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Claims  |
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We claim:
1. Dew point temperature measuring apparatus comprising:
heat flow sensor means having a condensing surface means and operative upon
a flow of heat through said condensing surface means to generate an output
signal proportional to the rate at which said heat flows through said
condensing surface means;
temperature change means to bring the temperature of said condensing
surface means to the dew point temperature of a gas flowing adjacent said
condensing surface means;
temperature sensor means to measure the surface temperature of said
condensing surface means; and
detecting means for determining a significant change in said output signal
from said heat flow sensor means thereby to determine the occurrence of
condensation on said condensing surface means whereby, the temperature
measured by said temperature sensor means, when said detecting means
detects said significant change in said output signal, is indicative of
the dew point temperature.
2. Dew point temperature measuring apparatus according to claim 1 wherein
said heat flow sensor means comprises a thermopile.
3. Dew point temperature measuring apparatus according to claim 1 wherein
said heat flow sensor means comprises a pair of matched heat flow sensors
and wherein said condensing surface means comprises a pair of condensing
surfaces for said heat flow sensors, respectively, arranged in the flow
path of said gas; wherein said temperature change means includes cooling
means operative to maintain one of said heat flow sensors at a temperature
slightly cooler than the other of said heat flow sensors; wherein said
detecting means includes means connecting the output signals of said heat
flow sensors to obtain the algebraic difference whereby a significant
change in the resulting composite output signal occurring upon said
condensation is more readily detectable; and wherein said temperature
sensor means is associated with the cooler one of said heat flow sensors.
4. Dew point temperature measuring apparatus according to claim 1 wherein
said temperature change means comprises a heat sink in thermal association
with said heat flow sensor means, and further comprises cooling means for
cooling said condensing surface of said heat flow sensor means.
5. Dew point temperature measuring apparatus according to claim 3 wherein
said temperature change means further includes heating means actuable for
heating the condensing surfaces of said pair of heat flow sensors; and
wherein said detecting means is responsive to a change in said composite
output signal resulting from said condensation to actuate said heating
means for raising the temperature of said condensing surfaces above said
dew point temperature, said detecting means further being responsive to a
change in said composite output signal resulting from a cessation in and
evaporation of said condensation to deactuate said heating means and
enable said cooling means to lower the temperature of said condensing
surfaces toward said dew point temperature.
6. Dew point temperature measuring apparatus according to claim 1 wherein
said heat flow sensor means comprises pairs of matched first and second
heat flow sensors having their condensing surfaces arranged in opposed
relation; said pairs being arranged in spaced relation along the flow path
of said gas; wherein said temperature sensor means comprises:
temperature sensors associated with the condensing surfaces of said first
heat flow sensors, respectively; wherein said temperature change means is
operative to maintain the condensing surfaces of said pairs at different
temperatures along said flow path, and is further operative to maintain
each of said first heat flow sensors at a temperature slightly cooler than
its associated second heat flow sensor; and wherein said detecting means
includes means connecting the output signals of said first and second heat
flow sensors of each of said pairs to obtain the algebraic difference
whereby a significant change in the resulting composite output signal
occurring upon condensation upon the condensing surface of said first heat
flow sensor of a particular one of said pairs indicates that the
temperature of said condensation surface of said first heat flow sensor of
said particular one of said pairs is the dew point temperature of said
gas.
7. Dew point temperature measuring apparatus according to claim 6 wherein
said temperature change means comprises first and second wedge shape heat
sinks spaced apart to define said flow path, with said first and second
heat sinks increasing in mass in a downstream direction; wherein said
temperature change means includes cooling means in thermal association
with said first heat sink; and wherein said first and second heat flow
sensors are in thermal association with said first and second heat sinks,
respectively.
