CA2066765A1 - Process and device for determining the dew point of components of a gas mixture - Google Patents
Process and device for determining the dew point of components of a gas mixtureInfo
- Publication number
- CA2066765A1 CA2066765A1 CA 2066765 CA2066765A CA2066765A1 CA 2066765 A1 CA2066765 A1 CA 2066765A1 CA 2066765 CA2066765 CA 2066765 CA 2066765 A CA2066765 A CA 2066765A CA 2066765 A1 CA2066765 A1 CA 2066765A1
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- Prior art keywords
- condensate collector
- temperature
- dew point
- humidity
- condensate
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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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- 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
Dew point hygrometers are either very precise and very expensive, for example mirror dew point hygrometers, or less costly but relatively imprecise, for example electrode arrangements for measuring electrical resistance. The disclosed appliance should be low-priced and at the same time have a measuring accuracy that corresponds to the measuring accuracy of mirror dew point hygrometers. A
moisture sensor (8) is spatially separated from but adjacent to a condensate collector (1). The condensate collector (1) is cooled down by a determined temperature difference by a cooling/heating device (2). When during the cooling process the sought dew point temperature is crossed, liquid deposits on the condensate collector. In the following heating phase, the condensate evaporates again and the resulting increase in the vapour content is detected by the moisture sensor (8) arranged above the condensate collector (1). By varying the temperature difference during the cooling phase, the sought dew temperature is iteratively determined. The invention has applications in industrial processes in general, in measurement techniques in general, and for monitoring compressed air installations.
moisture sensor (8) is spatially separated from but adjacent to a condensate collector (1). The condensate collector (1) is cooled down by a determined temperature difference by a cooling/heating device (2). When during the cooling process the sought dew point temperature is crossed, liquid deposits on the condensate collector. In the following heating phase, the condensate evaporates again and the resulting increase in the vapour content is detected by the moisture sensor (8) arranged above the condensate collector (1). By varying the temperature difference during the cooling phase, the sought dew temperature is iteratively determined. The invention has applications in industrial processes in general, in measurement techniques in general, and for monitoring compressed air installations.
Description
2 ~ 6 5 APPARATUS AND METHOD FOR DETERMI~ING T~E DEW POINT OF GAS
MIXTURE COMPONENTS
The invention is directed to an apparatus and a method for determining the dew point of components of a gas mixture.
Accurate determination of the vapor component in gas flows is highly important in many areas of process technology in order to avoid corrosion damage, to prevent unwanted emissions, or to condition the gas flow with respect to optimal implementation of the process.
A widely used arrangement, particularly for determining the water vapor component in air flows, consists in dew point hygrometers in which the air is cooled until the water vapor condenses. The dew point temperature determined in this way corresponds to the saturation temperature of the air, from which the relative humidity can also be derived when the sensible temperature of the air flow is known.
As is shown in the publication DE-Buch, Winfried Luck, Principles of Humidity, Measurement, Regulating [Feuchtigkeits-Grundlagen, Messen, Regeln], R. Oldenburg, Munich, Vienna 1964, pages 74 to 84, essentially three methods and apparatuses are known for this purpose:
a) dew point mirrors which mist when cooled below the dew point, b) cooled electrode arrangements which change electrical resistance or capacitance when condensation is precipitated, and c) fog chambers in which fog can be observed when the temperature of the measurement gas located in the chamber falls below the dew point.
In fog chamber hygrometers, the measurement gas is condensed and then expanded without falling below the dew point. The humid air cools considerably and virtually homogeneously due to the expansion. In an illuminated "fog chamber" the formation of fog can be observed in the event ~ o ~ ~ r~ 6 ~
that the temperature falls below the dew point of the measurement gas as a result of the drop in temperature due to the expansion. The precise overpressure at which fog is formed when the gas (e.g~ at atmospheric air pressure) is expanded is sought by repeated compression and subsequent expansion. The dew point temperature can be calculated from the humid gas temperature and the compression ratio.
In a mirror dew point measuring device with dew point mirrors the measurement gas is cooled to the boundary of dew precipitation by attempting to adjust the precise cooling at which the dew formation is detectable by repeated adjustment of the level of the cooling temperature. Devices are known in which a photocell arrangement always controls the cooling and/or heating of the dew mirror via a control amplifier in such a way that the dew limit is fixed automatically - i.e.
in a regulated manner. The task of automatically adjusting the dew point involves certain difficulties with respect to control technology because the dew formation rate is very highly dependent on the respective dew point temperature.
Besides the above-mentioned technical publication, different variants of mirror dew point measuring devices are also known from DE-OS 19 57 322, DE-AS 12 99 437, DE-OS 26 40 663, DE-OS 29 45 445 and US-PS 3 385 098.
To avoid the difficulties involved in photoelectric mirror scanning in mirror dew point measurement devices it is only necessary to make use of cooled electrode arrangements. Very low junction resistances which can supply powerful control quantities at low cost can be achieved by means of a meandering arrangement of a conductor path on an insulating substrate. Instead of measuring the resistance of the cooled electrode arrangement, the change in capacitance occurring when the electrical constant is changed as a result of the misting of the sensor can also be made use of. Such a capacitative dew point detector is known e.g. from DE-OS 32 31 534.
