Radio frequency nozzle bar dryer

Enhancement of the conventional nozzle bars in an air flotation, high speed air impingement web dryer by adding an electrode insulatedly mounted on the conductive surface of the nozzle bar which is in proximity to the web and establishing a plurality of radio frequency fringing electric fields to intercept the web and enhance the drying process is disclosed. The R.F. fields are powered by an integral R.F. generator electrically connected between the insulated electrode and the conductive nozzle bar housing. The system permits the beneficial characteristics of dielectric drying to be added to the conventional air impingement web dryer with minimum re-arrangement problems and maximum power profiling versatility. A further circuit characteristic provides for substantially constant dielectric power transfer to the web independent of variations in the nozzle bar to web spacing.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
This invention relates generally to methods for treating continuously 
moving webs. In particular, it is concerned with tunnel drying of webs 
previously coated with a liquid medium. It combines the effects of gaseous 
fluid impingement which treats the web and optionally supports it with 
high frequency energy input to the web, preferably radio frequency (R.F.), 
which enhances the treatment. 
2. Prior Art 
High speed air impingement dryers are widely used in industry to dry a 
variety of web products such as paper, photographic films, coated fabrics, 
et cetera. In their more advanced form, the air jets from the nozzle bars 
are used also to float and position the web as it moves along the drying 
path thus avoiding mechanical contact with the web and reducing web 
tension build up through the dryer. In this form, such dryers provide 
generally good service and drying speed. Like all air drying systems, 
however, they are subject to several inherent limitations: 
(a) The product must be over-dried in part to insure that any wetter or 
heavier coated areas are fully dried before exiting the dryer to other 
in-line processing steps or a windup. This is particularly troublesome 
when the process web contains anomalously heavier coated areas such as 
edges, coater skips, or splashes. In many cases, such anomalies determine 
the maximum process speed rather than the normal product drying. 
(b) To obtain higher drying speeds for a given dryer path length, the 
operator's only routes are to increase the air jet velocities or the air 
temperature. Neither can be increased indefinitely since excessive values 
of either may damage the coating or the base web. 
(c) As the drying speed is pushed higher, the problem of "skinning" of an 
initially wet coating surface becomes more pronounced. When all or most of 
the drying energy is transferred through the surface, a moisture gradient 
is set up in the coating thickness. This causes the surface to become 
drier than the bulk of the coating and thus lose mobility in the critical 
early drying phases. This prevents the surface tension in the coating from 
acting beneficially to smooth out surface irregularities that occur 
naturally in any coating process. The action described has been observed 
by most people in seeing brush marks gradually disappear from a slowly 
drying varnish coating. 
Many of the difficulties in air or radiant dryers described above can be 
alleviated by introducing dielectric heating energy into the web during 
the drying process. To be beneficial, this additional input need only 
represent a portion of the total energy needed to evaporate the solvent 
from the web. In most practical applications, a good volume of air 
impingement flow onto the web must be maintained to dilute and carry off 
the evaporated solvent. The general characteristics of dielectric drying 
which make it useful in process drying have been well covered in the 
literature and patent art. Briefly, these factors are: 
(a) In the typical case where the wet coating is the principal R.F. lossy 
energy receptor, dielectric heating will provides a compensating action to 
level the drying of coating anomalies across and along the web. This is a 
result of the energy being selectively absorbed proportional to the amount 
of the dielectrically lossy solvent locally present in the web. 
(b) Because the dielectric energy is liberated directly in the bulk of the 
web or coating, a more uniform moisture gradient in the thickness 
direction is achieved. This reduces the "skinning" effect described 
earlier and usually results in improved surface smoothness in coated webs. 
(c) A higher rate of energy input for faster drying can often be achieved 
because the dielectric coupling bypasses the limitations of conventional 
convective heat transfer. Thus, air velocities or temperatures of the air 
impingement dryer which may be excessively high for the product are 
avoided. 
Because the benefits of dielectric drying previously described are 
generally well known in the process drying industries, much effort has 
been expended over the years towards developing improved dielectric dryers 
both in the radio frequency and microwave areas. Some efforts have been 
quite successful but many have been abandoned for a variety of reasons. 
Those familiar with the art will generally agree the following 
difficulties are common: 
(a) A dielectric dryer design for a given application generally turns out 
to be a custom engineering and process development effort and is, 
therefore, usually time consuming and costly. 
(b) With conventional dielectric dryers, it is difficult to predict the 
energy input profile along the dryer path and just as difficult to adjust 
it after the process is put in operation. For sensitive products, this 
represents a very high technical risk for the plant operator who is 
attempting to obtain some of the the inherent benefits of dielectric 
heating. As a result, most process operators opt for a system using only 
air impingement drying since it can be engineered faster and more 
predictably. 
