Device for measuring a mass flow rate of a particulate material

A device for measuring a mass flow rate of a flow of particulate material which moves in a predetermined direction. The device includes a weighing cell, a first apparatus, a second apparatus, and a third apparatus. The weighing cell includes a tubular wall and is disposed to be passed through by a flow of particulate material. The first apparatus weighs the weighing cell and the quantity of particulate material contained in the weighing cell and produces a first signal. The second apparatus measures the speed of flow of the particulate material which flows through the weighing cell and produces a second signal. The third apparatus calculates the bulk density of the particulate material, calculates the mass flow rate of particulate material which flows through the weighing cell, and produces a third signal representing the mass flow rate of particulate material which flows through the weighing cell.

BACKGROUND OF THE INVENTION

This Application claims priority from EP 05100617.9 filed Jan. 31, 2005, the entire disclosure of which is incorporated herein by reference thereto.

1. Field of the Invention

The invention relates to a device for measuring a mass flow rate of a flow of particulate material which moves in a predetermined direction, said predetermined direction of flow.

The invention relates more specifically to a device for measuring a mass flow rate of a component introduced in an extruder, such as the main component, but also for measuring the mass flow rate of additional components such as colour masterbatches, if used in combination with a dosing system.

Designated by particulate material is particularly, but not exclusively, pellets, reground material, coarse powder, plastic masterbatch.

The invention is very useful for measuring the particulate material consumption rate of a machine such an extruder.

Measuring the mass flow rate of each component introduced in an extruder is particularly important when the components are used to extrude a plurality of superposed layers and when the thickness of each layer is difficult or impossible to measure on the extrusion line.

2. Description of the Background Art

Different devices are known for measuring a mass flow rate of a particulate material such as described in U.S. Pat. No. 6,732,597 and EP-A-0213524. These devices are generally based on “loss-in-weight” measurements of a hopper, and good accuracy is only possible when weight difference in the measuring hopper is sufficiently large compared to the total hopper weight.

SUMMARY OF THE INVENTION

An object of the invention is to obtain a device which makes it possible to measure the mass flow rate over a short period of time with an increased accuracy.

The use of such a device for measuring a mass flow rate with an increased accuracy is particularly important with a low flow rate, or during flow rate transitions.

Another object of the invention is a device for measuring a mass flow rate, which is compact, of a simple mechanical construction, robust and easy to clean.

To achieve these objects the invention has as its subject matter a device for measuring a mass flow rate of a flow of particulate material which moves in a predetermined direction, the predetermined direction of flow, the device being characterized in that it is situated in the flow of particulate material and has a weighing cell which is made up of a tubular wall defined between an internal face and an external face, the weighing cell having a volume of predetermined value, a cross section of predetermined value, a weight of predetermined value and being disposed to be passed through by the flow of particulate material, a first apparatus which, at least during a first predetermined time interval, weighs the whole made up of the weighing cell and a quantity of particulate material contained in the weighing cell, and produces a first signal representing at least the value of the weight of the whole made up of the weighing cell and the quantity of particulate material contained in said weighing cell, a second apparatus which, at least during the first predetermined time interval, measures the speed of flow of the particulate material which flows through the weighing cell and produces a second signal representing the speed of flow of the particulate material contained in said weighing cell, a third apparatus which uses the first signal, the second signal, the predetermined value of the volume of the weighing cell, the predetermined value of the cross section of said weighing cell and the weight of predetermined value of the weighing cell to calculate the bulk density of the particulate material, calculate the mass flow rate of particulate material which flows through the weighing cell during the first predetermined time interval, and produce a third signal representing the mass flow rate of particulate material which flows through the weighing cell during the first predetermined time interval.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawing, one sees a first device, said device1for measuring a mass flow rate DM of a flow40of particulate material3which moves in a predetermined direction which is a predetermined direction of flow63.

In a notable but non-limiting manner the particulate material3is intended to be introduced in a second device5, such an extruder.

