Multiple
Zone
Autoclaves
US2003085219
EP1462156
Also published as: US6872918 (B2) KR20050056933 (A) JP2005507774
(T) WO03039731 (A1) GB2381764 (A)
Abstract -- An autoclave
is provided for heat treatment of a load whose position relative
to the autoclave, whose cross-section and/or whose thermal
characteristics may vary along the load, e.g. large panels for an
airliner. The autoclave comprises a chamber for receiving the
load, a wall of said chamber providing one end thereof and a door
providing the other end of the chamber and giving access for
insertion and removal of the load. Means is provided for heating
gas in the chamber, and a plurality of gas circulation means are
provided spaced along the length of the autoclave and each
producing a zone for circulation of heating gas. Means is provided
for independent control of the rate of heat transfer between the
heating gas and the load in said zones and said gas circulation
means is arranged to produce a pattern of circulation in which
heating gas impinges non-axially onto the load.
FIELD OF THE INVENTION
[0001] The present invention relates to autoclaves and to their
use in the heat treatment of workpieces.
BACKGROUND TO THE INVENTION
[0002] EP-B-0176508 discloses a design for a gas-fired autoclave
which is useful e.g. in the production of articles from
fiber/resin mixtures and heat treatment of workpieces in the
glass, automotive and aerospace industries and which nowadays
typically have working temperatures of up to 450.degree. C. and
working pressures of up to 68 Bar. Autoclaves for use in curing
composites or heat-treating glass articles might typically have a
length of 3-4 meters, a diameter of 1-3 meters and a volume of
10-20 m.sup.3. For use in the automotive industry e.g. for heat
treating the chassis of a racing car an autoclave may typically
have a diameter of about 2.75 meters with a length of about 4.5
meters and an internal volume of about 25 m.sup.3. For use in the
heat treatment of aerospace components, an autoclave might
typically have a diameter of about 4.25 meters, a diameter of
about 12 meters and a volume of about 170 m.sup.3.
[0003] As shown in FIG. 1, a typical prior art autoclave is based
on a pressure vessel that has a length of about 3.7 meters (12
feet) and a diameter of about 1.5 meters (5 feet), the vessel
having a body 10 and a loading door 12. Vacuum lines 14 are
provided for connection to the mold side of a mold tool (not
shown) that is covered by a flexible diaphragm with a workpiece to
be molded located between the tool and the diaphragm. The tool is
connectable through valve 18 to vacuum and through valve 20 to
air. Valve 22 can be operated to admit air through pressure lines
16 to the interior of the pressure vessel. Heating is by exposed
radiant tubes 24 that run up and down the length of the pressure
vessel. The entry to each tube is provided with a gas-fired heater
34 and the discharge end of each tube is provided with an impeller
36 by which a negative pressure is produced towards the discharge
end and a flow of flue gas is maintained through the tube. A motor
38 mounted on the tank end wall drives a radial flow impeller 40
to produce a re-circulating flow of the gas within the pressure
vessel. Thermocouples 42 through the tank wall 10 responsive to
gas temperature are connected to a control unit 44 that is
operatively connected to the various heaters to turn them off or
on and maintain the gaseous atmosphere within the autoclave at
.+-.1.degree. of an intended value. The use of a variable speed
impeller to enable the same tubes to be used for heating and for
return to room temperature during the cooling part of the
operating cycle is disclosed in EP-A-0333389. Autoclaves of other
designs may be electrically heated, steam heated, oil-heated, hot
air heated or gas radiant-heated, but up to now they have relied
on an impeller in the end wall to produce a single generally axial
pattern of re-circulating gas flow as indicated by the arrows in
FIG. 1.
[0004] U.S. Pat. No. 6,240,333 (Lockheed-Martin) concerns the
fabrication of composite parts in an autoclave. Lockheed-Martin
explain that the F22 Raptor is an example of an aircraft made
largely from composite materials formed with flexible graphite
fibres, called a ply, that are impregnated with epoxy or BMI
resins which harden when subjected to the application of heat. The
uncured plies are placed on tools, each tool corresponding to a
composite part of the Raptor. Thus, when the graphite resin
mixture hardens over the tool, the composite part is formed with
the proper shape. Lockheed-Martin go on to explain that a number
of production techniques are available for forming composite
parts. Again, using the Raptor as an example, once the plies are
placed over the tool, a vacuum bag is used to hold the plies
securely to the tool during curing of the resin. The vacuum bag
forces the material to the tool and prevents the formation of
bubbles and other material deformities. The tools are then placed
in an autoclave for heating according to a schedule, adherence to
which may be essential in order to avoid the production of
defective parts.
[0005] Lockheed-Martin further explain that an autoclave operator
must carefully distribute tools in the heating chamber of the
autoclave to ensure that heating rate specifications are met, a
typical autoclave being 15 metres (50 feet) long but nevertheless
still being heated by blowing air with a large fan located at one
end of the heating chamber. They identify a number of difficulties
that this method of heating introduces into the production
process, amongst others that if an autoclave operator adjusts
heating rates to a lower level in order to avoid over-heating of a
part, the autoclave will require a greater time to cure other
parts, increasing the time required for the entire production run,
and that if the parts are distributed improperly, the autoclave
operator may have to violate the heating rate specifications for
some of the tools, thus wasting the parts on those tools, in order
to obtain useful parts from other tools. The solution suggested by
Lockheed-Martin is to provide load distribution software for
appropriate positioning of workpieces within a load to be
introduced into the autoclave. The software includes a layout
engine for determining the best layout of selected tools in an
autoclave heating container depending upon (a) the particular
tools selected, (b) the thermal performance of the tools and (c)
the thermal characteristics of the autoclave, the layout engine
generating the resulting pattern on a graphical user interface.
The layout pattern is determined depending on:
[0006] Thermal response of the tools stored in a database.
