United States Patent
`
`[191
`
`[11] Patent Number:
`
`4,645,450
`
`West
`
`[45] Date of Patent:
`
`Feb. 24, 1987
`
`[54] SYSTEM AND PROCESS FOR
`CONTROLLING THE FLOW OF AIR AND
`
`3/1984 Berkhof .
`4,436,506
`4,498,363 2/ 1985 Hanson et a1.
`
`............... ..... 431/90 X
`
`FUEL To A BURNER
`
`FOREIGN PATENT DOCUMENTS
`
`John S. West, Camp Hill, Pa.
`Inventor:
`[75]
`73 A'
`: C
`1T ht
`'
`I
`.
`]
`smgnee
`ec ronlcs, nc,
`[
`ontro
`Harr’Sburg’ 133'
`[21] Appl. No.: 645,337
`
`_
`
`50840
`88717
`54_129531
`848894
`909448
`
`5/1982 European Pat. Off,
`9/1983 European Pat. Off.
`8/1979 Japan.
`7/1981 U.S.S.R.
`2/1982 U.S.S.R.
`
`.
`.
`
`_
`.
`
`Aug. 29, 1984 _
`
`[56]
`
`[22] Filed:
`F23N 1/02
`[51]
`Int C14
`[52] us. Cl. ........................................ 431/12; 431/90;
`431/89
`.
`[58] Fleld of Search ....................... 431/12, 18, 76, 80,
`431/90’ 20’ 89; 236/15 BB, 15 BD! 15 E -
`References Cited
`U'S' PATENT DOCUMENTS
`2,963,082 12/1960 Binford et a1.
`.
`3,070,149 12/ 1962
`Irwin .
`341641201
`1/1965 IFWIQ -
`21269’448
`8/193? 3‘42”!“ '
`3,6321%; 32372 1:123:0th"""""""""""""
`’
`’
`.
`‘
`‘
`3,968,489 7/1976 Richards et a1.
`.
`4,067,684
`1/1978 McInerney ....................... 431/12 x
`4,097,218
`1/1978 Womack '
`4,252,300
`2/1981 Herder .
`............................. 431/76
`4,260,363 4/ 1981 Cratin, Jr.
`4,262,843 4/ 1981 Omori et a1.
`.
`442649297
`4/1981 Van Berkum -
`4,330,261
`5/1982 Surll
`................................... 431/76 X
`1,3223; 323%: Egg: ‘
`4’375’950
`3/1983 Dur1eny'H
`4,406,611
`9/1983 Michel
`.
`4,411,385 10/1983 Lamkewitz .
`4,421,473 12/1983 Londerville .
`
`Primary Examiner—Randall L. Green
`Attorney, Agent, or Firm—Sixbey, Friedman & Leedom
`[57]
`ABSTRACT
`'
`A flow controller s
`i
`ystem for optlmally controlllng the
`_
`flOW Of air and fuel to a burner in a plurality Of operat-
`ing modes throughout the firing range of the burner is
`dislclosed herein. The system ingludes a pfiir of diffegen-
`t1a pressure sensors connecte
`across t e air con uit
`and the burner, and the fuel conduit and the burner, as
`well as a pair of electrically operated air and fuel valves
`for controlling the pressure of the air and fuel destined
`for the burner. The system further includes a micro-
`processor control means electrically connected to both
`the pressure sensors and the air and fuel pressure regu-
`lating valves. Optimal air-to-fuel pressure ratios are
`.
`.
`.
`.
`.
`empmcally deflved at 63011 1301m along the mug range
`of the burner by means of detachably connectable flow-
`meters, oxygen sensors and thermocouples, and this
`information is stored within the memory of the micro-
`processor control means. The use of a microprocessor
`control means, in combination with a detachably con-
`nectable flowmeter and thermocouple, allows the Sys_
`tern to be easily retrofitted onto an existing burner sys-
`tem without the need for installation of orifice plate-
`tYPe flowmaers'
`'
`
`',
`19 Claims, 4 Drawing Figures
`
`431 76 X
`/
`
`.
`
`
`
`— — -— —
`I
`
`L ______ E
`
`1________ COMBUSTION
`
`fig
`INTERFACE
`
`CONTROLLER
`
`
`—
`
`-
`1n
`
`PROCESS
`CONTROLLER
`
`HONEYWELL - EXHIBIT 1008
`
`HONEYWELL - EXHIBIT 1008
`
`

