CA1282140C - Excess air control - Google Patents
Excess air controlInfo
- Publication number
- CA1282140C CA1282140C CA000544663A CA544663A CA1282140C CA 1282140 C CA1282140 C CA 1282140C CA 000544663 A CA000544663 A CA 000544663A CA 544663 A CA544663 A CA 544663A CA 1282140 C CA1282140 C CA 1282140C
- Authority
- CA
- Canada
- Prior art keywords
- speed
- motor
- excess air
- level
- heat exchanger
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
Classifications
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
- F23N—REGULATING OR CONTROLLING COMBUSTION
- F23N1/00—Regulating fuel supply
- F23N1/02—Regulating fuel supply conjointly with air supply
- F23N1/025—Regulating fuel supply conjointly with air supply using electrical or electromechanical means
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
- F23N—REGULATING OR CONTROLLING COMBUSTION
- F23N1/00—Regulating fuel supply
- F23N1/02—Regulating fuel supply conjointly with air supply
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
- F23N—REGULATING OR CONTROLLING COMBUSTION
- F23N2227/00—Ignition or checking
- F23N2227/04—Prepurge
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
- F23N—REGULATING OR CONTROLLING COMBUSTION
- F23N2233/00—Ventilators
- F23N2233/06—Ventilators at the air intake
- F23N2233/08—Ventilators at the air intake with variable speed
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
- F23N—REGULATING OR CONTROLLING COMBUSTION
- F23N2233/00—Ventilators
- F23N2233/10—Ventilators forcing air through heat exchangers
Abstract
EXCESS AIR CONTROL
ABSTRACT OF THE DISCLOSURE
Method and apparatus for maintaining excess air control in a gas furnace. A pressure switch is placed across a heat exchanger to indicate when, while accelerating the inducer motor speed during purging operation, the pressure drop reaches a predetermined level. When that occurs, the motor speed is sensed and recorded. When the furnace is subse-quently fired, the desired inducer motor speed is obtained by modifying the recorded motor speed by a correction factor derived from empirical data obtained from a gas furnace operating under selective variable conditions.
Further, in a two stage gas furnace system, low and high pressure switches are placed across the heat exchanger and are successively closed, as the inducer motor accelerates during purging, when the pressure drop reaches the respec-tive theoretically desired low and high firing pressure drop levels. As the switches are closed, the inducer motor speeds are sensed and recorded, with a ratio of the two then being calculated. After firing, the ratio is then directly applied to a desired high firing motor speed to obtain the desired low firing motor speed.
ABSTRACT OF THE DISCLOSURE
Method and apparatus for maintaining excess air control in a gas furnace. A pressure switch is placed across a heat exchanger to indicate when, while accelerating the inducer motor speed during purging operation, the pressure drop reaches a predetermined level. When that occurs, the motor speed is sensed and recorded. When the furnace is subse-quently fired, the desired inducer motor speed is obtained by modifying the recorded motor speed by a correction factor derived from empirical data obtained from a gas furnace operating under selective variable conditions.
Further, in a two stage gas furnace system, low and high pressure switches are placed across the heat exchanger and are successively closed, as the inducer motor accelerates during purging, when the pressure drop reaches the respec-tive theoretically desired low and high firing pressure drop levels. As the switches are closed, the inducer motor speeds are sensed and recorded, with a ratio of the two then being calculated. After firing, the ratio is then directly applied to a desired high firing motor speed to obtain the desired low firing motor speed.
Description
EXCESS AIR CONTROL
Background of the Invention This inventi~n relates generally to gas furnaces and, more particularly, to con~rol of exces6 air in 8 gas furnace hsving a variable ~peed inducer motor.
In the operPtion of a gas-fired furnace, combustion efficiency can be optinlzed by maintaining the proper ratlo of the gas input rate and the combustion air flow rate.
G~nerally, the ideal ratio i5 offset somewha~ for 6afety purpo6es by providin~ for slightly more combustion sir (i.e., exces6 air) than tha~ required for optimum combu6tion efficiency conditions. In order that furnace heat 108ses are mini~ized, it is ~mportant th~s excess air level i~
controlled.
Since the pressure nrop acro66 the heat exchanger is proportional to excess air, it i6 maintained at a predetermined constant level for a given gas inp~t rate. In one method of maintaining such a constant pressure drop is in an arrangement wherein sansors are provided at the inlet and the outlet of the heat exchanger, and B pres~ure tran6ducer i8 provided to receive ~ignals from those sensors to calculate a pressure drop 6ignal which is then provided to the furnace control to responsivcly vary the speed of the inducer motor 80 as to maintain a con~tant pres6ure drop and thereby maintain the excess air at a constant level. One of the problems with the use of such a tran6ducer i6 i~
rel~tively high cost. Further, the reliability of ~uch a tr~nsducer wa found to be less than that desired because of ~pparent thermal instabilitie~.
:/;( ~82~40 It has beco~e common practice in gas-fired furnaces to provide for two different firin~ 6tages where each stage has its own gas input ra~e. Two ~peed opera~ion can be accomplished with a fixed rate, two speed motor to drive the draft inducer motor ~nd blower motor; however, the electrical consumption of such motors limited to two sp~eds while operating at low speed would be significantly greater th~n that of a variabl~ speed electronically commutated motor (E~I), for example. Further, since the inducer motor would operate at only two fixed speeds, thc syfitem could not adapt to variable operating and ~ystem conditions ~uch as, for example, a variable length of vent system, such that ~e level of excess air could not be controlled ~o the degree desired unless the system was tuned for the particular 15 installation.
It is therefore an object of the present invention to provide an ~mproved method and apparatus for controlling the excess air in a gas-fired furnace without the need for field tuning 20 the combustion sys~em.
Another object of the present invention is the provision in a ~as-fired furnace for controlling the level of excess air without the use of a pressure transducer.
Yet another object of the present invention is the provision in a gas-fired urnace for controlling the level of excess air in a manner which ~akes into account ~he use o~ varlable length vent~.
