[0001] This invention relates to convective drying apparatus or ovens (hereinafter and in
the claims for convenience simply referred to as "apparatus") in which heated air
or other gas is directed onto material to be dried, processed, cooked or baked (hereinafter
and in the claims for convenience simply referred to as "dried").
[0002] In Patent Specification GB 2123537B (The Electricity Council) there is disclosed
a hot gas convective dryer having a plurality of jets or nozzles for directing a heated
gas onto material passed between 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.
[0003] In the aforesaid Patent Specification the electrodes of the radio frequency heating
means are discrete separate nozzle boxes incorporating the nozzles or jets for directing
heated air or other gas onto the material to be dried, for example a fabric web.
[0004] According to the present invention there is provided a hot gas convective drying
apparatus comprising a plurality of nozzles or a delivery chamber for directing hot
gas onto the material to be dried and radio frequency heating means adapted to create
a radio frequency field in the region through which the material to be dried travels,
the drying apparatus being characterised in that the electrode means of the radio
frequency heating means comprise a pair of spaced perforated plates between which
the material to be dried passes and with which are associated the hot gas directing
nozzles or chambers, or in that the electrode means of the radio frequency heating
means comprises a single perforated plate with an opposed array of individual nozzle
boxes incorporating hot gas directing nozzles.
[0005] Preferably, each perforated electrode plate includes a plurality of boxes or housings
each with rows of nozzle perforations.
[0006] Preferably, the nozzle perforations in each perforated electrode plate are smaller
in area than the perforations in the perforated electrode plate in the regions of
the latter between the nozzle boxes or housings.
[0007] The nozzle boxes or housings in each respective perforated electrode plate may be
staggered relative to the nozzle boxes or housings in the other perforated electrode
plate, or may be in line depending upon the nature of the material to be dried.
[0008] The advantages of employing continuous perforated plate electrodes as opposed to
discrete electrodes are threefold. Firstly, with certain material to be dried, there
is a better distribution into the material of the radio frequency energy. Secondly,
a perforated plate electrode is easier to clean and keep free from contaminants, especially
in a food environment. Thirdly, a perforated plate electrode is less likely to form
an impediment to passage of material to be dried if the latter has a tendency to make
contact with the electrode means.
[0009] Embodiments of the present invention will now be described, by way of example, with
reference to the accompanying drawings, in which:-
Fig. 1 is a diagrammatic illustration of hot gas convective drying apparatus according
to the invention;
Fig. 2 is a detail view of a perforated plate electrode;
Fig. 3 is a diagrammatic illustration of an alternative embodiment of drying apparatus;
and
Fig. 4 is a diagrammatic illustration of yet another embodiment of drying apparatus.
[0010] Referring to Figs. 1 and 2, the apparatus is used to dry moist material susceptible
to dielectric heating by means of radio waves, such, for example, as fabric or paper
webs or discrete items of material or foodstuff carried on a suitable conveyor system.
[0011] The conveyed material to be dried indicated at 10 in Fig. 1 passes between two perforated
plates 11, 12 which constitute the electrodes of a radio frequency heating means,
the perforated plates 11, 12 being connected to a radio frequency generator 13. The
radio frequency may be either a balanced feed or a coaxial feed arrangement as is
well known to those skilled in the art.
[0012] The plates 11, 12 have respectively connected thereto housings or boxes 14, 15, the
housings or boxes 14 being in staggered relationship to the housings or boxes 15,
the perforated plates 11, 12 and nozzle housings or boxes 14, 15 being disposed with
a drying enclosure 16.
[0013] Each housing or box 14, 15 has nozzle perforations 17 for directing heated pressurised
air onto the material 10 (see arrows 18). The nozzle housings or boxes 14, 15 are
connected by suitable ducts 19 to an air circulating fan and heater 20, which may
be located either within the drying enclosure 16 or external thereof.
[0014] The perforated electrode plates 11, 12 have, as aforesaid, nozzle perforations 17
from the housings or boxes 14, 15 and these are smaller in area than perforations
21 in the areas of the perforated electrode plates 11, 12 between the nozzle housings
or boxes 14, 15, the larger perforations 21 facilitating return of air (moisture-bearing
air) from the vicinity of the material 10 back into the air circulating fan and heater
20, a proportion of this air being exhausted through an outlet duct 22 incorporating
a suction fan (not shown).
[0015] In Fig. 1, parts similar to those of the embodiment of Figs. 1 and 2 are designated
by the same references with the suffix "A".
[0016] The difference in this embodiment is that each perforated plate 11A, 12A with its
associated housings or boxes 14A, 15A forms one wall of an air delivery chamber 23,
the air flow, in this instance, being from the air supply fan and heater 20A into
chambers 23, through the perforations in the plates 11A, 12A for impingement against
the material 10A and return into the housing or boxes 14A, 15A and from open ends
thereof into the drying enclosure 16A for return to the air supply fan and heater
20A with a proportion thereof exhausting through outlet duct 22A under the action
of its associated suction fan (not shown).
[0017] In this embodiment, the perforations in the plates 11A, 12A may be larger than those
in the plates between the housings or boxes 14A, 15A.
[0018] In Fig. 4, parts similar to those of the embodiment of Figs. 1 and 2 are designated
by the same references with the suffix "B".
[0019] In this embodiment there is only one perforated electrode plate 11B with opposed
individual and separate housings or boxes 14B incorporating nozzles.
[0020] In yet another arrangement (not shown), the air is externally ducted to exhaust after
impingement with the material to be dried, i.e. it is not returned into the drying
enclosure.
1. A hot gas convective drying apparatus comprising a plurality of nozzles or a delivery
chamber for directing hot gas onto the material to be dried and radio frequency heating
means adapted to create a radio frequency field in the region through which the material
to be dried travels, the drying apparatus being characterised in that the electrode
means of the radio frequency heating means comprise a pair of spaced perforated plates
between which the material to be dried passes and with which are associated the hot
gas directing nozzles or chambers, or in that the electrode means of the radio frequency
heating means comprises a single perforated plate incorporating hot gas directing
nozzles.
2. Apparatus as claimed in claim 1, in which each perforated electrode plate includes
a plurality of boxes or housings each with rows of nozzle perforations.
3. Apparatus as claimed in claim 2, in which the nozzle perforations in each perforated
electrode plate are smaller in area than the perforations in the perforated electrode
plate in the regions of the latter between the nozzle boxes or housings.
4. Apparatus as claimed in any one of claims 1 to 3, in which the nozzle housings
or boxes in each respective perforated electrode plate are staggered relative to the
nozzle boxes or housings in the other perforated electrode plate, or may be in line
depending upon the nature of the material to be dried.
5. Apparatus as claimed in claim 1, in which each hot gas delivery chambers has, as
one wall, one of the perforated electrode plate with associated housings or boxes
serving to receive gas after its impingement against the material to be dried.