2 ~
The substantial disadvantage of devices with dew point mirrors and fog chambers consists in the high cost in apparatus and control technology, which results in correspondingly high prices for such devices. On the other hand, cooled electrode arrangements have the disadvantage that the measuring accuracy is comparatively low. This is primarily a result of the hygroscopicity of all the materials conventionally used for conductor path carriers, which already leads to a drop in the surface resistance long before saturation and accordingly before the precipitation of liquid. The use of conductor path carriers with low hygroscopicity, e.g. paraffins or waxes, or the conditioning of conventional conductor path carriers with these materials likewise leads to unsatisfactory results because of the low melting point and low mechanical strength.
Another disadvantage of cooled electrode arrangements consists in that it is very difficult to determine the exact surface temperature of these materials, i.e. the exact surface temperature of the condensation surface, because of their poor heat conducting characteristics. Consequently, the dew point temperature can also not be exactly determined.
The described disadvantages of the known measuring devices and methods is also referred to in the article "Is My Compressed-Air Drier Working? [Funktioniert mein Drucklufttrockner?]" by E. Prumm, in the technical journal "Compressed-Air Technology [Drucklufttechnik]", 11-12/1989, pages 33 and 34.
Finally, an apparatus and a method for determining the dew point of components of a gas mixture from which the invention proceeds in the preamble of claim 1 is known from DE-OS 37 13 864. The known apparatus comprises a humidity sensor which can involve a dew pOillt mirror as well as a cooled electrode arrangement. Moreover, a condensate collector is provided which can be cooled or heated by means ~ .
;
2a~r~
of a Peltier element. A temperature sensor which measures the temperature of the condensate collector or condensation surface is arranged in thermal contact with the condensate collector. The humidity sensor, condensate collector and temperature sensor are integrated in an individual structural component part. For this reason and because the precipitation of liquid in this known system is determined either by a dew point mirror or by cooled electrode arrangements, this arrangement also has the aforementioned disadvantages of the respective measurement principles.
The object of the present invention is to provide an apparatus and a method for determining the dew point of components of a gas mixture which approximately achieve the measuring accuracy of mirror dew point measuring devices at a low price.
This object is met by the features of claims 1 and 12.
The substantial feature of the solution according to the present invention consists in that the temperature-controlled condensate collector is arranged so as to be spatially separated from the humidity sensor. Liquid is precipitated in the cooled condensate collector when the dew point temperature is reached. This li~uid is evaporated again during the heating of the condensate collector and leads to an increased vapor content in the surroundings of the condensate collector in which the humidity sensor is arranged. The increased vapor content is clearly indicated by the humidity sensor.
A substantial advantage of the invention consists in that it is no longer necessary to measure the temperature and occurrence of condensate at the same moment. Rather, according to the present invention, the conclusion is reached indirectly~ based on the occurrence of a significant change in humidity during the he'ating of the condensate collector, that the sought for dew point temperature has been passed through during the preceding cooling phase. The 2 ~
sought for dew point temperature can accordingly be determined iteratively by changing the temperature differences passed through and accordingly by changing the final cooling temperature. The initial temperatures at the start of the cooling phase can also be changed for this purpose.
To make the change in the vapor content more significant, it is advantageous to arrange at least the humidity sensor and the condensate collector in a measurement chamber. ~ further increase in significance can be achieved by preventing the flow of measurement gas at least during the heating phase. The measurement gas quantity necessary for determining the dew point is also reduced simultaneously.
Another possibility for increasing the significance of the indication consists in arranging the humidity sensor spatially directly above the condensate collector so that the condensate evaporating during the heating phase acts upon the humidity sensor directly.
A mirror or an electrode arrangement can be used as humidity sensor. Other component elements showing a sharp change in electrical resistance as a result of the increased vapor content can also be used. The peripheral equipment required for determining the significant change in humidity is governed by the utilized humidity sensor.
When using a mirror as humidity sensor, in which case the significant change in humidity is determined on the basis of the misting of the mirror, the control system expenditure and method implementation is substantially simplified in contrast to the ~cnown mirror dew point measuring devices. In the known mirror dew point measuring devices the difficulty with respect to control technology consists in determining as accurately as possible the temperature at the moment condensate is first precipitated.
That is, the exact time of the formation of dew and the ~a~ri~
temperature prevailing at this time must be determined together by measurement techniques. In contrast to this, the measurement of the humidity, the measurement of the occurrence of condensate and the measurement of temperature are so-to-speak uncoupled in terms of measurement technology in the present invention. It is on:Ly necessary to determine whether or not a sharp change in humidity occurs within a comparatively large temperature and time interval. The measurement of the respective de~:l point temperature is effected by iteration.
Also, as a result of the use of an electrode arrangement as humidity sensor, the hygroscopicity of the conductor path carrier plays no role, since the change in the humidity content need only be qualitatively determined.
In an advantageous manner, the hygroscopicity of the material used for the condensate collector also plays only a subordinate role, since the amount of liquid which is present prior to reaching the saturation temperature by hygroscopicity is so s~.all that the resulting indica~ion pulses are clearly distinguished from those of a condensate collector after the temperature falls below the dew point or when the dew point temperature is reached.
Another advantage of the present invention consists in that the humidity sensor itself need not be heated or cooled and only insignificant amounts of condensate are precipitated on it. A chief source of dirt and corrosion is accordingly eliminated.
When using an electrode arrangement as humidity sensor corrosion can be prevented in that the humidity sensor is acted upon by alternating current during measurement so that no galvanic changes are possible.
