GB2245371A

GB2245371A - A flow sensor

Info

Publication number: GB2245371A
Application number: GB9011043A
Authority: GB
Inventors: Paul Rolando
Original assignee: HUNTER INTERNATIONAL
Current assignee: HUNTER INTERNATIONAL
Priority date: 1990-05-17
Filing date: 1990-05-17
Publication date: 1992-01-02
Anticipated expiration: 2010-05-17
Also published as: GB2245371B; GB9011043D0

Abstract

A sensor for determining air flow conditions in a pipe or duct (6) comprises a tube (3) which is closed at one end and which is connected to pressure measuring means (12) at the other end. The tube has a plurality of bores (11) in a direction aligned with the direction of flow in the pipe. The portion of the tube (3) which is received within the pipe lies in a cross-sectional plane of the pipe and is substantially triangular when viewed in that cross-sectional plane and is disposed substantially symmetrically with respect to the central axis of the pipe. A similar tube (4) with bores (1) facing in the opposite direction to bores (11) is arranged downstream of tube (3) and connected to the means (12). <IMAGE>

Description

DESCRIPTION OF INVENTION A Flow Sensor.

THE PRESENT INVENTION relates to a flow sensor and more particularly to a flow sensor to be incorporated in a pipe or duct forming part of a heating, ventilating or air conditioning system in order to determine air flow conditions, such as pressure or velocity at a particular point in the system.

It has been previously proposed to provide a flow sensor comprising a tube mounted within a pipe or duct, the tube defining one or more bores permitting air to flow into the tube and generate pressure within the tube which may be measured using conventional apparatus connected to one end of the tube. Using tubes at different positions within a pipe enables pressure differentials and flow velocities to be determined.

The present invention seeks to provide an improved flow sensor which is of simple design and which provides an accurate measurement of flow conditions.

According to the present invention there is provided a flow sensor comprising a tube adapted to be mounted in a pipe or duct, the tube being closed at one end and being connected to measuring apparatus at the other end, the tube having a plurality of bores extending into the tube in a direction substantially aligned with the direction of flow in the pipe or duct, the portion of the tube which is received within the pipe or duct lying substantially in a cross-sectional plane of the pipe or duct, the tube being substantially triangular when viewed in said crosssectional plane, and being disposed substantially symmetrically with respect to the central axis of the pipe or duct.

Preferably the sensor further incorporates a second tube adapted to be mounted in the pipe or duct at a position spaced from the first tube in the direction of flow within the pipe or duct, the second tube also being closed at one end and connected to measuring apparatus at the other end, the second tube also having a plurality of bores extending into the tube.

Conveniently the bores extending into the tube which is located upstream are located on the upstream side of that tube and the bores which extend into the tube located downstream are located on the downstream side of that tube.

Advantageously the triangular portion of each tube comprises three linear portions interconnected by arcuate corners.

Preferably each of the three sides of the triangular portions of the or each tube is provided with a plurality of bores extending into the tube.

Conveniently the triangular portion of the or each tube is mounted within the pipe or duct by fixing means which connect the tube to the pipe or duct.

Advantageously the end of the or each tube which is connected to measuring apparatus extends out through the pipe or duct wall via a bore formed in the wall, the bore being sealed by means of a resilient grommet or the like through which the tube passes. Alternatively both ends of the or each tube extend out.through the pipe or duct wall via bores sealed by means of resilient grommets or the like.

Preferably the closed end of the or each tube is closed by means of a plug.

In order that the present invention may be more readily understood and so that further features thereof may be appreciated, the invention will now be described by way of example, with reference to the accompanying drawings, in which: FIGURE 1 is a graph showing flow velocity as a function of pipe diameter for laminar and tubulent flow conditions; and FIGURE 2 is a schematic, perspective view of a flow sensor in accordance with the present invention when'mounted in a section of pipe.

Referring initially to Figure 1 of the drawings line 1 represents the velocity distribution across a circular sectioned pipe for laminar flow conditions. Line 1A represents the corresponding velocity profile at a position just beyond a bend in the pipe, where the momentum of the air causes it to be 'pushed' to one side of the pipe. The peak velocity is positioned closer to the pipe wall, rather than being at the centre of the pipe. Line 2 represents the flow distribution across the same pipe for tubulent flow conditions. The vertical axis represents velocity whilst the horizontal axis represents the pipe cross section, with the two vertical lines representing the pipe walls. It will be appreciated from this graph that the flow velocity across a pipe varies from zero at the pipe walls to a maximum at the centre of the pipe.It will therefore be appreciated that if flow measurements are taken at different points across the diameter of a pipe then different results will be obtained. Thus, when determining flow conditions it is particularly important to take measurements at the correct point in order to obtain a true reflection of the average flow conditions across the pipe diameter.

Figure 2 illustrates a flow sensor in accordance with this invention when mounted in a section of pipe. The flow sensor comprises first and second tubes 3, 4, one end of each tube being formed in the shape of triangle and being closed by means of a plug 5. The triangular portion of each tube is dimensioned to be received within a pipe 6 so that the open end of each tube may pass out through the wall of the pipe via bores 7, 8. The bores 7, 8 are sealed by passing the open end of each tube through a resilient grommet 9 which is mounted in the bore and which forms a seal around the tube. The triangular portion of each tube 3, 4 is mounted within the pipe 6 by means of appropriate mounting clips 10 which retain the tube in the desired position.The triangular portion of the first tube 3 is provided at intervals along its three sides on the upstream side of the tube with bores 11 which extend into the tube.

