Methods Of And Means For Determining The Calorific Value Of Combustible Gases

Abstract

Methods of and means for measuring the calorific value of combustible gases wherein a mixture of a combustible gas and a combustion-supporting gas is burned in one or more flames, the temperature or temperatures of the burned gases being monitored and the volume ratio of the combustion-supporting gas to the combustible gas being adjusted so as to maintain said temperature or the average of said temperatures at substantially maximum; the volume ratio of said gases which produces said maximum temperature varying substantially directly with the calorific value of said combustible gas.

Parent Case Text

This application is a continuation-in-part of my copending application Ser. No. 21,410, filed Mar. 20, 1970, now abandoned.

Description

BACKGROUND OF THE INVENTION

The calorific value of a combustible gas has been defined as the quantity of heat in British Thermal Units (BTU) which is released when one standard cubic foot of gas is completely oxidized at a temperature of 60.degree. F. and any water produced by oxidation is in the liquid state. When the gas is a hydrocarbon or a mixture of hydrocarbons, the oxidation products for complete oxidation are carbon dioxide and water; and when one standard cubic foot of said gas is mixed with a sufficient quantity of oxygen at 60.degree. F. to completely oxidize said gas, oxidation is carried out and the products thereof, carbon dioxide and water, are cooled to 60.degree. F. and all water is condensed to liquid state, the total heat given off, including the heat transferred in cooling the said products and in condensing all of the water, is the calorific value of said gas. As defined in this manner, the calorific value is a function only of the chemical composition of the combustible gas, whereby it is possible to determine the calorific value when only the chemical composition of said gas is known.

Calorific value so defined is used extensively in industry as a measure of the quality of a gas or other fuel. IF a If is to be fed to a burner, the proper operation of the latter is often highly dependent upon calorific value whereby it is essential to control such value within narrow limits. Accordingly, it is common practice for suppliers and users of combustible gas to monitor the calorific value thereof.

Since the calorific value of a combustible gas depends only on its chemical composition, said value can be determined by a complete chemical analysis of the gas if the calorific value of each of its constituents is known; however, this method is time-consuming and impractical for continuously monitoring the calorific value. All standard methods for measuring calorific value involve mixing and burning a known volume of a combustible gas with an excess of oxygen-containing or combustion-supporting gas, transferring the resultant heat to a heat absorbing fluid and measuring the quantity of heat transferred. The conditions for these operations would ideally be the same as in the above definitions of calorific value. Any deviation from these conditions will cause the resultant heat transferred per standard cubic foot of combustible gas in the measurement to be different from the calorific value.

In practice, it is difficult to maintain the rigid conditions required for correct measurement, since the initial temperatures of the combustible and oxygen-containing gases are seldom, if ever, 60.degree. F. Also, the temperature of the combustion products after heat transfer is not 60.degree. F. and, usually, is higher than the initial temperature of the gases. The water produced is seldom condensed to the liquid state and the heat absorbing fluid never absorbs all of the heat transferred from the combustion products, since some heat is always lost by radiation and conduction.

Each of the foregoing deviations is a source of measurement error and correction thereof requires complicated, expensive apparatus and, often, special environmental control.

SUMMARY OF THE INVENTION

The basic method of this invention includes the following steps:

1. the combustible gas is mixed with dry air or other combustion-supporting or oxygen-containing gas;

2. the mixture is burned in one or more flames;

3. the temperature of these flames or burned gases is monitored;

4. the volume ratio of the gases is adjusted so as to maintain said temperature at substantially maximum; and

5. said volume ratio of said gases which produces said maximum temperature is measured.

This measured volume ratio will be referred to hereinafter as "critical combustion ratio" and may be defined as that volume ratio of the gases which produces maximum flame temperature when said gases are premixed and burned. It has been found that the critical combustion ratio varies substantially directly with the calorific value of the combustible gas and that a very accurate indication of calorific value can be obtained by measuring said critical combustion ratio.

The physical nature of the critical combustion ratio may be more readily understood upon consideration of known flame phenomena. It is well known that the adiabatic temperature of a flame that is produced by burning a mixture of combustible and combustion-supporting gases is a function only of the initial temperature, pressure and chemical composition of the mixture and that said adiabatic temperature would be reached in the combustion zone of the flame only if there were no heat losses from the burning gases. Also, it is well known that if the ratio of combustion-supporting gas to combustible gas is varied in the initial mixture, the adiabatic flame temperature varies and that a critical ratio between the gases exists at which said adiabatic flame temperature is at maximum. If the initial mixture contains less combustion-supporting gas than required to achieve this critical ratio, the adiabatic flame temperature is lower and this is generally due to there being insufficient oxygen to achieve complete combustion whereby less heat is released. In the event that the initial mixture contains combustion-supporting gas in excess of that required to achieve the critical ratio, the adiabatic flame temperature is again lower and is generally due to the necessity of heating such excess. Thus, the critical ratio between the gases is equal to the critical combustion ratio defined hereinbefore.

Objects of the invention are to provide improved methods of and means for measuring the calorific value of combustible gases which do not depend upon measuring the amount of heat released in combustion, which are not affected by the errors set forth hereinbefore, which are simple, which are capable of continuous simple operation and which are not affected by ambient temperature and other varying environmental factors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view illustrating an apparatus for carrying out one of the methods of my invention,

FIG. 2 is a view, similar to FIG. 1, of a modified apparatus for carrying out another method of the invention,

FIG. 2A is a diagrammatic view illustrating the time-dependent behavior of the total flow of air to the burner of FIG. 2,

FIG. 2B is a diagrammatic view illustrating the effect of continual small perturbations in the air flow rate on the burned gas temperature,

FIG. 3 is a view, similar to FIG. 1, of another modified apparatus for carrying out a third method of the invention,

FIG. 4 is a view, similar to FIG. 1, of a further modified apparatus for carrying out a fourth method of the invention,

FIG. 4A is an enlarged elevational view, taken on the line 4A--4A of FIG. 4, showing the recording chart and stylus,

FIG. 5 is a diagrammatic view of the components of the controller and thermocouple of FIG. 1,

FIG. 6 is a view, similar to FIG. 5, of the components of the controller and thermocouple of FIG. 2,

FIG. 7 is a view similar to FIG. 5, of the components of the controller and thermocouple of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred method of measuring the critical combustion ratio, i.e., the measured volume ratio between the combustible gas and dry air or combustion-supporting or oxygen-containing gas which produces the maximum flame temperature when said gases are premixed and burned in a flame, can be best understood by referring to FIG. 1 wherein a suitable apparatus 1 is illustrated diagrammatically. For convenience, the combustion-supporting or oxygen-containing gas will be referred to as "dry air" or "air" with the understanding that any suitable gas is included in this designation. Likewise, the combustible gas will be merely termed "gas."

