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] FIGS. 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. ----