Still another object of the prese~lt invention i~ the provision for con~rolling a variable speed motor ~o as to maintain desirable levels of excess air when operating with either ~ single or multi-stage sys~em.
- - -~L~f~
Yet another object of the present invention is the provision in a gas-fired furnace for an excess air control system which is economical to manufacture and effective in use.
These objects and advantages become more readily apparenk upon reference to the following description when ~akèn in conjunction with the appended drawings.
Summary of the Inventio_ Briefly, in accordance with one aspect of the invention, there is provided in the furnace heat exchanger, at least one pressure switch, which is responsive at a pressure level commensurate with a desired theoretical level of excess air ~hen operating in a firing condition. During the purging cycle, the inducer motor is accelerated to increase the pressure in the heat exchanger such that the pressure switch closes when its make point is reached.
As the switch closes, the inducer motor speed is sensed and recorded by a microprocessor. After purging, the furnace is fired and the inducer motor is allowed to stabilize. After a short period of time the inducer motor speed is reduced to a level which is based on the recorded motor speed and on a formula derived from data experimentally obtained from an exemplary operating system demonstrating the desired excess air level under high fired conditions.
In this way, the pressure switch is used during pre-ignition operation to obtain a motor speed which can be subsequently applied to obtain the desired operating speed of the inducer motor under firing conditions.
In accordance with another aspect of the invention, there are provided in the furnace heat exchanger, a pair of pressure switches, with a low pressure switch being responsive at a pressure level commensurate with the desired theoretical level of excess air when operating in a low flring condition, and a high pressure switch that is responsive at a pressure ~L~8~4g~
3 a commensurate with a desired theoretical excess air level when operating in a high firing condition. During the purging cycle the inducer motor is accelerated to increase the pressure in the heat exchanger such that the low pressure and high pressure switches close in succession. As the switches close, the inducer motor speeds are sensed and recorded b~ a microprocessor. A ratio is then calculated between the speed at which the low pressure switch closed and that at which the high pressure switch closed. This ratio is then recorded for subsequent application. After purging, the furnace is fired at the high firing rate and the inducer motor is allowed to stabilize. After a short period of time the inducer motor speed is reduced to a level which is based on the recorded motor speed and on a formula derived from data experimentally obtained from an exemplary operating system demonstrating the desired excess air level under variable high fire operating conditions. After a short period of time, the furnace transitions to a low fire operating condition with the inducer motor speed being reduced to a speed which is calculated by multiplying the previous high fire motor speed by the ratio which was previ-ously calculated. In this way, the pressure switches are used during pre-ignition operation to obtain a ratio which can be subsequently applied to obtain the desired operating speed of the inducer motor for low fire operation.
In the drawings as hereinafter described, a preferred embodiment is depicted. However, various other modifica-tions and alternate constructions can be made thereto without departing from the true spirit and scope of the invention.
8X14~3 Brief Description of the Drawin~
Figure 1 is a perspective view vf a gas furnace having the present invention incorporated therein.
Figure 2 is a schemstic illustration of the two installed pressure switches thereof as applied to the heat exchanger system.
Figures 3-5 are graphic illustrations of ~hAnges in the inducer motor speeds during eypical cycles of operation.
Description of the Preferred Embodim nt Referring now to Figure 1, there is shown 8 furnace of the general type with which the present invention can be employed. A burner assembly 11 communicates with a burner box 12 of a primary heat exchanger 13. Fluidly connected at the other end of the primary heat exchanger 13 is a condensing heat exchanger 14 whose discharge end is luidly connected to a collector bo~ 16 and an exhaust vent 17. In operation, a gas valve 18 me~ers the flow of gas to the burner assembly 11 where combustion air from the air inlet 19 is mixed and ignited by the ignition assembly 21. The hot gas is then passed through the primary heat exchanger 13 and the condensing heat exchanger 14 as shown by the arrows. The 2.5 relatively cool exhaust gases then pass through the collector box 16 and the exhaust vènt 17 to be vented to the atmosphere~ while the condensate flows from the collector bo~
16 through a condensate drain line 22 from where it is ~uitably drained to a sewer connection or the like. Flow o~
the combustion air into the air inlet 19, ~hrough ~he heat exchangers 13 and 14, and the exhaust vent 17 is enhanced by a draft inducer blower 23 which i8 driven by a motor 24 in response to control signals from the microprocessor.
The household air is drawn into a blower 26 which is driven by a drive motor 27, ~gain in response ~o signals received from the microprocessor. The discharge air from the blo~er 26 passes over the condensing heat exchanger 14 and the primary heat cxchsnger 13, in coun~erflow relationship wi~h the hot combustion gases, to thereby heat up the household air, which then flows from the discharge opening 28 to thc duct sy6tem within the home.
The microprocessor mentioned hereinaboYe i8 contained in the microprocessor control assembly 29. In re&ponse to electrical sign~ls from the thermos~at, and from other ~ignals to be ~iscussed hereinafter, the microprocessor control assembly 29 operates to control the inducer motor 24 and the blower motor 27 in such a way as to promote an efficient combustion process at two different firing rates.
- To aid in the control of excess air, a psir of pressure ~witches 31 and 32 are placed across burner box 12 and the collector box 16, respectively, so ~s to permit the me~surement o~ the pressure drop across ~he heat exchanger system. The switches 31 and 32 are mechanically connected within the system to sense the heat exchanger pressure drop shown in Figure 2.
As will be seen, a burner box tube 33 leads from the pressure tap 36 and a collector box tube 34 leads from the pressure tap 37. Fluidly connected therebetween, in parallel relationship, are the low pressure switch 31 and high pressure switch 32. These switches are calibrated to make, or close, at ~pecific pressure differentials as determined in a manner which will be more fully described hereinafter.
Switche~ that ha~e been found satisf~ctory for use in this manner are commercially available from Tridelta a3 part numbers FS 6003-250 (high pressure) ~nd FS 6002-249 (low pressurc).
