ELECTRICAL APPARATUS

Abstract: Ohmic heating apparatus has a flow through chamber (28) constructed such that in a heating zone two opposed chamber walls are constituted by titanium electrodes (24, 26) compressed between a pair of steel walls (20, 22) lined by sheets (44) of temperature resistant non-elastomeric plastics forming a sealed joint between the electrodes and the walls (20, 22) by the use of high compressive forces, so avoiding the formation of crevices in which materials to be heated may be retained.

Description: ELECTRICAL APPARATUS The present invention relates to ohmic heating apparatus, i.e. apparatus of the kind having a chamber for containing a liquid through which an electric current is to be passed, said chamber comprising a first electrode and a second electrode spaced apart to define a current path therebetween for the passage of heating current through the liquid in use. The liquid through which the electric current is to be passed will be referred to in the specification as "electrolyte" for convenience irrespective of the mode of conduction in the liquid and of whether any electrolysis occurs . Various electrode layouts have been proposed for apparatus of this kind. GB-A-2068199 for example has cylindrical electrodes running transversely through a cylindrical heating chamber at spaced locations. An advantageous electrode layout described in WO-A-89/10438 has plate electrodes set on opposed walls of a rectangular cross- section chamber in face to face relationship. The walls of the chamber are insulated. In practice they will be of metal and the electrodes will have to be insulated from the walls on which they are positioned. We have found that this gives rise to difficulties. It is essential in apparatus used for heating sensitive materials, e.g. foodstuffs, that there are no crevices in which the foodstuff can lodge and be subject to excessive heating causing burning or other degradation. The joint between a plate-like electrode and an underlying insulation sheet is we have found a source of difficulty in this regard. The problem is made worse by the poor response of known elastomers to the elevated temperatures employed and the thermal cycling involved when the apparatus is turned on and off. Accordingly, the present invention now provides ohmic heating apparatus having a chamber for containing a liquid through which an electric current is to be passed to heat the liquid, said chamber being generally tubular and, in a neatmg zone, having a first pair of opposed insulating walls and a second pair of opposed walls connecting said first pair of walls characterised in that said second pa r of wails are constituted by said electrodes. The performance cf apparatus of this kind is often limited by the maximum current density whicn the electrode can sustain. It is often found that the current density is not constant over the whole of the surface area of the electrode exposed for current flow but tends to increase substantially at certain parts of the electrode, typically at edges of the electrode which are exposed. Generally, additional current paths are available extending from the edges of the electrode through the volume of electrolyte situated laterally beyond the edge of the electrode and this is responsible for this "edge effect" . In order to keep the maximum current density to which any part of the electrode is subjected down to an acceptable level, the bulk of the surface area of the electrode has to be operated at a current density which is considerably less than would be preferred. This problem is especially severe in ohmic heating operations in which it is essential not to overheat any part of the product stream as in food processing. This is because the conductivity of the electrolyte generally rises with temperatures so that local overheating encourages still higher local current flow and further overheating in a runaway cycle. It has been proposed previously in WO89/10438 to achieve a more uniform current density in ohmic heating apparatus by including between edge regions of opposing electrodes shield members which obstruct the passage of current at the edges of the electrodes to some degree. Such an arrangement is capable of providing substantial improvements from the point df view of current carrying capacity but the presence of the shield members is necessarily disadvantageous from the point of view of causing some degree of obstruction to material flowing through the apparatus and complicates tne pnysical construction of the apparatus. We have now discovered that a similar effect to that produced by the inclusion of shield members can be ootamed DV an arrangement of the electrodes themselves wnich overcomes these disadvantages of the use of shield members. Preferably therefore, in regions at each end of said heating zone said electrodes progressively diverge from one another going