[0007] Radial and axial variance in autoclave heating, the slow
responding tools being laid out in regions of high heating and the
fast responding tools being located in regions of low heating.
[0008] Uniform airflow around the load.
[0009] Feasibility of loading in the indicated pattern.
[0010] However, Lockheed-Martin give no detailed directions about
how a layout engine should be written and what calculations it
should perform, particularly as regards uniformity of airflow.
SUMMARY OF THE INVENTION
[0011] The present invention is based on the premise that in order
to be in a position to treat loads that differ in mass, shape and
cross-section along their length and to improve the chance that
the whole load can receive the intended heat treatment, it is
inherently better to modify the characteristics of an autoclave to
take account of the characteristics of the load rather than to
accept whatever characteristics the autoclave happens to have and
modify the characteristics of the load.
[0012] One problem that arises when complex loads are heat treated
in autoclaves is that at different positions along the autoclave
there may be differences in the relative position or the
cross-section of the load, said differences in an autoclave with
axial gas circulation changing the speed of the circulating gas
and hence of heat transfer to the load.
[0013] That problem is solved according to the invention by a
method of heat treating a load as aforesaid in an autoclave, which
method includes circulating heated gas within the load space by a
plurality of gas circulation means spaced along its length and
each causing the heated gas to circulate generally non-axially of
the load space and/or to impinge non-axially onto the load.
[0014] Thus the invention may comprise an autoclave for
heat-treating a load, said autoclave comprising:
[0015] a shell defining a pressurizable heating chamber;
[0016] means within the shell defining a load space;
[0017] at least one door for closure of the heating chamber and
for permitting entry of loads into and discharge of loads from the
load space;
[0018] means for heating the gas within the load space, and
[0019] a plurality of impellers and respective driving means
spaced apart at intervals along the heating chamber each for
non-axial circulation of gas in a respective zones of said load
space.
[0020] The invention further comprises an autoclave for heat
treatment of a load whose position relative to the autoclave
and/or whose cross-section may vary along the load, said autoclave
comprising:
[0021] a chamber for receiving the load, said chamber having first
and second ends and an axis that passes through said first and
second ends, the wall of said chamber providing the first end;
[0022] a door providing the second end of the chamber and giving
access for insertion and removal of the load;
[0023] means for heating gas in said chamber; and
[0024] heated gas circulation means arranged to produce a pattern
of circulation in which heating gas circulates generally
non-axially of the load space and/or impinges non-axially onto the
load.
[0025] Another problem that arises when complex loads are heat
treated in autoclaves is that at different positions along the
autoclave there may be differences in the thermal characteristics
of the load, which in an autoclave with axial gas circulation may
be difficult to overcome merely by adjusting the distribution of
the load to take account of known or forecast differences in heat
transfer rate with position.
[0026] That problem is solved according to the invention by a
method of heat treating a load whose thermal characteristics vary
with position along the load, which method comprises heating the
load in an autoclave having a plurality of gas circulation means
spaced along its length and each producing a zone for circulation
of heating gas, the gas circulation in said zones being
independently controllable. With this method, a load of variable
geometry and mass can be heated at different temperatures along
its length or at different speeds of gas circulation in order to
raise the temperature of the mass as a whole at a uniform rate.
[0027] The invention further provides an autoclave or oven for
heat treatment of a load whose thermal characteristics may vary
along its length, said autoclave comprising:
[0028] a chamber for receiving the load, said chamber having first
and second ends, the wall of said chamber providing the first end;
[0029] a door providing the second end of the chamber and giving
access for insertion and removal of the load;
[0030] means for heating gas in said chamber; and
[0031] a plurality of gas circulation means spaced along the
length of the autoclave and each producing a zone for circulation
of heating gas, the gas circulation in said zones being
independently controllable.
[0032] The aforesaid problems are not mutually exclusive, and
indeed will commonly occur together.
[0033] Thus in a further aspect the invention provides an
autoclave for heat treatment of a load whose position relative to
the autoclave, whose cross-section and/or whose thermal
characteristics may vary along the load, said autoclave
comprising:
[0034] a chamber for receiving the load, said chamber having first
and second ends and an axis that passes through said first and
second ends, the wall of said chamber providing the first end;
[0035] a door providing the second end of the chamber and giving
access for insertion and removal of the load;
[0036] means for heating gas in said chamber; and
[0037] a plurality of gas circulation means spaced along the
length of the autoclave and each producing a zone for circulation
of heating gas, the gas circulation in said zones being
independently controllable and said gas circulation means being
arranged to produce a pattern of circulation in which heating gas
impinges non-axially onto the load.
DESCRIPTION OF PREFERRED FEATURES
[0038] The above autoclave is divided longitudinally into a
sequence of treatment zones, and preferably the means for
controlling the rate of heat transfer between the heating gas and
the load in each zone comprises an impeller. It has been found
that the impeller can provide a dual function: firstly adjusting
the speed of the circulating gas and hence the coefficient of heat
transfer to the load and secondly acting as a source of heat for
the heating gas because of the high power input which is required
in practice to produce gas circulation at the required velocity or
mass flow, especially at the typical working pressures of 5-25 bar
found in the autoclave, means preferably being provided for
independently adjusting the friction heat generated in said
heating gas by the impeller of each treatment zone. It has been
found in practice that providing one or more thermocouples in the
autoclave measuring gas temperature and load temperature and using
a difference between measured and required temperatures to
generate a difference signals to adjust the impeller speeds and
hence the amount of friction heat that the impellers generate
provides fine temperature control and can enable load temperatures
of .+-.1.degree. C. to be achieved during the load heating phase
of the autoclave processing cycle. The means for controlling the
rate of heat transfer between the heating gas and the load in each
zone preferably also comprises cooling means for cooling gas
circulating in said zone. The ability to adjust the rate of gas
flow in zones along the length of the autoclave and optionally in
different regions within a single zone is of particular value
during the cooling part of a treatment cycle in order to take
account of differences in gas flow path around different regions
along the load and also differences in the heat capacities of tool
and workpiece at different regions along the load. The provision
of independent adjustments for primary heat zone-wise or in a
group of zones, mass flow rate in each zone, friction heat
generation in each zone and cooling in each zone enables a high
degree of stability to be achieved.
