CA1073976A - Dewpointmeters - Google Patents
DewpointmetersInfo
- Publication number
- CA1073976A CA1073976A CA255,019A CA255019A CA1073976A CA 1073976 A CA1073976 A CA 1073976A CA 255019 A CA255019 A CA 255019A CA 1073976 A CA1073976 A CA 1073976A
- Authority
- CA
- Canada
- Prior art keywords
- dewpointmeter
- probe
- motor
- thermocouple
- tube
- Prior art date
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- Expired
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-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N25/00—Investigating or analyzing materials by the use of thermal means
- G01N25/56—Investigating or analyzing materials by the use of thermal means by investigating moisture content
- G01N25/66—Investigating or analyzing materials by the use of thermal means by investigating moisture content by investigating dew-point
- G01N25/68—Investigating or analyzing materials by the use of thermal means by investigating moisture content by investigating dew-point by varying the temperature of a condensing surface
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- Physics & Mathematics (AREA)
- Health & Medical Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- Chemical & Material Sciences (AREA)
- Analytical Chemistry (AREA)
- Biochemistry (AREA)
- General Health & Medical Sciences (AREA)
- General Physics & Mathematics (AREA)
- Immunology (AREA)
- Pathology (AREA)
- Investigating Or Analyzing Materials Using Thermal Means (AREA)
Abstract
Abstract of the Disclosure A dewpointmeter comprises a detecting element or probe, a thermocouple adapted to determine the temperature of the probe, means for directing a flow of cooling air at the probe, and an electrical circuit extending to the probe and adapted to be completed by condensate formed on the probe, there being a motor driven regulator adapted to control the rate of flow of cooling air directed at the probe, which motor is adapted to be driven in accordance with the current flowing through the circuit.
Description
- - .
1(~73~76 This invention relates to dewpointmeters.
It is important in various instances to provide a means of determining the dewpoint of a gas, which has been defined as the highest surface temperature at which it is possible to condense a liquid film on a surface exposed to the gas. Thus knowledge of the dewpoint of a flue gas is important in such areas as the study of low temperature corrosion in boilers, it being the case that if the flue gas is at a temperature lower than the dewpoint, the gas will condense and the acidic nature of the condensate can cause corrosion of the metal parts of a flue. Also, when the inner sur-face of the flue is wet, the fine solid particle content of the gas is caused to adhere on the inner surface - a feature which should be avoided.
Hitherto, it has been usual to utilise a dewpointmeter to determine the dewpoint of, e.g. a flue gas, and predominantly such dewpointmeters have been manually operated, although attempts have been made to provide an automatic dewpointmeter. Standard manual dewpointmeters consist of two base eléments, a detecting element including an electrical circuit to be completed by con-densate formed on it, to be inserted in the gas stream, and againstwhich cooling air is directed, and a box containing measuring ins-truments and the controls. With the detecting element inserted in the gas stream, and allowed to warm up cooling air is then di-rected against the detecting element to cool the element until a conducting film has condensed on it. The air flow is then manually regulated until a steady current reading is obtained at which point the rate of condensation and the rate of evaporation of the condensate is equal, and when the temperature of the detecting element is at the dewpoint temperature of the flue gas. The steady current reading can then be utilised to determine the dewpoint of the gas. Whilst such devices function adequately, it is a major disadvantage that the operation is non continuous. An operative
1(~73~76 This invention relates to dewpointmeters.
It is important in various instances to provide a means of determining the dewpoint of a gas, which has been defined as the highest surface temperature at which it is possible to condense a liquid film on a surface exposed to the gas. Thus knowledge of the dewpoint of a flue gas is important in such areas as the study of low temperature corrosion in boilers, it being the case that if the flue gas is at a temperature lower than the dewpoint, the gas will condense and the acidic nature of the condensate can cause corrosion of the metal parts of a flue. Also, when the inner sur-face of the flue is wet, the fine solid particle content of the gas is caused to adhere on the inner surface - a feature which should be avoided.