toward the ends of said heating zone, so producing a variation m mter-electrode spacing. Also, we have appreciated that as the liquid passed through ohmic heating apparatus is heated, generally its conductivity will rise, causing an increase in the current flowing. Where the process conditions are current limited, this may mean that overall current has to be set so that the current at the cold end of the heater is lower than maximum in order to keep the increased current at the hot end within acceptable limits. Preferably, there is also a central region between said end regions of said heating zone in which said inter-electrode spacing is uniform or varies to counteract temperature induced changes in conductivity as described below. Generally, heating apparatus according to the invention will further comprise at each end of said electrode containing heating zone a respective furtαer zone in end to end connection with said heating zone, each said further zone having electrically insulative walls. The electrically msulative walls of said further zones and the insulating walls of the heating zone may be of electrically insulative material or may be constructed from electrically conducting material provided with an electrically insulating lining. To operate at temperatures of about 130°C when heating water based materials such as foodstuffs, the chamber will need to be able to resist internal pressures in the region of 10 6 N/m 2 (lObar) . ■ ά - The insulating lining is preferably loose, i.e not bonded to the underlying walls and is formed from a polymer wh cn is resistant to absorbing moisture nto its surface under the conditions of use. In order to facilitate making a liquid tight seal, it would be normal practice to use an elastomeric material for the lining that would readily deform to f t irregularities in the mating surfaces of the cnamoer walls . However, we have found that elastomeric materials do not resist the conditions of use as well as would be desired in that the surface of such materials after thermal cycling tends to form pockets or pores which are penetrated by the liquid whicn is then locally overheated. We therefore prefer to employ as the insulative lining sneet a material which is resistant to the heat and electrical stress involved, even though this means that the material is non-elastomeric and needs to be placed under higher pressure to form a seal . The non-elastomeric liner as preferably placed under a pressure of more than 600 N/m 2 , e.g. 10000 to 15000 N/m 2 or more. Suitable heat resistant plastics include PEEK (polyether ether ketone) e.g. that sold under the Trade Name VICTREX (ICI) , which has the formula:- Formula 1 An alternative suitable polymer is the methylpentene copolymer TPX sold under the Trade Name OPULENT (Mitsui Seka) . Optionally, such polymers may be provided with a coating of a release material such as PTFE on that face which contacts the contents of the chamber. Preferably, thickness for the lining sheet is from 0.25 to 0.75 mm, e.g. about 0.5 mm. One side of the lining faces the interior of the chamber, (the "wet" side) and the other faces the chamber walls (the "dry" side) . Preferably, the pressure is applied in the dry side via an elastomeric material to spread the pressure evenly. This elastomeric material is protected from the action of heat combined with moisture and electrical stress by the lining. Preferably, electrical • apparatus according to the invention comprises a plurality of said heating zones joined end to end by intervening ones of said further zones, i.e. further zones having electrically insulative walls. This enables different electrical currents to be applied through the electrodes in the different electrode containing heating zones so as to obtain better control over the overall treatment of the conductive liquid. The conductive liquid may contain solids either dissolved or carried in particulate form in the liquid. Such particles may be fine and suspended or they may be large, for instance where the material to be electrically treated is a foodstuff, such as chunks of meat carried in a gravy. The spacing between said electrodes may vary progressively over a major portion of the length of said electrodes in such a way as to counteract a tendency for increased current flow between downstream portions of the electrodes caused by higher electrical conductivity exhibited by the liquid when heated. Variations of inter-electrode spacing in regions where substantial heating is occurring or in which the liquid is hot may be undesirable because there will, all things being equal, be a consequent increase in the cross-sectional area of the flow path througn the chamber and nence a corresponding progressive decrease in linear flow rate and hence more of the overall heating effect will occur m the downstream part cf the heater, which may be undesirable from a control