Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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HEATER BUNDLE FOR ADAPTIVE CONTROL AND
METHOD OF REDUCING CURRENT LEAKAGE
FIELD
[0001] The
present disclosure relates to electric heaters, and more particularly
to heaters for heating a fluid flow such as heat exchangers and the control
thereof.
BACKGROUND
[0002] The
statements in this section merely provide background information
related to the present disclosure and may not constitute prior art.
[0003] A fluid
heater may be in the form of a cartridge heater, which has a rod
configuration to heat fluid that flows along or past an exterior surface of
the cartridge
heater. The cartridge heater may be disposed inside a heat exchanger for
heating
the fluid flowing through the heat exchanger. If the cartridge heater is not
properly
sealed, moisture and fluid may enter the cartridge heater to contaminate the
insulation material that electrically insulates a resistive heating element
from the
metal sheath of the cartridge heater, resulting in dielectric breakdown and
consequently heater failure. The moisture can also cause short circuiting
between
power conductors and the outer metal sheath. The failure of the cartridge
heater
may cause costly downtime of the apparatus that uses the cartridge heater.
[0004]
Further, during operation, some heaters may experience "current
leakage," which is generally the flow of current through to a ground. The
current
leaks by way of insulation surrounding conductors in electrical heaters and
this
condition can cause a rise in voltage and over-heating.
SUMMARY
[0005] In one
form of the present disclosure, a method of controlling a heating
system is provided that comprises providing at least one heater assembly, the
heater
assembly comprising a plurality of heater units, each heater unit defining at
least one
independently controlled heating zone. Power is supplied to each of the heater
units
through power conductors electrically connected to each of the independently
controlled heating zones in each of the heater units, and this power is
modulated to
each of the independently controlled heating zones, wherein a voltage is
selectively
supplied to each of the independently controlled heating zones such that a
reduced
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number of independently controlled heating zones receives the voltage at a
time or
at least a portion of the independently controlled heating zones receive a
reduced
voltage at all times.
[0006] In another form, a method of reducing current leakage in a heating
system is provided that comprises providing at least one heater assembly, the
heater
assembly comprising a plurality of heater units, each heater unit defining at
least one
independently controlled heating zone, supplying power to each of the heater
units
through power conductors electrically connected to each of the independently
controlled heating zones in each of the heater units, and modulating power
supplied
to each of the independently controlled heating zones, wherein a voltage is
selectively supplied to each of the independently controlled heating zones
such that
a total area of the independently controlled heating zones that receives
voltage at a
time is reduced or at least a portion of the independently controlled heating
zones
receive a reduced voltage at all times.
[0007] In another form, a heater system is provided that comprises a
heater
bundle having a plurality of heater assemblies, each heater assembly
comprising a
plurality of heater units, each heater unit defining at least one
independently
controlled heating zone, and power conductors electrically connected to each
of the
independently controlled heating zones in each of the heater units. A power
supply
device is configured to modulate power to each of the independently controlled
heater zones of the heater units through the power conductors, wherein a
voltage is
selectively supplied to each of the independently controlled heating zones
such that
a reduced number of independently controlled heating zones receives the
voltage at
a time or at least a portion of the independently controlled heating zones
receive a
reduced voltage at all times.
[0008] In still another form, a heater system is provided that comprises a
heater assembly having a plurality of heater units, each heater unit defining
at least
one independently controlled heating zone. Power conductors are electrically
connected to each of the independently controlled heating zones in each of the
heater units, and a power supply device is configured to modulate power to
each of
the independently controlled heater zones of the heater units through the
power
conductors. A voltage is selectively supplied to each of the independently
controlled
heating zones such that a reduced number of independently controlled heating
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zones receives the voltage at a time or at least a portion of the
independently
controlled heating zones receive a reduced voltage at all times.
[0009] Further areas of applicability will become apparent from the
description
provided herein. It should be understood that the description and specific
examples
are intended for purposes of illustration only and are not intended to limit
the scope
of the present disclosure.
