GB2362306A

GB2362306A - Eddy current heating of fluid flow impeller

Info

Publication number: GB2362306A
Application number: GB0003802A
Authority: GB
Inventors: Malcolm Robert Snowball
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-02-19
Filing date: 2000-02-19
Publication date: 2001-11-14
Also published as: EP1130336A3; EP1130336A2; GB0003802D0

Abstract

An electrically inductive impeller 13, disposed in the flow duct and rotated by a motor 10 to cause the fluid to flow along the flow duct, is inductively heated by coil 21. The coil wire is of the Litz type and comprises copper rope or braid. A non-stick coating enhances the expansion and contraction of the impeller inhibiting scale. Alternatively the impeller may be positioned in a cooking chamber (Fig2) and water sprayed onto the heated impeller to create steam. Fig 3 show a similar embodiment where a heated ring 40 mounted radially externally to the heated fan passes through a water trough 42.

Description

2362306 High Efficiency Fluid Heatinq Apparatus.

This invention relates to an apparatus for heating fluids.

Fluid heating apparatus generally rely on either an electric element disposed in the liquid or gas to be heated 5 fluid or a low efficiency heat exchanger.

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 require a relatively large amount of space.

It is often desirable to be able to circulate or pump the heated fluid and this is generally achieved using a pump or fan having an impeller. However, the combined cost of the heating apparatus and the pump or f an can be high. Furthermore, the combined system is bulky.

1 have now devised a compact and highly efficient apparatus for heating fluids, in which the heat can be applied to the fluid in a controlled manner.

In accordance with this invention, there is provided a heating apparatus comprising a flow duct for the fluid to be heated, an electrically inductive impeller disposed in the flow duct, drive means arranged to rotate the impeller to cause the fluid to flow along the flow duct and a 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 J2 R, where I is the current in the impeller and R is the electrical resistance of the impeller.

The heating apparatus could be used to heat and circulate any kind of fluid. In one embodiment, the apparatus could form a combined pump and heater in a central heating 2 system. In another embodiment, the apparatus could form a means for heating and circulating cooking oil or fat in a frier. In yet a further embodiment, the apparatus could form a fan for heating and circulating air in an oven.

The resistivity of the impeller depends on the material that it is made from. Thus, it will he appreciated that the temperature which the impeller reaches will be dependent on the material of the impeller.

Preferably the impeller is formed from metal.

Preferably the coil is disposed outside the flow duct on the opposite side of a wall thereof to the impeller.

Preferably the wall is formed of a magnetically permeable material such as plastics or glass.

Preferably the impeller is of the radial flow type.

is Preferably the radial flow impeller comprises a base lying normal to the axis of rotation of the impeller and plurality of axially extending vanes.

Preferably the solid support plate of the centrifugal impeller and hence the impeller itself is efficiently heated as it is rotated.

Preferably, the base extends parallel to a flat portion of the wall of the flow duct, the coil being disposed on the opposite side of said flat wall portion.

Preferably the coil is flat. Preferably the flat coil lies in a plane normal to the axis of rotation of the impeller.

Preferably the heat input to the impeller is controlled by a closed loop feedback, temperature control system.

Combination ovens have been part of the product range available to catering professionals and establishments for the last fifteen years. Such ovens constitute a highly versatile cooking tools that allow the user to use convected hot air and/or steam. Thus, preferably means are provided for directing steam and/or water onto the impeller, so that hot steam-laden 3 air can be distributed by the impeller. Embodiments of this invention will now be described by way of examples only and with reference to the accompanying drawings, in which: 5 FIGURE 1 is a sectional view through a water pump in accordance with this invention; FIGURE 2 is a sectional view through an embodiment of oven in accordance with this invention; and FIGURE 3 is a sectional view through an alternative embodiment of oven in accordance with this invention.

Referring to Figure 1 of the drawings, there is shown a water pump comprising an electric motor 10 having a pump assembly 11 mounted to one end thereof. The assembly 11 comprises cylindrical casing 12 and an impeller 13 rotatably is mounted inside a chamber of the casing 12. The casing comprises a radially projecting inlet 14 and an axially extending outlet is.

The impeller 13 is a one piece formation of metal comprising a circular base 16 and a plurality of axially extending vanes 17 each lying in plane which extends substantially radially of the impeller. In use, as the impeller 4 is rotated, water is drawn from the outside of the impeller chamber, radially through its vanes 5 and axially out through the central part of the impeller 6. A shaft 18 extends through the rear casing wall 19 and is secured such that it can rotate freely by means of a bearing 20. Preferably the shaft 18 is a poor thermal conductor so that heat does not substantially conduct into the rotational drive arrangement.

