Note: Descriptions are shown in the official language in which they were submitted.
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DIGITAL HIGH TURNDOWN BURNER
FIELD OF THE INVENTION
The present invention relates to heating devices and, more particularly, to
gas fueled, indirect-fired burners.
BACKGROUND OF THE INVENTION
Gas burners, incorporated for example into indirect heating devices,
utilize the combustion of a gas or similar fuel (e.g., propane, natural gas,
or fuel
oil) for heating a work substance, oftentimes a flowable substance such as air
or
water. For example, heated air may be directed into the interior of a home for
general comfort heating purposes. In operation, natural gas or other fuel is
controllably forced through a nozzle or jet portion of the burner, where it is
intermixed (most typically) with air, forming a gas spray or aerosol for
enhancing combustion. In the case of an indirect heater, the gas spray is
ignited,
and the combustion product (heated air/plasma) is directed into a heat
exchanger, where the energy produced by the combustion process is transferred
to the work substance to be heated. The combustion exhaust is then moved to
an exhaust exit, possibly after one or more recirculation steps or the like to
further recapture heat from the combustion product.
For a =gas heating device, the amount of fuel burned per unit time (e.g.,
liters or btu per hour) is referred to as the firing rate. Simple heating
devices are
configured to run at a single firing rate, with the heater being cycled on and
off
in cases where it is desired to achieve an average output that is less than
the
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maximum possible output. If a heating device is capable of steady state
operation at two or more firing rates within acceptable combustion parameters
(e.g., combustion byproducts are kept to below a desired level, according to
ANSI safety and performance standards or the like), this is referred to in the
industry as "turndown." In other words, while keeping within acceptable
operational parameters, it is possible to "turn down" the heating device from
the
maximum possible firing rate to one or more lower firing rates. The ratio of
the
highest firing rate to the lowest firing rate in a heating device, at steady
state
operation and keeping within acceptable operational parameters, is referred to
as the "turndown ratio" of the heating device.
High turndown ratios are desirable for achieving greater levels of
efficiency in a heating device. For example, although it is possible to vary
the
average actual heat output of a single firing rate heating device by cycling
the
device between on and off operational modes, this can result in low levels of
combustion efficiency, higher levels of fuel use per unit heat output, and a
greater level of undesirable combustion byproducts. Among other reasons, this
is because the conditions in the combustion chamber vary widely over time as
the combustion process is turned on and off. When combustion is ongoing, the
gas spray produced by the burner is consistent, and the temperature in the
combustion chamber is high, factors that favor efficient operation. However,
when combustion is turned off or restarted, this results in temperature
variations in the combustion chamber, and variances in the quality of the gas
spray input, factors that inhibit efficient operation.
High turndown burners exist in the industry, typically for use in process
heating, that is, for heating a work substance for an industrial or
manufacturing
process. Current technology tends to use techniques such as pre-mixing of the
combustion air and gas mixture to assure a proper air/fuel ratio prior to
ignition, ceramic liners or "targets" that retain heat in the combustion
chambers
to assure ignition at low gas flow rates, multiple individual burners that are
"staged" using electromechanical or electronic controls, rudimentary
mechanical
linkages between the gas valve and a damper on the combustion air blower, or
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simple electronic controls that modulate the gas valve and blower together but
are auxiliary or add-on systems to the basic HVAC unit controls. The published
range of operation of any of these systems tends to peak at a turndown ratio
of
20:1 for a single burner, again, referring to the ratio of highest firing rate
to
lowest firing rate. (Higher turndown ratios than this may be achieved in a
heating device by using a multiple burner approach, where the turndown ratio
is directly related to the number of burners. However, such devices are not
directly relevant to the present case, since each individual burner has a low
or
unitary turndown ratio.)
One of the limiting factors in achieving higher turndown ratios is the loss
of control of the air/fuel mixture at low flow rates. Failing to achieve the
theoretically ideal fuel/ air mixture can result in emissions of carbon
monoxide,
aliphatic aldehydes, nitrous oxides, and other contaminants that are judged to
be
harmful. Producing those contaminants will cause a burner design to fail ANSI
(American National Standards Institute) safety and performance standard tests.
A second limitation of these designs is the inability to consistently ignite
or
maintain combustion of the air/fuel mixture at very low flow rates. The
ability
to do so is also part of the ANSI safety and performance standards.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a burner system having
a very high turndown ratio, typically 30:1, 60:1, or 90:1 that meets all
applicable
safety and performance standards across an entire range of firing rates.
