WO1989000384A1

WO1989000384A1 - Apparatus for heating electrically conductive flowable media and methods of use of such apparatus

Info

Publication number: WO1989000384A1
Application number: PCT/GB1988/000566
Authority: WO
Inventors: Richard Wolfgang Emil Mosse; Jan Rupert Van Der Post
Original assignee: Beckswift Limited
Priority date: 1987-07-15
Filing date: 1988-07-14
Publication date: 1989-01-26
Also published as: AU2085788A; EP0375694A1; GB9000410D0; JPH02504331A; GB2229900A

Abstract

Apparatus for ohmic heating of an electrically conductive flowable medium comprises pipe means (9) through which such a medium can be arranged to flow, and at least two electrodes (21) spaced apart in the pipe means insulating shield members (211) to reduce current density at the electrode edges, and means (25, 26, 27) movable in said pipe means for dislodging material deposited from said medium on the electrodes (24) and on the interior of the pipe means in use, which may be a scraper which comprises a plurality of scraper members (26, 27) spaced lengthwise of the pipe means connected to a reciprocable carrier member (25), a portion of which extends from the upstream end of the pipe means and is connected to the means (32, 33) for producing reciprocation of said scraper members.

Description

APPARATUS FOR HEATING ELECTRICALLY CONDUCTIVE FLOWABLE MEDIA AND METHODS. OF DSE OF SUCH APPARATUS

The present invention relates to apparatus for heating electrically conductive flowable media and to methods of using such apparatus.

There are many circumstances in which it is necessary to heat flowable media to elevated temperatures. Examples are the heating of foodstuffs for cooking or for sterilising as well as the heating of chemical reactants to perform a chemical reaction. It is known to heat flowable media by causing an electric current to flow directly in the media between electrodes. Advantages of such direct electric ohmic heating of the medium are that relatively rapid heating rates can be obtained and that to a first approximation the heating is at a uniform rate throughout the material. This is in distinction to heating systems such as heat exchangers, ovens and steam or hot water jacketed vessels where the heating of the medium is by transfer of heat from a hot surface and temperature gradients between the medium more directly in contact with the surface and the remainder of the medium are inevitable. In obtaining satisfactory heating rates, such heating systems are likely to cause overheating of the medium in closest contact with the heating surface. Difficulties have also however been experienced in the ohmic heating of some flowable materials, particularly foodstuff materials, through the build up of deposits from the flowable material on the interior of the heating apparatus, particularly the heating electrodes.

United Kingdom Patent Specification 2067390B describes an ohmic heating apparatus in which electrodes are spaced longitudinally of a pipe through which a flowable medium to be heated is conducted, the electrodes typically being cylindrical in shape and passing transversely through enlarged portions of the pipe. This is intended to cause the electric field strength at the exposed surface of each electrode to be such that substantially all current flows from a smooth portion of the exposed surface of the electrode spaced from the pipe itself to minimise electric field strength discontinuities which are thought to be associated with the build up of material on the electrodes. It is proposed also that the electrodes may be reciprocated in a direction pependicular to the axis of the pipe with a view to scraping the surface of the electrode to remove fouling. High voltages are employed since the area of the electrodes is small and the spacing between the electrodes is large.

Accordingly, high current densities at the electrodes surfaces are employed. The problem of deposition of the flowable medium on the walls of the pipe, rather than on the electrodes, is moreover not addressed.

No detailed description of means for reciprocating the electrodes to scrape them is provided. The constructon of apparatus of that kind would probably be complex because of the difficulty in providing a flexible lead able to carry the very high currents involved, or of maintaining a sliding contact clear of contamination and oxidation.

Such a system of electrode movement for scraping could not moreover be applied to the type of ohmic heating apparatus in which the electrodes face one another across the direction of fluid flow rather than being spaced lengthwise of it. Systems of the former kind are advantageous in that the area of the electrode is much less restricted so that one has the option of operating such an apparatus at relatively low voltages and high currents. The use of voltages of the order of hundreds of volts rather than thousands of volts is highly advantageous from the point of view of simplicity of construction of the apparatus and safety of its operation.

Accordingly, the present invention provides apparatus for ohmic heating of an electrically conductive flowable medium comprising pipe means through which such a medium can be arranged to flow. the pipe means being made of or internally lined with a material having an electrical conductivity sufficiently low to allow ohmic heating of a flowable medium therein in use, at least two electrodes spaced apart in the pipe means and arranged to make electrical contact in use with a medium flowing therethrough, and means movable in said pipe means to act on the electrodes and those if any of the internal wall surfaces of the pipe means which are in contact with the flowable medium within the zone or zones in

10 the pipe means in which heating of the flowable medium takes place in use, so as to dislodge or deflect therefrom material tending to deposit thereon from the flowable material as it is heated.

Preferably, the movable dislodging means is a scraper comprising one or more scraper members mounted

^•-^•-> for scraping contact with the interior of the pipe means.

Such a scraper may comprise a portion extending out from the upstream end of said pipe means and connected to means for producing scraping movement of said scraper in said pipe means. 20

The scraper may comprise a plurality of scraper members spaced lengthwise of the pipe means. Such scraper members may for instance be connected to a reciprocable carrier member, a portion of which extends from the upstream end of the pipe means and is

25 connected to means for producing reciprocation of said scraper members.

For instance, the carrier member may be a rod or equivalent means extending along the pipe means and carrying projecting scraper members extending to or toward the interior walls of the pipe means.