8. Dew point temperature measuring apparatus comprising:
first and second adjacent matched heat flow sensors having first and second
condensing surfaces, respectively, arranged in a flow path of a gas and
operative upon a flow of heat through said condensing surfaces to generate
output signals proportional to the rate at which said heat flows through
said condensing surfaces, respectively;
temperature change means to bring the temperature of the condensing
surfaces of said first and second heat flow sensors toward the dew point
temperature of said gas, and including cooling means operative to maintain
said first condensing surface at a temperature slightly lower than said
second condensing surface whereby condensation at said dew point
temperature will first occur upon said first condensing surface;
temperature sensor means to measure the surface temperature of said first
condensing surface; and
detecting means connecting the output signals of said first and second heat
flow sensors in series opposition whereby said output signals are
algebraically summed to provide a composite control signal which changes
abruptly upon occurrence of said condensation whereby, the temperature
measured by said temperature sensor means, when said detecting means
detects an abrubt change in said composite control signal, is indicative
of the dew point temperature.
9. Dew point temperature measuring apparatus according to claim 8 wherein
said first and second heat flow sensors are thermopiles.
10. Dew point temperature measuring apparatus according to claim 8 wherein
said cooling means comprises heat sink means in thermal association with
said first and second heat flow sensors for cooling said first and second
condensing surfaces toward said dew point temperature, and thermal barrier
means disposed in the heat flow path between said second condensing
surface and said heat sink means for maintaining said first condensing
surface at said temperature slightly cooler than said second condensing
surface, said first and second condensing surfaces being otherwise
essentially isothermal.
11. Dew point temperature measuring apparatus according to claim 8 and
including means defining said flow path and operative to pass a sample of
said gas along said flow path and across said first and second condensing
surfaces in a substantially steady state condition.
12. Dew point temperature measuring apparatus according to claim 8 wherein
said temperature change means further includes heating means actuable for
heating said first and second condensing surfaces; and
wherein said detecting means is responsive to a change in said composite
control signal resulting from said condensation to actuate said heating
means for raising the temperature of said condensing surfaces above said
dew point temperature, said detecting means further being responsive to a
change in said composite control signal resulting from a cessation in and
evaporation of said condensation to deactuate said heating means and
enable said cooling means to lower the temperature of said condensing
surfaces toward said dew point temperature.
13. Dew point temperature measuring apparatus according to claim 8
including heating means actuable for heating said first and second
condensing surfaces, and wherein said detecting means comprises:
first logic means operative to produce said composite control signal in
response to a detected difference between the output signals of said first
and second heat flow sensors;
second logic means operative to compute a desired dew point temperature in
response to a temperature signal and a pressure signal, and to a
predetermined relative humidity of the gas which is to be maintained;
third logic means, responsive to said first and second logic means, for
actuating means for introducing dry gas into said gas when said
temperature of said first condensing surface is at least as high as said
desired dew point temperature at the time said first logic means produces
said composite control signal; and
means responsive to said first logic means to actuate said heating means at
the time said first logic means products said composite control signal.
14. Dew point temperature measuring apparatus according to claim 13 wherein
said first logic means is operative to detect both electrically positive
and electrically negative differences between the output signals of said
first and second heat flow sensors, and said means is responsive to said
first logic means to actuate also deactuates said heating means,
respectively, depending upon whether said composite control signal
represents an electrically positive or electrically negative difference,
whereby said first and second condensing surfaces are heated and cooled
cyclically above and below said dew point temperature. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to apparatus for measuring the dew
point temperature of a gaseous atmosphere for establishing its relative
humidity, and in particular to apparatus for measuring the dew point
temperature of hot, dirty air of the kind commonly found in industrial
environments such as drying systems.
2. Prior Art
Industrial process drying is typically accomplished by circulating hot, dry
air over or through a wet product. When the air becomes saturated with
moisture, it must be replaced with fresh air. Large quantities of energy
are used in heating this replacement air, much of which energy will be
wasted if a given quantity of air is replaced before its full drying
capacity has been used. On the other hand, the drying process will become
inefficient if the air is allowed to remain in the system after it has
become saturated with moisture. To achieve optimum efficiency, therefore,
it is necessary to use a system controller that can cause the air in the
dryer to be replaced at the proper time, and this controller must in turn
be provided with a sensor to measure the relative humidity of the air in
the dryer. In this regard, "humidity" is the amount of water vapor present
in a given quantity of air. If the amount of water vapor present is less
than the maximum possible at a given temperature, the amount is expressed
in terms of relative humidity. "Relative humidity" is the actual mass of
moisture in a given quantity of air divided by the mass of moisture that
the same volume and temperature of air would contain if it were completely
saturated.
