GB2095411A

GB2095411A - Paper porosity

Info

Publication number: GB2095411A
Application number: GB8103923A
Authority: GB
Original assignee: Filtrona PLC
Current assignee: Essentra PLC
Priority date: 1981-02-09
Filing date: 1981-02-09
Publication date: 1982-09-29
Also published as: GB2095411B

Abstract

A gas flow control device comprises a plurality of contiguous chamber communicating in series, the up-stream chamber having a gas inlet and the gas outlets from one chamber to the next and from the downstream chamber being constituted by respective porous walls of sintered material for which gas flow is directly proportional to pressure drop over respective different flow value ranges whose maxima increase downstream from chamber to chamber. The control device may be used in a method of measuring the porosity of a gas permeable sample in which gas from a pressure source is passed through the device and then through the sample, the pressure drop across the sample is measured and the pressure drop across the porous wall through which laminar flow is achieved is measured, the air flow is derived from the air flow/pressure drop ratio for the wall and the permeability of the sample is derived from the air flow and the pressure drop across the sample.

Description

SPECIFICATION Paper porosity This invention relates to control and measurement of gas flow, especially in the measurement of the gas permeability (porosity) of materials, e.g. of paper or other gas-permeable sheet material.

Whilst the invention is described below mainly in connection with measurement of the gas permeability of paper, especially cigarette or cigarette filter wrapping paper, it is to be understood that it is applicable in like manner to any gas permeable test material and to the control or measurement of gas flow in general.

Measurement of paper permeability is of importance in the paper making and using industries as a check on product quality. For the cigarette industry it is especially important since the air permeability of cigarette and filter wrapper can be an important factor in determining the tar and nicotine produced on smoking, and the tar and nicotine values published for a brand must be maintained.

The permeability or porosity P of a sample sheet is given by the relationship p F AxPD where F is the volume gas flow per unit time through the sample, A is the exposed area of the sample and PD is the pressure drop across it.

In one measurement procedure the gas flow upstream of the sample of known exposed area is maintained at a predetermined value, the pressure drop across the sample is measured, and P is calculated as above. It is preferred, however, to measure instead the gas flow F for a given predetermined pressure drop PD.

The range of permeabilities amongst filter plugwraps has recently increased markedly with the use of the air dilution technique, in which air is drawn in through the wrapper to dilute the smoke, the degree of air dilution (and hence wrapper permeability) being especially important in determining tar and nicotine values.

There is thus a need for accurate and reliable gas flow measurement over a wide range of flow values, e.g.

from 5 ml/min. to 300 I/min. and there is no current device which meets these requirements satisfactorily.

Various devices are being used imdustriallyto measure air flow, e.g. variable area flowmeters, hot wire anemometers, turbine meters, critical flow orifices etc. To cater for a wide flow range present apparatus generally has a plurality of such devices arranged in a respective plurality of parallel flow paths reliable for measurement over respective different flow ranges, the appropriate path for a given measurement being selectable by a valve or valves. The disadvantage of such an arrangement is that in a parallel network bends in the conduit create rotational flow, and valves and flowmeters in the flow line add to the turbulence. This interference to the main gas stream can seriously affect the accuracy of a measuring device, especially a turbine meter.It is important that the sample, e.g. paper, under test be presented with a non-turbulent flow, because permeability is a property of the sample relating the flow through it to the pressure difference across it, and the latter is disturbed by turbulent flow which is non-linear and erratic.

The present invention provides a gas flow control device comprising a plurality of contiguous chambers communicating in series, the upstream chamber having a gas inlet and the gas outlets from one chamber to the next and from the downstream chamber being constituted by respective porous walls of sintered material for which gas flow is directly proportional to pressure drop over respective different flow value ranges whose maxima increase downstream from chamber to chamber.

Whn a gas passed through a wall of porous sintered material it is fragmented into a multiplicity of small channels, tending to cause its flow to become laminar so that the pressure drop due to resistance offered by the small channels is proportional to flow. The nature of the flow is dependent on the area, thickness and porous structure of the wall of sintered material e.g. a given wall area of sintered material starts to produce a back pressure signal as a flow of gas is presented to it, and this increases in proportion to flow until a point is reached when the gas flow starts to become non-laminar.