DRAWINGS
[0010] In order that the disclosure may be well understood, there will now
be
described various forms thereof, given by way of example, reference being made
to
the accompanying drawings, in which:
[0011] FIG. 1 is a perspective view of a heater bundle constructed in
accordance with the teachings of the present disclosure;
[0012] FIG. 2 is a perspective view of a heater assembly of the heater
bundle
of FIG. 1;
[0013] FIG. 3 is a perspective view of a variant of a heater assembly of
the
heater bundle of FIG. 1;
[0014] FIG. 4 is a perspective view of the heater assembly of FIG. 3,
wherein
the outer sheath of the heater assembly is removed for clarity;
[0015] FIG. 5 is a perspective view of a core body of the heater assembly
of
FIG. 3;
[0016] FIG. 6 is a perspective view of a heat exchanger including the
heater
bundle of FIG. 1, wherein the heater bundle is partially disassembled from the
heat
exchanger to expose the heater bundle for illustration purposes; and
[0017] FIG. 7 is a block diagram of a method of operating a heater system
including a heater bundle constructed in accordance with the teachings of the
present disclosure.
[0018] The drawings described herein are for illustration purposes only
and
are not intended to limit the scope of the present disclosure in any way.
DETAILED DESCRIPTION
[0019] The following description is merely exemplary in nature and is not
intended to limit the present disclosure, application, or uses.
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[0020] Referring to FIG. 1, a heater system constructed in accordance with
the teachings of the present disclosure is generally indicated by reference
10. The
heater system 10 includes a heater bundle 12 and a power supply device 14
electrically connected to the heater bundle 12. The power supply device 14
includes
a controller 15 for controlling power supply to the heater bundle 12. A
"heater
bundle", as used in the present disclosure, refers to a heater apparatus
including two
or more physically distinct heating devices that can be independently
controlled.
Therefore, when one of the heating devices in the heater bundle fails or
degrades,
the remaining heating devices in the heater bundle 12 can continue to operate.
[0021] In one form, the heater bundle 12 includes a mounting flange 16 and
a
plurality of heater assemblies 18 secured to the mounting flange 16. The
mounting
flange 16 includes a plurality of apertures 20 through which the heater
assemblies 18
extend. Although the heater assemblies 18 are arranged to be parallel in this
form, it
should be understood that alternate positions/arrangements of the heater
assemblies
18 are within the scope of the present disclosure.
[0022] As further shown, the mounting flange 16 includes a plurality of
mounting holes 22. By using screws or bolts (not shown) through the mounting
holes 22, the mounting flange 16 may be assembled to a wall of a vessel or a
pipe
(not shown) that carries a fluid to be heated. At least a portion of the
heater
assemblies 18 are be immersed in the fluid inside the vessel or pipe to heat
the fluid
in this form of the present disclosure.
[0023] Referring to FIG. 2, the heater assemblies 18 according to one form
may be in the form of a cartridge heater 30. The cartridge heater 30 is a tube-
shaped heater that generally includes a core body 32, a resistive heating wire
34
wrapped around the core body 32, a metal sheath 36 enclosing the core body 32
and the resistive heating wire 34 therein, and an insulating material 38
filling in the
space in the metal sheath 36 to electrically insulate the resistive heating
wire 34 from
the metal sheath 36 and to thermally conduct the heat from the resistive
heating wire
34 to the metal sheath 36. The core body 32 may be made of ceramic. The
insulation material 38 may be compacted Magnesium Oxide (MgO). A plurality of
power conductors 42 extend through the core body 32 along a longitudinal
direction
and are electrically connected to the resistive heating wires 34. The power
conductors 42 also extend through an end piece 44 that seals the outer sheath
36.