The "dew point" is the temperature at which air with a given moisture
content becomes saturated. This measurement is accomplished by
artificially lowering the temperature of a surface and then noting the
temperature at which moisture first condenses. Dew point temperatures can
be converted directly to relative humidity measurements by reference to
appropriate charts or tables. Although the foregoing discussion relates to
water and air, it is equally applicable to the condensation of other
liquids from other gases.
A dew point sensor for use in industrial environments should be able to
operate with an accuracy of 5% or better in an atmosphere having a
temperature between 200 and 300 degrees Fahrenheit and a relative humidity
between 50% and 100% The sensor must be unaffected by the contaminants
that are often present in industrial drying operations.
Many types of dew point sensors are known to the art. They indicate the
relative humidity of an atmosphere indirectly by directly measuring the
dew point temperature, as previously discussed.
Some dew point sensors are capacitive and resistive devices and employ a
moisture-sensitive material to sense condensation. The moisture-sensitive
material is formed into a condensation surface on the dew point sensor,
and heat is slowly removed from the sensor until condensation begins. The
moisture-sensitive material reacts to the presence of the moisture of
condensation, for example by changing resistance. The temperature of the
sensor is continuously monitored, and when this change of resistance is
detected the temperature is recorded. From this temperature, the relative
humidity can be determined by reference to the tables mentioned.
Another prior art form of dew point sensor is a condensing mirror type
provided with a reflective surface. When the temperature of the reflective
surface is reduced to the dew point temperature, condensation takes place
The moisture of condensation is detected either by visual inspection or by
the use of a photocell or other light-sensitive device that responds to
the change in reflectivity caused by the presence of moisture
Unfortunately, none of the dew point measuring instruments just described
is able adequately to satisfy the requirements for use in industrial
processes. The high temperatures that occur in industrial processes damage
the components used in many sensors, and the contaminants often found in
such processes alter the absorption properties of many moisture-sensitive
sensors and cloud the reflecting surfaces of reflector type sensors.
Various other methods of achieving acceptable accuracy under the conditions
of industrial processes have been attempted. One such device is disclosed
in U.S. Pat. No. 2,680,371 issued to Donath on June 8, 1954. The Donath
apparatus takes advantage of the fact that water gives up heat when it
changes from its gaseous to its liquid state. This effect is the converse
of the well-known cooling effect experienced when water evaporates. Just
as water chills the surface from which it evaporates, so it warms the
surface upon which it condenses. The heat given off by condensing water is
called the "heat of condensation", and the presence of this heat is
indirectly detected by the Donath apparatus.
The Donath device is comprised of a reflecting condensation element; means
to measure the temperature of the element; means to slowly reduce the
temperature of the element by applying a coolant to its back side; and
means to expose the front side of the element to the gaseous mixture to be
tested. As the element cools past the dew point of the mixture of gases
under test, condensation takes place. The temperature of the element rises
slightly as the element absorbs the heat of condensation, and this rise in
temperature is used to indicate the occurrence of condensation
The principal drawback to the Donath apparatus is that it does not actually
detect the heat of condensation. Rather, it measures the temperature of
the sensing element. Although the heat of condensation will cause the
temperature of the sensing element to rise, there is a finite time delay
between the occurrence of condensation and the diffusion of the heat of
condensation throughout the element. Since the temperature of the element
rises only in response to the latter event, the accuracy of the Donath
apparatus is inherently limited.
The apparatus disclosed in U.S. Pat. No. 2,904,95, issued to Obermaier on
Dec. 10, 1953, seeks to overcome the drawbacks of the Donath apparatus by
providing a split sensing element. One part of the element is insulated
from moisture such that its temperature is unaffected by the occurrence of
condensation, and the difference in temperature between the insulated and
the uninsulated parts is then compared. Although this arrangement is an
improvement over the Donath apparatus, it still suffers from the inherent
limitation that what is actually measured is the change in the temperature
of the element rather than the heat of condensation.