Most of these difficulties stem from the design of the conventional radio 
frequency web dryer. Typically it consists of a ladder-like array of 
alternating electrically "hot" and grounded electrodes which establish a 
fringing electric field to intercept the proximate process web. The 
electrodes are bussed together on heavy R.F. conductor systems to a common 
radio frequency power generator. R.F. generator powers in the 10 kilowatt 
to 50 kilowatt range are fairly common in the industry. In this 
arrangement, it is difficult to extend the applicator length or remote the 
common generator more than about one-quarter wavelength of the operating 
frequency because of the voltage standing wave effects that are 
encountered. In such an arrangement, the voltage level across each 
electrode pair and hence the R.F. electric field is substantially the same 
throughout the whole array. The overall level can be adjusted from the 
common generator or through various circuit coupling means known in the 
art. With this arrangement, the difficulty in predicting or controlling 
the power input to the web along its drying path through the electrode 
system stems from two major effects. First, the local energy coupling to 
the web will vary strongly as the inverse of the gap between the electrode 
pair and the web. Thus, if the planarity or positioning of the web is less 
than perfectly controllable, the local energy input will also be 
uncontrollable. The second factor has to do with the complex nature of the 
dielectric loss factor of the material which is the receptor for the 
energy. In R.F. systems, this is usually a partially conductive solvent 
containing ionic solutes. In microwave systems, additional mechanisms come 
into play such as polar molecule coupling. The amount of energy locally 
transferred depends simultaneously on the local conditions of solvent 
quantity, its solute concentration, and its temperature. All these factors 
are varying along the dryer path in a complex and interdependent manner. 
What is needed by the drying industry is an efficient approach to combining 
the best features of air impingement drying with the best features of 
dielectric drying. When used in combination, a synergism results to 
produce a drying system superior to either approach used alone. Both 
mediums contribute to the total energy transfer to the web. The R.F. 
contributes to leveling of coating anomalies while the air impingement 
carries off evaporated solvent and helps maintain the coating temperature 
nearer the dew point rather than its boiling point. 
There have been some efforts in the industry to achieve this goal and get 
around the engineering and process problems associated with combining air 
impingement and dielectric drying. One such approach is described in U.S. 
Pat. No. 4,257,167 issued to H. C. Grassmann. In that approach, individual 
air impingement nozzle bars of an approximately conventional design are 
made to act as alternate polarity electrode bars of a stray field R.F. 
coupler. The active fringing R.F. field is established between the 
separate nozzle bars. This generally leads to inefficient dielectric 
energy coupling because the optimum nozzle bar spacing is usually too 
large to establish an optimum R.F. field. Grassman partially overcomes 
this by showing optional satellite electrode bars added between the nozzle 
bars. The entire set of nozzle bars acting as electrodes is driven from a 
common R.F. power generator as in a conventional R.F. stray field ladder 
electrode. 
Although the arrangement described by Grassman will provide a route to 
achieving a potentially useful combination of air impingement and 
dielectric drying, it is still subject to all the engineering and process 
problems ascribed earlier to dielectric dryers. These include the problems 
of distributing substantial amount of R.F. power from a remote generator 
over the length of a tunnel dryer which might extend hundreds of feet. The 
arrangement also precludes much versatility in experimenting with the 
optimum number and placement of R.F. heating zones in an existing long 
tunnel dryer and establishing a controllable power transfer profile along 
that length. 
Others have attempted to achieve combined air flow and dielectric drying by 
utilizing microwave power sources. Such microwave applicators, typically 
utilizing serpentine wave guide sections experience great difficulty in 
providing controlled distribution of energy input both across and along 
the web in the processing zone. 
3. Definitions 
Nozzle Bar, as referred to herein, means a structure, usually elongated, 
disposed transversely to the path of the moving web and in close proximity 
thereto. It provides jets of gas through one or more slot or hole orifices 
to impinge on a proximate web. This impinging flow is typically used to 
heat, condition, or dry the web. In addition, the kinetic energy of the 
gas flow may be directed so as to create a zone of pressure higher than 
ambient to provide mechanical positioning and/or support of the moving 
web. 
Air, as referred to herein, means any gaseous fluid capable of transporting 
sensible or latent heat to or from the web and usually is capable of 
transporting any solvent vapors released from the web to collection points 
away from the processing area. In a typical application, the gas is air of 
a controlled temperature and humidity. 
