Note: Descriptions are shown in the official language in which they were submitted.
CA 02235963 1998-04-27
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PROCESS FOR THE BIOLOGICAL-THERIMAL
TREATMENT OF WASTE
The invention relates to a process for the biological-thermal treatment
) of organic constituents-containing wastes in a tank, wherein the exhaust
air drawn off from the tank is again fed back to the tank. In other words,
a circulating air system is used, wherein all or part of the exhaust air
leaving the tank is again fed back to the tank. The process can be used
for the biological-thermal composting or stabilising of organic
constituents-containing wastes.
The tank is a closed tank with forced ventilation. Normally, in the
bottom part of the tank an essentially horizontally extending perforated
bottom is provided, on which the waste rests as rotting material or
rotting mixture in the form of a pile or heap. By means of an air supply
system, which normally consists of several fans, a gas mixture consisting
mainly of air or re-circulated air is supplied to the area underneath the
perforated bottom. The gas mixture flows upwards through the holes or
other openings of the perforated bottom and from there into the rotting
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mixture. It flows through the rotting mixture substantially from the
bottom upwards and is then drawn off from the upper part of the tank
as exhaust air. The exhaust air can be treated before all or part of it is
again fed back to the rotting material. It_is possible, for example, to
~? .
separate water from the circulating air, e.g. by cooling th e circulating air
and separating the water that occurs as condensate.
When composting bio-wastes, i.e. wastes that contain organic
constituents, for the purpose of making compost or stabilising material,
the rotting mixture is 'ventilated more or less for so long until the
biologically easily decomposable organic substances contained therein
are decomposed. It is decomposed mainly by bacteria which under
aerobic conditions produce the greatest possible metabolic effect. With
the known processes for the composting by an intensive rotting process
(i.e. a process in a closed tank with forced ventilation), to speed up the
composting the supply of oxygen is optimised. With this, from solid,
amorphous or liquid substances the gaseous substances C02, water
vapour and ammonia are produced. These metabolic products together
with the air flowing through the rotting mixture are discharged as
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exhaust air, wherein the temperature and oxygen content in the rotting
mixture are controlled by way of the air (i.e. the condition of the inlet
air and of the exhaust air) in such a way that microbial life can unfold
unhindered and can decompose the existing potential of easily -
decomposable organic substance in the shortest -possible time.
Organic substances that are difficult to decompose are in the first
instance decomposed mainly by fungi which - the same as most bacteria
take the oxygen required for the metabolism from the water of the
substrate on which they grow (see Schlegel, General Microbiology, 5th
edition, page 169). During the decomposition of this potential new bio-
~ mass occurs in the form of a microbial cellular substance which in turn
consists of easily decomposable hydrocarbon compounds. The newly
formed cellular substance is under aerobic conditions approximately 20
times larger than under anaerobic conditions. It consists - the same as
the easily decomposable organic substance of the basic substrate - also
of approximately 85 % biologically easily decomposable organic substance
and at the end of the decomposition of the easily decomposable organic
substances present in the substrate represents a decomposable potential
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the quantity of which should not be under-estimated. The reduction of
the substrate mass as well as of the newly formed bio-mass (i.e. the
biological decomposition) takes place according to a declining
exponential function, i.e. it will not lead to a complete dissolving of the
entire bio-mass (consisting of basic substrate _ and formed cellular
substance), but it is possible, after a suitable period, to decompose the
organic part of the basic substrate (i.e. of the bio-waste) completely.
Most present composting works operators complain that on completion
of the technical treatment most composts display :, it is true, a self-
heating according to the rotting degree 4 or 5, but that after a dry
storage a self-heating according to the rotting degree 1 can again be
observed. This can be attributed to the fact that micro-organisms of
organic substances that are difficult to decompose change info easily
decomposable organic substances (cellular substances) which, when
moistened again, then react immediately and, the same as fresh
substrate, in a short while release large quantities of heat (as metabolic
product), wherein the temperature in the compost substrate increases
sharply within a short time. Until now the time and the temperature
maximum reached in this time is the main criterion for determining the
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rotting degree. This, it is true, is only correct when the test conditions
with regard to the substrate mass and ambient conditions are always the
same, but until now as a simple standard method it determines the
quality of the compost. The correct testing method must accurately
determine the quantity of heat generated during a specific time or
another metabolic product (see DE-OS 43 36 497). This is the only way
in which the rotting degree or decomposition degree of an organic
substance can be determined.
