Electric heating – Inductive heating – With heat exchange
Reexamination Certificate
2000-09-21
2002-07-23
Walberg, Teresa (Department: 3742)
Electric heating
Inductive heating
With heat exchange
C428S357000, C442S132000
Reexamination Certificate
active
06423953
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to heating agents that heat in alternating magnetic fields, and more specifically to fibers that generate unexpectedly high heating rates and do so despite being smaller in diameter then previously thought necessary to generate significant heating.
2. Description of Prior Art
Radio frequency alternating magnetic field have been used for some time to generate heat in heating agents. The process is related to and uses equipment similar to the induction heating generators used in the heat treating of metals. The heating agents are electrically conductive and/or magnetic. An electrically non-conductive, non-magnetic material or matrix is transparent to a magnetic field in the radio frequency range and therefore cannot be heated by the field. The heating agents are normally added to or placed upon or in materials or matrices that would otherwise not heat or not heat efficiently in an alternating field of a given frequency and intensity (the resulting combinations hereinafter termed “heating matrices”). Applications include: bonding or welding of thermoplastics; curing of thermosets; melting or curing of adhesives e.g. thermoplastics, thermosets, thermoplastic/thermosets, elastomerics, etc.; activating foaming agents; initiating polymerization; curing ceramics; generation of heat in containers, inserts or tooling which, in turn, transfer heat to materials in thermal contact therewith; starting or accelerating catalytic reactions; heating sealing, compression and transfer molding and numerous other applications.
The frequencies utilized in practical applications range from: 50 KHz+for the heating of metal screens; to 2-5 MHz for the heating of ferromagnetic particles e.g. fine iron oxides; to 5-30 MHz for the heating of iron and other ferromagnetic particles. These processes are distinguished from dielectric and microwave heating. Dielectric heating operates in the 27 MHz to high MHz frequency range and generates heat by exciting the electric dipoles in the dielectric material as they try to align with the rapidly alternating electric field. Microwave heating operates at frequencies from the high MHz to the GHz range where water dipoles resonate. Neither the dielectric nor the microwave processes require heating agents in order to generate heat as long as the materials to be heated are sufficiently polar. Heating agents have been added to materials for use in both electric and microwave processes, where the heat needed to be concentrated or intensified. The very high frequencies involved allowed the use of very small particles that would not heat as efficiently, if at all, at lower frequencies.
Heat is predominantly generated in heating agents by either hysteresis or eddy current losses. Hysteresis losses occur in any magnetic material. Magnetic dipoles within each magnetic domain of the particle attempt to align themselves with the rapidly alternating magnetic field. The energy required to rotate them is dissipated as heat, the rate at which the heat is generated increases with the rate of reversal of the magnetic field—i.e. the frequency of the alternating current. The hysteresis loop differs for each magnetic material and depends upon the strength of the magnetic field and the properties of the material. The area within the hysteresis loop reflects the magnitude of the hysteresis losses, which are manifested as heat. As long as the particle size is larger than one magnetic domain, hysteresis losses do not depend on particle size. Hysteresis occurs in non-conductive ferromagnetic materials such as oxides and ferrites as well as ferromagnetic materials.
Eddy currents, as the name implies, are circulating currents that appear to flow in swirls or eddies in electrically conductive materials, which need not be magnetic and thus include copper and aluminum, for example. Eddy currents, like other electrical currents, require a complete electrical path. For a given eddy current there is an associated voltage drop V, which, for a pure resistance R, is given by Ohm's Law, V=IR, where I denotes current. When a voltage drop occurs, electrical energy is converted into thermal energy or heat. Eddy current heating is based on P=I
2
R, thus the (P) power (heat) in watts is proportional to the square of the (I) current in amps and to the (R) resistance in ohms. Ferromagnetic materials can have both eddy current and hysteresis losses however, if the material is large enough to allow the flow of eddy currents, the heat generated by the eddy currents is generally greater than the heat generated by hysteresis.
Whether a heating agent is large enough to be heated by eddy currents is determined by its size relative to its reference depth. When a solid round bar is placed in a solenoid coil, the alternating current in the coil induces current in the bar. The bar is most easily visualized as consisting of numerous concentric sleeves. The current induced in the outermost sleeve is greater than the current induced in the second sleeve. The effective depth of the current carrying layers is the reference depth or skin depth. The reference depth is dependent on the frequency of the alternating current through the coil, and the electrical resistivity and relative magnetic permeability of the workpiece or heating agent. The definition of d is:
d=
3160
{square root over (p/&mgr;f)}
(English units)
or
d=
5000
{square root over (p/&mgr;f)}
(metric units)
Where d is the reference depth, in inches or centimeters; p is the resistivity of the workpiece, in ohm-inches or ohm-centimeters; &mgr; is the relative magnetic permeability of the workpiece or heating agent (dimensionless); and f is the frequency of the field in the work coil, in hertz. The reference depth is the distance from the surface of the material to the depth where the induced field strength and current are reduced to 1/e, or 37% of their surface value. The power density at this point is 1/e
2
, or 14% of its value at the surface (e=base of the natural logarithm=2.718)
As noted above, the heating efficiency of a heating agent is much higher if it is large enough for eddy currents to flow. That, in turn is determined by the electrical diameter of the workpiece or heating agent which equals a/d or the ratio of the diameter of the heating agent to the reference depth. A heating agent with a ratio of >4 is efficient at heating, one with a ratio of >2>4 is far less efficient and one with a ratio of >2 is considered “no good” or unusable.
This is the basis for the definition of a “critical frequency” below which induction heating efficiency drops rapidly. Thus for a round bar the critical frequency is that at which the ratio of workpiece diameter to reference depth is approximately 4+/1. Below the critical frequency, efficiency drops rapidly because less current is induced due to current cancellation. Current cancellation becomes significant when the reference depth is such that the eddy currents induced from either side of a workpiece “impinge” upon each other and, being of opposite sign, cancel each other.
The equation for calculating the critical frequency (f
c
) for a round bar is:
f
c
=1.6×10
8
p/&mgr;a
2
(
a
in in.)
f
c
=4×10
8
p/&mgr;a
2
(
a
in cm.)
Where a=diameter of the bar; &mgr;=permeability of the material making up the bar and p=resistivity in ohm-inches or ohm-centimeters of the material making up the bar.
Rather than do the calculations each time, the recommend practice in induction heating is to refer to a table or graph to determine the minimum diameters efficiently heated at a given frequency. The following example is from C. A. Tudbury,
Basics of Induction Heating,
John F. Rider Inc., New York, 1960.
Approximate Smallest Diameter (converted to microns—&mgr;) Which Can Be Heated Efficiently By The Equipment Indicated To The Temperature Shown
Final
Spark-Gap
Vacuum Tube
Temp.
Motor Generator
Oscillator
Oscillator
Material
° F.
3 KHz
10 KHz
50
Stevens & Showalter LLP
Van Quang
Walberg Teresa
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