`

`US. Patent
`
`Feb. 24, 1987
`
`Sheet 1 of 3
`
`4,645,450
`
`
`
`MN
`
`
`
`29.539200
`
`munjomhzoo
`
`wo<umwkz_
`
`

`

`US. Patent
`
`Feb. '24, 1987
`
`Sheet 2 of 3
`
`4,645,450
`
`N6t
`
`mmqqomhzoo
`
`mmmuoma
`
`m?—
`
`
`
`ZOFmDmEOQ
`
`wo<mmwhz_
`
`mMJJOmHZOo
`
`
`
`
`

`

`US. Patent
`
`Feb. 24, 1987
`
`Sheet 3 of3
`
`4,645,450
`
`pERS
`
`IGNITED
`?
`
`I
`
`Y s
`
`
`WARM
`
`UP TIME
`EXI;IRED
`
`
`
`-
`
`READ DESIRED
`HEATER TEMP
`
`204
`
`206
`
`208
`
`READ ACTUAL
`HEATER TEMP
`
`2'0
`
`I
`
`
`
`
`MAINTAIN
`
`AIR AND FUEL
` DESI RED =
`
`ACTUAL TEMP
`PRESSURES .
`
`
`
`FOR TIME T
`?
`
`
`
`
`
`
`
`
`
`
`
`DESIRED>
`
`ACTUAL TEMP
`
`
`
`P
`
`
`
`DECREASE FUEL
`INCREASE AIR
`PRESSURE DROP
`PRESSURE DROP
`
`AT INCREMENTAL
`AT INCREMENTAL
`
`
`RATE DEPENDENT
`RATE DEPENDENT
`
`
`ON A ’TEMP
`ON A TEMP
`
`
`
`
`DECREASE AIR
`
`INCREASE FUEL
`
`PRESSURE TO
`PRESSURE TO
`
`ACHIEVE PRE-
`
`ACHIEVE PRE-
`
`
`PROGRAMMED
`PROGRAMMED
`
`
`A/ F RATIOS
`A/F RATIOS
`
`
`
`
`
`