It is another object of the present invention to provide a gas burner
system having a very high turndown ratio, typically of 90:1 or greater, that
meets both ANSI safety and performance standards (or similar standards) across
an entire range of firing rates.
To achieve this and other objects, an embodiment of the present invention
relates to a gas burner system for a heating device, e.g., an air heater. The
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system includes a burner, a valve assembly for controlling a flow of fuel to
the
burner, a blower assembly for directing air to the burner, and a control
system.
The burner includes one or more burner plates for facilitating the mixture of
air
and fuel to be combusted in a combustion chamber portion of the heating
device. The control system is configured to independently control the valve
assembly and the blower assembly according to a control profile, for
generating
a plurality of air/fuel mixtures each for operation of the burner at a
different
firing rate, with a turndown ratio of 30:1, 60:1, or 90:1 and higher.
In another embodiment, the control profile in effect maps an optimal
range of operation of the gas valve assembly to an optimal range of operation
of
the blower assembly. The control profile is generated by testing the gas
burner
system across the operational ranges (or portion thereof) of both the valve
assembly and the blower assembly, which enables data to be captured for any
non-linear operational modes. Thus, the burner system is assured of operating
at the proper air/fuel mixture over a very wide range of firing rates, i.e.,
for each
air/fuel mixture, the burner system/heating device operates within ANSI safety
and performance standards or similar official standards in countries other
than
the United States.
In another embodiment, the control system receives a control signal that
indicates a desired or designated firing rate. For example, the control signal
might be generated by an HVAC system, based on a user control input of a
desired heat output. The control system cross-references the control signal to
the control profile, for determining a first operational signal to apply to
the valve
assembly, and another, second operational signal to apply to the blower
assembly. Application of the respective operational signals to the valve
assembly and the blower assembly results in the generation of the proper
air/fuel mixture for the firing rate designated by way of the control signal.
In another embodiment, the burner includes two burner or aeration
plates. The burner plates are arranged in a generally V-shaped configuration,
and each has a number of circular aeration apertures for facilitating the
mixture
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of air and fuel. the apertures are sized so as to help with achieving the
proper
air/fuel mix across a wide range of firing rates.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be better understood from reading the
. following description of non-limiting embodiments, with reference to the
attached drawings, wherein below:
FIG. 1 is a perspective view of a burner system according to an
embodiment of the present invention;
FIG. 2 is a top plan view of the burner system;
1 5 FIG. 3 is a side elevation view of the burner system;
FIG. 4 is a front elevation view of the burner system;
FIG. 5 is a cross-section view of the burner system, taken generally along
line 5-5 in FIG. 1;
FIG. 6 is a perspective view of a burner plate portion of the burner
system;
FIG. 7 is a schematic view of the burner system;
FIGS. 8 and 9 are schematic views of a control profile; and
FIGS. 10A-10C are various schematic views illustrating one method for
determining the control profile.
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DETAILED DESCRIPTION
With reference to FIGS. 1-10C, an embodiment of the present invention
relates to a digital high-turndown burner system 20, for use as part of an
indirect-fired heating device 22, e.g., for comfort or make-up air heating.
The
system 20 includes a burner unit 24 attached to and disposed within a housing
26, a gas valve assembly 28 for controlling a flow of natural gas or other
fuel 30
to the burner unit 24, and a blower assembly 32 attached to the housing 26 for
directing air 34 to the burner unit 24. The high-turndown burner system 20
also
includes a control system 36, which independently controls the gas valve
assembly 28 and blower assembly 32 according to a control profile 38. The
control profile 38 maps an operational range 40 of the gas valve assembly to
an
operational range 42 of the blower assembly, for generating a plurality of
air/fuel mixtures 44 each for operation of the burner at a different firing
rate.
The control profile 38 is customized according to the particular physical
characteristics of the burner system 20, such that: (i) the ratio of highest
firing
rate of the burner system 20 to the lowest firing rate (i.e., the turndown
ratio) is
30:1, 60:1 or 90:1 and greater; and (ii) the burner system 20 meets ANSI
safety
and performance standards across the entire turndown ratio ranges. In
particular, for each air/fuel mixture generated according to the control
profile 38
and combusted in the heating device 22, the air/fuel mixture is consistently
ignited, combustion is consistently maintained, and harmful combustion
exhaust byproducts are kept below designated limits.
