Note: Descriptions are shown in the official language in which they were submitted.
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The invention relates to a method for drying
feedstock while it is being treated with pressurized
steam in an autoclave according to the precharacteriz-
ing clause of Claim 1 and to an autoclave according to
the precharacterizing clause of Claim 5.
EP O S38 756 B1 discloses an autoclave for the
pressure-steam treatment of feedstock, for example
moulded blanks made of porous concrete or foamed sili-
cate, comprising an essentially horizontal cylindrical
inner chamber enclosed by an autoclave jacket, and
heating means which serve to generate superheated steam
and whose heat exchanger is disposed in the lateral re-
gions of the inner chamber and is provided with a flue,
thus resulting in a circulating free convection flow.
With existing autoclaves the space available is not al-
ways adequate for optimum positioning and size of the
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heat exchangers with a view to homogeneous drying while
maintaining the permissible top!fIoor temperature dif-
ference. The design of autoclaves to be newly installed
will reflect this.
s It is an object of the invention to provide a
method and an autoclave according to the precharacter-
izing clauses of Claims 1 and 5, which enable a sub-
stantially homogeneous temperature distribution to be
achieved in the feedstock during drying.
This object is achieved according to the char-
acterizing part of Claims 1 and 5.
In the process, the energy transfer, required
for drying, from the heating medium to the feedstock is
ensured with the aid of forced convection in an super-
heated steam atmosphere, the forced convection being
driven via steam ejectors connected to a driving-steam
compressor. During the essentially isobaric drying
phase of, for example, 12 bar a part-volume flow of wa-
ter vapour is abstracted from the autoclave to drive
the steam ejectors, said part-volume flow being small
compared with the volume circulated in the autoclave
for drying, preferably having a ratio of from 1:5 to
1:20, so that the energy input required for forced cir-
culation in the autoclave is kept low. This part-volume
flow is compressed, in a driving-steam compressor (for
example a vapour blower) situated inside or outside the
autoclave, to a slightly elevated pressure level. This
pressure level preferably exceeds the autoclave pres-
sure expected in the drying phase by from about 100 to
900 mbar, but is relatively high according to the ratio
of part-volume flow abstracted to the circulated vol-
ume. Owing to the préssure difference, this driving
steam is then injected, with a laterally uniform dis-
tribution via the steam ejectors disposed in the lat-
eral regions of the inner chamber, so that because ofthe high outflow velocity of the driving steam of, for
example, from 70 to 200 m/s, water vapour is entrained
from the surroundings and the steam mixture thus formed
from driving steam and ambient water vapour (intake
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steam stream) is directed as a steam stream, which may
be many times larger than the driving-steam volume
flow, as far as underneath the feedstock.
Since the driving steam enters the autoclave ln
an essentially vertically downward direction via the
respective outlet orifices, the floor region of the
autoclave can be maintained at the desired temperature,
and flow through the feedstock takes place upward from
below. Because of the considerably larger circulating
steam flow rates, compared with natural convection, the
absolute superheating temperature and the temperature
spread between inlet and outlet of the heat exchanger
will drop while the drying capacity remains the same.
This results in more homogeneous drying between the
various layers of the feedstock and in a reduced
top/floor temperature difference.
Another consequence of the steam ejectors being
used is that, firstly, they can be positioned centrally
with respect to the feedstock, thus ensuring largely
homogeneous drying along the longitudinal axis of the
autoclave, and that secondly only a fraction of the to-
tal circulating steam flow rate has to be passed
through a duct system or piping system and via a
(mechanical or thermal) driving-steam compressor.
The number and arrangement of the steam ejec-
tors in the autoclave can be chosen as required. If the
number of steam ejectors is relatively small, the vol-
ume flow per steam ejector for constant total volume
flow is correspondingly higher, which leads to corre-
sponding lengths of the steam ejectors and, given that
they are of jet-pump type design, especially of their
diffusers, so that, depending on the space available,
they may have to be arranged in an inclined or horizon-
tal position. This may also involve nesting.
Such a method and such an autoclave are espe-
cially suitable for drying porous and preferably min-
eral feedstock, for example made of porous concrete or
foam-silicate.
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Further refinements of the invention can be
gathered from the following description and the depend-
ent claims.
The invention is explained below in more detail
with reference to the illustrative embodiments depicted
in the accompanying figures.
Figure 1 schematically shows a cross-section of
an autoclave comprising one embodiment of a steam cir-
culation system.
Figure 2 schematically shows a cross-section of
an autoclave comprising a further embodiment of a steam
circulation system,
Figure 3 schematically shows an additional em-
bodiment of a steam circulation system.
