Note: Descriptions are shown in the official language in which they were submitted.
218~181
Method for autoclaving porous, molded products
The invention relates to a method for autoclaving
porous, molded products according to the preamble of Claim
1.
Such a method, particularly suitable for autocla-
ving porous concrete blocks is known from EP 0 538 755
Bl. This, after a heating phase, provides for a combined
o setting and drying phase followed by a pressure reduction
phase, steam-containing heating medium, for drying
purposes, being blown off in a controlled manner from the
autoclave while the latter is in the setting and drying
phase, and being reused at least for partial heating of
a further autoclave to be heated.
The isobaric drying of porous concrete blocks
with superheated steam initially involves a transition of
the water from the liquid into the gaseous phase at the
block surface, whereas within the block the liquid water
is passed to the surface via a capillary transport. At
this stage the block is isothermal, temperature corres-
ponding to the boiling temperature of the water vapour
partial pressure of the atmosphere surrounding the block.
Delayed heating processes may even result in the tempera-
ture of the block core being below the boiling tempera-
ture.
Eventually, drying at the block surfaces proceeds
so rapidly that the block surfaces can no longer be kept
moist by capillary transport. The peripheral zones of the
block dry out. The material temperature rises in the
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peripheral zones and here leads to superheating with
respect to the as yet moist core of the porous concrete
block. As drying progresses, the peripheral zone grows,
being distinguished by a moisture content below the
equilibrium moisture content and temperatures above the
boiling point.
As a result of the re-evaporation in the follow-
ing pressure reduction phase, the core temperature of the
porous concrete block follows the boiling temperature and
thus results in the core being cooled to 100~C at ambient
pressure.
The superheated peripheral zones do not cool as
rapidly, so that the temperature difference between
periphery and core increases during evaporation. Owing to
the thermal expansion this leads to a release of the
tensile stresses within the peripheral zone, which result
from the drying as a result of shrinkage.
Said re-evaporation causes the dry peripheral
zone to grow, which may lead to an increase in the
tensile stresses and thus to cracking during evaporation.
When the autoclave charge is discharged from a
superheated atmosphere, the block experiences a thermal
shock. After drying and pressure reduction, the block,
having a core temperature of 100~C and material tempera-
tures in the peripheral layers of from 130~C to 180~C, is
in a superheated atmosphere of from 180~C to 200~C.
During discharge the block is exposed, within a few
seconds or minutes, to an environment of about 20~C (shop
floor). This results in rapid cooling of the block
surface and thus in a thermal shock involving an increase
in the tensile stress in the peripheral zones, with the
consequent risk of cracking. The tensile stresses pro-
duced by shrinkage within the peripheral zones are
enhanced by tensile stress due to thermal expansion
(contraction) of the periphery. The risk of cracking is
greatest when the autoclaved material is pulled out since
tensile stresses due to shrinkage and thermal expansion
in the peripheral zones are additive. The operations
proceed very rapidly, so that creeping and relaxation
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cannot contribute significantly to stress relief in the
block.
The object of the invention is to provide a
method of the type mentioned at the outset, which avoids
s cracking of the autoclaved material.
This object is achieved according to the charac-
terizing part of Claim 1.
By virtue of saturated steam being caused to wash
around the autoclaved material after drying, the super-
heating of the peripheral zones can be brought down
effectively, i.e. the temperature difference between the
peripheral zones and the core temperature in the interior
of the autoclaved, material (with a temperature
profile of corresponding shape) is considerably reduced,
so that stresses which occur as a result of local tem-
perature gradients in the material when the material is
pulled out from the autoclave are lowered so considerably
that they can be absorbed by the material without
the risk of cracking.
The temperature difference, thus lowered, between
peripheral zone and core is retained even during the
pressure reduction phase, the core temperature dropping,
in line with the saturated steam curve, down to 100~C at
ambient pressure. Expediently, the cooling by means of
causing saturated steam to wash around the material is
continued into the pressure reduction phase and, if
required, until the material is removed from the
autoclave, causing a further reduction in the temperature
difference between core and peripheral zone. By lowering
the surface temperature, the thermal shock upon opening
of the autoclave and, consequently, the internal stresses
in the block are diminished. As a result, cracking of the
autoclaved material can be avoided.