U.S. Pat. No. 3,396,574, issued to Hanlein, et al., on July 9, 1965, also
discloses apparatus for measuring the increase in temperature of a sensing
element that results from the heat of condensation. The Hanlein apparatus
employs a Peltier device to accomplish slow cooling of the sensing
element, and it uses two thermocouples to detect the increase in the
temperature of the sensing element that occurs after the heat of
condensation has diffused through the sensing element. Like the Donath and
Obermaier inventions, the Hanlein invention uses the event of the increase
in temperature of the sensing element to approximate the event of the
transfer of the heat of condensation to the element. The Hanlein device is
therefore subject to the same limitations of accuracy.
It will be apparent from the foregoing that there is a need for a dew point
measuring apparatus that can function in the hostile environment commonly
present in industrial systems and that is not subject to the errors
inherent in sensing devices known to the art. The present invention
satisfies this need.
SUMMARY OF THE INVENTION
The dew point measuring apparatus of the present invention is characterized
by a heat flow sensor which, in one embodiment, comprises a condensation
surface and an embedded differential thermopile. The heat flow sensor
measures the flow of heat into the surface rather than the temperature of
the surface. It senses the exothermic release or endothermic absorption of
heat as moisture condenses on or evaporates from the surface at the dew
point temperature. When the apparatus is used to determine the relative
humidity of a gaseous atmosphere, the temperature of the sensor is slowly
lowered until the dew point of the atmosphere is reached. As soon as
condensation begins, heat of condensation is released from the condensate
and flows into the condensation surface. The heat flow sensor responds
virtually instantaneously to this heat flow, thereby permitting a much
more precise determination of the atmosphere's dew point, and hence of its
relative humidity, than can be obtained by sensors known to the art.
In one practical application the apparatus of the invention includes a pair
of heat flow sensors mounted adjacent a heat sink. A coolant is circulated
through the heat sink to slowly reduce the temperature of the sensors. A
heater is provided which is actuable to raise the temperature of the
sensors. By actuating and deactuating the heater the temperature of the
sensors can be cycled up and down through the dew point of a gas or
atmosphere under test, thereby permitting the atmosphere's dew point to be
tracked or followed, and measured as often as desired. One of the sensors
is kept at a slightly higher temperature than the other, and the output
signals from the two sensors are compared, as by connection in series
opposition. A stream of gas under test is passed over the sensors. A
relatively sudden change or imbalance in the signal from the oppositely
electrically connected sensors occurs as moisture condenses on the cooler
sensor and the exothermic heat of condensation is released. The surface
temperature of the cooler sensor at that instant is the dew point of the
gas. This can be used to determine from an appropriate table the relative
humidity of the atmosphere.
The use of two heat flow sensors rather than only one has been found to
give much better noise immunity. A first sensor is thermally directly
associated with the heat sink so that its temperature falls relatively
fast in response to the coolant circulating through the heat sink. The
second sensor is also thermally associated with the heat sink, but through
a path providing a higher thermal resistance. Consequently, its
temperature always lags that of the first sensor in response to the
coolant.
Since the first sensor is at all times slightly cooler than the second
sensor, when the sensors are cooled condensation occurs first on the first
sensor because the first sensor reaches the dew point temperature before
the second sensor. When such condensation occurs on the first sensor, the
heat of condensation is released. This heat flows through the surface and,
by continuity of thermal conduction, through the first sensor and into the
heat sink, causing an immediate change in the signal from the first heat
flow sensor. However, since the second sensor is slightly warmer there is
no condensation on it and it generates no change in signal due to
condensation. The resulting overall change in output signal from the two
heat flow sensors indicates condensation has occurred.
A temperature sensing element is located in the surface of the first sensor
and its temperature at the instant of output signal change constitutes the
dew point temperature.
The differential thermopile employed in the embodiment mentioned is made up
of a number of individual thermocouple junctions connected in series with
the differential junctions placed on opposite sides of a very thin thermal
barrier. As heat flows across the barrier the thermopile generates an
electrical signal proportional to the rate of heat flowing through it. It
should be understood that other forms of heat flow sensors could be used
instead, if desired.
