Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a method of utilizing the enthalpy of
cooling water in a suction drying process of a paper, board, or
other porous or fibrous web, in which process the wet web and the
drying felt carrying the web are over the whole width of the web
enclosed between two air-tight surfaces having good thermal conduc-
tivity, the surface which is in contact with the web is heated in
order to evaporate water from the web and the surface which is in
contact with the drying felt is cooled by cooling water in order
to condense the water evaporating from the web to the drying felt,
and the drying felt is thereafter separated from the dry web and
released from the condensed water.
In present paper or board drying methods heat is recovered from the
mixture of air and steam which is obtained from the hood surrounding
the conventional drying cylinder or group of drying cylinders.
Usually the water which is heated by such gases flows out from the
heat recovery equipment at a temperature of 30 - 55C. This water
is mainly used in the same factory, for example as shower water for
the wet end of the board machine, and,/or for heating the building.
Said water has not been warm enough to be utilized - at least not
to an appreciable extent - as district heating water or as a heat
source for heat pumping.
Developments of heat pumping in connection with drying of a paper
or board web have been described, for example in some patents (US
patent 2 933 826, Swedish patent 7111908-5, German Patent 2630853).
These inventions have the common feature that heat is recovered
by transferring heat from said mixture of air and steam directly
into water or an other medium which is flowing or evaporating at
a low pressure in a heat exchanger. In each case the recovered
heat can be utilized to evaporate water, and the steam thereby
generated is compressed by a compressor or compressors to pressures
needed in the drying cylinder.
These methods, however, have not resulted in marketable products
due to the relatively small economic advantage gained in comparison
with the use of back pressure steam, and due to operational diffi-
culties encountered, for example when the web is brokenl with hoods
sealed more tightly than normally.
Suction drying between two bands, or between a cyiinder and a band
(Finnish patent 54514, US patent 4112586, British patent 1502040,
Finnish patent 59636) differs very much from conventional drying.
The heat energy needed to evaporate water from the web is brought
as saturated steam to the inner surface of the cylinder shell or to
the other side of the hot band, from where it passes through a
relatively thin metal layer into the web. Inside the web heat pene-
trates to the depth where evaporation takes p1ace. The evaporated
steam passes through the rest of the web and through the felt, and
condcnses onto the cold metal surface which is cooled with cooling
water from the outside. The cooling water leaves the process at a
temperature below 40C.
It is clear that the entha1py in the cooling water discharged from
the above described, ~own, suction drying process could be better
utilized, for instance in heat pumping, if the temperature of this
water could be increased. This kind of development, however, has
been prevented by the suspicion that the increased discharge tempe-
rature of the cooling water might slow down the drying process. In
many heat apparatuses the heat flux is directly proportional to
the temperature difference. From this might be concluded that
also in the above described suction drying process the heat flux
through the hot metal surface into the web being dried would be
almost directly proportional to the difference in temperature
between the hot and the cold metal surfaces.
A decrease of the drying rate could lead the harmful consequences.
In practice it is important that the drying rate does not decrease
much, because this would necPssitate larger drying surfaces (more
cylinders in many cases), which is expensive.
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It may be noted that if the temperature of the heating steam
is increased from loo& to 180 C, the drying rate becomes
sixfold. The optimum steam temperature results from considera-
tions involving the total economy.
The object of the present invention is to improve the utilization
of the cooling water enthalpy in the above described suction
drying process. According to the invention this is realized
when the cooling water leaves the process at a temperature of
60 - loo&.
In one aspect of the present invention there is provided a
method of utillzing the enthalpy of cooling water in a suction
drying process of a paper, board, or other porous or fibrous
web, in which process the wet web and the drying felt carrying
the web are over the whole width of the web enclosed between
two air-tight surfaces having good thermal conductivity, the
surface which is in contact with the web is heated in order to
evaporate water from the web and the surface which is in contact
with the drying felt is cooled by cooling water in order to
condense the water evaporating from the web to the drying felt,
and the drying felt is thereafter separated from the dry web
and released from the condensed water, characterized in that
said cooling water is discharged from the drying process at a
temperature of 60 - 100 C.
The invention is based on the surprising fact, proved by tests
that in suction drying the cooling water can be fed into the
drying process at a temperature of up to 80 & and discharged
at loo& without decreasing the drying rate with more than a
few percent compared with the situation where the discharge
temperature of the cooling water is between lo& and 30 & .