`

`1
`
`4,645,450
`
`SYSTEM AND PROCESS FOR CONTROLLING
`THE FLOW OF AIR AND FUEL TO A BURNER
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`This invention relates to a controller for regulating
`the air and fuel flow to a burner. The controller utilizes
`a combustion interface controller having a micro-
`processor for maintaining the pressure drops of both the
`air flow and the fuel flow across the burner at desired,
`optimal rates at each point along the firing range of the
`burner.
`
`2. Description of the Prior Art
`Control systems for regulating the flow of air and fuel
`to burners and furnaces are well known in the prior art.
`One of the best known types of these control systems is
`known as a pressure balanced, constant ratio system.
`This system operates by balancing the pressures of the
`air and fuel flow into the burners throughout the firing
`range of the burners in such a manner that the ratio of
`the flow rate of the air to the flow rate of the fuel re-
`mains at a constant, stoichiometrically optimal value.
`Some of the first pressure-balanced ratio systems
`employed “jack-shafts” which mechanically coupled
`the air regulating valve and the fuel regulating valve of
`the system so that when the air valve was set at a differ-
`ent point along the firing range of the burner, the fuel
`valve automatically mechanically readjusted itself into
`a position commensurate with the optimal ratio of air
`flow to fuel flow. Later, mechanical air-to-fuel ratio
`regulators were developed which worked in conjunc-
`tion with motor-operated valves in the air conduit.
`However, such linked valves and mechanical pressure-
`balanced controls are accompanied by a number of
`‘ shortcomings. For example, as the mechanical linkage
`between the air flow and the fuel flow valves wears ad
`loosens over time, the ability of the system to accurately
`maintain an optimal air-to-fuel ratio throughout the
`firing range of the burner diminishes. Similarly,
`the
`wear of the diaphragms in the mechanical regulators
`ultimately impairs the ability of mechanical air-to-fuel
`ratios to function optimally. Additionally, such me-
`chanical regulators were often inaccurate across every
`point in the firing range of the burner, even when new.
`The inaccuracies caused by such wear invariably lead to
`burning ratios which are less than optimal, and hence
`fuel-wasting.
`To compensate for the inaccuracies which adversely
`affect such mechanical, pressure balancing controls
`over time, electronic mass-flow pressure balance con-
`trols were developed. These electronic systems gener-
`ally incorporate flowmeters in both the air and fuel
`conduits which consist of a calibrated orifice plate
`mounted in the flowpath of both the air and fuel flows
`destined for the burner, and a differential pressure sen-
`sor which is pneumatically connected across this cali-
`brated orifice plate. The differential pressure sensor
`transmits an electrical output indicative of the pressure
`drop across the plate. This electronic output is in turn
`connected to a microprocessor, which computes the
`flow rates by calculating the square root of these pres—
`sure differential signals. Next, the microprocessor com-
`pares these actual air and fuel flow rates with pre-pro-
`grammed “ideal” optimal ratio set-point rates which
`have been previously stored in the memory of the mi-
`croprocessor. The microprocessor then sends signals to
`motor-operated flow control valves located in both the
`
`5
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`30
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`35
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`40
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`60
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`
`2
`air and fuel conduits in order to correct any error which
`it perceives between the actual and set—point air and fuel
`flows. Some prior art electronic mass-flow pressure
`balance controls are capable of shifting to a non-stoichi-
`ometric “excess air” mode at lower firing rates. Such
`non-stoichiometric firing rates have been found to in-
`crease the heat-producing efficiency of the burner (de-
`spite the fact that the resulting air and fuel ratio is not
`stoichiometrically optimal) because the mixture of ex-
`cess air and fuel flowing to the burner generates con—
`vection currents in the furnace which more effectively
`and uniformly transfer the heat generated by the burner
`to the output vent of the furnace.
`Despite the superior accuracy that such electronic
`mass-flow systems have over mechanical-type pressure—
`balancing systems, certain problems remain. For exam-
`ple, in order for the flowmeters used in such systems to
`accurately monitor the air and fuel flows destined for
`the burner, both the inlet and outlet of the orifice plate
`mounted across the air and fuel conduits must be ad-
`joined to a straight section of conduit at least ten con-
`duit-diameters in length. If such straight lengths of con—
`duit do not adjoin both the inlet and outlet portions of
`the orifice plate, the flow of the air or fuel through the
`orifice plate may not have a symmetrical profile across
`the diameter of the conduit, which in turn will greatly
`reduce the ability of the flowmeter to relay an accurate
`flow rate. The requirement that each of the air and fuel
`sections include a straight section of conduit at least
`twenty conduit-diameters in length often poses prob-
`lems when one attempts to retrofit an electronic mass-
`flow control system onto an older burner. Straight sec-
`tions having a twenty-diameter length or more may be
`exist in these older systems, or if they do, such sections
`may be inaccessible. Hence,
`the installation of such
`mass-flow control systems in older burner systems often
`necessitates the installation of straight sections of con-
`duit in order that the flowmeters necessary for the oper-
`ation of these systems may function properly. Addition-
`ally, the orifice plates of these flowmeters create consid-
`erable flow resistances in the air conduit which often
`necessitates the installation of a new and more powerful
`air blower which is capable of generating the air flow
`required at “high fire”. Finally, while the accuracy of
`such electronic mass-flow control systems is generally
`better than mechanical-type pressure ratio systems,
`certain inaccuracies are still present even in the best of
`such systems. Such inaccuracies arise from the fact that
`the computation of the flow rate is based upon a pres-
`sure drop in the air and fuel conduits which is usually
`considerably upstream of the burner, rather than across
`the burner itself. Any measured pressure drop upstream
`of the burner is going to be considerably smaller than
`the pressure drop across the burner itself. The smaller
`the pressure drop used to operate the flowmeter, the
`more difficult it is for the differential pressure sensor to
`accurately relay differential pressure at the low end of
`the firing range, which in turn limits the turn-down
`range of the control system.
`Clearly, there is a need for an electronic control sys-
`tem which may be easily retrofitted onto an existing
`burner system without
`the necessity of installing
`straight lengths of conduit in the air or fuel pressure
`lines, and without replacing the existing blower. Ide-
`ally, such a system would be capable of measuring the
`flow rate of both the air and fuel by accurately measur-
`ing the differential pressure drop of the air and fuel
`
`