point of view. This effect can be compensated for by tapering the width of the chamber transverse to the electrode spacing direction in opposition to the taper of the electrode spacing. Thus, the width of the chamber transverse to the direction of mter-electrode spacing may vary progressively n opposition to the said variation in mter-electrode spacing. To exactly counteract the effect on current flow caused by variations with temperature m the conductivity of the liquid flowing in the apparatu's, the mter-electrode spacing would have to vary so as to mirror the conductivity/ temperature relationship, even where this is non-linear. It will generally be an acceptable compromise to taper the mter- electrode spacing linearly to ease construction of the apparatus . Suitably, the inter-electrode distance varies by a factor of from 1.5 to 4, e.g. more preferably from 2 to 3, e.g. about 2.5, between the colder end of the apparatus and the hotter end within said major-portion of the apparatus to counteract the change in conductivity caused by heating. Similarly, to offset such a lmear variation in electrode spacing so as to maintain a constant cross-section would require a non-linear variation in chamber width but it will generally be adequate to taper the chamber width in a linear manner. Preferably, the transverse cross-sectional area of the flow path, in the region over which said inter-electrode spacing is varied to counteract thermal effects on the conductivity of the liquid, is kept constant to at least the extent that the percentage variation in cross-sectional area is no more than 20% of the percentage variation in mter- electrode spacing. One may in the same apparatus seek to deal with edge effects and with the variation with temperature the conductivity of the liquid. The invention includes a method of heating a liquid (optionally containing solids as described above) comprising placing said liquid in the chamber of electrical apparatus as described above and passing an ohmic heating current between said electrodes. Preferably, the liquid is flowed through the chamber. The invention will be further described and illustrated w th reference to the following description of a preferred embodiment with reference to the accompanying drawings in which: - Figure 1 is a side elevation of electrical apparatus according to the invention; Figure 2 is a cross-section on the line A-A of Figure 1; Figure 3 is a cross-section on the line B-B of Figure 1; Figure 4 is an enlarged view of the area circled at the left-hand end of Figure 1; and Figure 5 is an englarged view of the area circled toward the right-hand end of Figure 1. The ohmic heating apparatus illustrated in the drawings comprises an electrical heating chamber 10 which has a heating zone 11 connected at each end via further zones 12, 14 to respectively an inlet conduit 16 -and outlet conduit 18. In use, material to be heated in introduced at the left- hand end of Figure 1 to the inlet conduit 16, passes through the electrical heating chamber 10 and is subjected to ohmic heating in zone 11 before passing out through the further zone 14 and the outlet conduit 18. Looking at Figure 3 it can be seen that the heating zone 11 comprises a pair of side walls 20, 22 which are spaced apart by electrodes 24, 26 to define a rectangular section cavity 28. Suitably, each side wall 20 is provided by a plate-like steel casting reinforced by ribs 30 running into orthogonal directions. On each of the side walls 20 and 22 there is provided a line of bolt receiving through apertures 32. Each said aperture 32 contains an insulating collar 34 Eacn of the electrodes 24, 26 consists of a titanium billet having a pair of opposed parallel side faces 36 which fit face to face against the side walls 20 and having a electrode face 38 joining said side faces and facing into the cavity 28. A line of bolt receiving apertures 40 extends along each of the electrodes 24, 26 to match the line of apertures 32 in the side walls 20, 22 and the side walls 20, 22 are bolted up to the electrodes 24, 26 by bolts 42 passing through said apertures and through the collars 34. A plastics insulating sheet 44 of 0.5 mm PEEK is sandwiched between the electrodes 24, 26 and the side walls 20, 22 and is provided with apertures to accommodate the passage of the bolts 42. As can be seen in Figure 1, the electrodes 24, 26 are shaped so as to diverge from one another towards their ends. Approximately the middle two-thirds of the zone 11 is of constant spacing between the electrodes The electrodes diverge at a constant rate from one another over the remaining one-sixth of the length of the heating zone at each end. The spacing between the electrodes the central uniform region is approximately one-third of the spacing at the widest point of divergence. So as provide a fluid tight s al for the cavity 28, slots 46 are provided the side