Abstrict The mass flow rate of particles conveyed by a gaseous medium through
a conduit is measured by an electrostatic flow meter comprising
a grounded electrical conductor in contact with a semiconductive
static charge-generating element exposed to the moving particles.
As the particles pass the semi-conductive, static charge-generating
element, they impart a static charge to the element which is transferred
by the conductor to ground. The resulting current, which can be
measured by any suitable current measuring means, is proportional
to the mass flow rate of the particles.
Claims What is claimed is:
1. A device for measuring the flow of cellulosic particulates transported
within a conduit by a gaseous medium comprising:
(a) a plastic semi-conductive static charge-generating element,
electrically insulated from the conduit and which replaces a portion
or segment of the conduit wall, said element having a surface on
which static charges are generated when exposed to the flow of particulates;
(b) a grounded electrical conductor which is electrically insulated
from the conduit and in contact with the static charge-generating
element; and
(c) means for measuring electrostatic current flowing
through the conductor from the static
charge-generating element to ground, wherein the flow of particulates
within the conduit generates a measurable electrical current through
the conductor which is proportional in magnitude to the mass flow
rate of particles in the conduit.
2. The device of claim 1 wherein the static charge-generating element
is a hollow cylinder having an electrical conductor embedded therein
and which is adapted for having particulates flowing therethrough.
3. The device of claim 1 wherein the means for measuring the electrostatic
current comprises a constant voltage source electrically connected
to the grounded electrical conductor, which provides a constant
voltage to the conductor to offset the voltage drop caused by the
current measuring means.
4. The device of claim 1 wherein the material of the semi-conductive
charge generating element is an acrylic polymer.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and apparatus for measuring
the mass of fibrous particulates being transported through or with
a gaseous medium.
2. Description of the Prior Art
Several apparatus are available for determining the mass of particles
being transported in or through a gas. Green et al. ("A Low
Cost Solids Flow Meter for Industrial Use,"Journal of Physics
E: Scientific Instruments, Vol. 22 No. 10 October, 1978 pp. 1005-1010),
describes an instrument for measuring the mass flow rate of dry
conducting or nonconducting granular solids or powder. As solids
are transported into and through a capacitor formed by the walls
of a conveyor, the dielectric constant of the air in the field of
the capacitor changes and the change is converted to an electrical
signal that may be correlated with the mass flow of the solids.
West German Auslegeschrift No. 1121822 issued on Jan. 11 1962
to Moller et al., discloses an apparatus for measuring the quantity
of material conveyed in a gas through a pipeline. A wire extends
along the axis of the pipeline, and an electric field is produced
between the wire and the pipe wall. The wire is connected to a capacity-measuring
bridge, and changes in the electric field are related to the solids
flow between the wire and pipe wall.
U.S. Pat. No. 4312180 issued to Reif et al. on Jan. 26 1982
and entitled "Detecting Particles," describes an apparatus
comprising a pair of electrically conductive members spaced apart
and means for providing an electrical potential adjacent one of
the members. This potential produces ions which transmit a charge
to particles being transported past the ion-producing member. The
charged particles are moved downstream within the apparatus and
detected by charge detecting means.
Also somewhat related is U.S. Pat. No. 3478261 to Forster et
al., which teaches a means for measuring static charge in a pipe
carrying a liquid hydrocarbon steam comprising a pair oppositely-charged
electrodes in the pipe, a bias voltage source, a ground connected
to the negative side of the bias voltage, and a pair of electrometers
between the electrodes and the ground. The algebraic sum of the
currents flowing from the two electrodes to the ground indicates
the amount of static charge.
However, none of the prior art teaches a simple means for detecting
the mass flow rate of particles in a conduit by simply measuring
electrostatic current resulting from static charge formation.
SUMMARY OF THE INVENTION
In general, the invention resides in the discovery that certain
particles being conveyed through a conduit by a gaseous medium can
be made to generate a measurable electrostatic current which is
proportional to the mass flow rate of the particles through the
conduit. A uniform distribution of particles in the conduit is not
necessary. Particles for which this discovery is applicable particularly
include cellulosic fibers and fragments which are commonly conveyed
by air streams in industrial processes.
More specifically, the invention resides in a method for indirectly
measuring the mass flow rate of particles transported through a
conduit by a gaseous medium comprising: (a) exposing the surface
of a semi-conductive material to the flow of particles in the conduit,
wherein static charges are formed on said surface; (b) dissipating
the static charges from said surface through an electrical conductor,
wherein the resulting electrical current is proportional to the
mass flow rate of particles through the conduit; and (c) measuring
the current.
In a further aspect, the invention resides in an electrostatic
flowmeter for measuring the flow of particulates transported within
a conduit by a gaseous medium comprising: (a) a semiconductive static
charge-generating element, electrically insulated from the conduit,
having a surface on which static charges are generated when exposed
to the flow of particulates; (b) a grounded electrical conductor,
electrically insulated from the conduit, in contact with the semi-conductive
static charge generating element; and (c) means for measuring electrical
current flowing through the conductor, wherein the flow of particles
within the conduit generates a measurable electrical current through
the conductor which is proportional in magnitude to the mass flow
rate of particles in the conduit.
