DETERMINING THE FLOW RATE OF A FLOWING FLUID

Abstract

A flow measurement device for determining the flow rate of a fluid flowing in a line is provided, wherein the flow measurement device has a measurement element arranged at a measurement point in the line for a selective detection of a measurement value of the flowing fluid, a control and evaluation unit to determine the flow rate from the measurement value, and a flow guidance element arranged upstream of the measurement point with respect to the direction of flow. In this respect, the flow guidance element supplies a representative portion of the flow to the measurement point.

Claims

1. A flow measurement device for determining the flow rate of a fluid flowing in a line, wherein the flow measurement device has a measurement element arranged at a measurement point in the line for a selective detection of a measurement value of the flowing fluid, a control and evaluation unit to determine the flow rate from the measurement value, and a flow guidance element arranged upstream of the measurement point with respect to the direction of flow, wherein the flow guidance element supplies a representative portion of the flow to the measurement point.

2. The flow measurement device in accordance with claim 1, wherein the measurement element has at least one heating element and at least one temperature sensor for a calorimetric flow rate measurement.

3. The flow measurement device in accordance with claim 1, wherein the measurement element has a pressure measurement element.

4. The flow measurement device in accordance with claim 1, wherein the measurement point is arranged within the flow guidance element or follows on directly from the flow guidance element.

5. The flow measurement device in accordance with claim 1, wherein the flow guidance element has at least one aperture to allow an uninfluenced portion of the fluid complementary to the representative portion to pass.

6. The flow measurement device in accordance with claim 5, wherein the representative portion corresponds to at most 75%, at most 60%, at most 50%, at most 40%, at most 30%, at most 25%, at most 20%, at most 10%, or at most 5% of the cross-section of the flowing fluid.

7. The flow measurement device in accordance with claim 1, wherein the flow guidance element has a plurality of arms arranged in spoke form and having guide slots for picking up the representative portion.

8. The flow measurement device in accordance with claim 7, wherein the guide slots are continued in the longitudinal direction of the line as guide channels to the measurement point.

9. The flow measurement device in accordance with claim 7, wherein the flow guidance element has four arms arranged to form a cross.

10. The flow measurement device in accordance with claim 7, wherein the flow guidance element has a plurality of support elements at an angle offset from the arms.

11. The flow measurement device in accordance with claim 10, wherein the flow guidance element has in each case a support element centrally between two arms.

12. The flow measurement device in accordance with claim 1, wherein the flow guidance element has a central blocking element.

13. The flow measurement device in accordance with claim 1, wherein the flow guidance element has a central guide channel toward the measurement point.

14. The flow measurement device in accordance with claim 1, wherein the measurement point is arranged off center.

15. The flow measurement device in accordance with claim 8, wherein the guide channels are not formed symmetrical to a central longitudinal axis of the line.

16. The flow measurement device in accordance with claim 1, wherein the flow guidance element is formed in a first cross-section presenting itself to the onflowing fluid symmetrical to the center point of the lone cross-section.

17. A method of measuring a flow rate of a fluid flowing in a line in which a measurement value of the flowing fluid is selectively detected at a measurement point by a measurement element arranged in the line and the flow rate is determined from the measurement value, wherein the flow is varied by a flow guidance element upstream of the measurement point, wherein the flow guidance element supplies a representative portion of the flow to the measurement point.

Description

[0037] FIG. 1 a schematic overview representation of a flow measurement device in a longitudinal section of a line with flowing fluid;

[0038] FIG. 2 a front view of a flow guidance element;

[0039] FIG. 3 a rear view of the flow measurement device;

[0040] FIG. 4 a longitudinal section of the flow guidance element in an upright sectional plane;

[0041] FIG. 5 a longitudinal section of the flow guidance element in a horizontal sectional plane that is tilted by 90° against the upright sectional plane of FIG. 4; and

[0042] FIG. 6 a sketch to illustrate measurement errors with a selective conventional measurement without a flow guidance element in accordance with the invention.

[0043] FIG. 1 shows a flow measurement device 10 in a longitudinal sectional representation of a line 12 in which a fluid 14 flows in the direction of flow marked by arrows 16. A measurement element 20 is arranged at a measurement point 18. A measurement value of the fluid 14 is determined by it that is evaluated in a control and evaluation unit 22. Different technologies are known by which the flow velocity or the flow rate of the fluid 14 can be determined by a selective measurement. Selective measurement means that measurement only takes place at the measurement point 18 so that the flow profile over the cross-section of the line 12 is only detected at a single point. It is not precluded here to arrange a plurality of measurement elements at a plurality of measurement points, but this multiplication of the measurement effort should preferably be avoided, that is there should only be the one measurement point 18 with the one measurement element 20.

[0044] The example looked at in more detail here of a selective measurement is the thermal or calorimetric flow rate measurement. A further alternative named by way of example is a flow rate measurement using the pressure or a pressure drop. The thermal flow rate measurement has already been briefly presented in the introduction; the flow measurement device 10 can in this respect be formed as in the prior art. The measurement element 20, for example, has a substructure hang at least one heating element and at least one temperature sensor that are preferably manufactured in thin film technology. The control and evaluation unit 22 is connected to the at least one heating element and the at least one temperature sensor to evaluate the temperature measurements, to control the heating power, and to determine a flow velocity or a flow rate of the fluid 14. In principle every known method can be considered for a thermal flow rate measurement. For example, in a CTA (constant temperature anemometry) process, the heating element is regulated to a fixed overtemperature with respect to the temperature at the temperature sensor. In other words, the temperature of the unheated fluid 14 is measured by the temperature sensor and a certain difference temperature thereto is specified as the control variable at the heating element. The heating power required for this can be converted into a flow rate using a characteristic.