With reference to FIG. 7 initially, and as mentioned above, the burner
system 20 will typically be used as part of an indirect-fired heating device
22.
Most such heating devices include a heat exchange unit 48, a combustion
chamber 46 inside or downstream from the heat exchange unit 48, and an
exhaust system 50. In operation, an air/fuel mixture from a burner system
(such
as the system 20 described herein) is ignited in or just before entering the
combustion chamber 46. The combustion product is directed to the combustion
chamber 46, where it heats a working substance outside the heat exchange unit.
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For example, the working substance might be ambient air, kept separate from
the combustion air, which is heated and directed to a particular location for
comfort purposes. The combustion exhaust is then vented through the exhaust
system 50 (Which may simply be a vent or passage) to ambient.
The housing 26 is a generally rectangular enclosure, made of sheet metal
or the like, which is connected directly to the end of the combustion chamber
46.
The blower assembly 32 indudes a blower fan 52, an AC or other motor 54, and
a blower motor controller 56. An output shaft of the motor 54 rotatably drives
an impeller portion of the fan 52, which is located inside a fan housing 58
connected to the burner housing 26, for moving air from outside the fan
housing
58 (and outside the burner housing) to inside the burner housing 26. The
blower
motor controller 56 has a control input for receiving a blower operational
signal
60 from the control system 36, e.g., a 0-10 VDC signal, and a power output
terminal electrically connected to the blower motor 54. The controller 56
outputs
a PWM (pulse width modulation) power signal to the motor 54, for controlling
the speed of the motor, based on the blower operational signal 60 as received
from the control system. For example, it may be a linear relationship, such
that a
0 V operational signal 60 from the control system 36 corresponds to a "motor
off" condition, and a 10 V operational signal 60 from the control system 36
corresponds to a maximum speed of the motor. Typically, the power output
terminal of the blower motor controller 56 will include from 2 to 3 electrical
outputs, each of which is attached to one of the electrical terminals of the
motor
by an electrical line or cable. The PWM power signal applied to the motor by
the blower motor controller 56 includes one or more separate electrical power
signals applied to each of these cables/lines, according to a standard power
waveform for powering the type of motor 54 in question. Thus, in operation,
the
blower motor controller 56 converts the 0-10 VDC blower operational signal 60
into an appropriate proportional power output signal for power the fan motor
54.
The gas valve assembly 28 includes an AC gas valve actuator 62 and a
ball-type valve 64, which is interfaced with a gas supply line 66. The gas
supply
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line 66 runs from a gas main (or other gas source), through the housing 26,
and
into the burner unit 24, as discussed in more detail below. The ball-type
valve
64 is operably disposed in the path of the supply line 66, and the gas valve
actuator 62 positions the ball-type valve 64 to control the flow rate of gas
30
through the supply line 66 and into the burner unit 24. The gas valve actuator
62 receives a valve operational signal 68 from the control system 36, which
governs the position of the ball-type valve 64 and therefore the flow rate of
gas
into the burner. Like the blower operational signal 60, the valve operational
signal 68 may be a 0-10 VDC signal, with the gas valve actuator 62 controlling
the valve 64 proportional to the level of the received signal 68. For example,
if
the valve actuator 62 receives a 0 VDC signal 68 from the control system 36,
the
gas valve actuator 62 closes the valve 64 (or maintains the valve in a closed
state), and if the valve actuator 62 receives a 10 VDC signal 68 from the
control
system 36, the gas valve actuator 62 opens the valve 64 to a fully open state.
The
gas supply line and/or gas valve assembly may be outfitted with other standard
components for safety or operational purposes, such as a gas pressure
regulator
(not shown).