The apparatus 1 includes a suitable flame using burner 2, which may have a top 3 of the Meker type and which has a flame-supporting grid 4 at its outlet, a dry air inlet line 5 and a gas inlet line 6, both leading to the burner. A Y 7 connects the inner ends of the lines 5 and 6 to a common line or conduit 8 which functions as a mixing chamber for feeding the mixed gas and air to the burner 2. Suitable control valves 9 and 10 are mounted in the outer ends of the air and gas lines for accurately regulating the flow through said lines. Between the Y 7 and control valves 9 and 10, the lines 5 and 6 have have suitable metering devices, such as orifice flow meters 11 and 12, connected therein. As shown by the numeral 13, a large number of small flames or flamelets emanate from the grid 4 upon burning of mixed gas and air and for many carbon-containing fuels the carbon monoxide combustion product burns in a large flame 14 as it mixes with ambient air.

A thermocouple 15 is disposed within the carbon monoxide flame 14 or immediately above the flamelets 13 to provide an input signal to a controller 16 which is adapted to operate the air conrol valve 9 in such manner as to maximize the electrical signal from the thermocouple. As a result, the temperature of the burned gas measured by the thermocouple 15 and adiabatic flame temperature of the flamelets 13 are always at maximum. It is noted, however, that the inlet lines may be reversed, the dry air passing through the valve 10 and inlet line 6 and the gas passing through the valve 9 and inlet line 5. In this case, the controller 16 is adapted to operate the gas control valve 9 in such manner as to maximize the electrical signal from the thermocouple. The resulting burned gas temperature measured by the thermocouple 15 and the adiabatic flame temperature of the flamelets 13 are always at maximum. An electric or pneumatic signal FA, proportional to the air flow rate measured by the meter 11, is transmitted through a conductor 17 to a divider 18. Likewise, an electric or pneumatic signal FG, proportional to the gas flow rate measured by the meter 12, is transmitted to the divider 18 through a conductor 19. The divider is a suitable calculating device whose output is FA divided by FG or the critical combustion ratio between the gas and air.

Another method is illustrated by the apparatus of FIG. 2 which is similar to the apparatus 1 and wherein additional dry air or gas, preferably air, is added intermittently to the mixture of gas and air. In this embodiment of the invention, a modified controller 16' is interposed between the thermocouple and the air control valve or the gas control valve for imparting opening and closing movement to said valve. A common or mixing chamber line 20 is provided at the inner ends of the air and gas inlet lines 5 and 6 and has an additional air inlet line 21 communicating therewith between the burner 2 and Y 7. Mounted in the line 21 is a flow control valve 22 operated by a pulse controller 23 which is adapted to open the valve for brief periods of time at regular intervals.

As shown by the curve of FIG. 2A, the resulting total flow of air to the burner 2 has a time-dependent behavior. The pulse controller 23 operates continually so that the total air flow through the burner is the sum of the air flow through the air inlet valve 9 and a continuing series of air pulses through the valve 22, the amplitude of these air pulses being small compared with the air flow rate through said valve 9. The effect of the air pulses is to provide a continuing series of small perturbations on the total air flow rate through the burner 2. In the event that the temperature of the burned gases, as measured by the thermocouple 15, is not at maximum, the continual small perturbations in the air flow rate cause a continuing series of small perturbations in the temperature of the burned gases. This effect on the burned gas temperature is illustrated by the curves of FIG. 2B.

The first or uppermost curve of the latter depicts the conditions when the air flow rate through the air inlet valve 9 is in excess of that required for the burned gas temperature to be at maximum whereby said temperature is less than its maximum value. A small pulse of air through the valve 22 temporarily increases the air flow rate through the burner, thereby temporarily lowering the burned gas temperature further. This corresponds to the continuing series of air pulses producing a continuing series of negative temperature pulses shown in the first curve of FIG. 2B. When the control 16' senses these negative temperature pulses by means of the thermocouple 15, said controller operates the valve 9 so as to reduce the total air flow to burner 2.

The second curve of FIG. 2B depicts the situation when the air flow rate through the air inlet valve 9 is less than that required for the burned gas temperature to be at maximum. In this event, a small pulse of air through the valve 22 temporarily increases the air flow rate to the burner and thereby temporarily increases the burned gas temperature. This corresponds to the continuing series of air pulses producing a continuing series of positive temperature pulses. When the modified controller senses these positive temperature pulses through the thermocouple 15, said controller operates the valve 9 so as to increase the total air flow to the burner.

If the burned gas temperature is at maximum, then small perturbations in the total air flow have essentially no effect on this temperature. As illustrated by the last or lowermost curve of FIG. 2B, the burned gas temperature is not affected by the continuing series of air pulses (FIG. 2A). In this case, the controller 16' senses no temperature pulses and does not operate the valve 9. Accordingly, it is readily apparent that the controller operates the air inlet valve in such manner as to maintain the burned gas temperature at maximum.

Alternatively, it may be desirable to use air pulses through the valve 22 of sufficient magnitude to produce small negative temperature pulses when the burned gas temperature is at maximum. In this event, the modified controller is so designated that it operated the valve 9 only when the thermocouple 15 senses temperature pulses of a magnitude or sign different from those produced when the burned gas temperature is at maximum.