Hitherto, it has been usual to utilise a dewpointmeter to determine the dewpoint of, e.g. a flue gas, and predominantly such dewpointmeters have been manually operated, although attempts have been made to provide an automatic dewpointmeter. Standard manual dewpointmeters consist of two base eléments, a detecting element including an electrical circuit to be completed by con-densate formed on it, to be inserted in the gas stream, and againstwhich cooling air is directed, and a box containing measuring ins-truments and the controls. With the detecting element inserted in the gas stream, and allowed to warm up cooling air is then di-rected against the detecting element to cool the element until a conducting film has condensed on it. The air flow is then manually regulated until a steady current reading is obtained at which point the rate of condensation and the rate of evaporation of the condensate is equal, and when the temperature of the detecting element is at the dewpoint temperature of the flue gas. The steady current reading can then be utilised to determine the dewpoint of the gas. Whilst such devices function adequately, it is a major disadvantage that the operation is non continuous. An operative
-2- ~_ 1~73976 is required to switch off the air flow to allow the element to heat up, reapply the air flow and adjust the air flow to obtain the steady reading each time a reading is required to be taken.
In an attempt to provide an automatic dewpointmeter, it has been proposed to provide a detection circuit to sense the onset of condensation on the element, the circuit causing the temperature of the element to be taken simultaneously with the onset of current, and at the same time activate a solenoid valve to switch off the flow of cooling air to the element. The tempe-rature of the element thus begins to increase until it is raisedabove the dewpoint temperature and all the condensate evaporates.
The cooling air is then automatically switched on again to cool the element until again there is the onset of condensation and thus current, and the cycle repeated. Thus, a semi-continuous measu-rement of dewpoint temperature is obtained in that at each point in the cycle that the element is at the dewpoint temperature, condensate appears and the temperature read. The disadvantage of this method is that, apart from being semi-continuous, the time lag involved in detecting the onset of condensation causes the temperature to be read at a point below the dewpoint tempera-ture, and this error, which can be the order of 5C to 10C is dependent on the rate of build up of condensate and the rate of cooling of the element.
It has also been proposed, in an attempt to provide an automatic system, to provide a circuit that measures the current in a circuit completed by condensate on the element, the circuit controlling the switching on and off of the cooling air. Thus, when current in the circuit reaches a pre-set maximum, a solenoid valve is activated to switch off the cooling air to the element.
As the temperature of the element rises condensate begins to evaporate and proportionately less current flows in the circuit until it reaches a preset minimum, when the solenoid valve is 107397tj operated to switch on the cooling air. In this way the current oscillates between the preset value, and the temperature of the element oscillates above and below the dewpoint temperature~ This temperature oscillation can be as high as + 15C about the dewpoint temperature and makes accurate determination of the dewpoint tem-perature almost impossible.
According to the present invention, a dewpointmeter com-prises a detecting element or probe, a thermocouple adapted to determine the temperature of the probe, means for directing a flow of cooling air at the probe, and an electrical circuit ex-tending to the probe and adapted to be completed by condensate formed on the probe, there being a motor driven regulator adapted to control the rate of flow of cooling air directed at the probe, which motor is adapted to be driven in accordance with the current flowing through the circuit-Preferably, in the circuit completed by the condensate,electronic means are provided to determine the differential of the measured current, it being the sign of the differential (ne-gative or positive) and its magnitude that is utilised to control the speed and direction of the drive of the motor driven regulator.
Thus, when the differential of the current is positive, the element must be below the dewpoint temperature, and the motor is driven to decrease the flow of cooling air to the element. Thus the element heats up to a point where the condensate begins to eva-porate, and when the differential of the measured current becomes negative, showing that the element is above the dewpoint tempera-ture. That signal causes the direction of the motor to be reversed, to increase the flow of cooling air to the element, and thus cool it down. By utilising a motor driven regulator, controlled as defined above, the cooling air can be blown at the element at a rate to maintain the temperatureof the element substantially cons-tant, that temperature being the one at which the current in the 1073~
circuit is substantially constant, and at which the rate con-densate is forming is equal to the rate at which it is evapo-rating, i.e. , the dewpoint temperature.
It is however possible to utilise a so-called proportio-nal control for the motor. Thus, means can be provided in the circuit to detect when thé current in the circuit is above or below a pre-set threshold current of a value at a level high enough not to be affected by the slow increase of stray current due to contamination of the element surface with solid particles when inserted in the flue gas. As condensate builds up on the probe, the current flowing in the circuit increases until the probe reaches a temperature sufficient to start to evaporate the conden-sate and when the current flowing starts to fall and, if left un-checked, would heat up to a temperature at which all the conden-sate had evaporated and the current would be zero. However, by providing a motor driven regulator in accordance with the inven-tion, the direction of drive of which is in accordance wi*h whether the current flowing in the circuit is above or below the threshold figure and the rate in accordance with the extent to which the cur-rent is above or below the threshold, cooling air can be continuous-ly blown aga-inst the probe at a rate to maintain the probe at a temperature at which the current has a steady value, equal to the predetermined threshold value, and the rate of condensation of the flue gas on it is equal to the rate of evaporation, which tempera-ture is the d~wpoint temperature of the gas. Therefore, the ther-mocouple detecting the temperature of the probe can provide a continuous reading of the dewpoint temperature.