Turning in particular to FIGS. 4-6, the burner unit 24 is a cast "line
burner" that includes upper and lower stainless steel aeration plates 70a,
70b,
two side plates 72a, 72b, and a gas inlet manifold 74. The manifold 74 has a
generally tubular main body 76, each end of which is outfitted with a mounting
flange 78. At least one end of the main body 76 is open, for providing an
inlet
aperture 80 for the passage of natural gas 30 or other fuel. A longitudinal
slot 82
extends partway down one side of the main body 76, for providing a
passageway that extends from the interior of the body 76 through to the space
or
area located between the aeration plates 70a, 70b. The upper aeration plate
70a,
70b includes a first planar portion 84 attached to the manifold main body 76
just
above the longitudinal slot 82. A second planar portion 86, either angled
slightly
with respect to the first planar portion 84 or coplanar therewith, is
integrally
attached to the trailing edge of the first planar portion 84, and extends
towards
the outlet end of the burner system, in the direction of the combustion
chamber
46. A third planar portion 88 is integrally attached to the trailing edge of
the
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second planar portion 86, but is angled significantly upwards, e.g., 30-500
with
respect to the second planar portion 86, so as to meet an upper inner surface
of
an outlet chute portion 90 of the housing 26. The lower aeration plate 70b is
configured symmetrically to the upper aeration plate 70a, but is angled
downwards for extending from just below the longitudinal slot 82 to a lower
inner surface of the outlet chute 90. As indicated in the figures, the upper
and
lower aeration plates 70a, 70b are generally oriented relative to one another
to
form a "V"-like shape. Additionally, each aeration plate 70a, 70b is provided
with a plurality of circular aeration holes or apertures 92, 94. Each of a
first set
of the aeration apertures 92 is located either in the trailing edge area of
the
second planar portion 86 of the aeration plate, or in the third planar portion
88.
Each of the second set of the aeration apertures 94 is located generally in
the
leading area of the second planar portion 86 of the aeration plate, or in the
first
planar portion 84. In each aeration plate 70a, 70b, there is an area of the
aeration
plate, between the trailing line of small-diameter aeration apertures 94 and
the
leading line of large-diameter aeration apertures 92, in which the aeration
plate
has no apertures.
The aeration plates 70a, 70b are sized according to the desired heat
capacity and/or gas input range of the burner system. For example, a 200-400
MBH burner system might utilize 6-inch (width) aeration plates, and a 600 MBH
burner system might utilize 12-inch aeration plates. ("MBH" refers to
thousands
of BTU's per hour, e.g., 1 MBH = 1 MBTU/hour, where "MBTU" is a standard
abbreviation for 1000 BTU's.)
The burner unit side plates 72a, 72b are attached to and extend between
the side edges of the aeration plates 70a, 70b and the main body 76 of the
manifold 74. Thereby, the side plates 72a, 72b enclose the sides of burner
unit,
for facilitating the passage of air from the interior of the housing 26 (e.g.,
air
blown therein by the blower assembly 32) through the aeration apertures 92, 94
of the aeration plates 70a, 70b. The entire burner unit 24 is disposed in the
housing 26 as best shown in FIG. 5. As indicated, the manifold 74 is connected
to and extends between the two side walls of the housing 26. The aeration
plates
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70a, 70b and side plates 72a, 72b extend from the manifold 74 to the inner
surface area of the housing chute 90. The trailing edges of all four of the
aeration plates 70a, 70b and side plates 72a, 72b are attached to the inner
surface
of the chute 90, such that the only fluidic passageway (e.g., passage for air)
from
the interior of the housing 26 to the exit end of the chute 90 (e.g., the end
of the
chute that lies in the combustion chamber) is through the aeration apertures
92,
94 of the aeration plates 70a, 70b. The mounting flange 78 of the inlet
aperture
end 80 of the manifold 74 is attached to the gas supply line 66 by way of one
or
more adapters, gaskets, or the like 96 that create a gas-tight connection
between
the supply line 66 and inlet aperture 80, i.e., the purpose of the gaskets 96
is to
prevent gas from leaking into the housing interior.
As should be appreciated, the chute 90 is a rectangular extension of the
housing 26, made of sheet metal or otherwise, which provides an exit from the
housing and burner unit for passage of the combustion product into the
combustion chamber 48. Because the chute 90 "sticks out" from the housing
proper, it also acts to project the exit or trailing end of the burner unit,
defined
by the trailing ends of the aeration plates and side plates attached to the
inner
surface of the chute, further into the combustion chamber 46 than if the
burner
terminated coextensively with the housing side walls.
In general operation (without yet referring to the control profile 38, which
is discussed in more detail below), the control system 36 receives a control
signal
98 from an HVAC controller 100 or otherwise. The control signal 98 contains
information relating to a desired heat output level of the heating device 22.
For
example, the control signal 98 may be a DC voltage signal having a range from
"Vmin" to "Vmax," with the intended or designated heat output of the heating
device being linearly proportional to the DC voltage level of the signal 98.