[0039] As regards heating the circulating gas, electricity is one
possible heat source, in which case it is convenient to provide an
independent heater for heating gas circulating in each zone. In
the case of gas, steam or oil heating e.g. using radiant tubes,
the heating means may comprise at least a first heater that is
common to a group of zones and typically at least first and second
heaters for first and second groups of zones. Control means may be
adapted to create differential conditions in at least one zone in
a time-varying pattern, thereby to transfer gas axially between
zones.
[0040] The pattern of gas circulation is non-axial and is
transverse to the axis or longitudinal dimension of the load space
which typically has an aspect ratio greater than one. Means are
preferably provided in each zone for establishing a
circumferential bilobal circulation of gas, the plane of said
circumferential bilobal circulation being generally at right
angles to said longitudinal direction or axis. In order to achieve
such a circulation pattern, the autoclave may further comprise
spaced oppositely facing inner wall portions defining with a side
wall of the chamber spaces for flow of gas along the circumference
of said chamber, a first aperture defined between said inner wall
portions for entry of gas into said flow spaces, and a second
aperture defined between said inner wall portions opposite the
first aperture for gas leaving the flow spaces and flowing through
said chamber towards the first aperture. In order to increase the
mass flow of heated gas traveling over the load and hence the
coefficient of heat transfer, it is preferred to provide means for
reducing the volume of gas above the load and hence increasing the
speed of the gas. For this purpose at least one gas deflection
means is preferably provided in said autoclave for varying the
velocity of gas adjacent to the load, and actuator means is
preferably connected to the gas deflection means for adjustment of
the position thereof from the exterior of said chamber.
[0041] The above autoclave may be used for heat treating an
elongated article with its longest dimension directed generally
parallel to the axis of the autoclave, and the heat treatment is
carried out so as to heat the article according to a predetermined
pattern, usually so that it rises in temperature evenly along its
length. The article may be non-linear in its longitudinal
direction e.g. a panel for an aircraft wing having both
longitudinal and transverse curvature.
[0042] Typically the load comprises articles each consisting of a
workpiece in contact with a tool, the workpiece being heat treated
and being shaped by contact with the tool as in the forming of
composites by a combination of evacuation of the interface between
the composite and the forming tool and application of pressure of
the hot gaseous atmosphere within the autoclave. The autoclave has
as one of its main uses the treatment of a single workpiece and a
single forming tool extending along a major part of the internal
space of the autoclave. It can also be used for the het treatment
of a plurality of workpieces and forming tools extending in
side-by-side relationship along a major portion of the internal
space of the autoclave. It may also be used for the heat treatment
of a plurality of workpieces and forming tools disposed end to end
in series along the internal space. Use of an autoclave to make
shaped parts is not limited to the production of parts in curable
plastics or composite materials, but also includes parts made in
metal that are required to undergo a heat treatment to change
their shape or improve their properties.
[0043] Age creep forming is a process that can be used for forming
metallic plates into a desired contoured shape, for example to
give an aluminium or alloy wing panel its aerofoil shape. The
practical steps involved in age creep forming are closely
analogous to those involved in moulding a curable composition.
Following machining, a metal panel is placed onto a mould and
covered with a sheet of a plastics material that resists high
temperatures. The assembly is placed in an autoclave, the air
beneath the sheet is evacuated and the interior of the autoclave
is pressurised, forcing the panel tightly onto the mould, and the
autoclave is heated e.g. to about 220.degree. C. After a period of
e.g. 24 hours the panel is cooled to room temperature and removed
from the autoclave. U.S. Pat. No. 4,188,811 (Chem-tronics)
discloses a process for shaping a metallic workpiece that uses a
single-faced die and the use of heat and pressure to conform the
workpiece to the shape of the die surface by creep forming. In
particular, the patent discloses a process for altering the shape
of a metallic workpiece which comprises the steps of: placing the
workpiece on the face of a die which face has a configuration
wanted in the workpiece and concurrently heating said workpiece
and applying pressure thereto via a compliant body composed of
discrete pieces of a heat resistant, pressure transmitting
material and located on that side of the workpiece opposite the
die, the temperature to which the workpiece is heated and the
pressure applied thereto being so correlated as to cause the
workpiece metal to flow plastically at a stress below its yield
strength into contact with the face of said die to thereby impart
the wanted configuration to the workpiece. More recent references
to creep forming occur in U.S. Pat. Nos. 5,345,799 (Aliteco AG)
and 6,264,771 (Bornschlegel).
[0044] As previously mentioned, one preferred gas circulation
pattern within the zones of the autoclave is bilobal with a plane
of bilobal circulation in each zone directed transversely of the
axis of the autoclave, and wherein gas at a central region of said
bilobal circulation impinges onto and/or passes through the tool.
