WO1995017649A1

WO1995017649A1 - Flowmeter

Info

Publication number: WO1995017649A1
Application number: PCT/DK1994/000472
Authority: WO
Inventors: Henning Max Hansen
Original assignee: Danfoss A/S
Priority date: 1993-12-21
Filing date: 1994-12-19
Publication date: 1995-06-29
Also published as: DK142693A; EP0737302A1; DK142693D0; AU1309595A

Abstract

The invention relates to a method and a flowmeter for measuring the mass flow rate of a medium. By generating an oscillating volume flow between spatially separated points in the flow of medium, and measuring a resulting pressure difference, the mass flow rate through the tube can be established. Hereby a measurement of flow can be made with minimal flow restriction and with simple means independently of the design of the tube.

Description

Flowmeter

The present invention relates to a method and a flowmeter for measuring the mass flow rate of a medium by creating a volume flow between spatially separated points in the flow of medium, and measuring a resulting pressure difference.

From US 3,921,448 it is known to measure mass flow rates by means of a pump with fixed displacement, drawing a medium from two ducts, which are connected to the main flow of medium before and after the place where the pump delivers its flow of medium to the main flow of medium. The main flow of medium is subject to a flow restriction in that the tube is restricted in the area where the ducts are con- nected, and the measuring of differential pressure takes place. The mass flow rate results from a differential pressure measurement between two points in the main flow of medium. By adding flows of medium to the main flow of medium, according to Bernoulli's law a pressure difference will be created, which reflects the mass flow rate of the flow of medium.

US 3,232,104 and US 3,232,105 describe mass flow rate meters, where a flow of medium is divided into two part flows, and a measuring bridge with four branches is cre¬ ated. Each of the branches has an orifice plate which causes an unintentional flow restriction. Across the bridge there is a pump with fixed displacement, so that the flow of medium in the branches of the bridge consists of the sum of the flow to be measured, and the flow of medium gener¬ ated by the pump. The differential pressure across the bridge is an expression of the mass flow rate, and two different possibilities of placing differential pressure meters are demonstrated.

The object of the present invention is to provide a method and a device for measuring mass flow rates, where the measurement can be made with minimum flow restriction across a short, straight tube connection.

This can be achieved by a method as described in the intro- duction, where the volume flow is generated as an oscillat¬ ing flow.

Bernoulli's law may be used for the calculation of pressure drops both in static and in oscillating flows. Hereby flow measurements can be made with minimum flow restriction, and by simple means independently of the shape of the tube. It is possible in a simple manner to create oscillating flows of medium, for example a reciprocating piston may be used. The differential pressure may be measured as the pressure difference at the same time at pressure transducers, dis¬ placed in the direction of the flow of medium. Thus only a distance between the pressure actuator and the pressure transducer determines the length of the flowmeter, so that the flowmeter may be short.

To the flow of medium or parts of it, several oscillating volume flows may be added with different phases or fre¬ quencies. This ensures that the oscillating flow proceeds mainly in the main flow of medium between the points where the oscillating flow is induced. Thus the measuring of differential pressure can be carried out in the flow of medium in an area, where it is certain that the oscillating volume flow is mixed with the main flow of medium, and the measurements can be made with great accuracy.

The flow of medium can be divided into part flows of medium, and oscillating volume flows with different phase can be added to at least two part flows of medium, and the pressure difference can be measured between the part flows of medium. Thus it becomes possible to make several measurements of the pressure difference by using several pressure transducers in the various part flows of medium. The measuring results can be treated by a CPU, and with several measurements it is possible to suppress inaccur¬ acies.

The differential pressure can be measured as the difference between pressures at different times. Hereby the invention can be implemented with a pressure meter, which makes measurements at times adapted to the oscillating volume flow. The differential pressure can be measured at times when the velocity of the volume flow is highest in both directions. Only one pressure meter is used for this, and the length of he tube is determined only by the distance between the points where the oscillating flow is induced that is required to ensure that the oscillating volume flow is mixed with the main flow of medium.

The differential pressure can be measured as the difference in pressure between spatially separated points at the same time. Thereby a pressure difference can be measured as the difference in pressure between different pressure trans¬ ducers. When the volume flow is oscillating, the result of the measurement will reflect the oscillating flow of medium, and the greatest pressure difference is reached, when the difference in velocity is the greatest. Therefore it will be appropriate to carry out the measurements synchronously with the pressure actuators.

