Abstrict A compact dual microanemometer-based flow meter for a gaseous fluid
includes a dynamic microanemometer flow sensor located in a measuring
nozzle of reduced diameter directly exposed to the main fluid flow
stream and a static microanemometer sensor located so as to be in
communication with the main flow stream but not in direct contact
with the main flow stream. Particulate matter is excluded by a particulate
trapping system located upstream of the flow sensors and has a plurality
of sequentially encountered particulate traps including dead-end
trap and settling traps and a honeycomb laminarizing device is positioned
in the stream between the particulate trapping system and the microanemometers.
Claims What is claimed is:
1. A compact hot film microanemometer-based flow meter for a gaseous
fluid of interest comprising:
inlet and outlet access for connecting the flow meter to a distribution
system for the gaseous fluid;
generally hollow meter body means connected between the inlet and
outlet accesses and describing an internal fluid flow path;
a reduced diameter nozzle-shaped measuring section describing a
segment of the fluid flow path channel in the meter body having
at least one microbridge sensor located therein and being in communication
with the fluid;
a particular trapping system including trap means having the form
of a fluid conduit having a closed end and a plurality of outlet
openings in said conduit spaced apart from said closed end, said
openings in fluid communication with a plurality of sequentially
disposed settling chambers, all disposed in the fluid flow path
between the inlet means and the measuring section containing the
microbridge sensor; and
at least a first laminarizing means disposed in the stream between
the particulate trapping system and the measuring section.
2. The flow meter of claim 1 further comprising a coarse screen
means disposed between the particulate trapping system and the first
laminarizing means.
3. The flow meter of claim 1 further comprising second laminarizing
means disposed upstream of the particular trapping system.
4. The flow meter of claim 3 wherein the first and second laminarizing
means are honeycomb structures.
5. The flow meter of claim 1 wherein the measuring section contains
at least two sensors, a dynamic flow measuring microbridge sensor
and a static condition measuring microbridge sensor being disposed
on opposite sides of the main fluid passage.
6. The flow meter of claim 1 wherein the measuring section contains
at least two sensors, a dynamic flow measuring microbridge sensor
and a static condition measuring microbridge sensor both being contained
in one package mounted on one side of the flow conduit.
7. The flow meter of claim 1 wherein the first laminarizing means
is an elongated honeycomb structure.
8. The flow meter of claim 1 wherein the microanemometer sensors
further comprise a pluralityo f electrically conductive leads disposed
to connect outside the meter body.
9. The flow meter of claim 8 wherein the electrically conductive
leads do not protrude into the gaseous fluid of interest.
10. A compact hot film microanemometer-based flow meter for a gaseous
fluid of interest comprising:
inlet and outlet access for connecting the flow meter to a distribution
system for the gaseous fluid;
generally hollow meter body means connected between the inlet and
outlet accesses and describing an internal fluid flow path;
a reduced diameter nozzle-shaped measuring section describing a
segment of the fluid flow path channel in the meter body;
a dynamic hot film icrobridge sensor disposed in the nozzle-shaped
measuring section and directly exposed to the main fluid flow stream;
a condition measuring static hot film microbridge sensor positioned
in communication with the main flow stream but not in direct contact
with the main flow stream and at a position relative to the dynamic
sensor such that the temperature of the fluid of interest encountered
by both sensors is substantially the same;
a particulate trapping system including a plurality of sequentially
encountered particulate trap means disposed in the fluid flow path
between the inelt means and the measuring section containing the
first and second microbridge sensor means; and
at least a first laminarizing means disposed in the stream between
the particulate trapping system and the measuring section, said
particular trap means has a dead-end trap in the form of a fluid
conduit having a closed end and a plurality of radially disposed
outlet openings spaced from the closed end which communicate with
a plurality of sequentially disposed settling traps.
11. The flow meter of claim 10 wherein the area of the total radial
outlet openings spaced from the closed end is at least equal to
the internal cross-section of the fluid flow conduit.
12. In a compact hot film microbridge-based flow meter for a gaseous
fluid of interest comprising inlet and outlet accesses for connecting
the flow meter to a distribution system for the gaseous fluid of
interest, a generally hollow meter body means connected between
the inlet and outlet accesses and describing an internal fluid flow
path having a nozzle-shaped necked down measuring channel therein,
a first hot film microbridge flow sensor associated with the flow
meter and disposed in the measuring channel so as to be directly
exposed to the mainf luid flow stream and a second hot film microbridge
sensor associated with the flow meter disposed so as to be in communication
with the main flow stream but not in direct contact with the mani
flow stream, the improvement comprising:
a particulate trapping system having a fluid conduit as a first
element thereof for communicating said fluid to a dead end, having
openings in said conduit spaced apart from said dead end, said openings
being in fluid communication with a plurality of sequentially encountered
particulate trap settling chambers, all disposed in the fluid path
between the inlet means and the measuring channel; and
at least a first laminarizing means disposed in the stram between
the particular trapping system and the measuring channel.