Preferably however, the carrier member is a cage of preferably parallel elongate carrier means, such as rods or equivalent means on which scraper members are carried. Preferably, at least some of the scraper members bridge between pairs of elongate carrier means. Other ones of the scraper members may extend from one elongate carrier means toward but not reaching another elongate carrier means, especially in cases where shield members are present between opposed electrodes as will be described hereafter.

Where the pipe means is of rectangular internal cross-section, sets of scraper members may be disposed along the length of the carrier member successively for scraping different opposed pairs of wall surfaces of the pipe means.

Other forms of scraper may however be adopted. For instance, if the interior of the pipe means is of generally circular cross-section, the scraper may take the form of an auger spiral rotatable within the pipe means to produce the necessary scraping action.

Such an auger spiral may be connected to a drive member extending from the upstream end of the pipe means and connected to means for producing the rotational movement. Alternatively, the scraper may comprise a plurality of brush members, radially extending blades, or other radially extending members mounted to and extending from a carrier member adapted for rotation within the pipe means to produce the required scraping motion. Once again, the carrier member may extend from the upstream end of the pipe means for connection to means for producing the required motion.

The scraper means referred to above may be designed to produce a high degree of turbulence in the flowable medium to assist in prevent deposition of material in the interior of the pipe means to prevent any tendency for the medium to deposit on the scrapers, they may be made of or coated with a material which the flowable medium in question will not adhere to or will adhere to only to a reduced extent. A suitable material for the surface of such scraper in many instances will be for instance silicone rubber or polytetraflurorethylene. The scraper members are preferably made from an electrically insulating material to avoid short circuiting the electrodes and to avoid altering the effective electrode area.

Preferably the interior of the pipe means is of polygonal cross-section, e.g. rectangular cross- section, providing a plurality of flat surfaces on which the scraper members may act and in which electrodes may be mounted so as to provide a constant inter-electrode distance over the whole area of a given pair of electrodes. Instead of scraper members contacting the pipe means, one may employ generally similar members mounted to pass close to, but not in contact with, the electrodes and pipe means interior to cause turbulence in the flowable medium to dislodge material therefrom.

Preferably, said electrodes form one or more electrode pairs, members of each pair of electrodes facing one another across the width of the pipe means. Preferably there are at least three pairs of electrodes.

Means may be provided for connecting each of said three electrode pairs between earth and one phase of a three phase alternating voltage electricity supply. However, other arrangements of the connection between the electrodes and an electrical supply may be used.

For instance the electrode may be connected between phases, with neutral at the end of the pipe means. However, the edge current density on an electrode has been found to be a significant factor in electrode life and can limit the maximum current passed in use through the electrode. In a multi-stage system, e.g. where there are three pairs of electrodes arranged sequentially along the pipe means, each electrode sees not only the opposite electrode in its pair but also the next and/or previous electrode along the pipe means. The adjacent electrode can have a significant effect on edge current density in the electrode under consideration, depending on how the electrodes are supplied with power. If there are phase differences between successive electrodes, quite large potential differences will occur between adjacent electrodes which may make a substantial contribution to the edge current density. Accordingly, it is preferred that all the electrodes on one side of the pipe means be supplied in phase, the amplitudes of the supplies to the individual electrodes preferably being separately controlled. This allows the gaps between electrode sets to be minimised.

Preferably, the apparatus is operated at a voltage between electrode pairs of less than 650 volts, preferably about 240 volts. Preferably, said voltage is an AC voltage of low frequency, preferably about 50 cycles. Preferably, the apparatus is such as to operate conveniently at current densities of from 0.25 to 1.2 amps per cm², more preferably from 0.4 to 0.6 amps per cm², e.g. 0.5 amps per cm². An advantage of the use of relatively low voltages is that the electrolytic production of oxygen is less likely to occur. The production of oxygen electrolytically in foodstuffs is liable to produce off flavours, probably by oxidation of the sulphur content of the material being heated producing sulphur dioxide.

Optionally, the length of electrodes measured lengthwise of the pipe means decreases between successive pairs of electrodes in the flow direction whereby a relatively even current distribution is obtained in use when equal voltages are applied across each of said electrode pairs despite an increase in the conductivity of the flowable medium with increasing temperature. Preferably, the percentage length decrease between successive pairs is from 5 to 15% , e.g. about 10%, when the flowable medium is milk. The optimum change in electrode length will depend upon the nature of the flowable medium and the temperature changes being produced. By way of example, for milk, suitable lengths would be 89, 80, and 71 cm to heat milk from 80^* to 140^*C. Preferably, each electrode is elongate lengthwise of the pipe means.

Preferably, each electrode is recessed into the interior surface of the pipe means so as to lie substantially flush with areas of said surface adjacent the respective electrode.

Preferably, the interior surface of the pipe means is formed by a lining of electrically insulating plastics material. Preferably, the electrodes are recessed into said plastics material.

Preferably, said plastics material is elastically stretched prior to the insertion of the respective electrode therein and is then allowed to relax to grip said electrode so as to form a good seal on edges thereof. Said stretching is preferably in the flow direction of the apparatus when assembled.

Preferably, said means for dislodging material from the interior of the pipe means is adapted to dislodge material from the interior of the pipe means over substantially the whole of the length within which ohmic heating takes place in use.

