Induction heater having an alternating current conductor

An induction heater has a conductor loop (1) in which a large alternating current is induced by a toroidal primary transformer (8). An annular secondary magnetic core (4) encircles a straight length of the conductor loop (1) and is itself enclosed by inner and outer cylinders (2 and 5) interconnected by annular end plates (6 and 9). The current in the conductor (1) produces magnetic flux in the core (4) which in turn produces axial heating currents in the cylinders (2 and 5). The inner cylinder (5) may have heated fins (3). An alternative embodiment has an inductively heated screw.

BACKGROUND OF THE INVENTION 
This invention relates to an induction heater wherein material is heated by 
contact with an inductively heated heating element. 
Bulk or continuous flow heaters are employed as dryers or calciners. 
Typically, a heating member contacts material to be heated so that heat is 
efficiently transferred from the heating element to the material to be 
heated. Some degree of mixing action to improve contact can also be 
incorporated with the heating action of the heating element, for example 
by providing the heating element with fin members, as a result contact 
between the heating element and the material to be heated is enhanced. 
However, it is preferably to provide substantially uniform heating of the 
material to be heated. Consequently, means aree required to supply heat 
uniformly to the heating member and the fin members thereof. 
The supply of heat to the heating element is particularly problematical 
where fin members are included. Known dryers incorporate heating by means 
of gas jets or hot fluid. Consequently, in order to heat such fin members 
complicated supply tubing must be engineered into the fin member. It is 
also known to adapt the fin member to enhance the mixing resulting from 
movement of the heating element. This only serves to further complicate 
the design of the fin member. Particular problems are encountered with 
rotary drum dryers and calciners since it is necessary to incorporate 
rotary couplings for supply of the hot fluid or gases. Consequently, there 
are a number of drawbacks for heating the heating element of known bulk or 
continuous flow heaters. 
SUMMARY OF THE INVENTION 
The present invention provides an induction heater having an alternating 
current carrying conductor extending along an axis, a core means 
substantially encircling said axis to guide magnetic flux resulting from 
said alternating current, and a heating element for contacting and 
transferring heat to material to be heated, the heating element comprising 
an electrically conducting closed loop encircling magnetic fluix in the 
core means and being heated by electrical current induced thereby. 
In this way, the heating element is remotely heated by induced electrical 
currents produced from the magnetic flux flowing in the core means. There 
is then no requirement to provide mechanical coupling between the heat 
energy source and the heating element. Furthermore, the heating element 
can have different shape arrangements to be heated by the electrical 
currents flowing therein. 
Preferably the heating element is formed to encircle said axis so that 
material to be heated is enclosed within the heating element. In this way, 
for example, dust resulting from the induction heater can more easily be 
contained. 
Preferably the heater includes means to deliver material into contact with 
the heating element, the heating element being adapted to effect relative 
movement between the material in contact and said axis. Therefore, contact 
and transfer of heat between the heating element and the material to be 
heated can be enhanced thereby providing more efficient transfer of heat 
from the heating element to the material to be heated. The heater can 
effect relative movement by causing the heating element to be moved 
periodically along or about said axis, or alternatively by causing the 
heating element to vibrate. Since the heating element is inductively 
heated, the provision of effecting relative movement does not further 
complicate supply of heat to the heating element. 
Conveniently, the heating element includes at least one fin member whereby 
the surface area capable of contacting the material is increased. The fin 
member forms part of the electrically conducting closed loop encircling 
the magnetic flux and therefore some electrical current is induced in the 
fin member to heat it. It can therefore be seen that supply of heat 
specifically to the fin member is considerably simplified compared with 
the above mentioned bulk or continuous flow heaters. 
In a preferred embodiment, a part of said loop formed by the heating 
element has an electrical resistance, per unit length in the direction 
around the lopp encircling the flux, which is higher than the resistance 
of the remainder of the loop. With this arrangement, a major part of the 
resistive heating of the heating element loop can take place at said 
higher resistance part, allowing a temperature gradient to be set up 
between this part and the remainder of the loop. As a result the magnetic 
core means located in the loop between the higher resistance part and the 
remainder, can be at a temperature below that of the high resistance part. 
Preferably, heat insulation means is provided between said higher 
resistance part of the loop and the core means. Also, cooling means may be 
provided in or adjacent said remainder of the loop. 
Examples of the present invention will now be described with reference to 
the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a current carrying conductor 1 is shaped as a loop to 
pass through a primary transformer 8 at a convenient point. The conductor 
is typically made of copper and may be laminated to reduce the AC 
resistance. A portion of the conductor 1 forms an axis about which is 
provided a ferromagnetic core 4. The core 4 is enclosed within a metal 
skin formed from concentrically aligned inner cylinder 5 and outer 
cylinder 7 and end plates 6 and 9. In this way, the skin forms a closed 
electrically conducting loop about the core 4. The core 4 is typically 
formed from a laminated ferromagnetic material. 
Alternating current set up in the conductor 1 by the toroidally wound 
transformer 8 sets up an alternating magnetic flux which is guided by the 
core 4. In turn, the alternating flux in core 4 induces currents to flow 
around the above mentioned electrically conducting closed loop. These 
currents flow in the direction of the axes of the cylinders 5 and 7. 