Proximate, as referred to herein, means the region between the nozzle bar 
and the web within the area projected by the individual nozzle bar 
structure on the web. Depending on the portion of the discussion, it may 
be used in conjunction with the web surface, working surface of the nozzle 
bar, or the space between these two. This is the region wherein the 
impingement gas jets provide the bulk of their heat transfer action and, 
also, provide any pressure support for web positioning if that feature is 
included in the design. 
Radio Frequency (or R.F.), as referred to herein, means an electrical 
voltage or current whose polarity is reversing periodically with time. A 
generally accepted frequency of reversal for R.F. is approximately 0.5 
Megahertz to approximately 500 Megahertz. For industrial heating 
applications there are legal and technical preferences for operating at 
one of the ISM (Industrial, Scientific, and Medical) bands allocated in 
Part 18 of the Federal Communications Commission regulations. 
SUMMARY OF THE INVENTION 
It is the object of the invention to provide a method and apparatus to 
permit a process web drying operator to obtain the benefits inherent in 
the combination of air impingement and dielectric drying in a manner which 
avoids the major technical and economic problems of previously existing 
approaches. In addition to its process advantages, the new approach 
described below will provide significant commercial advantages to the 
owner of an existing conventional air impingement/flotation dryer wishing 
to upgrade its performance with minimum technical risk and lost production 
time. 
This objective is accomplished by the instant invention of an apparatus 
which combines both the ducting and impingement air jets of a conventional 
nozzle bar with a self-contained radio frequency energy applicator. The 
conductive, typically metallic, sides and top edge of the housing and 
nozzle edges are utilized to form a pair of electrodes, conveniently 
operated at electrical ground potential. A single or multiple conductor 
bar is located centrally on the surface of the nozzle bar assembly which 
is proximate to the process web. This conductive bar acts as the 
electrically "hot" electrode. A radio frequency generator, preferably 
located inside the envelope of the nozzle bar housing, supplies high 
frequency power to this electrode and sets up fringing R.F. electric 
fields with each of the side ground electrodes which are the adjacent 
areas of the nozzle structure which are connected to the generator as 
well. These fringing electric fields are confined substantially to the 
proximate space between the nozzle bar and the web. They intercept and 
penetrate the proximate web undergoing drying and couple additional energy 
to it by the same dielectric loss mechanisms as other dielectric dryers. 
A unique feature of the electrical circuitry of the invention is that it 
intrinsically senses and adjusts the R.F. electric field intensity 
existing across the electrode pairs if the proximate web varies its 
distance from the electrode faces. It does this in such a manner that the 
nominal level of the power transfer from the device to the web remains 
substantially constant over normal gap variations. It is possible, also, 
to adjust the nominal power level of transfer by externally varying the 
D.C. supply voltage to the circuit thereby permitting the power level 
along the drying path to be profiled for optimum process drying results. 
The above features can be realized in an assembly whose external envelope 
is substantially the same as a conventional nozzle bar. In most cases, the 
assembly can be arranged to bolt into the mounting position and substitute 
for any conventional nozzle bar in an existing air flotation dryer. It can 
connect, without modification, to the existing air supply duct connector. 
The only required modifications to an existing machine would be the 
addition of low voltage D.C. power supply and optional control cables 
through the dryer tunnel. Operating safety would also make it prudent to 
add electrical interlocks on close by operator access doors. It will be 
necessary also to verify the electrical shielding integrity of the dryer 
tunnel or process enclosure to insure no excessive level of 
electromagnetic radiation can escape to the environment. 
Given the features described above, the objectives of the invention can be 
realized in a straight forward manner. 
(a) The unitized assemblies, if used as direct, bolt-in replacements for 
conventional nozzle bars, will permit simple, low risk development of 
critical process/product parameters by direct on-line tests. The units can 
be quickly installed in various numbers or positions and evaluated. So 
called "A/B" test comparisons may be obtained with "air only" drying 
simply by turning off the electrical power. If for some reason, the 
supplemental R.F. energy input causes detrimental effects, normal 
production can be resumed simply by turning off the electrical power or 
re-bolting the conventional nozzle bars in place. 
(b) By providing integral R.F. generators in each R.F./Nozzle bar module, 
the assemblies can be utilized over very long tunnel lengths in any 
positional arrangement without encountering the technical problems and 
costs attendant to bussing all the bars from a remote common R.F. 
generator. 