13. The flow meter of claim 12 further comprising a coarse screen
means disposed between the particulate trapping system and the laminarizing
or flow straightening structure.
14. The flow meter of claim 12 further comprising second laminarizing
means disposed upstream of the particulate trapping system.
15. The flow meter of claim 12 wherein the first laminarizing means
is an elongated honeycomb structure.
Description CROSS REFERENCE TO RELATED APPLICATION
Cross reference is made to co-pending application Ser. No. 07/727415
filed of even date and assigned to the same assignee as the present
application.
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to the flow measurement of gaseous
fluids using electronic hot film microsensors and, more particularly,
addresses the problems associated with retaining maximum microsensor
sensitivity while avoiding microsensor damage due to debris and
particulate matter carried by the flowing stream of interest. The
system further avoids clogging and provides a minimum pressure drop
for the achieved sensitivity to flow. The invention eliminates noise
errors normally associated with fluid flow measuring devices occasioned
by turbulent flow in the fluid of interest or in the placement of
the fluid sensor with respect to proximate upstream and downstream
interfering pipe fittings such as valves, elbows and the like utilized
in the distribution system.
RELATED ART
Hot film microanemometer packages for general use are known for
both uni- and bi-directional flow applications. An example of such
a device is illustrated and described in U.S. Pat. No. 4501144
to Higashi, et al. The microanemometers or "microbridges"
themselves are quite inexpensive to produce. The small size and
relatively fragile undercut suspended bridges of the sensors, however,
are vulnerable to damage from particulate matter and debris carried
in the fluid stream. For that reason, packages containing devices
of the class described have had to be constructed to be protective
of the sensing elements. Because applications vary, they have been
designed for any possible adverse condition or placement, i.e.,
for the "worst case". This requirement has tended to make
the packages very high cost items.
The devices, like conventional orifice meters, for example, further
have had severe use limitations with respect to placement in distribution
networks of fluids to be metered. For example, most of the known
packages are not designed to prevent or address the problem of flow
transitions from laminar to turbulent flow within the required flow
range. This leads to serious calibration and readout errors or severely
limits use of the device. Also, these devices normally do not address
the problems of minimizing pressure drop or permanent pressure loss
in the conduit of the measured flow to thereby minimize aberrations
in the fluid distribution network caused by metering.
Large particles (generally .gtoreq.200 microns in size) and debris
need to be eliminated from the gas flow to be sensed by such a microbridge-type
flow meter sensor, and the flow needs to be reasonably laminar to
minimize the noise amplitude of turbulence in order to make use
of the high degree of sensitivity characterized by these devices.
Prior solutions to the problems of debris involved the use of a
bypass or the insertion of fine screens to eliminate particulate
matter. However, these solutions themselves caused a loss of sensitivity
in the case of the bypass technique or clogging problems with respect
to the fine mesh screens which, in turn, also produced a large increase
in the associated pressure drop.
Clearly, the need exists for a low-cost microanemometer-based fluid
flowmeter characterized by a low pressure drop which, at the same
time, takes advantage of the extreme sensitivity of the microbridge
throughout its full measurement range to produce accurate results.
This is especially true with respect to a compact meter for natural
gas which can be retrofitted into existing distribution systems.
These often have tight quarters with interfering pipe fittings such
as proximate elbows, couplings, reducers and the like.
SUMMARY OF THE lNVENTlON
The present invention overcomes disadvantages associated with previous
hot film or hot element microanemometer packages for gaseous fluid
flow sensing. The invention provides a compact, simple, low cost
microanemometer gas flow meter package which retains the maximum
sensitivity of the microbridge sensor, minimizes turbulence noise
effects, avoids clogging and provides a low pressure drop in relation
to the achieved sensitivity to flow. Its compactness further distinguishes
it from prior bulky, mechanical bellows or diaphragm-based measurement
devices. The system is not affected by proximate obstacles or pipe
fittings such as elbows upstream or downstream of the flow meter
in the gas distribution network as an isolation system prevents
the effects from reaching through to the location of the microbridge
sensor. The electronics design provides for a repeatable, low noise
operation and the design of the support structure allows for low
cost manufacturing and integration with, for example, a shut-off
valve and regulator, if desired. The system is well suited for the
precise measurement of gas flows including the metering of fuel
gases, natural, city, propane, butane, or the like.
