Abstract:
An apparatus for heating liquids, such as cooking fat, includes a vessel for the liquid to be heated, an electrically inductive impeller disposed in the vessel, a motor for rotating the impeller, to cause the liquid to circulate around the vessel, and a electrical coil on the opposite side of a wall of the vessel to the impeller. A high frequency signal is applied to the coil, which generates a magnetic field that induces eddy currents in impeller. The impeller is not an ideal conductor and, therefore, the electrical energy is dissipated as heat, as current, flows through the impeller. The heat generated in the impeller is transferred to the liquid as it circulated around the vessel by the impeller.

Description:
This application is a continuation-in-part of application Ser. No. 09/784,513 filed Feb. 15, 2001 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an apparatus for deep frying food products. 
     2. Related Background Art 
     Liquid heating apparatus generally rely on either an electric element disposed in the liquid to be heated or a low efficiency heat exchanger which indirectly heats the liquid by means of gas or electricity. 
     Such known apparatus are not energy efficient due to the many thermal interfaces involved in the process, they are expensive to run and in general occupy a relatively large amount of space. 
     It is therefore an object of the present invention to provide a liquid heating apparatus which is inexpensive to run and which does not occupy a large amount of space. 
     Another disadvantage of known liquid heating apparatus is that there is often an uneven temperature distribution throughout the heated liquid and this problem is particularly apparent in large heating vessels. Pumps are known which can be used to pump the heated liquid to evenly distribute the temperature. Another advantage of providing a pump is that the heated liquid can be distributed or passed though a treatment element such as a filter. However, the inclusion of a pump in the apparatus adds to the cost and physical size of the apparatus. 
     Many liquids such as wax and cooking fat solidify or become extremely viscous when cool and a problem with this is that the rotation of the impeller of any pump in the liquid will be inhibited when the liquid is cold. This can damage the motor which drives the impeller. 
     It is therefore an object of the present invention to provide a liquid heating apparatus which is able to provide an even temperature distribution throughout the liquid and which avoids the above problems associated with conventional circulation pumps. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, there is provided a liquid heating apparatus comprising an electrically inductive impeller disposed in a chamber arranged to contain the liquid to be heated, drive means arranged to rotate the impeller to induce a flow in the liquid in the chamber, and an electrical coil disposed adjacent the impeller and arranged to induce eddy currents therein. 
     In use, a high frequency signal (in excess of 20 kHz) is applied to the coil, which generates a magnetic field that induces eddy currents in impeller. The impeller is not an ideal conductor, and thus the electrical energy is dissipated as heat as current flows through the impeller. Thus, the heating effect is proportional to I 2 R, where I is the current in the impeller and R is the electrical resistance of the impeller. 
     The resistivity of the impeller depends on the material that it is made from. Thus, it will be appreciated that the temperature which the impeller reaches will be dependent on the material of the impeller. The impeller directly heats the liquid and thus the apparatus is efficient. The impeller also acts to circulate the liquid and thus an even temperature distribution can be achieved without the requirement for a pump and separate heating element. The impeller can also be used to distribute the heated liquid or to pass it through a treatment element such as a filter. The apparatus will not be damaged if the material to be heated is of the kind whose viscosity is inversely proportional to temperature by virtue of the fact that the impeller rapidly heats up, thereby quickly heating the surrounding liquid and allowing the impeller to rotate normally. The impeller helps to distribute the locally heated liquid around the apparatus so that all of the material soon becomes fully flowable. 
     In a preferred embodiment, means may be provided for energizing the coil prior to rotation of the impeller, so as to reduce any risk of damage to the drive means before the surrounding material becomes fully flowable. 
     Many liquids expand as they change in temperature and it will be appreciated that this can damage the apparatus. Accordingly, preferably a wall of the chamber is resiliently deformable in order to allow expansion of the liquid as it changes in temperature. 
     Preferably the coil is disposed outside the chamber on an opposite side wall thereof to the impeller. 
     Preferably the wall is formed of a magnetically permeable material such as plastics or glass. 
     The amount of power required to heat a liquid is much greater than that for a gas and thus a large current has to be applied to the coil in order to quickly heat the liquid. furthermore, the temperature to which the liquid is to be heated is often high and this again necessitates a large coil current. 