Preferably, said electrodes in aggregate have a surface area exposed for current flow which is greater than 10% of the internal surface area of that part of said pipe means within which ohmic heating takes place in use. Preferably, said aggregate surface area is greater than 30% of said internal surface area.

Preferably, said aggregate surface area is about 40% of said internal surface area. As discussed above, the current density at the edges of the electrodes may be the limiting factor deciding the maximum current that can be passed and hence the maximum rate of heating that can be obtained. Depending on the elctrode's shape and position however, the maximum current density may be other than at the edge.

Preferably, in the apparatus according to the invention there is included one or more shield member, each said shield member preferably being thin relative to the length of the current path in the apparatus and being so positioned with respect to a respective electrode or electrode pair as to reduce current density at the part or parts of the electrode surface which in the absence of the shield member would experience the highest current density, the shield member thereby serving to increase the current carrying capacity of the electrode or electrodes.

Electrodes in apparatus according to the invention may have a wide variety of shapes including plate-like, cylindrical or spherical. The areas on the electrodes which would normally experience the maximum current density will depend on the electrode shape and sometimes on the nature of the counter electrode with which the electrode defines a current path. The current density maximum may occur at the closest approach of the electrode to the counter electrode, where current paths are shortest. Often it will occur at an edge of the electrode where flowable medium outside of the direct path between the electrode and its opposite electrode provides additional current paths. Generally, the shield member will be positioned between the electrode and its opposite electrode, overlying that area of the electrode surface which would otherwise suffer the greatest current density. By restricting the current density at these areas relative to the rest of the electrode surface, paradoxically the current carrying capacity of the apparatus can be increased. The average current density can be raised without the maximum current density exceeding permitted limits. Preferably, the shield member extends face to face with and spaced from the shielded area of the electrode.

Preferably, the shield member extends substantially beyond the area of the electrode to be shielded. Preferably, the shield member overlaps the said edge region of the electrode area exposed for current flow, i.e. extends both beyond said edge region outside the said electrode area and towards the middle of the electrode.

Preferably, the shield member overlaps the edge region of the electrode area exposed for current flow by a distance which is from 10 to 300% of the spacing between the electrode and the opposite electrode. More preferably, said overlap is from 10 to 100%, more preferably 20 to 50% of said spacing, for instance about 30%.

The shield member preferably extends outwardly beyond the electrode area exposed for current flow by a distance at least as great as the spacing between the electrode and its opposite electrode, more preferably by at least twice said spacing, for instance by at least four times said spacing.

The electrode may be of any shape and may present any number of edges or any shape of edge. Shield members may preferably be provided overlapping any edge of the electrode bordered by the flowable medium in use.

Usually it will be preferred that the or each shield member is positioned substantially midway between the respective electrode and an opposite electrode. However, the or each said shield member may be positioned nearer to the respective electrode than to the opposite electrode and, optionally, for the or each said shield member associated with the electrode, a corresponding shield member may be provided for the opposite electrode.

Preferably the or each shield member is a substantially planar member.

Generally speaking, the thickness of the shield member in the between electrodes direction will not be critical but may be minimised to avoid obstructing the flow of material through the apparatus. The shield member may have to sustain substantially all of the voltage applied to the cell and must therefore have sufficient dielectric strength to stand the voltage in question.

The maximum current density of the edge of each electrode may be restricted to no greater than twice the average current density when the apparatus is in use, in some cases to no greater than 1.5 times, and in some cases to as low as 0.65 times the average."

It is desirable to prevent leakage of current to plant outside the ohmic heating chamber. If a neutral ring or collector electrode is placed in the inlet or outlet pipes of the heater, this will shunt leakage currents to neutral before they reach external plant. This net current to neutral can be virtually eliminated in apparatus where electrodes are arranged in opposed pairs along the pipe means of the apparatus if the pair of electrodes at the end of the pipe means i_s driven by symmetrically opposed A.C. voltages and a shield member is provided overlapping the outer of the pair of electrodes and stopping a distance before the neutral collector electrode or ring is reached. Current then passes from one electrode, around the outer end of the shield member and back to the opposite electrode rather than to the neutral collector. Of the current reaching the neutral collector electrode, almost all will return to the opposite electrode. An appropriate electrical supply may be for example an invertor powered by a six pulse rectifier system or a centre neutral single phase transformer, or a six phase supply.

The advantage of such an arrangement is that the end volumes in the pipe means (the volumes upstream and downstream of the electrode) may be minimised. A large downstream end volume is not only waste volume but may increase the minimum hold time at high temperature undesirably. Also, smaller, less rigorously specified neutral connections may be employed. The apparatus may further include a heat exchanger connected to the downstream end of said pipe means for cooling said flowable medium by heat exchange with a coolant stream. The apparatus may also further include a pressure relief valve downstream of the flow path for said flowable medium through said heat exchanger. The use of a pressure relief valve enables superatmospheric pressure maintained in the heating apparatus to prevent boiling to be relieved so that the heated and recooled medium may then be treated at about atmospheric pressure in subsequent processing such as packaging.

Although the heating produced by ohmic heating apparatus is intrinsically more even than in other forms of heating apparatus, we have found that some departure from even heating occurs. The temperature attained by materials in the heater is dependent on the current density and the length of time for which the material is exposed to the current. Liquids and slurries do not move through a pipe with uniform velocity. The velocity distribution across the pipe section will depend on the flow characteristics of the material being heated. Even slight overheating of some materials, especially food products, may cause deleterious effects. We have found that the effects due to non-uniform flow are of practical significance when temperature sensitive materials are being heated.