The power conductors 42 are connected to the external power supply device 14
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(shown in FIG. 1) to supply power from the external power supply device 14 to
the
resistive heating wire 32. While FIG. 2 shows only two power conductors 42
extending through the end piece 44, more than two power conductors 42 can
extend
through the end piece 44. The power conductors 42 may be in the form of
conductive pins. Various constructions and further structural and electrical
details of
cartridge heaters are set forth in greater detail in U.S. Patent Nos.
2,831,951 and
3,970,822, which are commonly assigned with the present application and the
contents of which are incorporated herein by reference in their entirety.
Therefore, it
should be understood that the form illustrated herein is merely exemplary and
should
not be construed as limiting the scope of the present disclosure.
[0024] Alternatively, multiple resistive heating wires 34 and multiple
pairs of
power conductors 42 may be used to form multiple heating circuits that can be
independently controlled to enhance reliability of the cartridge heater 30.
Therefore,
when one of the resistive heating wires 34 fails, the remaining resistive
wires 34 may
continue to generate heat without causing the entire cartridge heater 30 to
fail and
without causing costly machine downtime.
[0025] Referring to FIGS. 3 to 5, the heater assemblies 50 may be in the
form
of a cartridge heater having a configuration similar to that of FIG. 2 except
for the
number of core bodies and number of power conductors used. More specifically,
the
heater assemblies 50 each include a plurality of heater units 52, and an outer
metal
sheath 54 enclosing the plurality of heater units 52 therein, along with a
plurality of
power conductors 56. An insulating material (not shown in FIGS. 3 to 5) is
provided
between the plurality of heating units 52 and the outer metal sheath 54 to
electrically
insulate the heater units 52 from the outer metal sheath 54. The plurality of
heater
units 52 each include a core body 58 and a resistive heating element 60
surrounding
the core body 58. The resistive heating element 60 of each heater unit 52 may
define one or more heating circuits to define one or more heating zones 62.
[0026] In the present form, each heater unit 52 defines one heating zone
62
and the plurality of heater units 52 in each heater assembly 50 are aligned
along a
longitudinal direction X. Therefore, each heater assembly 50 defines a
plurality of
heating zones 62 aligned along the longitudinal direction X. The core body 58
of
each heater unit 52 defines a plurality of through holes/apertures 64 to allow
power
conductors 56 to extend therethrough. The resistive heating elements 60 of the
heater units 52 are connected to the power conductors 56, which, in turn, are
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connected to an external power supply device 14. The power conductors 56
supply
the power from the power supply device 14 to the plurality of heater units 50.
By
properly connecting the power conductors 56 to the resistive heating elements
60,
the resistive heating elements 60 of the plurality of heating units 52 can be
independently controlled by the controller 15 of the power supply device 14.
As
such, failure of one resistive heating element 60 for a particular heating
zone 62 will
not affect the proper functioning of the remaining resistive heating elements
60 for
the remaining heating zones 62. Further, the heater units 52 and the heater
assemblies 50 may be interchangeable for ease of repair or assembly.
[0027] In the present form, six power conductors 56 are used for each
heater
assembly 50 to supply power to five independent electrical heating circuits on
the
five heater units 52. Alternatively, six power conductors 56 may be connected
to the
resistive heating elements 60 in a way to define three fully independent
circuits on
the five heater units 52. It is possible to have any number of power
conductors 56 to
form any number of independently controlled heating circuits and independently
controlled heating zones 62. For example, seven power conductors 56 may be
used
to provide six heating zones 62. Eight power conductors 56 may be used to
provide
seven heating zones 62.
[0028] The power conductors 56 may include a plurality of power supply and
power return conductors, a plurality of power return conductors and a single
power
supply conductor, or a plurality of power supply conductors and a single power
return
conductor. If the number of heater zones is n, the number of power supply and
return conductors is n +1.
[0029] Alternatively, a higher number of electrically distinct heating
zones 62
may be created through multiplexing, polarity sensitive switching and other
circuit
topologies by the controller 15 of the external power supply device 14. Use
of
multiplexing or various arrangements of thermal arrays to increase the number
of
heating zones within the cartridge heater 50 for a given number of power
conductors
(e.g. a cartridge heater with six power conductors for 15 or 30 zones.) is
disclosed in
U.S. Patent Nos. 9,123,755, 9,123,756, 9,177,840, 9,196,513, and their related
applications, which are commonly assigned with the present application and the
contents of which are incorporated herein by reference in their entirety.