~ -- \
o Since ~he system is normally oper~tin~ under negative pressure conditions, it is nece~s~ry to fluidly connect the vent of gas valve 18 with tube 38 to tubes 33 ana 39 via a "T" fit~ing 40 so as to reference low pressure switch 31, high pressure switch 32, and gas valve 18 to the negative pressure inherent in burner bo~ 12 while inducer motor ~4 i8 in operation.
Since the pressure drop across ~he heat exchangers is indicative of the level of excess air in the combustion 8y8tem, the low and high pressure switches 31 and 32 are used to determine when the level of e~cess air falls below the minimum desired theoretical levels for low and high firing conditions, respectively. For example, the low pressure switch 31 is so calibrated that it will close at the point when the excess ~ir level is equal to the desired theoretical value for low firing conditions. At that time, the closing of the switch causes ~ signal to be transmitted to the microprocessor, which in turn initiates a ~ensing and recording of the inducer motor speed, RPM 1. Similarly, as the speed of the inducer motor is increased, the level of excess air is increa~ed until it fin~lly reaches the desired theoretical value for high firing conditions, at which time the high pressure switch 32 closes and a signal is sent to the microprocessor. The inducer motor speed is again sensed and recorded at RPM 2. These speeds RPM 1 and RPM 2 are then mathematically altered to obtain the. desired motor speeds in accordance with the present lnvention.
Referring now to Figure 3, a typical cycle of opera~ion will be described. Upon a call for heat, the contral checks the status of the high and low pres~ure switche6 32 and 31. If both of the switches are open as they 6hould be, then the inducer motor is accelerated unti~ the pressure drop equals Pl, at which time the low pressure switch 31 is closed and the inducer motor speed RPM 1 is recorded. The inducer motor 8~ ~ 4 speed is allowed to continue to accelerate until ~he pressure drop equals P~, at which time th~ high pressure switch 32 cloaes and the inducer motor speed RPM 2 i~ recorded. The microprocessor control 29 then calculates the ra~io of the inducer ~peeds at low and high firing switch clo6ure polnt~
as follow6:
RATIO = RP~ 1 ~Eq. 1 The RATIO is then recorded for subsequent appllcation.
Afeer the high pressure switch 32 clo~es, the system undergoes a vent purge and the pilot is ignited by the furnace control. Shortly ater the pilot proves, and the main burnel-s ignite, the control then calculates RPM 4 using RPM 2 as wlll be described more fully hereinafter, after wh~ch it reduces the inducer motor speed to RPM 4.
20 It will be understood that when ignition occurs, the bul~;
temperature of the heat exchange sy~tem increases and th~
bulk density decreases. This, in turn causes a substanti&l increase in the pressure drop as shown in Figure 3. In order to reduce the pressure drop to the level at which RPM 2 was sensed the speed of the inducer motor must be reduced accordingly. However, it is not obvious as to how much that speed can be reduced. The various factors that are involved include: the difference in temperature and density between flue gas and air, the gas valve opening characteristics, and the length of the system vent.
In order ~o determine nominal operating point8 for various 6ystems a pressure drop P5 commensurate with desired theoretical level of excess air was used An exemplary 35 8ystem was experimentally run ur.der various operatin~
condi~ions (i.e., wanm and cold s~rts for each of minimum 1.40 and maximum length vent conditions), with RPM 1 - RPM 4, as well as the heat exchanger pres~ure drop (HXDP), being recorded. The resulting data was thQn analyzed and modified to make the heat exchan~er pressure drop repeatable from cycle to cycle for minimum and maximum vent lengths. For thls purpose, a nominal high firing rate heat exchanger pre~sur~ drop of .72 inches w.c. was used. Thus, where the variation from this nominal value was above a predetermined threshold, the following equation was ~pplied to correct the RPM 4 v~lues:
RPMCor = RPM x ~ ~YDP (Eq. 2) Taking the average of the experimental data o obtained, the corrected RPM 4 values were determined to be related to RP~i 2 values, for minimum and maximum vent condi~ions, as shown ir.
Table I.
TABLE I
RP~l 2 ~PM 4 Min. Ver.t 2574 2469 ~ax. Vent 3429 3124 As6uming now, a linear relationshlp between minlmum and maximum vent conditions, a best fit straight line equation using RP~ 2 and RP~l 4 values was determined as follows:
RP~ 4 = 497.11 + ~.766 x RPM 2) (Eq. 3) The speed of the inducer motor is therefore held at RP~l 4 until the end of the high firing period. ~en th~ hea~
exchanger has been war~ed up and the blower motor has been calibrated, the control the~ switches to a low firing -2 ~
condi~ion. This is accomplished by first calculating inducer motor speed RP~; 5 using inducer motor spePd RPM 4. The blower motor speed i8 ~hen reduced to a low firing speed and the furnace control reduces the Lnduc~r ~otor speed to ~PM 5, where:
RPM 5 = RPM 4 x RATIO (Eq. 4) Where: RATIO is defined as Equation 1, measured during vent purge.
As the inducer motor speed is reduced from RPM 4, thP high pressur~ fiwitch 32 opens and the high firing solenoid is de-energized. The inducer motor speed is thus reduced to RP~
5 and remains at that level during the period of low firing operation. If the thermostat is not sati6fied within a prescribDd ~eriod of time, the control will switch from a low - firin~ t~ 8 high firing condition. This i8 done by first accelerating the inducer mo~or until the high firing pressure switch closes and thereby energizes the high firing solenoid.
The speed of the inducer motor RPM 6 is then recorded. The blower th~n goes to high firing speed and the control increases the inducer motor speed to RPM 7. The rel~tionship between RPM 6 and XPM 7 values are experimentally determined in the same manner as described for RPM 2 and RPM 4 above, with the average RP~i's for a mlnimum and maximum vent l~ngths being shown in Table II.
