MEASURING DEVICE FOR DETERMINING THE FLOW RATE OF A FLOWING MEDIUM THROUGH A CHANNEL

Description

[0001] The invention relates to a measuring device for determining the flow rate of a flowing medium through a channel according to the preamble of claim 1. State of the art

[0002] Flow measuring devices for flowing media are used in many industrial sectors. These media are essentially fluids, i.e., vapors, gases, or liquids. The present invention does not relate to the measurement of the flow behavior of solids.

[0003] Suitable measuring devices include, for example, appropriately designed nozzles, vortex flow meters, and especially pitot tubes. The latter are particularly preferred because they cause only minimal permanent pressure losses in the flowing medium.

[0004] Typical pitot tubes have two so-called differential pressure channels, each equipped with differential pressure openings in the upstream and downstream directions. When the tube is immersed in a flowing medium, a pressure differential is created between these two channels. From this differential pressure, the flow velocity of the medium passing the pitot tube can be determined if the resistance coefficient of the pitot tube is known.

[0005] A corresponding pitot tube is described in particular in DE 195 09 208 A1. This document shows a pipeline with two channels separated by a partition. Crucially, the pitot tube is equipped with laterally projecting ribs in the flow direction, featuring essentially linear separation edges for the flowing medium.

[0006] US Patent 4,453,419 A shows a sensor in an air conditioning system. Air intake openings are arranged in a cross of sensor units, which are connected centrally via a node. The air volumes recorded at the front and separately at the rear are then directed to the corresponding measuring instruments.

[0007] US 4 481 829 A shows a plurality of measuring crosses in an air duct, which are also connected to each other via a node, from which the corresponding different pressure volumes are discharged.

[0008] German patent DE 41 18 404 A1 discloses a medical electronic device for assessing pneumocardial processes during exercise and for estimating pneumocardial function during static testing. The device consists of a pair of hollow, tubular ribs within a tubular mouthpiece, intersecting each other at their centers to form a cross. In cross-section, each rib has a pair of cavities that are separated from one another. While the cavities of one rib are isolated from each other, each rib is individually in fluid communication with two cavities of another rib. During use, a pair of hollow needle probes are inserted through membranes into the separate cavities of the rib. The hollow needle probes are then connected to pressure transducers via appropriate tubing. Task

[0009] The object of the present invention is to significantly improve such a measuring device with a pitot tube and to make it usable for measuring flowing media at very low speeds. Solution to the task

[0010] The features of the characterizing part of claim 1 lead to the solution of the problem.

[0011] According to the present invention, the aim is to achieve the largest possible differential pressure signal, or a sufficient pressure signal, even at very low flow velocities (0.7 m / s). This is made possible by the symmetrical design of the measuring cross or sensor. With a single bar-shaped sensor, the airflow can sometimes bypass the sensor, for example, after a bend in the air duct, resulting in no measurement. This risk does not exist with a measuring cross. With a measuring cross, the airflow always encounters at least one measuring unit, and usually two, which significantly improves accuracy.

[0012] A key advantage of the present invention is that the measuring range of the volume flow controller is extended, particularly towards lower air velocities. The differential pressure between the two differential pressure channels is sufficiently high, even at low air velocities (approx. 0.7 m / s), to ensure controllability in this velocity range. This high differential pressure, even at low air velocities, can be physically explained by the fact that the air velocity at the separation edge is approximately three times higher than the average velocity in the channel.

[0013] In addition to a high differential pressure, the selected sensor profile reduces measurement uncertainties at low flow velocities. The measured values ​​are stable across the entire measuring range. This is due to the previously mentioned fact that flow separation always occurs at the same point over a wide velocity range.

[0014] Laboratory measurements have confirmed the controller's position-independent operation. It was found that the new measuring system exhibits high measurement accuracy even with disrupted airflow, such as that found immediately after fittings (90° bends, T-pieces).

[0015] In the immediate vicinity of the sensor is a thin layer, the so-called boundary layer. Two flow regimes can occur in this layer: the flow can be turbulent or laminar. The laminar-turbulent transition of the boundary layer flow (boundary layer separation, where the flow can no longer follow the geometric contour) depends on the Reynolds number or the air velocity.

[0016] This boundary layer separation of the medium from the flow probe always results in vortex formation and high energy loss, leading to a so-called dead zone with backflow. Regardless of the flow velocity, the airflow always separates at the so-called separation edge in the lateral region of the profile being traversed. This results in the measured C-value (the geometric constant of the measuring unit), which significantly influences the flow rate measurement, assuming a nearly constant value over a wide velocity range. This enables a more precise measurement of the air velocity or volume flow, ensuring high process suitability.

