Molded flow restrictor |
|||||||
申请号 | US13074932 | 申请日 | 2011-03-29 | 公开(公告)号 | US08756990B2 | 公开(公告)日 | 2014-06-24 |
申请人 | Jamie Speldrich; | 发明人 | Jamie Speldrich; | ||||
摘要 | The present disclosure relates generally to flow sensors, and more particularly, to devices and methods for providing a pressure drop through a flow sensor at a given flow rate. In one illustrative embodiment, a sensor assembly includes a housing with a first flow port and a second flow port. The housing may define a fluid channel extending between the first flow port and the second flow port, with a sensor positioned in the housing and exposed to the fluid channel. The illustrative sensor may be configured to sense a measure related to the flow rate of a fluid flowing through the fluid channel. A flow restrictor may be situated in and integrally molded with at least one of the first flow port and the second flow port. The flow restrictor may be configured to accurately provide a pressure drop through the flow sensor at a given flow rate. | ||||||
权利要求 | What is claimed is: |
||||||
说明书全文 | This application claims the benefit of U.S. Provisional Application Ser. No. 61/322,754, filed Apr. 9, 2010, entitled “Molded Flow Restrictor”, which is hereby incorporated by reference. The present disclosure relates generally to flow sensors, and more particularly, to devices and methods for providing a precisely controlled pressure drop through a flow sensor at a given flow rate. Flow sensors are often used to sense the flow rate of a fluid (e.g. gas or liquid) traveling through a fluid channel. Such flow sensors are often used in a wide variety of applications including, for example, medical applications, flight control applications, industrial process applications, combustion control applications, weather monitoring applications, as well as many others. The present disclosure relates generally to flow sensors, and more particularly, to devices and methods for providing a pressure drop through a flow sensor at a given flow rate. In one illustrative embodiment, a flow sensor assembly includes a housing with a first flow port and a second flow port. The housing may define a fluid channel extending between the first flow port and the second flow port, with a flow sensor positioned in the housing exposed to the fluid channel. The flow sensor may sense a measure related to the flow rate of a fluid flowing through the fluid channel. A flow restrictor may be situated in and integrally molded with at least one of the first flow port and the second flow port. The flow restrictor may be configured to accurately and/or controllably provide a pressure drop through the flow sensor at a given flow rate. In some instances, the flow restrictor may be molded to have flash formed on an upstream and/or downstream surface of the flow restrictor. The flash may be configured and/or oriented such that the flash fails to partially obstruct a designed opening of the flow restrictor. The preceding summary is provided to facilitate an understanding of some of the innovative features unique to the present disclosure and is not intended to be a full description. A full appreciation of the disclosure can be gained by taking the entire specification, claims, drawings, and abstract as a whole. The disclosure may be more completely understood in consideration of the following detailed description of various illustrative embodiments of the disclosure in connection with the accompanying drawings, in which: The following description should be read with reference to the drawings wherein like reference numerals indicate like elements throughout the several views. The description and drawings show several embodiments which are meant to be illustrative of the claimed disclosure. For merely illustrative purposes, the present disclosure has been described with reference to relative terms including, for example, left, right, top, bottom, front, back, upper, lower, up, and down, as well as others. It is to be understood that these terms are merely used for illustrative purposes and are not meant to be limiting in any manner. For some applications, it may be desirable to have a particular pressure drop across the flow sensor at a given flow rate of the fluid. In some instances, the pressure drop at a given flow may be based, at least in part, on a diameter of the fluid channel. Variations in the diameter of the fluid channel during manufacturing (e.g. molding) or other processing of the flow sensor may impact the ability of the flow sensor to accurately and precisely achieve a specified pressure drop through the flow sensor at a given flow rate of the fluid. For example, In the illustrative example, the fluid channel 12 may experience a range of flow rates of fluid flow 14. For example, the fluid channel 12 may include a high-volume fluid flow, a mid-volume fluid flow, or a low-volume fluid flow. Example fluid flow applications can include, but are not limited to, medical applications (e.g. respirometers, ventilators, spirometers, oxygen concentrators, spectrometry applications, gas chromatography applications, sleep apnea machines, nebulizers, anesthesia delivery machines, etc.), flight control applications, industrial applications (e.g. air-to-fuel ratio, spectrometry, fuel cells, gas leak detection, gas meters, HVAC applications), combustion control applications, weather monitoring applications, as well as any other suitable fluid flow