In its most preferred form the dewpointmeter of the invention includes both forms of control over the motor driven regu-lator, the differential control preventing any tendency for the tem-perature of the element to oscillate above and below the dewpointtemperature as may be possible in certain circumstances when pro-portional control only is provided, and the proportional control ~73976 ensuring that if in certain circumstances the condensate is reduced to zero by the element temperature, and thus the current and the rate of change of current being zero, the differential control will become inoperative, leaving the proportional control to reduce the temperature of the element to the dewpoint temperature whereupon condensate is again formed and the differential control becomes operative once more.
The motor of the motor driven pressure regulator may be a variable speed motor operated by a continuous drive signal, or a synchronous motor operated by feeding in a train of pulses, and when the speed of the motor is proportional to the "ON/OFF"
ratio of the train of pulses.
The reading from the thermocouple may be visually display-ed, or arranged to provide a permanent record on a chart recorder.
Obviously, both a visual and permanent record can be provided if desired.
To ensure that the probe does not become fouled by dirt, it is preferred to provide for the cleaning of the probe at re-gular intervals. Thus, an adjustable timer may be provided, and the probe fitted with a cleaning air or other fluid tube directed at the detector element of the probe, so that at regular intervals, cleaning air or other suitable fluid, e.g. water is blown against the element. Typically a cleaning blow will last for 10 seconds with a frequency of one blow every 3 hours.
If desired, the probe may additionally be provided with an external thermocouple to obtain approximate reading of gas tem-peratures in the vicinity of the ~ be.
One embodiment of the invention will now be described with reference to the accompanying drawings in which:- -Figure 1 is a general arrangement showing, schematically a dewpoint in accordance with the invention;
Figure 2 is a sectional side elevation of the detecting element in the end of the probe tubes of Figure l;
Figure 3 is a sectional side elevation of the probe con-necting means; and Figure ~ is a section side elevation of the connection of the probe tube to a probe adaptor tube.
In the drawings, a dewpointmeter includes a probe tube 1 for insertion in a stream of e.g. flue gas, the probe tube being connected to a source of cooling air governed by a motor-driven regulator unit 2, and there being an electronic control unit 3.
As in shown particularly by Figures 2 and 3, the probe tube 1 is provided with a detecting element 4 ~ormed by a thimble S of borosilicate glass, having a collar 6 lying between a shoulder on a stainless steel terminal block 7 mounted on the probe tube, and an internal shoulder on a locking collar 8, there being as-bestos seals 9 between the collar 6, the terminal block 7 and the locking collar 8. The end of the thimble 5 is formed by a disc 10 of sintered glass into which is fused a platinum/rhodium thermocouple, and an annular platinum electrode, the platinum leg of the thermocouple and the annular platinum electrode forming the two electrodes across which a stabilised A.C. potential is applied. Three platinum leads 11 extending from the detecting element are insulated by ceramic material and are soldered to ter-minals 12 mounted on the terminal block 7.
The probe tube 1, of stainless steel, is provided with an inner tube 13 carrying cooling air to the thimble 5, to direct cooling air at the inner face of the glass disc 10. Air admitted to the thimble exhausts through apertures in terminal block into the probe tube 1, and to atmosphere through holes 14 in the probe tube (see Figure 3). Also as is shown more particularly by Figure
In an attempt to provide an automatic dewpointmeter, it has been proposed to provide a detection circuit to sense the onset of condensation on the element, the circuit causing the temperature of the element to be taken simultaneously with the onset of current, and at the same time activate a solenoid valve to switch off the flow of cooling air to the element. The tempe-rature of the element thus begins to increase until it is raisedabove the dewpoint temperature and all the condensate evaporates.
The cooling air is then automatically switched on again to cool the element until again there is the onset of condensation and thus current, and the cycle repeated. Thus, a semi-continuous measu-rement of dewpoint temperature is obtained in that at each point in the cycle that the element is at the dewpoint temperature, condensate appears and the temperature read. The disadvantage of this method is that, apart from being semi-continuous, the time lag involved in detecting the onset of condensation causes the temperature to be read at a point below the dewpoint tempera-ture, and this error, which can be the order of 5C to 10C is dependent on the rate of build up of condensate and the rate of cooling of the element.