The rear casing wall 19 lies parallel to the base 16 of the impeller 13. The rear casing wall 19 is made of a material which allows electromagnetic waves to pass through it, such as plastic or glass. on the other side of the casing wall 19 and positioned around the shaft 18 is a substantially flat coil 21.

4 Preferably the coil 21 is made from copper rope or braid and is of the Litz construction, whereby the coil 21 is multi-stranded with each strand electrically insulated from each other. The coil 21 is positioned adjacent to the pump impeller 13 and forms part of the resonant tank circuit of a high frequency power generator 22, which could be of the series resonant inverter type. When the coil 21 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 impeller support plate 16. 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 pump impeller 13 heats up.

The water is pumped with high turbulence, which is important to achieve high heat transfer efficiency. This, in conjunction with the heat generated in the pump impeller 13 by the coil 21 and the induction generator 22 provides a very efficient water heating apparatus.

Preferably there is a small space between the coil 11 and the casing wall 19, which is either naturally or force ventilated to keep the coil 21 cool. The continual heating up and cooling down of the pump impeller 13 and its motion remove any scale build up on the pump impeller 13: This can be enhanced by coating the impeller 13 with a non- stick surface such as PTFE. The continual expansion and contraction of the pump impeller 13, cracks the scale off its surface, which is then spun off by centrifugal force, hence maintaining a high heat transfer efficiency.

In use, when the motor 10 is operating and the impeller 13 is rotating in a direction which draws the water from the inlet 14 and delivers it to the outlet 15, the high frequency generator 22 feeds the induction coil 21 with high frequency current and the pump impeller 13 thus heats up.

Water at the inlet 14 is drawn through the hot impeller vanes 17 and out through the outlet 15. The water flows through the vanes 17 in a turbulent manner and therefor extracts heat from the impeller 13 very efficiently. It is then pushed out 5 of the outlet 15.

If the unit is part of a re-circulating system then water will flow back into the inlet and through the impeller to pick up more heat. A temperature sensor 23 placed in the outlet or inlet stream will detect the water temperature and can be used to control the water temperature by regulating the motor speed and/or the power supplied to the induction coil 21.

In this manner a compact, energy efficient and cost effective water heating/re-circulating system can be produced.

Referring to Figure 2 of the drawings, there is shown an oven comprising a thermally insulated housing 24 having an aperture 25, over which is positioned a plate 26 formed of a non-metallic and thermally insulating material. Protruding through the plate 26 is a shaft 27 secured such that it can rotate freely by means of a bearing 28. Preferably the shaft 27 is a poor thermal conductor so that heat does not substantially conduct into the rotational drive arrangement.

Attached to the shaft 27 is a metal fan impeller 29 of the radial flow or centrifugal type. The base 30 of the impeller 29 lies substantially flat against the cover plate 26.

On the other side of the cover plate 26 and positioned around the shaft 27 is a substantially flat coil 31. Preferably the coil 31 is made from copper rope or braid and is of the Litz construction, whereby the coil 31 is multi-stranded with each strand electrically insulated from each other. The coil 31 is positioned adjacent to the fan impeller 29 and forms part of the resonant tank circuit of a high frequency power generator 32, which could be of the series resonant inverter type.

When the coil 31 is powered with high frequency current 6 a high frequency magnetic field is produced. The magnetic lines of force in the magnetic field produce eddy currents in the base 30 of the fan impeller 29. 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 fan impeller 29 heats up.

Means (not shown) are provided to rotate the fan impeller 29 in a way that air inside the oven cavity is drawn into the centre of the centrifugal fan impeller 29 and then blown out at its periphery, hence circulating the air in the oven. This in conjunction with the heat generated in the fan impeller 29 by the coil 31 and the induction generator 32 provides a very efficient convected air heating system.

Preferably there is a small space 33 between the coil 31 and the cover plate 26, which is either naturally or force ventilated to keep the coil 31 cool. Projecting through the wall of the cavity is a pipe 34, one end of which points into the fan impeller 29. The other end of the pipe 34 is attached to a valve 35 which when actuated allows water to pass from a water tank 36 to the pipe 34. The water is then delivered via the pipe 34 onto the hot fan impeller 29 where it is converted into steam; this steam is then dispersed throughout the oven by the fan impeller 29.

The fact that the water is converted to steam on the fan impeller 29 makes it resistant to scaling problems. Scale builds up on the fan impeller 29, which is removed by the continual heating up and cooling down of the fan impeller 29. This continual expansion and contraction of the fan impeller 29, cracks the scale off the surface of the fan impeller 29, which is then spun off by centrifugal force and finally removed by normal oven cleaning.

It is relatively straightforward for those skilled in the art to provide control systems, which controls oven 7 temperature, humidity and timing sequences to provide combinations of convected hot air and/or steam.