[0017] Depending on the sensor geometry and air velocity, the location of the separation edge is always different, meaning the separation edge moves. By geometrically adjusting the sensor geometry (especially the radius), the dead water area behind the profile can be directly influenced.

[0018] The differential pressure between the two differential pressure channels, or the dynamic pressure P_dyn, results from the total dynamic pressure P_total and the static pressure behind the profile P_static. The air velocity v can then be derived from P_dyn.

[0019] The radius is intended to influence the separation edge and thus also the dead water area behind the airfoil; that is, the radius is set and the flow separation is influenced in such a way that the backflow behind the airfoil is minimized. This is because the backflow increases the static pressure p_static and thus reduces the dynamic pressure p_dyn.

[0020] The reason is that, due to the specially curved contour of the side edge of the sensor profile, the flow can always follow the contour up to the separation edge over a large speed range and the flow does not separate in front of it.

[0021] The advantages of the present invention begin as early as the flow of the medium. An oblique flow angle of + / - 10° onto the profile results in virtually no change in the achieved accuracy. This positive characteristic is particularly interesting for the use of the profile downstream of fittings (90° bends, tees, etc.). The specially curved, flow-optimized profile shape ensures a smooth flow, which offers particular advantages with regard to noise (flow noise) and pressure loss. The low pressure loss also leads to energy cost savings.

[0022] Due to the flow-optimized design, the flow accelerates approximately threefold towards the trailing edge. This acceleration of the flow in the area near the profile results in an increase in differential pressure.

[0023] In a volume flow controller, the new measuring device offers, in addition to the conventional principle of differential pressure measurement, the possibility of measuring the differential pressure in the area immediately before and after the flap blade (keyword: characteristic map control), where the air experiences a different acceleration depending on the flap position.

[0024] The fact that the number of measuring axes can be reduced to one for small channel diameters and round designs allows for savings in both assembly and material costs. The newly developed measuring cross now contains only nine individual parts. If only one measuring axis is required, the number of individual parts is reduced to just three. This results in savings in material costs, reduced assembly time, and lower tooling and maintenance costs for the supplier.

[0025] The special design of the plastic and / or aluminum parts means that, compared to other measuring elements, more force is required to separate the parts. This special design also gives the new measuring element a high degree of stability.

[0026] The entire geometric design of the sensor has been improved. Due to its mirror-symmetrical design with respect to a plane running along the edges of the flow path, the sensor can be used in both directions of flow. This symmetrical design is therefore advantageous for installation in a volume flow controller, reducing the risk of errors (due to incorrect installation) when fitting the sensor into the controller. Character description

[0027] Further advantages, features and details of the invention will become apparent from the following description of preferred embodiments and from the drawing; this shows in Figure 1 a perspective view of a measuring device according to the invention in its assembled position; Figure 2 a perspective view of the measuring device according to Figure 1 in exploded view; Figure 3a cross-section through a measuring sensor according to the invention; Figure 4 Schematic representations of different flow patterns on a sensor.

[0028] According to the Figures 1 and 2 A sensor 2 of a measuring device for the velocity of a medium flowing in a channel 1 is inserted into the channel 1. The sensor 2 is secured by means of a plastic cover 4 and a pressure tap 5, which is connected via corresponding pressure lines 6 to a measuring unit (not shown).

[0029] In Figure 1 In the assembled position, it can be seen that the sensor 2 extends through the channel 1 along its entire diameter and that corresponding inlet openings 7 are exposed to the flowing medium.

[0030] According to Figure 3These inlet openings 7 in the sensor 2 are opposite outlet openings 8, which are open in the direction of flow X. Air entering the inlet openings 7 is fed to the measuring unit via a first differential pressure channel 9, the pressure pickup 5, and the pressure line 6. Similarly, air entering through the outlet openings 8 is fed to the measuring unit via a further differential pressure channel 10 and also via the pressure pickup 5 to the pressure line 6, with both channels 9 and 10 being separated from each other by a partition 11. The supply to the measuring unit is also separate.

[0031] A key point is the cross-sectional geometry of the sensor 2 or its individual sensors 2.1-2.4. This is also determined with the aid of the Figure 3The following is described in more detail below. An inlet surface 12, which has the inlet openings 7, and an outlet surface 13, into which the outlet openings 8 are formed, are rounded so that a slight inclination of the sensor 2 in the range of + / - 10 degrees does not lead to any change in the achieved measuring accuracy. This is particularly advantageous when using the sensor 2 downstream of fittings, such as a bend in a duct. Here, the flow towards the sensor 2 is usually not straight, but inclined to a limited extent.