According to Figure 1 an autoclave is provided
which has an autoclave jacket 1 which encloses an es-
sentially horizontal, circular cylindrical inner cham-
ber 2. At one end, the autoclave jacket 1 is usually
sealed by a dished end, whereas the other end, the
charging end, is provided with a hinged cover. At the
bottom, rails 3 are arranged via which, when the cover
is swung open, a multiplicity of carriages 4 with feed-
stock 5 arranged thereon can be moved into the auto-
clave and moved out of it.
In this arrangement, a steam exhaust 6 in the
top region of the autoclave is provided which leads to
a driving-steam compressor 7 with an upstream or down-
stream heat exchanger 8, which is designed to raise the
steam pressure to a level above the essentially iso-
baric level in the autoclave and to superheat the steam
flowing through. The superheated, elevated-pressure
steam which later serves as driving steam is recycled
into the autoclave via feed lines 9. The feed lines 9
are connected, via T pieces, to driving-steam lines 10
which extend, in the lateral curved regions of the
autoclave in the longitudinal direction thereof and es-
sentially over its length and which are provided with
essentially downwardly directed nozzle-like outlet ori-
fices 11 for driving steam and thus form steam ejec-
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tors. Thus the injected driving steam aspirates water
vapour from the surroundings and directs the steam mix-
ture thus formed as far as underneath the feedstock 5.
The outlet orifices 11 may have any suitable
shape, for example round or in the form of slits ex-
tending in the longitudinal direction of the autoclave.
As can be gathered from Figure 2, the heat ex-
changer 8 for superheating the steam may alternatively,
in contrast to the embodiment of Figure 1, be disposed
inside the autoclave in such a way that the intake
steam stream formed from driving steam and ambient wa-
ter vapour flows through it before coming into contact
with the feedstock 5. In this arrangement, one heat ex-
changer 8 each extends in the corresponding lateral
curved region of the autoclave jacket 1, essentially
over the length of the autoclave.
Expediently, to avoid short-circuit flows, the
respective lateral curved region of the autoclave
jacket 1 is screened off by a baffle 12 which is dis-
posed at the top and the bottom at a distance from theautoclave jacket 1, so that the steam stream, as a re-
sult of forced convection, will flow through the inner
chamber 2 which accommodates the feedstock 5; compare
Figure 2.
Furthermore, it is expedient for the steam
ejectors, as depicted in Figure 2, to be of jet-pump
type design, that respective nozzle-like driving-steam
outlet orifice 11 which is situated at the end of a
branch pipe 13 debouching centrally into an intake pipe
14, which is open at the top and narrows in a funnel-
like manner, and which is adjoined, in a downward di-
rection, by a mixing tube 15 and then a diffuser 16
which is flared towards the steam outlet side. If the
heat exchanger 8 is disposed inside the autoclave, the
diffuser 16 expediently debouches via the heat ex-
changer, as depicted in Figure 2. The steam ejectors,
which are thus of jet-pump type design, are distributed
- uniformly over the length of the autoclave, in particu-
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lar being lined up next to one another without gapsover the length of the autoclave.
If appropriate, a reciprocating mode of opera-
tion may also be envisaged, this being achieved by fur-
ther nozzle-like driving-steam outlet orifices 11',
each located at the end of a branch pipe 13' connected
to a feed line 9', debouching centrally into the dif-
fuser 16 in an upward direction; shown in Figure 2 by
dashed lines. Via an appropriate venting system it is
possible to effect intermittent switch-over in this ar-
rangement, to achieve even more homogeneous, vertical
temperature distributlon in the feedstock 5. In this
arrangement it is expedient for intake pipe 14, mixing
tube 15 and diffuser 16 to have an at least essentially
symmetrical shape with respect to their central plane
transverse to their axis.
The outlet cross-section of the steam ejectors
may, in particular, have a ratio of length to width
greater than 10, so that their flow is of largely, as
it were, two-dimensional character.
Alternatively, intake pipe 14, mixing tube 15
and diffuser 16 may form a uniform pipe having a con-
stant, round or rectangular cross-section.
According to Figure 3, intake, mixing and dif-
fuser section of the steam ejector of jet-pump type de-
sign may, in the longitudinal direction of the auto-
clave, form a single section or a plurality of elongate
sections, a plurality or all of the outlet orifices 11
arranged in the longitudinal direction of the autoclave
debouching into the intake section. To this end, a mul-
tiplicity of essentially vertical branch pipes 13 may
each, at the bottom end, carry a horizontal distributor
pipe 17, so that the latter essentially occupy the
length of the autoclave and each carry a plurality of
spaced outlet orifices 11 or a slit-like orifice 11 ex-
tending in the longitudinal direction of the distribu-
tor pipe 17.
To achieve an essentially homogeneous distribu-
tion of the steam stream, individual control of the
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steam ejectors can be effected with the aid of control
valves between the driving-steam supply and the outlet
- orifice(s) 11.