One effect of the scrapers described above will be to produce turbulence in the flowable material, so encouraging a uniform flow rate and even heating. By preventing non-uniform heating, the tendency to form deposits of material on the pipe walls may in many instances be greatly reduced so that scraping as such is not necessary. Also, damage to the flavour or nutritional value of foodstuffs may be reduced. Accordingly, we have found that there is value in creating a high level of stirring of the flowable medium in the pipe means, even without scraping of the walls of the pipe means and the electrodes.

Accordingly, in a second,main aspect, the invention provides apparatus for ohmic heating of an electrically conductive flowable medium comprising pipe means through which such a medium can be arranged to flow, the pipe means being made of or internally lined with a material having an electrical conductivity sufficiently low to allow ohmic heating of a flowable medium therein in use, at least two electrodes spaced apart in the pipe means and arranged to make electrical contact in use with a medium flowing therethrough and means for producing stirring or turbulence of the flowable material to provide a substantially uniform time of passage through the pipe means for all the flowable material.

Such stirring means may be fixed flow deflectors but more preferably are moving stirrer members, e.g. scraper members as previously described.

Such stirring means should generally be designed to divert material away from the walls of the. pipe means into the centre of the pipe means and to ensure thorough mixing.

As explained above, temperature control is of great importance in the heating of food materials.

Accordingly, apparatus according to the present invention preferably includes means for measuring the flow rate of said flowable material, means for measuring the temperature of said flowable material entering said pipe means, control means responsive to said flow rate and temperature measurements to regulate the passage of heating current and means for measuring the outlet temperature to which said flowable material is heated in said pipe means, said control means being further responsive to -said outlet temperature to further regulate the passage of current. It has been found that by first adjusting the heating energy supplied according to the inlet temperature and flow rate and then using the outlet temperature in a feed back loop to further regulate performance, the temperature is more closely controlled both as to uniformity and absolute level. In an ohmic heater, the requirements for temperature contol are much more critical than in a conventional heater controlled by gas or liquid heat exchange.

The control means identified above will preferably be a microprocessor control means.

The invention includes a method of heating a flowable medium comprising passing the flowable medium through the pipe means of an apparatus of the invention as described above and passing electric current through the flowable medium between said electrodes to heat the flowable medium. Heating apparatus as described above may be connected directly to a packing machine capable of receiving the flowable medium from the apparatus directly or coupled via a heat exchanger provided for cooling the flowable medium to a packing machine capable of receiving the cooled flowable medium and packing the flowable medium in any desired manner.

Apparatus may be provided upstream of the ohmic heating appratus according to the invention in which the flowable medium is previously heated. Where the flowable medium is a food product requiring cooking, the food produce may be partially cooked before being introduced into the ohmic heating apparatus according to the invention for further heating, for instance for sterilisation.

The invention will be further explained and illustrated by the following description of a preferred embodiment with reference to the accompanying drawings in which:-

Figure 1 is a schematic view of apparatus according to the invention installed as part of a food product sterilisation apparatus; Figure 2 is a sectional elevation of the ohmic heating apparatus of Figure 1;

Figure 3 is a sectional elevation of the pipe means of the apparatus of Figure 2;

Figure 4 is a section on the line IV-IV of Figure; 3;

Figure 5 is a section on the line V-V of Figure 2 Figure 6 is a corresponding section on the line VI-VI of Figure 2;

Figure 7 is a transverse cross-section of the heat exchanger of the apparatus of Figure 1.

Figure 8 is a schematic side view of a second embodiment of heating apparatus according to the invention;

Figure 9 is a simplified longitudinal cross-section through the central element of the apparatus of Figure 8, omitting details of the electrical connection to the electrodes; Figure 10 is a transverse cross-section on the line XX-XX in Figure 9;

Figure 11 is a plan view of the element shown in Figures 8 and 9, showing the electrical connection of 05 one electrode;

Figure 12 is a longitudinal cross-section on the line XII-XII in Figure 11;

Figure 13 is a longitudinal cross-section through the inlet end region of the apparatus of Figures 8 to 10 12;

Figure 14 is a longitudinal cross-section through the outlet end region of the apparatus of Figures 8 to 13.

Figure 15 is a computer simulation of the 15 equipotential distribution in the apparatus of Figures 8 to 14 in use; and

Figure 16 is a computer simulation of the equipotential distribution in the apparatus of Figures 8 to 14 in use with the central electrode pair 20 switched off.

As shown in Figure 1, apparatus for sterilising a flowable food product may comprise a first product holding tank 1 having an inlet 2 for food product to be sterilised and an inlet 3 for a pressurising *25 atmosphere of gas. The apparatus may comprise a further food product holding tank 4 having a similar inlet 5 for food product and inlet 6 for pressurising gas.

The food product holding tanks 1 and 4 may be connected via a selector valve 7 to the inlet of ohmic heating apparatus 8 according to the invention which will be described in greater detail hereafter. The heating apparatus comprises a tube 9 constituting the pipe means of the apparatus and having an inlet 10 for flowable food product and an outlet end 11 for sterilised food product.

The outlet end 11 of the heating apparatus 8 is connected to a counter current heat exchanger 12 which will also be described in greater detail hereafter. Heat exchanger 12 has an inlet 13 for a coolant stream of liquid and an outlet 14 for the coolant stream which is connected to a heating coil 15 in the food product holding tank 4.