The semi-conductive, static charge-generating element (hereinafter
referred to simply as the "element") can be constructed
of any material which will generate and conduct static charges.
Suitable materials include, for example, any common resinous plastics
or polymers such as acrylics, fiberglass, polycarbonates, etc. In
order to generate measurable quantities of static charges, it is
preferable that the exposed surface of the element be as large as
possible without substantially interfering with the flow of particles
through the conduit. Hence the specific design or shape can vary.
For example, a rod-like cylindrical probe can be used in those situations
where it is desirable or necessary to have a device which can be
inserted through the wall(s) of the conduit. Alternately, the element
can take a planar shape, such as a flat sheet, which is oriented
within the conduit such that the plane of the sheet is parallel
to the axis of the conduit. In this manner, the surface area of
the element is large, but the restriction to particulate flow is
minimized. A still further alternative element design is a hollow
cylinder, which can be inserted within the conduit if the diameter
is smaller than that of the conduit, or which can be sized to actually
replace a segment of the conduit.
The electrical conductor which carries the static charge away from
the element can be of any electrically conductive material, such
as copper wire, capable of removing the static charge as fast as
it is created. In this regard it is preferable that the conductor
be positioned on or within the element in a manner which efficiently
removes the charges. For example, a single point contact between
the conductor and an element having a very large surface area would
probably be insufficient to drain away all of the static charges
as fast as they are formed due to the large distance some of the
static charges would have to travel through the semiconductor material
to reach the conductor. A better design is to widely distribute
the conductor over the entire surface of the element. In order to
assure a good electrical connection, a preferred design has the
conductor imbedded into the element just below its surface to assure
instantaneous charge removal.
As mentioned, the conductor is grounded at one end, thereby causing
a flow of charges through the conductor from the element to ground.
Any suitable means for measuring electrical current can be used
to detect this charge flow, such as an electrometer or ammeter.
However, if the resistance of the electrometer is high relative
to the voltage drop between the element and ground, it will be necessary
to connect the nongrounded end of the conductor to a constant voltage
source (V) which compensates for the resistance in the circuit due
to the presence of the electrometer. This will be clearly illustrated
with reference to the Drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 illustrates one embodiment of this invention wherein the
semi-conductive element is in the form of a hollow tube adapted
to be inserted in-line to replace a segment of the conduit carrying
the particles.
FIG. 2 is a cross-sectional view of a conduit illustrating another
embodiment of this invention wherein the element is in the form
of a rod inserted through the wall of the conduit carrying the particles.
DETAILED DESCRIPTION OF THE DRAWING
FIG. 1 illustrates one embodiment the invention wherein the element
1 is a hollow acrylic tube adapted to be inserted in-line into the
conduit 2 transporting the particles. Each end of the element is
provided with an electrically insulating sleeve 3 and/or flange
3' which prevents contact between the conduit 2 and the element.
A suitable material for this insulator is rubber. The conductor
4 which in this embodiment is a strip of copper, is suitably secured
to the inner wall of the acrylic tube and aligned parallel to the
axis of the tube. The conductor is also electrically insulated from
the conduit. It should be noted that the conductor can be secured
to the semiconductive acrylic tube in many different configurations.
It should also be noted that any substantial intrusion of the conductor
into the flow path of the particles may cause material buildup upon
and fouling on the conductor and result in inaccurate measurement.
It is therefore preferred that the conductor be embedded into the
element just below the surface. One end of the conductor is wired
through an electrometer 5 to ground. The other end of the conductor
is connected to a constant voltage source V.sub.c which compensates
for the voltage drop in the circuit caused by the presence of the
electrometer. The constant voltage source may not be necessary if
the electrometer has sufficiently low resistance relative to the
magnitude of the current generated by the static charges formed
on the element.
In operation, as particles flow through the conduit and the acrylic
tube, static charges are formed on the inner surface of the acrylic
tube and are dissipated to ground by the conductor. The resulting
current, which is proportional to the mass flow rate of the particles
flowing through the conduit, is measured by the electrometer.
FIG. 2 illustrates another embodiment of this invention in which
the element 1 is in the form of an acrylic rod. Shown is a cross-section
of the conduit 2 carrying particles 6. The conductor 4 is embedded
in the acrylic rod. In this case the conductor is simply a copper
wire which extends through opposite sides of the wall of the conduit.
The element 1 does not touch the wall of the conduit and the conductor
is suitably insulated from the wall of the conduit by a rubber sleeve
7. As with the embodiment of FIG. 1 the conductor is grounded at
one end through an electrometer which measures current flow. The
other end of the conductor is preferably wired into a constant voltage
source V.sub.c. If the conduit is of a material which creates or
conducts static charge, it is also preferable to ground the conduit
as shown in order to avoid leaking of charge from the conduit to
the element.
It will be appreciated that the specific configuration of the element
and conductor can vary widely from the foregoing examples, shown
for purposes of illustration, without departing from the scope of
this invention. |