In the accompanying drawings -
Figure 1 is a section through a preferred
particulate feeder embodying the present;invention;
Figure 2 schematically illustratès the
operation of the particulate feeder shown in Figure l;
and
Figure 3 is a diagrammatic representation of a
feeder system in accordance with the invention.
In Figure 1, a housing 1 formed of a gas
impervious material such as stainless steel or
aluminium has cylindrical sides 2 and an
. upper radial flange 3. The housing has a base 4 which
opens into an exit pipe 5 leading to a pneumatic
conveying line (not shown). The flange 3 is adapted to
abut the base of a mechanical feeder device outlet (not
shown) and be fixed thereto by means of bolts placed
through.holes 6 provided in said flange. Contained
. inside the housing 1 is a truncated conical funnel 7 :
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WO ~/08723 PCT/GB90/0010~_
having sides at an angle of 45 to the vertical and
formed of a gas Dervious material. The funnel is
lû'2 ed in the hsusing so as to provid~ 2 smooth
tr~nsition in si,e from its wide end 8 positioned near
5 ~ o~ ope"ir~ he ~,uu;ing ;o its n~rro~ end
whi:h communicates with the exit pipe S. A plenum
chamDer 10 is defined by the sides 2 and base 4 of the
housing together with the sides of the conical funnel
7. The housing is provided with an inlet 11 ror a
supply of a conveying gas introduced under pressure
when the particulate feeder is in operation. As can
seen from Figure 2, during operation, particulate
material 12 free-falls into the particulate feeder from
a mechanical feeding device (not shown). Conveying gas
supplied und~er pressure to the inlet 11 in the housing
1 enters the plenum chamber 10 from where it passes
through the Wdlls of the conical funnel 7. The
particulate material 12 falling into the conical funnel
meets the conveying gas at or ne~r to the funnel walls
from where it is carried by the gas flow down the
interior of the funnel and flushed into the exit pipe 5
; to the pneumatic conveying line (not shown).
In Figure 3 a particulate feeder of the type
shown in Figures 1 and 2 is engaged by f]ange 3 at the
outlet 13 of a motor driven screwfeeder 14. The
screwfeeder 14 in operation feeds particulate material
1~ from a sealed bulk storage hopper 15 to the outlet
13 from where the particulate material free-falls into
the conical funnel 7 of the particulate feeder. In
order that the conveying gas introduced into the plenum
chamber 10, Iying between the housing 1 and the funnel
7, via gas inlet 11 flows in the desired direction and
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W090~08723 - 10 - PCT/GB90/0010
at the desired flowrate, a pipe 16 is preferably
provided to connect the screwfeeder outlet region 17 to
the airspace 18 above the level of particulate material
in the hopper. Thus, when the conveying gas is flowins,
the gas pr~ssures at 17 and 18 will be equalized so
~ that particulat~ fecd is nc' adversely affected
by any pressure build-up in the system. Typically, the
pipe 16 will contain or be provided with a device to
restrict the flow of particulate material and conveyins
gas through the pipe but which still allows the
equalisation of pressure between the bulk storagè
hopper and the particulate feeder. For example, the
pipe 16 may contain or be fitted with a pressure
equalisation valve which only need be opened
periodically to relieve any pressure buildup in the
system. Such a feature would be advantageous at low
flow rates. Alternatively, instead of providing a
return line 16, with or without a control valve, to
connect the funnel inlet (17) to the top space (18) of
the hopper, the top space of the hopper may be
connected to a separate gas feed (not shown) which
would be controlled automatically to maintain the gas
pressure at the top of the hopper at a value equal to
the gas pressure at 17. In the case where the volume
of gas in the hopper is large, such a system having a
separate gas feed to the top space of the hopper would
be preferable to a system having a return line 16 since
in the latter system pressure equalisation would cause
i the flow of gas to be diverted in the funnel with the
result that the uniform transport of solids in the
3 conYeying line would be disrupted.
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Woso/08723 1 PCTJGB90/0010
A, mentioned earlier, spray co-deposition
processes require a maximum amount of particulate
material with a minimum amount Qr carrier gas. This
ratio is termed "phase density" and is given the symbcl
"~", where
-
~ = mass flowrate of solids
.
mass flowrate of gas
Some powders will convey in "dense phase",
i.e. ~= 50-200 (approx) at velocities of about 1 m/s.
Experience of handlin3 such powders in this mode is
essential as operating conditions are approaching
conditions in blocked pipes. However, to ensure
accurate operation of a "Loss in Weight" feeder, it is
desirable to operate the conveying line at the opposite
end of the phase density range (i.e. ~= 1-10 and
v = 20-40 m/s) which conditions are analogous to
conditions in an open pipe. The other advantage of
this regime is that even small discontinuities in
powder flow within the conveying line (causing line
pressure fluctuations) can be measured easily relative
to the normally low line pressure. In "dense" phase
conveying, large fluctuations in solids flowrates can
go unnoticed since they are masked by the normally high
line pressures required.
The feeder system of the present invention
canbe made to work quite satisfactorily using phase
densities of 20 to 50 at velocities of 10 m/s. This
mode of transport (i.e. moving/sliding beds and dunes),
however, may not give desirable results in terms of
metal matrix composite product structure. The present
invention has been used successfully to convey SiC
I powder (F230 grit, F600 grit and F1000 grit) over a
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wO9Ot08723 PCT/GB9O/OO10~
jt~, ~`J .~ 3 - 1 2
range of feedrates from 0.5 to 3.5 kg/min using
transport gas flows of from 45 x 10~3 to 85 x 10~3
Nm3/min through nominal 8 mm diameter tubing. Th^
terms "F23C, 600 and 1000 grits" are described in FE?A
Standard 42-G3-1984 and US S~andard ANSI B.7~.12-1975.
As soon as the feeder motor is turned off, the
conveying line clears rapidly rather ~han undergoing
any gradual reduction in powder level concentration in
the conveyin3 line.
EXAMPLE
The feeder system as described above was used
to convey various particulate ma~rials at various flow
rates. The system was found to work at high solids
densities and yet still maintain uniform flow. The
1~ results are shown in the following Taùle-
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WO 90/08723 ~ 3 ~ '' ~ PCr/GB90~00l0
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woso/o8723 ~38 PCT/GB90~00l0~
Notes to Table
(1) The solids density values are obtained by dividins
"Flowrate" by "Gas Flow" x103.
(2) 50:50 wt' mixtur
(3) Screwfeeder pressure 1.0 bar 9 ~ 2 kg/min
(4) Screwfeeder pressure 3.0 bar 9 ~ 3.5 kg/min
(5) Screwfeeder pressure 2.5 bar 9
The first six runs shown in the Table, i.e. those
for SiC (F600) at 0.5, 2.0, 3.0 and 3.5 kg/min demonstrate
that high solids densitites can be achieved. The other
runs in the Table demonstrate the ability of the feeder of
the invention to handle a range fo materials and particle
sizes. In all cases, a satisfactory uniform flow was
achieved.
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