[0030] With this structure, each heater assembly 50 includes a plurality
of
heating zones 62 that can be independently controlled to vary the power output
or
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heat distribution along the length of the heater assembly 50. The heater
bundle 12
includes a plurality of such heater assemblies 50. Therefore, the heater
bundle 12
provides a plurality of heating zones 62 and a tailored heat distribution for
heating
the fluid that flows through the heater bundle 12 to be adapted for specific
applications. The power supply device 14 can be configured to modulate power
to
each of the independently controlled heating zones 62.
[0031] For
example, a heating assembly 50 may define an "m" heating zones,
and the heater bundle may include "k" heating assemblies 50. Therefore, the
heater
bundle 12 may define mxk heating zones. The plurality of heating zones 62 in
the
heater bundle 12 can be individually and dynamically controlled in response to
heating conditions and/or heating requirements, including but not limited to,
the life
and the reliability of the individual heater units 52, the sizes and costs of
the heater
units 52, local heater flux, characteristics and operation of the heater units
52, and
the entire power output.
[0032] Each
circuit is individually controlled at a desired temperature or a
desired power level so that the distribution of temperature and/or power
adapts to
variations in system parameters (e.g. manufacturing variation/tolerances,
changing
environmental conditions, changing inlet flow conditions such as inlet
temperature,
inlet temperature distribution, flow velocity, velocity distribution, fluid
composition,
fluid heat capacity, etc.). More specifically, the heater units 52 may not
generate the
same heat output when operated under the same power level due to manufacturing
variations as well as varied degrees of heater degradation over time. The
heater
units 52 may be independently controlled to adjust the heat output according
to a
desired heat distribution. The individual manufacturing tolerances of
components of
the heater system and assembly tolerances of the heater system are increased
as a
function of the modulated power of the power supply, or in other words,
because of
the high fidelity of heater control, manufacturing tolerance of individual
components
need not be as tight/narrow.
[0033] The
heater units 52 may each include a temperature sensor (not
shown) for measuring the temperature of the heater units 52. When a hot spot
in the
heater units 52 is detected, the power supply device 14 may reduce or turn off
the
power to the particular heater unit 52 on which the hot spot is detected to
avoid
overheating or failure of the particular heater unit 52. The power supply
device 14
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may modulate the power to the heater units 52 adjacent to the disabled heater
unit
52 to compensate for the reduced heat output from the particular heater unit
52.
[0034] The power supply device 14 may include multi-zone algorithms to
turn
off or turn down the power level delivered to any particular zone, and to
increase the
power to the heating zones adjacent to the particular heating zone that is
disabled
and has a reduced heat output. By carefully modulating the power to each
heating
zone, the overall reliability of the system can be improved. By detecting the
hot spot
and controlling the power supply accordingly, the heater system 10 has
improved
safety.
[0035] The heater bundle 12 with the multiple independently controlled
heating zones 62 can accomplish improved heating. For example, some circuits
on
the heater units 52 may be operated at a nominal (or "typical") duty cycle of
less than
100% (or at an average power level that is a fraction of the power that would
be
produced by the heater with line voltage applied). The lower duty cycles allow
for
the use of resistive heating wires with a larger diameter, thereby improving
reliability.
[0036] Normally, smaller zones would employ a finer wire size to achieve a
given resistance. Variable power control allows a larger wire size to be used,
and a
lower resistance value can be accommodated, while protecting the heater from
over-
loading with a duty cycle limit tied to the power dissipation capacity of the
heater.