The tool advantageously has a gas-receiving opening that faces a
location where gas that has been traveling along a circumferential
part of its circulation path enters a central part of its
circulation path in which the gas travels across the load space A
second preferred gas circulation is tetra-lobal with first and
second impellers disposed, when the autoclave is viewed in
section, at the 0.degree. and 180.degree. positions and having
discharge outlets defined by discontinuities in the inner
load-space defining wall at the 90.degree. and 270.degree.
positions so that first and second inward flows of heated air can
be produced which can impinge on a workpiece from opposite
directions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0045] How the invention may be put into effect will now be
described, by way of example only, with reference to the
accompanying drawings, in which:
[0046]
FIG. 1 is a
simplified diagrammatic view in longitudinal vertical section of a
known autoclave;
[0047]
FIG. 2 is a
simplified diagrammatic view in vertical section of an autoclave
with air circulation from one end and with an elongated workpiece
that is curved at least along its longitudinal direction;
[0048]
FIG. 3 is a
simplified diagrammatic view of an autoclave and workpiece similar
to that in FIG. 2 except that an air circulation at least part of
which is radial is produced by means of a series of impellers
located at intervals along the autoclave;
[0049] F
IGS. 4a-4h are
views of the autoclave in transverse section showing the pattern
of air circulation;
[0050]
FIGS. 5a-5c are
upper, middle and lower portions of a diagram of the autoclave and
its associated control systems;
[0051]
FIG. 6 is a view
of another autoclave according to the invention in transverse
section;
[0052]
FIGS. 7a-7b and 8a-8c
are views of the autoclave of FIGS. 3 and 4a-4h showing schemes
for the creation of pressure waves for bringing about movement of
air axially from one zone to another;
[0053]
FIGS. 9 and 10 are
respectively
a side elevation and a view in transverse section of a further
autoclave according to the invention; and
[0054]
FIGS. 11 and 12
are respectively a diagrammatic partly sectioned side view and a
view in transverse section of a yet further autoclave according to
the invention,
[0055]
FIG. 13 is a
circuit diagram of one of the burner and heat exchanger units that
form part of the autoclave; and FIG. 14 is a view of one of the
heating and cooling zones of the autoclave showing one form of
cooler.
DETAILED DESCRIPTION OF PREFERRED
EMBODIMENTS
[0056] The present invention is particularly, though not
exclusively applicable to autoclaves of high aspect ratio, high
volume or both high volume and high aspect ratio. An autoclave of
relatively small size but high aspect ratio might be used, for
example, in the heat treatment or shaping of yacht masts and could
have a length of e.g. 20 meters and a diameter of 1 meter, with an
internal volume of about 12 m.sup.3. In the case of both high
volume and high aspect ratio, the autoclave may be 15 meters in
length and in a typical installation may be about 35 meters in
length, there being no specific upper limit in length because of
the non-axial flow pattern that has been selected. The volume of
the autoclave may be more than 250 cubic meters, often more than
500 cubic meters and in a typical installation more than 750 cubic
meters. The aspect ratio of the load space within the autoclave
(length to diameter or maximum transverse dimension) may be more
than three, usually more than 5 and in a typical installation
about 7.
[0057] The problems that arise when a component such as a panel
for a large aircraft wing, said panel arising from where the wing
joins the fuselage of the aircraft, is to be heat treated in a
conventional autoclave 50 with axial air circulation via fan 52
are shown in FIG. 2. The wing panel 54 may be, for example, of
aluminum alloy of section typically 40 mm towards its base and 4
mm towards its tip with curvature both transversely and
longitudinally and with change of section gradually all along its
length. Tooling of steel plate that is typically about 10 mm thick
supports the panel 54 that is to be creep formed and the panel or
workpiece is to be pulled down onto a datum surface defined by
screw jacks distributed along and across the tool. The panel is
covered by a rubber sheet and is pulled down onto the datum
surface by vacuum and by the pressure of the gas within the
autoclave, which will typically be at a pressure of up to 20 bar
and up to 200.degree. C.
[0058] For creep forming, a typical specification for the thermal
regime to be undergone by the panel 54 is that it should be heated
to .+-.2.degree. C. of its target temperature and that the
thickest part of the panel should achieve its target temperature
within one hour of the thinnest part. Heat reaches the panel
mainly by impingement of the heating gas on the rubber cover
sheet, so that it is necessary to model convection in air,
conduction through the rubber cover sheet and the thermal capacity
of the aluminum panel.
[0059] In FIG. 2 the tooling is omitted for the sake of clarity.
As apparent, gas flows axially away from fan 52 between the
sidewall 56 of the autoclave and inner wall 58 as indicated by
arrows 60 and returns inward to provide an axial return flow 62.
Radiant tube heating elements (not shown) are provided between
walls 56 and 58. The autoclave is formed in three segments, with a
segment 62 furthest from the fan, a central segment 64 and a
segment 66 nearest to the fan. In the segment 62 furthest from fan
52 the panel 54 is at a relatively wide spacing from inner wall 58
and gas flow is relatively slow. In the middle segment 64 the gap
between panel 54 and inner wall 58 has narrowed and gas flow has
accelerated with a corresponding increase in heat transfer
coefficient. In the segment 66 nearest the fan 52 because of the
reverse curvature of the panel 54, the heating gas no longer
impinges directly on the rubber over-layer and instead part of it
by-passes it to return directly to the fan as indicated by arrow
68, while the remainder becomes turbulent as indicated by arrows
70. In order to overcome the problems imposed by the differing gas
flow regimes and consequential differences in load (workpiece
and/or tool) heat transfer coefficient, the fan 52 has to produce
a very high gas flow which is against a high static head resulting
from the length of the flow paths and obstruction provided by the
heaters in the outflow part of the path and the load in the return
part of the path. Gas flow through the tooling does not contribute
significantly to processing because the predominant gas flow is
over the surface of the panel 54 as shown. Inevitably one end of
the load is cooler than the other. MW of fan power is required,
with high capital cost, and there is a zero diversity factor.