Again referring to FIG. 2, the electric or pneumatic signals FA and FG, proportional to the air and gas flow rates respectively measured by the meters 11 and 12, are transmitted by the conductors 17 and 19 to the divider 18 which produces an output of FA divided by FG or the critical combustion ratio between the gas and air.

A third method of measuring the critical combustion ratio, or calorific value of the gas, is shown by the apparatus circuit FIG. 3 which includes a second designs 2' similar or identical to the constant current source and having an identical or similar top 3' and grid 4' so as to produce identical small flames or flamelets 13', and for many carbon-containing fuels, a carbon monoxide flame 14'. A pair of Ys 27 and 27' and a pair of common or mixing chamber conduits 28 and 28' connect the inner end portions of the dry air and gas inlet lines 5 and 6 to the burners 2 and 2' respectively, whereby the air is supplied equally to both burners. A flow restriction orifice 29 is mounted in the gas inlet line 6 between the leg of the Y 27' leading to the common conduit 28' and burner 2' and the leg of the Y 27 leading to the common conduit 28 and burner 2 whereby the flow rate of gas through said conduit 28 to said burner 2 is slightly less than the gas flow rate through the said conduit 28' to said burner 2'. Although it is not necessary for the burners to be identical, it is essential that both burners have the same heat loss and that equal amounts of air are fed thereto. A pair of thermocouples 25 and 25' are disposed within the carbon monoxide flames 14 and 14' or above the flamelets 13 and 13' and are connected in parallel to a controller 26 which operates the gas inlet control valve 10 in such manner as to hold the electric signal from the thermocouples at a characteristic value. Except for the foregoing, this embodiment of the invention is identical to the apparatus 1 of FIG. 1.

The temperature difference between the thermocouples 25 and 25' is equal to the temperature of the thermocouple 25' above the flamelets 13' minus the temperature of the thermocouple 25 above the flamelets 13. When the ratio of the flow of dry air passing through the meter 11 to the flow rate of gas passing through the meter 12 is equal to the critical combustion ratio, then the adiabatic flame temperatures for the mixtures going to the two burners will be essentially the same. Due to the operation of the orifice 29, the dry air of air flow rate to gas flow rate in the conduit 28' leading to burner 2' is slightly lower than the critical combustion ratio and the ratio of dry air flow rate to gas flow rate in the conduit 28 leading to the burner 2 is slightly higher than the critical combustion ratio. Thus, the adiabatic flame temperatures of the mixtures being burned in the burners 2 and 2' are each slightly less than maximum and essentially the same. The average temperature of the burned gases in the two burners is at substantially maximum, and any charge in the flow rate will either lower the temperature of both flames or lower the temperature of one flame more than enough to offset any increase in the temperature of the other. Any difference between the temperatures measured by the thermocouples 25 and 25' is due to a difference in their relative position with respect to the flamelets 13 and 13' and this relative position is fixed. Thus, when the ratio of the flow of dry air passing through the meter 11 to the flow rate of gas passing through the meter 12 is equal to the critical combustion ratio, the temperature difference between the two thermocouples has a characteristic value which is independent of the composition of said gas.

The controller 26 is set so that it does not activate the valve 10 when the difference between the thermocouples is at this characteristic value. If the ratio of the air flow rate through the meter 11 to the gas flow rate through the meter 12 is greater than the critical combustion ratio, the temperature difference between the thermocouples 25 and 25' is greater than this characteristic value. In this event, the gaseous mixture in the burner 2 always contains a greater excess of air than the burner 2'. The thermocouples sense the resulting temperature difference between the two flames 14 and 14' and the resulting signal causes the controller 26 to operate the valve 10 so as to increase the total gas flow through the meter 12.

When the ratio of the flow rate of air passing through the meter 11 to the flow rate of gas passing through the meter 12 is less than the critical combustion ratio, the temperature difference between the thermocouples 25 and 25' is less than the aforesaid characteristic value. In this case, the gaseous mixture in burner 2' always contains a greater excess of gas than the burner 2. The thermocouples sense the resulting temperature difference between the flames and the resulting signal causes the controller 26 to operate the valve 10 so as to decrease the total gas flow through the meter 12. As described hereinbefore, the action of the controller is such as to maintain the total air flow rate through the meter 11 in a ratio to the total gas flow rate through the meter 12. This ratio is essentially equal to the critical combustion ratio which is produced by the divider 18 whose output is the electric or pneumatic signal FA divided by the electric or pneumatic signal FG.

A fourth method of measuring the critical combustion ratio and thereby the calorific value of the gas is illustrated diagrammatically in FIG. 4 wherein a dry air inlet line 30 and a combustible gas inlet line 31 are connected by a Y 32 and a common or mixing chamber conduit 33 to a burner 34 which may be substantially identical to the burner 2. Pressure regulators 35 and 36 are mounted in the air and gas inlet lines 30 and 31 for maintaining constant pressures in said lines. Inwardly of the pressure regulator 35, the air inlet line has a control valve 37 connected therein for providing a specific flow ratio of air to gas. The valve 37 includes a valve element 38 mounted on one end of an elongate valve stem or shaft 39 screwthreaded into the bonnet 40 of said valve, whereby rotation of the shaft imparts opening and closing movement to the valve element. Inwardly of the pressure regulator 36 a flow restriction orifice 41 is disposed in the gas inlet line. The pressure regulator 36 keeps the pressure drop across the orifice 41 constant, thereby achieving a constant flow rate of combustible gas in the conduit 31. Preferably, the burner 34 includes a Meker type top 42 and a grid 43 for producing a plurality of flamelets 44 and, for many carbon-containing fuels, a carbon monoxide flame 45.

