The invention relates to a deep-frying device, comprising a deep-frying vessel, a separating or filtering system, a feedstock reservoir, means for heating the oil, a pump and a pipe system which con- nects these parts to each other in such a way that oil can be fed from the feedstock reservoir into the deep-frying vessel and can be returned from the deep-frying vessel, via the separating system, to the feedstock reservoir.

A deep-frying device of this kind is disclosed in Dutch Patent Application 8701760, and in US Patent 4,34,173.

In the deep-frying device disclosed in the said Dutch Applica¬ tion 8701760, the aim is to keep the dwell time of the frying fat or oil in the deep-frying vessel short, such that no reduction of the quality of the oil can occur, or can occur only to a very limited extent, for example as a result of oxidation in air and as a result of contaminants being introduced, fatty acid oxidation, inter alia, being thereby combated or delayed. In the said device, the frying agent, namely fat or oil, is led in a continuous process through an extensive filtering system which ensures that all contaminants formed or introduced are separated off from the oil or the fat, so that no harmful compounds can be formed. The aim is thus to prolong the service life of the oil. Deep-frying in oil which remains of good quality for a long period is good for the health of the consumer and good for the environment, since there is less unpleasant odour. Time is required for effective filtering, as the filtering speed has to be slow, and this in turn has the result that the filter surface has to be large and thus occupies considerable space, and in addition the effective filter surface is reduced fairly rapidly. This known system is readily usable for large industrial equipment but is less so for small devices. The aim is to remove the residual food sediment by a continuous method, before this sediment burns, and to prevent the occurrence of chemical oxidation reactions which give rise to polymer formation which makes the oil prematurely unusable with regard to its frying properties, taste, health and extremely unpleasant odour, (which can happen after only a few hours) .

These reactions of the oil with oxygen arise as a result of the introduction of very small food sediment, in conjunction with the high frying temperature and the frequently still much higher transfer tempera ^¬ ture, with oxygen and light.

The invention has the aim of providing a deep-frying device having a closed system, by means of which a compact construction is possible.

This aim is firstly achieved, according to the invention, in that the separating system between the deep-frying vessel and the feedstock reservoir consists of an overflow centrifuge, the axis of rotation of which forms an acute angle with the vertical, and of which the drum, provided with a' closed wall, accommodates a disposable bag which bears against the closed inner wall of the drum and can be removed, via the opening having an overflow edge, after the deep-frying-vessel has been emptied, via the centrifuge, into .the feedstock reservoir by means of a regulating system.

An overflow centrifuge makes effective separation possible between contaminated and clean oil. As a result of the inclined position of the axis of rotation, which in practice deviates relatively little, for example by approximately 15 degrees, from the horizontal, the centri- fuging drum is easily accessible after it has come to a stop, as there is then still only little oil left in. the centrifuging drum. By using a disposable bag as the lining of the wall of the centrifuging drum, it is possible to remove the collected contaminants very rapidly. The device can be provided with a front door, such as, for example, in a washing machine, which after being opened gives access to the centrifuging drum and thereby allows the dirty bag to be pulled out and replaced by a clean bag. The rotary speed of the centrifuging drum of course depends on the circumstances and is preferably adjustable. It has been observed that in the case of a drum having a diameter of 150 mm and a depth of 160 mm, the most suitable rotary speed is of the order of magnitude of 4400 revo¬ lutions per minute. The higher the rotary speed, the smaller are the particles which are flung against the wall, so that the oil located in the centre of the drum, which oil can escape via the overflow, is very clean.

The abovementioned dimensions of the centrifuging drum, and the preferred rotary speed associated therewith, are suitable for a deep-fry- ing vessel containing from 6 to 18 litres, this being associated with a feedstock reservoir of from 10 to 22 litres and a circulating speed of the oil through the system of from 1 to 6 litres per minute.

The combination of cross-section/length/raised edge/rotary speed of the drum determines, with the flow and viscosity of the oil, the

separation of the sediment which is deposited on the wall of the drum, in this case on the collecting bag, removable in a simple manner, which is placed therein.

According to the invention, the centrifuging drum preferably has a core whereby the drum is seated on the drive spindle. This promotes balance, in that said drum is secured near to the centre of gravity thereof.

The disposable bag is preferably a bag of the type having a base which fits over the core. A flat bag having a pointed base is suit- able if the point is directed inwards when the bag is opened to form a cylinder. A bag of this kind also promotes the preservation of balance, since it can bear tightly against the inner wall and rear wall of the drum.