Consequently, material to be heated can be placed within the inner cylinder 
5 and be heated by the energy produced in the cylinder by the induced 
axial currents. To enhance contact between the material to be heated and 
the cylinder 5, fins 3 are provided on the axially facing side of the 
cylinder 5. It will be apparent that a suitable protective tube may be 
required to protect the conductor 1. The structure comprising the 
cylinders 5 and 7 and core 4 can be rotated in the direction of the arrow 
A. In this way, the material to be heated is moved into and out of contact 
with the cylinder 5 to allow uniform transfer of heat from the cylinder to 
the material to be heated. Heating the fin members 3 is advantageous 
because the material is in contact with a large heated surface area and is 
continually agitated or mixed. Such a heater can be employed as a bulk 
heater or can be tilted so that material gravitates towards one or other 
of the end plates 6 and 9 as the structure 2 is rotated. This form of 
heater has particular advantages when high air flows through the heater 
are undesirable for example when drying fine powders. 
It will be apparent that by suitable arrangement of the induced currents 
flowing in the skin of the structure 2 high temperatures can be achieved, 
for example up to the Curie temperature of the core, whilst at the same 
time maintaining a high surface contact area. This has particular uses in 
claciners. 
Referring to FIG. 2, a continuous flow induction heater is illustrated. A 
motor 21 rotates a screw structure 22. The screw structure 22 comprises an 
outer wall 23 which has a spiral slot cut in it to receive screw flight 
24. The structure 22 also has an inner wall 25. A toroidal ferromagnetic 
core 26 is sandwiched between the inner and outer walls 23 and 25. To 
retain mechanical integrity between the inner and outer walls, the 
toroidal core is formed of several individual ring cores with stiffening 
ribs 27 disposed between adjacent rings. An electrically conducting 
conductor 1, corresponding to that shown in FIG. 1, runs along the axis of 
the structure 22. Thus, when alternating electrical current passes through 
conductor 1 magnetic flux is induced in the toroidal rings 26. The inner 
and outer walls 23 and 25 are arranged to form an electrically conducting 
closed loop around the toroidal cores 26 so that induced magnetic flux in 
the cores also induces electrical currents to flow in the walls 23 and 25 
and the screw flight 24. The structure 22 is located within a can 28 
having an inlet 29 and an outlet 30 as shown. Consequently, material 
entering at 29 contacts the structure and is urged towards outlet 30 by 
the screw flight 24 when the motor 21 rotates the structure 22 in the 
direction of the arrow A. 
Thus, FIG. 2 illustrates a continuous flow induction heater having a heated 
screw flight. The heating of the flight and the supply of heat to the 
walls 23 and 25 is provided without complex rotary couplings. 
It will be apparent that the heaters shown in FIGS. 1 and 2 are merely 
examples of induction heaters embodying the invention. For example, the 
rotary device shown in FIG. 2 could be applied to an extruder. The 
induction heater embodying the invention can be used to mix materials and 
heat them at the same time and has the advantage of providing a high heat 
transfer area between the heating element of the heater and the material 
to be heated. 
The maximum operating temperature of the heater is limited by the Curie 
temperature of the iron core. With the designs illustrated the inner and 
outer skins and the core tend in time to reach approximately the same 
temperature. FIG. 3 illustrates an example of a modification which can 
allow the hot elements of the heater to operate at temperatures above the 
temperature of the magnetic core. 
In FIG. 3, the modification is shown as applied to the embodiment described 
above with respect to FIG. 1. The outer cylinder of the electrically 
conducting closed loop is formed to have an electrical resistance in the 
direction of the axis which is less than that of the inner cylinder 5 with 
fins 3. Thus, the outer cylinder is formed in this example of a copper 
cylinder 40 which is fastened at each end by means of bolts 41 to a copper 
mounting ring 42. The mounting ring 42 is secured, e.g. by electron beam 
welding to the associated annular end plate 6 or 9. 
The inner cylinder 5 and fins 3 are made of a metal having the necessary 
mechanical strength and heat resistant qualities and also having an 
electrical resistivity such that the resistance along the axis of the 
combined inner cylinder 5 and fins 3 is higher, preferably substantially 
higher than the resistance of the copper cylinder 40. The inner cylinder 5 
and fins 3 may be made of steel for example. 
Because of the higher resistance of the inner cylinder 5 and fins 3, most 
of the heating energy is delivered to the inner cylinder 5 and fins 3 
rather than the copper outer cylinder 40. As a result the outer cylinder 
40 may be cooler than the inner cylinder 5, permitting a temperature 
gradient to be established between the inner and outer cylinders. The 
magnetic core 43 located between the inner and outer cylinder may thus be 
maintained at a temperature below that of the inner cylinder 5. 
Preferably, a thermal insulation 44 is provided between the inner cylinder 
5 and the magnetic core 43 to maximise the temperature difference between 
the inner cylinder 5 and the core. Further, the outer copper cylinder 40 
may be perforated to permit air ventilation of the magnetic core 43. 
An additional outer cylindrical casing 45, typically of steel, may be 
provided interconnecting the annular end plates 6 and 9 to provide 
additional mechanical rigidity and strength to the structure. The outer 
casing 45 may then also be perforated to permit passage of ventilating air 
to cool the core 43. 
It will be appreciated that the above preferred construction may also be 
applied to other embodiments of the invention such as that illustrated in 
FIG. 2. In FIG. 2, the inner wall 25 would be made with an axial 
resistance lower than that of the outer wall 23 with screw flight 24, and 
thermal insulation would be provided between the outer wall 23 and the 
magnetic core 26. Ventilating air would be passed through the interior of 
the inner wall 25, which might be perforated, to cool the magnetic core.