Suitable bearings 47 and 47 rotatably support the shaft 39 and a recording chart support or disk 48 fixed on the free end of said shaft for rotation therewith. As best shown in FIG. 4A, a conventional circular chart 49 is adapted to be detachably mounted on the disk 48 for coaction with a stylus 50 which is pivotally connected to an actuator 51. The latter is activated electrically by an amplifier 52 which is energized by a thermocouple 53 disposed above the flamelets 44 and/or within the carbon monoxide flame 45. An electric motor 54 is provided for rotating the shaft in either direction and a cycling unit 55 is interposed between the motor and its power source for controlling the direction and extent of such rotation. The motor 54 has its drive shaft 56 operatively connected to the shaft 39 by coacting gears 57 and 58.

It is noted that each position of the chart 49 and it supporting disk 48 corresponds to a specific setting of the control valve 37 and, thus, to a specific flow rate ratio of dry air to gas. The stylus 50 indicates and records on the chart the temperature of the burned gases for each position of said chart and this temperature is characteristic of the specific flow rate ratio of dry air to gas corresponding to each such position. When it is desired to measure the critical combustion ratio of the gas, the motor 54 is actuated by the cycling unit 55 to rotate the shaft in first one direction and then in the other. This rotation may be a complete revolution or a portion thereof, such as 30.degree., in each direction. The stylus scribes twice, once for each direction of rotation. The shaft 39 is rotated through an angle or arc sufficient for one of its positions during such rotation to correspond to a specific flow rate ratio of dry air to gas which equals the critical combustion ratio.

The coaction of the rotating chart 49 and recording stylus 50 produces a graph of the burned gas temperature as a function of the ratio of the air flow rate to the gas flow rate. The ratio which produces the maximum temperature or critical combustion ratio can be read directly from the chart after the shaft has traversed its rotational path. If desired, the chart 49 may be printed so that the circular coordinate thereon reads directly in units of calorific value such as BTU/SCF, since caloric value is proportional to the critical combustion ratio. Also, it is manifest that the control valve 37 and orifice restriction 41 may be reversed in effect with the gas being fed through line 30 and the air through the line 31 so as to provide a specific flow rate of gas to air rather than air to gas. Manifestly, the first three methods (FIG. 1) are adaptable to the continuous recording of this method.

Although the controllers 16, 26 and 16' are not available as standard stock items, they may be constructed from stock items or readily built by competent instrument companies from available components. In each controller, the gas flow is controlled by a reversible direct current motor and in controllers 16 and 26, the power connections to the motor pass through a pair of relays which in turn are operated by an electric current from an electronic control circuit. The relays have different threshold current values at which they operate. If this current is above the threshold values of both relays, the latter are placed in such position that the power drives the motor in a direction which closes one of the inlet valves of the apparatus. If this current is below the threshold values of both relays, they are moved to a different position whereby the power drives the motor in the opposite direction. In the event this current is between the two threshold values, the relay positions are such that the motor does not operate. The input to the electronic control circuit is connected to the thermocouple wires. In the case of element 26, the electronic control circuit is an EMF or voltage to current converter which is a stock item.

Referring again to FIG. 5, the controller 16 of FIG. 1 comprises a reversible direct current motor 60 having the sprocket 61 of its drive shaft 62 meshing with a driven sprocket 63 fixed on a rotatable shaft or stem 64 for opening and closing the dry air inlet valve 9 or the gas inlet valve 10. The polarity of the power connections of the motor are arranged to be reversed in order to reverse the direction of rotation of the shaft of said motor and thereby open and close one of the valves. A pair of commercially available alarm switching units 65 and 66, such as Foxboro M/63R Electronic Consotrol Alarm Units, are connected in series by an electrical lead or conductor 69 and includes a pair of single pole double throw switches or relays 67 and 68, respectively, having their poles 70 and 71 connected by respective electrical conductors or leads 72 and 73 to the power connections of the reversible motor 60. The threshold value of each alarm unit is adjustale and is slightly higher in the unit 65 than in the unit 66. Each of the relays 67 and 68 have contacts A and B, with the contact B of the relay 67 and the contact A of the relay 68 connected by a positive electrical lead or conductor 74 to a power supply 76. A negative lead or conductor 75 electrically connects the power supply to the contact A of the relay 67 and to the contact B of the relay 68, whereby the motor shaft 62 is rotated in one direction when the poles 70 and 71 are engaged with the contacts A and in the opposite direction when said poles engage the contacts B to impart closing and opening movement to the valve 9 or 10.

The electronic control circuit of the controller 16 is constituted of the remaining elements of said controller and includes a double pole double throw electronically controlled switch or relay 77 electrically connected in a conductor 78 extending between the alarm unit 65 and a suitable constant current source 79 and controlled at its terminal 80 by the output signals of a differentiating circuit 81 which is connected directly to the thermocouple 15. It is noted that the design of the differentiating circit is subject to variation and such designa are well-known in this art. Also, the constatcurrent asource is electrically connected by a conductor or lead 82 to the alarm unit 68.

For imparting a series of positive current pulses to one set 83 of dual contacts of the switch 77, a pair of electrical leads or conductors 84 connect an oscillator 85 to said contacts. Another electrical oscillator 86 imparts a series of negative current pulses to a second set 87 of dual contacts through a pair of conductors or leads 88. Since the switch 77 has its double pole 89 interposed in the conductor 78, a constant current is delivered by the source 79 to the double pole of the electronic switch. A combined current is supplied to the units 65 and 66, due to said units being connected in series by the lead 69 and is composed of the constant current and either a series of positive or negative current pulses dependent upon whether the double pole 89 is engaged with the contacts 83 or 87. This combined current forms the output of the electronic control circuit and, at any given time, the output from either one or the other of the oscillators 85 and 86 is superimposed on a constant current from the source 79 and the latter current is between the threshold values of the alarm switching units.