`

`4,645,450
`
`3
`across the burner itself, rather than at a point considera-
`bly upstream of the burner, in order to extend the poten-
`tial turn-down range of the system and to reduce the
`opportunity for inaccurate flow rate measurements to
`occur. Finally, 'it would be desirable if such a system
`was simple and inexpensive in construction, and capable
`of operating in a hybrid optimum mode consisting of a
`“splicing together” of various types of optimum modes
`over the entire firing range of the system.
`
`._
`
`SUMMARY OF THE INVENTION
`
`In its broadest sense, the invention is a system for
`controlling the flow of air and fuel to a burner in a
`variety of operating modes throughout the firing range
`of the burner in order to maximize fuel efficiency. The
`system generally comprises a pressure sensing means for
`sensing the pressure of air flowing into the burner, first
`and second valves for modulating the flow of air and
`fuel, respectively, to the burner, and a control means
`operatively connected to both the first and second
`valves and the pressure sensing means for maintaining
`the air-to-fuel pressure ratios at selected optimal values
`which depend upon the point on the firing range at
`which the burner is operated.
`The pressure sensing means may include first and
`second differential pressure sensors fluidly connected
`across the air conduit and the burner, and the fuel con-
`duit and the burner, respectively. In the alternative,
`. when there is a fuel meter present in the fuel conduit
`i" which is capable of generating an electricl signal indica-
`tive of the flow rate of the fuel, the pressure sensing
`means may only include a differential pressure sensor
`connected across the air conduit and the burner. In
`either case, the pressure sensing means is capable of
`sensing a pressure drop and generating a signal which is
`accurately indicative of at least of the flow rate of the
`' air to the burner.
`The control means of the invention may be a combus—
`tion interface controller which includes a microproces-
`sor. The control means may further be electrically con-
`nected to a process controller (which in many cases is
`merely a programmable thermostat which normally
`operates the furnace system) and may coact with the
`combustion interface controller in order that the burner
`of the system arrives at a desired heat output with a
`maximum amount of fuel and process efficiency. Both
`the first and second valves and the output of the differ-
`ential pressure-sensing means are electrically connected
`to the combustion interface controller, which is pro-
`grammed to operate the burner at a specific optimal
`air-to-fuel pressure ratio at each point along the firing
`range of the burner. The air-to-fuel pressure ratios may
`be identical at each point along the firing range of the
`burner, or they may vary.
`The flow controller system may include a flowmeter
`which is detachably connectable to the fuel conduit for
`correlating the various fuel pressures along the firing
`range with specific fuel flow rates. In the preferred
`embodiment, the flowmeter used is a vortex-shedding
`flowmeter which is detachably connectable to the fuel
`conduit by means of a arrangement of T-joints and
`globe valves. Unlike permanently connected orifice-
`plate flowmeters,
`the detachably connected vortex-
`shedding flowmeters create no efficiency~reducing flow
`resistances in the fuel conduit. Additionally, because
`ultrasonic-type flowmeters may be used at the high-
`pressure sides of the fuel conduits which are typically
`part of most existing furnace systems, the temporary
`
`4
`installation of such flowmeters is usually far simpler
`than the permanent installation of orifice—plate flowme-
`ters since the amount of straight—length upstream and
`downstream piping which must be connected to the
`inlet and outlet of the fiowmeter in order to obtain
`accurate flow readings is much shorter.
`After the control system of the invention is initially-
`installed onto an existing furnace system, flow readings
`are taken from the vortex-shedding flowmeter at se-
`lected points along the firing range of the burner. These
`readings are correlated with the fuel conduit differential
`pressure readings which correspond to these selected
`points. Additionally, air flow rates are computed along
`a series of selected points throughout the firing range of
`the burner by noting the air pressures at these points,
`and computing the air flows corresponding to these
`pressures by means of charts which are usually pro—
`vided by the blower and burner manufacturers. Both of
`these sets of data points are read into the microproces-
`sor of the combustion interface controller, which inter-
`polates each of these sets of points into lines correlating
`specific pressures with specific flow rates of both fuel
`and air. The system is then calibrated by computing the
`optimum stoichiometric combinations of air and fuel
`throughout the entire firing range of the furnace system,
`and running the system at these computed stoichiomet-
`ric ratios with an oxygen probe placed in the flue of the
`furnace in order to empirically correct these ratios to an
`optimum value at each point along the firing range of
`the system. Next, the furnace system is run at the empir-
`ically derived air-to-fuel ratios at the upper part of the
`firing range, and at various “excess air” modes at the
`lower end of the firing range in order to empirically
`locate the most effective “excess air” mode at the lower
`end of the firing range. A final “hybrid” optimal mode
`is then spliced together at the end of thesetests and
`entered into the memory of the microprocessor of the
`combustion interface controller.
`In operation,
`the process controller compares the
`actual temperature of the furnace which houses the
`burners with the desired temperature. If the desired
`temperature does not equal the actual temperature, the
`combustion interface controller incrementally adjusts
`the pressure drops sensed by the differential pressure
`sensors at a rate dependent upon the perceived differ-
`ence by adjusting the air and fuel valves until the de-
`sired and actual temperatures are equal.
`BRIEF DESCRIPTION OF THE SEVERAL
`FIGURES
`
`FIG. 1 is a schematic diagram illustrating the control
`system of the invention retrofitted onto a dual-burner
`furnace capable of operating on both gaseous and liquid
`fuels;
`FIG. 2 is an alternate embodiment of the control
`system of the invention retrofitted onto a furnace hav-
`ing a single, gaseous fuel burner and a fuel meter on its
`fuel conduit;
`FIG. 3 is a side view of the detachably mountable
`flowmeters used to calibrate the control system of the
`invention, illustrating both the flowmeter, fittings and
`conduits used to temporarily connect it to the high
`pressure side of a fuel conduit, and
`FIG. 4 is a flow chart illustrating the operation of the
`combustion interface controller of the system.
`
`10
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`15
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`20
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`25
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`30
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`35
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`55
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`60
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`65
`
`