The pressure difference can be measured by a combination of measurements of pressure at different times, and measure- ents of pressure in different parts of the flow of medium. By making several, independent measurements of pressure, it is rendered possible to outbalance pressure drops due to viscosity, or pressure drops from surface roughness in tubes or ducts.

The present invention also describes a flowmeter for measuring mass flow rates in a flow of medium where the flowmeter has means of producing a volume flow between spatially separated points, and the flowmeter has pressure transducers for measuring at least one pressure difference, where the flowmeter has at least one oscillating pressure actuator.

In this manner it is possible to design a flowmeter without appreciable flow restriction in a straight tube, where the length is determined by the distance between the pressure actuator and one or several pressure transducers. On the basis of Bernouilli's law measuring of differential pres- sure caused by the oscillating volume flow can be used for measuring the mass flow rate through the tube.

The flowmeter can be built so that the oscillating pressure actuator or actuators form oscillating volume flows in counterphase. Hereby a reciprocating membrane or a piston can exploit the same movement for simultaneous formation of two oscillating flows of medium in counterphase.

The flow of medium may be divided into two, separate part flows of medium, where the part flows of medium are influ¬ enced in different phases by oscillating pressure actuat¬ ors. In this manner it is rendered possible to place a pressure actuator in a cross connection between the part flows of medium, so that the pressure actuator influences the part flows of medium in counterphase simultaneously. By letting the oscillating elements work in counterphase it is possible to achieve outbalancing of the oscillating system.

The oscillating flows may be formed by a reciprocating piston, placed in a cross connection between the two flows of medium. The piston may be driven by a rotating motor through a crankshaft connection, whereby the piston can form sinusoidal oscillation in counterphase in the part flows of medium. There are pressure transducers which measure a pressure difference in the flows of medium before and/or after the oscillating pressure actuator or actuators. Hereby the differential pressure may be measured between the part flows of medium both before and after the oscillating element. By making several measurements a higher solution can be achieved, and even very small mass flow rates can be registered. At the same time the effect can be eliminated by part flows of different magnitudes, for example due to different roughness of the duct or a skew flow profile in the inlet.

The oscillating flows can be formed by at least one oscil¬ lating plate. This may achieve actuation of the plate by a reciprocating movement from a linear motor.

The part flows of medium may be formed by plates which divide the flow of medium, and one plate is made to oscil¬ late by an actuator, while other plates measure pressure differences between the part flows of medium. A minimum flow resistance is achieved in this way, because the plates bar the flow with their edges only. It becomes possible to design a flowmeter independently of the shape of the tube, and the flowmeter may be fastened internally in an existing tube.

The method can be implemented by providing fixed plates before, after, and between measuring plates and the oscil¬ lating plate. This may achieve that the medium does not flow on both sides of the movable plates, thereby reducing the accuracy of measurement.

The movable plates may be fastened in a tube wall, and the plates may have extensions outside the tube wall, upon which an oscillation actuator and pressure oscillation measuring units are fastened. Hereby the invention can be implemented without introducing electrical connections into the flow of medium.

The movable plates may be fastened in their oscillation nodes. Hereby the plates can be fastened by relatively rigid means of fastening, without appreciable damping of the induced oscillation. It may be possible to weld the plates where they pass the tube wall.

With advantage the plates may be made to oscillate at their resonant frequency. This reduces the force required for driving the plates, and it becomes possible to determine the density of the medium on the basis of the resonant frequency.

Instead the plates may be made to oscillate at a frequency that is different from the resonant frequency. Hereby the frequency of oscillation can be chosen freely, and an arbitrary, easily available frequency may be used. The frequency may be chosen with a view to the electrical environment, so as to avoid interference from or to other electronic equipment.

The dividing plates may be shaped from a common plate in which slots are cut to divide them, whereby the plates join where the plates are to be fastened to the tube wall. In this manner it is also easy to obtain simple sealing where the plates pass the tube wall, when there are no gaps between the individual plate sections. The flowmeter can be manufactured cheaply with narrow slots, which can be cut with a laser, and the plate with the slots can be fastened to the tube by welding.

In the following the present invention is explained in detail on the basis of the drawings, which show: Fig. 1 A possible embodiment of the invention with a side duct.

Fig. 2 Another possible embodiment of the invention with actuators placed in the flow duct.

Fig. 3 An embodiment where the flow is divided into two flows of medium.

Fig. 4 The invention designed with plates in a tube.

Fig. 5 Shows a sectional view through fig.4.

Fig. 6 An embodiment with several dividing plates.