According to the invention the device1for measuring a mass flow rate1is situated in the flow40of particulate material3and has:a weighing cell6which is made up of a tubular wall60defined between an internal face61and an external face62, this weighing cell6having a volume of predetermined value V, a cross section of predetermined value C, a weight of predetermined value W and being disposed to be passed through by the flow40of particulate material3,a first apparatus7which, at least during a first predetermined time interval Z,weighs the whole made up of the weighing cell6and a quantity of particulate material3contained in this weighing cell6, andproduces a first signal S1representing at least the value P of the weight of the whole made up of the weighing cell6and the quantity of particulate material3contained in said weighing cell6,a second apparatus8which, at least during the first predetermined time interval Z, measures the speed of flow E of the particulate material3which flows through the weighing cell6and produces a second signal S2representing the speed of flow E of the particulate material3contained in said weighing cell6,a third apparatus9which uses the first signal S1, the second signal S2, the predetermined value V of the volume of the weighing cell6, the predetermined value C of the cross section of said weighing cell6and the weight of predetermined value W of the weighing cell6tocalculate the bulk density Q of the particulate material3,calculate the mass flow rate DM of particulate material3which flows through the weighing cell6during the first predetermined time interval Z, andproduce a third signal S3representing the mass flow rate DM of particulate material3which flows through the weighing cell6during the first predetermined time interval Z.

In a preferable but non-limiting manner, the flow40of particulate material3which moves in the predetermined direction of flow63, is moving through a pipe4and is going out of said pipe4through an opening, said third opening41which is situated at an end42of the pipe4.

The device1for measuring a mass flow rate is situated downstream from the third opening41of said pipe4.

The word “pipe” designates any apparatus intented to convey the granular material upstream to the device1for measuring the mass flow.

Notably, the weighing cell6has a tubular wall60at least locally transparent in order to allow observation, from outside this weighing cell6, of the particulate material3which flows against its internal face61, and the second apparatus8has:a fourth apparatus10to capture and register, through the tubular wall60, during at least the first predetermined time interval Z, two successive images of the particulate material3in contact with the internal face61of the wall60of the weighing cell6, these two successive images constituting a group2of a first image201and a second image202,a fifth apparatus11tocompare the first image201and the second image202of the group2of two successive images, anddetermine the value of a displacement L of the particulate material3during the first predetermined time interval Z, andproduce a fourth signal S4representing the value L of the displacement of the particulate material3which flows through the weighing cell6during the first predetermined time interval Z,a sixth apparatus12to use the fourth signal S4and, as a function of the value of the first predetermined time interval Z,calculate the speed of flow E of the particulate material3which flows through the weighing cell6andproduce the second signal S2representing the speed of flow E of the particulate material3contained in said weighing cell6.

In the drawing,the reference symbol “P” for the value P of the weight of the whole made up of the weighing cell6and the particulate material3contained in said weighing cell6is associated with the reference symbol “S1” for the first signal S1,the reference symbol “E” for the speed flow E is associated with the reference symbol “S2” for the second signal S2,the reference symbol “DM” for the mass flow rate is associated with the reference symbol “S3” for the third signal S3,the reference symbol “L” for the displacement L is associated with the reference symbol “S4” for the fourth signal S4.

The third signal S3may, for example, be directed to a seventh apparatus13for registering the values of this third signal during a predetermined period.

One skilled in the art is able to take these measures to fulfil this function.

Since the particulate material has a certain bulk density Q calculated by apparatus9, and the weighing cell6also has a cross section of known value C, the value of the mass flow rate DM during the first predetermined time interval Z is a function of the value of the speed of flow E and of the value of the bulk density Q.

Notably, the first apparatus7which weighs the whole made up of the weighing cell6and the quantity of particulate material3contained in this weighing cell6:obtains a plurality of weights during the first predetermined time interval Z, andproduces a first signal S1representing the average weight of the whole made up of the weighing cell6and the quantity of particulate material3contained in said weighing cell6during said first predetermined time interval Z.