When the EMF of the thermocouple or flame temperature is increasing with time, the output of the differentiating circuit 81 is such that the electronic switch is not activated or triggered and the double pole of the latter remains in the same position. In this event, the output of the electronic control circuit is a continual series of either positive or negative current pulses from the oscillator 85 or 86 so as to effect an intermittent opening or closing movement of the inlet valve due to the intermittent operation of the motor 60 and thereby continually increase the flame temperature. After the flame temperature has passed through maximum, the output signal of the differentiating circuit 81 reverses and activates or triggers the double pole 89 of the electronic switch and the other oscillator is connected to the electronic control circuit output so as to reverse the polarity or sign of the current pulses which form said output. The effect is to reverse the direction in which the inlet valve is moved whereby the flame temperature is caused to reapproach its maximum value.

If the current or output from the electronic control circuit is above the upper thresholds of the units 65 and 66, the poles 70 and 71 move into engagement with the respective contacts B of the relays 67 and 68 whereby the current from the power supply 76 flows through the conductors 74 and 72 and drives the motor shaft 62 in a direction imparting closing movement to the inlet valve. The poles of the relays engage the contacts A of said relays so that the current from the power supply flows through the conductors 94 and 93 so as to reverse the motor 60 and impart opening movement to the valve. Thus, the controller 16 functions to maintain the flame temperature at or near its maximum value.

As shown in FIG. 7, the controller 26 of the apparatus of FIG. 3 includes a reversible direct current motor 90 having the sprocket 91 of its drive shaft 92 meshing with a driven sprocket 93 fixed on a rotatable shaft or stem 94 for opening and closing the combustible gas inlet valve 10. The direction of rotation of the motor is adapted to be reversed by reversing the polarity of its power connections. A pair of alarm switching units 95 and 96, such as Foxboro M/63R Electronic Consotrol Alarm Units, are connected in series by a lead or conductor 99 and include a pair of single pole double throw relays or switches 97 and 98, respectively, having their respective poles 100 and 101 connected by electrical conductors or leads 102 and 103 to the power connections of the reversible motor 90. Each of the relays 97 and 98 have a pair of opposed contacts A and B, with the contacts A and B and B and A of the relays 97 and 98, respectively, connected by respective positive and negative electrical conductors or leads 104 and 105 to a power supply 106 whereby the motor shaft 92 is rotated in one direction when the poles 100 and 101 are engaged with the contacts A and in the opposite direction when the contacts B are engaged by said poles. Each of the units 95 and 96 has an adjustable threshold current at which its relay or switch is activated and this threshold current is slightly higher in the unit 95 than in the unit 96. The burner temperature difference, which is sensed by the thermocouples 25 and 25' and which corresponds to the critical combustion ratio or maximum flame temperature, is intermediate or between the temperature differences corresponding to the threshold values of the alarm units.

Electrical leads or conductors 107 and 108 respectively connect the units 95 and 96 to an electronic control circuit or converter 109, commercially available as a Foxboro EMF-To-Current Converter and preferably set at a current output range from 10 to 50 ma when the voltage input ranges between 1.5 and 3.5 mv. For producing a current which is a linear function of the thermocouple 25' temperature minus the temperature of the thermocouple 25, the converter 109 is connected directly to said thermocouples and this output current is conducted to the alarm switching units through the conductors 107 and 108. When the flow of the combustible gas is too low to maximize the flame temperature, the difference between the temperatures of the thermocouples is higher than the characteristic value which does not activate the valve 10 and the current supplied by the converter exceeds the threshold values of both units 95 and 96. The poles of the relays 97 and 98 engage the contacts B thereof so as to connect the motor 90 to the power supply 106 through the conductors 102-105 and thereby rotate said motor shaft 92 in a direction to open the valve 10 and increase the combustible gas flow, the electrical current flowing through the conductors 104 and 102 and said relay 97 from said power supply to said motor.

If the latter flow is too high to maximize the flame temperature, the difference between the temperatures of the thermocouples 25' and 25 is lower than the aforesaid characteristic value and the current resulting from the converter 109 is below the threshold value of each alarm unit. The pole of each relay is engaged with the contact A and the motor 90 is connected to the power supply 106 so that the electrical current flows through the conductors 104 and 103 as well as the relay 98 from said power source to said motor so as to drive the motor shaft 92 in an opposite or reverse direction and move the valve 10 toward its closed position for decreasing the flow of combustible gas. Since the threshold value of the unit 96 is lower than that of the unit 95, its pole 101 moves toward the contact B of the relay 98 before the pole 100 of the relay 97 moves toward its contact B upon the increasing of the current from the converter above said threshold values and said pole 100 moves toward the contact A of said relay 97 prior to the movement of said pole 101 toward the contact A of said relay 98 when said converter current decreases below said threshold values.

The operation of the controller 26 is such that the final value of the optimum ratio of air to combustible gas produces a temperature difference between the thermocouples and a resulting current from the converter 109 which activates only the unit 96 and not the unit 95, whereby the pole of the relay 98 engages its contact B and the contact A of the relay 97 is engaged by its pole. Consequently, both leads 102 and 103 to the motor are connected to and only to the same terminal of the power supply by the negative conductor 105, thereby rendering said motor inoperative and maintaining the combustible gas flow constant at its final desired value. Likewise, the motor 90 is inoperative when the positions of the poles 100 and 101 are reversed so as to engage the contact B of the relay 97 and the contact A of the relay 98 whereby the motor leads are connected and only to the same terminal of the power supply 106 by the positive conductor 104. Manifestly, the fluctuation of the current from the converter 109 in accordance with the thermocouple temperature difference provides the ratio of air to combustible gas at essentially that value which maximizes the flame temperature. It is noted that the controller 26 is of a preferred design, although other switching circuits can be utilized to control the combustible gas inlet valve, and that the important feature of this design is its three different states which depend upon the voltage output of the thermocuples 25 and 25' and which effect three modes of operation of the valve 10, namely, stationary, opening and closing movement. Of course, the air or combustion supporting gas inlet valve 9 may be controlled rather than the combustible gas inlet valve.

