Note: Descriptions are shown in the official language in which they were submitted.
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ELECTROSTATIC ASSISTED WEH COOLING AND REMOISTENING DEVICE
BACKGROUND OF THE INVENTION
The present invention relates to apparatus for cooling
and/or remoistening of a moving web. In drying a moving web
of material, such as paper, film or other sheet material, it
is often desirable that the web be contactlessly supported
during the drying operation in order to avoid damage to the web
itself or to any ink or coating on the web surface. A
conventional arrangement for contactlessly supporting and
drying a moving web includes upper and lower sets of air bars
extending along a substantially horizontal stretch of the web.
Heated air issuing from the air bars floatingly supports the
web and expedites web drying. The air bar array is typically
inside a dryer housing which can be maintained at a slightly
sub-atmospheric pressure by an exhaust blower that draws off
the volatiles emanating from the web as a result of the drying
of the ink thereon, for example.
It is often necessary to cool and/or remoisten the web
after it has been dried. For example, U.S. Patent No.
5, 333, 395 discloses a drying apparatus for traveling webs which
includes a cooling tunnel directly connected with the dryer,
a combustion chamber for combusting solvent which becomes
volatile during drying of the web, heat exchangers, etc. U.S.
Patent No. 5,038,495 discloses a cooling device for cooling a
web of material exiting a dryer. The cooling device comprises
a substantially closed housing with an inlet and an outlet slit
for the web of material. The housing includes a feed aperture
at the outlet slit side for feeding outside air into the
housing, and a discharge aperture at the inlet slit side for
discharging air from the housing into the dryer. Air is fed
through the housing counterflow to the direction of web travel.
A series of nozzles bring the infed air into contact with the
web of material. U.S. Patent Nos. 4,702,015, 4,689,895 and
4,763,424 disclose apparatus and a method for providing a
shower of fog onto a web or machine component in contact with
the web. The fog evaporates on the hotter surface and cools
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that surface. U.S. Patent No. 5,881,647 discloses the use of
electrostatic fields to direct a water spray towards a web.
Smoke tunnels are conveniently used in web dryers to
address the generation of smoke during processing. Smoke
tunnels are typically located between the flotation dryer and
the chill stand. More efficient handling of the web as it
exits the dryer would be desirable.
It therefore would be desirable to lower the bulk
temperature of the web in order to decrease the heat load of
the cooling or chill rolls, or even eliminate the chill rolls
or other cooling means such as an air-based cooling zone.
Lowered web bulk temperature also would decrease the
evaporation rate of the solvent mixture coating the web,
thereby reducing the visible vapors evolving from the web and
eliminating the need for a smoke tunnel. Condensation that
normally occurs at the dryer exit and on the cooling rolls
could be controlled to a minimum, and the product quality of
the web could be improved in view of the absence of excessive
moisture loss from the web.
SUMMARY OF THE INVENTION
The problems of the prior art have been overcome by the
present invention, which provides apparatus and a method for
enhancing the effectiveness of a water spray to cool and/or
remoisten a web of material. More specifically, the preferred
embodiment of the apparatus in accordance with the present
invention includes a web dryer, preferably a flotation dryer,
an electrostatic charge generating device or devices, a water
spray, an integrated power supply to supply high voltage power
to the charge device, and a drainage system for handling excess
liquid generated during the cooling and/or remoistening of the
web. The cooling apparatus is capable of a modular arrangement
to optimize spacing and facilitate the addition of cooling
capacity where needed, such as with faster web speeds or
heavier web weights. One or more temperature sensors can be
used to optimize the amount of and rate of fluid fed to the
spray nozzles. The spray nozzles and charge bars can be
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retractable with respect to the web to facilitate web up
procedures.
In its method aspects, the present invention is directed
towards a method for drying and cooling a web, typically a
moving web, by heating the web such as with a plurality of air
flotation nozzles, and enhancing the effectiveness of a water
spray by directing the water spray onto the web, by controlling
the amount of water sprayed onto the web based upon the web
temperature, by removing excess liquid generated during the
cooling and/or remoistening process, and by removing steam
generated by the cooling process.