`

`5
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`Structure of the Invention
`
`4,645,450
`
`FIG. 1 illustrates a preferred embodiment of the con-
`trol system 1 as installed onto a dual-burner furnace
`system 2 having a pair of burners 29a, 29b. Each of the
`burners 29a, 29b includes a branch air conduit 13a, 13b,
`a liquid fuel branch conduit 49a, 49b, and a gaseous fuel
`branch conduit 78a, 78b, respectively, for guiding a
`flow of air and liquid or gaseous fuel thereto. Generally,
`the control system 1 includes six motor-operated valves
`15a, 15b, Sla, 51b, and 80a, 80b which are mounted in
`the air conduits 13a, 13b, liquid fuel conduits 49a, 49b,
`and gaseous fuel conduits 780, 78b of the furnace system
`2. Each of these valves is electrically connected to the
`combustion interface controller 23 as indicated.
`In the preferred embodiment, the combustion inter-
`face controller 23 includes a microprocessor formed by
`a Z-80 chip manufactured by Zilog, Inc., of Campbell,
`Calif, which is appropriately connected to a Radio
`Shack Model TRS-SO microcomputer (black and white
`version). Radio Shack is a division of Tandy Corpora-
`tion of Ft. Worth, Tex. The output of the microcom-
`puter is preferably connected to the control cables of
`the motor-operated valves 150, 15b. 510, 52b and 80a,
`80b through an appropriate, commercially available
`interface card.
`,
`The control system 2 of the invention further in-
`cludes six differential pressure sensors 25a, 25b, 57a,
`57b, and 88a, 88b which are fluidly connected across the
`air branch conduits 13a, 13b, liquid fuel conduits 49a,
`49b, and gaseous fuel conduits 780, 78b and the burners
`29a, 29b, respectively. Like the previously discussed
`motor-operated valves, each of the differential pressure
`sensors is electrically connected to the combustion in-
`terfacecontroller 23.
`After the control system 1 has been properly cali-
`brated so that optimal combinations of air and fuel pres—
`sure drops have been entered into the memory of the
`microprocessor of the combustion interface controller
`23 for each point along the firing range of the burner
`assemblies 29a, 29b, the control system 1 maintains an
`optimal flow rate of air and fuel at each selected point
`along the firing range by adjusting the valves 15a, 15b,
`51:1, 51b, and 800, 80b until the pressure drops sensed by
`the differential pressure sensors 25a, 25b, 57a, 57b, and
`88a, 88b are achieved. Normally, the air-to-fuel ratio
`will not remain constant over the entire span of the
`firing range, but will vary between the low end of the
`firing range and the medium-to-high ends of this range.
`At the outset, it should be noted that while the furnace
`system 2 is capable of operating on either gaseous or
`liquid fuel, it will normally operate on one fuel or the
`other, but not both. Accordingly, at any one given time,
`the combustion interface controller 23 will be control-
`ling either the liquid fuel motor-operated valves Sla,
`51b or the gaseous fuel operated valves 80a, 80b, but not
`both simultaneously.
`Turning now to a more specific description of the
`control system 1 in the context of the dual-burner fur-
`nace system 2, the air blower 3 of the furnace system 2
`is connected to a main blower conduit 5 which prefera-
`bly includes a heat recuperator 7. The recuperator 7
`preheats the pressurized air which the air blower 3
`pumps into the burner assemblies 29a, 29b by reclaiming
`some of the heat present in the flue gases which escape
`from the furnace 100. Recuperator 7 preferably includes
`
`6
`a flue gas inlet 9 as indicated in order to thermally cou-
`ple the air flowing through it to the relatively hotter