Another advantage of the present invention consists in that it is comparatively simple to measure the temperature on the surface of the condensate collector, since good heat 2~7~
conducting and corrosion-resistant metals can preferably be used in this case.
Another advantage consists in that soiling which occurs as a result of the frequent condensation and subsequent re-evaporation of liquid in the condensate collector has only an insubstantial influence on the functioning and measuring accuracy of the prèsent invention.
A particularly simple and compact construction results when a Peltier element is used for cooling and heating the condensate collector.
A compact construction also results when one side of a heat exchanger, particularly an evaporation heat exchanger, is constructed as condensate collector. When the cooling of the condensate collector is effected by means of an evaporation heat exchanger, the heating of the condensate collector can be effected g. by means of resistance heating or by microwave radiation.
In particular, if the dew point of a plurality of components in a gas mixture are to be determined one after the other it is reasonable to use a sintered metal heat exchanger as condensate collector. In this case the component with the highest dew point must first be completely removed from the measurement gas volume. Only when this is done can the next lowest dew point of a component be determined. That is, the condensate collector simultaneously acts as a cooling drier.
In such cases it is also sensible to provide a plurality of humidity sensors and/or a plurality of assigned condensate collectors. For example, the time required for determining the sought for dew point temperature is minimized when a plurality of humidity sensors with assigned condensate collectors with different starting temperatures and/or temperature differences during cooling are operated simultaneously.
.
~ r~ 3 The invention is explained in more detail ln the following with reference to an embodiment example shown in the drawing:
Fig. 1 shows a preferred embodiment: form of the present invention.
The dew point measuring device according to Fig. 1 comprises a condensate collector 1 in the form of an aluminum plate. Naturally, differently shaped condensate collectors consisting of other materials with good heat conduction can also be used. One side of the condensate collector 1 is connected with good heat conduction with a device in the form of a Peltier element 2 for regulating the temperature of the condensate collector 1. Condensate can precipitate on the other side or on an active surface 3. A
temperature sensor 4 by which the temperature of the condensate collector 1, or more exactly the temperature of the active surface 3, can be monitored and by which the sought for dew point temperature is measured is provlded on the active surface 3 of the condensate collector 1. The Peltier element 2 has electrical connections 6 and 7. A
humidity sensor 8 in the form of an electrode arrangement is provided spatially above the condensate collector 1. The humidity sensor 8 comprises an insulator 10 on which the two poles 11 and 12 of the electrode arrangement are arranged in the form of conductor paths which mesh with one another in the manner of cogs.
Humidity sensor 8 and condensate collector 1 are arranged in a measurement chamber 14. The measurement chamber 14 comprises a feed line 16 and a drain line 17.
The drain line 17 can be blocked by a valve 18. A delivery device 20 by which the measurement gas is conveyed into the measurement chamber 14 is shown schematically in the feed line 16. Desired devlces SUCll as filter, heat exchanger, volume flow controller, etc. which are not shown in more detail in the drawing can be inserted between the delivery device 20 and the measurement chamber 14 and between the measurement chamber 14 and the valve 18.
It is quite simple to regulate and operate the described embodiment form. At the moment the measuring device is put into operation or at the start of a measurement cycle, the active surface 3 of the condensate collector 1 is at the starting temperature level To~ The desired starting temperature To is usually the ambient temperature or the temperature of the measurement gas when it enters the measurement chamber 14. The condensate collector 1 or its active surface 3 can be cooled by the temperature difference ~T to a temperature T1 by cooling the condensate collector 1 by means of the Peltier element 2.
If the dew point measuring device according to the invention is used e.g. in a compressed-air supply system, the temperature Tl is a temperature defined as a warning threshold value or the desired dew point temperature of the respective monitored drier.
When the temperature T1 is reached the heating phase is initiated by reversing the poles of the electrical connections 6 and 7 of the Peltier element 2. The duration of the cooling phase or the duration of the heating phase and the holding time at the respective temperature level are governed by the case of application in question. During the cooling phase it must be ensured that sufficient condensate can precipitate and it must be ensured during the heating phase that the condensate which has formed is completely evaporated again. If the humidity sensor 8 shows an indication pulse, e.g. in the form of a sharp drop in resistance, before reachiny the freely selectable heating temperature governed by the area of application, the final cooling temperature reached during the preceding cooling 2~'7~
phase is the sought for dew point temperature or a temperature below the dew point temperature.
If the humidity sensor ~ shows no indication pulse, that is, if the resistance remains e.g. constant, the dew point temperature has not been rea~hed.
In the first case a higher temperature, e.g. +1 ~, is proceeded toward; a lower temperature is proceeded toward in the second case.
In this way it is ensured that the series of measurements is always carried out in the direction of the actually present dew point temperature.
The exact parameters of the control strategy, that is, e.g. the temperature steps, cooling and heating speeds are governed by the respective case of use.
Insofar as a permanent indication is desired the present dew point temperature is defined e.g. in such a way that the last temperature at which there was an indicatlon corresponds to the respective dew point temperature.
The allowable or suitable length of time of the measurement interval is likewise governed by the respective application and can be influenced by the output of the installed cooling device. Further, it is also possible to install a plurality of cooling surfaces at one sensor or to place a plurality of arrangements in a measuring chamber so that a quasi-continuous measurement is possible.