Methods for carrying out a composting process are known from the prior
publications DE-PS 36 37 393, DE-PS 40 21 865, DE-PS 43 34 435, EP-
PS 322 424, DE-PS 38 29 018, DE-PS 41 24 880, DE-PS 43 22 688, DE-
PS 41 07 340, DE-GM 93 00 127 and DE-PS 42 15 267, to which
reference is made here. All these prior publications have in common the
use of atmosptieric oxygen from the air flowing through the rotting
mixture.
In the German patent application No. 195 13 262.9-41 a completely
closed circulation of the air and of the gases generated during the
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fermentation is disclosed, wherein the oxygen required for the breathing
is taken from the water content of the waste, the chemical compounds
or from fresh water fed in from the outside, and not from the air. Here
the drying of the rotting mixture after the decomposition of the easily
~ decomposable organic substances takes place by feeding in fresh air after
separating water from the circulating air. However, this method of
operation has the disadvantage that by feeding in fresh air the relative
moisture of the circulated gases is reduced and accordingly the
water absorption capacity increased, but the heat required for the
evaporation of residual water in the substrate is missing. If - due to the
absence of easily decomposable organic substances - the biologically
generated heat decreases, then, due to a lack of thermal energy of the
rotting mixture, a complete extraction of moisture and accordingly an
environmentally neutral storage stability of the end product will not be
ensured. Furthermore, the method of operation used until now has the
disadvantage that the moisture extraction from the exhaust air takes
place in the condenser at the dew point limit. As a result, when the
temperature in the rotting mixture drops, the water absorption capacity
of the circulating air is limited, and the final water content values
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obtainable in the rotting mixture are detrimental for ensuring the long-
term storage stability as well as for the calorific value.
Proceeding from this it is the object of the invention to eliminate the
shortcomings that occur according to the prior art, and to propose an
improved process for the biological-thermal treatment of organic
constituents-containing wastes of the type described at the outset.
According to the invention this object is achieved in that the exhaust air
is heated before it re-enters the tank. The exhaust air or circulating air
is heated in such a way that in the rotting mixture a residual moisture
content is obtained which leads to a diffusion equilibrium with the air
which under normal circumstances surrounds the rotting mixture when
it is stored.
As heat sources which can be used for this re-heating, in particular the
following can be used: the feeding back of the condensation heat energy
drawn off from the circulating air, preferably by means of air/air heat
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exchangers; changing the condition of the air by increasing the pressure;
sluicing the heat of condensation out of the air circulation system, which
heat by means of a heat pump is brought up to a higher temperature
level; the heat from the combustion process of a dry stabilising product
or other intermediate product produced from the wastes; the heat from
the combustion process of screen overflows; the heat from combustion
processes of other energy carriers; the heat from solar installations; the
heat from electrical transformation processes. The heat sources can be
operated as central plants or also as decentralised energy stations which
supply every tank in a plant comprising several tanks.
Advantageous further developments are described below.
Preferably the composition of the circulating air, i.e. the exhaust air,
which is fed back to the tank, is controlled in dependence on the partial
pressures of its components. The composition of the circulating gas
volume is, therefore, controlled in dependence on the partial pressure
of the gas components. As a result thereof it is possible to ensure an
aerobic breathing without having to supply gaseous oxygen from outside
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{'it is possible, however, to supply gaseous oxygen from outside). During
the mass balancing of rotting processes it was found surprisingly that no
significant consumption of gaseous oxygen could be detected, although
all characteristics of an aerobic metabolism were present. From this it
can be concluded that the oxygen required for the aerobic breathing for
the decomposition of easily decomposable organic substances in the sub-
strate is already present and suffices for the microbial phase of the
aerobic decomposition or can be obtained by feeding in water .