TABLE Il RPM 6, 8 RPM- 7, 9 Min. Vent 2398 2482 Mfix. Vent 3044 3080 41~
Again, a~suming a rel&tionship between minimum and maximum vent conditions, a best fit straight line equation using RP~
6 and RPM 7 was deter~ined to be:
RPM 7 = 262.18 ~ (.926 x RP~l 6) tEq. 5) The inducer motor ~peed ifi then held constant at RPM 7 for high firing operation until such tim~ as the thermostat conditionæ are met or the system aga~n change6 to a low firing operating condition.
If, for example, an obstruction was temporarily placed over the system vent, the pressure drop would be reduced to the point where the high pressure switch 32 would open9 causing the high firing ~olenoid to be de-energized. This is neces6ary because of the reduced combustion airflow as shown in Figure 5. The control then ¢auses the inducer motor speed to be increased until the high pressure swi~ch 32 reclo~es and re-energizes the high fire solenoid. At tha~ time, the inducer motor speed RPM 8 is recorded and the furnace control increases the inducer motor speed to RPM 9 where:
RPM 9 = 262.18 ~ (.926 x RPM 8) (Eq. 6) As will be ~een, ~he inducer motor speed RPM 9 is determined as a function of the speed RPM 8 with the use of the same mathematical relationship found between RP~ 6 and RPM 7 as expressed in Equation 5.
It will be understood that throughout the operation described hereinabove, controlling limits are operative in the various operating modes, and the relevant conditions are monitored such that if the limits are exceeded, a failure is lndica~ed and the cycle is readjusted accordingly. For e~ample, during the period of initial acceleration tO RPM 1 as shown in Figure 3, if either the low pressure switch does not close 8~ 1 4 within a prescr$bed period of time or the RPM 1 value i8 outside its prescribed limits, a fault is signalled7 the unit shut6 down and tries again. If the high pressure switch closes before the low pressure switch close~, a fault i9 signalled and the unit locks out. Similar limit8 ~nd modified operating modes are provided during the other phases of operation to ensure that the system is operating within the intended parameters.
1~ While ~h~ present invention has been described in terms of use with a two stage 8y8tem, it should be understood that certain aspects thereof can just as well be used with a single or other multi stage systems. For example, while the Equations 3, 4, 5 and 6 have been applied to obtain the desired inducer motor operatin~ speeds for high firing conditions in a two stage system, they are equally applicable for use in de~ermining the inducer motor speeds for operation under firing conditions in a single or o~her multi-stag~
systems.
It will be understood that the pres~nt invention has been described in ~erms of a preferred embodlment. However, it may take on any number of other forms while remaining within the scope and intent of the invention.
Background of the Invention This inventi~n relates generally to gas furnaces and, more particularly, to con~rol of exces6 air in 8 gas furnace hsving a variable ~peed inducer motor.
In the operPtion of a gas-fired furnace, combustion efficiency can be optinlzed by maintaining the proper ratlo of the gas input rate and the combustion air flow rate.
G~nerally, the ideal ratio i5 offset somewha~ for 6afety purpo6es by providin~ for slightly more combustion sir (i.e., exces6 air) than tha~ required for optimum combu6tion efficiency conditions. In order that furnace heat 108ses are mini~ized, it is ~mportant th~s excess air level i~
controlled.
Since the pressure nrop acro66 the heat exchanger is proportional to excess air, it i6 maintained at a predetermined constant level for a given gas inp~t rate. In one method of maintaining such a constant pressure drop is in an arrangement wherein sansors are provided at the inlet and the outlet of the heat exchanger, and B pres~ure tran6ducer i8 provided to receive ~ignals from those sensors to calculate a pressure drop 6ignal which is then provided to the furnace control to responsivcly vary the speed of the inducer motor 80 as to maintain a con~tant pres6ure drop and thereby maintain the excess air at a constant level. One of the problems with the use of such a tran6ducer i6 i~
rel~tively high cost. Further, the reliability of ~uch a tr~nsducer wa found to be less than that desired because of ~pparent thermal instabilitie~.
:/;( ~82~40 It has beco~e common practice in gas-fired furnaces to provide for two different firin~ 6tages where each stage has its own gas input ra~e. Two ~peed opera~ion can be accomplished with a fixed rate, two speed motor to drive the draft inducer motor ~nd blower motor; however, the electrical consumption of such motors limited to two sp~eds while operating at low speed would be significantly greater th~n that of a variabl~ speed electronically commutated motor (E~I), for example. Further, since the inducer motor would operate at only two fixed speeds, thc syfitem could not adapt to variable operating and ~ystem conditions ~uch as, for example, a variable length of vent system, such that ~e level of excess air could not be controlled ~o the degree desired unless the system was tuned for the particular 15 installation.
It is therefore an object of the present invention to provide an ~mproved method and apparatus for controlling the excess air in a gas-fired furnace without the need for field tuning 20 the combustion sys~em.
Another object of the present invention is the provision in a ~as-fired furnace for controlling the level of excess air without the use of a pressure transducer.
Yet another object of the present invention is the provision in a gas-fired urnace for controlling the level of excess air in a manner which ~akes into account ~he use o~ varlable length vent~.
Still another object of the prese~lt invention i~ the provision for con~rolling a variable speed motor ~o as to maintain desirable levels of excess air when operating with either ~ single or multi-stage sys~em.
- - -~L~f~
Yet another object of the present invention is the provision in a gas-fired furnace for an excess air control system which is economical to manufacture and effective in use.
These objects and advantages become more readily apparenk upon reference to the following description when ~akèn in conjunction with the appended drawings.
Summary of the Inventio_ Briefly, in accordance with one aspect of the invention, there is provided in the furnace heat exchanger, at least one pressure switch, which is responsive at a pressure level commensurate with a desired theoretical level of excess air ~hen operating in a firing condition. During the purging cycle, the inducer motor is accelerated to increase the pressure in the heat exchanger such that the pressure switch closes when its make point is reached.
As the switch closes, the inducer motor speed is sensed and recorded by a microprocessor. After purging, the furnace is fired and the inducer motor is allowed to stabilize. After a short period of time the inducer motor speed is reduced to a level which is based on the recorded motor speed and on a formula derived from data experimentally obtained from an exemplary operating system demonstrating the desired excess air level under high fired conditions.