Fig. 7 An embodiment with through-going dividing plates.

Fig. 8 An enlarged section through fig. 7.

Fig. 9 A possible embodiment of the dividing plates.

Fig. 1 shows a possible embodiment of the present invention with a flowmeter 1. The flowmeter has a medium inlet 2 and a medium outlet 3, between which the medium flows in a flow duct 4. A side flow duct 7 is shown, in which an actuator 5 is fastened. The actuator 5 is illustrated here as a recip¬ rocating membrane, which generates an oscillating flow of medium v in the side branch 7. An oscillating flow of medium v will therefore be added near the inlet 2 to the main flow of medium V, and another oscillating flow of medium v in counterphase is added to the main flow of medium V near the outlet 3. The oscillating flows of medium v will thus be added to the main flow of medium V through the duct 4. A single pressure transducer 6 is shown, which can be used for measuring a differential pressure by carry¬ ing out several measurements displaced in time. The measurements must be made at times when the oscillating volume flow has the same velocity in different directions. This can be done if the measurements are made synchronously with the pressure actuator. The greatest differential pressure can be measured at the greatest difference in velocity, and the oscillating volume flow v has its greatest velocity at the zero passing of the pressure actuator 5. Depending on the direction of movement of the pressure actuator, a maximum and a minimum value are obtained, and the differential pressure is the difference between the minimum and the maximum values.

According to Bernouilli's law P₀ = P, + p(V+v)², where

P₀ is the static pressure of medium in N/m² before the flowmeter,

Pj is the pressure measured between the outlet points of the duct at a time Tj in N/m² , p is the density of the medium in kg/m³ ,

V is the velocity of the medium in m/s, v is the velocity of the induced volume flow in m/s.

If V and v have the same direction, the above equation applies, but because v is oscillating, the induced volume flow currently changes direction, and at times when V and v have opposite directions, the following equation applies: ^p _o = P₂ + p(V-v) ² , where

P₂ is the pressure measured at a time T₂ in N/m .

In this case T-, and T₂ are times when the volume flow v has the same velocity, but opposite direction.

The differential pressure can be expressed as the differ¬ ence between Pj and P₂, and the equations can be rewritten into:

P_. = ₀ - P(V+v)² P₂ = P₀ - %P(V-V)²

The equation for the differential pressure can be set up after the mass flow rate, which is equal to p*V in the following manner:

P, - P₂ = (P₀ - p(V+v)²) - (P₀ - p(V-v)²) p. - p₂ = p((V-v)² - (V+v)²)

P, - P₂ = -2*p*V*v, resulting in the following expression of the mass flow rate:

Based upon knowledge of the velocity v of the induced volume flow, the mass flow rate can be calculated on the basis of the measured differential pressure. The velocity of the volume flow can be established by the structural design of the pressure actuator 5.

Fig. 2 shows another embodiment with two pressure actuators 8 and 9, which can work in counterphase and which influence the main flow of medium in the duct 4. As described under fig. 1, this creates an oscillating flow of medium v, which is added to the main flow of medium V in the duct 4. The invention shown in fig. 2 differs from fig. 1 in that two pressure transducers 10 and 11 are used, which measure the pressure in the duct 4 simultaneously. Thus it becomes possible to measure a differential pressure continuously.

The same equations can be set up as for fig. 1, and the mass flow rate: p*N = (Pj - P₂)/-2*v.

Fig. 3 shows a third possible embodiment of the present invention. Here the flow of medium V is divided after the inlet 2 into two part flows of medium 12 and 13, where the flows of medium 12 and 13 join again before the outlet 3. Between the two part flows of medium 12 and 13 there is a cross duct 14, which contains an oscillating actuator 15, which may generate a pulsing flow of medium v in the cross connection 14. If the actuator 15 consists of a reciprocat¬ ing piston, oscillating flows of medium v may be generated in the two branches of the cross connection 14, which flows of medium v are in counterphase. The oscillating flows of medium v are mixed with part flows of medium 12 and 13 at the outlet of the duct. In the part flows of medium 12 and 13, pressure transducers 16, 17, 18 and 19 are placed on both sides of the cross duct 14. By using more transducers a higher degree of measuring accuracy can be obtained, and the measurements can be carried out continuously by an electronic control unit 20, which is connected to the actuator 15 as well as with the four pressure transducers 16, 17, 18 and 19, which measure P,, P₂, P₃ and P₄, respect¬ ively.