walls 20, 22 facing the sides of the electrodes 24, 26 toward the corners thereof. Within the slots 46 behind the liner sheet 44 there is positioned a metal strip 48 having elastomeric facing 50. A plurality of drillings 52 penetrate to the rear of the slots 46 from the outside surface of the side walls 20, 22 and are threaded to receive bolts 54 by means of which the strip 48 may be jacked against the liner sheet 44 and side face of the electrodes 24, 26 to provide a good liquid tight seal by applying a pressure of about 1200 N/m 2 or more. In the further zones 12, 14 as shown in Figure 2, the structure is similar except that the electrodes 24, 26 are replaced by top walls 56, 58 of material bearing an insulative lining layer 60, suitably of the same material used for lining 44. The connection between the further zone 14 and the heating zone 11 is illustrated detail n Figure 4. The side walls 20, 22 extend continuously through the further zones 12 and 14. The electrodes 24, 26 stop at the end of the heating zone and abut the ends of the top and bottom walls 56, 58 of the further zones 12, 14. To form a liquid-tight seal between the electrodes and the top and bottom walls 56, 58, a slot 62 is provided at each end of each electrode 24, 26. A corresponding rebate 64 is 'provided at the end of the top and bottom walls 56, 58. The slots 62 and the rebate 64 together face to face produce a channel which contains a transversely extending block 66. Threaded bores 68 are provided extending down to the rear of the block 66 to accommodate jacking bolts 70 by means of which the block 66 in each case may be jacked toward the cavity 28. The insulative lining 60 on the interior face of each of the top and bottom walls 56, 58 is continued past the end of the rebate 64 under the lock 66 and into the slot 62 of the electrode against which it is squeezed by the block 66. The end face of the electrode is chamfered to connect at a shallow angle with the liner 60 supported on the block 66. The connection between the further zones 12 and 14 and the inlet and outlet conduits 16, 18 is illustrated in Figure 5 in detail . The lower wall 58 of the further zone 12 carrying its insulating liner 60 has at its end a flange portion 72. The inlet conduit 16 has a flared outlet end terminating in a transversely extending flange 74 which is butted up against the flange 72 with an insulating ring 76 and a metal ring 77 and a further insulating ring 79 sandwiched between them. Flanges similar to flange 72 are provided on the other walls of the further zone 12. The ring 77 has an internal flange 78 directed into the cavity defined by the further zone 12 overlying the interior faces of the wall 58 and the other walls of the further zone 12. A number of hinged clamps 80 are provided on the outlet end of the inlet conduit 16 which reach around to the rear face of the flange 72 and by a screw adjuster 82 bearing against an insulating pad member 84 are used to clamp the flanges 72 and 74 tightly together. A plurality of bores 86 each provided with an insulating liner 88 and containing bolts 90 are used to pull the flange 78 outwardly against insulative linings 60 of the wall 58 to form a liquid tight seal. A similar connection is made between the further zone 14 and the outlet conduit 18. It can be seen that the cavity through the apparatus is entirely free of internal obstructions. In use, electrical voltage is applied across the electrodes 24, 26 in a known manner. Generally, alternating current is employed m apparatus of this type and relatively high frequencies are preferred. By virtue of the variations in the inter-electrode spacing along the cavity, a current density of much improved uniformity is obtained as compared to the situation which would prevail if the electrode spacing were constant . Whilst the invention has beta described with reference to the illustrated embodiment by way of example, many modifications and variations thereof are possible within the scope of the invention. Thus, the inter-electrode spacing in the region covering the middle two thirds of the heating zone 11 may be increased linearly in the direction of the outlet and the width of the chamber may be narrowed progressively to produce the same cross-section at the end of this region as at its start. The width of the chamber may be varied also in the end regions of the heating zone where the electrodes diverge to mitigate end effects, so as to counteract the effect upon the cross- sectional area which the changes in electrode spacing would otherwise produce. Shield members may be employed to further modify the current density in the apparatus, although this will generally not be desirable. Scraper members may be employed to prevent a build-up of material on the internal walls of the heating chamber or further chamber, although again this will generally not be necessary or desirable.