Preferably, in order to achieve a simple device, the centrifuge is placed lower than the deep-frying vessel and a regulating valve is located in the pipe from the deep-frying vessel to the centrifuge, which regulating valve determines the flow per unit of time and is preferably controlled by a thermostat, in order to ensure that oil supplied to the deep-frying vessel on start-up is not returned too rapidly to the centri- fuge and feedstock reservoir if the correct temperature has not yet been reached.

According to the invention, the regulating system can fill the deep-frying vessel, by means of the pump, as soon as the "frying operation" is switched on and can then ensure a continuous feed to the deep-frying vessel from the feedstock reservoir, and discharge from the deep-frying vessel, via the centrifuge, to the feedstock reservoir.

This regulating system thus ensures continuous circulation and also, as already stated earlier, the step of freeing the access to the drum when the latter has come to a stop and the deep-frying vessel has been emptied.

This emptying of the deep-frying vessel at the end of a frying operation is also useful if a coarse filter is located in the deep-frying vessel, which filter can then be cleaned, while the return feed ensures that the hot fat or oil cannot come into contact with oxygen and light. A device is preferred, however, in which there is no filter located in the deep-frying vessel and the latter has a smooth inner wall, as will be explained in more detail.

According to the invention, the deep-frying vessel can be con¬ nected to at least two feedstock reservoirs and the centrifuge can

discharge to each feedstock reservoir, in which case a selection switch determines which feedstock reservoir is associated with the deep-frying vessel, in the pumping circuit, for feed and discharge.

The invention offers a complete system which is affordable. The good properties which are achieved with the invention, by means of a compact arrangement, are; a. a longer life and thereby a considerable reduction in costs; b. a healthier and ^' better-tasting product; c. standardisation of the end product; d. favourable effects as regards the environment:

- no more unpleasant oxidation odour,

- a considerable reduction in the flow of waste oil etc. , due to the much longer life,

- as a result no contamination by oxidation in the frying vats, no use of chemically aggressive cleansing agents which reach the drains. The bag can be made of bulky thermoplastic material. It is noted that US Patent 3.827,985 discloses a frying device in which use is made of an overflow centrifuge for cleaning the frying fat or oil.

This centrifuge, however, stands vertically and is fed by a pump which applies suction from the deep-frying vessel and feeds the cleaned oil, which overflows, back to the deep-frying vessel. It is therefore not a closed system which is concerned here, but an open system. By removing the oil, to be cleaned, from the deep-frying vessel by suction, there is no guarantee that all the oil is treated by the cen¬ trifuge. During a standstill, for the purpose of removing collected con¬ tamination, it is impossible to avoid the drum being completely filled with oil. Cleaning is therefore a difficult and dirty operation. There is no certainty that complete cleaning has been achieved.

In the case of the device according to the invention, heating can be carried out in a normal manner with the aid of an electrical heat¬ ing element placed in the feedstock reservoir. This ensures, with the aid of a thermostat, that the temperature in the feedstock reservoir remains above the coagulation temperature, for example of the order of magnitude of 40 to 0"C or above, when no frying takes place over a relatively long period, for example at night or during other periods of non-operation. During use, the desired frying temperature is then maintained.

Furthermore, a heating element can be accommodated in the

deep-frying vessel itself, and this element ensures that the correct temperature always prevails in the deep-frying vessel.

This means, however, that heating elements of this kind, in particular when they are located in the deep-frying vessel itself, them- selves become contaminated and must be cleaned, which may be a problem. A device is thus preferred, according to the invention, in which the deep-frying vessel comprises a double base and is double-walled at least over part of the height of the side walls, corresponding to the level of the oil in the deep-frying vessel, and the interior of the double base and double wall is filled with a heatable fluid. By heating in an indirect manner, there is no longer any heating element located in the deep-frying vessel and this vessel can have a smooth base and smooth side walls. Heating the fluid in the double wall or base can take place at any desired position. With regard to the feedstock reservoir, -it is also preferred that this is constructed with a double wall and base, in which a heated or heatable fluid is also located.

A preferred embodiment is achieved, in accordance with the invention, if the double base and wall of the deep-frying vessel and the double base and wall of the feedstock reservoir are incorporated in a circulation system having a pump which feeds the heated fluid from the space defined by the double wall and/or base of the feedstock reservoir, to the space formed by the double wall and/or base of the deep-frying vessel and, from there, back to the space defined by the double wall and/or base of the feedstock reservoir, respectively.