Another advantageous further development is characterised in that the
gaseous metabolic products are separated from the circulating air. This
is done preferably by molecular sieves. To control the partial pressures
of the circulating scavenging gases, the gaseous metabolic products are,
therefore, separated from the gas mixtures, which preferably takes place
by suitable molecular sieves.
After coming out of the rotting mixture the gases, which have absorbed
moisture, are cooled so that the condensable andlor sublimatable
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substances carried along in same, e.g. water, ammonia and the like, can
be separated and removed in a liquid from the circulation system. The
partial pressure ratio of the "carrier gases" (nitrogen and oxygen) in the
_ climatic environment of the micro-organisms to the "metabolism gases"
(carbon dioxide and water vapour) preferably is controlled in that, when
the overall pressure increases to above the atmospheric pressure (of
1013 mbar = hectopascal), excess quantities of CO2, ammonia and water
vapour are drawn off. This can take place by a washing operation.
Subsequently the gases are not, as before, mixed with oxygen-containing,
-dry air to increase the water absorption capacity, but they are heated
from the outside by a heat source. As a result the partial pressure ratios
of the gas components are changed. Furthermore, the water and carbon
dioxide absorption capacity of the gases is increased and the thermal
energy required for the evaporation of the water is supplied. This is in
particular extremely advantageous when the microbial decomposition
capacity drops drastically due to a slowly occurring dryness rigidity. If
bio-masses are to be stabilised in such a way that microbial
decomposition comes to a complete stop, the water content at the
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surfaces of the waste particles must be brought down to values below the
normal condition of the air (H20 < 3,9 g/kga, corresponding to
approximately 15 % relative substance and air moisture), and the easily
decomposable compounds, -e.g. carbohydrates, must be biologically - -
~~ -
decomposed. With the methods used until now (composting with
circulating air without re-heating or mechanical thermal drying) this is
not possible in a short time (i.e. in less than about seven days), but it
can be done with the process according to the invention.
Another advantageous further development is characterised in that
aerobic and anaerobic gas mixtures flow alternately through the rotting
~ mixture, in dependence on the nitrogen content.
Furthermore it is advantageous when a water-saturated and a water-
absorbable gas mixture flow alternately through the rotting mixture, in
dependence on the prevailing type of micro-organisms.
In certain cases it may be advantageous to enrich the circulating air with
CA 02235963 1998-04-27
_ 12 _
pure oxygen.
Another advantageous further development is characterised in that
condensate is separated from the exhaust air and this condensate is
i~ controlled with respect to its volume and/or _ its constituents. The
condensate may be separated from the exhaust air by cooling. It may be
collected and measured and following this be controlled with respect to
its volume and/or constituents. Subsequently the circulating air is
heated.
The process control preferably takes place in dependence on the
conductivity, the chemical oxygen requirement and/or the pH-value of
the condensate.
The circulated gases are preferably moved by several inlet air fans and
without exhaust air fan. To further reduce the newly forming bio-mass,
the rotting mixture can be removed from the tank after a fermentation
cycle, preferably re-comminuted, preferably moistened again and fed
back again to the tank (fermenter) and subjected to another rotting
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cycle. When doing so, as a result of a rapidly occurring heat generation
an increase in temperature can be observed, the progression of which is
the greater, the greater the as a result of the preceding biological
decomposition newly formed bio-mass of strictly aerobic micro-
organisms. -
The invention is based on the following considerations:
~
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' 14 -
Bacteria, fungi and yeasts which carry out the biological decomposition,
unfold their metabolic maximum under different conditions of life.
Bacteria prefer a high aw,-value of 0,98 (the aw-value represents the
- relative air moisture over water with oxygen and nutrients dissolved in
same). Mould fungi with a preferred aa,-value of 0,8 and yeasts with a
preferred aw-value of 0,6, on the other hand, can live at a considerably
lower relative moisture, i.e. less water and accordingly also oxygen in
their environment.