[0037] The use of a scaling factor may be tied to the capacity of the
heater
units 52 or the heating zone 62. The multiple heating zones 62 allow for more
accurate determination and control of the heater bundle 12. The use of a
specific
scaling factor for a particular heating circuit/zone will allow for a more
aggressive (i.e.
higher) temperature (or power level) at almost all zones, which, in turn, lead
to a
smaller, less costly design for the heater bundle 12. Such a scaling factor
and
method is disclosed in U.S. Patent No. 7,257,464, which is commonly assigned
with
the present application and the contents of which are incorporated herein by
reference in its entirety.
[0038] The sizes of the heating zones controlled by the individual
circuits can
be made equal or different to reduce the total number of zones needed to
control the
distribution of temperature or power to a desired accuracy.
[0039] Referring back to FIG. 1, the heater assemblies 18 are shown to be
a
single end heater, i.e., the conductive pin extends through only one
longitudinal end
of the heater assemblies 18. The heater assembly 18 may extend through the
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mounting flange 16 or a bulkhead (not shown) and sealed to the flange 16 or
bulkhead. As such, the heater assemblies 18 can be individually removed and
replaced without removing the mounting flange 16 from the vessel or tube.
[0040]
Alternatively, the heater assembly 18 may be a "double ended"
heater. In a double-ended heater, the metal sheath are bent into a hairpin
shape
and the power conductors pass through both longitudinal ends of the metal
sheath
so that both longitudinal ends of the metal sheath pass through and are sealed
to the
flange or bulkhead. In this structure, the flange or the bulkhead need to be
removed
from the housing or the vessel before the individual heater assembly 18 can be
replaced.
[0041]
Referring to FIG. 6, a heater bundle 12 is incorporated in a heat
exchanger 70. The heat exchanger 70 includes a sealed housing 72 defining an
internal chamber (not shown), a heater bundle 12 disposed within the internal
chamber of the housing 72. The sealed housing 72 includes a fluid inlet 76 and
a
fluid outlet 78 through which fluid is directed into and out of the internal
chamber of
the sealed housing 72. The fluid is heated by the heater bundle 12 disposed in
the
sealed housing 72. The heater bundle 12 may be arranged for either cross-flow
or
for flow parallel to their length.
[0042] The
heater bundle 12 is connected to an external power supply device
14 which may include a means to modulate power, such as a switching means or a
variable transformer, to modulate the power supplied to an individual zone.
The
power modulation may be performed as a function of time or based on detected
temperature of each heating zone.
[0043] The
resistive heating wire may also function as a sensor using the
resistance of the resistive wire to measure the temperature of the resistive
wire and
using the same power conductors to send temperature measurement information to
the power supply device 14. A means of sensing temperature for each zone would
allow the control of temperature along the length of each heater assembly 18
in the
heater bundle 12 (down to the resolution of the individual zone). Therefore,
the
additional temperature sensing circuits and sensing means can be dispensed
with,
thereby reducing the manufacturing costs. Direct measurement of the heater
circuit
temperature is a distinct advantage when trying to maximize heat flux in a
given
circuit while maintaining a desired reliability level for the system because
it
eliminates or minimizes many of the measurement errors associated with using a
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separate sensor. The heating element temperature is the characteristic that
has the
strongest influence on heater reliability. Using a resistive element to
function as both
a heater and a sensor is disclosed in U.S. Patent No. 7,196,295, which is
commonly
assigned with the present application and the contents of which are
incorporated
herein by reference in its entirety.
[0044] Alternatively, the power conductors 56 may be made of dissimilar
metals such that the power conductors 56 of dissimilar metals may create a
thermocouple for measuring the temperature of the resistive heating elements.
For
example, at least one set of a power supply and a power return conductor may
include different materials such that a junction is formed between the
different
materials and a resistive heating element of a heater unit and is used to
determine
temperature of one or more zones. Use of "integrated" and "highly thermally
coupled" sensing, such as using different metals for the heater leads to
generation of
a thermocouple-like signal. The use of the integrated and coupled power
conductors
for temperature measurement is disclosed in U.S. Application No. 14/725,537,
which
is commonly assigned with the present application and the contents of which
are
incorporated herein by reference in its entirety.
