GB2123564A

GB2123564A - Fluid flow measurement

Info

Publication number: GB2123564A
Application number: GB08209179A
Authority: GB
Inventors: Henry Rosenberg
Original assignee: Furness Controls Ltd
Current assignee: Furness Controls Ltd
Priority date: 1982-03-29
Filing date: 1982-03-29
Publication date: 1984-02-01

Abstract

A flow measuring device which comprises a stack of generally flat plates 43 spaced from one another, the flat plates being mounted in side plate 32 having slots, the side edges of the plates being sealed so as to form generally rectangular through flow passages, and means 38, 39 being provided to connect a pressure measuring device 20 either to at least one of the through flow passages at two points spaced from the ends or to the inlet and outlet of the through flow passages. <IMAGE>

Description

SPECIFICATION Flow measuring device The present invention relates to a flow measuring device. A number of methods for measuring flow of gases or liquids are known, but the present invention relates particularly to those using Poiseuille's law. Thus, where a fluid is flowing through a tube, there is a pressure differential along the tube depending upon the flow rate of the fluid through the tube. A measure of this pressure differential can therefore be used to measure the flow rate through the tube.

Apparatus utilising this principle have been well known for, probably, a hundred years.

The law only holds true for certain viscosities, size of tube and flow rate (basically the flow must be laminar flow) and to measure the flow rate through large tubes, it is necessary to separate the flow into a plurality of separate flow paths and to measure the pressure differential in one of these plurality of flow paths and then extrapolate to provide the total flow rate. In an alternative arrangement, the pressure difference between the inlets and outlets to the tubes is measured.

Figure 1 of the drawings shows a cross section through a known type of flow measuring device utilising this principle. In Figure 1 fluid flows from left to right through the apparatus, the inlet 11 being provided by a suitable fitting, the diameter of the outer end 12 of the inlet 11 being the same as the diameter of the bore of the tube to which it is attached, and the diameter of the bore of the inner end 13 of the inlet 11 being of a wider diameter and including flow directing means in a form of vanes 1 4.Extending along the length of the apparatus are a plurality of circular section tubes 1 5 which are packed together and held together by being embedded in resin 1 6. Clearly because the resin 1 6 blocks off the parts of the cross section between the tubes 15, the overall diameter of this section must be greater than the diameter of the outer end 12 of the inlet 11 if the apparatus is not to unduly restrict the flow of fluid therethrough. The total cross sectional area of the tubes 15 must be approximately the same as the cross sectional area of the bore of the outer end 1 2. An outlet fitting 1 6 is provided generally similar to the inlet fitting 11 except that the vanes 14 are not required.

One of the tubes 1 5A includes, intermediate its ends, two holes through its wall which are connected to fittings 18, 1 9 and the fittings 18, 1 9 are connected to pressure differential measuring apparatus 20. The flow rate of the fluid through the particular tube 1 5A to which the pressure differential measuring apparatus 20 is attached, can be used to measure the flow rate through that one tube 1 SA and therefore the total flow rate through the apparatus can be determined by multiplying that measurement by the total number of tubes 15.

Figure 2 shows a transverse section on the line 2-2 of Figure 1, from which the cross section of the tubes 1 5 and their arrangements can be seen.

Such an apparatus is difficult to build because it is difficult to space the tubes 1 5 accurately. Until recently the tubes were mounted in an end plate which was made of metal and which required some considerable dexterity in assembly, but more recently the tubes are embedded in a resin which simplifies the process of manufacture slightly.

There are a number of other disadvantages of this well known arrangement. The apparatus is difficult to calibrate because it is not easy to adjust the apparatus once manufactured. In practice, a substantial proportion of apparatus manufactured by the above method have to be rejected.

Because of the problems associated with this type of apparatus, other types have been proposed.

For example, it has been proposed to use a honeycomb material to provide separate flow passages instead of the multiple tubes, but it is difficult and extremely expensive to produce honeycombs which produce passages of sufficient length. Generally the process of manufacturing honeycomb lends itself to producing only short passages. It will be appreciated therefore that the disadvantages of the apparatus described with respect to Figures 1 and 2, have been known for a long time but no sensible solution to them has been proposed.

The present invention provides a flow measuring device comprising a stack of generally flat plates spaced from one another, means for sealing the side edges of the plates together to form spaces forming generally rectangular through flow passages, and means for connecting a pressure measuring device either to at least one of the through flow passages at two points spaced from the ends of said through flow passage or to the inlet and outlet of said through flow passages.

The means for sealing the side edges may comprise two further plates with means for mounting and spacing the generally flat plates from one another. This mounting and spacing means may be in the form of slots in which the flat plates may be mounted.