`flue gases flowing out of the furnace 100. These flue
`gases are of course expelled from the recuperator 7
`through an appropriate outlet (not shown).
`Downstream of the recuperator 7, the main blower
`conduit 5 bifurcates into branch conduits 130, 13b, each
`of which ultimately communicates with one of the pre-
`viously mentioned burner assemblies 29a, 29b. Each of
`the branch conduits 13a, 13b includes a motor-operated
`butterfly valve 151:, 15b, respectively. Each of these
`butterfly valves includes a pivotable vent element 17a,
`17b whose position may be modulated by means of an
`electric motor 19a, 19b. Each of the electric motors 19a,
`19b are electrically connected to the combustion inter-
`face controller 23 by means of output cables 21a and
`21b. Each of the branch conduits 13a, 13b further in-
`cludes a differential pressure sensor 25a, 25b for measur-
`ing the differential pressure between the pressurized air
`in the branch conduits 13:2, 13b and the flame of the
`burner assemblies 29a, 29b. Each of these differential
`pressure sensors 25a, 25b includes an upstream pressure
`conduit 27a, 27b which is pneumatically coupled to its
`respective branch conduit 130, 13b in the position
`shown, as well as a downstream pressure conduit 31a,
`31b which is pneumatically coupled across the burner
`assemblies 29a, 29b through the burner pressure con-
`duits 33a, 33b, respectively. In the preferred embodi-
`ment, differential pressure sensors 25a, 25b (as well as
`all of the other differential pressure sensors of the con-
`trol system 1) may be either the linear-variable differen-
`tial
`transformer type as manufactured by Robinson-
`Halpern of Plymouth Meeting, Pa., or the solid-state
`piezoresistive silicone chip type (Model PR-270), as
`manufactured by Manac Systems of Minneapolis, Minn.
`The electrical outputs of both of the differential pres-
`sure sensors 25a, 25b are connected to the combustion
`interface controller 23 through input cables 36a and
`36b. Finally, each one of the branch air conduits 13a,
`13b includes both an air pressure gauge 38a, 38b and a
`thermocouple 400, 40b. The air pressure gauges 38a,
`38b each provide a visual indication of the absolute
`pressure within the branch conduits 13a, 13b. Neither of
`the air pressure gauges 380, 38b are necessary for the
`operation of the control system 1 of the invention.
`However, the provision of such gauges in each of the
`branch air conduits 13a, 13b assists the operator in the
`initial calibration of the system 1 after it has been in-
`stalled within a particular burner system 2, and also
`provides a double-check on the pressure drop readings
`obtained from the differential pressure sensors 25:1, 25b.
`The thermocouples 40a, 40b are each connected to the
`combustion interface controller 23 via input cables 420,
`42b, respectively, and generate an electric signal indica-
`tive of the temperature of the pressurized air entering
`the burner assemblies 29a, 29b. Such temperature read-
`ings are important because they provide data which
`allows the combustion interface controller 23 to infer
`the density of the air entering the burner assemblies 29a,
`29b, which is necessary if the air flow rate into the
`burner assemblies is to be accurately computed.
`As previously mentioned,
`the furnace system 2 is
`capable of burning both liquid and gaseous fuels. Under
`normal circumstances, the furnace system 2 would burn
`natural gas. However, in the event that fuel oil should
`become less expensive than natural gas, or the local
`utility service should terminate natural gas service to
`
`5
`
`10
`
`15
`
`20
`
`25
`
`3O
`
`35
`
`4O
`
`45
`
`50
`
`55
`
`60
`
`65
`
`