The heating of the heat-transferring fluid in the said double walls or bases preferably takes place at the location of the feedstock reservoir, for example by placing a heating element in the double base of the feedstock reservoir. Using this system, heating of the oil in the feedstock reservoir is achieved in an indirect manner, by heating the fluid in the double wall or base of the feedstock reservoir, this fluid thus ensuring, in the circulation system comprising the double wall or base of the deep- frying vessel, by heat transfer there, that the temperature of the oil in the deep-frying vessel is maintained. There are thus, for example, two circulation systems, one for the oil in which the frying takes place and one for the fluid which ensures the heat transfer, the latter fluid preferably being an oil which is resistant to high temperatures of the order of magnitude of, for example, 300°C, i.e. a so-called thermal oil.

To achieve the longest possible service life of the frying oil, it has proved advantageous to keep the difference between the desired temperature in the frying pan and the temperature in the double wall as small as possible. If, according to the invention, a plurality of feedstock reser¬ voirs are used which can optionally be included in a circuit with the deep-frying vessel, products can then be treated with various frying agents in the deep-frying vessel. Examples of frying agents are rape oil, walnut oil, palm oil, coconut oil, animal fats or mixtures thereof. By means of the device according to the invention, the oil remains clean for a particularly long period, so that various types of products can usually be treated without difficulty. Fish, however, requires separate treatment, since the fish, introduced into the frying medium, itself exudes an oil which could spoil the taste of another product. In that case, a plurality of feedstock reservoirs connected to the same deep-frying vessel are therefore useful. It does not matter that the contaminants in the centrifuge may have different origins, as long as the correct oil is returned in a very clean state to each feedstock reservoir. By means of the device according to the invention, separation of contaminants from the oil is achieved in a manner requiring very little space. This is of great importance for installations which have to be small, such as in restaurant kitchens, street vending stalls and the like. Furthermore, a plurality of deep-frying vessels can also be connected to the same feedstock reservoir.

The invention will now be described in more detail with the aid of the drawings.

Figure 1 shows diagrammaticall , in side view, a deep-frying device according to the invention.

Figure 2 is the front view associated therewith. Figure 3 shows a side view and end view of a bag to be placed in the centrifuging drum, both folded flat and in the opened state.

Figure 4 shows a variant of the device shown in Figure 1, in side view.

Figure 5 is the front view associated therewith. In Figures 1, 2, 4 and 5, the side wall or front wall, respectively, is cut away. The access door, comparable with that of a washing machine, is not shown.

Figures 1 and 2 show a casing 1 containing, at the top, a deep- frying vessel 2 provided with a heating element, and, underneath, a feedstock reservoir 3. A pump 4' driven by a motor 4 is located in the feedstock reservoir 3, which pump introduces melted fat or oil, via the pipe 5, into the deep-frying vessel 2. Instead of a vessel, the feedstock reservoir 3 can also consist of a heat exchanger which is closed and insulated.

The level in the deep-frying vessel can be controlled by a level switch which cooperates, for example, with a valve 7 located between pipes 6 and 8, through which fat or oil can flow from the vessel into the drum of a centrifuge 9. the axis of rotation of which forms an acute angle with the vertical.

In the example shown, this centrifuge has a cylindrical drum with an edge 12 which surrounds the access aperture and acts as an over- flow edge. A disposable container or bag 12' is located in the vessel. Contaminated, oil or fat which reaches the drum 9 via the pipes 6 or 8 and the valve 7 is separated off, clean oil passing via the over¬ flow edge 12 into the casing 10, which surrounds the centrifuging drum and possesses a discharge pipe 13 which in turn opens into the feedstock reservoir 3- The contaminants are deposited on the wall of the bag placed therein.

This feedstock reservoir can be provided with a heating element 14 which is controlled by a thermostat (not shown) and ensures that the temperature in the feedstock reservoir, during "non-operation", is above the coagulation temperature of the fat, for example of the order of magnitude of 40° to 50°C or higher, and is at the desired frying tempera¬ ture during operation.

The present invention also relates to a continuous system in which the pump 4' ensures that in a relatively small device, for example for street vending, the oil, in an amount of, for example, 10 litres, is circulated at a flow rate of, for example, 180 to 200 litres per hour.

The valve 7 can be such that, whether in conjunction with a level switch or not, it regulates the flow quantity so as to keep the level in the deep-frying vessel constant. The valve 7 can also be closed completely, which is desirable when, on start-up of the frying device, unmelted fat has to be melted in the deep-frying vessel 2 with the aid of a heating element present therein.