(c) The use of low power individual generators in each module provides two 
useful characteristics that were not available in the previous art. First, 
the power level of each assembly may be made different or remotely 
adjustable so that a specific R.F. power input profile can be established 
along the dryer path length. This can be particularly useful in the 
processing of sensitive products. Secondly, as each module power supply is 
limited in its total output, it is unlikely an anomalous heavy coating 
defect can cause a catastrophic energy input wherein all the available 
power of a large common R.F. generator is drawn into one small portion of 
the web. 
(d) The electrical circuit features which maintain the transferred power 
substantially constant independent of typical gap variations will provide 
the dryer operator a wider process latitude than has been available with 
previous art. By reducing the effects of web tension, product changes et 
cetera which can all affect the gap variations, it should be possible to 
improve process efficiency and quality control. 
(e) By incorporating the fringing field electrode elements in the proximate 
face of the nozzle bar several important advantages are obtained over 
previous art. 
The nominal electrode to web gap is generally smaller and better controlled 
in this region than in the space between nozzle bars. The smaller gap 
provides better electrical coupling than larger gaps. Also, the 
proportioning between the spacing of the electrode elements, gap, and 
material dielectric loss characteristics may be optimized for a given 
application independent of the impingement air jet design or the nozzle 
bar to nozzle bar spacing. 
The overall result of the present invention as a consequence of the 
features described is to provide the web drying industry with a approach 
which will secure the maximum benefits of combined air impingement and 
R.F. drying without the costly detriments and technical risks associated 
with the previous art.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 shows the schematic arrangement of a typical air impingement 
flotation dryer. A base web 1 is supplied from either a continuous 
production process or a supply roll 2. The web 1 passes through a coating 
station 3 where some material to enhance the final product properties is 
applied to one or both sides of the base web surface. The design of the 
coating station may take many forms depending on the nature of the 
coating. From the coating station, the coated web passes in a dryer tunnel 
4 whose length is appropriate for the web speed and drying rate. In the 
tunnel, energy is transferred into the web to effect heating, drying, and 
possibly curing of the coated product. Typically, the major thermal load 
is evaporating a carrier solvent which is applied as part of the coating 
mix. In other cases, a liquid may be present in the base web which must be 
dried even without a coating applied. An example of this is evaporating 
the water from a web of paper. 
Upon completing the path through the dryer tunnel 4, the product web should 
be satisfactorily dried. After exiting, it usually passes across rollers 5 
to provide traction, tension isolation, or web guidance. From there, the 
web typically is passed to a product windup 6 or directly to further 
in-line finishing steps. 
In such an air impingement flotation dryer, the energy transfer is effected 
by the action of high velocity gas streams 7 impinging on one or both 
sides of the web from a series of nozzle bars 8 spaced along the the web 
path on one or both sides. Generally the gas is air whose temperature and 
humidity is controlled and supplied to the individual nozzle bars by one 
or more distribution ducts 9. Velocities of the impingement air issuing 
from the nozzle slots may range from approximately 200 to 6,000 feet per 
minute depending on the application. In addition to impinging the gas 
stream on the product web for good heat transfer, the nozzle bars 8 also 
may perform the function of supporting the web as it passes through the 
dryer tunnel length. This is accomplished by creating a zone 10 of 
increased gas pressure between the working face of the nozzle bar and the 
adjacent section of product web. In general, for this flotation support 
action to be achieved, nozzle jets on opposing sides of the working face 
must have a component of tangential velocity directed inward to the 
centerline of the nozzle bar. This is to overcome the Bernoulli effect of 
the accelerating gas flow as it escapes the zone 10 between the bar face 
and the web which, if not counteracted, would cause the web to be drawn 
against the bar. Requirements and various design approaches to accomplish 
a stable web support with good gas-to-web heat transfer are well known in 
the art. 
In the present invention, the physical envelope, air impingement action, 
and web flotation action are still present in essentially the same form as 
the prior art. The nozzle bar, however, provides for the addition of a 
high frequency electric field to be present in the proximate space of the 
nozzle bar and web. That is to say the space between the working face of 
the bar and its projection on the product web. This is illustrated in FIG. 
2. The nozzle bar assembly 8 is typically of similar physical outline to 
regular air impingement nozzle bar and mechanically supported on tunnel 
rails etc. in the same way. Conditioned drying air 11 is transferred from 
the common distribution duct 9 via an aperture connection to the bar not 
shown in this view and is then distributed internally within the bar. 
After traversing the internal passages within the nozzle bar body, the air 
exits uniformly from nozzles 12 along the bar length. These nozzles are 
shown as the typical slot jets but may take the form of a series of hole 
orifices or other perforations as is known in the art. Air flow 7 exiting 
the nozzles is partially indicated by the arrows. The nozzle bar assembly 
8 is positioned transversely across the dirction of motion of the product 
web 1 as is known in the art. 