`

`4,645,450
`
`’
`
`7
`industrial users (as sometimes happens during cold
`waves in the northeast, when the utility services cannot
`serve the heating needs of both homeowners and indus-
`try), the furnace system 2 is provided with a liquid fuel
`system as a backup. This liquid fuel system includes a
`liquid fuel source 45, which is usually fuel oil pressur-
`ized by means of a pump. The source 45 of pressurized
`liquid fuel is fluidly connected to a main liquid fuel
`conduit 46 which ultimately connects with the burner
`assemblies 29a, 29b and includes a means for detachably
`connecting a flowmeter 48. This flowmeter 48 is part of
`the flowmeter assembly 180, is best seen with reference
`to FIG. 3, and will be described in greater detail at a
`later point in the specification. The flowmeter 48 is
`connected to the combustion interface controller 23 by
`means of an input cable 47. Generally speaking, the
`flowmeter 48 is not a permanent part of the control
`system 1 of the invention, but is detachably connected
`to the main liquid fuel conduit 46 for calibration pur-
`poses only, whereupon it is removed. The flowmeter 48
`is used to empirically ascertain the liquid fuel flow rates
`which correspond to each value of the liquid fuel differ-
`ential pressures along the firing range of the burner
`assemblies 29a, 29b. In the preferred embodiment, flow-
`meter 48 is a Model VTX 900 vortex-shedding ultra-
`sonic flowmeter manufactured by Brooks Instrument
`Division of Emerson Electric Company located in Hat-
`field, Pa. The use of a detachably connectable ultrasonic
`flowmeter on the main liquid fuel conduit 46 provides
`the combustion interface controller 23 with an accurate
`correlation between actual liquid fuel flow rates and
`liquid fuel pressures across the entire operating range of
`' the burner system 2 without the need for a permanently
`installed flowmeter, which not only introduces un-
`wanted obstructions in the flowpath of the fuel, but is
`‘ often expensive to install.
`Downstream of the flowmeter 48, the main liquid fuel
`conduit 46 bifurcates into two branch fuel conduits 49a
`and 49b. Each of the branch fuel conduits 49a, 49b
`includes a motor-operated fuel valve 51a, 51b which in
`turn includes its own valve motor 53a, 53b for modulat-
`ing the flow of fuel through these valves. Each of the
`motor-operated flow valves 510, 51b is connected to the
`combustion interface controller 23 by way of an output
`control cable 55a and 55b, as indicated in FIG. 1. In
`addition to the valves Sla, 51b, each of the liquid fuel
`branch conduits 49a, 49b includes its own differential
`pressure sensor 57a, 57b for ascertaining the differential
`pressure'of the liquid fuel across the burner assemblies
`29a, 29b, respectively. Each of the differential pressure
`sensors 57a, 57b includes upstream pressure conduits
`59a, 59b which are directly connected to the liquid fuel
`branch conduits 49a, 49b, and downstream pressure
`conduits 610, 61b which are connected to the burner
`pressure conduits 330, 33b by way of pneumatic inter-
`sections 35a, 35b. As was the case with the differential
`pressure sensors 25a, 25b located on the air branch
`conduits 13a, 13b, the output of each of the differential
`pressure sensors 57a, 57b is connected to the combus-
`tion interface controller 23 by means of an input cable
`630, 63b. Finally, each of the liquid fuel branch conduits
`49a, 49b includes its own pressure gauge 65a, 65b. Each
`of these gauges serves the same function as the air pres-
`sure gauges 38a, 38b serve with respect to their branch
`conduits 13a, 13b, i.e., they facilitate the initial calibra-
`tion of the system 1 and assist in detecting spurious
`readings of the differential pressure sensors 57a, 57b in
`the event either of these sensors malfunctions.
`
`‘
`
`8
`Turning now to the components of the gaseous fuel
