Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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
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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.
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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
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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
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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
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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
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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.
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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
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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
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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.
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12
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.