The outlet for food product from the heat exchanger 12 is connected to an aseptic holding tank 16 having an outlet 17 for food product to be connected to a packaging machine and an inlet 18 for pressurising gas.

The ohmic heating apparatus according to the invention is illustrated in greater detail in Figures 2 to 6. The apparatus comprises a square section tube 9 constituting a pipe means built up in a manner best seen with reference to Figure 4. The tube 9 comprises upper and lower plates 19 which may be of metal, provided with an insulating interior lining plate 20. As can be seen in Figure 2, electrodes 21 are recessed into the inner surface of the lining 20. The upper and lower assemblies of plates 19, lining 20 and, where present, electrodes 21 are spaced by insulating spacer wall members 22 which are backed by metal wall members 23 having laterally extended upper and lower flanges which lie between laterally extending marginal portions of the plates 19. Bolts 24 pass through aligned apertures in the plates 19 and the flanges of the wall members 23 to hold the assembled components together in a sealing relationship. The heating apparatus illustrated contains three pairs of electrodes 21 spaced along the length thereof as best seen in Figure 3. Each electrode 21 lies recessed into the inner surface of the insulating liner 20 so that the interior surface of the pipe is smooth and uninterrupted.

Contact with the electrodes 21 is made through apertures 35 in the walls 19 through which pass contact members 36 connecting the electrodes with contact plates 37 to which connection is made from the electrical supply. Each of the three pairs of electrodes shown may be connected between earth .and one phase of a three phase supply which may be a conventional 440 volt, 50Hz three phase mains supply as is standard in the United Kingdom, or an essentially equivalent mains supply such as may be standard in any other country. A different phase, but equivalent voltage, is therefore applied across each pair of electrodes. As shown, the electrodes are all of the same length. If the electrical conductivity of the liquid or other flowable material being treated rises with temperature, then more current will tend to flow between a downstream pair of electrodes than between a more upstream pair of electrodes. This can conveniently be countered by constructing the apparatus so that the electrodes are of decreasing length as one goes further downstream, so evening out the current consumption between the three phases of the supply. A rod 25 extends axially of the pipe 9 and carries a plurality of scraper members 26 and 27 best seen in Figures 2, 5 and 6. Each scraper member comprises a central boss 28, a pair of radially extending arms 29 and a pair of blade members 30 of electrically insulating material adapted to sweep the whole exposed surface area of the respective wall of - 25 - the pipe 9. In the scraper members 26, the radial arms 29 extend horizontally and the blade edges vertically. In the scraper members 27, the radial arms extend vertically and the scraper edges horizontally.

Other forms of scraper may be employed. Preferably the design is such as to minimise deflection of the scraper itself by the forces acting on it, avoid trapping material, and conveniently incorporate means for diverting the material into the centre of the pipe means from the edges thereof.

Rod 25 extends out from the upstream end of the pipe 9 and out through a bushing 31 in the wall of an L-shaped inlet member 10 attached to the pipe 9. As seen in Figure 1, the rod 25 is connected to means for producing reciprocation of the rod, here illustrated schematically as a motor driven flywheel 32 connected eccentrically by a rod 33 to the end of rod 25. The distance over which rod 25 is reciprocated in use should be such that the area swept by successive scrapers 26 in the pipe 9 overlaps and similarly such that the area is swept by successive scrapers 27 overlaps also. The whole interior length of the pipe 9 from the most upstream end of the first electrode down to the most downstream end of the last electrode should therefore be swept by the scrapers acting on all four walls. Since the rod 25 is introduced into the apparatus from the upstream end of the heating pipe 9 it is not critical that a completely sterile seal is maintained at all times at the bushing 31 since any slight degree of contamination which might enter the system along the rod 25 will only add marginally to any bacterial contamination already present in the material to be sterilised.

The apparatus includes an inlet thermocouple 38 an outlet thermcouple 39, and a flow rate monitor 40, such as a doppler flow rate measuring device, linked to a control unit (not shown) wherein the signals from the inlet thermocouple and the flow rate monitor are used to derive an outlet signal which in turn regulates the application of voltage to the electrodes to regulate the energy input in the heater. The signal from the outlet thermcouple 39 is then used in a feed back loop to refine the temperature control effected by the control means. The heat exchanger 12 illustrated in Figures 1 and 7 comprises a path for sterilised food product which runs counter current to a path for coolant liquid. The heat exchanger comprises an outer shell 38 divided internally into layers by partition plates 34 which each extend from one end of the shell 38 to adjacent the other end leaving a gap at the other end between the end of the plate 34 and the shell 38. Each plate 34 commences from a different end of the shell 38 from the preceding plate as one works down the stack of plates so that the plates define a serpentine path for coolant liquid from the inlet 13 to the outlet 14. Within the spaces between the plates 34 the path for the sterilised food produce is provided by a tube arranged to run along a serpentine path in each layer between the plates 34 and to pass from one layer to the next through the spaces between plates 34 and the ends of the shell 38. The tube forms an overall flow path through the heat exchanger extending through each layer in succession to the outlet from the heat exchanger for the sterilised product.

