Control of a furnace

Claim 2

Apparatus for controlling the amount of oxygen supplied to a combustion zone in a furnace comprising: means for establishing a first signal representative of the calculated efficiency of said furnace during a first time period; means for establishing a second signal representative of the actual oxygen concentration in the furnace flue gas during said first time period; means for establishing a third signal representative of the calculated efficiency of said furnace during a second time period, wherein said second time period is later in time then said first time period and wherein the amount of oxygen supplied to said combustion zone in said furnace has been changed after said first time period; means for establishing a fourth signal representative of the actual oxygen concentration in said furnace flue gas during said second time period; means for establishing a fifth signal representative of the desired oxygen concentration in said furnace flue gas during a third time period in response to said first, second, third and fourth signals, wherein said third time period is later in time then said second time period; means for establishing a sixth signal representative of the most current measurement of the actual oxygen concentration in said furnace flue gas; means for comparing said fifth signal and sixth signal and for establishing a control signal which is responsive to the difference between said fifth signal and said sixth signal; and means for manipulating the amount of oxygen supplied to said combustion zone during said third time period in response to said control signal.

Claim 3

Apparatus in accordance with claim 1 wherein said first signal is the average of a plurality of furnace efficiencies calculated periodically during said first time period, wherein said second signal is the average of a plurality of actual oxygen concentrations in the furnace flue gas measured periodically during said first time period, wherein said third signal is the average of a plurality of furnace efficiencies calculated periodically during said second time period, and wherein said fourth signal is the average of a plurality of actual oxygen concentrations in said furnace flue gas measured periodically during said second time period.

Claim 4

Apparatus in accordance with claim 2 wherein a fuel is provided to said furnace with the combustion of said fuel being utilized to supply heat to a fluid stream flowing through said furnace and wherein said means for establishing said first signal and said third signal comprises: means for establishing a plurality of seventh signals representative of the heat absorbed by said fluid stream in said furnace per unit time for different time periods during said first time period; means for establishing a plurality of eighth signals representative of the heat absorbed by said fluid stream in said furnace per unit time for different time periods during said second time period; means for establishing a plurality of ninth signals representative of the heat provided to said furnace by the combustion of said fuel per unit time for different time periods during said first time period which correspond to the different time periods for the establishing of said plurality of seventh signals; means for establishing a plurality of tenth signals representative of the heat provided to said furnace by the combustion of said fuel per unit time for different time periods during said second time period which correspond to the different time periods for the establishing of said plurality of eighth signals; means for dividing corresponding ones of said plurality of seventh signals by corresponding ones of said plurality of ninth signals to establish said first signal and for dividing corresponding ones of said plurality of eighth signals by corresponding ones of said plurality of tenth signals; and means for averaging the results of the division of said plurality of seventh signals by said plurality of ninth signals to establish said first signal and for averaging the results of the division of said plurality of eighth signals by said plurality of tenth signals to establish said third signal.

Claim 5

Apparatus in accordance with claim 3 wherein means for establishing a particular one of said plurality of seventh and eighth signals comprises: means for establishing an eleventh signal representative of the flow rate (F p ) of said fluid stream; means for establishing a twelfth signal representative of the specific heat (C p ) of said fluid stream; means for multiplying said eleventh signal by said twelfth signal to establish a thirteenth signal representative of (F p ) (C p ); means for establishing a fourteenth signal representative of temperature T 0 of the fluid stream as said fluid stream flows from said furnace; means for establishing a fifteenth signal representative of the temperature T i of said fluid stream before said fluid stream enters said furnace; means for subtracting said fifteenth signal from said fourteenth signal to establish a sixteenth signal representative of (T 0 -T i ); and means for multiplying said thirteenth signal by said sixteenth signal to establish a particular one of said plurality of seventh and eighth signals.

Claim 6

Apparatus in accordance with claim 4 wherein means for establishing a particular one of said plurality of ninth and tenth signals comprises: means for establishing a seventeenth signal representative of flow rate (F 9 ) of said fuel to said furnace; means for establishing an eighteenth signal representative of the BTU content (BTU g ) of a unit volume of said fuel; and means for multiplying said seventeenth signal by said eighteenth signal to establish a particular one of said plurality of ninth and tenth signals.

Claim 7

Apparatus in accordance with claim 1 additionally comprising: means for establishing a seventh signal representative of the most current measurement of the flow rate of said fuel; lag means; means for providing said seventh signal through said lag means to thereby establish an eighth signal; delay means; means for passing said eighth signal through said delay means to thereby establish a ninth signal; means for subtracting said ninth signal from said eighth signal to establish a tenth signal; means for establishing an eleventh signal representative of the volume of air which must be provided to said furnace per unit volume of said fuel; means for multiplying said tenth signal and said eleventh signal to establish a twelth signal; and means for biasing said control signal with said twelfth signal.

Claim 8

Apparatus in accordance with claim 6 wherein a damper is utilized to manipulate the flow of oxygen to said combusition zone and wherein said control signal biased with said twelfth signal is scaled so as to be representative of the change of position of said damper required to provide an actual oxygen concentration in said furnace flue gas substantially equal to the desired oxygen concentration represented by said fifth signal.

Claim 9

Apparatus in accordance with claim 7 additionally comprising: means for establishing a thirteenth signal representative of the most current measurement of the pressure in said furnace; means for establishing a fourteenth signal representative of the maximum allowable pressure in said furnace; means for comparing said thirteenth signal and said fourteenth signal and for establishing a fifteenth signal which is responsive to the difference between said thirteenth signal and said fourteenth signal, wherein said fifteenth signal is scaled so as to be representative of the change of position of said damper which will maintain said actual pressure substantially equal to the maximum allowable actual pressure; low select means; means for providing said fifteenth signal and said control signal biased by said twelfth signal to said low select means, said low select means selecting the lower of said thirteenth signal and said control signal biased by said twelfth signal to establish a sixteenth signal; and means for manipulating the position of said damper in response to said sixteenth signal.