Thus in a device according to the invention, for a low flow, e.g. up to 2 I/min. the flow through the outlet wall of the upstream chamber will be laminar and measurement of the PD across the outlet (readily accomplished accurately) allows the air flow to be obtained accurately from the known PD/flow characteristics of the outlet wall. At higher flows, e.g. 2 to 20 I/min., the flow through the outlet wall of the upstream chamber will be turbulent, but that through the outlet wall of the adjacent downstream chamber will still be laminar, so that the airflow can be derived as above. Athird chamber will allow derivation of greater air flows (e.g. up to 300 I/min.) in like manner, with the range measurable being increased if desired by a subsequent chamber or chambers.The limiting factor in extending to higher gas flows is the ability of the first chamber to allow a sufficiently high gas flow therethrough. The range of operation of the device can be varied according to needs, however, when it is constructed so that chambers can be added or removed at either end.

The difference in laminarflow range between one outlet wall and another may be due to difference in any one or more of the exposed area, thickness and porous structure (diameter, spacing, number, interconnection etc. of the internal channels) of the walls.

For use in measurement of the gas permeability of a sample the device according to the invention preferably includes a housing contiguous and communicating in series with the most downstream chamber and having its outlet constituted by means for mounting the sample across the gas flow - e.g. a clamp for mounting a permeable sheet for flow of the gas therethrough from the chamber. The upstream chamber can then have its gas inlet connected to a gas supply with means being provided for measuring the PD across the sample and that across the outlet wall of a selected chamber.Such an arrangement can be used to measure gas permeability of the sample at a given constant flow or at a given constant PD across the sample, the measuring means preferably operating to generate electrical signals from which a digital or analogue indicator can be calibrated to give a direct permeability reading in the required units.

Each chamber outlet wall can be of any of numerous materials and configurations; it might for example be a flat (e.g. disc) or profiled (e.g. conical or part spherical) wall at the downstream end of an otherwise impervious chamber, or the chambers may be nested and wholly of porous sintered material apart from a common downstram impervious wall having a gas inlet for the innermost (upstream) cylinder.

Particulate materials which can be sintered to provide the chamber outlet walls include metals (e.g.

stainless steel, copper, brass), glass, plastics and ceramics. The graded particles can be filled into a mould of the appropriate shape and size and heat treated if necessary under pressure to effect sintering to a coherent porous body.

The invention is illustrated, by way of example only, in the accompanying drawing which shows schematically a device according to the invention forming part of an apparatus for measuring the gas permeability of paper.

In the drawing, the device according to the invention is a linear flow transducer indicated generaily by the reference numeral 2 having spaced nesting cylinders 3,4 and 5 of sintered bronze particles soldered to a common upstream metal base 6 having a gas inlet 8 to the innermost of the nesting cylinders. The side and downstream end walls of each of cylinders 4 are porous so that gas supplied through inlet 8 passes successively through the cylinders from left to right. The nesting cylinders 4 are in turn surrounded by an impervious housing 10 also secured to common base 6 and having an outlet constituted by a clamp 12 for holding a gas permeable sheet 14 across the gas flow.Gas (usually air) is supplied to the inlet 8 as shown from a source 16 via pressure regulator 18, solenoid valve S1, and a fourth porous cylinder 20 of sintered brass particles housed in impervious housing 22 having inlet 24; cylinder 20 and housing 22 are secured to base 6 about inlet 8 on the face opposite from cylinders 3 to 5. Regulator 18 is controlled by motor 26, and 28 is a low flow control valve operated by motor 30.

Cylinders 3,4 and 5 are in open communication with respective channels 32,34 and 36 in base 6, and the back pressure signal 38 from sample 14 and selected cylinder internai gas pressures are transmitted as indicated schematically to transducers T1 and T2 via solenoid valves S2, S3, S4, S5, and S6 according to the settings of these individual valves between their a, 6, and c settings.