For an automated regulating of the dew point measurement device, according to the invention, or an automated determination of the dew point temperature the indication of the humidity sensor 8, the signal from the temperature sensor 4 and the controlling of the Peltier element 2 are combined in a control device which is not shown in more detail.
If the duration of the coolin~ phases is optimized, measuring accuracies of the mirror dew point measurlng device can be achieved with the described dew point 2 ~
measuring device according to Fig. 1, i.e. measuring accuracies of + 0.2 oc.
In a concrete construction of the dew point measuring device according to Fig. 1, e.g. a plate of epoxy resin (15 x 15 mm) as insulator 10 with conductor paths of copper having a width of 300 ~m and a thickness of 35 ~m and arranyed at a distance of 150 ~m from one another as poles 11 and 12 are used as humidity sensor 8. An aluminum plate with dimensions of 3 x 15 x 15 mm is used as condensate collector. The distance between the humidity sensor 8 and the condensate collector 1 is 2 mm. The Peltier element 2 has a current consumption of 1.8 A at a maximum 3 Volt voltage. The measurement chamber 14 has a volume of 20 cm3.
When measuring in compressed air of 8 bar at dew point temperatures between 1.5 and 2.5 C and at an average sensible temperature of the compressed air of 22 C, 12 measuring cycles were carried out per hour. The volume flow through the measurement chamber 14 amounted to 150 nltr./h.
(normal liters per hour). The following resistance changes were achieved during the heating of the condensate collector 1:
starting conditions >50 * 106 Ohm 0.2 X above dew point temperature >50 * 106 Ohm 0.2 K below dew point temperature <0.5* 106 Ohm Accordingly, there is a significant resistance ratio of 1:100 with the aforementioned measuring accuracy of 0.2 X.
A substantially smaller resistance ratio would also be significant, so that the measuring accuracy could be further increased.
Optical sensors, ultrasonic sensors and other component elements suitable for capacitative or resistance measurements can also be used in addition to the shown conductor path sensor.
2 ~
The measurement gas chamber can be designed in principle for optionally high pressures.
The arrangement described as preferred is very robust and, in contrast to the conventional optical and capacitative devices, is resistant to soiling, does not require calibration and can be produced from inexpensive industry standard components, including the peripheral devices, so that the prices which can be realized are in the order of magnitude of 1/10 of the mirror dew point measuring devices.
~ he described basic arrangement allows many variants, e.g. the cascade connection of a plurality of Peltier elements for achieving particularly low cooling temperatures or high re-cooling temperatures or the use of other cooling systems.
Other variants are conceivable iIl such a way that the sensor also undergoes a temperature treatment, e.g. for selectively determining the presence of a plurality of condensable vapors, or a plurality of sensors are placed around a cooling surface or a plurality of cooling surfaces are placed around one or more sensors.
MIXTURE COMPONENTS
The invention is directed to an apparatus and a method for determining the dew point of components of a gas mixture.
Accurate determination of the vapor component in gas flows is highly important in many areas of process technology in order to avoid corrosion damage, to prevent unwanted emissions, or to condition the gas flow with respect to optimal implementation of the process.
A widely used arrangement, particularly for determining the water vapor component in air flows, consists in dew point hygrometers in which the air is cooled until the water vapor condenses. The dew point temperature determined in this way corresponds to the saturation temperature of the air, from which the relative humidity can also be derived when the sensible temperature of the air flow is known.
As is shown in the publication DE-Buch, Winfried Luck, Principles of Humidity, Measurement, Regulating [Feuchtigkeits-Grundlagen, Messen, Regeln], R. Oldenburg, Munich, Vienna 1964, pages 74 to 84, essentially three methods and apparatuses are known for this purpose:
a) dew point mirrors which mist when cooled below the dew point, b) cooled electrode arrangements which change electrical resistance or capacitance when condensation is precipitated, and c) fog chambers in which fog can be observed when the temperature of the measurement gas located in the chamber falls below the dew point.
In fog chamber hygrometers, the measurement gas is condensed and then expanded without falling below the dew point. The humid air cools considerably and virtually homogeneously due to the expansion. In an illuminated "fog chamber" the formation of fog can be observed in the event ~ o ~ ~ r~ 6 ~
that the temperature falls below the dew point of the measurement gas as a result of the drop in temperature due to the expansion. The precise overpressure at which fog is formed when the gas (e.g~ at atmospheric air pressure) is expanded is sought by repeated compression and subsequent expansion. The dew point temperature can be calculated from the humid gas temperature and the compression ratio.
In a mirror dew point measuring device with dew point mirrors the measurement gas is cooled to the boundary of dew precipitation by attempting to adjust the precise cooling at which the dew formation is detectable by repeated adjustment of the level of the cooling temperature. Devices are known in which a photocell arrangement always controls the cooling and/or heating of the dew mirror via a control amplifier in such a way that the dew limit is fixed automatically - i.e.
in a regulated manner. The task of automatically adjusting the dew point involves certain difficulties with respect to control technology because the dew formation rate is very highly dependent on the respective dew point temperature.
Besides the above-mentioned technical publication, different variants of mirror dew point measuring devices are also known from DE-OS 19 57 322, DE-AS 12 99 437, DE-OS 26 40 663, DE-OS 29 45 445 and US-PS 3 385 098.