In an atmosphere witli a low oxygen content less new cellular substance
is formed, which therefore also need not be decomposed to produce a
~ matured or storable stable compost.
The formation of the bio-gas CH4 can be prevented when an
acidification of the substrate is avoided by controlling the circulating gas
composition in respect of its moisture (aw,-value < 85 %), C02-content (>
10%), oxygen content (< 10%) and ammonia content (< 1%) in such
a way that for fungi and fungi-like micro-organisms (actinomycetes)
advantageous intermediate decomposition products and climatic
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.
- 15 -
conditions occur.
An intermittent method of operation with a change in the substrate
moisture has the advantage that bio-masses in the dryness rigidity are
more easily broken up into smaller pieces and accordingly can be
attacked more easily by bacteria. After the fungi phase comminution
measures are, therefore, carried out and the climatic moisture required
for an optimum decomposition metabolism is produced. According to
the invention this is achieved in that in chronological sequence, by
changing the oxygen partial pressure in conjunction with water,
facultative anaerobic micro-organisms are given the opportunity to utilise
organically bound oxygen, for example by de-nitrification.
Another advantageous further development is characterised in that water
is fed to the exhaust air before it re-enters the tank. Preferably fresh
water is fed in. The feeding in of water preferably takes place after
heating the exhaust air re-entering the tank. The water is preferably
sprayed into the air flow.
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According to another advantageous further development several tanks
are provided, the exhaust air pipes of which open out into an exhaust air
manifold and the inlet air pipes of which branch off from an inlet air
manifold. As a result thereof the process can be carried out particularly_
effectively. Furthermore, there is the added advantage that as a result
of the manifolds the C02-content of various tanks can be balanced out.
If, for example, the C02-content in the circulating air of a specific tank
is very high, air with a lower C02-content can be fed to this tank from
the manifold. As a result thereof the exhaust air volume can be reduced
further, possibly down to zero.
It is advantageous when the air of the exhaust air manifold is first fed
to a heat exchanger, preferably an air/air heat exchanger, by which the -
heat given off by the exhaust air manifold is transferred to the inlet air
manifold or to the air or circulating air flowing through same. It is
furthermore advantageous when the heat from the exhaust air manifold,
instead thereof or in addition thereto, by an optional further heat
exchanger, preferably an air/water heat exchanger, is given off to a
cooling system, preferably a cooling water circulation system.
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It is furthermore advantageous when the air or circulating air in the inlet
air
manifold is heated by a preheating device, which preferably takes place in the
direction of flow behind the aforementioned air/air heat exchanger.
The invention further more relates to an apparatus for implementing the
process
according to the invention with several tanks, inlet air and exhaust air pipes
leading to and from the tanks and an exhaust air manifold as well as an inlet
air
mani fold.
In accordance with a first aspect of the present invention, there is provided
a
process for biothermal treatment of waste materials containing organic
constituents in a container, the process comprising the steps of:
(i) removing exhaust air from the container;
(ii) retaining at least a partial amount of the exhaust air as circulating air
for reuse;
(iii) separating condensate from the circulating air;
(iv) collecting and monitoring the condensate to determine at least one of
quantity and content of the condensate;
(v) heating the circulating air; and
(vi) returning the circulating air to be circulated through the waste
materials in the container.
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In accordance with a second aspect of the present invention, there is provided
an
apparatus for biothermal treatment of waste materials containing organic
constituents comprising at least one waste material container tank, having
(i) an air exhaust means and an air supply means; and
(ii) an air recirculating system connected to the air exhaust means and the
air supply means, the system comprising a condensate separation means and at
least one air reheating means.