Claim 12

A method in accordance with claim 9 wherein said first signal is the average of a plurality of furnace efficiencies calculated periodically during said first time period, wherein said second signal is the average of a plurality of actual oxygen concentrations in the furnace flue gas measured periodically during said first time period, wherein said third signal is the average of a plurality of furnace efficiencies calculated periodically during said second time period, and wherein said fourth signal is the average of a plurality of actual oxygen concentrations in said furnace flue gas measured periodically during said second time period.

Claim 13

A method in accordance with claim 10 wherein said first and second signals establish a point on a plot of furnace efficiency as a function of oxygen concentration in said furnace flue gas, wherein said third and fourth signals establish a second point on said plot, and wherein said step of establishing said fifth signal comprises the steps of: determining the slope of a line between the point determined for said time T 1 and the point determined for the time T 2 on said plot; increasing the desired oxygen concentration during said time period T 2 by a desired incremental value to establish said fifth signal if the slope of said line is positive; and decreasing the desired oxygen concentration during said time period T 2 by said desired incremental value to establish said fifth signal if the slope of said line is negative.

Claim 14

A method in accordance with claim 10 wherein a fuel is provided to said furnace with the combustion of said fuel being utilized to supply heat to a fluid stream flowing through said furnace and wherein said steps of establishing said first signal and said third signal comprises: establishing a plurality of seventh signals representative of the heat absorbed by said fluid stream in said furnace per unit time for different time periods during said first time period; establishing a plurality of eighth signals representative of the heat absorbed by said fluid stream in said furnace per unit time for different time periods during said second time period; establishing a plurality of ninth signals representative of the heat provided to said furnace by the combustion of said fuel per unit time for different time periods during said first time period which correspond to the different time periods for the establishing of said plurality of seventh signals; establishing a plurality of tenth signals representative of the heat provided to said furnace by the combustion of said fuel per unit time for different time periods during said second time period which correspond to the different time periods for the establishing of said plurality of eighth signals; dividing corresponding ones of said plurality of seventh signals by corresponding ones of said plurality of ninth signals to establish said first signal and for dividing corresponding ones of said plurality of eighth signals by corresponding ones of said plurality of tenth signals; and averaging the results of the division of said plurality of seventh signals by said plurality of ninth signals to establish said first signal and for averaging the results of the division of said plurality of eighth signals by said plurality of tenth signals to establish said third signal.

Claim 15

A method in accordance with claim 12 wherein establishing a particular one of said plurality of seventh and eighth signals comprises the steps of: establishing an eleventh signal representative of the flow rate (F p ) of said fluid stream; establishing a twelfth signal representative of the specific heat (C p ) of said fluid stream; multiplying said eleventh signal by said twelfth signal to establish a thirteenth signal representative of (F p ) (C p ); establishing a fourteenth signal representative of temperature T 0 of the fluid stream as said fluid stream flows from said furnace; establishing a fifteenth signal representative of the temperature T i of said fluid stream before said fluid stream enters said furnace; subtracting said fifteenth signal from said fourteenth signal to establish a sixteenth signal representative of (T 0 -T i ); and multiplying said thirteenth signal by said sixteenth signal to establish a particular one of said plurality of seventh and eighth signals.

Claim 16

A method in accordance with claim 13 wherein establishing a particular one of said plurality of ninth and tenth signals comprises the steps of: establishing a seventeenth signal representative of flow rate (F 9 ) of said fuel to said furnace; establishing an eighteenth signal representative of the BTU content (BTU g ) of a unit volume of said fuel; and multiplying said seventeenth signal by said eighteenth signal to establish a particular one of said plurality of ninth and tenth signals.

Claim 17

A method in accordance with claim 9 additionally comprising the steps of: establishing a seventh signal representative of the most current measurement of the flow rate of said fuel; lagging said seventh signal to thereby establish an eighth signal; delaying said eighth signal to thereby establish a ninth signal; subtracting said ninth signal from said eighth signal to establish a tenth signal; establishing an eleventh signal representative of the volume of oxygen which must be provided to said furnace per unit volume of said fuel; multiplying said tenth signal and said eleventh signal to establish a twelfth signal; and biasing said control signal with said twelfth signal.

Claim 18

A method in accordance with claim 15 wherein a damper is utilized to manipulate the flow of oxygen to said combusition zone and wherein said control signal biased with said twelfth signal is scaled so as to be representative of the change of position of said damper required to provide an actual oxygen concentration in said furnace flue gas substantially equal to the desired oxygen concentration represented by said fifth signal.

Claim 19

A method in accordance with claim 16 additionally comprising the steps of: establishing a thirteenth signal representative of the most current measurement of the pressure in said furnace; establishing a fourteenth signal representative of the maximum allowable pressure in said furnace; comparing said thirteenth signal and said fourteenth signal and establishing a fifteenth signal which is responsive to the difference between said thirteenth signal and said fourteenth signal, wherein said fifteenth signal is scaled so as to be representative of the change of position of said damper which will maintain said actual pressure substantially equal to the maximum allowable actual pressure; low select means; selecting the lower of said thirteenth signal and said control signal biased by said twelfth signal to establish a sixteenth signal; and manipulating the position of said damper in response to said sixteenth signal.