[0032] Following the inflow surface 12 is an arc curvature 14, which transitions into an S-shaped curvature 15. The end of this S-shaped curvature 15 forms a vertex-shaped separation edge 16 with a predetermined optimized radius.

[0033] This means that the arc curvature 14 results in a gentle division of the flowing medium, leading to significantly lower pressure losses than with known probes. However, in the region of the S-shaped curvature 15, the medium is accelerated, which is due to a Bernoulli effect. This acceleration, in conjunction with a descent 17 following the separation edge 16 that is curved in the opposite direction to an S (mirror image of S), causes the medium to detach from the probe 2 at the separation edge 16 and then transition into turbulent flow, which only merges with the remaining medium, which must flow around the opposite separation edge, well downstream of the probe 2.

[0034] According to the present invention, it has been found that with a radial, vertex-shaped design of the separation edge 16 with a radius of 0.5, the so-called "dead water zone" following the two opposing separation edges is kept very small. In this dead water zone, the turbulent flow is reduced to a minimum, so that only a very small backflow of air through the outflow opening 8 into the differential pressure channel 10 occurs. This means that a pressure difference ΔP between the incoming air in the differential pressure channel 9 and the outgoing air in the differential pressure channel 10 is maximized. According to the formula V = C • √ΔP divided by P, this ΔP essentially determines the volume flow rate V (C = geometric constant, so-called C-factor); P = air density [kg / m³ < ].

[0035] To enable more precise flow measurement, the invention provides that the measuring sensors 2.1-2.4 are connected to each other via a connecting node 18 to form a measuring cross, with the pressure sampling element 5 being provided on only one measuring sensor 2.4. The corresponding differential pressure channels 9 and 10 are, of course, connected to each other via this connecting node 18. They are then each jointly connected to the pressure sampling element 5.

[0036] Especially in Figure 4It is evident how the sensor according to the invention affects the measurement of flow values. The flow is measured by at least two sensors in all four cardinal directions, which in practice leads to significantly more and more accurate measurements. For example, it was found in the laboratory that the actual velocity value, measured in m / s, deviates from the corresponding target value by less than 10%, and this was the case at a large number of different pressures, measured in Pascals. Reference symbol list

[0037] 1 channel 34 67 2 Sensor 35 68 2.1-2.4 Unit of measurement 3 seal 36 69 4 plastic cover 37 70 5 Pressure sampling unit 38 71 6 Pressure line 39 72 7 Inlet opening 40 73 8 exhaust opening 41 74 9 Effective pressure channel 42 75 10 Effective pressure channel 43 76 11 septum 44 77 12 flow surface 45 78 13 Outflow surface 46 79 14 curvature 47 15 S-shaped curvature 48 16 tear-off edge 49 17 descent 50 18 node 51 19 52 20 53 21 54 22 55 23 56 24 57 25 58 26 59 27 60 28 61 29 62 30 63 31 64 32 65 33 66

Claims

1. Measuring device for determining the flow rate of a flowing medium through a channel (1), wherein a measuring sensor (2) is exposed to the flowing medium in the channel (1) and comprises inflow openings (7) which are open opposite to the direction of flow of the medium and outflow openings (8) which are open in the direction of flow (X) of the medium, wherein the inflow openings (7) and the outflow openings (8) are located in a measuring sensor (2.1 - 2.4) separated by a partition wall (11) and are connected via a pressure extraction part (5) to a measuring unit, wherein several measuring units (2.1 - 2.4) are connected via a node (18) to form a measuring sensor, and wherein only one pressure extraction part (5) is connected to one measuring unit (2.4), and wherein the measuring sensor (2) has, on both sides of the inflow openings (7) and the outflow openings (8), a tear-off edge (16) which is curved in a rounded radius (R) in the form of an apex, characterized in that between an inflow surface (12) with the inflow openings (7) and the tear-off edge (16), there follows an oppositely S-shaped curvature (14), followed by an S-shaped curvature up to the tear-off edge (16), and adjoining the tear-off edge (16) an oppositely directed descent (17), which in turn merges into an S-shaped curvature towards an outflow surface (13) with the outflow openings (8), and both ascents (15) and descents (17) merge at their ends into respective further radii in which the inflow openings (7) and the outflow openings (8) are located.

2. Device according to claim 1, characterized in that differential pressure channels (9, 10) between the inflow openings (7) and the outflow openings (8) and the pressure extraction part (5) are formed identically in the measuring sensor.

3. Device according to claim 1 or 2, characterized in that the measuring units (2.1 - 2.4) are integrally formed from metal or plastic.