Exemplified embodiments of the invention will be explained in greater detail
in
the following with reference to the attached drawing, wherein:
Fig. I is a diagrammatic view of two tanks including the other components for
implementing the process,
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- Z'8 -
Fig. 2 is a diagrammatic view of a tank with the other components for
implementing the process, and
Fig. 3 shows an apparatus for implementing- the process, _comprising
several tanks. -
The bio-mass to be fermented is filled daily into the tanks (rotting
boxes) RB1 and RB2 shown in Fig. 1, seeing that also the waste disposal
cycles display a daily rhythm. Still further tanks may be present in a
plant. Every tank is fitted in its bottom part with an essentially
horizontally extending perforated bottom 1, in which holes or other
~ openings are provided. Underneath the perforated bottom 1 a plurality
of smaller air chambers 2 are provided, to which air can be admitted, i.e.
which can be controlled. The air is fed by fans 4 into the air chambers
2 and flows through the holes in the perforated bottom 1 into the rotting
mixture 3 that rests on the perforated bottom 1. Because of the
multitude of individual air chambers 2 an uneven gas flows through the
rotting mixture 3 with the attendant risk of a breaking through of the
entire gas volume at one point or in a small area is avoided. The
CA 02235963 1998-04-27
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perforated bottom 1 may consist of solid perforated plates, perforated
bricks, pendulum bottom profiles or air-permeable belt conveyors.
On the perforated bottoin 1 rests the ratting mixture 3 in gas-permeable
form. The fan 4 circulates the gas volume enclosezl in the system via the
inlet air heat exchanger 5 through the rotting mixture and the exhaust
air heat exchanger 6.
When in special cases CO2 must be discharged and oxygen let in, the
valves 7 and 8 are opened.
~ The condensate drawn off from the heat exchanger 6 by cooling is fed
through a pipe 9 to a condensate treatment plant with oxygen
enrichment. Thereafter it can be used to again moisten the rotting
material for a further rotting cycle.
The heat carried off from the heat exchanger 6 is fed via the storage
unit 10, the heat pump 11 and the other storage unit 12 to the heat
exchanger 5 and there is transferred to the inlet air. The drying process
CA 02235963 1998-04-27
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2U-
is at an end when the water absorption of the circulating gas is close to
zero glkg. If heat is available from thermal processes, e.g. from the
combustion of the stabilising material, this can be fed into the heat
exchanger 12 at point 13.
t...! _
As can be noted from Fig. 1, fresh water 15 can be fed in. This fresh
water is sprayed into the air flow after it has been heated in the heat
exchangers 5 and 6. The air moistened in this manner then flows into
the tanks.
The tanks are filled daily, corresponding to the waste disposal cycle of
the refuse disposal industry, which means that the individual tanks at
daily intervals and in the course of the decomposition kinetics reach a
maximum in the release of the metabolic product "heat". This is
illustrated in the time-heat diagram 14. Every day a tank Rbn is filled.
The tank RB1 reaches the heat maximum after approximately one day,
the tank RB2 after two days, the tank RB3 after three days and so on.
Seeing that after about seven days the heat release has reached a
minimum, the process can be ended here. From the diagram 14 it can
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- 21 -
also be noted that for the summation of the quantities of heat to be
eliminated daily, not the respective daily maximums should be taken as
a basis, but a diminishing pattern. From this it follows that when up to
seven rntting tanks are connected to a heat exchanger combination with
heat pump, a favourable cost-usefulness ratio can be obtained.
Figure 2 illustrates an exemplified embodiment with a rotting tank which
basically is constructed in the same way as that of Fig. 1. The rotting
tank is supplied with air by a fan, which air passes through the
perforated bottom and the rotting mixture resting on same. Subsequently
the exhaust air is drawn off by another fan and fed to an air/air heat
~ exchanger. There it gives off heat to the air to be fed to the rotting
mixture. Subsequently the air is fed to a water/air heat excfianger, where
it gives off further heat to an air-cooled cooling tower. The air then
flows through the other side of the air/air heat exchanger and as
circulating air is again fed to the rotting mixture. If required, fresh air
can be fed into the circulation system through an adjustable valve. It is
furthermore possible to remove exhaust air from the circulation system
by way of a valve and connected filter. Water of condensation can be
CA 02235963 1998-04-27
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drawn off from the heat exchangers.