Unlike a conventional nozzle bar, the present invention provides one or 
more electrically isolated electrode bars 13 which extend along the 
assembly length on its working face 14, said face 14 being the one which 
is in proximity to the process web 1. A high frequency electrical 
generator, not shown in this figure, preferably housed inside the body of 
the nozzle bar 8, is connected to establish a high frequency, preferably 
in the radio frequency range, high intensity, alternating electric field 
15 between the electrode 13 and the electrically conductive nozzle bar 
assembly body components 8. Preferably, the nozzle bar assembly is held at 
ground potential for safety and electrical convenience. The electric field 
15 which is partially shown as dotted lines has field lines which extend 
into the electrically non-conductive dielectric space between the 
electrode 13 and the nozzle body 8. These field lines fringe out into this 
general space as is covered in texts on electric fields and partially 
intercept the proximate product web 1. Electrical power for driving the 
high frequency generator is conveniently supplied via cable 47 from a 
remote direct current power supply 41. 
A more detailed view is shown in FIG. 3. This figure shows an end elevation 
of the nozzle body of this invention with the end cover removed. The 
product web 1 is positioned above the working face of the R.F. enhanced 
nozzle bar assembly at a gap spacing G typically about 0.062 to 0.500 
inches. Conditioned air 11 enters the bar assembly via the connecting 
aperture 20, shown here in section for clarity. Usually some type of 
locating collar and air sealing gasket 21 are provided to prevent air 
leakage. The air first enters a plenum or distribution chamber 22 inside 
the body of the bar. From this space it can communicate along the length 
of the nozzle bar assembly and be distributed to the nozzles slots in a 
uniform manner. Uniformity in the design shown is improved by providing a 
small pressure drop by means of a series of distribution holes 23 along 
the length of the skirts of the internal baffles 24. After passing through 
the distribution holes, the air flows upward in the spaces 25 between the 
walls formed by the outer housing 26 and the inner baffles 24. From the 
spaces 25, the air flow continues to the impingement nozzles 12 which, for 
the design shown, are slot orifices whose sides are formed from extensions 
of the housing body 26 and the baffle plates 24. The slow width is 
generally narrower than approach channel spaces 25 and is sized to provide 
the desired volume of air flow for the design value of jet velocity. The 
spacers 28 which are spaced at intervals along the housing wall illustrate 
one method of fastening the assembly and providing rigidity for the nozzle 
gap. Internal air pressure in the duct 9 might range up to approximately 
several inches of water pressure to provide commercially useful 
impingement velocities. As the air 7 issues from the nozzle orifices 12 to 
impinge on the product web 1, it is provided with a component of inward 
velocity to oppose that of the jet from the other side. This helps create 
a higher pressure zone 10 across more of the proximate working face 14 of 
the nozzle bar and thus both support the product web 1 and provide a 
restoring force to prevent a still wet web from mechanically contacting 
parts of the nozzle bar. 
The electronic components of the high frequency generator are located 
inside the envelope of the structure as shown by the area 30. Those 
components may be mounted on a printed circuit board 31 or could be 
mounted directly on the septum plate 32. One output terminal of the 
oscillator circuit is electrically connected to the electrode bar 13 via a 
conductor 33. The other output terminal is connected to the nozzle body 
housing via conductor 35. Electrode 13 is supported on an insulating board 
34 which provides for both the mechanical support and sealing of air 
leakage from the working face area 14. Practically, this insulating board 
34 must be a high quality dielectric material which will have negligible 
dielectric loss at the frequency and electric field strength present in 
the area. Also, it should have adequate mechanical properties for the 
temperature and environment present in the area. 
FIG. 4 shows a schematic cross section view that more fully illustrates the 
positioning and dielectric heating action of the high frequency electric 
field that is established between the electrode 13 and the nozzle 
components 24 and 26. The R.F. generator circuit 36 creates a high 
frequency voltage V(RF) across its output terminals. One side is connected 
to the nozzle components, 24, 26, typically sheet metal parts, and 
conveniently tied to the machine frame electrical ground. The other R.F. 
generator terminal is connected to the electrically isolated electrode bar 
13. This establishes fringing electric fields 15 in the electrically 
non-conductive dielectric space surrounding the conductors as indicated by 
the dashed lines in FIG. 4. If the product web 1 is in reasonable 
proximity, field lines will enter it and, in fact, will tend to be 
concentrated therein since the product, typically, will have a dielectric 
constant greater than the air and therefore present a lower dielectric 
impedance. The action of this field in the product is to cause a 
displacement electric current to flow in the product at each reversal of 
the applied field polarity. If the product web or its coating possesses a 
dielectric loss factor e.sub.r " of a reasonably high value, a useful 
portion of this alternating current flow will be transformed into heat 
directly in the body of the product. This heating effect will be 
concentrated in the areas of highest displacement current, typically the 
projection of the gap S between the electrode elements 13, 24 onto the web 
1 in the regions indicated by 37. 