The present invention thus reduces or eliminates dryer
smoking from the dryer exit end without the use of a
conventional smoke tunnel, reduces or eliminates solvent
condensation problems on the chill rolls, reduces chilled water
use at the chill stand, has less sensitivity to dryer/printing
press process variations, and has lower dryer operating
temperature and associated web exit temperature. It is also
believed that with paper webs, the paper gloss is improved and
the handling in the folder is improved due to slightly higher
residual solvent in the paper web.
Successful operation of the unit requires careful
management of the cooling fluid spray. It is important to
monitor the web temperature and use that temperature
measurement to limit the amount of spray. Excess spray is not
desired as it can migrate into the upper part of the hood and
reduce the life of the charge bars. In addition, extra mist
can pressurize the unit and leak out the web slot and condense
on chill rolls and other surfaces. This mist may also contain
solvent vapor which forms deposits on the surfaces it condenses
on.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic view of one embodiment of the
apparatus in accordance with the present invention;
Figure 2 is a section view of the apparatus of Figure 1
in accordance with the present invention;
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Figure 3 is a schematic view of the apparatus in modular
form in accordance with one embodiment of the present
invention;
Figure 4 is a schematic view of a web flotation dryer with
an electrostatic cooling apparatus in accordance with the
present invention;
Figure 5 is a perspective view of the fan inlet
modification for evaporating excess water generated in the
electrostatic cooling device in accordance with the present
invention;
Figure 6 is a schematic view of an alternative embodiment
of the present invention;
Figure 7 is a schematic view of the apparatus in modular
form in accordance with another embodiment of the present
invention;
Figure 8 is a perspective view of a spray nozzle manifold
in accordance with one embodiment of the present invention;
Figure 9 is a side view of the retractable manifold
arrangement shown in the retracted position in accordance with
one embodiment of the present invention;
Figure 10 is a side view of the retractable manifold
arrangement shown in the unretracted position in accordance
with one embodiment of the present invention;
Figure 11 is a perspective view of the retractable
manifold in accordance with one embodiment of the present
invention; and
Figure 12 is a cross-sectional view of the entrained air
flow in accordance with one embodiment of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
Turning first to Figure 1, there is shown a running web
12 entering a housing 10 through a first web slot 13 and
exiting the housing 10 through a second web slot 14 spaced from
the first web slot and horizontally aligned therewith. A web
slot seal 15 can be used to minimize the transfer of gases
(air) between the housing 10 and outside the housing 10.
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Preferably the housing 10 is thermally insulated, and also
includes an electrical insulator 17 between the metal cladding
of the dryer end frame and the housing 10 to prevent an
unwanted ground path for charge bars 31. The web 12 enters the
housing 10 after having exited a web flotation dryer (Figure
4) attached to the housing 10, passes through the housing 10,
and exits the housing 10 and continues to cooling apparatus
(not shown), for example, such as a chill stand. Preferably
the housing 10 is dimensioned such that it can replace an
existing smoke tunnel and thus be retrofitted into an existing
dryer upon removal of the smoke tunnel which is rendered
obsolete by the present invention. Housing having lengths of
from 20 to 40 inches are generally suitable for this purpose.
The cooling device is also capable of modular design, allowing
two or more modules to be aligned in series as shown in Figure
3. This optimizes spacing of the charge bars and spray nozzles
to minimize cost and maximize spray effectiveness. As faster
web speeds and heavier web weights are encountered, additional
modules can be added. The cross-web dimensions of the device
will vary with web width. Top and bottom access to the
internal components in the housing 10 is provided.
The design of the web flotation dryer can be conventional,
and preferably includes a plurality of upper and lower Coanda
air bars to floatingly dry the running web 12, thereby raising
its temperature.
In a preferred embodiment of the present invention,
integrated into the housing 10 is a power supply 20. By
integrating the power supply to the housing 10, significant
advantages are realized compared to a device having a remote
power supply. Since high voltage (30-50 kV) is required to
operate the apparatus, ease of connecting high voltage to the
charge bars and of supplying residual heat to the enclosure via
a fan 22 to prevent any solvent condensation from occurring is
achieved. This is in contrast to expensive high voltage
connectors or custom fit wiring on-site where the power supply
is remotely located.