Transducers T1 and T2 measure the pressure drop across the sample 14 and across a selected cylinder 3,4 and 5. The two transducers T1 and T2 have different operating ranges to allow for accuracy of measurement over a wider range than is possible with a single transducer.

In the constant flow method of operation the pressure drop across sample 14 and also across the selected cylinder is applied to both transducers and the output from which ever transducer provides the better resolution of results is selected. In the constant pressure method of operation, a constant pressure drop of appropriate value is maintained across the sample 14 and is measured on whichever of the transducers is most appropriate for the particular pressure drop chosen while the pressure drop across the selected cylinder is measured on whichever of transducers T1 and T2 is most appropriate for the resolution required.

If greater accuracy of resolution is required it is, of course, possible to employ more than two transducers, the operating characteristics of the various transducers being selected to respond within desired ranges.

The signals from transducers T1 and T2 preferably operate a visual display or print-out (not shown) giving a direct digital or analogue reading of the pressure drop across sample 14 and ofthe airflow, and/or ofthe sample permeability calculated therefrom.

Cylinders 3,4 and 5 have respective different airflow ranges over which they show a constant air flow/pressure difference ration, with the maxima of these ranges increasing from cylinder 3 to cylinder 4 to cylinder 5. In one particular embodiment, this air flow range for cylinder 3 is from 5 to 2000 ml/min, that for cylinder 4 is up to 20000 ml/min, and that for cylinder 5 is up to 300000 ml/min. The flow/pressure drop characteristics of each cylinder are previously ascertained by experiment to find its airflow/PD ratio and the airflow range over which this ratio is constant, and this constant ratio can be built into the transducer/display or print-out arrangement to provide a direct read-out of airflow in the required units from the pressure differences sensed.

The illustrated apparatus can be used for measuring permeability of a sample 14 either at constant flow or at constant pressure drop across the sample. In the following two Examples, cylinders 3,4 and 5 have linear fiow/pressure drop behaviour over the respective ranges mentioned above.

EXAMPLE 1 Constant flow The sample is a sheet of pervious paper 14 which is mounted tautly in clamp 12. Motor 26 sets pressure regulator i8to provide an air pressure from source 16 of 40 psi.

Valve S3 is switched to its position c, valve S4 to its position b, valve S5 to its position c, and valve S1 to its position c; motor 30 then adjusts valve 28 until the pressure drop across cylinder 3 as indicated by transducer T1 corresponds to a flow of 1050 ml/min.

Valves S2, S3, S4 and S5 are then switched to their b positions and valve S6 to its c position, and the pressure drop across paper 14 as indicated by transducer T2 is then measured. With the air flow F measured in ml/min. and the pressure drop PD across paper 14 measured in mm.Wg., the permeability P of paper 14 in usual units is calculated as 10 100 P=Fx-10x 100 A PD where A is the area in square centimetres of the paper exposed to the gas flow.

If it should be required to measure the PD across paper 14 at air flows above that at which the air flow/pressure drop ratio for cylinder 3 is constant, then the solenoid valves would be set instead to allow sensing of the pressure drop across cylinder 4 or cylinder 5 as appropriate.

EXAMPLE 2 Constant pressure The test sample is paper which is mounted as in Example 1.

Valves S1 to S5 are switched to their positions b, and valve S6 to its position c, and motor 26 adjusts regulator 18 until the pressure drop across paper 14 as indicated by transducer T1 is 1 cb (101.7 mm.Wg).

Then valves S2 to S4 and S6 are switched to their positions b and valve S5 to its position c.

If the pressure drop across cylinder5 as sensed by transducer T2 is significant then the air flow deduced from the known air flow/PD characteristic for the cylinder (or indicated directly by a visual display operated by the transducer) can be used for determination of the permeability of paper 14. If not, the solenoid valves can be switched to give a sensing of the pressure drop across cylinder 4 (and then if necessary across cylinder 3) to obtain a reliable value for the air flow.

The paper permeability is then calculated from the air flow and the PD across paper 14 in the standard manner, with the permeability reading preferably being displayed visually as before.