applications, as desired. As shown in As illustrated, the thermal flow sensor 18 may include a heater element 22, a first sensor element 20 positioned upstream of the heater element 22, and a second sensor element 24 positioned downstream of the heater element 22. In this instance, when a fluid flow is present in fluid channel 12, at least some of the fluid 14 may flow through opening 30 into the sensor channel 26, across one or more sensor elements, such as sensor elements 20 and 24, and one or more heater elements, such as heater element 22, and through an opening 32 back into the fluid channel 12. While the first sensor element 20 is shown as upstream of the heater element 22, and the second sensor element 24 is shown as downstream of the heater element 22, this is not meant to be limiting. It is contemplated that, in some embodiments, the fluid channel 12 and sensor channel 26 may be bi-directional fluid channels such that, in some cases, the first sensor element 20 is downstream of the heater element 22 and the second sensor element 24 is upstream of the heater element 22. In this instance, when a fluid flow is present in fluid channel 12, at least some of the fluid 14 may flow through opening 32 into the sensor channel 26, across one or more sensor elements, such as sensor elements 20 and 24, and one or more heater elements, such as heater element 22, and through opening 30 back into the fluid channel 12. In some embodiment, it is contemplated that only one sensor element may be provided, and in other embodiments, three or more sensor elements may be provided. In some instances, both sensor elements 20 and 24 may be positioned upstream (or downstream) of the heater element 22, if desired. In some cases, the first sensor element 20 and the second sensor element 24 may be thermally sensitive resistors that have a relatively large positive or negative temperature coefficient, such that the resistance varies with temperature. In some cases, the first and second sensing elements 20 and 24 may be thermistors. In some instances, the first sensor element 20, the second sensor element 24, and any additional sensor elements may be arranged in a Wheatstone bridge configuration, but this is not required in all embodiments. In the example shown, when no fluid flow is present in the sensor channel 26 and the heater element 22 is heated to a temperature higher than the ambient temperature of the fluid in the fluid flow 14, a temperature distribution may be created in a generally symmetrical distribution about the heater element 22 to upstream sensor element 20 and downstream sensor element 24. In this example, upstream sensor element 20 and downstream sensor element 24 may sense the same or similar temperature (e.g. within 25 percent, 10 percent, 5 percent, 1 percent, 0.001 percent, etc.). In some cases, this may produce the same or similar output voltage in the first sensor element 20 and the second sensor element 24. When a fluid flow 14 is present in the sensor channel 26 and the heater element 22 is heated to a temperature higher than the ambient temperature of the fluid in the fluid flow 14, the temperature distribution may be asymmetrical and the amount of asymmetry may be related to the flow rate of the fluid flow 14 in the sensor channel 26 and, in some cases, the fluid channel 12. The flow rate of the fluid flow 14 may cause the upstream sensor element 20 to sense a relatively cooler temperature than the downstream sensor element 24. In other words, the flow rate of the fluid flow 14 may cause a temperature differential between the upstream sensor element 20 and the downstream sensor element 24 that is related to the flow rate of the fluid flow 14 in the sensor channel 26 and/or fluid channel 12. The temperature differential between the upstream sensor element 20 and the downstream sensor element 24 may result in an output voltage differential between the upstream sensor element 20 and the downstream sensor element 24. In another illustrative embodiment, the mass flow and/or velocity of the fluid flow 14 may be determined by providing a transient elevated temperature condition in the heater element 22, which in turn, causes a transient elevated temperature condition (e.g. heat pulse) in the fluid flow 14. When there is a non-zero flow rate in the fluid flow 14, the upstream sensor element 20 may receive a transient response later than the downstream sensor element 24. The flow rate of the fluid flow 14 can then be computed using the time lag between the upstream sensor element 20 and downstream sensor element 24, or between the time the heater is energized and when the corresponding elevated temperature condition (e.g. heat pulse) is sensed by one of the sensors, such as the downstream sensor 24. Further, it is to be understood that the illustrative heater element 22 and sensing elements 20 and 24 are merely illustrative and, in some embodiments, may not be present, as desired. For example, one or more pressure sensors, acoustical sensors, optical sensors, pitot tubes, and/or any other suitable sensor or sensor combination may be used to sense a measure related to a fluid flow in fluid channel 12, as desired. In the illustrative embodiments shown in As shown in In the illustrative embodiment, the flow restrictor 16 may be configured to precisely control and/or balance the pressure drop and mass flow rate of the fluid flow 14 through the fluid channel 12. For