[0060] The invention overcomes these problems, as shown in FIG. 3
by providing a generally non-axial flow pattern of heating gas
with gas circulating circumferentially between inner and outer
walls of the autoclave where it can be heated by flow past radiant
tubes and traveling across the load space so that the flow
impinges onto the load, as indicated by arrows 72. The pattern of
gas circulation in planes directed generally at right angles to
the axis of the autoclave provides the opportunity to divide the
load space into a multiplicity of processing zones in which gas
flow (speed and temperature) is independently controllable to
maintain uniform temperature of a load with diversity factors. In
the present embodiment, circumferential flow for each segment is
directed through and then downwardly from cooling units mounted at
and spaced axially along the uppermost region of the autoclave,
the cooling units being useful firstly for adjustment of the
temperature of the circulating gas during a heating part of a
treatment cycle and preventing over-heating of a lightly-loaded or
non-loaded region of the autoclave and secondly for assisting of
return of temperature to ambient during a cooling part of the
treatment cycle. Cooling units 74, 76 and 78 are provided in the
segment 62, cooling units 80, 82 and 84 are provided in the
segment 64 and cooling units 88, 90 and 92 are provided in the
segment 66. The circumferential flow enters the cooling units and
is then directed downwardly towards the load as shown. For the
return part of the travel of the gas, impellers in matching units
under the floor of the autoclave return gas from the load space
for flow circumferentially between the walls 56, 58.
[0061] Use of a multiplicity of impellers located at intervals
along the autoclave in addition to sharing the load gives rise to
a reduced static head at each impeller, so that smaller motors can
be used without compromising the air movement requirements of the
autoclave. Smaller motors are easier to manufacture and install
and provide improved control firstly because the transverse flow
path or paths controlled by each motor is or are relatively short
compared to the axial flow path of conventional autoclaves and
because adjustment of impeller speed can be used not only to
control mass flow but also to control the amount of friction heat
imparted at each impeller which especially at relatively high
autoclave internal pressures can provide a significant proportion
of the thermal input. The motors preferably have a rated power
output greater than that needed for gas circulation, so that
additional power can be used for friction heating of the gas in
the zone. Use of friction heating which may be important in the
dwell part of the processing cycle is facilitated if the duty is
shared by a plurality of motors and impellers located at intervals
along the autoclave and not simply by a single motor in an end
wall as in prior art autoclaves.
[0062] A cross-section of the autoclave of FIG. 3 is shown in
FIGS. 4a-4h in which it is apparent that the autoclave has a side
wall 56 and oppositely facing arcuate side walls 58, 58a defining
with the wall 56 circumferential gas circulation spaces 95
containing gas-fired radiant heater tubes 96. As is apparent from
FIG. 4a, each segment has six radiant tubes per side fired by six
gas burners giving twelve radiant tubes and gas burners per
segment. It will be appreciated that gas fired radiant tubes are
only an example and that other forms of heating may be employed. A
load space 98 is defined between the inner walls 58,58a, ceiling
100 and floor 102, the cooling units, in this case the unit 76
being ceiling-mounted and the impellers 104 being floor-mounted.
Load 106 is present in the load space and takes the form of a
panel to be formed and a forming tool with a blanket of deformable
material or a rigid second part of the tool covering the panel and
with means (not shown) for applying a vacuum under the panel to
assist the forming operation.
[0063] As shown by the arrows in successive figures, heating gas
from the underside of the load 106 passes into the impeller or fan
104 (FIG. 4b), from which it is discharged towards gas circulation
spaces 95 through which it flows circumferentially (FIGS. 4d, 4e),
until it reaches the cooling unit 76. The impeller or fan will
normally be a centrifugal fan having a casing, an inlet connected
to an opening in the floor (in this embodiment) and having
oppositely facing first and second outlets directed transversely
of the autoclave. In this way the gas from the load space flows
zone-wise through the surrounding space at opposite sides of the
autocalve towards the respective cooling unit e.g. 76. The gas
leaving the cooling unit passes downwardly onto or into and then
through a forming tool that forms part of the load 106 (FIGS. 4f,
4g, 4h) before returning to the underside of the load (FIG. 4b).
Accordingly there is established in each zone a circumferential
bilobal circulation pattern, with the load being in a central
region or load space where the gas flow from the two lobes becomes
combined and where the gas travels transversely of the load space,
in this case downwards and can impinge onto the tool to create a
local turbulent heat-transferring flow pattern.
[0064] Internal lagging 59 of rockwool or other inert thermally
resistant insulating material is provided as a lining to the outer
shell of the autoclave to reduce heat transfer the autoclave shell
during heating and hence the thermal stress on the shell, and also
to reduce heat transfer from the shell back into the load space
during the cooling phase of a treatment cycle. In this way the
energy requirement for each cycle is closer to that required for
heating and cooling the load or process mass and less energy goes
into heating and cooling the total mass of the autoclave which
includes the mass of the vessel or shell and its door or doors.
Energy that goes into heating or cooling the autoclave shell
during each treatment cycle is wasted energy and is desirably
minimized. The arcuate side walls 58,58a, ceiling 100 and floor
102 form a continuous surface so that all the air flow from
impeller 104 passes through the gas circulation space 95 to the
cooling unit 76 and there are no air gaps which could give rise to
overspill. The absence of air gaps is not critical and, for
example, ports could be formed in the sidewalls 58, 58a to direct
heated air onto particular regions of a workpiece and forming tool
but this is less preferred because such ports are likely to be
specific to a particular tool and workpiece, so that the autoclave
would have to be set up specifically for each job.