As shown in FIG. 6, the controller 16' of the apparatus of FIG. 2 includes a reversible direct current motor 110 having the sprocket 111 of its drive shaft 112 meshing with a driven sprocket 113 fixed on a rotatable shaft or stem 114 for opening and closing the air inlet valve 9 or combustible gas inlet valve 10. The direction of rotation of the motor is adapted to be reversed by reversing the polarity of its power connections. A double pole double throw electronic switch or relay 115 is electrically connected to the power input terminals of the motor through conductors or leads 116 and 117 and connected to the motor power supply 118 through electrical leads or conductors 119 and 120. The relay is controlled at its terminal 121 by the output signals of a pulse amplitude measuring circuit 122 which is connected directly to the thermocouple 15. It is noted that the design of the pulse amplitude measuring circuit is subject to variation and such designs are well-known in this art.

When the air flow rate is higher than required for maximum flame temperature, the thermocouple 15 senses a continuing series of negative temperature pulses. The input to the pulse amplitude measuring circuit 122 is a continuing series of negative voltage pulses produced by the thermocouple 15 in response to the temperature pulses. The output from the pulse amplitude measuring circuit 122 is a constant negative voltage equal to the amplitude of the negative voltage pulses and applied to the control terminal 121 of the switch 115.

When the voltage at the terminal 121 is negative, terminals 123 of the conductors 116 and 117 are connected to a pair of contacts 124 of the electronic switch. Current proceeds from the power supply 118 through the conductors 120 and 116 to the motor 110, causing said motor to drive the air control valve in a closing direction.

When the air flow rate is lower than required for maximum flame temperature, the thermocouple 15 senses a continuing series of positive temperature pulses. The input to the pulse amplitude measuring circuit 122 is a continuing series of positive voltage pulses produced by the thermocouple 15 in response to the temperature pulses. The output from the pulse amplitude measuring circuit 122 is a constant positive voltage equal to the amplitude of the aforesaid positive voltage pulses and is applied to the control terminal 121 of the switch 115.

When the voltage at the latter terminal is positive, the terminals 123 are connected to the contacts 125 of the electronic switch. Current flows from the power supply through the conductors 120 and 117 to the reversible motor, whereby the latter drives the air control valve in an opening direction.

Thus, the controller 16' functions to maintain the flame temperature at or near its maximum value.

A preferred embodiment of the invention has been illustrated for each of the four methods described heretofore. Many changes and modifications may be made therein, however, without departing from the spirit of the invention. In particular: any combustion-supporting gas can be used instead of dry air; any controllable method of metering the combustible gas and combustion-supporting gas prior to mixing and burning can be employed as long as the relative proportions of each are controlled as set forth herein; any method of determining the temperature of the burned gases may be used; whenever a numerical value of the ratio of combustion-supporting gas to combustible gas is required, any method of determining this numerical value may be utilized.

I have discovered that the critical combustion ratio measured in accordance with my invention varies directly with the calorific value of the combustible gas. The calorific value in BTU/SCF is determined by measuring the critical combustion ratio and multiplying this measurement by a suitable calibration constant. This method has been applied to the measurement of the calorific value of a number of combustible gases for the purpose of illustrating my invention. In each case, the calorific value was also obtained by complete chemical analysis of the combustible gas to determine its constituents. The contribution of each constituent to the total calorific value was determined by well-known thermodynamic methods using published heat of combustion and heat capacity data as set forth in "Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds," API Research Project 44, Carnegie Press, Pittsburgh, 1953.

The combustible gas mixtures were prepared by simultaneously flowing methane and a second gas through individual rotameters to measure their flow rates, mixing the two gases, and then passing the mixture to the combustible gas inlet of a Meker burner. The methane was C.P. (Chemically Pure) Grade obtained from a commercial source. The gases added to the methane to form the combustible mixtures were C.P. nitrogen, C.P. ethane, and Instrument Grade propane, all obtained from a commercial source. These gases are representative of major constituents that occur in natural gas. Each of these commercial gases was analyzed quantitatively by gas chromotography.

Table 1 shows the chemical composition of each combustible gas mixture tested and the calorific value of the mixture as determined by its composition. ##SPC1##

Natural gas has a nominal calorific value of about 1,000 BTU/SCF and the normal variation in such value is well within the range of the mixtures in Table 1. A standard ASTM (American Society For Testing Materials) method for measuring calorific value, in which air is used as a heat-absorbing fluid and upon which most commercial instruments are based, is limited in range to about 300 BTU/SCF without recalibration. Thus, as will be evident hereinafter, an advantage of the present invention is that a wider range of calorific values can be measured with a single calibration than has been heretofore possible.

The calorific value of each combustible gas mixture shown in Table 1 was measured by the first method of this invention and the results are shown in Table 2. ##SPC2##

First, the critical combustion ratio was determined for each combustible gas mixture using the apparatus illustrated in FIG. 1; however, gas flow rates were varied manually rather than automatically in the experiments. The flow of dry air which maximized the flame temperature was determined by measuring the temperature with the thermocouple 15 at three different air flow rates close to the maximum conditions. Then, these data points were fitted to a parabolic curve representing the variation of temperature with dry air flow. Although this technique was convenient in the laboratory, in a preferred embodiment of the invention the dry air flow would be continuously and automatically varied such as to maximize the signal from the thermocouple and, thus, the temperature. Gas and air flow rates were measured with rotameters, which had been calibrated using a wet test meter, and the critical combustion ratio was calculated from these flow rates using the following equation: ##SPC3##

Although in these experiments the dry air flow rate was varied to achieve maximum temperature, substantially the same results are obtained by varying the flow rate of the combustible gas mixture or both flow rates. It is only the ratio of the two flow rates that is significant.