One dielectric component not shown on FIG. 4 is the insulating support 
board 34 noted in FIG. 3. This was omitted from FIG. 4 for clarity. It 
will also intercept and concentrate a portion of the electric field 15. 
Because it is selected to possess a low dielectric loss factor, no 
significant heating of its material will occur. It will represent only a 
passive capacitance which may be electrically compensated in the R.F. 
generator circuit. 
Referring now to FIG. 5A, the schematic diagram shows the essential 
electrical components of two electrode gaps of an R.F. stray field 
electrode coupling system as might be used typically with the present 
invention. One objective of the design process is to obtain the maximum 
energy coupling for a given, acceptable electrode voltage V(RF) within the 
available working space, in this instance the width W1 of the working face 
14 of the nozzle bar assembly. Such coupling depends on the parameters of 
the average gap G, the electrode spacing S, the effective electrode widths 
W2 and W3, and the dielectric constant e.sub.r ' and loss factor e.sub.r " 
of the product load 1. Usually, the coupling characteristics are best 
determined by laboratory tests on mockups and the results will permit the 
designer to proportion the electrode widths W2 and W3 and the spacing S 
for optimum results. Typically, as the the gap G gets smaller, the optimum 
values for the spacing S and widths W2, W3 get smaller. Thus, if the 
application involves a product web running very close to the working face 
of the nozzle bar, an optimum design may require the use of more than the 
two fringing fields shown in FIG. 5A. One such alternate arrangement is 
illustrated schematically in FIG. 5B wherein multiple isolated "hot" 
electrodes are placed on the face 14 interspersed with grounded electrodes 
which are part of that face. 
The electrical coupling of the arrangements shown in FIG. 5A and FIG. 5B 
may be represented reasonably by an equivalent electrical circuit shown 
schematically in FIG. 5C. In this circuit, CS represents the shunt 
capacitances of the air gaps S and support insulator 34 between the 
electrodes. CA represents the air gap capacitances between the face of the 
electrode bars and the product web. CP represents the combined capacitance 
of the product web and its coating and RP represents the the equivalent 
parallel electrical loss of that same combination. For a given set of 
parameters and a given R.F. operating frequency, the equivalent circuit of 
FIG. 5C may be further simplified by standard electrical analysis 
techniques to that shown in FIG. 5D. In FIG. 5D, CE represents the 
equivalent capacitance of the load and RS represents the equivalent series 
loss resistance. This simpler form is useful in considering the load 
effects and the relationship between the voltage appearing across the 
electrodes V(RF) and the voltage supplied by the R.F. generator V(G). This 
will be used in later discussions covering an additional improvement of 
the present invention. 
In a typical dielectric web heater wherein multiple pairs of stray field 
electrodes are directly connected in parallel to the common R.F. 
generator, the capacitance of the electrode structure is the major 
capacitance of the generator's resonant tank circuit. In this arrangement, 
the electrodes, as pairs and as a group, tend to operate at essentially a 
constant voltage. That is to say, V(RF) in FIG. 5A or FIG. 5B is constant. 
This gives rise to a power transfer characteristic such as shown in FIG. 
6A by curve A. As the gap distance G between the product web and the 
electrodes is increased, the power P transferred to the web decreases 
about as an inverse function. This is a natural consequence of holding the 
electric field gradient constant [V(RF)=constant ] and moving the film 
outward where it intercepts less and less of the electric field lines. It 
may be interpreted also in the schematic circuit of FIG. 5C as an increase 
in the impedances of CA. Prior art devices operate in this manner. For a 
typical drying machine, this gives rise to unpredictability in the 
performance because it is impossible to always control the web-to-nozzle 
bar spacing G in an exact manner. A variety of factors such as changes in 
web tension, web curling, and aerodynamic fluttering can all conspire to 
periodically change the gap G in an unpredictable manner. 