[0045] The controller 15 for modulating the electrical power delivered to
each
zone may be a closed-loop automatic control system. The closed-loop automatic
control system 15 receives the temperature feedback from each zone and
automatically and dynamically controls the delivery of power to each zone,
thereby
automatically and dynamically controlling the power distribution and
temperature
along the length of each heater assembly 18 in the heater bundle 12 without
continuous or frequent human monitoring and adjustment.
[0046] The heater units 52 as disclosed herein may also be calibrated
using a
variety of methods including but not limited to energizing and sampling each
heater
unit 52 to calculate its resistance. The calculated resistance can then be
compared
to a calibrated resistance to determine a resistance ratio, or a value to then
determine actual heater unit temperatures. Exemplary methods are disclosed in
U.S. Patent Nos. 5,280,422 and 5,552,998, which are commonly assigned with the
present application and the contents of which are incorporated herein by
reference in
their entirety.
[0047] One form of calibration includes operating the heater system 10 in
at
least one mode of operation, controlling the heater system 10 to generate a
desired
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temperature for at least one of the independently controlled heating zones 62,
collecting and recording data for the at least one independently controlled
heating
zones 62 for the mode of operation, then accessing the recorded data to
determine
operating specifications for a heating system having a reduced number of
independently controlled heating zones, and then using the heating system with
the
reduced number of independently controlled heating zones. The data may
include,
by way of example, power levels and/or temperature information, among other
operational data from the heater system 10 having its data collected and
recorded.
[0048] In a
variation of the present disclosure, the heater system may include
a single heater assembly 18, rather than a plurality of heater assemblies in a
bundle
12. The single heater assembly 18 would comprise a plurality of heater units
52,
each heater unit 52 defining at least one independently controlled heating
zone.
Similarly, power conductors 56 are electrically connected to each of the
independently controlled heating zones 62 in each of the heater units 62, and
the
power supply device is configured to modulate power to each of the
independently
controlled heater zones 62 of the heater units through the power conductors
56.
[0049]
Referring to FIG. 7, a method 100 of controlling a heater system
includes providing a heater bundle comprising a plurality of heater assemblies
in
step 102. Each heater assembly includes a plurality of heater units. Each
heater
unit defines at least one independently controlled heating circuit (and
consequently
heating zone). The power to each of the heater units is supplied through power
conductors electrically connected to each of the independently controlled
heating
zones in each of the heater units in step 104. The temperature within each of
the
zones is detected in step 106. The temperature may be determined using a
change
in resistance of a resistive heating element of at least one of the heater
units. The
zone temperature may be initially determined by measuring the zone resistance
(or,
by measurement of circuit voltage, if appropriate materials are used).
[0050] The
temperature values may be digitalized. The signals may be
communicated to a microprocessor. The measured (detected) temperature values
may be compared to a target (desired) temperature for each zone in step 108.
The
power supplied to each of the heater units may be modulated based on the
measured temperature to achieve the target temperatures in step 110.
[0051]
Optionally, the method may further include using a scaling factor to
adjust the modulating power. The scaling factor may be a function of a heating
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capacity of each heating zone. The controller 15 may include an algorithm,
potentially including a scaling factor and/or a mathematical model of the
dynamic
behavior of the system (including knowledge of the update time of the system),
to
determine the amount of power to be provided (via duty cycle, phase angle
firing,
voltage modulation or similar techniques) to each zone until the next update.
The
desired power may be converted to a signal, which is sent to a switch or other
power
modulating device for controlling power output to the individual heating
zones.
[0052] In the
present form, when at least one heating zone is turned off due to
an anomalous condition, the remaining zones continue to provide a desired
wattage
without failure. Power is modulated to a functional heating zone to provide a
desired
wattage when an anomalous condition is detected in at least one heating zone.