Second, the calorific value was calculated from the measured critical combustion ratio using the following equation:

(Calorific Value) = (Calibration Factor) X ("Critical Combustion Ratio")

This equation gives the calorific value as measured by the method of my invention employing the apparatus illustrated in FIG. 1. In practice, the calibration factor is determined by applying the method to combustible gas mixtures of known calorific value. Calibrating the apparatus in this manner and utilizing the above equation tends to correct for constant errors made in metering the gases and, thus, further improve the precision of the method. To obtain the results of Table 2, a single calibration factor was used and was chosen by the least squares method so as to maximize the agreement between the calorific values measured by my method and those values determined from the known composition of the combustible gas mixture. The agreement is within 0.5 percent over the entire range of calorific values and the results show that the calorific value is indeed proportional to the critical combustion ratio.

It has also been discovered that my methods for measuring critical combustion ratio have two unexpected advantages. First, the measurement is independent of the exact position of the thermocouples shown in FIGS. 1-4. As long as the thermocouple is close to and above the flamelets and/or within the cone or flame formed by the burning of the carbon monoxide combustion product, the measured value of critical combustion ratio is not affected by the thermocouple position even though the thermocouple temperature varies with its position whereby the construction of the depicted apparatuses is greatly simplified. Second, the measured value of the critical combustion ratio is independent of temperature over a wide range, and this is a very significant advantage.

Consequently, my methods of measuring calorific value and the illustrated apparatuses can be used in a wide range of ambient temperatures without taking special precautions to control the initial temperature of the combustible gas and ambient air. This is one of the prime disadvantages of prior known methods of and means for measuring calorific value. In prior known methods and means, it usually is necessary to operate the calorimetric apparatus in a temperature controlled environment, held to within 1.degree.-2.degree.F., in order to achieve a precision of 0.5 percent. In some cases, a special building is required for this purpose. No such precautions need be taken with the methods of my invention.

That the measured calorific value using my methods is independent of initial gas temperature and thermocouple position is shown by the data in Table 3. ##SPC4##

The apparatuses of FIGS. 1-3, which measure the critical combustion ratio, may also be employed in automatic control systems. These apparatuses have application in any system in which the mixing ratio of various gases is controlled either to achieve a constant calorific value or to maximize the combustion temperature in a furnace.

For example, assume it is desired to control the addition of propane to natural gas in order to keep the calorific value of the resulting combustible gas constant. The controllers in the apparatuses of FIGS. 1-3 can be caused to directly control propane addition instead of controlling the gas. In this event, the flow rate of a sample of the combustible gas and the flow rate of dry air to the apparatuses would be held constant. The controllers would vary the percent propane in the combustible gas rather than the dry air or combustible gas flow rate and said controllers would operate so as to keep the calorific value of the combustible gas constant. This constant calorific value corresponds to a critical combustion ratio which is equal to the constant ratio of dry air to combustible gas entering the apparatuses.

If it is desired to control the air or gas flow to a furnace in order to maximize combustion temperature, the apparatuses of FIGS. 1-3 could be utilized. The controllers would function not only as shown and described, but would also directly control the appropriate gas flow rate to the furnace.

In summary, methods have bpen discovered for measuring the calorific value of combustible gases which require relatively simpler apparatuses than prior methods. Errors due to heat losses have been completely eliminated since the only thermal quantity measured is a relative temperature. It is not necessary to measure the absolute value of any temperature, but only the conditions for which the temperature of burned gases produced in a flame is a maximum and these conditions are not affected by heat losses from the flame. Other than calibration against known mixtures, no corrections are required to achieve precision comparable to that provided by prior, more complex methods. My methods are insensitive to ambient temperature which may vary between -40.degree. F. and 130.degree. F. which has a large effect on the results of prior methods. In addition, methods have been discovered for automatically controlling the mixing of various gases in order to hold the calorific value constant or maximize combustion temperature.

Claims

I claim:

1. A method of determining the calorific value of a combustible gas including

mixing a combustible gas with a combustion supporting gas and burning the resulting mixture,

measuring the temperature of the burned mixture,

adjusting the volume ratio of the gases until the temperature of the burned mixture is at substantially maximum,

and ascertaining the volume ratio of said gases which produces said maximum temperature and which is proportional to the calorific value of the combustible gas.

2. A method as defined in claim 1 wherein

the gases prior to burning are at approximately ambient temperature.

3. A method as defined in claim 1 wherein

said method is performed at ambient temperatures of from approximately -40.degree. F. to 130.degree. F.

4. A method as defined in claim 1 wherein

the combustion supporting gas is dry air.

5. A method as defined in claim 1 wherein

the step of adjusting the volume ratio of the gases comprises regulating the rate of supply of at least one of said gases to the mixture until the temperature of the burned mixture is at substantially maximum.

6. A method as defined in claim 5 wherein

the rate of supply of the combustion supporting gas is regulated.

7. A method as defined in claim 5 wherein

the rate of supply of the combustible gas is regulated.

8. A method as defined in claim 1 wherein

the step of ascertaining the volume ratio of the gases comprises separately metering the rate of supply of said gases to said mixture.

9. A method as defined in claim 1 including

continually supplying to the mixture additional volumes of one of the gases in small pulses to provide a continuing series of small perturbations on the total flow rate of such gas,

measuring the pulses of the temperature of the burned mixture related to the pulses of the flow rate,

adjusting the flow rate of one of said gases so as to provide a predetermined temperature pulse amplitude.

10. A method as defined in claim 1 including

varying the volume of one of the gases mixed with the other gas to provide different mixtures,

and recording the different temperatures of the burned mixtures.

11. A method of determining the calorific value of a combustible gas including

separating a volume of combustion supporting gas into two equal divisions,

separately mixing the separated divisions with separate unequal divisions of a volume of combustible gas whereby the division of the combustible gas supplied to one of the separate mixtures is slightly greater than the division of said gas supplied to the other separate mixture,

separately burning said mixtures,

measuring the temperature difference between the burned mixtures,

adjusting the volume ratio of the gases prior to division thereof until the temperature difference of the burned mixtures is at a predetermined value,

and ascertaining the volume ratio of said gases which produces said temperature difference and which is proportional to the calorific value of said combustible gas.

12. A method as defined in claim 11 wherein

the gases prior to burning are at approximately ambient temperature.

