[0001] The invention relates to methods of and apparatus for convective drying of materials
in which heated air or other gas is directed onto the material to be dried.
[0002] Such a dryer may comprise a plurality of nozzles or jets for. the heated air or gas
and the material is passed between or below these jets or nozzles. The material may
be a web, e.g. of fabric, which is passed between the jets or nozzles or it may be
material, e.g. sheet material or pieces, which is carried on a belt or other carrier
between or below the jets or nozzles.
[0003] According to the present invention, in a convective dryer having a plurality of jets
or nozzles for directing a heated gas onto material passed over the jets or nozzles,
these jets or nozzles are formed of electrically conductive material and are arranged
to constitute the electrodes or at least some of the electrodes of radio frequency
heating means providing a radio frequency field in the region through which the material
to be dried is passed.
[0004] By this arrangement, the radio frequency heating means heats the material internally
so that heat transfer to the material occurs within the body of the material as well
as on the surface thereof where the convective heating is effected.
[0005] The internal heating of the material causes moisture from the interior to move to
the surface of the material, keeping this surface wet and so allowing higher air (or
other gas) temperatures to be used than would be the case in the absence of radio
frequency heating. Thus the overall drying efficiency is increased, resulting in increased
drying speeds and reduced dryer length and may give improved material quality.
[0006] The invention furthermore includes within its scope a method of drying a material
comprising the steps of passing the material through a heated gas stream and simultaneously
subjecting the material to a radio frequency field.
[0007] The material to be dried would usually be electrically non-conductive and thus the
radio frequency field would give dielectric heating. The frequency of operation may
be chosen in accordance with known practice for dielectric heating.
[0008] The radio frequency heating may be "stray field" heating in which the electrodes
are all arranged beneath the material to be dried. Preferably however "through field"
heating is effected using electrodes above the material co-operating with electrodes
below the material. The electrodes in such an arrangement preferably are staggered,
that is to say a upper electrodes are arranged above spaces between lower electrodes.
[0009] Preferably the major part of the heat input is applied to heating the air or other
gas for the convective drying. Typically the amount of power input to the radio frequency
heating is between 1/5th and 1/10th of the.total energy input to the dryer.
[0010] The jets or nozzles for the heated gas may be arranged, in the known way, to form
an air flotation device to support the material to be dried. Such an arrangement may
conveniently be used for the drying of webs of material.
[0011] For materials which have to be carried through the dryer on a belt, it is convenient
to provide jets or nozzles both above and below the belt, these jets or nozzles being
arranged to form electrodes of a "through field" radio frequency heating system. Such
an arrangement may conveniently be applied to the drying of sheet materials such as
insulation board and plaster board and to other products such as pulp or fibre mouldings.
The technique may also be applied to the baking or processing of a wide range of foodstuffs.
[0012] In the following description of one embodiment of the invention, reference will be
made to the accompanying drawing which illustrates diagrammatically in side elevation
a dryer for web material.
[0013] Referring to the drawing, the material to be dried is shown diagrammatically as a
web 10 of electrically non-conductive material which is moved through the dryer in
the direction of the arrow A. Above the web is a plurality of spaced nozzle boxes
11 each constituting an electrode and each having one or more nozzles 12 for directing
heated air onto the web. Similarly, below the web, there are nozzle boxes 13, these
nozzle boxes constituting electrodes and having nozzles 14 to direct heated air onto
the web.
[0014] Electrically the nozzle boxes 11 are connected as shown at 15 in parallel to one
output terminal 16 of a radio frequency generator 17. The nozzle boxes 13 are connected
electrically in parallel as shown at 18 to the second terminal 19 of the radio frequency
generator. The various nozzles are fed from an air heater 20 in which the heat may
be generated from any convenient fuel source for example, gas, oil, steam banks or
electrical resistance heaters.
[0015] The resonant frequency of the output circuit of the radio frequency heater may be
made controllable, as is well known in radio frequency heating, e.g. by the use of
adjustable capacitors or inductors in the radio frequency circuit.
[0016] The positional arrangement of the electrodes in which the upper electrodes 11 are
staggered with respect to the lower electrodes 13, provides a "through field" radio
frequency heating system which permits relatively thick materials to be processed
without substantial differential heating occurring. The ratio of the vertical spacing
between the upper and lower electrodes (as shown at a in the figure) to the horizontal
spacing (as shown at b in the figure) is chosen to suit the particular material being
dried, this ratio being chosen in accordance with the thickness of the material,_
the loss factor and the vapour diffusion characteristics through the material, as
well as to give optimum web stability and convective heat transfer. The radio frequency
heating heats the material internally and so tends to drive moisture towards the surface
of the material thereby keeping the surface of the material wet. This allows the drying
to take place at a much higher air temperature than would be possible if there was
no radio frequency heating and hence gives an improved drying efficiency and increased
drying speeds.
[0017] The major part of the energy input to the system is into the air heaters. The amount
of energy provided by the radio frequency heating is typically between 1/5th and 1/10th
of the total energy input to the dryer.
[0018] Although in the embodiment illustrated, the electrodes and nozzles are spaced along
the length of the web to be dried, they may additionally or alternatively be spaced
across the width of the web.
1. A convective dryer having a plurality of jets or nozzles for directing a heated
gas onto material passed over the jets or nozzles wherein the jets or nozzles are
formed of electrically conductive material and are arranged to constitute the electrodes
or at least some of the electrodes of radio frequency heating means providing a radio
frequency field in the region through which the material to be dried is passed.
2. A convective dryer as claimed in claim 1 wherein the electrodes are all arranged
beneath the material to be dried.
3. A convective dryer as claimed in claim 1 wherein electrodes are provided above
the material co-operating with electrodes below the material.
4. A convective dryer as claimed in claim 3 wherein the electrodes are staggered,
with upper electrodes arranged-above spaces between lower electrodes.
5. A convective dryer as claimed in any of the preceding claims and wherein the major
part of the heat input is applied to heating the gas for the convective drying.
6. A convective dryer as claimed in any of the preceding claims wherein the amount
of power input to the radio frequency heating is between 1/5th and 1/10th of the total
energy input to the dryer.
7. A convective dryer as claimed in any of the preceding claims. wherein the jets
or nozzles for the heated gas are arranged to form an air flotation device to support
the material to be dried.
8. A method of drying a material comprising the steps of passing the material through
a heated gas stream and simultaneously subjecting the material to a radio frequency
field.
9. A method as claimed in claim 8 wherein the major part of the heat input is applied
to heating the gas.
10. A method as claimed in claim 8 wherein the power input to the radio frequency
heating is between 1/5th and 1/10th of the total energy input to the dryer.