It has also been proposed, in an attempt to provide an automatic system, to provide a circuit that measures the current in a circuit completed by condensate on the element, the circuit controlling the switching on and off of the cooling air. Thus, when current in the circuit reaches a pre-set maximum, a solenoid valve is activated to switch off the cooling air to the element.
As the temperature of the element rises condensate begins to evaporate and proportionately less current flows in the circuit until it reaches a preset minimum, when the solenoid valve is 107397tj operated to switch on the cooling air. In this way the current oscillates between the preset value, and the temperature of the element oscillates above and below the dewpoint temperature~ This temperature oscillation can be as high as + 15C about the dewpoint temperature and makes accurate determination of the dewpoint tem-perature almost impossible.
According to the present invention, a dewpointmeter com-prises a detecting element or probe, a thermocouple adapted to determine the temperature of the probe, means for directing a flow of cooling air at the probe, and an electrical circuit ex-tending to the probe and adapted to be completed by condensate formed on the probe, there being a motor driven regulator adapted to control the rate of flow of cooling air directed at the probe, which motor is adapted to be driven in accordance with the current flowing through the circuit-Preferably, in the circuit completed by the condensate,electronic means are provided to determine the differential of the measured current, it being the sign of the differential (ne-gative or positive) and its magnitude that is utilised to control the speed and direction of the drive of the motor driven regulator.
Thus, when the differential of the current is positive, the element must be below the dewpoint temperature, and the motor is driven to decrease the flow of cooling air to the element. Thus the element heats up to a point where the condensate begins to eva-porate, and when the differential of the measured current becomes negative, showing that the element is above the dewpoint tempera-ture. That signal causes the direction of the motor to be reversed, to increase the flow of cooling air to the element, and thus cool it down. By utilising a motor driven regulator, controlled as defined above, the cooling air can be blown at the element at a rate to maintain the temperatureof the element substantially cons-tant, that temperature being the one at which the current in the 1073~
circuit is substantially constant, and at which the rate con-densate is forming is equal to the rate at which it is evapo-rating, i.e. , the dewpoint temperature.
It is however possible to utilise a so-called proportio-nal control for the motor. Thus, means can be provided in the circuit to detect when thé current in the circuit is above or below a pre-set threshold current of a value at a level high enough not to be affected by the slow increase of stray current due to contamination of the element surface with solid particles when inserted in the flue gas. As condensate builds up on the probe, the current flowing in the circuit increases until the probe reaches a temperature sufficient to start to evaporate the conden-sate and when the current flowing starts to fall and, if left un-checked, would heat up to a temperature at which all the conden-sate had evaporated and the current would be zero. However, by providing a motor driven regulator in accordance with the inven-tion, the direction of drive of which is in accordance wi*h whether the current flowing in the circuit is above or below the threshold figure and the rate in accordance with the extent to which the cur-rent is above or below the threshold, cooling air can be continuous-ly blown aga-inst the probe at a rate to maintain the probe at a temperature at which the current has a steady value, equal to the predetermined threshold value, and the rate of condensation of the flue gas on it is equal to the rate of evaporation, which tempera-ture is the d~wpoint temperature of the gas. Therefore, the ther-mocouple detecting the temperature of the probe can provide a continuous reading of the dewpoint temperature.
In its most preferred form the dewpointmeter of the invention includes both forms of control over the motor driven regu-lator, the differential control preventing any tendency for the tem-perature of the element to oscillate above and below the dewpointtemperature as may be possible in certain circumstances when pro-portional control only is provided, and the proportional control ~73976 ensuring that if in certain circumstances the condensate is reduced to zero by the element temperature, and thus the current and the rate of change of current being zero, the differential control will become inoperative, leaving the proportional control to reduce the temperature of the element to the dewpoint temperature whereupon condensate is again formed and the differential control becomes operative once more.
The motor of the motor driven pressure regulator may be a variable speed motor operated by a continuous drive signal, or a synchronous motor operated by feeding in a train of pulses, and when the speed of the motor is proportional to the "ON/OFF"
ratio of the train of pulses.
The reading from the thermocouple may be visually display-ed, or arranged to provide a permanent record on a chart recorder.
Obviously, both a visual and permanent record can be provided if desired.
