Note: Descriptions are shown in the official language in which they were submitted.
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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
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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
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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
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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
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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
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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.
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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
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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
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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.
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