Claims: CLAIMS 1. Ohmic heating apparatus having a chamber (10) for containing a liquid through which an electric current is to be passed to heat the liquid, said chamber being generally tubular and, in a heating zone, having a first pair of opposed insulating walls (20, 22) and a second pair of opposed walls connecting said first pair of walls characterised in that said second pair of walls are constituted by said electrodes (24, 26) . 2. Apparatus as claimed in Claim 1, wherein regions at each end of said heating zone said electrodes progressively diverge from one another going toward 'said ends of said zone. 3. Electrical apparatus as claimed in Claim 2, wherein in a central region between said end regions of said heating zone, the spacing between the electrodes is uniform or grows progressively toward one end of said zone at a lesser rate than in the said end regions . 4. Electrical apparatus as claimed in any one of Claims 1 to 3, further comprising at each end of said heating zone (10) a respective further zone (12, 14) in end to end connection with said heating zone, each said further zone having electrically insulative walls. 5. Electrical apparatus as clai.πed in Claim 4, comprising a plurality of said heating zones (10) joined end to end by intervening ones of said further zones (14) , each said further zone having electrically insulative walls. 6. Apparatus as claimed in any preceding claim, wherein the spacing between said electrodes (24, 26) varies progressively over a major portion of the length of said electrodes in such a way as to counteract a tendency for increased current flow between downstream portions of the electrodes caused by higher electrical conductivity exhibited by the liquid when heated. 7. Apparatus as claimed in Claim 6 , wherein the width cf tne chamber (10) transverse to the direction of mter-electrode spacing varies progressively m opposition to the said variation inter-electrode spacing. 8. Apparatus as claimed in Claim 7, wherein the transverse cross-sectional area of the chamber (10) , in the region over which said mter-electrode spacing s varied to counteract thermal effects on the conductivity of the liquid, s kept constant to at least the extent that the percentage variation in cross-sectional area is no more than 20% of the percentage variation mter-electrode spacing. 9. Apparatus as claimed m any one of Claims 6 to 8 , wherein in addition to said variation in mter-electrode spacing to counteract the effect of variations in the conductivity of the liquid with temperature, in regions at each end of said electrodes said electrodes progressively diverge from one another going toward the ends of said heating zone to counteract the tendency for increased current density at the ends of the electrodes . 10. Apparatus as claimed in any preceding claim, wherein said electrodes are compressed between said first pair of walls. 11. Apparatus as claimed in any preceding claim, wherein said first pair of walls are lined by an insulating sheet material (44) which is compressed between said first pair of walls and said electrodes to form a liquid "-ight seal for the chamber. 12. Apparatus as claimed in Claim 11, wherein said insulating sheet material is a non-elastomeric plastics material which is compressed by a pressure in excess of 6000 N/m 2 between said electrodes and said first pair of walls. 13. Apparatus as claimed in Claim 11 or Claim 12, wherein said compression is applied to said sheet material on the faces thereof remote from the interior of said chamber via an elastomeric material . 14. Ohmic heating apparatus substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings . 15. A method of ohmic heating comprising passing an electric heating current througn a liquid (optionally containing suspended solids) in the cnamoer of ohmic heating apparatus as claimed m any one of Claims 1 to 1 .

Inventors: MOSSE RICHARD WOLFGANG EMIL, VAN DER POST JAN RUPERT, MOSSE, RICHARD, WOLFGANG, EMIL, VAN DER POST, JAN, RUPERT
Applicants: BECKSWIFT LTD, MOSSE RICHARD WOLFGANG EMIL, POST JAN RUPERT V D, BECKSWIFT LIMITED, MOSSE, RICHARD, WOLFGANG, EMIL, VAN DER POST, JAN, RUPERT
Application year: 1993
Main classification: F24H1
IPC codes: A23L1, A23L3, A47J27, F24H1