[0065] The layout of the autoclave of FIG. 3 is diagrammatically
shown in FIG. 5, which is a schematic view of the autoclave and an
associated control system. The segments 62, 64, 66 are heated by
radiant tubes 110 and fired by gas burners 112 as described e.g.
in EP-B-0176508 and EP-B-0333389. The radiant tubes are
represented in the diagrammatic section that forms part of FIG. 5c
as G1-G12, and are directed axially, each passing through three
heating zones each defined by independently controllable coolers
74, 76, 78, 80, 82, 84, 88, 90, 92 and by independently
controllable impellers 114, 116, 118; 120, 122, 124; and 126, 128,
130. The gas burners for each segment have associated
thermocouples G1-G12 which measure the temperature of the
circumferential air and pass signals to a respective one of
segment heater logic units ICU 7, ICU 9 and ICU 11 that in turn
pass command signals to progressive gas burner controllers 132,
134, 136 associated with the respective segments (cold<SP). The
three heater logic units receive heat enable commands 138, 140,
142 from fan and cooler logic units ICU 6, ICU 8 and ICU 10 for
the three segments.
[0066] In the first zone, thermocouples A1 and A2 measure the
temperature of the flow exiting impeller 114, and thermocouples A3
and A4 measure the flow entering cooler unit 74 the difference
providing a measure of the heat taken up by the load or during a
cooling part of the cycle energy released from the load, the
thermocouples occurring in pairs because of the bilobal flow
pattern. In the second zone thermocouples A5 and A6 measure the
temperature of air exiting impeller 116 and thermocouples A7 and
A8 monitor the temperature of air entering cooler unit 76. In the
third zone, thermocouples A9 and A10 monitor the temperature of
air leaving impeller 118 and thermocouples A11 and A12 monitor the
temperature of air entering cooler unit 78. Signals from the
twelve thermocouples are supplied to the unit ICU6, ICU7 or ICU8
which in addition to providing gas burner command signals also
provides command signals Z1, Z2, Z3 to proportional cooling valves
144, 146 and 148 (Hot>SP) and similar signals to friction heat
inverters 150, 152, 154 (Hot.+-.SP) for the impellers of each
zone. Accordingly if the gas in any zone is sufficiently below the
set point, then the gas burners 112 of the radiant tubes 1109 can
be switched on. If the temperature of the gas in any zone is above
the set point, cooling can be initiated, and adjusting inverter
power for each zone can compensate fine deviations in gas
temperature.
[0067] The operation of the autoclave depends not only on
measurements of heating system temperature and of gas temperature
but also on measurement of load (tool or workpiece) temperature.
For that purpose, load sensor thermocouples 1-33 and reference
thermocouples 1-4 are allocated to segment 62, load sensor
thermocouples 34-67 and reference thermocouples 5-8 are allocated
to segment 64 and load sensor thermocouples 68-100 and reference
thermocouples 9-12 are allocated to segment 66. Logic units ICU
1-ICU 4 feed signals for the hottest and the coldest of groups of
thermocouples that they monitor to temperature control logic ICU
5. In this way the temperature of the process mass
(tooling+workpiece) may be sensed zone-wise and the control unit
can respond both to deviations of the whole process mass from the
intended temperature and also to deviations from the intended
temperature within individual zones.
[0068] As shown at 156 (FIG. 5c), the processing cycle to be
carried out by the autoclave which is stored at device 156 will
normally include a relatively simple pressure cycle 158 that
provides for pressurization of the autoclave, dwell at pressure
and release of pressure on completion of the cooling phase of the
treatment cycle. A workpiece processing cycle 160 that coincides
with the pressure cycle has a predetermined rate of temperature
rise, dwell time at the intended processing temperature and a
predetermined rate of temperature return to ambient. The cycle of
processing gas temperature 162 is usually more complex, with the
gas temperature leading the workpiece temperature during the
heating part of the cycle, and with a ratio between those
temperatures being a factor that determines the coefficient of
heat transfer. From device 156, information is supplied to
pressure controllers 164 for the three segments, and temperature
set points are supplied to air/load temperature ratio controllers
166, 168, 170.
[0069] If any of the workpiece or control thermocouples indicate
too low a temperature, then logic ICU5 (FIG. 5c) supplies
information to dwell/cold/hold logic 165 which is also supplied
with the temperature set-point and which may return a signal to
device 156 to vary e.g. the air temperature set point. Device ICU
5 is also concerned with spread control. If one of the
thermocouples in the group is at or near the required temperature
while others are at too low a temperature, then the logic causes
supply of additional heat to be reduced (Hold) until the
temperature of the cold areas has caught up. If a thermocouple in
any of the segments is hot, then a signal is sent to the ratio
controller 166, 168 or 170 to reduce heating in the segment where
the thermocouple in question is sited. The signal is passed both
to the segment heater logic unit ICU 7, 9 or 11 and also to the
fan and cooler logic units for the adjoining segments. Thus if one
of the thermocouples 1-33 or one of the reference thermocouples
1-4 gives a HOT signal, then a signal is passed to the ratio
controller 166 for segment 62 for reduction of gas burner heat
and/or impeller friction heat and to pulse speed input of fan and
cooler logic ICU 8 for adjoining segment 64 to adjust the friction
heat developed any or all of the zones in that segment. Similarly,
if one of the thermocouples 34-67 or one of the reference
thermocouples 5-8 gives a HOT signal, then a signal is passed to
the ratio controller 168 for segment 64 for reduction of gas
burner heat and/or impeller friction heat and to pulse speed input
of fan and cooler logic units ICU 6 and ICU 10 for adjoining
segments 62 and 66 to adjust the friction heat developed any or
all of the zones in those segments. Again, if one of the
thermocouples 68-100 or one of the reference thermocouples 9-12
gives a HOT signal, then a signal is passed to the ratio
controller 170 for segment 66 for reduction of gas burner heat
and/or impeller friction heat and to pulse speed input of fan and
cooler logic ICU 8 for adjoining segment 64 to adjust the friction
heat developed any or all of the zones in that segment. The
control circuit therefore enables a relatively coarse response to
be carried out segment-wise to major low or high deviations of
gas, tool or workpiece temperature, and more finely tuned
zone-wise responses to smaller temperature fluctuations from the
intended heat treatment cycle using zone-wise cooling, zone-wise
adjustment of the rate of mass flow by change in impeller speed
and zone-wise change in friction heat generation.