To ensure that the probe does not become fouled by dirt, it is preferred to provide for the cleaning of the probe at re-gular intervals. Thus, an adjustable timer may be provided, and the probe fitted with a cleaning air or other fluid tube directed at the detector element of the probe, so that at regular intervals, cleaning air or other suitable fluid, e.g. water is blown against the element. Typically a cleaning blow will last for 10 seconds with a frequency of one blow every 3 hours.
If desired, the probe may additionally be provided with an external thermocouple to obtain approximate reading of gas tem-peratures in the vicinity of the ~ be.
One embodiment of the invention will now be described with reference to the accompanying drawings in which:- -Figure 1 is a general arrangement showing, schematically a dewpoint in accordance with the invention;
Figure 2 is a sectional side elevation of the detecting element in the end of the probe tubes of Figure l;
Figure 3 is a sectional side elevation of the probe con-necting means; and Figure ~ is a section side elevation of the connection of the probe tube to a probe adaptor tube.
In the drawings, a dewpointmeter includes a probe tube 1 for insertion in a stream of e.g. flue gas, the probe tube being connected to a source of cooling air governed by a motor-driven regulator unit 2, and there being an electronic control unit 3.
As in shown particularly by Figures 2 and 3, the probe tube 1 is provided with a detecting element 4 ~ormed by a thimble S of borosilicate glass, having a collar 6 lying between a shoulder on a stainless steel terminal block 7 mounted on the probe tube, and an internal shoulder on a locking collar 8, there being as-bestos seals 9 between the collar 6, the terminal block 7 and the locking collar 8. The end of the thimble 5 is formed by a disc 10 of sintered glass into which is fused a platinum/rhodium thermocouple, and an annular platinum electrode, the platinum leg of the thermocouple and the annular platinum electrode forming the two electrodes across which a stabilised A.C. potential is applied. Three platinum leads 11 extending from the detecting element are insulated by ceramic material and are soldered to ter-minals 12 mounted on the terminal block 7.
The probe tube 1, of stainless steel, is provided with an inner tube 13 carrying cooling air to the thimble 5, to direct cooling air at the inner face of the glass disc 10. Air admitted to the thimble exhausts through apertures in terminal block into the probe tube 1, and to atmosphere through holes 14 in the probe tube (see Figure 3). Also as is shown more particularly by Figure
3, the inner tube 13 has an inlet 15 extending out of the probe tube 1 to an air supply pipe 16 terminating in a quick release coupling 17 incorporating a selfsealing valve.
10739~;~6 Extension leads 17 from the terminals 5 pass down the probe tube 1 to a terminal board 18, the board having appropriate leads to a socket 19, to enable connection of the probe to the electronic control unit 3. Preferably, a thermistor not shown, is provided on the board 18 to constitute a cold junction compensator for the thermocouple.
As is shown particularly by Pigure 4, the probe tube 1 is located in position by a probe adaptor tube 20. Thus, an end cap 21 is provided on the adaptor tube 20, having an aperture through which passes the probe tube 1, the end cap also locating a flanged member 22 provided with appropriate holes 23 by which the adaptor tube can be bolted in position. As is shown particu-larly by Figure 2, the locking collar 8 is a close fit in the oppo-site end of the adaptor tube 20. The flange 24 of the member 22 has an air passageway 25 connected to a source of compressed air, and to which an air passageway or tube 26 is connected, the tube 26 having its opposite end 27 directed against the outer surface of the thimble 5.
As is shown schematically by Figure 1, the air supply pipe 16 is connected to an air flow controller 28 within the motor driven regulator 2, the controller 28 being driven by a motor 29.
Within the unit 2 is a solenoid valve 30 controlling the passage of air to the tube 26. The electronic control unit 3 comprises a control unit 31 for the motor 29 activated by current measurement means 32 connected to the detecting element 4 on the probe tube.
The thermocouple on the glass disc 10 is connected to an amplifier 33 in the unit 3, which is turn is connected to a linearizer 34, the linéarizer feeding a device 35 providing a temperature read-out facility. Also within the unit 3 is a timing device 36 coupled to the solenoid valve 23.