As mentioned at fig. 1 the following four equations can be set up, based upon Bernouilli's law:

P_c = Pi + hp (V+v) ² Po = P₂ + HP (V-v) Po = P₃ + HP (v+v)

P₀ = P₄ + hp(V-v)

The differential pressure can be calculated as:

P₂ - Pj + P₃ - P₄. Therefore the following equation can be set up:

P₂ - P, + P₃ - P₄ = (P₀-%p(V-v) )-(P₀-%p(V+v) )+ (P- p (N-v) )-(P₀-%p(V+v)²)

= hp(-(v-v)²+(v+v)²-(v-v)²+(v+v)²)

= %p(2V*v+2V*v+2V*v+2V*v) = 4*p*v*v

with the following result for the mass flow rate: (P₂ - P, + P₃ - P₄) / 4*V = p *V

As in fig. 1, v is a known, induced value, and the mass flow rate can be calculated on the basis of the pressure measurements.

Fig. 4 shows a longitudinal section of an alternative embodiment of the present invention from fig. 3. Fig. 4 differs in that the two flow ducts have been formed by dividing a tube 121 or a duct by a number of plates 115,

122 and 123. The plates are seen from the end and are therefore shown as a line. The ducts are shown in fig. 4 and indicated by 112 and 113. The plate 115 is made to oscillate by an actuator not shown. When the plate is moved upwards, medium is displaced away in both directions, while at the same time on its bottom side the plate generates an underpressure, which makes medium flow towards the plate from both sides. Due to the oscillation, the plate is subsequently moved in the opposite direction, and the flows occur in the opposite direction. Thereby the plate 115 generates an oscillating volume flow v. The plates 122 and

123 are connected with pressure transducers (see fig. 7) and are used for measuring differential pressure. The plates 122 and 123 are influenced transversally by the difference in pressure caused by the volume flow v. The equations shown for fig. 1 can be applied here, and there¬ fore the calculations are not repeated.

Fig. 5 shows a section through a flowmeter like the one shown in fig. 4. It is shown here that the plate 115 is made to oscillate. The plate 115 may with advantage be made to oscillate at a resonant frequency, so that oscillation nodes 129 are created, which may be used for suspending the plate 115. The plates 122 and 123 may be suspended in the same manner, and if the plates are of the same material with the same design, due to influence by the medium the measuring plates 122 and 123 will have a higher resonant frequency that the actuator plate 115.

Fig. 6 shows an embodiment of the present invention where, differently from the invention according to fig. 4, seven plates are used to divide the flow of medium.

If the invention is implemented according to fig. 4, the flow meter will have disadvantages in that the volume flow generated by the plate 115 will be reduced by the fact that part of the flow will stream through the spaces between the plates. Another problem is that the oscillating volume flow streams around the plates 122 and 123, thereby reducing the influence on the pressure transducers.

If the present invention is implemented according to fig. 6, the distance between the plates is made as short as possible, so that the flow in the gap between the plates is reduced. In order to reduce the flow around the measuring plates 122, 123 there are fixed plates 124 and 127 outside the measuring plates 122 and 123. Between the measuring plates 122 and 123 and the actuator plate 115 there may be fixed plates 126 and 127. Thus, oscillations from the plate 115 are prevented from being transferred mechanically to the plates 122 and 123.

Fig. 7 shows an embodiment where the plates 115, 122, 123, 124, 125, 126 and 127 are mounted in a gap in the tube 128, and a number of the plates partially protrude through the tube wall. This makes it possible to mount the pressure actuator 121 and the pressure transducers 116 and 118 outside the tube, thereby avoiding contact with the medium. The plates 115, 122, 123, 124, 125, 126 and 127 may be designed so that nodes for their oscillation are found in the place where they pass the wall. Hereby the plates can be secured where their movement is minimal, and they can be fastened with an elastic glue. Fig. 8 shows the plate 115 fastened to the tube 128 with a weld along the oscillation node 129 of the plate 115. The other plates can be welded in the same manner. The pressure actuator 121 is shown here between the plate 115 and a mechanical fixation 131. The fixation 131 may extend in the length of the flowmeter, and be used as reference for the pressure transducers 116 and 118. At the same time the fixed dividing plates may be fastened to 131.

Fig. 9 shows how the plates 115, 116, 125, 126 and 127 are produced by cutting sections in a common plate. The plate is through-going where it passes the tube, and the weld may be in the shape of a continuous seam. The sections must be narrow, and the plate may be cut advantageously with a laser. Hereby the plates 115, 116, 125 and 127 may be produced in a simple and cheap manner.