To avoid the difficulties involved in photoelectric mirror scanning in mirror dew point measurement devices it is only necessary to make use of cooled electrode arrangements. Very low junction resistances which can supply powerful control quantities at low cost can be achieved by means of a meandering arrangement of a conductor path on an insulating substrate. Instead of measuring the resistance of the cooled electrode arrangement, the change in capacitance occurring when the electrical constant is changed as a result of the misting of the sensor can also be made use of. Such a capacitative dew point detector is known e.g. from DE-OS 32 31 534.
2 ~
The substantial disadvantage of devices with dew point mirrors and fog chambers consists in the high cost in apparatus and control technology, which results in correspondingly high prices for such devices. On the other hand, cooled electrode arrangements have the disadvantage that the measuring accuracy is comparatively low. This is primarily a result of the hygroscopicity of all the materials conventionally used for conductor path carriers, which already leads to a drop in the surface resistance long before saturation and accordingly before the precipitation of liquid. The use of conductor path carriers with low hygroscopicity, e.g. paraffins or waxes, or the conditioning of conventional conductor path carriers with these materials likewise leads to unsatisfactory results because of the low melting point and low mechanical strength.
Another disadvantage of cooled electrode arrangements consists in that it is very difficult to determine the exact surface temperature of these materials, i.e. the exact surface temperature of the condensation surface, because of their poor heat conducting characteristics. Consequently, the dew point temperature can also not be exactly determined.
The described disadvantages of the known measuring devices and methods is also referred to in the article "Is My Compressed-Air Drier Working? [Funktioniert mein Drucklufttrockner?]" by E. Prumm, in the technical journal "Compressed-Air Technology [Drucklufttechnik]", 11-12/1989, pages 33 and 34.
Finally, an apparatus and a method for determining the dew point of components of a gas mixture from which the invention proceeds in the preamble of claim 1 is known from DE-OS 37 13 864. The known apparatus comprises a humidity sensor which can involve a dew pOillt mirror as well as a cooled electrode arrangement. Moreover, a condensate collector is provided which can be cooled or heated by means ~ .
;
2a~r~
of a Peltier element. A temperature sensor which measures the temperature of the condensate collector or condensation surface is arranged in thermal contact with the condensate collector. The humidity sensor, condensate collector and temperature sensor are integrated in an individual structural component part. For this reason and because the precipitation of liquid in this known system is determined either by a dew point mirror or by cooled electrode arrangements, this arrangement also has the aforementioned disadvantages of the respective measurement principles.
The object of the present invention is to provide an apparatus and a method for determining the dew point of components of a gas mixture which approximately achieve the measuring accuracy of mirror dew point measuring devices at a low price.
This object is met by the features of claims 1 and 12.
The substantial feature of the solution according to the present invention consists in that the temperature-controlled condensate collector is arranged so as to be spatially separated from the humidity sensor. Liquid is precipitated in the cooled condensate collector when the dew point temperature is reached. This li~uid is evaporated again during the heating of the condensate collector and leads to an increased vapor content in the surroundings of the condensate collector in which the humidity sensor is arranged. The increased vapor content is clearly indicated by the humidity sensor.
A substantial advantage of the invention consists in that it is no longer necessary to measure the temperature and occurrence of condensate at the same moment. Rather, according to the present invention, the conclusion is reached indirectly~ based on the occurrence of a significant change in humidity during the he'ating of the condensate collector, that the sought for dew point temperature has been passed through during the preceding cooling phase. The 2 ~
sought for dew point temperature can accordingly be determined iteratively by changing the temperature differences passed through and accordingly by changing the final cooling temperature. The initial temperatures at the start of the cooling phase can also be changed for this purpose.
To make the change in the vapor content more significant, it is advantageous to arrange at least the humidity sensor and the condensate collector in a measurement chamber. ~ further increase in significance can be achieved by preventing the flow of measurement gas at least during the heating phase. The measurement gas quantity necessary for determining the dew point is also reduced simultaneously.
Another possibility for increasing the significance of the indication consists in arranging the humidity sensor spatially directly above the condensate collector so that the condensate evaporating during the heating phase acts upon the humidity sensor directly.
A mirror or an electrode arrangement can be used as humidity sensor. Other component elements showing a sharp change in electrical resistance as a result of the increased vapor content can also be used. The peripheral equipment required for determining the significant change in humidity is governed by the utilized humidity sensor.
When using a mirror as humidity sensor, in which case the significant change in humidity is determined on the basis of the misting of the mirror, the control system expenditure and method implementation is substantially simplified in contrast to the ~cnown mirror dew point measuring devices. In the known mirror dew point measuring devices the difficulty with respect to control technology consists in determining as accurately as possible the temperature at the moment condensate is first precipitated.
That is, the exact time of the formation of dew and the ~a~ri~
temperature prevailing at this time must be determined together by measurement techniques. In contrast to this, the measurement of the humidity, the measurement of the occurrence of condensate and the measurement of temperature are so-to-speak uncoupled in terms of measurement technology in the present invention. It is on:Ly necessary to determine whether or not a sharp change in humidity occurs within a comparatively large temperature and time interval. The measurement of the respective de~:l point temperature is effected by iteration.
Also, as a result of the use of an electrode arrangement as humidity sensor, the hygroscopicity of the conductor path carrier plays no role, since the change in the humidity content need only be qualitatively determined.