Thus, with the probe tube 1 secured by the adaptor tube 20 to, e.g. the wall of a furnace flue (not shown) such that the 1(~73976 detecting element 4 is situated in the flow of flue gas, compressed air is fed to the air flow controller 28 and thus to the thimble 5. Cooling of the thimble causes condensate to form on the outer surface of the thimble to complete an electrical circuit from the electronic control unit by bridging the platinum leg of the ther-mocouple and the platinum annulus on the glass disc 10. Therefore current fed to the circuit from the control unit 3 passes through the circuit at a rate determined by the amount of condensate on the thimble. The current measuring device 32 serves two additional functions. Firstly, it includes electronic means to determine the differential of the current measured, and secondly, it includes means to determine whether or not the current measured is above or below a pre-set threshold level. Whilst current is flowing in the circuit, the differential of the current may be positive, zero or negative. A positive differential indicates that the tempera-ture of the thimble is below dewpoint, and a negative differential that the thimble is above dewpoint. In either instance, a signal is fed to the controller 31 which in turn controls the direction of drive of the motor 29 and the speed of drive in accordance with the sign of the differential and its magnitude. A zero differen-tial, in normal circumstances indicates that the thimble is at the dewpoint temperature, and no signal is passed to controller 31 causing the drive of the motor and hence the setting of the regu-lator 28 to be maintained in the condition that has created the zero differential of the current. However, in abnormal circums-tances, the thimble could be brought very rapidly to a temperature at which all the condensate has evaporated. Thus, a zero reading for current would be taken, and the electronic means within the device 32 produce a zero differential. This could lead to the drive of the motor 29 and the setting of the regulator 2~ to be maintained in a condition whereby the thimble is maintained at a temperature where no condensate can form. However, by providing means in the device 32 to determine that the current is above or below a threshold level, a zero reading of current would cause a signal to be passed to the controller 31 to cause the drive of the motor 29 and hence the setting of the regulator 28 to increase ~ the flow of cooling air to the thimble to cool it down and when condensate would again be formed. As soon as condensate is formed, the differential form of control would then become the predominant control factor once more.
By effectively controlling the motor 29 by the current in the circuit completed by the condensate on the thimble, there is continuous correction of the motor drive and hence continuous adjustment of the rate at which cooling air is directed into the thimble 5, the result of which is that the thimble is held at substantially a constant temperature, that at which the rate of condensation is equal to the rate of evaporation of the condensate, this being the dewpoint temperature of the particular flue gas being measured.
The thermocouple on the glass disc 10 continuously reads the temperature of the gas, and feeds its signal to the ~mplifier 33, from where the signal is directed through the linearizer 34 and to the read-out device 35. If required a temperature gauge 37 can be provided in addition to the read-out (e.g. a pen recorder) from the device 35.
The invention, therefore, provides a relatively simple means of providing for the continuous detection measurement and display of the dewpoint temperature of any gas.
To provide for the cleaning of the outer face of the glass disc 10, the timing device is set such that at regular in-tervals, the solenoid valve is operated to allow a blast of clean-ing air to be directed at the disc 10, the timer also controllingthe duration of the blast.
10739~;~6 Extension leads 17 from the terminals 5 pass down the probe tube 1 to a terminal board 18, the board having appropriate leads to a socket 19, to enable connection of the probe to the electronic control unit 3. Preferably, a thermistor not shown, is provided on the board 18 to constitute a cold junction compensator for the thermocouple.
As is shown particularly by Pigure 4, the probe tube 1 is located in position by a probe adaptor tube 20. Thus, an end cap 21 is provided on the adaptor tube 20, having an aperture through which passes the probe tube 1, the end cap also locating a flanged member 22 provided with appropriate holes 23 by which the adaptor tube can be bolted in position. As is shown particu-larly by Figure 2, the locking collar 8 is a close fit in the oppo-site end of the adaptor tube 20. The flange 24 of the member 22 has an air passageway 25 connected to a source of compressed air, and to which an air passageway or tube 26 is connected, the tube 26 having its opposite end 27 directed against the outer surface of the thimble 5.
As is shown schematically by Figure 1, the air supply pipe 16 is connected to an air flow controller 28 within the motor driven regulator 2, the controller 28 being driven by a motor 29.
Within the unit 2 is a solenoid valve 30 controlling the passage of air to the tube 26. The electronic control unit 3 comprises a control unit 31 for the motor 29 activated by current measurement means 32 connected to the detecting element 4 on the probe tube.
The thermocouple on the glass disc 10 is connected to an amplifier 33 in the unit 3, which is turn is connected to a linearizer 34, the linéarizer feeding a device 35 providing a temperature read-out facility. Also within the unit 3 is a timing device 36 coupled to the solenoid valve 23.