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- 3a -
Said test results indicating such a minor effect of the
temperature of the cooling water may seem surprising- They
can be explained as follows. The evaporation rate from the
web is roughly directly proportional to the heat flux from
the hot surface to the web. This flux depends on the
difference in temperature between the hot surface and the
evaporation point inside the web. If the temperature of the
hot surface is the same in two different situations, and the
evaporation rate, and therefore also the heat flux, are
almost the same, then also the temperature at the evaporating
zone of the web must be nearly the same in both cases. This
means also that the evaporation pressure in these two cases
is nearly equal-
The evaporated steam passes through a part of the web andthrough the felt, and is then condensed on the cold metal
surface. I'he pressure of the steam when it reaches the cold
surface depends on the temperature of this surface, if this
temperature is for example 29 C in one case (corresponding
to cooling water of about 17 C) and 87 C in another case
(corresponding to cooling water of 75C), steam pressure
in the first case is about 5 '~Pa and in the second case
about 70 }~a-
Correspondingly, the specific volume of the steam in the first casets sixfold compared with the specific colume in the second case.
Because the cold surface in both cases received nearly the same mass
flow of steam, the speed of this steam is in the first case about
s;x times as high as in the second case. The higher velocity steam
suffers a greater pressure drop in its way through part of the web
and through the felt. Thus it is possible (this can be proved by
calculations) that the same mass flows of steam in said two cases
start from the same evaporation pressure and reach the cold metal
sùrface for condensation with different pressures. This explains
the above-mentioned test results where the drying rate was only
little affected by the big changes in the temperature of the cooling
water.
.
The cooling water discharged according to the invention from the
drying process at a high temperature can be utilized as source
water for heat pumping or for district heating.
When the cooling water is used for heat pumping it is preferably
discharged from the drying process at a temperature of 60-85C.
The lowest possible temperature for econo`mical heat pumping is
about 60C, and the preferable upper limit temperature when drying
paper or board is about 85C. A part of the discharged cooling
water is evaporated at a low pressure in a suitable expansion unit,
for example in a packed column or a cyclone. The latent enthalpy
needed for the evaporation is supplied by that part of the water
which does not evaporate. This water comes out from the evaporation
unit at a lower temperature, 50-65C, than- at which it was fed
into the unit and the water is brought back into the drying unit as
cooling water. The evaporated steam is compressed by compressors
to a desired pressure for the drying process.
As desribed above, steam is fed to the heat pumping compressors
under a considerably higher pressure than what is possible to ob-
tain from the evaporation unit of a heat pumping process in connec-
tion with the known suction drying process where cooling water
leaves the drying process at a temperature below 40C. The higher
pressure steam results in better heat pumping efficiency and impro~-
ed economy for the whole drying process.
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- ~hen the cooling water is used for distric~ heating~ it is supplied
to the drying apparatus at a low temperature, 0-50C, and out at a
high temperature, preferably 75-100C. After having been used for
heating at the district heating locale, the water can be either
returned at a low temperature to the drying unit, where it is used
again as cooling water, or it can be fed somewhere else out of the
drying process. In the latter case new cooling water must be con-
tinuously fed into the drying process.
The discharge temperature of the cooling water can be controlled
by altering the cooling water flow rate. If the drying rate of the
web remains unchanged, the heat flux through the cold band into the
cooling water remains also unchanged; if the inlet temperature of
the cooling water is ~ept unchanged, the discharge temperature of
the cooling water rises when the cooling water flow rate is decreas-
ed. Thus it is easy to control the discharge temperature of the
cooling water under different operating conditions even within wide
margins.
The invention will be described in more detail in the following
with reference to the enclosed drawing, which shows schematically
a heat pumping arrangement in connection with a suction drying
process according to British patent 1502040.
The upper part of the drawing shows a suction dryer 1 comprising
an upper, endless, air-tight metal band 2 with good thermal conduc-
tivity, and a similar lower metal band 39 these ~wo bands running
in parallel over a certain zone. The paper web 4 to be dried is led
between the bands so that it is completely enclosed by the bands
within ~he said parallel zone. Between the paper web and the lower
band 3 runs an endless drying felt which carries the web. On the
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inner side of the upper band 2 there is a heating box 6, the lowèr
side of which is open against the upper surface of the band in the
parallel zone. The box is provided with an inlet 7 for the heating
steam and with outlets 8 for the condensate. On the inner side of
the lower band 3 there is a cooling box 9, the upper side of which
is open against the lower surface of the band in the parallel zone.
The box is provided with an inlet 10 for ~he cooling water and with
an outlet 11 for the used water.