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At least one row of spray nozzles 30 are provided, which
receive water (or other suitable cooling fluid, such silicone
and/or other surfactants or fluids which contain silicone or
other surfactants) from a source, such as a reservoir (not
shown), and spray the fluid towards the web 12. A water
softener device may be used for the water supply to prevent
scale buildup from plugging the nozzles. Preferably the spray
nozzles 30 are an axial flow hollow cone type with 0.15 mm
orifice diameters, and are spaced 2 inches between nozzle
centers. The amount an rate of fluid sprayed from the spray
nozzles 30 can be controlled to achieve a desired cooling rate.
A web temperature sensor 24, preferably located at or near the
exit end of the housing, allows the web cooling to be monitored
and maintained at a desired (e.g., predetermined) setpoint by
adjusting pump pressure and flow of fluid to the spray nozzles
30. More specifically, a web temperature setpoint is selected
and the fluid spray pressure is increased until the temperature
is reached or the pump output is at a maximum. A controller
can be used to increase the pump speed which is controlled by
a variable speed drive. Thus, a closed loop pump output
control system using the temperature sensor can be provided to
optimize web cooling. A second web temperature sensor 25
spaced from the sensor 24 also can be used; the preferred
arrangement locating one sensor at the web inlet and the second
at the web outlet. In the two sensor embodiment, a
differential setpoint between inlet and outlet temperatures,
for example, can be used. A suitable differential setpoint
between inlet and outlet temperatures is 40 to 50°, for
example.
A second row of spray nozzles 30A can be provided to
increase the cooling capacity of the apparatus where necessary
or desirable. Suitable valving may be used to allow the fluid
flow to one or more individual nozzles 30, 30A to be
interrupted, especially where a 3/4 or 1/2 width web is run
rather than a full web width, as less fluid will be wasted if
the unneeded nozzles are shut down, and the overhead charge
devices will be protected from the direct fluid spray
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(excessive water spray on the charge devices can cause them to
short out). Water spray also may be applied on both sides of
the web, such as to reduce smoking from the web by quickly
ending the solvent evaporation process. This feature is shown
in Figure 7, which is similar to Figures 1 and 3 except with
the provision for water spray nozzles 30 on the top of the web
as well as the bottom.
A plurality of electrostatic charge generators or bars 31
are provided in the housing 10 above one side of the web 12,
which generate an electric charge or electrostatic field within
the housing 10. A plurality of field director or ground bars
32 are provided in the housing 10 on the opposite side of the
web 12 to direct the fluid droplets exiting the spray nozzles
30 toward the web 12. Preferably ground bar 32 is located
opposite a charge bar 31. The combination of the electrostatic
charge generators 31 and field directors 32 limits the flow
path of the droplets exiting the spray nozzles, ensuring that
most or all of the droplets impinge upon the web 12. The
electrostatic charge on the web 12 created by the bars 31
attracts the spray and causes the droplets to break down into
extremely small particles . These two actions allow a very high
percentage (80-950) of the spray to be used in cooling the web
12. Because the process uses the latent heat of vaporization
of water, efficient cooling of the web is achieved relative to
its temperature when exiting the dryer.
It is an advantage when "webbing up" the device not to
have protrusions which the web can catch on and cause the web
to break. In order to aid the web up process, the nozzles and
bars can be retractable. Figure 8 shows a spray nozzle
manifold 100 having a plurality of nozzles 30 and provisions
for retraction. The nozzles 30 are preferably linearly aligned
along each manifold 100 in the direction across the width of
the web. The opposite ends of the manifold 100 each include
an actuation arm 101 and a pivot point 102. Similar manifolds
can be used for each linear array of ground bars 32 which are
also located below the web line in the cooling assembly.