[0070] As indicated above load sensor couples TC-1 to TC-100 are
provided may be positioned e.g. at the underside of the tool,
and/or at the interface between the tool and the workpiece and/or
to the free surface of the workpiece. The ability to control the
friction heat supplied via the impellers on the basis of measured
local tool temperature is a significant advantage of the
invention. It has also been found that although the heat from the
gas burners is important during heating to the required process
temperature, when working at pressures of e.g. 7-15 bar the
friction heat from the impellers can provide most if not all of
the heat required to maintain the required stable temperature
within the load space. A practical embodiment of the autoclave
described above has been constructed by the applicants and has
achieved a total spatial uniformity within a 5.5 meter diameter
and a 40 meter length of .+-.1.2.degree. C. immediately on
installation, and with a tool in place has achieved a total
spatial uniformity of .+-.1.8.degree. C. The applicants expect to
achieve even greater spatial uniformity when the autoclave is
finally configured and tuned and in particular to achieve a
spatial uniformity with a tool in place of .+-.1.0.degree. C. or
better.
[0071] FIG. 6 is a cross-section of an alternative embodiment of
an autoclave of the invention in which each zone is electrically
heated with an overhead heater and cooled as required by a floor
radiator, the flow of gas across the load chamber impinging onto a
molding tool from its underside. One possible use of the autoclave
is for the molding and curing of large panels of resin reinforced
carbon fiber or other composite materials for use in airliners.
The autoclave has sidewall 180 and inner walls 182, 182a defining
passages 184, 184a for circumferential flow of heating gas
together with a ceiling 186 and a floor 188. An impeller 190
brings about flow of gas from load space 191 through electrical
heaters 192, 192a and through passages 184, 184a to radiator 194
which contains cooling elements and from which the gas returns to
the load space 191. Gas entering the load space passes through
trolley 196 and tool 198 so that it impinges on the underside of
the molding surface of the tool. The panel to be molded is on the
upper surface of the tool and is of negligible thickness, and it
has been omitted for the sake of clarity. The upper face of the
panel may also need to be molded e.g. because it has one or more
upstanding integrally formed ribs, and for that purpose the carbon
fiber lay-up may be covered with a second part of the tool, also
omitted for clarity. Gas flows along the underside of the tool
towards the periphery of the autoclave as shown by the arrows, and
is returned towards the tool so as to heat the upper mold part
that rests on the carbon fiber lay-up by means of movable baffles
200, 200a whose positions are adjustable from outside the
autoclave by actuators 202, 202a. A gap between the baffles 200,
200a permits the gas to return to the impeller 190 as shown. The
baffles 200, 200a reduce the volume of gas above the tool 198,
with the consequences that the velocity of the gas and hence its
coefficient of heat transfer to the tool is increased.
[0072] In order to minimize variations in load temperature axially
of the autoclave, it may be desirable to provide means for
conveying gas axially from one zone to the other. In order to
achieve such conveyance, a cyclically varying pattern of
circulation conditions may be applied to at least one zone that
shifts axially of the autoclave. For example, adjacent zones may
vary in temperature individually and cyclically as in FIGS. 7a and
7b. Alternatively a zone of high temperature may be followed by
two zones of lower temperature as in FIGS. 8a-8c. The cyclical
variation in temperature from zone to zone may conveniently be
achieved by adjustment of the friction heat of the impellers
114-130 via logic units ICU 6-ICU 10 and inverters 150-154.
[0073] Various modifications may be made to the illustrated
embodiments without departing from the invention.
[0074] For example, the drawings have illustrated cases where
firstly the heater is in the ceiling and the cooling radiators are
in the floor, gas flowing downwardly through the load space to
provide hot air impingement from above, and secondly the cooling
radiators are positioned in the ceiling and the hot air is ducted
to rise from under the floor into the load space through which it
flows upwardly into the base of the tooling to provide hot air
impingement from below. Although these airflow directions may
often be convenient, the direction of airflow is arbitrary and
could for example be side to side, the heater and cooling radiator
being correspondingly placed. Furthermore, the flow can be
established using more than one impeller per zone, the fans being
located above and below the workpiece and tool or to either side
of the workpiece and tool to provide hot air imingement from above
and below or from opposed sides.
[0075] In FIG. 9 there is shown an autoclave having a single large
vessel rather than three vessels joined in series, and having an
aspect ratio of less than 3. One end 222 of the vessel is closed
and the other end is closable by a door 224 which can be
manipulated by gantry crane 226. The autoclave is divided
longitudinally into heating zones as in the previous embodiments,
and a cross-section of one such zone appears in FIG. 10. A pair of
impellers 228, 230 are located 180.degree. apart within the shell
232 which is internally insulated by insulation 234 e.g. of
rockwool. Sidewalls 229, 231 separate the load space from heaters
236, 238 which occur in two banks each associated with a
respective impeller. Also associated with each impeller is a
cooler (not shown) for forced cooling of the gas within the
autoclave during the cooling phase of each tratment cycle. The
walls 229 and 231 are spaced apart at an equatorial region of the
autoclave to define therebetween openings 240, 242 for jets of air
into the load space as shown by arrows 244, 246. These jets can
impinge from opposite directions onto a workpiece and tool when
present in the load space, and a gas flow can be eatablished which
is 4-lobal when viewed in transverse cross-section. Such a
multiple zone arrangement may be desirable where it is required to
maintain a high level of mass air flow whilst retaining motors of
sensible proportions. The speeds of the impellers 228 and 230 may
be controlled individually by respective control means to provide,
if desired, not only separately controllable heating and cooling
for each individual zone along the autoclave but also separately
controllable heating and cooling for individual regions within
each such zone.