The system features a combination of a plurality of sequentially
encountered particulate trap systems, coarse screen and a laminarizing
or flow straightening structure upstream of the sensors to reduce
the turbulent effects and allow the use of an unprotected dynamic
or flow measuring microbridge sensor. The dynamic microbridge sensor
is used in combination with a recessed or remotely mounted static
microbridge sensor utilized to make certain static measurements
which can be combined with the dynamic measurements to produce accurate
mass or standard volumetric flow readings which are valid and fully
compensated against density and composition changes in the fluid
over a wide range of these fluid parameters.
One successful embodiment includes a plurality of features which
combine to produce an unique advantageous metering performance at
low manufacturing cost. The system features convenient inlet and
outlet pipe fittings for each retrofit or original installation.
The inlet of the device features a "deadend trap" which
functions not only to trap large particles but also to prevent upstream
disturbances from reaching the microbridge sensor. The dead-end
trap is in the form of a blind conduit having a closed end and a
plurality of radial outlet openings spaced from the closed end connect
with a plurality of settling traps. A coarse screen having approximately
ten to twenty mesh per inch is located beyond the settling traps
together with a laminarizing honeycomb just upstream of an exposed
microbridge flow sensor. The honeycomb is generally about 0.5-2.0
inches in length and has a cell size of about 0.125 inches (3.5
mm) and a wall thickness of 0.001-0.002 inches (20-50 microns).
The honeycomb functions to reduce turbulence and further reduce
the effect of proximate upstream pipe fittings.
The flow sensing microbridge is normally located near the entry
to a channel section having a reduced cross-section to achieve the
desired flow speed, laminarity and uniformity in the vicinity of
the sensing microbridge. With respect to the flow channel, the surface
of the flow measuring microbridge may be mounted to be flush with
the wall, but preferably protrudes into the flow channel as much
as about 1 mm in order to emerge from the boundary layer. An additional
microbridge static specific condition sensor which may determine
gravity or composition is remotely located and separated from the
main flow channel by a short access channel and a protecting screen.
The static or condition sensing microbridge sensor may be positioned
in any manner with respect to the dynamic microbridge sensor in
the device so long as the temperature of the sensed fluid is the
same or substantially the same as that flowing past the dynamic
sensor.
The entire assembly is packaged together in a compact arrangement
sealed in a gas-tight fashion. The only difference between the mounting
of the flow and composition sensors, of course, is based on the
exposure to the flowing stream. The components can be packaged together
in an arrangement such that the gas inlet and outlets are vertical,
horizontal or mixed while providing for the appropriate pipe connections.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings, in which like numerals are utilized to designate
like parts throughout the same:
FIG. 1 is a schematic view of the compact flow measurement system
of the invention;
FIG. 2 is a greatly enlarged fragmentary cross-sectional view of
a gas-tight microbridge sensor package illustrating both dynamic
and static i.e., in-channel and remote location mounting; and
FIGS. 3a, 3b and 3c illustrate a two sensor package which can be
used with an embodiment of the invention in which both sensors are
mounted on the same side and location in the flow channel.
DETAILED DESCRIPTION
FIG. 1 depicts a simple schematic view of a compact flow meter
in accordance with the principles of the present invention. The
system is shown generally at 10 and includes a main body between
an inlet and outlet 12. Inlet conduit 13 may be provided with an
optional laminarizing honeycomb 14 and further has a closed end
15 which provides a dead-end trap for particles. The fluid passage
is through a plurality of radial openings 16 located well above
the closed end of the conduit. The fluid path downstream of openings
16 include a second series of particle traps in the form of the
settling traps 17. A coarse screen member 18 typically a screen
of ten to twenty mesh, is positioned upstream of and protects a
honeycomb 19 which further laminarizes the flow and directs it into
nozzle 20 i.e., a channel of a reduced diameter which contains
a dynamic flow sensor system 21 and a static composition sensor
system 21a. A further coarse screen 23 is provided downstream of
the sensors to protect them from the effects of any reverse flow.
The second honeycomb 14 located upstream of the dead-end trap is
an optional honeycomb which can be used to provide additional protection
against upstream turbulence and make the dead-end trap more effective.