The oscillating pressure actuators 5, 8, 9, 15 and 121 may work according to one of the following principles: -electromagnetic

-magnetostrictive -piezoelectric -capacitive -thermal or by combinations of these.

The pressure transducers 6, 10, 11, 16, 17, 18, 19, 116 and 118 may work according to one of the following principles: -electromagnetic -piezoceramic

-capacitive -optical -strain gauge or by a combination of these.

Claims

P A T E N T C L A I M S

1. A method of measuring the mass flow rate of a flow of medium by creating a volume flow between spatially separated points in the flow of medium, and measuring a resulting pressure difference, c h a ¬ r a c t e r i s e d i n t h a t the volume flow is generated as an oscillating flow.

2. A method according to claim 1, c h a r a c t ¬ e r i s e d i n t h a t to the flow of medium or parts of it, several oscillating volume flows are added with different phases or frequencies.

3. A method according to claim 1 or 2, c h a r a c t ¬ e r i s e d i n t h a t the flow of medium can be divided into part flows of medium, and oscillating volume flows with different phases can be added to at least two part flows of medium, and the pressure difference is measured between the part flows of medium.

4. A method according to one of the claims 1-3, c h a r a c t e r i s e d i n t h a t the pres¬ sure difference can be measured as the difference between pressures at different times.

5. A method according to claims 1-3 c h a r a c t - e r i s e d i n t h a t the pressure difference is measured as the difference in pressure between spatially separated points at the same time.

6. A method according to claim 4 or 5, c h a r a c t - e r i s e d i n t h a t the pressure difference is measured by a combination of measurements of pressure at different times, and measurements of pressure between spatially separated parts of the flow of medium.

7. A flowmeter for measuring mass flow rates in a flow of medium, where the flowmeter has means of producing a volume flow between spatially separated points, and the flowmeter has pressure transducers for measuring at least one pressure difference, c h a r a c t ¬ e r i s e d i n t h a t the flowmeter has at least one oscillating pressure actuator.

8. A flowmeter according to claim 7, c h a r a c t ¬ e r i s e d i n t h a t the oscillating pres¬ sure actuator or actuators generate an oscillating volume flow in counterphase.

9. A flowmeter according to claim 7 or 8, c h a r ¬ a c t e r i s e d i n t h a t the flowmeter has means of dividing the flow of medium into two, separ- ate part flows of medium, where the part flows of medium are influenced in different phases by at least one oscillating pressure actuator.

10. A flowmeter according to one of the claims 7-9, c h a r a c t e r i s e d i n t h a t the pressure actuator is formed by a reciprocating pis¬ ton, placed in a cross connection between two flows of medium.

11. A flowmeter according to one of the claims 7-10, c h a r a c t e r i s e d i n t h a t there are pressure transducers which measure a pressure differ¬ ence in the flows of medium before and/or after the oscillating pressure actuator or actuators.

12. A flowmeter according to one of the claims 7-11, c h a r a c t e r i s e d i n t h a t the oscillating flows are generated by at least one oscillating plate.

13. A flowmeter according to one of the claims 7-12, c h a r a c t e r i s e d i n t h a t the part flows of medium are formed by plates which divide the flow of medium, and at least one plate is made to oscillate by one or several actuators, while at least one plate measures pressure differences between the part flows of medium.

14. A flowmeter according to one of the claims 7-13, c h a r a c t e r i s e d i n t h a t before, after, and between measuring plates and the oscil¬ lating plate there are fixed plates.

15. A flowmeter according to one of the claims 7-14, c h a r a c t e r i s e d i n t h a t the mov- able plates are .fastened in a tube wall, and the plates have extensions outside the tube wall, upon which an oscillation actuator and pressure oscilla¬ tion measuring units are fastened.

16. A flowmeter according to one of the claims 7-15, c h a r a c t e r i s e d i n t h a t the plates are made to oscillate at their resonant frequencies.

17. A flowmeter according to one of the claims 7-16, c h a r a c t e r i s e d i n t h a t the movable plates are fastened in their oscillation nodes.

18. A flowmeter according to one of the claims 7-17, c h a r a c t e r i s e d i n t h a t the plates are made to oscillate at a frequency that is differ¬ ent from the resonant frequency.

19. A flowmeter according to one of the claims 7-18, c h a r a c t e r i s e d i n t h a t the dividing plates are formed by a common plate in which slots are cut to divide them where they join where the plates are fastened to the tube wall.