In an advantageous manner, the hygroscopicity of the material used for the condensate collector also plays only a subordinate role, since the amount of liquid which is present prior to reaching the saturation temperature by hygroscopicity is so s~.all that the resulting indica~ion pulses are clearly distinguished from those of a condensate collector after the temperature falls below the dew point or when the dew point temperature is reached.
Another advantage of the present invention consists in that the humidity sensor itself need not be heated or cooled and only insignificant amounts of condensate are precipitated on it. A chief source of dirt and corrosion is accordingly eliminated.
When using an electrode arrangement as humidity sensor corrosion can be prevented in that the humidity sensor is acted upon by alternating current during measurement so that no galvanic changes are possible.
Another advantage of the present invention consists in that it is comparatively simple to measure the temperature on the surface of the condensate collector, since good heat 2~7~
conducting and corrosion-resistant metals can preferably be used in this case.
Another advantage consists in that soiling which occurs as a result of the frequent condensation and subsequent re-evaporation of liquid in the condensate collector has only an insubstantial influence on the functioning and measuring accuracy of the prèsent invention.
A particularly simple and compact construction results when a Peltier element is used for cooling and heating the condensate collector.
A compact construction also results when one side of a heat exchanger, particularly an evaporation heat exchanger, is constructed as condensate collector. When the cooling of the condensate collector is effected by means of an evaporation heat exchanger, the heating of the condensate collector can be effected g. by means of resistance heating or by microwave radiation.
In particular, if the dew point of a plurality of components in a gas mixture are to be determined one after the other it is reasonable to use a sintered metal heat exchanger as condensate collector. In this case the component with the highest dew point must first be completely removed from the measurement gas volume. Only when this is done can the next lowest dew point of a component be determined. That is, the condensate collector simultaneously acts as a cooling drier.
In such cases it is also sensible to provide a plurality of humidity sensors and/or a plurality of assigned condensate collectors. For example, the time required for determining the sought for dew point temperature is minimized when a plurality of humidity sensors with assigned condensate collectors with different starting temperatures and/or temperature differences during cooling are operated simultaneously.
.
~ r~ 3 The invention is explained in more detail ln the following with reference to an embodiment example shown in the drawing:
Fig. 1 shows a preferred embodiment: form of the present invention.
The dew point measuring device according to Fig. 1 comprises a condensate collector 1 in the form of an aluminum plate. Naturally, differently shaped condensate collectors consisting of other materials with good heat conduction can also be used. One side of the condensate collector 1 is connected with good heat conduction with a device in the form of a Peltier element 2 for regulating the temperature of the condensate collector 1. Condensate can precipitate on the other side or on an active surface 3. A
temperature sensor 4 by which the temperature of the condensate collector 1, or more exactly the temperature of the active surface 3, can be monitored and by which the sought for dew point temperature is measured is provlded on the active surface 3 of the condensate collector 1. The Peltier element 2 has electrical connections 6 and 7. A
humidity sensor 8 in the form of an electrode arrangement is provided spatially above the condensate collector 1. The humidity sensor 8 comprises an insulator 10 on which the two poles 11 and 12 of the electrode arrangement are arranged in the form of conductor paths which mesh with one another in the manner of cogs.
Humidity sensor 8 and condensate collector 1 are arranged in a measurement chamber 14. The measurement chamber 14 comprises a feed line 16 and a drain line 17.
The drain line 17 can be blocked by a valve 18. A delivery device 20 by which the measurement gas is conveyed into the measurement chamber 14 is shown schematically in the feed line 16. Desired devlces SUCll as filter, heat exchanger, volume flow controller, etc. which are not shown in more detail in the drawing can be inserted between the delivery device 20 and the measurement chamber 14 and between the measurement chamber 14 and the valve 18.
It is quite simple to regulate and operate the described embodiment form. At the moment the measuring device is put into operation or at the start of a measurement cycle, the active surface 3 of the condensate collector 1 is at the starting temperature level To~ The desired starting temperature To is usually the ambient temperature or the temperature of the measurement gas when it enters the measurement chamber 14. The condensate collector 1 or its active surface 3 can be cooled by the temperature difference ~T to a temperature T1 by cooling the condensate collector 1 by means of the Peltier element 2.
If the dew point measuring device according to the invention is used e.g. in a compressed-air supply system, the temperature Tl is a temperature defined as a warning threshold value or the desired dew point temperature of the respective monitored drier.
When the temperature T1 is reached the heating phase is initiated by reversing the poles of the electrical connections 6 and 7 of the Peltier element 2. The duration of the cooling phase or the duration of the heating phase and the holding time at the respective temperature level are governed by the case of application in question. During the cooling phase it must be ensured that sufficient condensate can precipitate and it must be ensured during the heating phase that the condensate which has formed is completely evaporated again. If the humidity sensor 8 shows an indication pulse, e.g. in the form of a sharp drop in resistance, before reachiny the freely selectable heating temperature governed by the area of application, the final cooling temperature reached during the preceding cooling 2~'7~
phase is the sought for dew point temperature or a temperature below the dew point temperature.
If the humidity sensor ~ shows no indication pulse, that is, if the resistance remains e.g. constant, the dew point temperature has not been rea~hed.
In the first case a higher temperature, e.g. +1 ~, is proceeded toward; a lower temperature is proceeded toward in the second case.
In this way it is ensured that the series of measurements is always carried out in the direction of the actually present dew point temperature.