Thus,
"Vmin" might indicate a minimum heating output level, or that the heating
device remain or enter a "turned off" state, whereas "Vmax" might indicate a
maximum heating output level of the heating device. Based on the control
signal 98, the control system 36 outputs a blower operational signal 60 and a
valve operational signal 68, for independent control of the blower assembly 32
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and the gas valve assembly 28, respectively, so as to produce the desired heat
output of the heating device 22.
Based on the blower operational signal 60 received from the control
system 36, the blower assembly motor controller 56 powers the motor 54 for
operation of the blower fan 52. The fan 52 draws in air 34 from an ambient
external source, and blows it into the interior of the housing 26. Because the
aeration apertures 92, 94 of the aeration plates 70a, 70b represent the only
egress
for air 34 in the housing (considering the positive pressure generated by the
fan
output), the air is forced through the aeration apertures 92, 94 and into the
space
between the two aeration plates 70a, 70b. Concurrently, based on the valve
operational signal 68 received from the control system 36, the gas valve
actuator
62 operates the ball-type valve 64 for allowing natural gas or other fuel 30
to
flow through the supply line 66 at a particular rate. The gas 30 passes into
the
interior of the burner manifold 74, where it is directed through the
longitudinal
slot 82 for passage into the space located between the V-oriented aeration
plates
70a, 70b. The air 34 mixes with the gas 30 to form an air/fuel mixture 44, and
is
ignited by a standard ignition system 102. The ignition system may include,
for
example, a spark igniter, a flame rod, and/or the like. The ignited air/fuel
mixture 44 then passes out the exit end of the burner system, out of the chute
90,
and into the combustion chamber 46 for transferring heat energy to the working
substance in the heat exchanger. The aeration plates 70a, 70b generally
facilitate
the mixing of air and fuel in the burner, before, during, and after ignition,
and it
has been found that the particular aeration aperture size and pattern
discussed
above enhances this mixing effect across a very wide range of air and fuel
flow
rates.
As noted above, the control profile 38 in effect maps the operational
ranges of the gas valve assembly 28 and the blower assembly 32 to one another,
for operation of the burner system at the proper air/fuel mixture over a very
wide range of flow rates. As has been shown in many cases of prior
development of burner technology, maintaining a proper air/fuel mixture is not
a linear relationship. This is due to the non-linear nature of fluid flows
through
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valves and fans. By "mapping" the characteristics of both the gas valve
assembly and the blower assembly in effect independently, and then embedding
those characteristics in the control system, the burner is assured of
operating at
the proper air/fuel mixture over a very wide range of flows. The control
system
uses two independent output channels 60, 68 to achieve this result.
As implemented, the control profile 38 can take several forms. In a first,
with reference to FIG. 8, the control profile is a lookup table mapping the
range
of HVAC control signals 98 to a range of gas valve assembly operational
signals
68 and to a range of blower assembly operational signals 60. Thus, for each of
a
plurality of discreet values of the control signal, from "Vmin" to "Vmax"
divided into, e.g., 0.1 V intervals, there is a corresponding pair 104 of
signals that
include a valve operational signal 68 and a blower assembly operational signal
60. For example, as shown in FIG. 8, the maximum control signal Vmax
correlates to a valve operational signal "m" VDC and a blower operational
signal "x" VDC, where m and x are within the range of operational signals for
the gas valve assembly and blower assembly, respectively. Similarly: (i) the
minimum control signal Vmin correlates to another valve operational signal "n"
and another blower operational signal "y"; and (ii) each intermediate control
signal, e.g., a control signal "A" volts, where Vmin < A < Vmax, correlates to
an
intermediate valve operational signal "B" and a blower operational signal "C,"
where n (and B) and y (and C) are within the range of operational signals for
the
gas valve assembly and blower assembly, respectively. The valve operational
signals m and n may correspond to the maximum and minimum operational
signals, respectively, but this is not necessarily the case. The same is true
for the
blower assembly operational signals.
