Electric heating – Inductive heating – With diverse-type heating
Reexamination Certificate
2002-12-21
2004-04-06
Hoang, Tu Ba (Department: 3742)
Electric heating
Inductive heating
With diverse-type heating
C219S628000
Reexamination Certificate
active
06717118
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an apparatus for heating an object. More particularly, this invention relates to an apparatus that heats an object both inductively and resistively. The invention is particularly useful for heating large tubular objects, such as barrels and nozzles of machinery, such as extruders and injection molding machines, for processing molten material, such as plastic and metal.
Referring to
FIG. 1
, a typical resistive heater circuit
10
in accordance with the prior art is shown. A power supply
12
may provide a DC or AC voltage, typically line frequency to a heater coil
14
which is wrapped around in close proximity to a heated article
20
. Typically, the heater coil
14
is made up of an electrically resistive element with an insulation layer
18
applied to prevent it from shorting out. It is also common to have the entire heater coil encased in a cover
16
to form a modular heating subassembly. The prior art is replete with examples of ways to apply heat to material and raise the temperature of the heated article
20
to a predetermined level. Most of these examples center around the use of resistive or ohmic heat generators that are in mechanical and thermal communication with the article to be heated.
Resistive heaters are the predominate method used today. Resistive heat is generated by the ohmic or resistive losses that occur when current flows through a wire. The heat generated in the coil of the resistive type heater must then be transmitted to the workpice by conduction or radiation. The use and construction of resistive heaters is well known and in most cases is easier and cheaper to use than inductive heaters. Most resistive heaters are made from helically wound coils, wrapped onto a form, or formed into sinuous loop elements.
A typical invention using a resistive type heater can be found in U.S. Pat. No. 5,973,296 to Juliano et al. which teaches a thick film heater apparatus that generates heat through ohmic losses in a resistive trace that is printed on the surface of a cylindrical substrate. The heat generated by the ohmic losses is transferred to molten plastic in a nozzle to maintain the plastic in a free flowing state. While resistive type heaters are relatively inexpensive, they have some considerable drawbacks. Close tolerance fits, hot spots, oxidation of the coil and slower heat up times are just a few. For this method of heating, the maximum heating power can not exceed P
R(max)
=(I
R(max)
)
2
x
R
c
, where I
R(max)
is equal to the maximum current the resistive wire can carry and R
c
is the resistance of the coil. In addition, minimum time to heat up a particular article is governed by t
R(min)
=(cM&Dgr;T)/P
R(max)
, where c is the specific heat of the article, M is the mass of the article and &Dgr;T is the change in temperature desired. For resistive heating, total energy losses at the heater coil is essentially equal to zero because all of the energy from the power supply that enters the coil is converted to heat energy, therefore P
R(losses)
=0.
Now referring to
FIG. 2
, a typical induction heating circuit
30
according to the prior art is shown. A variable frequency AC power supply
32
is connected in parallel to a tuning capacitor
34
. Tuning capacitor
34
makes up for the reactive losses in the load and minimizes any such losses. Induction heater coil
36
is typically comprised of a hollow copper tube, having an electrically insulating coating
18
applied to its outer surface and a cooling fluid
39
running inside the tube. The cooling fluid
39
is communicated to a cooling system
38
to remove heat away from the induction heater coil
36
. The heater coil
36
is not generally in contact with the article to be heated
20
. As the current flows through the coil
36
, lines of magnetic flux are created as depicted by arrows
40
a
and
40
b.
Induction heating is a method of heating electrically conducting materials with alternating current (AC) electric power. Alternating current electric power is applied to an electrical conducting coil, like copper, to create an alternating magnetic field. This alternating magnetic field induces alternating electric voltages and current in a workpiece that is closely coupled to the coil. These alternating currents generate electrical resistance losses and thereby heat the workpiece. Therefore, an important characteristic of induction heating is the ability to deliver heat into electrical conductive materials without direct contact between the heating element and the workpiece.
If an alternating current flows through a coil, a magnetic field is produced that varies with the amount of current. If an electrically conductive load is placed inside the coil, eddy currents will be induced inside the load. The eddy currents will flow in a direction opposite to the current flow in the coil. These induced currents in the load produce a magnetic field in the direction opposite to the field produced by the coil and prevent the field from penetrating to the center of the load. The eddy currents are therefore concentrated at the surface of the load an decrease dramatically towards the center. As shown in
FIG. 3A
, the induction heater coil
36
is wrapped around a cylindrical heated body
20
. The current density J
x
is shown by line
41
of the graph. As a result of this phenomenon, almost all the current is generated in the area
22
of the cylindrical heated body
20
, and the material
24
contained central to the heated body is not utilized for the generation of heat. This phenomenon is often referred to as “skin effect”.
Within this art, the depth where current density in the load drops to a value of 37% of its maximum is called the penetration depth (&dgr;). As a simplifying assumption, all of the current in the load can be safely assumed to be within the penetration depth. This simplifying assumption is useful in calculating the resistance of the current path in the load. Since the load has inherent resistance to current flow, heat will be generated in the load. The amount of heat generated (Q) is a function of the product of resistance {circle around (R)} and the eddy current (I) squared and time (t), Q=I
2
Rt.
The depth of penetration is one of the most important factors in the design of an induction heating system. The general formula for depth of penetration &dgr; is given by:
δ
=
ρ
/
πμ
μ
υ
f
where
&mgr;
&ugr;
=magnetic permeability of a vacuum
&mgr;=relative magnetic permeability of the load
&rgr;=resistivity of the load
f=frequency of alternating current
Thus, the depth of penetration is a function of three variables, two of which are related to the load. The variables are the electrical resistivity of the load &rgr;, the magnetic permeability of the load &mgr;, and the frequency f of the alternating current in the coil. The magnetic permeability of a vacuum is a constant equal to 4Π×10
−7
(Wb/A m).
A major reason for calculating the depth of penetration is to determine how much current will flow within the load of a given size. Since the heat generated is related to the square of the eddy current (I
2
), it is imperative to have as large a current flow in the load as possible.
In the prior art, induction heating coils are almost exclusively made of hollow copper tubes with water cooling running therein. Induction coils, like resistive heaters, exhibit some level of resistive heat generation. This phenomenon is undesirable because as heat builds in the coil it effects all of the physical properties of the coil and directly impacts heater efficiency. Additionally, as heat rises in the coil, oxidation of the coil material increases and this severely limits the life of the coil. This is why the prior art has employed means to draw heat away from the induction coil by use of a fluid transfer medium. This unused heat, according to the prior art, is wasted heat energy which lowers the overall efficiency of the induction heater. In a
Kagan Valery G.
Pilavdzic Jim Izudin
Von Buren Stefan
Hoang Tu Ba
Husky Injection Molding Systems LTD
Kotula Steven J.
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