     A disadvantage of large coil currents is that the coil itself can become very hot and potentially damaged due to I 2 R losses. This problem is exacerbated by the heat radiating from the heated liquid within the chamber. In order to overcome this problem, the coil is preferably separated from the wall of the chamber by an insulating layer of magnetically permeable material. 
     Preferably the layer of magnetically permeable material comprises air. Preferably a fan is provided for causing flow of the air in said layer. 
     Preferably the windings of the coil are open. Preferably the fan causes a flow of air through the coil windings. 
     Preferably the impeller is driven by a shaft, the fan being mounted on said shaft. 
     At high frequencies in the order of those used in the present invention, the current is confined to the skin of the coil winding owing to the so-called skin effect. This has the result of reducing the effective cross-sectional area of the winding carrying the current. Hence, the heating of the coil is further increased due to the corresponding increase in resistance of the coil. In order to overcome this problem, the coil preferably comprises windings which each comprise a plurality of electrically insulated conductors connected in parallel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of this invention will now be described by way of examples only and with reference to the accompanying drawings, in which: 
     FIG. 1 is a sectional view through an embodiment of deep fat frier in accordance with this invention; 
     FIG. 2 is a sectional view through an alternative embodiment of deep fat frier in accordance with this invention; and 
     FIG. 3 is a sectional view through an embodiment of apparatus in accordance with this invention for heating chemicals; 
     FIG. 4 is a sectional view through an alternative embodiment of apparatus in accordance with this invention for heating chemicals. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1 of the drawings, there is shown a deep fat fryer comprising a flying vessel  10  for containing cooking fat. An electric motor  11  having a vertically extending rotary output shaft  12  is mounted to the underside of the bottom wall  15  of the vessel  10 . The shaft  12  extends into the vessel  10  through a bearing and seal  13 . Preferably the shaft  12  is a poor thermal conductor so that heat does not substantially conduct into the motor  11 . 
     An impeller  14  mounted to the upper end of the shaft  12  inside the vessel for rotation about a vertical axis. The impeller  14  is a one piece formation of metal comprising a circular base lying normal to the axis of the shaft  12  and a plurality of axially extending vanes each lying in plane which extends substantially radially of the impeller. In use, as the impeller  14  is rotated, fat is drawn axially downwardly towards its center and is then expelled radially outwardly through its vanes. 
     The bottom wall  15  of the vessel  10  lies parallel to the base of the impeller  14 . A substantially flat coil  16  is mounted adjacent the bottom wall  15 , on the opposite side thereof to the impeller  14 . The flat coil  16  lies normal to the axis of the shaft  12 . The wall  15  is made of a material which allows electromagnetic waves to pass through it, such as plastic or glass. 
     Preferably the coil  16  is made from copper rope or braid, such as Litz wire, whereby the coil  16  is multi-stranded with each strand electrically insulated from each other. 
     The coil  16  is positioned adjacent to the impeller  14  and forms part of the resonant tank circuit of a high frequency power generator (not shown), which could be of the series resonant inverter type. When the coil  16  is powered with high frequency current a high frequency magnetic field is produced. The magnetic lines of force in the magnetic field produce eddy currents in the base of the impeller  14 . These eddy currents flow in a circular path around each line of force in the metal and create heat in the metal due to its electrical resistance; hence the whole impeller  14  heats up. 
     The fat is circulated with high turbulence, which is important to achieve high heat transfer efficiency. This, in conjunction with the heat generated in the impeller  14  by the coil  16  provides a very efficient apparatus for heating the fat in the vessel  10 . 
     A small gap  17  extends between the coil  16  and bottom wall  15  of the vessel in order to provide thermal isolation between the coil  16  and the vessel  10  of hot fat. The coil is supported by a former  18  which keeps adjacent turns of the coil windings apart. A fan  19  is mounted on the shaft  12  below the coil  16  and in use is arranged to direct a flow of air onto the coil  16  as the shaft  12  rotates. The flow of air flows through the open coil windings and thereby keeps the coil  16  cool. 
     A temperature sensor (not shown) may be used to control the fat temperature by regulating the motor speed and/or the power supplied to the induction coil  16 . When the fat in the vessel  10  is cold it may solidify or become extremely viscous and it will be appreciated that this will inhibit rotation of the impeller  14  with the result that the motor  11  could be damaged. In order to overcome this problem, the coil  16  may be energised for a short period prior to energization of the motor, in order heat the fat surrounding the impeller  14  sufficiently for the impeller to turn relatively freely. Following energization of the motor  11 , the heated fat soon heats the surrounding fat and the apparatus functions normally. 