When at least one heating zone is turned off based on the determined
temperature,
the remaining zones continue to provide a desired wattage. The power is
modulated
to each of the heating zones as a function of at least one of received
signals, a
model, and as a function of time.
[0053] For
safety or process control reasons, typical heaters are generally
operated to be below a maximum allowable temperature in order to prevent a
particular location of the heater from exceeding a given temperature due to
unwanted chemical or physical reactions at the particular location, such as
combustion/fire/oxidation, coking boiling etc.). Therefore, this is normally
accommodated by a conservative heater design (e.g., large heaters with low
power
density and much of their surface area loaded with a much lower heat flux than
might
otherwise be possible).
[0054]
However, with the heater bundle of the present disclosure, it is possible
to measure and limit the temperature of any location within the heater down to
a
resolution on the order of the size of the individual heating zones. A hot
spot large
enough to influence the temperature of an individual circuit can be detected.
[0055] Since
the temperature of the individual heating zones can be
automatically adjusted and consequently limited, the dynamic and automatic
limitation of temperature in each zone will maintain this zone and all other
zones to
be operating at an optimum power/heat flux level without fear of exceeding the
desired temperature limit in any zone. This brings an advantage in high-limit
temperature measurement accuracy over the current practice of clamping a
separate
thermocouple to the sheath of one of the elements in a bundle. The reduced
margin
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and the ability to modulate the power to individual zones can be selectively
applied
to the heating zones, selectively and individually, rather than applied to an
entire
heater assembly, thereby reducing the risk of exceeding a predetermined
temperature limit.
[0056] The
characteristics of the cartridge heater may vary with time. This
time varying characteristic would otherwise require that the cartridge heater
be
designed for a single selected (worse-case) flow regime and therefore that the
cartridge heater would operate at a sub-optimum state for other states of
flow.
[0057]
However, with dynamic control of the power distribution over the entire
bundle down to a resolution of the core size due to the multiple heating units
provided in the heater assembly, an optimized power distribution for various
states of
flow can be achieved, as opposed to only one power distribution corresponding
to
only one flow state in the typical cartridge heater. Therefore, the heater
bundle of the
present application allows for an increase in the total heat flux for all
other states of
flow.
[0058]
Further, variable power control can increase heater design flexibility.
The voltage can be de-coupled from resistance (to a great degree) in heater
design
and the heaters may be designed with the maximum wire diameter that can be
fitted
into the heater. It allows for increased capacity for power dissipation for a
given
heater size and level of reliability (or life of the heater) and allows for
the size of the
bundle to be decreased for a given overall power level. Power in this
arrangement
can be modulated by a variable duty cycle that is a part of the variable
wattage
controllers currently available or under development. The heater bundle can be
protected by a programmable (or pre-programmed if desired) limit to the duty
cycle
for a given zone to prevent "overloading" the heater bundle.
[0059] In
still another form of the present disclosure, a method and apparatus
to reduce current leakage is provided. One method of controlling a heating
system
comprises providing at least one heater assembly, the heater assembly
comprising a
plurality of heater units, each heater unit defining at least one
independently
controlled heating zone as set forth above. Power is supplied to each of the
heater
units through power conductors electrically connected to each of the
independently
controlled heating zones in each of the heater units, and the power supplied
is
modulated to each of the independently controlled heating zones. In order to
reduce
current leakage, a voltage from the power supply is selectively supplied to
each of
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the independently controlled heating zones such that a reduced number of
independently controlled heating zones receives the voltage at a time, or at
least a
portion (or a subset) of the independently controlled heating zones receive a
reduced
voltage at all times. In one example, the voltage may be selectively supplied
by a
variable transformer.