The exact parameters of the control strategy, that is, e.g. the temperature steps, cooling and heating speeds are governed by the respective case of use.
Insofar as a permanent indication is desired the present dew point temperature is defined e.g. in such a way that the last temperature at which there was an indicatlon corresponds to the respective dew point temperature.
The allowable or suitable length of time of the measurement interval is likewise governed by the respective application and can be influenced by the output of the installed cooling device. Further, it is also possible to install a plurality of cooling surfaces at one sensor or to place a plurality of arrangements in a measuring chamber so that a quasi-continuous measurement is possible.
For an automated regulating of the dew point measurement device, according to the invention, or an automated determination of the dew point temperature the indication of the humidity sensor 8, the signal from the temperature sensor 4 and the controlling of the Peltier element 2 are combined in a control device which is not shown in more detail.
If the duration of the coolin~ phases is optimized, measuring accuracies of the mirror dew point measurlng device can be achieved with the described dew point 2 ~
measuring device according to Fig. 1, i.e. measuring accuracies of + 0.2 oc.
In a concrete construction of the dew point measuring device according to Fig. 1, e.g. a plate of epoxy resin (15 x 15 mm) as insulator 10 with conductor paths of copper having a width of 300 ~m and a thickness of 35 ~m and arranyed at a distance of 150 ~m from one another as poles 11 and 12 are used as humidity sensor 8. An aluminum plate with dimensions of 3 x 15 x 15 mm is used as condensate collector. The distance between the humidity sensor 8 and the condensate collector 1 is 2 mm. The Peltier element 2 has a current consumption of 1.8 A at a maximum 3 Volt voltage. The measurement chamber 14 has a volume of 20 cm3.
When measuring in compressed air of 8 bar at dew point temperatures between 1.5 and 2.5 C and at an average sensible temperature of the compressed air of 22 C, 12 measuring cycles were carried out per hour. The volume flow through the measurement chamber 14 amounted to 150 nltr./h.
(normal liters per hour). The following resistance changes were achieved during the heating of the condensate collector 1:
starting conditions >50 * 106 Ohm 0.2 X above dew point temperature >50 * 106 Ohm 0.2 K below dew point temperature <0.5* 106 Ohm Accordingly, there is a significant resistance ratio of 1:100 with the aforementioned measuring accuracy of 0.2 X.
A substantially smaller resistance ratio would also be significant, so that the measuring accuracy could be further increased.
Optical sensors, ultrasonic sensors and other component elements suitable for capacitative or resistance measurements can also be used in addition to the shown conductor path sensor.
2 ~
The measurement gas chamber can be designed in principle for optionally high pressures.
The arrangement described as preferred is very robust and, in contrast to the conventional optical and capacitative devices, is resistant to soiling, does not require calibration and can be produced from inexpensive industry standard components, including the peripheral devices, so that the prices which can be realized are in the order of magnitude of 1/10 of the mirror dew point measuring devices.
~ he described basic arrangement allows many variants, e.g. the cascade connection of a plurality of Peltier elements for achieving particularly low cooling temperatures or high re-cooling temperatures or the use of other cooling systems.
Other variants are conceivable iIl such a way that the sensor also undergoes a temperature treatment, e.g. for selectively determining the presence of a plurality of condensable vapors, or a plurality of sensors are placed around a cooling surface or a plurality of cooling surfaces are placed around one or more sensors.
Claims (16)
1. Apparatus for determining the dew point of components of a gas mixture comprising - a humidity sensor (8), - a condensate collector (1), - a device (2) which is thermally coupled with the condensate collector (1) for regulating the temperature of the condensate collector (1), and a temperature sensor (4) which thermally contacts the condensate collector (1), characterized in that the humidity sensor (8) measures the gas humidity and is spatially separated from, but adjacent to, the condensate collector (1).
2. Apparatus according to claim 1, characterized in that the humidity sensor (8) is placed spatially directly above the condensate collector (1).
3. Apparatus according to one of the preceding claims, characterized in that at least the humidity sensor (8) and condensate collector (1) are arranged in a measurement chamber (14).
4. Apparatus according to claim 3, characterized in that the measurement chamber (14) has a feed line (16) and a drain line (17), and in that at least the drain line (17) can be blocked by a valve (18).
5. Apparatus according to one of the preceding claims, characterized in that the humidity sensor (8) comprises a mirror which indicates a high vapor content by misting.
6. Apparatus according to one of claims 1 to 4, characterized in that the humidity sensor (8) comprises a substrate (10) of an electrical insulator on which at least two conductor paths (11, 12) are arranged which are spatially separated from one another.
7. Apparatus according to one of the preceding claims, characterized in that the device for regulating the temperature of the condensate collector (1) comprises at least one Peltier element (2) by means of which the condensate collector can be cooled or heated.
8. Apparatus according to one of claims 1 to 6, characterized in that the device (2) for regulating the temperature of the condensate collector (1) comprises a heat exchanger, particularly an evaporation heat exchanger, one side of which forms the condensate collector.
9. Apparatus according to claim 8, characterized in that the heat exchanger is a sintered metal heat exchanger.
10. Apparatus according to one of the preceding claims, characterized in that a plurality of humidity sensors are provided.
11. Apparatus according to one of the preceding claims, characterized in that a plurality of condensate collectors are provided.