In the present invention, the electrode circuit as represented by CE and RS 
of FIG. 5D is combined with a a series inductor L as shown in FIG. 6B. The 
value of the series inductor L is selected so that L and CE are series 
resonant at the operating frequency F. As is covered in standard 
electrical engineering texts, such a circuit has a number of distinct 
characteristics. First, when driven at its resonant frequency, the 
alternating voltage V(RF) appearing across CE and RS, which is the voltage 
across the electrodes, can be much larger than the driving voltage V(G) of 
the generator which is applied to the input terminals of the circuit. More 
specifically, the electrode voltage V(RF) is equal to the generator 
voltage V(G) multiplied by the circuit quality factor Q. This quality 
factor Q for resonant circuits is covered in standard electrical circuit 
texts and may be enumerated for the series circuit by dividing the circuit 
reactance of either inductor L or capacitance CE by the total series 
circuit resistance. This voltage gain characteristic provides a practical 
engineering convenience by supplying a method of obtaining the several 
hundred to several thousand volts desired across the electrodes from a 
solid state high frequency generator which most practically is operated at 
D.C. supply voltages from about 12 to 50 volts. 
With the circuit arrangement of FIG. 6B, assuming the R.F. generator 
voltage V(G) is constant, the power transfer characteristics are shown on 
FIG. 6C on the curve labeled B. As shown, the power transfer actually 
increases as the the gap increases. This is because, as the gap increases, 
the equivalent series loss resistor RS decreases in value, Q increases, 
and the electrode voltage V(RF) increases. Because the dielectric heating 
effect increases as the square of the electric field strength, the effect 
is to increase the power coupling as the gap increases and the generator 
voltage V(G) is held constant. 
A power transfer that rises with increasing gap could be as troublesome as 
the falling characteristic shown in FIG. 6A. Also, it might lead to 
excessively high electrode voltages and arcing if the load loss 
represented by resistor RS gets too low. An element of the present 
invention is the addition of a fourth element in the output circuit to 
provide a more desirable characteristic. This is the addition of a ballast 
resistance RB in the series resonant output circuit shown in FIG. 6D. With 
the addition of this element, when sized appropriately, a power coupling 
characteristic as shown in curve C of FIG. 6C can be obtained. The 
arrangement of FIG. 6D thus can be made to provide substantially constant 
power transfer to the product web even as the gap varies over its 
practical extremes. This characteristic is obtained because the added 
ballast resistor places limits on how fast the circuit Q can rise as the 
load moves into a lower field region. Thus, the electrode voltage rises at 
a rate just sufficient to maintain the substantially constant coupling. 
It will be realized that the ohmic R.F. impedance of the ballast resistor 
RB does not have to be lumped in one discrete component but may utilize 
the inherent residual losses of the inductor L or the general circuit 
conductors to form part or all of the required value. 
The addition of this feature providing substantially constant power 
transfer to the product web provides the process operator with a system 
which will exhibit more predictable drying rate performance and more 
operating tolerance relative to the various factors affecting the web 
positioning. 
FIG. 7 shows a schematic drawing of one radio frequency generator system 
which is the preferred embodiment of the present invention. Referring to 
this diagram, electrical power is conveniently supplied from single or 
multiple phase plant mains 40 by normal practice. It enters a power supply 
41 which is conveniently a separate assembly and located outside the dryer 
tunnel 4 as illustrated in FIGS. 1 and 2. In one portion 42 of the power 
supply 41, the incoming mains power is transformed into one or more direct 
current sources whose voltage may be fixed or adjustable. The power supply 
may also contain additional circuitry 43 to monitor current flow or other 
parameters of interest. Optionally, voltage level adjustment, current 
monitoring, on/off control, et cetera may all be handled at a remote 
location 44 via electrical cabling 45. Such a remote location might be a 
process control room or computer. All the power supply actions mentioned 
are well known in the existing art. 
The output of the D.C. power supply 41 is connected to one or more 
R.F./nozzle bar assemblies of the invention. In this figure, the 
electrical elements of one such bar are those shown enclosed by the dashed 
line 46. The connection between the power supply 41 and a nozzle bar 
electrical elements 46 could be accomplished conveniently by a 
multi-conductor insulated cable 47. The primary D.C. power flow V1(DC) 
enters the individual nozzle bar assembly elements 46 via the terminal 48. 
For the particular circuit devices used in this illustration, the terminal 
48 would have a positive polarity. From terminal 48, the current flows 
through a low pass network composed of the capacitors C1, C2, C3, and 
inductor L1. The function of this conventional network is to pass the 
average D.C. power easily into the R.F. oscillator circuit while 
preventing the high frequency power present in the oscillator from flowing 
backward into the power supply. The generation of high frequency 
oscillations is accomplished by the combination of the active solid state 
transistor Q1, feedback inductor LF, and the the series resonant output 
circuit. The series resonant output circuit is composed of the series 
inductor L and the capacitance and equivalent load loss resistance of the 
applicator electrode 13, these latter being represented in FIG. 6B as CE 
and RS. The oscillator action is relatively conventional and is well 
covered in electrical engineering texts. Briefly, the action proceeds as 
the following sequence. Assume the transistor Q1 is near cutoff and a 
rising current I1 is flowing into the series resonant output circuit. As 
it passes through the series inductor L, a magnetic mutual coupling M 
between it and the feedback inductor coil LF induces a voltage in coil LF. 