As shown in FIG. 8, each pair 104 of gas valve and blower assembly
operational signals results in a particular air/fuel mixture 44 for a
designated
firing rate, e.g., valve signal B and blower signal C result in an air/fuel
mixture
"d." (Generally speaking, the firing rate is proportional to the received
control
signal 98, so that the higher the control signal, the greater the rate of gas
or other
fuel burned in the burner system and the greater the heat output of the
heating
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device.) In operation, the control system 36 receives a control signal 98 from
the
HVAC controller 100 or otherwise. The control system 36 cross-references the
received control signal 98 to the control profile 38, which results in a pair
of
operational signals 60, 68, one for the gas valve assembly and one for the
blower
assembly, that correlate to the received control signal. The operational
signals
are then applied to the gas valve assembly and blower assembly, which output
gas and air into the burner unit 24. The burner unit 24 mixes the gas and air,
thereby producing an air/fuel mixture 44, which is ignited and combusted in
the
combustion chamber 46 for heat exchange. The control profile 38 is configured
for producing a high turndown ratio for the single-burner unit 24, e.g., the
ratio
of the highest firing rate (typically corresponding to Vmax and air/fuel
mixture
"e" in FIG. 8) to the lowest firing rate (typically corresponding to Vmin and
air/fuel mixture "f" in FIG. 8) is 30:1, 60:1 or even 90:1 and greater.
Additionally, the control profile 38 is configured so that across the entire
turndown range, for each air/fuel mixture generated according to the control
profile 38 and combusted in the heating device 22, the burner unit and heating
device meet ANSI safety and performance standards, namely, the air/fuel
mixture is consistently ignited, combustion is consistently maintained, and
harmful combustion exhaust byproducts are kept below designated limits.
FIG. 9 shows a sample graph of the range of blower assembly and valve
assembly operational signals, such as those that would be found in a lookup
table as in FIG. 8. As indicated, it will typically be the case that the
relationship
between the blower assembly operational signals and the valve assembly
operational signals is non-linear, which enables the system to meet ANSI
safety
and performance standards across all the firing rates of a very high turndown
ratio.
Whereas, FIG. 8 shows a control profile that maps each value in a range of
control signals 98 to a pair of blower assembly and valve assembly operational
signals, the control profile may be arranged in other manners. For example,
each value in the range of control signals may be mapped to a particular gas
valve assembly flow rate and a particular blower assembly flow rate. In turn,
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the range of possible flow rates for the gas valve assembly, and the range of
possible flow rates for the blower assembly, are each mapped to a set or range
of
operational signals. Thus, in operation: (i) based on a received control
signal,
the control system correlates the control signal to a gas valve assembly flow
rate
and to a blower assembly flow rate; (ii) based on the gas valve assembly flow
rate, the control system determines a gas valve assembly operational signal
for
achieving that flow rate, based on a graph, lookup table, algorithm
calculation,
etc.; (iii) based on the blower assembly flow rate, the control system
determines
a blower assembly operational signal for achieving that flow rate, based on a
graph, lookup table, algorithm calculation, etc.; and (iv) the blower assembly
operational signal and gas valve operational signal are applied to the blower
assembly and gas valve assembly, respectively. As should be appreciated by
way of this example, the term "control profile" as used herein therefore
includes
any mapping or other relationship or group of relationships, such as graphs,
lookup tables, databases, algorithms, or the like, for determining a blower
assembly operational signal and gas valve operational signal for each of a
plurality of possible control signal values, again, for operation of the
burner
unit/ heating device according to ANSI safety and performance standards across
a very high turndown ratio, e.g., 30:1, 60:1 or even 90:1 and greater.
In a preferred embodiment, the gas valve operational signal is the
primary control signal and the blower operational signal is secondary. That
is,
the control system employs the gas valve operational signal before the blower
operational signal when generating air/fuel mixtures. It will be appreciated,
however, that either the gas or blower operational signals may be employed as
the primary signal without departing form the scope of the invention. It will
also be appreciated that the sequence of the claimed components, in
particular,
the blower and valve assemblies, may be varied without departing from the
present invention.
Moreover, while the present invention facilitates very high turndown
ratios of 30:1, 60:1, or 90:1 and greater, other intermediate ratios, e.g.,
40:1, 50:1
or 70:1, are possible. As stated, the inventive system allows for very high
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turndown ratios while meeting or exceeding both ANSI safety and performance
standards across the entire ratio.
As mentioned above, because blower assemblies, gas valve assemblies,
burner units, combustion chambers, etc. all involve the control, flow, and
mixing of one or more fluids (e.g., gas/fuel or air) in a confined space with
a
varying geometry (e.g., flow through a valve opening), heating devices of the
type disclosed herein are non-linear systems. Thus, the combustion output of
the heating device is not a linear function of the control inputs, across the
operational range of the heating device. In certain heating devices, non-
linear
effects are minimized by having a small turndown ratio. For achieving a high-
turndown ratio, however, the burner system of the present invention takes into
account non-linear system effects. Thus, despite the fact that very low flow
rates
may be especially non-linear, the system of the present invention is able to
achieve such low flow rates, and thereby achieve a very high turndown ratio
(again, while meeting ANSI standards across the entire range of operation).