`system of the furnace system 2, a source 70 of pressur-
`ized gaseous fuel (which is typically natural gas) is con-
`nected to the burner assemblies 29a, 29b via-a main
`gaseous fuel conduit 72. Like the previously-described
`main liquid fuel conduit 46, conduit 72 likewise includes
`means for detachably connecting a flowmeter 74, which
`is also a vortex shedding ultrasonic flowmeter. How-
`ever, in contrast to the Brooks Instruments ultrasonic
`flowmeter used in connection with the liquid fuel
`source 45, flowmeter 74 is preferably a VP series, gase-
`ous-type vortex-shedding ultrasonic flowmeter manu-
`factured by J-Tec Associates, Inc. of Cedar Rapids,
`Iowa. The use of a detachably connectable ultrasonic
`flowmeter 74 in gaseous fuel conduit 72 obviates the
`installation of an expensive orifice plate flowmeter,
`which could require the permanent installation of a
`straight length of conduit over twenty diameters in
`length. Additionally, the use of an ultrasonic flowmeter
`74 in the gaseous fuel conduit 72 has the effect of ex-
`tending the firing ratio of the furnace 2, since such
`flowmeters are sensitive over a much greater range than
`orifice-plate flowmeters. As is indicated in FIG. 1, flow-
`meter 74 is electrically connected to the combustion
`interface controller 23 by means of an input control
`cable 76. The main gaseous fuel bifurcates into a pair of
`gaseous fuel branch conduits 78a, 78b. Each of the
`branch conduits 78a, 78b includes a motor-operated,
`butterfly—type valve 80:1, 80b which is modulated by
`means of an electric motor 84a, 84b, respectively. Each
`of these motor-operated valves 80a, 80b is connected to
`an output cable 86a, 86b connected to the combustion 7
`interface controller 23. In addition to having its own
`motor-operated butterfly-type valve, each of the gase-
`ous fuel branch conduits 78a, 78b further includes its
`own differential pressure sensor 88a, 88b for generating
`an electric signal indicative of the differential pressure
`drop across the gaseous fuel in the branch conduits 78a,
`78b and the flame in the burner assemblies 29a, 29b. To
`this end, each of the sensors 88:1, 88b includes an up-
`stream pressure conduit 90a, 90b connected to its re-
`spective gaseous fuel branch conduit 780, 78b, and a
`downstream pressure conduit 92a, 92b connected to the
`burner pressure conduits 33a, 33b via pneumatic inter-
`sections 35a, 35b, as shown in FIG. 1. The outputs of
`each of the gaseous fuel pressure differential sensors
`88a, 88b are connected to the input of the combustion
`interface controller 23 by way of input cables 95a, 95b.
`Finally, in order that the fluid resistance of each of the
`gaseous fuel branCh conduits 78a, 78b may be equalized,
`each of these branches includes a trim valve 99a, 99b.
`The furnace system 2 includes a heater 100 for hous-
`ing the previously mentioned burner assemblies 29a,
`29b. A pair of thermocouples 102 and 106 are thermally
`coupled to the interior of the heater 100. These thermo-
`couples 102 and 106 transmit electrical signals indica-
`tive of the temperature of different regions of the heater
`100 to both the process controller 110 and the combus-
`tion interface controller 23 by way of parallel-con-
`nected input cables 1040, 104b and 108a, 108b, respec-
`tively. The output of the process controller 110 (which
`may be a pair of commercially available, programmable
`thermostatic controls) is in turn connected to the input
`of the combustion interface controller 23 by means of an
`input cable 109. The process controller 110 senses the
`difference between the actual temperature within the
`furnace 100 and the desired temperature to which the
`control system 1 is set, and transmits an electrical signal
`
`10
`
`15
`
`20
`
`25
`
`3O
`
`35
`
`40
`
`45
`
`50
`
`55
`
`65
`
`

`

`4,645,450
`
`9
`indicative of this difference in