Apparatus according to the invention as described above may be used in various ways. A product such as milk which requires to be heated quickly and cooled rapidly may be introduced into holding tank 1 and pressurised by air or nitrogen introduced through inlet 3 to a pressure of, say 5 atmospheres. The milk may then be heated as it passes through tube 9 by the direct passage through the milk of electrical current. The milk may then be rapidly cooled in the heat exchanger 12 the outlet of which may be connected to a holding tank as shown or may be connected directly through a pressure reduction valve to a packing machine so that the milk is packed, e.g. into cartons directly as it comes from the heat exchanger.

For materials such as flowable high solids products, e.g. pet foods, there may be a requirement for the product to be cooked and then to be heated briefly to a higher temperature for sterilisation. This may be conveniently achieved by passing the food product into holding tank 4 where it is heated by the • warmed coolant from the heat exchanger 12 passing through the heating coil 15. The product may be pre-cooked by this heating before being introduced through the selector valve 7 into the inlet of the heating apparatus 8 which is then used to raise the temperature of the food product still further by ohmic heating to produce sterilisation.

Heat extracted in the heat exchanger 12 may be used for other food processing operations in the same general installation, possibly on different foodstrea s which are not introduced into the apparatus 8.

Periodically, the apparatus will require cleaning and sterilising. In such a cleaning and sterilising procedure, it would be normal to finish with a rinse of clean water through the apparatus but in an ohmic heating apparatus of this kind, that presents a problem in that heating cannot be recommenced unless there is an electrically conductive medium filling the tube 9. Accordingly, it is preferred that following a sterilisation process, the apparatus is filled with an electrically conductive solution such as a sodium hydroxide or sodium chloride solution and that the solution is then progressively displaced by the introduction of the material to be treated through the inlet 10 of the pipe 9 after initiating ohmic heating. This minimises the waste of the flowable material during the start up of the machine.

As shown, the scraper blade members 30 are inclined outward of the pipe means in the direction of fluid flow. It may be preferable for them to be inclined in the opposite direction so that the flow of material acts to force the scraper blades into closer contact with the pipe means and so that greater turbulence is produced due to the flow acting against the scraper blade members and being deflected into the pipe interior. A further advantage of this arrangement is that rod 25 is under tension when most loaded and hence can be less resistant to buckling loads.

Amongst the advantages of the embodiment illustrated are that the provision of scrapers acting on the whole of the interior surface of the pipe in which heating occurs means that there is no requirement for cooling the^* walls of the pipe in an attempt to prevent material depositing on them as has been proposed previously in for instance Specification 2068200B. Cooling the walls of the pipe is of course in contradiction to the general purpose of the heating apparatus and is wasteful of energy.

The square cross-section of the pipe 9 enables it to be built up from flat sheet components so that it can be dismantled for inspection and for thorough cleaning in a way which is simply not possible with a system based on conventional circular pipes. The use of large electrodes running longitudinally of the pipe enables the use of low electrical voltages which are comparatively safe and hence the use of cheaper and simpler methods of construction.

The second embodiment of ohmic heating apparatus according to the invention shown in Figures 8 to 14 comprises a pipe means 109 shown in Figure 8 which is built up from five principal elements connected at flanges. These elements are an inlet end element 201, first, second and third heating elements 202, 203, 204 and an outlet end element 205. The structure of these elements is shown in greater detail in the subsequent

Figures. As can be seen in Figure 8, the pipe means

109 has an inlet 110 and an outlet 111. Figure 9 shows in cross-section the central one 203 of the three heating elements. The other two are similar. Each is of rectangular cross-section as shown in Figure 10 and has a pair of opposed outer wall members 119 which abut the edges of and are separated by a pair of opposed spacer wall members 123. Preferably, these wall members of the pipe means are of aluminium. Sealing gaskets are provided where the spacer wall members 123 abut the wall members 119. Immediately interior of the wall members 119 are insulating lining plates 120 which are spaced apart by insulating spacing lining plates 122 lying immediately interior of the spacer wall members 123. Recessed into the inner surface of each of the lining plates 120 can be seen electrodes 121, the surface of each of which lies flush with the bordering surfaces of the respective lining plate 120. Bolts 124 passing through appertures in the wall members 119 and 123 serve to compress the lining plates 120 against the spacing lining plates 122 so as to seal the interior cavity of the pipe means. To improve the sealing, longitudinally running grooves 206 are provided along each edge of the spacing lining plates 122.

Extending into the pipe means 109 from the inlet end is a set of four parallel rods 125 disposed, as seen in cross-section, at the corners of a rectangle. The four rods are mounted in a plate 207 outside the inlet end of the pipe means (Figure 13). A single rod

208 extends away from the rods 125 and parallel thereto from the centre of plate 207. Rods 125 enter the pipe means through bushings in an end plate of the pipe means so that the rods 125 are as reciprocable by movement of the rod 208.

Supported at matching intervals on each of the rods 125 are bearers 209 which carry scraper blades 210. The bearers 209 are generally spherical in shape having a dimetrical bore to receive the respective rod

125 and having a protruding neck upon which is mounted the respective scraper blade 210. Progressing down the pipe means 109 in the flow direction, scraper blades 210 are provided alternately for scraping the walls containing electrodes and for scraping the walls spacing the electrode containing walls.

The length of the reciprocating movement of the cage of rods 125 is sufficient that the areas of action of scraper blades 210 acting on a particular wall will overlap so that no area of the interior of the pipe means within the heating zone remains unscraped.

The use of a cage of four rods 125 to carry the scraper blades provides a natural springing action tending to spring the blades into contact with the walls for scraping.