[0045] In accordance with the invention, a flow guidance element 24 is arranged upstream of the measurement point 18; it Is only shown schematically in FIG. 1 and will be explained more exactly with reference to FIGS. 2-5. The flow guidance element 24 conducts a representative portion of the flow to the measurement point 18, as indicated by arrows 26. Representative portion means, on the one hand, that this portion is representative of the total flow so that an averaging effectively takes place over the flow cross-section thanks to the flow guidance element 24. The only selective measurement is thereby also able to detect a flow rate of the flow as a whole even with an irregular, unknown, or variable flow profile. On the other hand, it is only a portion of the flow; a further complementary portion flows through the flow guidance element 24 and in particular past the measurement points 18, as indicated by arrows 28, at least largely uninfluenced. The pressure loss of the flow guidance element 24 is thereby restricted.

[0046] FIGS. 2 to 5 show different views of the flow guidance element 24 by which its geometry and function will now be explained in detail. FIG. 2 here is a front view, FIG. 3 a rear view, FIG. 4 a longitudinal section in an upright or vertical sectional plane and FIG. 5 a longitudinal section in a sectional plane lying perpendicular or horizontal thereto.

[0047] The flow guidance element 24 is surrounded by a cylindrical frame 30 whose outer diameter corresponds to the inner diameter of the line 12. In the front cross-sectional area, that is the front best recognizable in FIG. 2 with an orientation toward the onflowing fluid 14, a central blocking element 32 is provided that does not allow any fluid 14 to flow through on the center longitudinal axis of the line 12. A plurality of arms 34 extend radially outwardly from the central blocking element; in the preferred embodiment shown, four arms 34 in a cross having right angles between the arms 34. The arms 34 have guide slots 35 toward the front through which fluid 14 can flow into the arms 34. For this purpose, the guide slots 36 are continued within the arms 34 in the further extent of the flow through guide channels 38 that can be seen in FIGS. 3 to 5. The guide channels 38 open in a central guide channel 40 that ends at the measurement point 18.

[0048] The front side of the flow guidance element 24 is preferably symmetrical with a centrally arranged central blocking element 32 and arms 34 of equal length of a regular cross in this cross-sectional plane. The measurement point 18 is, however, offset off center in the embodiment shown without restricting the universality due to a possible rotation of the line 12 toward the top so that the measurement element 20 becomes more easily accessible. The central guide channel 4 thus does not remain on the central longitudinal axis, but rather evades upwardly toward the measurement point 18. The upper one of the arms 34 is accordingly shorter on the upright diameter along the flow guidance element 34 and the lower one of the arms 34 is longer. In the two arms 34 arranged transversely thereto, the guide channels 38 travel upward, as can be recognized in FIG. 3. The described and shown asymmetry is only one conceivable embodiment. Alternatively, the measurement point 18 can be arranged centered, that is it can lie on the center longitudinal axis of the line 12. The measurement element 20 then as to be arranged and connected at the center.

[0049] The flow guidance element 24 has apertures 42 through which the fluid 14 can flow in an uninfluenced manner between the arms 34. As can be recognized in FIGS. 2 and 3, the common area of these apertures 42 makes up a large portion of the cross-sectional area of the tine 12. A balance between a sufficiently representative portion of the flow that is picked up through the guide slots 36 in the arms 34 and a pressure loss through large openings 42 that is as small as possible can be found here. Support elements 44 are arranged in the apertures for an improved mechanical stability. They likewise take up as little cross-sectional area as possible with sufficient strength. A central arrangement within the apertures 42 has the smallest influence on the flow. The support elements 44 thus likewise form a cross, like the arms 34 and rotated by 45° thereto, in the preferred embodiment shown.

[0050] The flow guidance element 24 picks up partial cross-sections of the flow profile by means of the guide slots 36 and conducts these partial flows to the measurement point 18 by means of the guide channels 38 adjoining the guide slots 36 and by means of the central guide channel 40. The geometry of the guide slots 36 is selected such that the partial flow conducted to the measurement point 18 is representative for the total flow cross-section. The guide slots 38 extend radially over the total line 12 and a plurality of radial partial flows are picked up over the plurality of arms 34 in the peripheral direction. A good averaging thus takes place. At the at the same time, the guide slots 36 do not become too large. This would have the result that the flow accelerates by an unwanted amount at the measurement point 18. In addition a large pressure drop of the flow downstream of the flow guidance element 24 would be caused overall since then the apertures 42 through which the fluid 14 can flow without impediment would adopt too small an area in comparison with the guide slots 36.

[0051] Alternatively to the off center measurement point 18 explained up to now, a central arrangement thereof is likewise conceivable. This supports an uninterrupted flowing past of the flow portions flowing through the apertures 42. The interference effect is, however, also restricted with an off center measurement point 18, at least for so long as the front side remains in a symmetrical design and the offset from the center relative to the radius of the line cross-section remains small.

[0052] The situation shown in FIG. 6 with a 90° pipe curvature before the measurement point occurs very frequently in practice. The flow measurement device 10 would otherwise be oriented obliquely in space. Such a 90° curvature produces in a first approximation an offset of the center of the flow likewise in a 90° pattern. This is a reason why a cruciform arrangement of arms 34 in a reciprocal right angle is particularly advantageous.

[0053] It could be demonstrated in simulations that the partial flow picked up by the flow guidance element 24 and conducted to the measurement point 18 is actually representative, that is, for example, averaged after a 90° curvature of the line 12 over the flow profile. A very considerably smaller pressure loss is achieved here than would be possible with conventional flow converters that are directed to calming the total flow.