The phasing of the coils is such that this voltage acts to further reduce 
the base current of Q1 and thus drive it further into cutoff. This action 
continues until current saturation of the output circuit occurs controlled 
by its ohmic impedances. At this point the induced voltage in LF drops to 
zero and then begins to reverse. The action of the reversal is to increase 
the base current in the transistor Q1 and start it into its conductive 
state. As Q1 becomes conductive, it provides a return current path I2 for 
the current I1 which earlier flowed into and charged the output circuit. 
This current can now flow to the electrical ground via transistor Q1 and 
complete the circuit back to the power supply 41 via terminal 49. Finally, 
the capacitance of the output circuit is discharged and the feedback 
voltage again reverses. This starts the cycle again. 
The combination of the inductor L2 and capacitor C4 form a parallel 
resonant circuit which is broadly resonant at the operating frequency of 
the oscillator. This provides further isolation of the oscillator from the 
power supply and serves also as the source for the pulses of radio 
frequency current used to charge the series resonant output circuit at 
each cycle. 
A second D.C. source input V2(DC) enters assembly 46 via terminal 50. Its 
function is to provide an initial base drive current I3 to the transistor 
Q1. This is necessary to provide an initial quiescent circuit gain so that 
oscillations may start spontaneously. The amount of the initial base drive 
is controlled by the input voltage V2(DC), and resistors R1 and R2. After 
circuit oscillations begin, the R.F. component of the feedback signal 
flows through bypass capacitor C5 and is partially rectified at the 
base-emitter junction of transistor Q1. This action, in conjunction with 
resistor R2 and diodes D1 and D2, resets the transistor operating bias 
current I4 to provide for operation in the class C region. The meaning and 
characteristics of class C operation are well covered in the art. The 
opposed diodes D1 and D2 provide also a limiting action to protect the 
transistor base-emitter junction from excessive voltage after strong 
oscillations start. 
The small capacitor, C6 which is connected across the base-emitter junction 
of the transistor Q1 works in conjunction with the residual inductance of 
the transistor base lead not shown in this diagram. The combination serves 
to match the electrical impedance of the feedback signal with the 
impedance of the base-emitter junction as is known in the art. The action 
of the ballast resistor RB is to provide the output circuit with the 
constant power transfer characteristic explained earlier and illustrated 
by curve B of FIG. 6C. 
The transistor used in this circuit must be suitable for service at the 
circuit operating frequency. Generally, this will mean a transistor 
specifically designed for adequate gain and required power output at the 
operating R.F. frequency. Newer types of field effect transistors are also 
good candidates for this service. 
A typical set of component and operating values for the circuit in 
practical service are as follow: 
______________________________________ 
Nozzle Bar length 18 inches 
No. of "hot" electrodes 
1 
Q1 Output power 100 to 150 watts 
Voltage V1 (DC) 28 volts DC 
Voltage V2 (DC) 0 to 10 volts 
Operating Frequency (ISM) 
27.12 Megahertz 
C1 5 microfarads 
C2, C3, C5 .01 microfarads 
L1 (R.F. Choke) 5 turns #22 .25" I.D. 
L2 (Resonator) 1.5 turns #22 .25" dia. 
C4 68 picofarads 
C6 .05 microfarads 
Q1 MRF-327 
R1 180 ohms 
R2 10 ohms 
D1, D2 1N4005 
L 11 T #12 1.5" I.D. 
LF 2 T #12 1.5" I.D. 
RB 5 ohms 
______________________________________ 
It will be recognized by those familiar with circuit design that there are 
innumerable variations possible in the design of electronic power 
oscillators. The design shown is practical for the given application but 
could easily be changed as needed. For example, requirements for longer 
nozzle bars of higher electrical power may require the use of multiple 
transistors in parallel. Also, progress in the design of such devices is 
rapid and newer units might be used as they become available and offer 
technical advantages. It is also likely that the power supply for a given 
application may include additional safety and convenience features known 
in the art. These might include current limiting to prevent excessive 
electrode voltage excursions as the load RS decreases, thermal overload 
protection, et cetera.