Remoistening of the molded articles by the
saturated steam cannot take place, since the temperature
of the peripheral zone is above the boiling temperature
of water.
The saturated steam can advantageously be
introduced from below into the autoclave from a separate
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steam source, while the pressure relief valve of the
autoclave is opened to such an extent as to prevent
either the build-up of excess pressure or a drop in the
pressure prior to the pressure reduction phase. The
saturated steam thus passing through the autoclave in
order to cool the ~aterial is expediently used, at
least in part, to heat another autoclave to be heated.
The saturated steam can also be generated in the
autoclave itself, in which process, by controlled injec-
tion of liquid water into the autoclave, saturated steam
is formed therein with the reduction and utilization of
the superheat.
The cooling effect of the saturated steam can be
brought about by means of forced convection or by natural
convection.
It is further expedient, after the pressure
reduction phase, when ambient pressure has been reached,
to cool the steam atmosphere in the autoclave and thereby
generate a partial vacuum, preferably in the range of
from 0.1 to 0.5 bar absolute. This can be brought about,
for example, by water injection or via cooling surfaces,
while condensate produced as a result of cooling is
discharged.
The cooling surfaces may, for example, be formed
by cooling coils situated in the autoclave, which are
disposed to the side of and below the material, while the
condensate is discharged via separate collection trays or
a general water drainage system.
If cooling takes place via water injection, the
same water injection apparatus can be used as for genera-
ting saturated steam.
Cooling can further be achieved by using an
injection condenser or a surface condenser between a
waste steam line of the autoclave and a vacuum pump which
3s may be in the form of a jet pump or a water ring pump.
As well as causing additional drying of the
material, such a pressure reduction by cooling further
reduces the temperature both in the interior and in the
peripheral zones, thus further reducing the risk of
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cracklng .
This pressure reduction can take place relatively
rapidly, since the diffusion of steam from the pores of
the material is virtually unimpeded and the heat required
for evaporation does not first have to be transferred but
is already stored in the material. The advantage of this
post evacuation is that the heat potential in the moist
material is not just utilized as far as 100~C, but as far
as the boiling point of the lowest pressure. A cooling
section for the material taken from the autoclave can be
dispensed with, and the material taken out can imme-
diately be handled without difficulty.
Since prior to the autoclave being opened, air
must again be admitted to bring the autoclave to ambient
pressure, the air mixes with the steam atmosphere within
the autoclave, producing slow further cooling of said
atmosphere and consequently of the peripheral zones of
the material, thus causing an additional further
reduction in the stresses when the material is pulled
out.
The accompanying temperature-time diagram schema-
tically depicts an autoclaving operation, the core
temperature of the molded products, which follows the saturated
steam temperature, being shown as a continuous line and the peri-
pheral zone temperature of the molded products as a dashed line.
When the autoclave is run up, the core tempera-
ture lags behind the peripheral zone temperature until
the two are eventually, in the holding and drying phase,
equal at first. During drying by capillary transport the
two temperatures initially remain equal; subsequently,
capillary transport is no longer sufficient to keep the
peripheral zones moist. They dry out. The material
temperature rises in the peripheral zones and here leads
to superheating with respect to the as yet moist core
which is at saturated-steam temperature.
At the end of the drying process isobaric flush-
ing with saturated steam is initiated, resulting in a
corresponding reduction in the temperature difference
between core and peripheral zone. Said flushing can be
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maintained during the subsequent pressure reduction
phase, so that at the end of the pressure reduction
phase, when ambient pressure has been reached, the
temperature difference between core (100~C) and peri-
s pheral zone will be so low that the stresses occurring as
a result of the thermal shock in the course of the
autoclave being opened cannot result in cracks in the
moulded articles.
If, instead of the autoclave being opened when
said end of the pressure reduction phase is reached, the
steam atmosphere of the autoclave is cooled, this will
result in further reduction of the pressure, more water
will evaporate and the core temperature will follow the
vapour pressure curve, for example, as far as 60~C at
about 0.2 bar absolute, the peripheral zone temperature
also dropping further as a result. A subsequent, con-
trolled admission of air, in the course of which ambient
air mixes with the steam atmosphere and cools it further,
leads to cooling of the peripheral zones and thus a
further reduction in the temperature difference.