When the wet web passes through the suction dryer, the web is thus
enclosed between a steam heated upper band 2 and lower water-cooled
band 3. Therefore, the water contained in the web evaporates, passes
through the web and the felt, and is finally condensed on the sur-
face of the lower band, as is explained in said British patent
publication. After drying, the web is separated from the felt.
Cooling water Wj is discharged from the dryer at a temperature of
60-100C and led by a pipe 12 to a mixer 13. The condensate Wa is
led by a pipe 14 from the heating box 6 through a condensate
separator 15 and a pressure reducing valve 16 to the mixer. Upon
passing through the pressure reducing valve a part of the conden-
sate e~aporates. This small amount of steam is condensed in the
mixer immediately when it comes into contact with the cooling water.
From the mixer 13 most of the water is led through a pipe l7 and a
pressure reducing valve 18 into an evaporation column 19. This is
a conventional packed column operated under vacuum. It is fed from
the top; the e~aporating steam leaves through another conduit at
the top; and the remaining water, which has cooled by about 10C,
leaves through the bottom. This water Wj is pumped through a pipe
21 by means of a pump 20 back to the cooling box 9 of the suction
dryer as cooling water. From the evaporation column steam is led
through a pipe 22 to a first compressor 23, which is operated by
an electric motor. Either before, inside, or after the compressor
water is sprayed into the steam so that the steam leaving the
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compressor is saturated. This water is pumped by means of a pump 25
from the mixer through a pipe 24. The steam leaving the first column
passes through a pipe 26 to a second compressor 27, which operates
similarly. The number of compressors in series depends on economical
optimization. In practice this number is usually two, sometimes
possibly three. From the last compressor steam passes through a
pipe 28 to the heating box 6 of the suction dryer. This steam is
mixed with steam passing through pipe 30 from the blow-through re-
circulation compressor 29. This compressor receives steam from the
condensate separator 15.
The above described process is closed, except that the wet web
passes into the dryer, and the dry web and the evaporated water pass
out of tlle dryer. Electric power is needed for the electric motors
of the compressors and for the motors of the water circulation pumps.
There are minor heat losses from the pir)ing and the equipmant through
heat insulation into the surroundings Contrary to conventional
cylinder-hood drying, this drying process does not need any supply
of back pressure steam or any other steam from the outside.
The economy of the above described process, compared with the same
drying process without heat pumping and using back pressure steam
from sources outside the drying apparatus, is mainly dependent on
the price of electric power needed for heat pumping compared w;th
the price of back pressure steam, and on the capital cost of the
additional equipment required by the heat pumping. For example,
if the steam of 0.8 MPa is needed for the actual drying, the price
of back pressure steam is about 50 mark/(MWh latent enthalpy of
steam). If the required steam of 0.8 MPa is obtained by evaporating
water at 60C and compressing the evaporated steam to the desired
pressure by a compressor with an efficieny of 0.83, the price of the
required electric power is 69.0 mark/(MWh of latent enthalpy of
steam) provided that the price of electric power is 120 rnark/MWh.
If the steam is compressed from evaporating steam at 80C, the
price of the steam is 45.9 mark/(MWh of latent enthalpy of steam).
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The prices being as above mentioned, the drying steam obtained by
heat pumping from water at 80C is already cheaper than correspond-
ing back pressure steam, whereas steam obtained by heat pumping from
water at 60C is more expensive.
tn the future the price of electric power compared with fossile
fuels is likely to decrease. The above-described heat pumping pro-
cess will then become relatively more economical than now.
The above described heat recovery process is applicable also to a
suctlon dryer in which a rotating cylinder is substituted for one
of the bands, preferably the heated band, as described in Finnish
patent no 59636~ The invention can also be applied when
using the above described suction drying method for serial batch
drying.
The following table illustrates several examples of how a desired
discharge (outlet) temperature is obtained by controlling the
inlet temperature and flow rate of the coo1ing water. In all exam-
ples it is supposed that a constant amount of heat, 15000 kW is
transferred into the cooling water per time unit and that the other
conditions and the web quality are the same.
Example No. Inlet Temperature¦ Outlet Temperature¦ Flow rate
of cooling water
.. . _
1 50C 60C 359 kg/s
2 50C 70C 179 kg/s
3 50C 80 C 119 kg/s
4 80C 100 C 179 kg/s
0C 60C 60 kg/s
6 0C 70C 51 kg/s
7 0C 80C 45 kg/s
8 0C 100C 36 kg/s
In the above examples the drying rate of the web varies only a few
percent.
The object of the drawing and the description is only to il1ustrate
the idea of the invention. In its details the method according to
the invention can vary within the scope of the claims.