incompressible fluids flowing through an orifice type flow restrictor, such as restrictor 16, the mass flow rate “m” of the fluid flow 14 through the fluid channel 12 can be derived from Bernoulli's equation as the following equation: m=CA√{square root over (2p(P1−P2))} where: C is the orifice flow correction coefficient that characterizes the geometry of the orifice and the placement of the orifice in the fluid channel and in some cases may vary, for example, from 0.6 to 0.95; A is the cross-sectional area of the flow restrictor opening 17; p is the fluid density of the fluid flow 14; P1 is the fluid pressure upstream of the flow restrictor 16; and P2 is the fluid pressure downstream of the flow restrictor 16. Further, the cross-sectional area of the opening 17 of the flow restrictor 16 is given by: where: d2 is the diameter of the opening 17. Inserting this equation for the cross-sectional area of the flow restrictor into the mass flow equation, we can arrive at a mass flow equation of: From this equation, the relationship of the mass flow rate “m” and the pressure change (P1−P2) is governed primarily by the square of the flow restrictor opening 17 “d2”. By precisely and/or accurately sizing the diameter “d2” of the opening 17, the flow sensors 10 and 18 may precisely control and/or balance the relationship between the mass flow rate m and the change in pressure (P1−P2) of the fluid flow 14 through the fluid channel 12. In other words, by precisely and/or accurately designing the diameter “d2” of the opening 17, the flow sensors 10 and 18 may accurately achieve a particular pressure drop at a given mass flow rate m of the fluid flow 14. As shown in As illustrated in The bottom protective cover 37 and the top protective cover 36 may define a cavity for receiving package substrate 40 with the flow sensing element 42 mounted thereon. In the illustrative embodiment, a surface of the package substrate 40, which includes the flow sensing element 42, and an inner surface of the bottom housing cover 37 may define flow channel 46 of the flow sensor assembly 31. The flow channel 46 may extend from flow port 35 of the bottom protective cover 37, along the flow sensing element 42, and to flow port 33 of the bottom protective cover 37. The flow channel 46 may expose the flow sensing element 42 to a fluid flow. In some embodiments, as shown in In some embodiments (not shown), flow sensor assembly 31 may include one or more electrical leads electrically connected to the flow sensing element and extending external of the outer protective housing. For example, the one or more electrical leads may be configured to receive a signal transmitted from the flow sensing element corresponding to the sensed flow rate of a fluid flowing through flow channel, via one or more traces provided on the package substrate. In some cases, the one or more electrical leads may include a metal, however, any suitable conductive material may be used, as desired. In some embodiments, the outer protective housing may also include one or more mounting holes 38. As illustrated, top protective housing 36 includes two mounting holes 38, but any suitable number of mounting holes may be used, as desired. The mounting holes 38 may be configured to receive a fastener, such as a screw, bolt, or nail, to mount the top protective cover 36 to a desired surface to accommodate the particular equipment for which the flow sensor assembly 31 may be used. It is contemplated that top protective cover 36 or the bottom protective cover 37 may include additional mounting holes 38 or no mounting holes 38, as desired. In the illustrative embodiment, the flow sensor assembly 31 may include a flow restrictor 39 situated in the fluid channel 46 to provide a predetermined pressure drop in fluid flowing through the fluid channel 46 at a given flow rate. The flow restrictor 39 may include an opening 41 having a diameter that is reduced relative to adjacent portions of the fluid channel 46, and the flow restrictor may be designed (e.g., may have a precisely controlled diameter) to provide a specified pressure drop at a given flow level. As illustrated in In the illustrative embodiment, the flow restrictor 39 may be designed, formed, and/or molded to accurately control the pressure drop through fluid channel 46 at a given flow rate. In particular, the flow restrictor 39 may be formed (e.g., molded) in a manner that minimizes defects or irregularities that would impact flow of a fluid through the flow restrictor (e.g., defects or irregularities that would affect a pressure drop across the flow restrictor (i.e., by affecting the diameter of the orifice in the flow restrictor)). More specifically, and in the example context of a telescoping molding process through which the flow restrictor is formed in some embodiments, the molding process can be controlled in a manner that controls the shape and position of any flash. As used herein, flash can refer to excess material that is formed during a molding process, typically at the interface of mating pieces of a mold. For example, in a telescoping mold comprising two or more portions (e.g., die) that mate to form a cavity having the desired shape of a finished piece—into which cavity material (e.g., thermoplastic or other suitable material) is injected—excess material can work its way between the mating pieces, at the mating interface, to form flash. In a perfectly dimensioned mold, where the two