Midway between the opposed electrodes 121 insulating shield members 211 are provided at intervals along the pipe means. Four such shield members are provided. The first extends from within the inlet end element 201 to overlap the upstream portion of the electrode of the first heating element 201. The second extends from a postion overlapping the downstream end of the electrode of the first heating element so as to overlap the upstream end of the electrode of the second heating element 202. The third extends from overlapping the downstream end of the electrode of the second heating element 202 so as to overlap the upstream end of the electrode of the third heating element 203. The last extends from overlapping the downstream end of the electrode of the last heating element 203 into the outlet element 204. Each shield member is a relatively thin plate of insulating plastics material bridging between opposed longitudinally running grooves in the interior face of the spacing lining plates 122. As is more fully disclosed in our British Patent Application No. 8809750.6, the purpose of the shield members is to reduce the available current paths at and around the edges of the electrodes so as to reduce the tendency for current density to increase at the upstream and downstream edges of the electrodes. As a consequence, it is possible to run the electrodes at an average current density which is substantially greater than would be possible if there was a substantial rise in current density at these edges, as there would be in the absence of the shield members.

It can be seen that the shield members cover approximately 70% of the area of the electrodes. Where scraper blades 210 are disposed to scrape the interior face of the lining plates 122 in the region, where those plates carry the shield members 211, the scraper blades 210 are interrupted to accommodate the presence of the shield members 211. in other regions, the scraper blades bridge between the pairs of bearer members 209.

The lining plates 121 and 122 are preferably composed of a temperature resistant, food approved plastics material such as polyethersulphone or polyetheretherketone. The electrodes are preferably titanium coated with a proprietary electrode coating as known in this art, for instance an Eltech EC6 10 coating.

A problem arises in providing a junction between the electrode and the wall in which it is received such that no opportunity is given for food products to lodge in the junction between the electrode and wall. In the illustrated embodiment, the lining plates 120 are machined to receive the electrode 121 whilst in a longitudinally stretched condition and are then permitted to relax against the ends of the electrodes such that differential thermal expansion of the electrodes and the lining plates thereafter is accommodated by varying the elastic stress in the lining plates 120, thus avoiding the opportunity of opening up any gap between the end of the electrode and the lining plate 120.

This is accomplished by a method best described with reference to Figures 11 and 12. As shown in Figure 12, the wall member 119 of the pipe means is provided at its upstream end with a pair of abutment members 212 located on the interior face -of the wall member 119. At the opposite end of the wall member 119 a pair of screw adjustable abutment members 213 are provided connected to adjustment screws 214 which extend out through the flange at the end of the wall member 119. The lining plate 120 is provided with recesses for engagement over the abutments 212 and 213 and after the lining plate has been placed in contact with the wall member 119, the abutment 213 is screwed outwardly to stretch elastically the lining plate 120 in the flow direction of the apparatus. An aperture for receiving the electrode 121 is then machined in the lining plate 120 and holes for receiving an electrode 136 and a plurality of electrode mounting studs 213 are drilled in the lining plate 120. The electrode 121 is then mounted in position by studs 213 carried by the electrode 121 which are secured by nuts on the outside of the wall member 119 from which the studs are insulated by insulating collars. This holds the electrode flat against the interior face of the lining plate 120. The adjustment screws 214 of the movable abutments 213 are then slackened off so that the junction between the end of the electrode 121 and the end of the recess receiving the electrode in the lining plate 120 is allowed to close as some of the elastic stress in the lining plate 120 is released. The inlet end element of the pipe means 109 shown in Figure 3 has an outer metal casing consisting of a cylindrical outer wall 214 and a circular end plate 215 through bushes in which pass the rods 125 of the scraper mechanism. The downstream end of the wall 214 is flange mounted to the upstream end of the first heating element 202. Within the cylindrical wall 214, a block of PTFE 216 is provided.

The downstream end element 204 shown in Figure 14 has a generally similar structure except that of course the rods 125 do not pass through the end plate of the end element. An outlet tube 217 extends from the end plate of the end element 204 and is,flange mounted to the inlet tube 218 of a cooling apparatus from which it is electrically insulated. Preferably, the electrical supply to the apparatus takes the form of an invertor powered by a six pulse rectifier system or a centre neutral single phase transformer- or six phase supply arranged such that the electrodes on one side of the pipe means are driven by in phase supplies symmetrical about neutral which may be amplitude controlled independently, a neutral collector electrode being constituted by the end face of the downstream end element 205 which is provided with a connector terminal 219. It can be seen from Figure 14 that the most downstream one of the shield members 211 extends overlapping the downstream edge of the last pair of electrodes into the end element 205 stopping short of the neutral collector electrode. Control of the heating of a flowable product in the heater may be achieved by variation of the amplitude of the voltage applied to one or more of the pairs of electrodes.

For the purposes of control, one or more pairs of electrodes may momentarily be switched off. Also, where there is a potential risk of creating a high electrical load at a portion of an electrode where a scraper member is exposing only a thin strip of electrode to current flow from an adjacent electrode on the same side of the pipe means when there is a potential difference, it is possible to include means for sensing the scraper member position and using the resulting signal momentarily to cut off the electric supply at the potentially critical point to protect the electrode.