In this way, the pressure switch is used during pre-ignition operation to obtain a motor speed which can be subsequently applied to obtain the desired operating speed of the inducer motor under firing conditions.
In accordance with another aspect of the invention, there are provided in the furnace heat exchanger, a pair of pressure switches, with a low pressure switch being responsive at a pressure level commensurate with the desired theoretical level of excess air when operating in a low flring condition, and a high pressure switch that is responsive at a pressure ~L~8~4g~
3 a commensurate with a desired theoretical excess air level when operating in a high firing condition. During the purging cycle the inducer motor is accelerated to increase the pressure in the heat exchanger such that the low pressure and high pressure switches close in succession. As the switches close, the inducer motor speeds are sensed and recorded b~ a microprocessor. A ratio is then calculated between the speed at which the low pressure switch closed and that at which the high pressure switch closed. This ratio is then recorded for subsequent application. After purging, the furnace is fired at the high firing rate and the inducer motor is allowed to stabilize. After a short period of time the inducer motor speed is reduced to a level which is based on the recorded motor speed and on a formula derived from data experimentally obtained from an exemplary operating system demonstrating the desired excess air level under variable high fire operating conditions. After a short period of time, the furnace transitions to a low fire operating condition with the inducer motor speed being reduced to a speed which is calculated by multiplying the previous high fire motor speed by the ratio which was previ-ously calculated. In this way, the pressure switches are used during pre-ignition operation to obtain a ratio which can be subsequently applied to obtain the desired operating speed of the inducer motor for low fire operation.
In the drawings as hereinafter described, a preferred embodiment is depicted. However, various other modifica-tions and alternate constructions can be made thereto without departing from the true spirit and scope of the invention.
8X14~3 Brief Description of the Drawin~
Figure 1 is a perspective view vf a gas furnace having the present invention incorporated therein.
Figure 2 is a schemstic illustration of the two installed pressure switches thereof as applied to the heat exchanger system.
Figures 3-5 are graphic illustrations of ~hAnges in the inducer motor speeds during eypical cycles of operation.
Description of the Preferred Embodim nt Referring now to Figure 1, there is shown 8 furnace of the general type with which the present invention can be employed. A burner assembly 11 communicates with a burner box 12 of a primary heat exchanger 13. Fluidly connected at the other end of the primary heat exchanger 13 is a condensing heat exchanger 14 whose discharge end is luidly connected to a collector bo~ 16 and an exhaust vent 17. In operation, a gas valve 18 me~ers the flow of gas to the burner assembly 11 where combustion air from the air inlet 19 is mixed and ignited by the ignition assembly 21. The hot gas is then passed through the primary heat exchanger 13 and the condensing heat exchanger 14 as shown by the arrows. The 2.5 relatively cool exhaust gases then pass through the collector box 16 and the exhaust vènt 17 to be vented to the atmosphere~ while the condensate flows from the collector bo~
16 through a condensate drain line 22 from where it is ~uitably drained to a sewer connection or the like. Flow o~
the combustion air into the air inlet 19, ~hrough ~he heat exchangers 13 and 14, and the exhaust vent 17 is enhanced by a draft inducer blower 23 which i8 driven by a motor 24 in response to control signals from the microprocessor.
The household air is drawn into a blower 26 which is driven by a drive motor 27, ~gain in response ~o signals received from the microprocessor. The discharge air from the blo~er 26 passes over the condensing heat exchanger 14 and the primary heat cxchsnger 13, in coun~erflow relationship wi~h the hot combustion gases, to thereby heat up the household air, which then flows from the discharge opening 28 to thc duct sy6tem within the home.
The microprocessor mentioned hereinaboYe i8 contained in the microprocessor control assembly 29. In re&ponse to electrical sign~ls from the thermos~at, and from other ~ignals to be ~iscussed hereinafter, the microprocessor control assembly 29 operates to control the inducer motor 24 and the blower motor 27 in such a way as to promote an efficient combustion process at two different firing rates.
- To aid in the control of excess air, a psir of pressure ~witches 31 and 32 are placed across burner box 12 and the collector box 16, respectively, so ~s to permit the me~surement o~ the pressure drop across ~he heat exchanger system. The switches 31 and 32 are mechanically connected within the system to sense the heat exchanger pressure drop shown in Figure 2.
As will be seen, a burner box tube 33 leads from the pressure tap 36 and a collector box tube 34 leads from the pressure tap 37. Fluidly connected therebetween, in parallel relationship, are the low pressure switch 31 and high pressure switch 32. These switches are calibrated to make, or close, at ~pecific pressure differentials as determined in a manner which will be more fully described hereinafter.
Switche~ that ha~e been found satisf~ctory for use in this manner are commercially available from Tridelta a3 part numbers FS 6003-250 (high pressure) ~nd FS 6002-249 (low pressurc).
~ -- \
o Since ~he system is normally oper~tin~ under negative pressure conditions, it is nece~s~ry to fluidly connect the vent of gas valve 18 with tube 38 to tubes 33 ana 39 via a "T" fit~ing 40 so as to reference low pressure switch 31, high pressure switch 32, and gas valve 18 to the negative pressure inherent in burner bo~ 12 while inducer motor ~4 i8 in operation.
Since the pressure drop across ~he heat exchangers is indicative of the level of excess air in the combustion 8y8tem, the low and high pressure switches 31 and 32 are used to determine when the level of e~cess air falls below the minimum desired theoretical levels for low and high firing conditions, respectively. For example, the low pressure switch 31 is so calibrated that it will close at the point when the excess ~ir level is equal to the desired theoretical value for low firing conditions. At that time, the closing of the switch causes ~ signal to be transmitted to the microprocessor, which in turn initiates a ~ensing and recording of the inducer motor speed, RPM 1. Similarly, as the speed of the inducer motor is increased, the level of excess air is increa~ed until it fin~lly reaches the desired theoretical value for high firing conditions, at which time the high pressure switch 32 closes and a signal is sent to the microprocessor. The inducer motor speed is again sensed and recorded at RPM 2. These speeds RPM 1 and RPM 2 are then mathematically altered to obtain the. desired motor speeds in accordance with the present lnvention.
