Note: Descriptions are shown in the official language in which they were submitted.
CA 02312511 2000-06-27
PROCESS AND APPARATUS FOR DRYING GRANULATES
The invention consists of a process for drying granulates in at least two
drying bins by means of hot
drying air, which is circulated through parallel drying bins and through
drying air dryers and is heated
by passing through heaters, where the input stream of drying air to each of
the individual drying bins
is more or less throttled depending on the current temperature in the relevant
drying bins.
Such a process is known from DE 3131471 C2. There, three parallel drying bins
are provided where
a throttle valve is installed in the branch inlet pipe before each drying bin,
with a servomotor being
activated by a signal from a temperature probe installed in the branch inlet
pipe after the drying bin.
The drying air dryer has adsorption cells through which circulated drying air
and a regeneration agent
(hot air) are alternately fed. At the outlet from the drying air dryer, the
drying air is directed through
a single heater before it enters the feed pipe to the drying bins.
This known apparatus, though generally operating satisfactorily, has the
disadvantage that all of the
drying bins must be supplied with drying air at the same temperature. This is
unsuitable when
different materials are to be dried in the individual drying bins that,
because of the differing
temperature sensitivities, should be dried at different temperatures.
Moreover, with the known
apparatus, the relatively long pipes between the heater and the drying bins
must be insulated in order
to avoid excessive heat loss.
If, in order to overcome the abovementioned disadvantages, individual heaters
are installed for each
drying bin rather than a common heater, then care must be taken that the
quantity of air distributed
to the individual drying bins is determined by the amount of throttling of the
stream of drying air
carned to them. In some cases, this can lead to excessive demand being put on
one heater because
of a high drying air flow rate, so that the planned drying air heating
temperature might not be reached
there. To maintain the desired temperature under all operating conditions, two
or more inordinately
powerful heaters must be used, with the disadvantages that implies.
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Accordingly, the goal of this invention is to improve the known process so
that varying higher
temperature drying air can be made available to the individual drying bins and
the desired temperatures
of the various drying air streams can be attained under all operating
conditions without it being
necessary to use inordinately powerful heaters.
This goal is achieved by means of a process similar to the abovementioned type
by which each drying
air stream fed into the drying bins is heated by individual heaters and the
drying air streams depend,
not only on the temperature, but also on the heating level of the pertinent
individual heaters so that
when a predetermine heating level of the individual heaters is reached, the
appropriate drying air
stream is further throttled so that it is heated to a previously determined
desired temperature
independently from the quantity of drying air distributed to the individual
drying air streams.
The provision of individual heaters makes it possible to heat the drying air
streams fed into the various
drying bins to different temperatures depending on requirements. If the
capacity of an individual
heater is exceeded because the relevam drying air stream increases greatly
after other drying streams
are heavily throttled and/or a relatively high desired temperature is set for
the relevant drying air
stream, then the relevant drying air stream flow is throttled until the
relationship between the drying
air stream and the available heat is again sufficient to achieve the desired
temperature. Thus,
inordinately powerful heaters can be avoided without running the risk of the
drying air fed to any of
the drying bins failing to reach the desired temperature under extreme
operating conditions.
The process in this invention is carried out in such a way, using demand-
dependent, automatic
individual heaters that can be switched on and off, that the heat level-
dependent further throttling of
a drying air stream is triggered if the relevant individual heater is not
switched off within a
predetermined time period. The continuous operation of the individual heaters
detected during the
predetermined time period is interpreted as meaning that the heat level is not
su~cient to achieve the
desired temperature at the current flow rate.
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The invention also consists of an apparatus for carrying out the invention
process with at least two
drying bins, one drying air dryer and one heater in which a drying circuit
with a fan is installed, which
has a feed pipe, a return pipe and a branch pipe linking these for the
parallel arrangement of the drying
bins, where a throttle valve is installed in each branch input pipe and each
drying bin is equipped with
a temperature probe connected to the throttle valve adjuster by means of a
signal wire.
Such a device is also known from DE 31 31 471 C2. It suffers the same
disadvantages described
above.
To counter these disadvantages, the known device is improved upon with this
invention in that an
individual heater is installed in each branch input pipe to the drying bins
and each individual heater is
connected with the throttle valve control unit installed for the same drying
bin by means of a heat level
signal wire.
With the apparatus built in this way, it is possible to carry out the
invention process described above
and thus to achieve the corresponding advantages.
In a useful arrangement of the apparatus, the individual heaters are installed
after the throttle valve
in each freed branch pipe. Thus, the individual heaters are located close to
the appropriate drying bins,
decreasing heat loss and reducing insulation costs. In addition, the throttle
valves are subject to less
heat load.
In another appropriate arrangement, the temperature probes are installal in
each relevant drying bin.
In this way, dependable temperature measurements for controlling the apparatus
are achieved.
,An implementation example ofthe invention apparatus is illustrated more
clearly below in a schematic
drawing. The single figure shows essentially a top view of an apparatus with
two drying bins shown
in side view and with a drying air dryer.