or more portions mate without any space between, and in a perfectly controlled molding process, where just the right amount of material is injected, at just the right pressure, flash might be theoretically eliminated. However, most molds have, or develop over time, some spaces at the mating interface that facilitate the formation of flash. Given the likely existence of flash in a molded part such as the flow restrictor described herein, a mold that is employed to form the flow restrictor can be configured in such a way that any resulting flash is oriented in a direction that does not impact critical fluid properties of the flow restrictor. In particular, the mold can be configured such that flash (47 shown in As shown in In the illustrative embodiment, the flash 47 may not significantly impact the pressure drop of the fluid flowing through flow channel 46 at a given flow rate. In this embodiment, the flow sensor assembly 31 may be configured to accurately and precisely control and/or balance the pressure drop and the mass flow rate of a fluid flowing through the flow channel 46 at a given flow rate. As shown in In some embodiments, the opening 41 of the flow restrictor 39 may be generally circular in shape, as shown in As shown in As illustrated in While the mold has been described with mold piece 54 having opening 56 to receive at least part of portion 54 of mold piece 50, it is contemplated that mold piece 50 may include an opening to receive part of mold piece 52, if desired. In this embodiment, the flash 47 may be formed on a lower surface (45 shown in As shown in In some embodiments, the flow systems 60 and 80 may include a flow restrictor or laminar flow element (LFE), such as LFE 66 and LFE 82, situated in the main flow channel 68. The LFEs 66 and 82 may be configured to laminarize the fluid flow and/or create a pressure drop across the pressure inlet 70 and outlet 72 of the bypass sensing channel, which facilitates fluid flow into the bypass channel 44. The pressure drop across LFEs 66 and 82 may be dependent on restrictor geometry and placement of pressure taps 70 and 72 and may increase with flow rate. Furthermore, the fluid in the main flow channel 68 may become increasingly turbulent as the flow rate of the fluid increases. The LFEs 66 and 82 may be configured to straighten or laminarize the flow in the main flow channel 68 to reduce turbulence in the fluid flow by forcing the fluid to flow through a series of spaced orifices 64 and 84. In some embodiments, the orifices 64 and 84 can be circular and concentrically spaced about an axis of the main flow channel 68. However, other geometries of orifices adapted to arbitrary cross-sectional shapes of the main flow channel 68 and extending substantially parallel to the axis of the main flow channel 68 may be employed. In some embodiments, the orifices 64 and 84 may have a uniform repeating pattern of orifices of substantially identical hydraulic diameter throughout or a partially-repeating pattern in which orifices 64 and 84 are symmetrically aligned about the axis with other orifices of substantially the same hydraulic diameter. However, it is contemplated that other flow restrictors or LFEs may be used, as desired. In some embodiments, as shown in Laminarizing a fluid flow through the main flow channel 68 can be accomplished by adjusting the geometry of the orifices 64 and 68 to reduce a Reynolds number (Re), which is an index relating to turbulence. The Reynolds number is given by: Re=ρ*V*d/μ where: ρ is a fluid density; V is a mean velocity of flow; d is a characteristic distance (diameter or hydraulic radius); and μ is a viscosity. As shown from this equation, the mean velocity of the flow and the diameter of hydraulic radius are governed by the geometry of the LFE 64 and 82 and influence the Reynolds number and turbulence. The LFEs 64 and 82 can be configured to create a pressure differential to drive the fluid flow through flow sensor assembly 31. The pressure created by LFE 64 or 82 can be designed to precisely balance a pressure drop between taps 70 and 72. Imbalances between a mass flow pressure drop relationship of the LFE 66 or 82 and a mass flow pressure drop relationship of the sensor assembly 31 may cause variations in a sensor signal from flow sensor 42, which for some applications may be unacceptable. Wide variations in the sensor signal may cause signal extremes such as, for example, saturation due to excessive flow through the flow sensor assembly 31 or low signal having insufficient resolution due to insufficient flow through the flow sensor assembly 31. In either case, flow sensor 42 may imprecisely measure the flow rate of a fluid flowing through flow system 60 or 80. By precisely controlling the geometry of the flow restrictor 39, as discussed above and in some embodiments, a precise mass flow rate may be achieved at any given pressure differential created by LFE 66 or 82 leading to precise control of the flow sensor 42. Having thus described several illustrative embodiments of the present disclosure, those of skill in the art will readily appreciate that yet other embodiments may be made and used within the scope of the claims hereto attached. It will be understood that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of parts without exceeding the scope of the disclosure. The disclosure's scope is, of course, defined in the language in which the appended claims are expressed. |