The composition sensing microsensor 21a is exposed to the dead-ended
or static chamber 22 separated from the main flow channel 20 by
a narrow access passage 24. More detailed illustrations of a possible
construction of the microanemometer packages or supports are depicted
in FIG. 2 and FIGS. 3a-3c.
In FIG. 2 the greatly enlarged fragmentary structure 50 includes
a portion of the main meter body at 51 properly hollowed to accommodate
a sensor package 21 or 21a. The sensor package is sealed in gas-tight
relation to the meter body by means of O-ring 52 washer 53 and
hollow cylindrical set screw or nut 54. The sensor package further
contains a microbridge anemometer sensor element 55 contained on
a mounting base 56 with a mounting flange 57 and a plurality of
electrical leads as at 58 which may protrude into the fluid as at
59 or be mounted flush with the surface as at 60 connect the microbridge
chip with external devices, as desired. The package design provides
both a gas-tight seal and one which will protect the system at elevated
temperatures. The same construction can be used for both the remotely
located composition sensor and the flow sensor. As shown, the O-ring
seals the header 56 against the main flowmeter block 51 which may
be aluminum, while the screw or hollow nut 54 provides either for
metal-to-metal seal or using gasket or washer 53 completes the seal
between the header lip and the block 51.
As can be seen in FIG. 2 the only difference between the packages
for the flow and the composition sensor is based on the space in
front of the chip. In the case of the flow sensor, the header surface
is about flush with the inner pipe wall so that the microbridge
chip sticks out into the flow channel by as much as about the chip's
thickness. In the case of the composition sensor, the header surface
faces the cavity as at 22 which is connected to the flow channel
by the narrow passage 24 typically about 1 mm in diameter. A very
fine mesh protective screen 25 is held in place inside the cavity
22 as between a ledge 61 and a tight fitting washer 62. This, of
course, protects the composition sensor from debris that might enter
the narrow channel 24.
FIGS. 3a and 3b further illustrate details of an alternative dual
back-to-back microbridge sensor system which can be used in place
of the oppositely or otherwise differently disposed sensors illustrated
in FIG. 1 and which requires only one access to the flow channel.
The sensor system, shown generally at 26 includes housing members
31 and 32 which may be of high impact plastic material or the like.
A pair of thin film microbridge sensors 33 and 34 spanning etched
semiconductor chip substrates 35 and 36 are respectively spaced
by a member 37. The necessary electrical leads are illustrated as
by leads 38 and 39 associated with sensor 33 and 40 and 41 associated
with sensor 34.
With respect to the thin film microbridge or anemometer sensors
such as those depicted by reference numerals 21 21a, 33 and 34
very small and very accurate microbridge semiconductor chip sensors
of the class described in which etched semiconductor microbridges
are used as composition or flow sensors are well known and available.
Such sensors might include, for example, a pair of thin film sensors
flanking a thin film heater. Semiconductor chip sensors of the class
described are treated in a more detailed manner in one or more patents
including U.S. Pat. No. 4478076 4478077 4501144 4555939
4651564 and 4683159 all common of assignee with the present
invention. To the extent necessary, additional details with respect
to the microbridge sensors themselves may be incorporated by reference
from these cited documents.
For the purposes of the present application, it should suffice
to say that if the dynamic flow sensor 33 for example, comprises
a pair of thin film sensors symmetrically flanking a thin film heater,
the sensor can be used to sense flow in either direction. That is,
of course, provided that the chip assembly positions in the sensor
are in the proper positions in the sensor and the proper orientation
so that the flow meets the microbridge at a right angle in the assembled
meter. This further allows the flowmeter system of the present invention
to be constructed as reversible with respect to the conduit system
should such a configuration be desired. In any event, the system
is reversible with respect to the mode of measurement of the microbridge
system itself. Thus, for sensing dynamic flow, the sensor 21 or
33 is directly exposed to the stream of fluid flowing past it in
the conduit. By designing in adequate upstream particulate matter
and turbulent flow protection, the full sensitivity of the microbridge
system may be directly utilized in this manner. Thus, with the particulate
trapping system together with the screen and honeycomb, very accurate
flow measurements are made available to the metering or instrument
system with little danger for damage to the sensitive microbridge
unit itself.
The second microanemometer sensor 21a or 34 which may be mounted
away from or back-to-back with sensor 21 or 33 enables other parameters
of the fluid to be measured simultaneously with dynamic flow. While
the sensor 34 is not directly exposed to the flowing fluid, it is
in communication with that fluid and can measure certain parameters
related to the composition of the fluid which require a static environment.