Thus, with the probe tube 1 secured by the adaptor tube 20 to, e.g. the wall of a furnace flue (not shown) such that the 1(~73976 detecting element 4 is situated in the flow of flue gas, compressed air is fed to the air flow controller 28 and thus to the thimble 5. Cooling of the thimble causes condensate to form on the outer surface of the thimble to complete an electrical circuit from the electronic control unit by bridging the platinum leg of the ther-mocouple and the platinum annulus on the glass disc 10. Therefore current fed to the circuit from the control unit 3 passes through the circuit at a rate determined by the amount of condensate on the thimble. The current measuring device 32 serves two additional functions. Firstly, it includes electronic means to determine the differential of the current measured, and secondly, it includes means to determine whether or not the current measured is above or below a pre-set threshold level. Whilst current is flowing in the circuit, the differential of the current may be positive, zero or negative. A positive differential indicates that the tempera-ture of the thimble is below dewpoint, and a negative differential that the thimble is above dewpoint. In either instance, a signal is fed to the controller 31 which in turn controls the direction of drive of the motor 29 and the speed of drive in accordance with the sign of the differential and its magnitude. A zero differen-tial, in normal circumstances indicates that the thimble is at the dewpoint temperature, and no signal is passed to controller 31 causing the drive of the motor and hence the setting of the regu-lator 28 to be maintained in the condition that has created the zero differential of the current. However, in abnormal circums-tances, the thimble could be brought very rapidly to a temperature at which all the condensate has evaporated. Thus, a zero reading for current would be taken, and the electronic means within the device 32 produce a zero differential. This could lead to the drive of the motor 29 and the setting of the regulator 2~ to be maintained in a condition whereby the thimble is maintained at a temperature where no condensate can form. However, by providing means in the device 32 to determine that the current is above or below a threshold level, a zero reading of current would cause a signal to be passed to the controller 31 to cause the drive of the motor 29 and hence the setting of the regulator 28 to increase ~ the flow of cooling air to the thimble to cool it down and when condensate would again be formed. As soon as condensate is formed, the differential form of control would then become the predominant control factor once more.
By effectively controlling the motor 29 by the current in the circuit completed by the condensate on the thimble, there is continuous correction of the motor drive and hence continuous adjustment of the rate at which cooling air is directed into the thimble 5, the result of which is that the thimble is held at substantially a constant temperature, that at which the rate of condensation is equal to the rate of evaporation of the condensate, this being the dewpoint temperature of the particular flue gas being measured.
The thermocouple on the glass disc 10 continuously reads the temperature of the gas, and feeds its signal to the ~mplifier 33, from where the signal is directed through the linearizer 34 and to the read-out device 35. If required a temperature gauge 37 can be provided in addition to the read-out (e.g. a pen recorder) from the device 35.
The invention, therefore, provides a relatively simple means of providing for the continuous detection measurement and display of the dewpoint temperature of any gas.
To provide for the cleaning of the outer face of the glass disc 10, the timing device is set such that at regular in-tervals, the solenoid valve is operated to allow a blast of clean-ing air to be directed at the disc 10, the timer also controllingthe duration of the blast.
Claims (19)
1. A dewpointmeter comprising a detecting element or probe, a thermocouple adapted to determine the temperature of the probe, means for directing a flow of cooling air at the probe, and an electrical circuit extending to the probe and adapted to be completed by condensate formed on the probe, a regulator driven by a motor an adapted to control the rate of flow of said cooling air directed at the probe, said motor being adapted to be driven in accordance with the current flowing through said electrical circuit.
2. A dewpointmeter as in claim 1, wherein in said elec-trical circuit completed by the condensate, electronic means are provided to determine a differential signal of a measured current, the sign of said differential signal and its magnitude controlling the speed and direction of drive of the motor driven regulator.
3. A dewpointmeter as claim 1 or 2, wherein means are provided in said electrical circuit to detect when the current in said circuit is above or below a pre-set threshold current of a value at a level high enough not to be affected by a slow increase of stray current due to contamination of an element surface with solid particles when inserted in a flue gas.
4. A dewpointmeter as in claim 1 or 2, wherein the motor of the motor driven regulator is a variable speed motor ope-rated by a continuous drive signal.
5. A dewpointmeter as in claim 1 or 2, wherein the motor of the motor driven regulator is a synchronous motor operated by feeding in a train of pulses, and when the speed of the motor is proportional to a "ON/OFF" ratio of the train of pulses.