Because non-linear flow characteristics are dependent on the particular
geometry and configuration of a burner system, the control profile 38 is
tailored
for individual use with the burner system and heating device. (In other
words, for different burner systems, each system has its own customized
control
profile 38.) To prepare a control profile 38 for a particular burner system or
heating device, a prototype (i.e., physical implementation) of the heating
device
is first constructed, according to a desired heat capacity, size, operational
characteristics, and the like. Then, the prototype heating device is tested in
operation, across a designated, wide range of fuel and air flow rates, all the
while collecting data points relating to the combustion product or output of
the
heating device, in terms of heat output, combustion exhaust, and other
combustion performance characteristics. If the measured data points for a
particular fuel and air flow rate fall outside a desired range (e.g., ANSI
safety
and performance standards), that fuel and air flow rate is not used as part of
the
control profile. Instead, the control profile is an amalgam/grouping of those
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air/fuel data points that exhibit the best operational characteristics for
providing
the designated range of heat output.
The process is explained in more detail in FIGS. 10A-10C. As indicated at
Step 200 in FIG. 10A, and as mentioned above, for generating a control profile
38, a prototype heating device is first constructed. At Step 202, the heating
device is interfaced with a standard testing and data acquisition system. The
testing and data acquisition system includes a plurality of sensors disposed
in
the heating device at different locations of interest, and a computer system
(with
appropriate testing and data logging software) operably connected to the
sensors through a sensor interface device or the like. The sensors are
configured
to measure or sense various operational characteristics of the heating device
while in use, such as: (i) levels of CO (carbon monoxide) exiting the exhaust,
in
PPM or otherwise; (ii) actual gas/fuel input into the burner unit; (iii)
actual air
input; (iv) levels of NO (nitric oxide) and NOx (other mono-nitrogen oxides)
in the exhaust; (v) oxygen levels; (vi) heat output; and the like. Also
measured
is the operational signal 68 applied to the gas valve assembly, and the
operational signal 60 applied to the blower assembly. The computer system
monitors sensor output, records data values received from the sensors, and
organizes the data for further use. Suitable testing and data acquisition
systems
are available from National Instruments.
Once the prototype heating device is interfaced with the testing and data
acquisition system, it would be possible to commence testing by running the
heating device at every possible iteration and combination of possible
operational signals 60, 68. This could be done by dividing the gas valve
operational signal into a plurality of testing points, such as 100 or 1000
divisions
between 0-10 VDC. Each test signal would then be sequentially applied to the
gas valve assembly, e.g., 0 V, 0.1 V, 0.2 V, and so on. For each of these
input
signals, the blower assembly would be run across its entire operational signal
range, again, according to a designated level of granularity, such as 100 or
1000
divisions between 0-10 VDC. Thus, for a 0.1 V gas valve operational signal,
the
blower assembly would be sequentially run according to a 0 V, 0.1 V, 0.2 V,
0.3 V
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signal and so on. At each iteration, the testing and data acquisition system
would record data received from the sensors.
Because testing the prototype heating device in this manner would
generate a very large set of data, e.g., 1,000 points of iteration for two 0-
10 VDC
signals each divided into 100 increments, most of which would not be useful,
testing may be targeted or focused by first preparing a theoretical or
projected
characterization curve of burner unit performance, as in Step 204 in FIG. 10A.
Here, the theoretical characterization curve is estimated as being linear,
with
data points being assigned to the curve based on the heat capacity of the
burner
unit/heating device and theoretical relationships between desired heat output,
the fuel rate needed to achieve such a heat output, and the air rate needed
for
complete combustion at the fuel rate. Thus, for example, with reference to
FIG.