13. A method as defined in claim 11 wherein

said method is performed at ambient temperatures of from -40.degree. F. to 130.degree. F.

14. A method as defined in claim 11 wherein

the combustion supporting gas is dry air.

15. A method as defined in claim 11 wherein

the step of adjusting the volume ratio of the gases comprises regulating the rate of supply of at least one of said gases to the mixture until the average temperature of the burned mixtures is at substantially maximum.

16. A method as defined in claim 15 wherein

the rate of supply of the combustion supporting gas is regulated.

17. A method as defined in claim 15 wherein

the rate of supply of the combustible gas is regulated.

18. A method as defined in claim 11 wherein

the step of ascertaining the volume ratio of the gases comprises separately metering the rate of supply of said gases to said mixture.

19. A method as defined in claim 11 including

varying the volume of one of the gases mixed with the other gas to provide different mixtures,

and recording the different temperatures of the burned mixtures.

20. A method as defined in claim 11 wherein

the step of adjusting the volume ratio of the gases comprises regulating the rate of supply of at least one of said gases prior to division thereof in accordance with the temperature difference between the separately burned mixtures.

21. An apparatus for determining the calorific value of a combustible gas including

means for mixing a combustible gas and a combustion supporting gas,

means for burning the mixture,

means for supplying said mixture to the burning means,

means for sensing the temperature of the burned mixture and adjusting the volume ratio of the gases until the temperature is at substantially maximum,

and means for ascertaining the volume ratio of said gases which produces said maximum temperature and which is proportional to the calorific value of the combustible gas.

22. An apparatus as defined in claim 21 including

means for regulating the volume of at least one of the gases to adjust the volume ratio of said gases.

23. An apparatus as defined in claim 22 wherein

the volume of the combustion supporting gas is regulated.

24. An apparatus as defined in claim 22 wherein

the volume of the combustible gas is regulated.

25. An apparatus as defined in claim 21 wherein

the means for ascertaining the volume ratio of the gases comprises means for separately metering the rate of supply of said gases to the mixing means.

26. An apparatus as defined in claim 21 including

means for continually supplying to the mixing means additional volumes of one of the gases in small pulses to provide a continuing series of small perturbations on the total flow rate of such gas,

measuring means responsive to the polarity of the pulses for measuring the pulses of the temperature of the burned mixture related to the pulses of the flow rate and producing an output related to said polarity,

and adjusting means responsive to the output of the measuring means for adjusting the flow rate of one of said gases so as to provide a predetermined temperature pulse amplitude.

27. An apparatus as defined in claim 21 including

means for varying the volume of one of the gases mixed with the other gas to provide different mixtures,

and means for recording the different temperatures of the burned mixtures.

28. An apparatus as defined in claim 21 wherein

the burning means is exposed to ambient temperatures of from -40.degree. F. to 130.degree. F.

29. An apparatus as defined in claim 21 wherein

the combustion supporting gas is dry air.

30. An apparatus as defined in claim 21 wherein

the gases prior to burning are at approximately ambient temperatures.

31. An apparatus as defined in claim 21 wherein

the mixing means comprises a pair of separate mixers,

the burning means comprises a pair of separate burners in separate communication with the separate mixers,

the supplying means comprises means for equally dividing the combustion supporting gas and separate means for unequally dividing the combustible gas prior to separate mixing of the divisions of the gases whereby one of the mixtures contains a slightly greater quantity of said combustible gas than the other mixture,

the sensing means comprises a separate sensor for each burner,

the volume ratio of said gases being adjusted prior to division thereof to produce the substantially maximum average temperature of the burned mixtures.

32. An apparatus for determining the calorific value of a combustible gas including

a pair of separate mixer means,

means for supplying equal divisions of a combustion supporting gas to each mixer means,

means for supplying unequal divisions of a combustible gas to the separate mixer means whereby one of the resulting mixtures contains a slightly greater quantity of the combustible gas than the other mixture,

separate means separately communicating with said separate mixer means for separately burning said mixtures,

means for measuring the temperature difference of the burned mixtures and then adjusting the volume ratio of the gases prior to division thereof until the temperature difference is at a predetermined value,

and means for ascertaining the volume ratio of said gases which produces said temperature difference and which is proportional to the calorific value of said combustible gas.

33. An apparatus as defined in claim 32 including

means for regulating the volume of at least one of the gases to adjust the volume ratio of said gases.

34. An apparatus as defined in claim 33 wherein

the volume of the combustion supporting gas is regulated.

35. An apparatus as defined in claim 33 wherein

the volume of the combustible gas is regulated.

36. An apparatus as defined in claim 32 wherein

the means for ascertaining the volume ratio of the gases comprises means for separately metering the rate of supply of said gases to the mixing means.

37. An apparatus as defined in claim 32 including

means for varying the volume of one of the gases mixed with the other gas to provide different mixtures,

and means for recording the different temperatures of the burned mixtures.

38. An apparatus as defined in claim 32 wherein

the burner is exposed to ambient temperatures of from -40.degree. F. to 130.degree. F.

39. An apparatus as defined in claim 32 wherein

the combustion supporting gas is dry air.

40. An apparatus as defined in claim 32 wherein

the gases prior to burning are at approximately ambient temperatures.

41. An apparatus as defined in claim 32 including

means for regulating the rate of supply of at least one of the gases prior to division thereof until the temperature difference between the separately burned mixtures is at a predetermined value.

42. A method of determining the calorific value of combustible gases including

equally dividing a combustion supporting gas and separately mixing the separated equal divisions of the gas with separate unequal divisions of a combustible gas whereby the division of the combustible gas supplied to one of the mixtures is slightly greater than the division of said gas supplied to the other mixture,

burning said mixtures separately,

measuring the temperatures of the burned mixtures,

adjusting the volume ratios of the gases prior to division thereof to produce the substantially maximum average temperatures of the burned mixtures,

and ascertaining the volume ratios of said gases which produce said maximum average temperature and which is proportional to the calorific value of the combustible gas.