The effect of the use of the shield members described above is illustrated in Figures 15 and 16 which are plots of isopotentials produced by computer simulation. Figure 15 indicates that in normal use there will be virtually no current flow between electrodes on the same side of the pipe means and that the ratio between the maximum current density at any electrode and the average current density will be about 1.5. This will mean that the average current density may be much higher than would be case in the absence of the shields without the edge current density becoming excessive. The heating effect is greatest where the isopotentials are closest and it can be seen that the heating effect will therefore be greatest along the central axis of the heater. Since the flowable material is likely to flow fastest along the central axis, this current distribution is likely to result in more uniform beating. Furthermore, the positioning of the shields along the central axis will itself tend to reduce the flow rate along the centre of the pipe means, once again helping to produce uniform heating.

Figure 16 illustrates the condition which may momentarily arise when current is switched off at the centre electrodes. The plot indicates that there is not excessive current flowing between the upstream and downstream electrodes respectively and the central pair of electrodes and also indicates that the maximum current density at the upstream and downstream pairs of electrodes is even under these conditions not excessive in proportion to the average current density.

Both plots indicate that there will be virtually no current flow to the neutral electrodes at the end of the pipe means. It is an advantage of the scraper system described with reference to Figures 8 to 14 that the obstruction to flow in the pipe means is reduced even in comparison to the mechanism illustrated in Figures 1 to 7 because the presence of relatively long arms extending perpendicular to the flow direction is avoided. The design is good from the point of view of avoiding areas in which flowing material may be trapped with consequent risk of overheating. It also provides a built in spring to keep the scraper members in contact with the surfaces to be scraped with a low force by virtue of the length of the supporting rods which act as the spring.

The scraper acts on all these areas of the pipe means bordering zones in which heating takes place. Also, the design is such that where the interior of the pipe means is exposed to heated flowable medium and is not scraped, as in the end element 204, the flow is accelerated, tending to prevent deposition on the said interior.

The design of the apparatus in Figures 8 to 14 is such as to facilitate adjustment of the apparatus to cope with the heating of flowable materials of different conductivities. By shifting the shield members longitudinally or by removing the shield members illustrated and replacing them with shield members of a different length so as to mask the electrodes to a different extent, it is possible to alter the minimum or maximum resistivity of the flowable material which can conveniently be heated in the pipe means at a given power with a given current limit on the power supply. Whilst the invention has been illustrated by the description of particular embodiments, it should be appreciated that many modifications and variations thereof are possible within the scope of the invention.

Claims

1. Apparatus for ohmic heating of an electrically conductive flowable medium comprising pipe means (9) through which such a medium can be arranged to flow, the pipe means (9) being made of or internally lined with a material (20, 22) having an electrical conductivity sufficiently low to allow ohmic heating of a flowable medium therein in use, and at least two electrodes (21) spaced apart in the pipe means and arranged to make electrical contact in use with a medium flowing therethrough, characterised in that the apparatus includes means (25, 26, 27) movable in said pipe means to act on the electrodes and those if any of the internal wall surfaces of the pipe means which are in contact with the flowable medium within the zone or zones in the pipe means in which heating of the flowable medium takes place in use, so as to dislodge or deflect therefrom material tending to deposit thereon from the flowable material as it is heated.

2. Apparatus as claimed in Claim 1, wherein the movable dislodging means is a scraper comprising one or more scraper members (26, 27) mounted for scraping contact with the interior of the pipe means.

3. Apparatus as claimed in Claim 2, wherein the scraper comprises a plurality of scraper members (26, 27) spaced lengthwise of the pipe means connected to a reciprocable carrier member (25), a portion of which extends from the upstream end of the pipe means and is connected to the means (32, 33) for producing reciprocation of said scraper members.

4. Apparatus as claimed in Claim 3, wherein the reciprocable carrier member comprises a cage of elongate carrier means (125) and at least some of the scraper members (210) span between pairs of said elongate carrier means.

5. Apparatus as claimed in any preceding claim, wherein said means for dislodging material from the interior of the pipe means is adapted to act over substantially the whole of the length within which ohmic heating takes place in use.

6. Apparatus as claimed in any preceding claim wherein said electrodes (21) form one or more electrode pairs, members of each pair of electrodes facing one another across and width of the pipe means.

7. Apparatus as claimed in Claim 6, wherein each electrode (21) is recessed into the interior surface of the pipe means so as to lie substantially flush with areas of said surface adjacent the respective electrode.

8. Apparatus as claimed in any preceding claim, wherein said electrodes in.aggregate have a surface area exposed for current flow which is greater than 30% of said internal surface area.

9. Apparatus as claimed in any preceding claim, wherein the pipe means is straight and is of hollow rectangular cross-section, the interior faces of the pipe means being substantially flat.

10. Apparatus as claimed in any preceding claim, wherein the moveable dislodging means is a scraper mounted for scraping contact with the inside of the pipe means and such as to produce stirring or turbulence of the flowable material to provide a susbtantially uniform time of passage through the pipe means for all the flowable material.

11. Apparatus for ohmic heating of an electrically conductive flowable medium comprising pipe means (9) through which such a medium can be arranged to flow, the pipe means being made of or internally lined with a material (20, 22) having an electrical conductivity sufficiently low to allow ohmic heating of a flowable medium therein in use, and at least two electrodes spaced apart in the pipe means and arranged to make electrical contact in use with a medium flowing there- through characterised in that the apparatus includes means (25, 26, 27) for producing stirring or turbulence of the flowable material to provide a substantially uniform time-of passage through the pipe means for all the flowable material.