12. Method for determining the dew point of components of a gas mixture, particularly with an apparatus according to one of the preceding claims, comprising the following method steps:
a) cooling a condensate collector which is arranged in the gas mixture from an initial temperature To by a temperature differential .DELTA.T, b) heating the condensate collector, c) measuring the change in the gas humidity by means of a humidity sensor arranged so as to be separate from the condensate collector, d) repeating the preceding method steps with changed values for the initial temperature To and/or temperature differential T until a change in the humidity during the heating of the condensate collector is detected at least once, and e) determining the final temperature during the cooling phase as dew point temperature, at which there is no longer a change in humidity in the subsequent heating phase.
a) cooling a condensate collector which is arranged in the gas mixture from an initial temperature To by a temperature differential .DELTA.T, b) heating the condensate collector, c) measuring the change in the gas humidity by means of a humidity sensor arranged so as to be separate from the condensate collector, d) repeating the preceding method steps with changed values for the initial temperature To and/or temperature differential T until a change in the humidity during the heating of the condensate collector is detected at least once, and e) determining the final temperature during the cooling phase as dew point temperature, at which there is no longer a change in humidity in the subsequent heating phase.
13. Method according to claim 12, characterized in that there is a closed measurement volume at least during the heating phase or during the measurement of the humidity.
14. Method according to one of claims 12 to 13, characterized in that the condensate collector is heated to a temperature above the starting temperature To during the heating phase.
15. Method according to one of claims 12 to 14, characterized in that the individual method steps are carried out simultaneously with different starting values for To and .DELTA.T.
16. Method according to one of claims 12 to 15, characterized in that the change in humidity during the heating phase is determined by the change in resistance of an electrode arrangement (11, 12), wherein the electrode arrangement is acted upon by alternating current.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DEP4023796.6 | 1990-07-26 | ||
| DE19904023796 DE4023796C1 (en) | 1990-07-26 | 1990-07-26 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2066765A1 true CA2066765A1 (en) | 1992-01-27 |
Family
ID=6411072
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2066765 Abandoned CA2066765A1 (en) | 1990-07-26 | 1991-07-26 | Process and device for determining the dew point of components of a gas mixture |
Country Status (5)
| Country | Link |
|---|---|
| EP (1) | EP0494291A1 (en) |
| JP (1) | JPH05501764A (en) |
| CA (1) | CA2066765A1 (en) |
| DE (1) | DE4023796C1 (en) |
| WO (1) | WO1992001926A1 (en) |
Families Citing this family (5)
| 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 |
| DE19833586B4 (en) * | 1997-07-28 | 2007-05-16 | Ust Umweltsensortechnik Gmbh | Device for determining the moisture content in gases |
| DE19806476C1 (en) * | 1998-02-17 | 1999-10-28 | Deutsch Zentr Luft & Raumfahrt | Volumetric gas quantity determination in pipe conducting fluid and gaseous phase flow |
| DE102008024021B4 (en) | 2008-05-16 | 2010-02-11 | Rational Ag | Method for moisture measurement, condensing pressure chamber for this purpose and cooking appliance hereby |
| JP5408027B2 (en) * | 2010-04-28 | 2014-02-05 | トヨタ自動車株式会社 | Gas composition detection system and engine control system |
Family Cites Families (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3083565A (en) * | 1958-06-20 | 1963-04-02 | Bailey Meter Co | Dewpoint hygrometer |
| FR2231287A5 (en) * | 1973-05-23 | 1974-12-20 | Solvay | |
| US4224565A (en) * | 1978-06-05 | 1980-09-23 | Bell Telephone Laboratories, Incorporated | Moisture level determination in sealed packages |
| DE3243320C2 (en) * | 1982-11-23 | 1986-03-13 | Endress U. Hauser Gmbh U. Co, 7867 Maulburg | Dew point mirror hygrometer |
| DE3446277A1 (en) * | 1984-12-19 | 1986-06-19 | Forschungsinstitut Prof. Dr.-Ing.habil, Dr.phil.nat. Karl Otto Lehmann, Nachf. GmbH & Cie, 7570 Baden-Baden | Detector for measuring the dewpoint |
| US4579462A (en) * | 1985-05-20 | 1986-04-01 | Trans-Met Engineering, Inc. | Dew point measuring apparatus |
| DD269014A1 (en) * | 1986-07-31 | 1989-06-14 | Adw Ddr Inst Kosmosforschung | DEVICE FOR DETECTING CONDENSATE |
| DE3713864A1 (en) * | 1987-04-25 | 1988-11-17 | Hoelter Heinz | Method for the accurate measurement of the relative atmospheric humidity |
-
1990
- 1990-07-26 DE DE19904023796 patent/DE4023796C1/de not_active Expired - Fee Related
-
1991
- 1991-07-26 CA CA 2066765 patent/CA2066765A1/en not_active Abandoned
- 1991-07-26 EP EP19910913867 patent/EP0494291A1/en not_active Withdrawn
- 1991-07-26 WO PCT/EP1991/001415 patent/WO1992001926A1/en not_active Application Discontinuation
- 1991-07-26 JP JP51275591A patent/JPH05501764A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| JPH05501764A (en) | 1993-04-02 |
| EP0494291A1 (en) | 1992-07-15 |
| DE4023796C1 (en) | 1991-11-14 |
| WO1992001926A1 (en) | 1992-02-06 |
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