Measuring and testing – Volume or rate of flow – Using differential pressure
Patent
1983-11-29
1985-06-18
Ruehl, Charles A.
Measuring and testing
Volume or rate of flow
Using differential pressure
G01F 142
Patent
active
045234817
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a measurement unit for determining gas flow through a conduit by means of measuring pressure differential over an orifice means arranged in the conduit.
An air flow meter of this type is previously known by Swedish Patent Application No. 7909222-7. In many cases, this known meter functions entirely satisfactorily, but provides in certain cases values which are not completely reliable. The reason for this can be that the meter is mounted close to conduit features which affect the flow conditions (e.g. the flow velocity), such as a bend or a T-branch. Furthermore, the known meter is relatively expensive to manufacture.
The purpose of the present invention is to achieve a new type of measurement unit which does not have the above mentioned disadvantages of the known meter, and which has a simpler design, so that it can be manufactured at lower cost.
According to the invention, this purpose is achieved by a measurement unit as described by way of introduction above, which is characterized in that the orifice means comprises within a straight circular section of the conduit, a centrally mounted, flat circular disc, the plane of which is perpendicular to the central axis of the section of conduit, that the disc has a diameter less than the interior diameter of the conduit section but is at least 50% of said interior diameter, that on both sides of the disc there is arranged, coaxially to the disc, a rotationally symmetrical pressure measurement box, the lateral wall of which, which points towards the disc, is arranged with its edge portion in close proximity to the surface of the disc, that between said edge portion and the surface there are aperture means through which a pressure measurement chamber inside the box, and defined by the walls of the box and the surface of the disc, is in communication with the space in close proximity to the disc but outside the lateral wall of the box, and that in a wall of each box there is a pressure lead-through means with a portion outside the box to which there is connected a pressure lead-off line which runs through the wall of the conduit and can then be connected to a pressure differential meter.
The straight, circular conduit section of the measurement unit can be for example a circular conduit nipple mounted in an air duct or the like, but the measurement unit can of course also be mounted in a straight circular conduit.
The measurement unit, which makes it possible to measure airflow via fixed measurement lead-off lines, permits installation right next to a conduit detail, e.g. a bend or T-branch, virtually without practically affecting the measurement result at all. The special design also makes it possible to make the proper adjustments even at low air velocities in the conduit system. The measurement unit can be combined with an adjustment damper, thus producing an adjustable measurement unit.
The volumes in the two boxes (i.e. the spaces of their pressure chambers) provide an average value for the various pressures occurring over a cross-sectional area of the conduit in the vicinity of or upstream of the measurement unit. These variations in pressure can for example be caused by different flow velocities at the "inner radius" and the "outer radius" of a conduit elbow immediately upstream of the measurement unit.
The particular shape of the pressure measurement boxes on either side of the disc has the result that the disturbance in the gas flow (the air flow) intentionally achieved with the aid of the disc, and the uncontrolled disturbances generated by flow obstacles (e.g. a duct elbow) right next to the measurement unit, will produce a stable pressure field in the boxes, and the pressure differential between the pressures in the box chambers will be proportional to the gas flow through the conduit in which the measurement unit is mounted. Even rather extreme mountings of the measurement unit have proved to give measured values of the flow deviating less than 5% from the actual values.
The pressure lead-off lines from the pressure chambers
REFERENCES:
patent: 2687645 (1954-08-01), Velten et al.
patent: 4008611 (1977-02-01), Turocy
patent: 4290314 (1981-09-01), Geronime
patent: 4290315 (1981-09-01), Groberg
Ruehl Charles A.
Stifab AB
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