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In the drawing, a regeneration circuit (8) and a drying circuit (9) are shown,
which are brought
together inside a drying air dryer ( 10).
In the regeneration circuit (8), a suction filter ( 1 }, a regenerating fan
(2), a regenerating heater (3) and
two reverse valves (4a, 4b) are installed in sequence in the direction of the
flow, between which
two adsorption cells (Sa, Sb) are installed.
The two reverse valves (4a, 4b) and the two adsorption cells (Sa, Sb) are
likewise installed in drying
circuit (9) so that by switching the reverse valves (4a, 4b), the two
adsorption cells (Sa, Sb) are
alternately connected to regeneration circuit (8) and the drying circuit (9).
Drying circuit (9) is fitted with a fan (6) for the drying air and with a
filter (7) for the returning air.
The drying circuit (9) has a feed pipe ( 11 ) and a return pipe ( 12) for the
circulated drying air, as well
as a first branch pipe with a branch inlet pipe (21) and a branch outlet pipe
(28) and a second branch
pipe with a branch inlet pipe (31) and a branch outlet pipe (38) which are
each connected to a feed
pipe ( 11 ) and a return pipe ( 12) and connected to each other through a
first drying bin (20) and a
second drying bin (30). Both drying bins {20 and 30) are used to dry
granulates, which are brought
in and removed in the usual way, so that no further explanation is required.
The two drying bins (20, 30) are, as the drawing makes clear, of similar
construction as are the
apparatus parts. Thus, each branch inlet pipe (21, 31 ) has a throttle valve
(22a, 32a) and an individual
heater (23a, 33a). The heat level is regulated in such a way that drying air
introduced into drying bins
(20, 30) has a desired temperature corresponding to the granulates to be dried
as applicable. In
addition, temperature probes (23e, 33e) are installed in the branch inlet
pipes (21, 31) between the
individual heaters (23a, 33a) and drying bins (20, 30), which are connected to
control units (23c, 33c)
by signal wires (23d, 33d), which controls the heat intake through heat pipes
(23b, 33b) to the
individual heaters (23a, 33a).
CA 02312511 2000-06-27
The throttle valves (22a, 32a) make it possible to adjust the air flow to the
ganulate flow through
drying bins (20, 30) so as to optimize drying conditions (drying temperature).
In addition,
temperature probes (22e, 32e) are installed in drying bins (20, 30) which
report bin temperatures to
control units (22c, 32c) through signal wires (22d, 32d). This produces
setting signals for the throttle
valves (22a, 32a) which are sem through the control wires (22b, 32b).
Heat level control units (23c, 33c) are connected to the control units (22c,
32c) for the throttle valves
(22a, 32a) by signal wires (24, 34) and deliver the relevant heat level data
to them.
The illustrated and described apparatus works as follows:
The ganulates placed in drying bins (20, 30), which might be different
materials with different
characteristics, are dried using the drying air which is circulated through
the drying circuit (9) by the
fan (6) which, for example, is continuously dehumidified by the adsorption
cell (Sb). During this
operational phase, the adsorption cell (Sa) is connected in the regeneration
circuit (8) and is
appropriately regenerated. After a predetermined operational period and
ideally just before the
adsorption cell (Sb) is completely exhausted, the reverse valves (4a, 4b) are
switched over so that now
the freshly regenerated adsorption cell (Sa) is in the drying circuit (9) and
the more or less exhausted
adsorption cell (Sb) is in the regeneration circuit (8) and is regenerated.
This switching between
adsorption cells Sa and Sb contilrues as appropriate.
During the drying operation, the drying air fed into the drying bins (20, 30)
is heated separately in the
individual heaters (23b, 33b) to the predetermined desired temperature.
Depending on the
temperature value determined by the temperature probes (22e, 32e), the
throttle valves (22a, 32a) are
adjusted by the control units (22c, 32c) to an optimal drying air flow rate in
the appropriate drying
bins (20, 30).
Since, however, closing the throttle valve (22a or 32a) not only decreases the
drying air flow through
the connected drying bin (20 or 30) but also produces an increase in the
drying air flow through the
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other branch pipes with throttle valves (32a and 22a) and through the relevant
drying bins (30, 20),
there might be an excessive drying sir flow so that the connected individual
heaters (33a and 23a),
because of the limited heating capacity, are no longer capable of bringing the
heating level to the
desired temperature. Such a situation, which is characterized by continuous
operation (full load) of
the individual heaters 23a, 33a, is signalled to the control units (22c, 32c)
by the heat level signal wires
(24, 34). This causes an extra throttling of the drying air in the overloaded
branch pipe, so that the
temperature of the drying air again rises to the desired temperature.
It was stated above that air is circulated through the drying circuit (9).
This is not to be understood
as being restrictive, since other gases, such as nitrogen, may be used instead
of air in the drying circuit
(9) and circulated through the drying bins.