The plates are preferably considerably wider than the spacing between them.

The invention also provides flow measuring apparatus comprising a plurality of through flow passages, each having an inlet and outlet, and each being bounded by two generally flat walls and two side walls and means for connecting a pressure measuring device either to at least one of the through flow passages at two points spaced from the ends of said through flow passage or to the inlet and outlet ends of said through flow passages. The flat walls are generally of greater transverse extent than the two side walls. In both cases, therefore, the through flow passage may be rectangular.

It is found that such an arrangement is both simple to manufacture and is highly reproducable in that the reject rate is very much lower than with the arrangement of Figures 1 and 2. The manufacture is much simpler and the invention lends itself to a variety of simple methods of manufacture.

In the drawings.

Figure 1 aiready described, is an axial section through a flow measuring device of the prior art, Figure 2 is a transverse section through the flow measuring device of Figure 1, Figure 3 is a perspective cutaway view of a flow measuring apparatus according to the invention, and, Figures 4 to 6 show alternative arrangements of plates for use in a flow measuring apparatus of the invention.

In Figure 3 the flow measuring apparatus comprises a stack of (in this case) eight flat stainless steel plates 31, the width of each plate being (3.8 cms) its length (20.3 cms) and its thickness (0.71 mm).

The plates are mounted in edge plates 32, the edge plates 32 being of stainless steel and having slots 33 milled in one face. The slots 33 are each of a sufficient width to engage the edge of a plate 31. The slots 33 are spaced from one another in the two edge plates 32 by the same distance (1.59 mm) so that the plates 31 are held together as a stack 34 of parallel plates 31. Mounted above and below the stack 34 in Figure 3 are cover plates 36, 37, the upper cover plate 36 including two threaded bores 38, 39. These threaded bores 38, 39 each interconnect with a respective small bore (only one of which 41 is shown) in the upper plate 31.

It will be seen therefore that the stack 34 of plates 31 together with the edge plates 32 form a stack of through flow passages 43.

The cover plates 36, 37 extend beyond the open ends of the through flow passages 43 and seal with end fittings 46, 47, the end fittings including respectively an inlet fitting 48 and an outlet fitting 49. In this way an inlet chamber 51 and an outlet chamber 52 are formed.

In manufacturing the apparatus it is important that plates 31 should be accurately flat and also that their ends, where they extend to the chambers 51, 52 should be accurately formed.

In construction, the plates 31 are sealed to the edge plates 32 by a suitable sealing adhesive.

In use the small bores 41, 42 are connected to a differential pressure measuring apparatus which will measure differential pressure as low as 0.1 Pascals.

A gas or liquid whose flow rate is to be measured is passed from the inlet fitting 48 into the inlet chamber 51 and passes through the through flow passages 43. Because of their dimensions the through flow passages tend not to restrict the flow of fluid and the fluid then flows to the outlet chamber 52 and through the outlet fitting 49.

The pressure differential measuring apparatus measured the differential pressure of the fluid passing through the upper through flow passage 43A and by multiplying this value by 7 (there being 7 through flow passages) the total flow rate can be measured.

As will be understood the apparatus is extremely simple to construct. In tests it has been found that the apparatus is very reproducable so that manufacture is simplified. If the apparatus does not perform to required tolerances it can be easily disassembled and reassembled or if necessary the length of the through flow passages altered by means of changing the length of the plates 31.

Figures 4 to 6 show alternative arrangements of plates in diagrammatic form. The plates may be easily joined together.

Claims

1. A flow measuring device comprising a stack of generally flat plates spaced from one another, means for sealing the side edges of the plates together to form spaces forming generally rectangular through flow passages, and means for connecting a pressure measuring device either to at least one of the through flow passages at two points spaced from the ends of said through flow passage or to the inlet and outlet of said through flow passages.

2. A device as claimed in Claim 1 in which the means for sealing the side edges comprises two further plates with means for mounting and spacing the generally flat plates from one another.

3. A device as claimed in Claim 2 in which the mounting and spacing means is in the form of slots in which the flat plates are mounted.

4. A device as claimed in any of Claims 1 to 3 in which the plates are considerably wider than the spacing between them.

5; Flow measuring apparatus comprising a plurality of through flow passages, each having an inlet and outlet, and each being bounded by two generally flat wails and two side walls and means for connecting a pressure measuring device either to at least one of the through flow passages at two points spaced from the ends of said through flow passage or to the inlet and outlet ends of said through flow passages, the flat walls being generally of greater transverse extent than the two side walls.

6. A flow measuring apparatus substantially as hereinbefore described with reference to Figures 3 to 6 of the accompanying drawings.