Referring now to Figure 3, a typical cycle of opera~ion will be described. Upon a call for heat, the contral checks the status of the high and low pres~ure switche6 32 and 31. If both of the switches are open as they 6hould be, then the inducer motor is accelerated unti~ the pressure drop equals Pl, at which time the low pressure switch 31 is closed and the inducer motor speed RPM 1 is recorded. The inducer motor 8~ ~ 4 speed is allowed to continue to accelerate until ~he pressure drop equals P~, at which time th~ high pressure switch 32 cloaes and the inducer motor speed RPM 2 i~ recorded. The microprocessor control 29 then calculates the ra~io of the inducer ~peeds at low and high firing switch clo6ure polnt~
as follow6:
RATIO = RP~ 1 ~Eq. 1 The RATIO is then recorded for subsequent appllcation.
Afeer the high pressure switch 32 clo~es, the system undergoes a vent purge and the pilot is ignited by the furnace control. Shortly ater the pilot proves, and the main burnel-s ignite, the control then calculates RPM 4 using RPM 2 as wlll be described more fully hereinafter, after wh~ch it reduces the inducer motor speed to RPM 4.
20 It will be understood that when ignition occurs, the bul~;
temperature of the heat exchange sy~tem increases and th~
bulk density decreases. This, in turn causes a substanti&l increase in the pressure drop as shown in Figure 3. In order to reduce the pressure drop to the level at which RPM 2 was sensed the speed of the inducer motor must be reduced accordingly. However, it is not obvious as to how much that speed can be reduced. The various factors that are involved include: the difference in temperature and density between flue gas and air, the gas valve opening characteristics, and the length of the system vent.
In order ~o determine nominal operating point8 for various 6ystems a pressure drop P5 commensurate with desired theoretical level of excess air was used An exemplary 35 8ystem was experimentally run ur.der various operatin~
condi~ions (i.e., wanm and cold s~rts for each of minimum 1.40 and maximum length vent conditions), with RPM 1 - RPM 4, as well as the heat exchanger pres~ure drop (HXDP), being recorded. The resulting data was thQn analyzed and modified to make the heat exchan~er pressure drop repeatable from cycle to cycle for minimum and maximum vent lengths. For thls purpose, a nominal high firing rate heat exchanger pre~sur~ drop of .72 inches w.c. was used. Thus, where the variation from this nominal value was above a predetermined threshold, the following equation was ~pplied to correct the RPM 4 v~lues:
RPMCor = RPM x ~ ~YDP (Eq. 2) Taking the average of the experimental data o obtained, the corrected RPM 4 values were determined to be related to RP~i 2 values, for minimum and maximum vent condi~ions, as shown ir.
Table I.
TABLE I
RP~l 2 ~PM 4 Min. Ver.t 2574 2469 ~ax. Vent 3429 3124 As6uming now, a linear relationshlp between minlmum and maximum vent conditions, a best fit straight line equation using RP~ 2 and RP~l 4 values was determined as follows:
RP~ 4 = 497.11 + ~.766 x RPM 2) (Eq. 3) The speed of the inducer motor is therefore held at RP~l 4 until the end of the high firing period. ~en th~ hea~
exchanger has been war~ed up and the blower motor has been calibrated, the control the~ switches to a low firing -2 ~
condi~ion. This is accomplished by first calculating inducer motor speed RP~; 5 using inducer motor spePd RPM 4. The blower motor speed i8 ~hen reduced to a low firing speed and the furnace control reduces the Lnduc~r ~otor speed to ~PM 5, where:
RPM 5 = RPM 4 x RATIO (Eq. 4) Where: RATIO is defined as Equation 1, measured during vent purge.
As the inducer motor speed is reduced from RPM 4, thP high pressur~ fiwitch 32 opens and the high firing solenoid is de-energized. The inducer motor speed is thus reduced to RP~
5 and remains at that level during the period of low firing operation. If the thermostat is not sati6fied within a prescribDd ~eriod of time, the control will switch from a low - firin~ t~ 8 high firing condition. This i8 done by first accelerating the inducer mo~or until the high firing pressure switch closes and thereby energizes the high firing solenoid.
The speed of the inducer motor RPM 6 is then recorded. The blower th~n goes to high firing speed and the control increases the inducer motor speed to RPM 7. The rel~tionship between RPM 6 and XPM 7 values are experimentally determined in the same manner as described for RPM 2 and RPM 4 above, with the average RP~i's for a mlnimum and maximum vent l~ngths being shown in Table II.
TABLE Il RPM 6, 8 RPM- 7, 9 Min. Vent 2398 2482 Mfix. Vent 3044 3080 41~
Again, a~suming a rel&tionship between minimum and maximum vent conditions, a best fit straight line equation using RP~
6 and RPM 7 was deter~ined to be:
RPM 7 = 262.18 ~ (.926 x RP~l 6) tEq. 5) The inducer motor ~peed ifi then held constant at RPM 7 for high firing operation until such tim~ as the thermostat conditionæ are met or the system aga~n change6 to a low firing operating condition.
If, for example, an obstruction was temporarily placed over the system vent, the pressure drop would be reduced to the point where the high pressure switch 32 would open9 causing the high firing ~olenoid to be de-energized. This is neces6ary because of the reduced combustion airflow as shown in Figure 5. The control then ¢auses the inducer motor speed to be increased until the high pressure swi~ch 32 reclo~es and re-energizes the high fire solenoid. At tha~ time, the inducer motor speed RPM 8 is recorded and the furnace control increases the inducer motor speed to RPM 9 where:
RPM 9 = 262.18 ~ (.926 x RPM 8) (Eq. 6) As will be ~een, ~he inducer motor speed RPM 9 is determined as a function of the speed RPM 8 with the use of the same mathematical relationship found between RP~ 6 and RPM 7 as expressed in Equation 5.