Such a sensor can be used for the direct measurement of thermal
conductivity, k, and specific heat, c.sub.p, and from them can determine
the density, .rho., in accordance with a technique which allows
the accurate determination of both properties in a sample of interest
using a single sensing system.
That technique contemplates generating an energy or temperature
pulse in one or more heater elements disposed in and closely coupled
to the fluid medium of interest. Characteristic values of k and
c.sub.p of the fluid of interest then cause corresponding changes
in the time variable temperature response of the heater to the pulse.
Under relatively static conditions of the fluid, this, in turn,
induces corresponding changes in the time variable response of one
or more temperature responsive sensors coupled to the heater principally
via the fluid medium of interest.
A method and apparatus for determining both the thermal conductivity,
k, and the specific heat, c.sub.p, of a fluid of interest are more
fully disclosed in detail in U.S. Pat. No. 4944035. The use of
the thermal conductivity, k, and specific heat, c.sub.p, of a fluid
of interest to determine the density or specific gravity, .rho.,
of that fluid of interest is also possible as a function of specific
heat and thermal conductivity. A method and system for accomplishing
this is more fully illustrated and described in U.S. Pat. No. 4956793.
This, of course, allows one to monitor generally the composition
of the gaseous fluid of interest based, for example, on known hydrocarbon
constituents. The two references cited next above also are assigned
to the same assignee as the present invention and to the extent
necessary, details from them are deemed incorporated by reference
in the present application.
More particularly with respect to the thrust of the present invention,
however, the measurement of dynamic flow of the fluid in the system
is greatly enhanced by the provision of a substantially laminar
flow profile, i.e., one in which the Reynolds number, n.sub.Re,
is .ltoreq.2000 based either on the passage diameter or on the distance
to the leading edge of the sensor chip, past the microanemometer
device or based on the laminar flow pattern provided by the nozzle
itself. In addition to the elimination of turbulent flow at the
point of measurement, the system of the present invention has substantially
eliminated the probability of damage to the microanemometer due
to the presence of particles in the flowing fluid. In accordance
with the combination screen and trapping system of the present invention,
it has been shown in recent modeling work that less than 2% of particles
of a size 300 microns or greater escape the dead-end trap device
15 even under extreme flow rate conditions of one-third greater
than the maximum design flow rate for the meter. The openings 16
are normally of sufficient collective size to be equal to or greater
than the cross-section of the ID of pipe 13 and spaced highe nough
above the bottom of the dead-end trap to contain the expected amount
of particles, normally several diameters. It has further been experimentally
determined that the settling traps retain up to 99% of particulate
matter sized 300 microns or larger, which escape the dead-end trap.
The combination factors out to a total fraction of only about 0.0002
of entering particular of 300 microns or greater are transmitted
through both traps. Of course, the fraction is smaller for larger
particles and somewhat larger for smaller particles. The size 300
microns is singled out and used because in earlier experiments,
without the traps, large quantities of 0-100 100-200 and 200-300
micron-sized particles, when caused to impact the microbridge sensors
of the class used, did not harm the microbridge sensors themselves,
while particles in a class 300-400 microns in size did cause damage
to such a sensor.
The overall pressure drop of the systme of FIG. 1 has been measured
and found to be less than that of an earlier experimental embodiment
which did not use traps and honeycombs but which instead used a
total of five 30-40 mesh screens to achieve a similar reduction
in transmitted particular matter.
Of course, the schematic diagram of FIG. 1 is intended to show
the internal portion of a meter housing such as a gas meter housing
which would include in an overall structure, an associated shut-off
valve and regulator. However, the principles and operation of the
system can be adequately explained on the basis of the drawing FIG.
1. The gas thus enters the system through 13 flows through the
holes 16 across and around the further traps 17 and past the coarse
screen 18 through honeycomb 19 and into the nozzle-shaped section
of the flow channel 20. The gas flows through the channel 20 and
out the upper right connection after passing through the second
flow screen 23.
This invention has been described in this application in considerable
detail in order to comply with the Patent Statutes and to provide
those skilled in the art with the information needed to apply the
novel principles and to construct and use such specialized components
as are required. However, it is to be further understood that the
invention can be carried out by specifically different equipment
and devices and that various modifications both as to equipment
and procedure details can be accomplished without departing from
the scope of the invention itself. |