In a second embodiment of the invention, shown in Figure 3, an oven cavity 38 identical to the one described in the first embodiment is provided with a different fan impeller and coil arrangement. Preferably the fan impeller 39 is again of the metal centrifugal-flow type, and is fitted around its periphery is an annular metal ring 40. Preferably the metal ring 40 is thermally insulated from the fan impeller 39 by insulation spacers 41 so that when the ring 40 is heated, very little heat conducts back to the fan impeller 39.

Positioned at the edge of the ring 40 is a trough 42 suitable for carrying water. The trough 42 is positioned so that the edge of the ring 40 is inside the trough 42 and when the fan impeller 39 and ring 40 are rotated, the edge of the ring 40 revolves in the trough 42. On the other side of the cover plate 43 and positioned around the shaft 44 are two substantially flat coils 45,46. Preferably the coils 45,46 are made from copper rope or braid and are of the Litz construction as previously described in the first embodiment. Coil 45 is a flat circular coil positioned adjacent to the fan impeller 39 and coil 35 is a flat annular coil 46 positioned adjacent to the ring 40. The two copper coils 45,46 form part of the resonant tank circuits of two high frequency power generators 47,48 which could be of the Series Resonant Inverter type.

As described in the first embodiment, when high frequency current passes through the coils 45,46 high frequency magnetic fields are produced which project through the cover plate 43 and into the rear metal faces of the fan impeller 39 and the metal ring 40, thereby heating them up. As the coils 45,46 are connected to two separate high frequency power generators 47,48 and hence constitute two separate induction heating systems, the heat in the fan impeller 39 and in the 8 ring 40 can be adjusted independently of each other.

Fitted to the trough 42 is a pipe 49, which protrudes through the wall of the oven cavity, and supplies the trough 42 with water. Means are provided to rotate the fan impeller 39 and ring 40 in a way that air inside the oven cavity is drawn into the centre of the centrifugal fan impeller 39 and blown out at its periphery hence circulating the air in the oven. This in conjunction with the heat generated in the fan impeller 39 by the coil 45 and the induction generator 47 provides a very efficient convected air heating system.

Preferably there is a small space between the coils 45,46 and the cover plate 43 which is either naturally or force ventilated to keep the coils 45,46 cool. Heating the ring 40 via coil 46 and rotating the hot ring 40 in the trough 42, which contains water supplied through pipe 49 generates steam.

This arrangement is not susceptible to water scaling problems due to the continual heating and cooling of the metal ring 40 which sheds the accumulated scale as the ring 40 expands and contracts. 20 It is relatively easy for those skilled in the art to design control systems which control oven temperature, humidity and timing sequences to provide convected hot air and/steam. In a third embodiment (not shown). A system is provided the same as that disclosed above, except that spraying or 25 dropping the water onto the hot metal ring 40 generates the steam. Projecting through the wall of the oven cavity is a pipe, the end of which points onto the hot metal ring 40. Attached to the other end of the pipe is a valve which when actuated allows water to flow from a water tank through the pipe and 30 fall or spray onto the hot metal ring 40. In this manner steam is produced and is distributed around the oven via the re-circulating centrifugal fan impeller.

9

Claims

Claims

A fluid heating apparatus comprising a flow duct for the fluid to be heated, an electrically inductive impeller disposed in the flow duct, drive means arranged to rotate the impeller to cause the fluid to flow along the flow duct and a coil disposed adjacent the impeller and arranged to induce eddy currents therein.
2. A fluid heating apparatus as claimed in claim 1, in which the impeller is formed of metal.
3. A fluid heating apparatus as claimed in claims 1 or 2, in which the coil is disposed outside the f low duct on the opposite side of a wall thereof to the impeller.
4. A f luid heating apparatus as claimed in claim 3, in which the wall is formed of a magnetically permeable material.
5. A fluid heating apparatus as claimed in any preceding claim, in which the impeller is arranged to cause the fluid to flow radially of the axis of rotation of the impeller.
6. A fluid heating apparatus as claimed in claim 5, in which the impeller comprises a base lying normal to the axis 20 of rotation and plurality of axially extending vanes.
7. A fluid heating apparatus as claimed in claim 6, in which the base of the impeller is mounted adjacent a flat portion of the wall of the flow duct, the coil being disposed on the opposite side of said flat wall portion
8. A fluid heating apparatus as claimed in any preceding claim, in which the coil is flat.
9. A fluid heating apparatus as claimed in claim 8, in which the f lat coil lies in a plane normal to the axis of rotation of the impeller.
10. A fluid heating apparatus as claimed in any preceding claim, in which means are provided for directing steam or water onto the impeller.
11. A fluid heating apparatus substantially as herein described with reference to the Figure 1, Figure 2 or Figure 3 of the accompanying drawings.