It will be understood that throughout the operation described hereinabove, controlling limits are operative in the various operating modes, and the relevant conditions are monitored such that if the limits are exceeded, a failure is lndica~ed and the cycle is readjusted accordingly. For e~ample, during the period of initial acceleration tO RPM 1 as shown in Figure 3, if either the low pressure switch does not close 8~ 1 4 within a prescr$bed period of time or the RPM 1 value i8 outside its prescribed limits, a fault is signalled7 the unit shut6 down and tries again. If the high pressure switch closes before the low pressure switch close~, a fault i9 signalled and the unit locks out. Similar limit8 ~nd modified operating modes are provided during the other phases of operation to ensure that the system is operating within the intended parameters.
1~ While ~h~ present invention has been described in terms of use with a two stage 8y8tem, it should be understood that certain aspects thereof can just as well be used with a single or other multi stage systems. For example, while the Equations 3, 4, 5 and 6 have been applied to obtain the desired inducer motor operatin~ speeds for high firing conditions in a two stage system, they are equally applicable for use in de~ermining the inducer motor speeds for operation under firing conditions in a single or o~her multi-stag~
systems.
It will be understood that the pres~nt invention has been described in ~erms of a preferred embodlment. However, it may take on any number of other forms while remaining within the scope and intent of the invention.
Claims (14)
1. In a gas furnace of the type having a heat exchanger and a variable speed inducer motor, an improved method establishing a desired level of excess air comprising the steps of:
using empirical data obtained from a gas furnace operating under selective variable conditions, establishing a calibration factor for obtaining a desired excess air level for a furnace of nominal design characteristics;
providing a pressure switch that is responsive to a selected pressure drop level in the heat exchanger, said pressure drop level being selected so as to be commensurate with a theoretically desired excess air level under firing operating conditions;
accelerating the variable speed inducer motor until said pressure switch closes and recording the motor speed at that time; and applying said calibration factor to said recorded motor speed to obtain a desired induced motor speed.
using empirical data obtained from a gas furnace operating under selective variable conditions, establishing a calibration factor for obtaining a desired excess air level for a furnace of nominal design characteristics;
providing a pressure switch that is responsive to a selected pressure drop level in the heat exchanger, said pressure drop level being selected so as to be commensurate with a theoretically desired excess air level under firing operating conditions;
accelerating the variable speed inducer motor until said pressure switch closes and recording the motor speed at that time; and applying said calibration factor to said recorded motor speed to obtain a desired induced motor speed.
2. A method as set forth in claim 1 including the further step of maintaining said desired motor speed to obtain the desired level of excess air.
3. In a gas furnace of the type having a heat exchanger and variable speed inducer motor, apparatus for maintaining a desired excess air level during fired operating conditions comprising:
means for sensing when the pressure drop across the heat exchanger reaches a predetermined level while accelerating the inducer motor during purging operating conditions;
means for sensing and recording the actual inducer motor speed when said predetermined level is reached; and means for calculating, as the function of said actual inducer motor speed and as the function of performance data experimentally obtained, a desired inducer motor speed for operation under fired operating conditions.
means for sensing when the pressure drop across the heat exchanger reaches a predetermined level while accelerating the inducer motor during purging operating conditions;
means for sensing and recording the actual inducer motor speed when said predetermined level is reached; and means for calculating, as the function of said actual inducer motor speed and as the function of performance data experimentally obtained, a desired inducer motor speed for operation under fired operating conditions.
4. Apparatus as set forth in claim 3 wherein said predetermined level is that level which is commensurate with a theoretical desired excess air level under fired operating conditions.
5, Apparatus as set forth in claim 3 wherein said means for sensing and recording comprises a microprocessor.
6. In a gas furnace of the type having a heat exchanger and a variable speed inducer motor, a method for maintaining a desired excess air level comprising the steps of:
while accelerating the inducer motor during purging operation, sensing when pressure drop across the heat exchanger reaches a predetermined level;
sensing and recording the actual inducer motor speed when said predetermined level is reached; and calculating as a function of said actual motor speed and as a function of performance data experimentally obtained, a desired inducer motor speed for operation under fired operating conditions.
while accelerating the inducer motor during purging operation, sensing when pressure drop across the heat exchanger reaches a predetermined level;
sensing and recording the actual inducer motor speed when said predetermined level is reached; and calculating as a function of said actual motor speed and as a function of performance data experimentally obtained, a desired inducer motor speed for operation under fired operating conditions.
7. A method as set forth in claim 6 wherein said predetermined level is determined as being commensurate with a theoretically desired excess air level under fired operating conditions.
8. In a furnace of the type having a two stage gas valve, a two stage firing rate, a heat exchanger, and a variable speed inducer motor, a method of controlling excess air comprising the steps of:
providing a low pressure switch that is responsive to a selected first pressure drop level in the heat exchanger, said first pressure drop level being selected so as to be commensurate with a theoretically desired excess air level when operating the furnace in a low fire condition;
providing a high pressure switch that is responsive to a selected second pressure drop level in the heat exchanger, said second pressure drop level being selected so as to be commensurate with a theoretically desired excess air level when operating the furnace in a high fire condition;
accelerating the variable speed inducer motor until the low pressure switch closes, and sensing and recording a first motor speed at that time;
further accelerating the variable speed inducer motor until the high pressure switch closes, and sensing and recording a second motor speed at that time;
computing and recording the ratio of said first and second motor speeds;
operating the system in a high fire condition with the inducer motor operating at a desired third motor speed;
and transitioning to low fire operation, with the speed of the inducer motor being reduced to a fourth speed which is calculated by multiplying said third speed by said ratio.
providing a low pressure switch that is responsive to a selected first pressure drop level in the heat exchanger, said first pressure drop level being selected so as to be commensurate with a theoretically desired excess air level when operating the furnace in a low fire condition;
providing a high pressure switch that is responsive to a selected second pressure drop level in the heat exchanger, said second pressure drop level being selected so as to be commensurate with a theoretically desired excess air level when operating the furnace in a high fire condition;
accelerating the variable speed inducer motor until the low pressure switch closes, and sensing and recording a first motor speed at that time;
further accelerating the variable speed inducer motor until the high pressure switch closes, and sensing and recording a second motor speed at that time;
computing and recording the ratio of said first and second motor speeds;
operating the system in a high fire condition with the inducer motor operating at a desired third motor speed;
and transitioning to low fire operation, with the speed of the inducer motor being reduced to a fourth speed which is calculated by multiplying said third speed by said ratio.