Arrangements of chambers 3 to 5 other than that shown in the drawing are of course possible with similar advantage. Thus the device may have only two or more than three of the porous cylinders, and chambers of like function but quite different configuration could be used. In other arrangements, the chambers need not nest completely, with only that portion of a chamber which is enclosed being constituted by or including a porous wall of sintered material. In other arrangements, the chambers may be simply contiguous end to end - e.g. one might use a continous impervious tube separated into chambers by a transverse wall or walls of porous sintered material.

Whilst the above described arrangement in which the gas passes in eries through one porous sintered wall after another is preferred, there can also be advantage over prior apparatus and devices if a plurality of such walls of different permeabilities are disposed across paths connected in parallel between the gas supply and the test sample, with the appropriate path for the flow rate concerned in any particuiar case being selectable by means of a valve. This is because the appropriately selected path can convert turbulent to laminarflow on passage of the gas through its porous wall, so that, provided laminar flow between the wall and the test sample is maintained, a reliable air flow value can be obtained from the sensed pressure drop across the wall and used for derivation of the sample permeability in the manner indicated above.Indeed, there can be advantage in a permeability measuring apparatus having a single porous wall of sintered material across the path between gas supply and test sample, such apparatus being suitable for permeability measurement of the sample over the air flow range for which the air flow/pressure drop ratio for the porous wall is constant; such apparatus could be made more flexible by providing for the porous wall to be interchangeable with at least one other having a different flow range over which its flow/pressure drop ratio is constant.

Accordingly the present invention also provides a method of measuring the gas permeability of a gas-permeable sample, the method comprising passing gas from a pressure source firstly through a porous wall of sintered material and then through the sample so as to ensure laminar flow between the wall and sample, measuring the pressure drop across the sample, measuring the pressure drop across the porous wall and deriving the air flow from the known air flow/pressure drop ratio for the wall, and deriving the permeability of the sample from the said air flow and the pressure drop across the sample.

For the above mentioned parallel path and single porous wall embodiments, the presure drops may be sensed by transducers as in the illustrated embodiment, and the airflow may likewise be derived automatically from the known characteristics of the porous wall(s) and displayed.

In all embodiments the whole system of valves, motors, transducers etc. can be controlled by a microprocessor with the final permeability value being calculated automatically and displayed or printed out in chosen porosity units.

It will be appreciated that although the invention has been described wih reference to the use of a porous sintered material it will be appreciated that other multi-labyrinthine structures can serve as the medium for achieving laminarflow provided that the pressure drop characteristics of the material remain constant in use.

CLAIMS (filed 28.5.81) 1. A gas flow control device comprising a plurality of contiguous chambers communicating in series, the up-stream, chamber having a gas inlet and the gas outlets from one chamber to the next and from the downstream chamber being constituted by respective porous walls of sintered material for which gas flow is directly proportional to pressure drop over respective different flow value ranges whose maxima increase downstream from chamber to chamber.

2. A method of measuring the porosity of a gas permeable sample which comprises passing gas from a pressure source firstly through a porous wall of sintered material and then through the sample so as to ensure laminar flow between the wall and sample, measuring the pressure drop across the sample, measuring the pressure drop across the porous wall and deriving the air flow from the known air flow/pressure drop ratio for the wall, and deriving the permeability of the sample from the said air flow and the pressure drop across the sample.

3. A gas flow control device as claimed in claim 1, substantially as described herein with reference to and as shown in the drawings.

4. A method of measuring the porosity of a gas permeable sample conducted substantially as described herein with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. achieving laminarflow provided that the pressure drop characteristics of the material remain constant in use. CLAIMS (filed 28.5.81)

1. A gas flow control device comprising a plurality of contiguous chambers communicating in series, the up-stream, chamber having a gas inlet and the gas outlets from one chamber to the next and from the downstream chamber being constituted by respective porous walls of sintered material for which gas flow is directly proportional to pressure drop over respective different flow value ranges whose maxima increase downstream from chamber to chamber.