Likewise in a notable way:the fourth apparatus10captures and registers regularly, during the first predetermined time interval Z, successive images of the particulate material3which flows through the weighing cell6, andthe fifth apparatus11compares regularly the first image201and the second image202of each group2of two successive images to produce the fourth signal S4representing the value L of the displacement of the particulate material3which flows through the weighing cell6during the first predetermined time interval Z.

The fourth apparatus10has a video camera100and an image grabber101.

The fifth apparatus11has an image processing system110which analyzes the first image201and the second image202in order to determine the displacement of the particles30during the first predetermined time interval Z.

One skilled in the art is also able to take these last-mentioned measurements to fulfil these functions.

One sees that the proposed device for measuring a mass flow rate combines weight measurement with optical measurements of the translation rate of the particulate material3which flows through the weighing cell6.

In a preferable but non limitating manner, the weighing cell6is oriented in such a way that the particulate material3flows vertically in this weighing cell6, but such a vertical flow is not mandatory.

One knows that in the case of a tubular wall60filled with particulate material3, the displacement of said particulate material3is coherent at a reasonable distance from both ends of the tubular wall60, i.e. all the particles30making up said particulate material3displace themselves at the same speed as if these particles30constituted a block.

Notably the weighing cell6has:a first opening64for admission of the particulate material3in the weighing cell6and a second opening65for exit of the particulate material3from the weighing cell6anda first element66which, situated at the level of the first opening64, allows the flow of particulate material in the weighing cell6by limiting to a minimal value the action exerted upon the weighing cell6by the particulate material3situated upstream from said weighing cell6,a second element67which, situated at the level of the third opening, limits to a minimal value the interaction between weighing cell6and the particular material3which is situated downstream the said weighing cell6.

Also in a notable way, the wall60of the weighing cell6is at least partially made up of a material68which is selected in such a way that the coefficient of friction between the internal face61of the weighing cell6and the particulate material3has a value as low as possible so that the particles30in contact with said internal face61do not undergo relative displacement with respect to one another.

In the drawing, by way of example, the first apparatus, the second apparatus, the third apparatus, the fourth apparatus, the fifth apparatus, the sixth apparatus and the seventh apparatus are represented as being functional blocks which receive signals and inputs and/or product signals.

In the drawing, the known value of the first predetermined time interval Z, the value of density Q, the predetermined value V of the volume of the weighing cell6, the predetermined value C of the cross section of said weighing cell6and the weight of predetermined value W of the weighing cell6are respectively represented by the letters Z, Q, V, C W.

These values are considered as being inputs and are represented as being arrows which are attached to the appropriate reference numerals or symbols.

In a noteworthy way:the first element66hasa first part660which, situated axially with respect to the first opening64and upstream from this first opening64, in relation to the predetermined direction of flow63of the particulate material3,has a predetermined shape and a predetermined first cross section to bring about a first stream of particulate material3,having a second cross section with a first axial hollow space,a second part661which, situated peripherally with respect to the first opening64and at the level thereof,has a predetermined shape and a predetermined third cross section to transform the first stream of second cross section into a second stream which is of fourth cross section substantially equal to said first cross section,the second element hasa third part670which, situated axially with respect to the second opening65and upstream from this second opening65, in relation to the predetermined direction of flow63of the particulate material3,has a predetermined shape and a predetermined fifth cross section to bring about a third stream of particulate material3having a sixth cross section with a second axial hollow space,a fourth part671which, situated peripherally with respect to the second opening65and at the level thereof,

has a predetermined shape and a predetermined seventh cross section

to transform the third stream of sixth cross section into fourth flow which is of eighth cross section substantially equal to said fifth cross section.

According to the invention the device1for measuring a mass flow rate is situated between the opened end41of the said pipe4and an opening51, said fourth opening51, of the second device5.

In a preferable but non-limitative manner:the first cross section, the fourth cross section and the eighth cross section are circular, andthe second cross section and the sixth cross section are annular.