9. A method as set forth in claim 8 and including the step of calculating said desired third motor speed as a function of said second motor speed.
10. In a furnace of the type having a heat exchanger, a two stage firing rate, and a variable speed inducer motor, an excess air control apparatus, comprising:
a low pressure switch disposed across the heat exchanger and having a design threshold to close at a first pressure drop commensurate with a theoretically desired excess air level for operation under low fire conditions;
a high pressure switch disposed in the heat exchanger and having a design threshold to close at a second pressure drop commensurate with a theoretically desired excess air level for operation under high fire conditions;
means for sensing the two motor speeds when the inducer motor is accelerated to cause said low pressure and high pressure switches to close in succession;
means for calculating and storing a ratio of said two motor speeds;
means for arriving at a desired motor speed for high fire operation and for obtaining and recording said high fire speed; and means for applying said ratio to said high fire speed to obtain a desired low fire speed.
a low pressure switch disposed across the heat exchanger and having a design threshold to close at a first pressure drop commensurate with a theoretically desired excess air level for operation under low fire conditions;
a high pressure switch disposed in the heat exchanger and having a design threshold to close at a second pressure drop commensurate with a theoretically desired excess air level for operation under high fire conditions;
means for sensing the two motor speeds when the inducer motor is accelerated to cause said low pressure and high pressure switches to close in succession;
means for calculating and storing a ratio of said two motor speeds;
means for arriving at a desired motor speed for high fire operation and for obtaining and recording said high fire speed; and means for applying said ratio to said high fire speed to obtain a desired low fire speed.
11. An excess air control apparatus as set forth in claim 10 wherein said means for calculating and storing said ratio is a microprocessor.
12. An excess air control apparatus as set forth in claim 10 and including means for maintaining the inducer motor speed at said desired low fire speed during periods of low fire operation.
13. A method of controlling the level of excess air in a furnace of the type having a heat exchanger, a two stage firing rate, and a variable speed inducer motor, comprising the steps of:
establishing low and high fire heat exchanger pressure drops that are desirable for respective low and high fire operation;
while accelerating the variable speed inducer motor, sensing the pressure drop across the heat exchanger and sensing and recording the respective low and high motor speeds when said low and high fire heat exchnager pressure drops are reached;
calculating the ratio of said low and high motor speeds;
calculating a desired high motor speed for operation under stable high fire conditions; and calculating a desired low fire motor speed by multiplying said high fire speed by said ratio;
establishing low and high fire heat exchanger pressure drops that are desirable for respective low and high fire operation;
while accelerating the variable speed inducer motor, sensing the pressure drop across the heat exchanger and sensing and recording the respective low and high motor speeds when said low and high fire heat exchnager pressure drops are reached;
calculating the ratio of said low and high motor speeds;
calculating a desired high motor speed for operation under stable high fire conditions; and calculating a desired low fire motor speed by multiplying said high fire speed by said ratio;
14. A method as set forth in claim 13, wherein said recording and calculating steps are accomplished with the use of a microprocessor.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US06/908,474 US4729207A (en) | 1986-09-17 | 1986-09-17 | Excess air control with dual pressure switches |
| US908,476 | 1986-09-17 | ||
| US06/908,476 US4703747A (en) | 1986-09-17 | 1986-09-17 | Excess air control |
| US908,474 | 1992-07-06 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1282140C true CA1282140C (en) | 1991-03-26 |
Family
ID=27129501
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000544663A Expired - Lifetime CA1282140C (en) | 1986-09-17 | 1987-08-17 | Excess air control |
Country Status (5)
| Country | Link |
|---|---|
| AR (1) | AR240096A1 (en) |
| AU (1) | AU583745B2 (en) |
| CA (1) | CA1282140C (en) |
| DE (1) | DE3731318A1 (en) |
| GB (1) | GB2196152B (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| AT396829B (en) * | 1990-12-27 | 1993-12-27 | Vaillant Gmbh | METHOD FOR INITIATING THE COMBUSTION |
| DE102016117323B3 (en) * | 2016-09-14 | 2017-11-02 | Valeo Thermal Commercial Vehicles Germany GmbH | Method for keeping constant the combustion air mass flow supplied to the burner chamber of a mobile heater and heating device operating according to such a method |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE2925031A1 (en) * | 1979-06-21 | 1981-01-08 | Wilhelm Emmerich | Control for central heating boiler - keeps flue gas pressure constant and controls gas flow |
| JPS6047497B2 (en) * | 1981-05-25 | 1985-10-22 | 東プレ株式会社 | Air volume control device for central air conditioning equipment |
-
1987
- 1987-08-17 CA CA000544663A patent/CA1282140C/en not_active Expired - Lifetime
- 1987-09-04 GB GB8720780A patent/GB2196152B/en not_active Expired - Lifetime
- 1987-09-11 AR AR308692A patent/AR240096A1/en active
- 1987-09-14 AU AU78377/87A patent/AU583745B2/en not_active Ceased
- 1987-09-17 DE DE19873731318 patent/DE3731318A1/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| AU583745B2 (en) | 1989-05-04 |
| DE3731318C2 (en) | 1992-01-30 |
| AU7837787A (en) | 1988-04-14 |
| GB2196152B (en) | 1990-08-01 |
| AR240096A1 (en) | 1990-01-31 |
| GB8720780D0 (en) | 1987-10-14 |
| GB2196152A (en) | 1988-04-20 |
| DE3731318A1 (en) | 1988-04-07 |
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