[0060] The
independently controlled zones can be switched in sequence thus
limiting the number of zones (and the cross-sectional area of electrical
insulation that
is exposed to electrical potential). By limiting the number of zones (and the
area)
subjected to the electrical potential at any given time to a fraction of the
total number
of zones, we can reduce the current leakage by a similar fraction. For
example, if
the zones in a heater bundle are divided into four groups (not necessarily
geometrically contiguous) and if each of these groups covered approximately
1/4 of
the total area of the heater, and further, if the switching scheme is
configured so that
no more than one of the four zones is powered on at any given instant in time,
then
the overall leakage current from the heater can be reduced by a factor of 4
(to 25%
of its original value).
[0061] In
order to accomplish the selective supply of voltage, in one
form a scaling factor is employed. The scaling factor may be employed
according to
the teachings of U.S. Patent No. 7,257,464, which is commonly assigned with
the
present application and the entire contents of which are incorporated herein
by
reference in their entirety. The scaling factor may be employed for at least
one of
adjusting the modulating power, determining a magnitude of the voltage to be
selectively supplied, and determining a duration for which the voltage is
selectively
supplied.
[0062]
Further, the scaling factor may be a function of operational
characteristics of the heating system. For example, the scaling factor can be
a
function of power dissipation capacity of at least one independently
controlled
heating zone, a maximum allowable temperature of at least one independently
controlled heating zone, an exposed heating area of at least one independently
controlled heating zone, a thermal behavior model of the heating system,
characteristics of an environmental system producing fluid flow being heated
by the
heater system, a fluid flow rate across the heater assembly, an area of at
least one
independently controlled heating zone, electrical insulation resistance of at
least one
independently controlled heating zone, an electrical current leakage of at
least one
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independently controlled heating zone, a circuit resistance of at least one
independently controlled heating zone, a zone circuit EMF of at least one
independently controlled heating zone, and a dielectric constant of at least
one
independently controlled heating zone, among others.
[0063] In
another form, the scaling factor is a power limiting function
that limits a value that is one of wattage, magnitude of voltage selectively
supplied,
and duration for which the voltage is selectively supplied provided to each
heating
zone to multiple values less than that produced at a full line voltage through
the use
of a scaling function, the scaling function being a ratio between a desired
value and
the value full line voltage, wherein a power controller provides a scaled
output by
multiplying the percentage output by the scaling function.
[0064] The
order and/or location of the independently controlled
heating zones to which the voltage is sequentially supplied may be any of a
variety
depending on application requirements. For example, voltage may be
sequentially
supplied around a periphery or around edges of a heater first before being
next
supplied to other geometric areas of independently controlled heating zones.
Further, the voltage may be sequentially supplied to different heating zones
based
on a change in resistance of each heating zone.
[0065] In
another form, at least one heating zone is turned off based on
an anomalous condition, while remaining zones continue to receive voltage
selectively.
[0066] In
still another form, a rate of succesively supplying the voltage
to each of the heating zones is adjusted based on at least one operational
characteristic of at least one heating zone. The operational characteristics
may be,
by way of example, resistance, temperature, and change in resistance over time
of
at least one heating zone, a fluid flow rate across the heater assembly, an
area of an
independently controlled heating zone, electrical insulation resistance of at
least one
independently controlled heating zone, an electrical current leakage of at
least one
independently controlled heating zone, a circuit resistance of at least one
independently controlled heating zone, a zone circuit EMF of at least one
independently controlled heating zone, a dielectric constant of at least one
independently controlled heating zone, and characteristics of an environmental
system producing fluid flow being heated by the heater system.
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[0067] The methods according to this form of the present disclosure that
reduces leakage current may also be applied to at least one heater assembly,
the
heater assembly comprising a plurality of heater units, each heater unit
defining at
least one independently controlled heating zone. The methods can be employed
with any of the embodiments of heaters and heater systems disclosed herein
while
remaining within the scope of the present disclosure.
[0068] It should be noted that the disclosure is not limited to the
embodiment
described and illustrated as examples. A large variety of modifications have
been
described and more are part of the knowledge of the person skilled in the art.
These
and further modifications as well as any replacement by technical equivalents
may
be added to the description and figures, without leaving the scope of the
protection
of the disclosure and of the present patent.
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