Multiple nozzles and bars are tied together at the pivot points
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102 with a common bar 104. The actuating arms 102 of each
device are tied together with a second common bar 105. An
actuating device such as a pneumatic cylinder 106 can be used
to automatically retract the devices together. As shown in
Figure 11, one end of the cylinder 106 is coupled to the common
bar 104, and the opposite end is coupled to the common bar 105.
Alternatively, each manifold can be actuated independently.
The nozzles and bars are shown in the retracted position in
Figure 9, in which they are retracted 90° from their operable
position, and in the unretracted position in Figure 10.
Any excess fluid is removed through a drain 35, and also
by small amount of exhaust air (e.g., 100 scfm/ft of web
width). The exhaust air also removes the steam generated by
the evaporation of the water mist (typically web temperatures
encountered as the web exits the dryer heating section are
froma about 260°F to about 320°F, which is hot enough to cause
the mist to become steam). Without adequate exhaust, the unit
will pressurize and send mist out the web slot. The mist may
contain solvent vapor which can condense on surfaces such as
chill rolls and contaminate them. The fluid supply line can
include a valve which can be opened in the event of a web
break, or in order to prevent spray from spraying on a
stationary web (a wet web can easily break when web tension is
reapplied). The exhaust air may be supplied by a fan 22 or by
the normal flow of air through the web slot into a negatively
pressurized enclosure. An optional heater 29 (Figure 3) can
be used to raise the temperature of the atmosphere within the
housing 10.
A preferred method of exhausting air is to pressurize the
top half of the enclosure with the fan 22. This removes
excessive moisture from the area surrounding the charge devices
which may be harmful to them. The air can then travel into the
attached dryer, or be exhausted by independent means. One
embodiment of exhausting air in the present invention is
illustrated in Figure 12, which is particularly applicable for
dryers with low negative pressure at the web slot. A pair of
opposite seal bars 200 are positioned at the web slot to
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produce air jets. The air jets entrain surrounding air and
cause the air to flwo into the dryer through the web slot . The
shape of the bars encourages the jet to adhere to the bar
surface and not disturb the web. An optional suction box 205
which is ducted to the supply or exhaust fan inlet can be used
to produce a greater pressure differential at the web slot.
Figure 2 shows a section view of the apparatus. An edge
seal 28 is provided to establish the proper electrostatic
charge on the web 12. In the embodiment shown, two rows of
nozzles 30, 30A are used, spaced at 2 inch centers, with five
electrostatic charge bars (not shown) and three field director
bars 32. The preferred operating pressure range of the device
is 400 to 1000 psig, with a maximum water flow rate for a
device having 20 nozzles of about 0.3 gallons per minute at
1000 psig.
Figures 4 and 5 illustrate a two-module electrostatic
cooling device in communication with and adjacent to a web
flotation dryer 100 utilizing evaporation to evaporate the
excess water generated in the cooling device. In the
embodiment shown, excess water exiting drains 35 of each module
is directed into a dryer 100 by a suitable driving means such
as a pump 110 and associated piping. Figure 5 shows mounting
details of the optional evaporating device on the fan inlet,
which is the preferred arrangement for this feature. However,
the evaporating device also can be mounted on the fan outlet.
Excess water removed from the housing also can be filtered and
directed back to the water spray nozzles, or can be used for
other purposes such as as make-up water for an offset printing
press dampening water system.
Figure 6 shows another embodiment of the present invention
where the management of infiltration air is carried out. The
housing 10 is shown attached to dryer 100, with a traveling web
12 entering the housing 10 from the dryer 100 exit through a
web slot 13 the size of which may be adjustable. By adjusting
the opening of the web slot 13, the amount of air that flows
back into dryer 100 from the housing 10 due to the negative
pressure maintained in the dryer 100 can be controlled.
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Alternatively or in addition, a perforated plate 50 can be used
to allow air to flow into the housing 10, which air is then
also directed into the dryer enclosure 100 due to the negative
pressure in the dryer 100. Since air entering the housing 10
through the web exit slot 14 can disturb the spray being
applied to the web 12, management of infiltration air such as
by the use of an adjustable web slot or perforated plate can
minimize or eliminate the flow of air into the housing 10
through the web slot 14, thereby minimizing any disturbance of
the spray.