The present measuring apparatus is adapted for use in association with an
industrial process dryer for automatic control of the humidity of the
drying atmosphere. A typical dryer comprises a source of heat for heating
the atmosphere; a blower for bringing fresh, dry air into the dryer; a
controllable air outlet for letting saturated air out; and sensors to
monitor the temperature and pressure of the atmosphere. In operation, the
air outlet is initially closed to permit the atmosphere inside the dryer
to be warmed by the heat source. Articles in the dryer begin to dry and
the relative humidity of the atmosphere rises. A sample of the atmosphere
is brought past a pair of heat flow sensors, which are cooled to effect
condensation and establish the dew point. The detected dew point is
compared with a desired or programmed dew point that is automatically
computed from the predetermined desired relative humidity and the measured
temperature and pressure of the drying atmosphere. When the measured dew
point reaches the desired dew point, a signal is generated to actuate the
blower and the air outlet to expel moist air and admit new, dry air. After
each measurement of relative humidity, a heat source associated with the
sensors is actuated for a time sufficient to the heat sensors to a
temperature above the dew point. The cycle is repeated upon deactuation of
the heater and subsequent cooling of the sensors.
The present invention represents a significant advance in apparatus for
determining the relative humidity of an atmosphere. Heretofore, as
previously indicated, dew point measurement apparatus depended on
detection of a change in temperature of a sensor in response to
condensation. The present apparatus utilizes heat flow sensor means to
detect the change in the rate of heat flow which occurs on condensation,
giving an immediate indication of such condensation, well in advance of
the time it would take for a temperature sensor of the prior art to fully
react to such heat flow.
Other aspects and advantages of the present invention will become apparent
from the following more detailed description taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic showing of dew point measuring apparatus according to
the invention;
FIG. 2 is an enlarged detail view of the cooler one of the pair of heat
flow sensors;
FIG. 3 is a schematic showing of an automatic humidity control system
incorporating the present dew point detection apparatus;
FIG. 4 is a graph showing the heating and cooling cycles of the heat flow
sensors;
FIG. 5 is a graph showing the changes in heat flow signal corresponding to
the heating and cooling cycles of FIG. 4; and
FIG. 6 is a schematic showing of a second embodiment of the present dew
point measuring apparatus.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present apparatus provides a means for determining the relative
humidity of a gas or atmosphere by measuring the dew point of the
atmosphere. This is not accomplished by detection of a change in
temperature of a sensing element when condensation occurs on the sensing
element, but by direct detection of the release of the heat of
condensation at the dew point. This affords an immediate, more accurate
determination of relative humidity, compared to prior art temperature
sensing devices. Such temperature sensing devices are inherently less
accurate because of the delay between the heat release and the resulting
change in temperature of the temperature sensing element.
The invention has particular application in industrial process drying,
which involves heating air, circulating it over or through the wet
product, and exhausting it when it is water-laden. The hot exhaust air
must be replaced by heated fresh air, thus representing an appreciable
energy loss. Consequently, it is important that air not be exhausted until
its full drying capacity has been employed. Conversely, if air is
maintained in the system too long it becomes excessively wet, and
inefficient drying occurs. Reduced energy consumption and process
efficiency is obtained when the moisture content of the air is precisely
and continuously monitored and controlled.
As previously indicated, the broadly unique aspect of the present invention
is the use of a heat flow sensor which indicates the rate of heat flow
into a surface of the sensor so that it immediately detects the change in
rate of heat flow which occurs when condensation takes place upon such
surface. Simultaneous measurement of the temperature of the sensor surface
at that precise moment enables determination of the dew point.
As will be seen, although a single heat flow sensor can be employed, it is
preferable to use a pair of heat flow sensors. This is for the reason that
if the sensing surface temperature is allowed to decrease only a fraction
of a degree below dew-point, the heat flow due solely to condensation is
low relative to the heat flow which is normally present both during and in
the absence of condensation. By employing two matched heat flow sensors in
adjacent, side-by-side relation, and maintaining them at slightly
different temperatures, condensation takes place on the cooler heat flow
sensor. Connecting the sensors in series opposition results in a signal
due solely to condensation.
One embodiment of the invention provides a continuous dew point temperature
reading. This is done by using the condensation signal from the heat flow
sensor means to trigger a heating and cooling cycle operative to cause the
temperature of the heat flow sensors to oscillate about the condensation
or dew-point temperature. A digital logic circuit reads, stores and
up-dates the sensor signals to give the desired continuous record of dew
point.