Finally, the deep-frying vessel can be provided with a cover

15-

The device can be operated as follows:

After a main switch (not shown) has been switched on, the heat¬ ing element 16 in the deep-frying vessel comes into operation and ensures that fat, as yet unmelted, in the vessel is heated. In this way a supply of melted fat is formed in the vessel. New fat must be added since fat is also used up during the frying process.

When the fat is melted, the pump 4' can be switched on and the valve 7 can be opened. Circulation then begins, a relatively large quantity of melted fat or oil thus being circulated through the deep- frying vessel and through the centrifuge.

Fat discharged from the deep-frying vessel arrives in the drum 9 of the centrifuge, which is driven by a motor 11 from the moment when the pump comes into operation. Contaminants are deposited on the wall of the bag in the cen¬ trifuge and clean oil flows, via the overflow edge 12, into the casing 10 and back into the feedstock reservoir 3 via the pipe 13.

When the frying operation is finished and the heating element in the deep-frying vessel is switched off, the centrifuge, controlled by a time switch, continues running for a considerable time, for example 15 to 20 minutes, so that, via the valve 7 which is then open, all the fat or oil can pass from the deep-frying vessel, via the centrifuge, to the feedstock reservoir where it is kept in a fluid state or its temperature is maintained. After this period, controlled by a timing circuit, the deep- frying vessel is empty and the coarse filter 16 present therein can, if necessary, be cleaned.

The drum of the centrifuge can also be cleaned, via an aperture (not shown) in the casing 10 or in the casing 1. This cleaning operation can be carried out by using a disposable bag or container which is placed in the drum and consists of non-permeable material.

In relatively small installations, cleaning is necessary only once a day, so that the disposable bag is the most convenient solution. By means of the device according to the invention, frying can always be carried out in clean oil which has a considerably longer service life, this being achieved using a device which is relatively very small.

Frying odours which result from the burning of very small sedi ^¬ ment, scarcely arise at all and no oxidation occurs since, when the

device is not switched on, all the oil or fat is located in the feedstock reservoir 3 and is there closed off from oxygen and light.

Figure 3 shows a view of a bag 12' consisting of two layers which are secured to each other at the edges 17, 18, form an aperture at 19 and end in a point 20. When this flat bag is opened to form an approximately cylindrical shape, the point 20, as shown on the right in Figure 3. can be directed inwards in the manner shown by broken lines 21, whereby the bag inside the drum fits over the core 22 shown therein.

The embodiment of Figures 4 and 5 differs from that shown in Figures 1 and 2 in that the feedstock reservoir 3' has a double side wall 23 and double base 24, and the deep-frying vessel 2' has a double base 25 and double side walls 26, which end somewhat below the level of the oil in the deep-frying vessel, which level is indicated by a broken line 27- The spaces within the double walls indicated by 23, 24 and 25, 26, respectively, are included in a circuit 28 having pump 29, so that the oil in the double wall 23, 24 of the feedstock reservoir 3', which oil is heated, for example, by means of a heating element 30, can be fed via the pipe 28 to the double wall and base of the deep-frying vessel 2' and back again via the pipe 28' . The device shown can be so compact that it fits into a kitchen unit.

It should be noted, with regard to the regulating system, that it is obvious to any average expert in the field of regulating technology that a large number of solutions are possible, in this connection, which the person skilled in the art can perform using his normal knowledge.

A simple solution is that whereby a time switch is located in the electrical circuit of the motor 11 of the centrifuge, which switch, at least when the motor 4 of the pump 4' is switched off, allows the motor 11 of the centrifuge to continue running until sufficient time has elapsed for allowing the deep-frying vessel 2 or 2' to empty, via the regulating valve 7, and thereafter allows sufficient time for the oil in the centrifuging drum to be centrifuged out.

In the case of a deep-frying vessel having a capacity of 10 litres and a flow rate of, for example, 2 litres per minute, this means that the deep-frying vessel 2 can be empty within 5 minutes, so that in another 5 minutes the drum 9 can be stopped and the bag can be removed and replaced.

The regulating system can also be such that, when the motor 4 stops, the system sets the regulating valve 7 in the position fully open.

The centrifuge can have a brake which comes into operation as soon as the time switch switches off the motor 11.

The door (not shown) can have a safety switch, such as in washing machines.