[0076] FIGS. 11 and 12 show a further autoclave according to the
invention in which a generally cylindrical shell 300 is closed at
opposed ends by doors 302, 304 and has an internal load space
divided longitudinally into five independently controllable
heating zones 306a-306e. Each heating zone has an associated
impeller 308a-308e driven by motor 310a-310e, first heat exchanger
312a-312e for heating the gas in the load space by gaseous
combustion products and second heat exchanger 314c (FIG. 14; the
second heat exchangers are not shown in FIG. 11) for cooling the
gas in the load space by contact with water circulating through
cooling pipes. The gas in each zone is heated by heat exchange
with the combustion gas associated with the first heat exchanger
and by the friction heat imparted by the impeller and may be
cooled both during the heating part of the treatment cycle to
assist temperature control of the gas in that zone and during the
cooling part of the treatment cycle to speed up return to ambient
temperature. The control system may be generally as described with
reference to FIGS. 5a-5c, and in particular it may provide for
feedback control of the friction heat generated in each zone by
respective impellers 308a-308e in accordance with temperatures
sensed by thermocouples attached to the load within the autoclave.
The provision of doors 302,304 at each end of the autoclave shell
enables the autoclave to be positioned within a product flow line
so that the untreated products can enter at one end of the
autoclave and be discharged from the other end, the non-axial gas
flow of the invention enabling the doors of the autoclave to be
free of impellers and drive motors for the impellers which would
otherwise add weight and bulk. As is apparent in FIG. 12, the
vessel 300 is lined with one or more layers of thermally
insulating material 316, and the load space is defined by floor
318, sidewalls 320a, 320b and ceiling 322, an opening in the floor
318 leading to the impeller 308c and an opening in the ceiling 322
leading from the heat exchanger 310c into the load space.
Operation of the impeller establishes a toroidal circulation of
gas from the load space between the sidewalls 320a, 320b and then
returning from the first heat exchanger 310c into the load space
as indicated by arrows 324.
[0077] Referring to FIGS. 12 and 13 gas in line 330 and air in
line 332 are fed to burner 334 in combustion space 336. The
products of combustion are fed via line 338 to manifold 340 of
heat exchange pipe array 342. The gas from array 342 after heat
exchange with the gas in the combustion space is extracted via
manifold 344 and line 346 to optional fan 348 and then discharged.
Typically the gas mixture fed to burner 344 is about 300%
over-aerated to moderate the temperature of the products of
combustion passing through the heat exchanger tubes and hardness
and embrittlement of the heat exchanger tubes. Depending on the
heat transfer characteristics required in any particular
installation, the tubes of the array 342 may be plain or may carry
fins, as also may the tubes of the second heat exchanger. The
cooler for each zone may be one or more water-filled cooling tube
arrays located beneath the floor 318 on the upstream or downstream
side of the impeller 308, it may be a serpentine tube 314c (plain
or with cooling fins) located between the walls 320a, 320b and the
insulation 316 or it may be located in the space above the ceiling
322 upstream or downstream of the heat exchanger 312a-312e.
AUTOCLAVE
ASSEMBLIES
GB2216644
Also published as: // EP0333389 // EP0333389 // ZA890183 //
DE68920608
Abstract -- An autoclave
for heat curing of fibre reinforced plastics articles includes a
pressure and vacuum tight tank structure (10, 12) having exposed
internal heat exchange tubes (24) forming two or more loops
through which gas can flow in isolation from the interior of the
autoclave. In one embodiment The gas flow loops open to the
outside of the autoclave and are fed with hot gas from gas burners
(34) at their inlet ends the hot gas being drawn through the tubes
(24) by venturi nozzle air movers (36) at their discharge ends.
The gas within the autoclave is circulated by a fan (40). The use
of the venturi nozzle (36) in place of an impeller makes it
possible to work at higher discharge gas temperatures and to vary
the flow rate through the tubes (24) which can be used for heating
and at a higher gas flow rate for cooling.; In another embodiment
cooling of the autoclave chamber is effected by an air mover means
(36) in the form of an electrically driven variable speed fan (45)
associated with an electro-mechanically controlled solenoid
operated pneumatic value (50) attached to the burner (34).
IMPROVEMENTS
IN
OR RELATING TO AUTOCLAVES
WO8404725
Abstract -- An autoclave
for heat curing of fibre reinforced plastics articles includes a
pressure and vacuum tight tank structure (10, 12) having exposed
internal heat exchange tubes (24) forming two or more loops
through which gas can flow in isolation from the interior of the
autoclave. The gas flow loops open to the outside of the furnace
and are fed with hot gas from gas burners (34) at their inlet
ends, the hot gas being drawn through the tubes (24) by impellers
(36) at their discharge ends. The gas within the autoclave is
circulated by a fan (40). The autoclave has the advantage that the
burners (34) and other moving parts (36, 38) are outside the
autoclave and so are accessible in the event of failure which is
significant in the processing of high value materials such as
carbon fibre. It is of simple construction but provides highly
accurate temperature control. ---
Autoclave Treatment of Large Hollow Articles
GB2416326
Abstract -- A method of
curing or otherwise heat-treating a region of aircraft fuselage or
other large tubular article comprises producing a load by
providing a tubular tool 12 having an exterior forming surface and
contacting said article with said forming surface. A plug 16 is
provided that fits within said tool 12 so that gas flowing through
said tool bypasses said plug 16. The article is then heated and/or
cooled at least partly by flowing heated gas between the plug and
the forming surface. Tooling is also provided for carrying out the
above method. The article comprises resin bonded composites of
carbon fibre, aramid fibre, glass fibre etc. A heater pack 24,
which may be electrical, may be mounted. A cooling heat exchanger
26 removes heat energy from the tool and the article. A gas
impeller 28 is provided. ----
rexresearch.com