6. A dewpointmeter as in claim 1 or 2, further compri-sing means for visually displaying a reading from the thermocouple.
7. A dewpointmeter as in claim 1 or 2, further compri-sing means for providing a permanent record on a chart recorder of a reading from the thermocouple.
8. A dewpointmeter as in claim 1, wherein the probe is fitted with a cleaning air tube directed at a detector element of the probe.
9. A dewpointmeter as in claim 8, wherein an adjustable timer is provided to periodically admit the cleaning air along the tube.
10. A dewpointmeter as in claim 8, wherein an adjustable timer is provided to determine a length of time that the cleaning air is passed along the tube.
11. A dewpointmeter as in claim 1, wherein an external thermocouple is provided to obtain approximate readings of gas temperatures in the vicinity of the probe.
12. A dewpointmeter as in claim 1, wherein the probe is provided with a detecting element in the form of a thimble.
13. A dewpointmeter as in claim 12, wherein the thimble is formed from boro-silicate glass.
14. A dewpointmeter as in claim 12, wherein the end of the thimble is formed by a disc into which is fused the thermocouple and an annular electrode.
15. A dewpointmeter as in claim 14, wherein the disc is of sintered glass.
16. A dewpointmeter as in claim 14 or 15, wherein the thermocouple is a platinum/rhodium thermocouple.
17. A dewpointmeter as in claim 14, wherein the annular electrode is of platinum.
18. A dewpointmeter as in claim 1, wherein the probe is formed as a tube and is provided with an inner tube for passage of cooling air.
19. A dewpointmeter as in claim 1, wherein an adapter tube is provided to locate the probe in a position of use.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB2601375A GB1541441A (en) | 1975-06-18 | 1975-06-18 | Dew pont meters |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1073976A true CA1073976A (en) | 1980-03-18 |
Family
ID=10236987
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA255,019A Expired CA1073976A (en) | 1975-06-18 | 1976-06-16 | Dewpointmeters |
Country Status (7)
| Country | Link |
|---|---|
| JP (1) | JPS5947255B2 (en) |
| BE (1) | BE843122A (en) |
| CA (1) | CA1073976A (en) |
| DE (1) | DE2626699C3 (en) |
| FR (1) | FR2316587A1 (en) |
| GB (1) | GB1541441A (en) |
| NL (1) | NL175855C (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB9511204D0 (en) * | 1995-06-02 | 1995-07-26 | Sonander Sven O | Method and apparatus for measuring dew point temperature of a moist gas |
| FR2919310B1 (en) * | 2007-07-26 | 2009-11-06 | Total France Sa | PROCESS FOR THE ANTICORROSION TREATMENT OF AN INDUSTRIAL UNIT |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3252319A (en) * | 1964-01-06 | 1966-05-24 | Ind Instr Inc | Dew point device with proportional control circuit |
| GB1405492A (en) * | 1972-10-26 | 1975-09-10 | Ici Ltd | Dewpoint recorder |
-
1975
- 1975-06-18 GB GB2601375A patent/GB1541441A/en not_active Expired
-
1976
- 1976-06-15 DE DE19762626699 patent/DE2626699C3/en not_active Expired
- 1976-06-16 CA CA255,019A patent/CA1073976A/en not_active Expired
- 1976-06-18 JP JP7126376A patent/JPS5947255B2/en not_active Expired
- 1976-06-18 FR FR7619167A patent/FR2316587A1/en active Granted
- 1976-06-18 NL NL7606670A patent/NL175855C/en not_active IP Right Cessation
- 1976-06-18 BE BE168069A patent/BE843122A/en not_active IP Right Cessation
Also Published As
| Publication number | Publication date |
|---|---|
| DE2626699C3 (en) | 1980-08-28 |
| NL175855B (en) | 1984-08-01 |
| FR2316587B1 (en) | 1981-02-06 |
| NL175855C (en) | 1985-01-02 |
| JPS5214473A (en) | 1977-02-03 |
| FR2316587A1 (en) | 1977-01-28 |
| BE843122A (en) | 1976-10-18 |
| NL7606670A (en) | 1976-12-21 |
| JPS5947255B2 (en) | 1984-11-17 |
| GB1541441A (en) | 1979-02-28 |
| DE2626699B2 (en) | 1979-12-20 |
| DE2626699A1 (en) | 1977-01-20 |
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| Date | Code | Title | Description |
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| MKEX | Expiry |