     Referring to FIG. 2 of the drawings, there is shown an alternative embodiment of deep fat frier and like parts are given like reference numerals. In this embodiment, the vessel  10  comprises a main chamber  20  and a sub-chamber  21  connected thereto by an inlet duct  22 . The impeller  14  is mounted in the subchamber  21  with the center thereof in registration with the inlet duct  22 . An outlet duct  23  extends from a side wall of the sub-chamber  21 , radially of the impeller  14 . The outlet duct is connected via a filter  24  to the main chamber  20 . 
     In use, the apparatus functions exactly as before, except the fat is circulated through the filter  23  by the impeller  14 . 
     Referring to FIG. 3 of the drawings, there is shown an apparatus for heating chemicals which is similar in principle to the apparatus of FIGS. 1 and 2 and like parts are given like reference numerals. The impeller  14  is mounted in a chamber  30 , the bottom wall  15  of which is made of a material which allows electromagnetic waves to pass through it, such as plastic or glass. The upper wall  31  of the chamber  30  extends parallel to the lower wall  15  and is slidably mounted for movement perpendicular to its plane on a plurality of posts  32  extending perpendicularly from the bottom wall  15 . The slidable upper wall  31  is biased towards the impeller  14  by helical coil springs  33  mounted on the posts  32 . Helical coil springs  33  are fixed adjacent the top end of each post  32  by element  52 . End stops  34  are provided on the posts  32  for limiting the travel of the slidable upper wall  31  towards the impeller  14 . 
     An annular flexible diaphragm  35  extends around the impeller  14  between the upper and lower side walls  31 , 15  to form the side wall of the chamber. The impeller  14  is mounted in the chamber  30  with the center thereof in registration with an inlet duct  36  extending from the slidable upper wall  31 . An outlet duct  37  extends from the slidable upper wall  31  adjacent the radially outermost portion of the impeller  14 . 
     In use, the apparatus functions exactly as before, except the upper wall  31  of the chamber  30  moves away from the lower wall  15  to increase the volume of the chamber  30  as the chemical expands with change in temperature, thereby alleviating the risk of damage to the casing cased by the expansion. 
     Referring to FIG. 3 of the drawings, there is shown an apparatus for heating chemicals which is similar in principle to the apparatus of FIGS. 1,  2  and  3  and like parts are given like reference numerals. In this embodiment, two impellers  14  are mounted back-to-back on a hollow shaft  40  which extends through a pump chamber  41 . The coil  16  is sealingly mounted between the two impellers  14  in an inner chamber  42 . A cooling fan  43  is also mounted in the inner chamber  42  and comprises a flat disc mounted to the shaft and extending normal to the axis thereof A plurality of blades  44  are disposed circumferentially of the disc at its radially outermost point. A plurality of apertures  45  are formed in the hollow shaft  40  to communicate between the inner chamber  42  an the interior of the hollow shaft  40 . 
     The inner chamber  42  comprises opposite side walls  50  which are made of a material which allows electromagnetic waves to pass through them, such as plastic or glass. The disc of the fan  43  is made of a similar material. 
     The impellers  14  are mounted in the pump chamber  41  with the centers thereof in registration with respective inlet ducts  46  extending from a main inlet duct  47 . An outlet duct  47  extends radially outwards of the impellers  14  from the pump chamber  41 . 
     In use, when the shaft  40  is rotated, the liquid to be heated is drawn from the main duct  44  into the inlet ducts  46 , whereupon it is forced radially outwards through the blades of the impellers into the outlet duct  47  via the periphery of the pump chamber  41 . 
     The coil  16  inductively heats the impellers  14  and this heat is transferred to the liquid. In order to cool the coil  16 , the rotating fan  43  draws air axially along the hollow shaft  40  into the inner chamber  42  through the apertures  45 . The air then flows radially over the coil  16  to the periphery of the inner chamber  42 , whereupon the air is exhausted through an outlet duct (not shown). 
     The apparatus of FIG. 4 is capable of heating liquids rapidly to high temperatures owing to the use of two impellers  14  on respective opposite sides of the coil  16 . 
     Although the present invention has been described with reference to preferred embodiments, persons skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.