The bores 11 permit air to flow into the tube thereby generating pressure in the tube. When steady state flow conditions prevail and a state of equilibrium is reached, the pressure generated within the first tube 3 approximates to the total pressure of the fluid flowing within the pipe 6 i.e. the static pressure and the dynamic pressure combined, since fluid impinging against the forwardmost point of the tube, where the bores 11 are positioned, is effectively brought to rest, rather than flowing ar-ound the tube.

The second tube 4 is formed in similar manner to the first tube 3, having a triangular portion designed to be received within the pipe 6, the end of the triangular portion being closed by means of a plug 5. However, the second tube 4 differs from the first tube 3 in that the bores 11 extending into the tube 4 at intervals spaced along the three sides of the triangular portion of the tube are located on the downstream side of the tube when the tube is mounted in the pipe 6. The tube is mounted in the pipe in the same manner as the tube 3 with the open end of the tube passing through a resilient grommet 9 and out of the pipe.

Mounting clips 10 are provided in order to retain the tube 4 in the desired position within the pipe 6. When fluid flows along the pipe 6 a partial vacuum is generated within the second tube 3 due to the reduced pressure which prevails in the wake generated behind the tube 4 as fluid flows around the tube.

The first and second tubes 3, 4 are mounted within a pipe 6 at positions spaced apart along the pipe. The open ends of each tube 3, 4 are connected to conventional apparatus 12 for measuring the pressure differential in the two tubes and for calculating flow rates and other flow conditions.

It will be appreciated from Figure 1 and the description thereof that there is one particular radial position at which the correct average flow velocity can be measured when flow in a circular pipe is fully developed and laminar. Similarly there is one particular radial point at which measurement will give the correct average flow velocity when the flow in the pipe is fully developed and turbulent. These two points are different. When the flow passes through a bend in the pipe the radial point at which measurement will give the correct average flow rate will be different again. One is never absolutely certain as to precisely what type of flow conditions prevail within a pipe and thus there is a band or range of positions within which it is desirable to measure flow conditions.

It has been discovered that utilising a flow sensor tube having a measuring portion which is of triangular configuration as described and shown in Figure 2 provides a particularly accurate measurement of flow conditions since the bores 11 are located at positions across the range over which it is desired to measure the flow. Thus, the specific arrangement of the tube is effectively 'self-averaging'.

This configuration of the sensor tube has also been found to be particularly well suited to averaging out both the peak on a bend outer radius with the lower pressure on the inner radius. These factors are particularly important since it is not always possible to mount the sensor in the pipe at a position where ideal flow conditions prevail i.e. in a free undisturbed flow.

A modification of the sensor is illustrated in phantom in Figure 2. The closed end of the tube 3 or 4 may extend back out of the pipe at a position adjacent the bore 7, through a similar bore and grommet arrangement. mhis enables the plug 5 to be removed and an auxiliary calibration device to be coupled to this end of the tube during installation of the sensor to confirm that the apparatus 12 is operating correctly or to provide a direct readout of the flow conditions.

Claims

1. A flow sensor comprising a tube adapted to be mounted in a pipe or duct, the tube being closed at one end and being connected to measuring apparatus at the other end, the tube having a plurality of bores extending into the tube in a direction substantially aligned with the direction of flow in the pipe or duct, the portion of the tube which is received within the pipe or.duct lying substantially in a cross-sectional plane of the pipe or duct, the tube being substantially triangular when viewed in said crosssectional plane, and being disposed substantially symmetrically with respect to the central axis of the pipe or duct.

2. A flow sensor according to Claim 1 wherein the sensor further incorporates a second tube adapted to be mounted in the pipe or duct at a position spaced from the first tube in the direction of flow within the pipe or duct, the second tube also being closed at one end and connected to measuring apparatus at the other end, the second tube also having a plurality of bores extending into the tube.

3. A flow sensor according to Claim 2 wherein the bores extending into the tube which is located upstream are located on the upstream side of that tube and the bores which extend into the tube located downstream are located on the downstream side of that tube.

4. A flow sensor according to Claim 1, 2 or 3 wherein the triangular portion of each tube comprises three linear portions interconnected by arcuate corners.

5. A flow sensor according to any one of the preceding claims wherein each of the three sides of the triangular portions of the or each tube is provided with a plurality of bores extending into the tube.

6. A flow sensor according to any one of the preceding claims wherein the triangular portion of the or each tube is mounted within the pipe or duct by fixing means which connect the tube to the pipe or duct.

7. A flow sensor according to any one of the preceding claims wherein the end of the or each tube which is connected to measuring apparatus extends out through the pipe or duct wall via a bore formed in the wall, the bore being sealed by means of a resilient grommet or the like through which the tube passes.

8. A flow sensor according to Claim 7 wherein both ends of the or each tube extend out through the pipe or duct wal via bores sealed by means of resilient grommets or the like.

9. A flow sensor according to any one of the preceding claims wherein the closed end of the or each tube is close by means of a plug.

10. A flow sensor substantially as herein described with reference to and as shown in Figure 2 of the accompanying drawings.

11. Any novel feature or combination of features disclosed herein.