Referring now to the drawings, and particularly to FIGS. 1 and 2, a dew
point measuring apparatus 10 is illustrated which comprises, generally, a
thermally conductive reference body or heat sink 12; temperature change
means for altering the temperature of the heat sink 12, including cooling
coils 14 and an electrical resistence heating element embedded in a
portion of the heat sink 12 and constituting a heater 16; a pair of
matched heat flow sensors 18 and 20; and a temperature sensor 22.
The thermally conductive material of which the heat sink 12 is made can be
aluminum, copper or the like and is configured to fit within any suitable
external housing 24.
The heater 16 may be a resistance or any other suitable heater for raising
the temperature of the mass of the heat sink 12, the leads for the heater
16 being carried through a wall of the housing 24 for connection to a heat
controller 26.
The cooling coils 14 are suitably arranged within the heat sink 12 so that
circulation of cooling fluid through the coils 14 by means of a coolant
controller 28 is operative to drop the temperature of the heat sink 12.
The particular form of heat flow sensor 18 or 20 is not critical to the
present invention so long as it is capable of sensing the heat flow
through the condensing, sensing or operative faces of the sensors. It
could be, for example, a thermopile or an asymptotic type or the like. As
will be seen, condensation normally occurs upon the sensing surface of
only one of the heat flow sensors, but for convenience both sensing
surfaces are sometimes termed condensing surfaces.
The preferred form of heat sensor is a thermoelectric device or very thin
differential thermopile. Several hundred thermopile junctions are arranged
on either side of a thin thermal barrier and measure the very small
temperature gradient which develops across the barrier when heat flows
through the heat sensor operative face or condensing surface.
The sensor provides a self-generated signal which is bidirectional, that
is, heat flow from the gas into the condensing surface generates a
positive signal while heat flow from the condensing surface to the gas
generates a negative signal.
The thermopile used is well known in the art and for that reason will not
be described in detail, although it is important to note that, like other
suitable heat flow sensors, it is adapted to generate a voltage directly
proportional to the rate of heat flow through it. Although not shown in
detail, each of the heat flow sensors 18 and 20 is approximately one-half
inch wide and one inch long, and is potted or embedded in an epoxy resin
or the like in a recess in the upper side of the heat sink 12 flush with
the inner surface of the lower wall of a laterally elongated air duct or
conduit 30.
Suitable insulation coatings are provided to electrically insulate the flow
sensors from the adjacent heat sink. With the described arrangement, the
flush or operative faces of the sensors are exposed to any heat flow
resulting from condensation of moisture in the air flowing through the
interior of the duct 30. In the showing of FIG. 1 the air is being forced
through the conduit 30 out of the plane of the paper and toward the
viewer. Under steady state conditions the effect upon both sensors is the
same. Of course, the dimensions and configuration of the duct 30 are
arranged to establish laminar or steady state flow conditions across the
faces of the sensors 18 and 20, as will be obvious to those skilled in the
art.
The absolute velocity of air flow across the sensors is not critical, it
being primarily important that there be the same steady state flow past
both of them.
Alternate operation of the cooling coils 14 and heater 16 is adapted to
cycle the temperature of the heat sink 12 above and below the dew point
temperature, as will be seen.
The portions of the heat sink 12 with which the heat sensors 18 and 20 are
associated constitute isothermal bodies maintained as close as possible to
the same temperature, except for a slight temperature differential
provided by a higher thermal resistence material or layer 32 disposed in
the thermal path between the heat sensor 18 and the heat sink 12. Although
any suitable means could be employed to establish a temperature
differential between sensors 18 and 20, the means used is attachment of
the underside of sensor 20 directly to the top of the upper surface of the
right portion of the heat sink 12. However, the attachment of the
underside of the sensor 18 is to an upper left portion or block of the
heat sink 12. For convenience, this upper left portion is simply cut away
from the parent heat sink 12 and then replaced with the intervening layer
32 providing a thermal barrier in the heat sink 12 at its upper left
portion. The layer 32 could s | | |