10B, a theoretical or projected characterization curve might be populated by
first
assuming the desired heat output is linearly related to the control signal 98
and
to the gas valve operational signal 68, i.e., a heat output of zero
corresponds to a
control signal of 0 VDC and a valve signal of 0 VDC (point "A" in FIG. 10B),
and
a max heat output corresponds to a control signal of 10 VDC and a valve signal
of 10 VDC (point "B" in FIG. 10B), representing the maximum gas flow rate. The
blower operational signal data is then determined by assuming a linear
relationship between air flow rate and blower operational signal, and
calculating
the theoretical air flow rate needed for combustion at one or more of the gas
flow rates. For example: (i) it could be assumed that the blower should be in
an
"off" state when the gas flow rate is zero (data point 1min, AI in FIG. 10B);
(ii)
calculate the theoretical air flow rate needed for the maximum gas flow rate
at
point "B"; (iii) approximately determine the blower operational signal needed
to
achieve this air flow rate, resulting in data point {D, B} in FIG. 10B; and
(iv) map
the two points as a linear function.
As mentioned, the purpose of a theoretical or projected characterization
curve as in FIG. 10B is merely to approximate the operational range of the
heating device or burner unit, for narrowing the range of data points during
actual testing. (In other words, this method eliminates data outliers, such as
the
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case of the gas flow rate being at a maximum when the blower is turned off,
which is highly unlikely to result in an ideal operational state of the burner
unit.)
Thus, for testing the unit as at Step 206 in FIG. 10A, and with reference to
FIG.
10C, the range of gas valve operational signals 68 is again divided into a
plurality of testing points, according to a desired level of granularity,
e.g., 0.1
VDC signal increments, for a total of 100 testing points across a 0-10 VDC
signal
range. At each testing point of the gas valve assembly, e.g., point "Y" in
FIG.
10C (which is approximately a 3.7 VDC operational signal for 0-10 VDC range),
the projected curve is accessed for determining a projected blower assembly
operational signal 110. A window is established around this value, say from
"+X" to "-X," and the window is divided into a desired number of test points.
For example, "X" could be 2 volts, with the window around the "central"
operational signal 110 thereby being 4 volts total. While the gas valve
operational signal 68 is maintained at "Y" volts, each testing signal in the
window is sequentially applied to the blower assembly 32. (Therefore, if the
central operational signal 110 at point "Y" were "Z" volts, then the
operational
signal 60 applied to the blower assembly would range from Z+X volts to Z-X
volts, in 0.1 volt or other increments.) At each testing signal, the testing
and data
acquisition system collects data 112 relating to burner operation, as
described
above.
Once the data 112 is collected for all testing points of the gas valve
assembly and blower assembly, the data is analyzed as at Step 208 in FIG. 10A,
for generating a control profile as at Step 210. This may be done using data
analysis software (part of the testing and data acquisition system or
otherwise),
which determines, for each gas valve assembly testing point, the tested blower
signal that provides the best performance characteristics while staying within
ANSI safety and performance standards for CO output and the like. Other
optimization techniques may be employed, using standard software-based or
other methods, for taking into account all the variables present in the
system.
Once it is determined which operational signal pairs (e.g., blower operational
signal and valve operational signal) provide the best level of performance at
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each level of control signal/heating output, these are used to populate the
control profile.
The "window" testing approach discussed above might fall outside the
optimum performance level for a given testing point. If so, the window may be
adjusted until an optimum point is reached. Additionally, it will typically be
the
case that several iterations of the process is carried out for achieving the
most
fine-tuned control profile, e.g., the control profile generated in the first
run-
through (based on a theoretical characterization curve) is used as the
theoretical
1 0 characterization curve in the next run-through, and so on.
As should be appreciated, because the control profile 38 is generated
based on actual testing data acquired across a wide operational range of the
heating device, with independent control of the blower assembly and gas valve
assembly, non-linear system effects are fully accounted for, i.e., even in
instances
where the system acts particularly non-linear, such instances are identified
and
compensated for through the testing data. This also enables operation of the
system at a high turndown ratio. For example, in the case where a linear
control
profile might result in unsafe performance at a very low firing rate, vis-à-
vis a
high firing rate along the same linear profile, this is avoided by using a
control
profile according to the present invention, which can be non-linear.
The control system 36 is typically implemented as part of, and/ or
interfaced directly with, the HVAC controller 100.
Since certain changes may be made in the above-described digital high
turndown burner, it is intended that all of the subject matter of the above
description or shown in the accompanying drawings shall be interpreted merely
as
examples illustrating the inventive concept herein and shall not be construed
as
limiting the invention. The scope of the claims should not be limited by the
preferred embodiment set forth in the examples, but should be given the
broadest
interpretation consistent with the description set forth in the examples.