The closed rotting tanks preferably are thermally insulated. They are
-preferably made of reinforced concrete. In the tank, in a period of about
one week all substances that form biologicalI}T easily decomposable
compounds are released in gaseous form by oxygen dissolved and bound
in the substrate liquid, by aerobic breathing, and for the greater part are
removed from the rotting mixture with a circulating, continually moving
gas volume, the gas volume being fed through the rotting mixture from
the bottom upwards through a perforated bottom, which preferably is
provided with air chambers positioned underneath to which the air can
be supplied individually. The heating of the circulating air can take place
by the condensation heat of the condensate separated from the exhaust
air (see Fig. 2). It can take place by increasing the pressure in the
circulating gas flow or by a heat exchanger with known heat carriers.
With the process according to the invention the composting of bio-
masses can be carried out or the stabilising of wastes. The overall plant
for the implementation of the process can consist of inlet air fans,
rotting tanks (rotting boxes), exhaust air/inlet air heat exchangers,
CA 02235963 1998-04-27
23
exhaust air/heat carrier heat exchangers, exhaust air valve, exhaust air
purification plant, inlet air valve, water spraying nozzle, condensate
measuring device, temperature measuring device, programme regulators
and switch cabinet. It is possible to work mdth or without exhaust air fans
as well as with or without COZ measuring device The rotting tanks may
consist of reinforced concrete housings with and without thermal
insulation, a door that can be closed gas-tight, air-permeable bottom
plates and underneath same air boxes to which air can be supplied
individually. The air ducts underneath the perforated plates may run in
the longitudinal direction underneath the rotting boxes, extending
conically from the blowing-in point and/or can be supplied with different
air volumes, so that it is possible to influence edge flow losses on the
walls of the rotting tank.
Fig. 3 illustrates an apparatus with several tanks 21, 22, 23, an inlet air
pipe 24, 25, 26, in each of which a fan is arranged, leading to each one
of these tanks and an exhaust air pipe 27, 28, 29 leading away from each
tank. The exhaust air pipes 27, 28, 29 each lead via a slide valve to an
exhaust air manifold 30. The inlet air pipes 24, 25, 26, each of which is
CA 02235963 1998-04-27
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- 24
also provided with a slide valve, branch off from an inlet air manifold
31.
The exhaust.air manifold 30 runs through an air/air heat exchanger 32,
~ by which the heat of the air flowing through the exhaust air manifold 30
is given off to the air flowing through the inlet air manifold 31.
Subsequently the air in the exhaust air manifold 30 passes through three
air/water heat exchangers 33, 34, 35, by which the heat is given off to a
cooling water circulation system 36.
By way of a slide valve 37 in the branch pipe 38 from the exhaust air
,-~ manifold 30, air can be given off to a bio-filter.
The exhaust air manifold 30 changes over via a connecting pipe 39, in
which a slide valve 40 is arranged, to the inlet air manifold 31. A branch
pipe 41, in which a slide valve 42 is provided, which can be controlled
for the feeding in or mixing in of inlet air, opens out in the inlet air
manifold 31. Following this in the direction of flow a COZ measuring
device 43 is provided in the inlet air manifold 31.
CA 02235963 1998-04-27
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-
- 25
The inlet air manifold then leads agdin to the air/air heat exchanger 32.
A branch pipe 44 by-passes this air/air heat exchanger 32, so that the
.inlet air or part thereof can also be fed to the tanks 21, 22, 23 without
being heated in this heat exchanger 32, i.e. through other inlet air pipes
'-~ 45, 46, 47, each of which is also provided with a-slide valve.
From every exhaust air pipe 27, 28, 29 an individual circulating air pipe
with a slide valve leads to the respective inlet air pipe 24, 25, 26. As a
result thereof it is possible to individually circulate part of the
circulating
air for each tank and to let only part of the circulating air flow through
the manifolds.
_.~ .
In the inlet air manifold 31, in the direction of flow after the air/air heat
exchanger 32, a preheating device 48 is provided which consists of an
air/water heat exchanger to which outside heat. can be supplied.
