Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
  • G01F1/00—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow
  • G01F1/68—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow by using thermal effects
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
  • G01F1/00—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow
  • G01F1/68—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow by using thermal effects
  • G01F1/684—Structural arrangements; Mounting of elements, e.g. in relation to fluid flow
  • G01F1/6847—Structural arrangements; Mounting of elements, e.g. in relation to fluid flow where sensing or heating elements are not disturbing the fluid flow, e.g. elements mounted outside the flow duct
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
  • G01F5/00—Measuring a proportion of the volume flow
• • G—PHYSICS
  • G05—CONTROLLING; REGULATING
  • G05D—SYSTEMS FOR CONTROLLING OR REGULATING NON-ELECTRIC VARIABLES
  • G05D7/00—Control of flow
  • G05D7/06—Control of flow characterised by the use of electric means
  • G05D7/0617—Control of flow characterised by the use of electric means specially adapted for fluid materials
  • G05D7/0629—Control of flow characterised by the use of electric means specially adapted for fluid materials characterised by the type of regulator means
• • G—PHYSICS
  • G05—CONTROLLING; REGULATING
  • G05D—SYSTEMS FOR CONTROLLING OR REGULATING NON-ELECTRIC VARIABLES
  • G05D7/00—Control of flow
  • G05D7/06—Control of flow characterised by the use of electric means
  • G05D7/0617—Control of flow characterised by the use of electric means specially adapted for fluid materials
  • G05D7/0629—Control of flow characterised by the use of electric means specially adapted for fluid materials characterised by the type of regulator means
  • G05D7/0635—Control of flow characterised by the use of electric means specially adapted for fluid materials characterised by the type of regulator means by action on throttling means

Definitions

• Thermal siphoning in a mass flow controller may refer to a continuous circulation of gas caused by the free convection between the heated thermal flow sensor and the bypass. Thermal siphoning may result in a non-zero output signal for the flow rate that resembles zero point drift, even when the actual output flow rate is zero. In some MFC designs, thermal siphoning effects maybe more likely to occur if the mass flow controller is installed vertically, and may vary in proportion with the molecular weight and pressure of the fluid whose flow rate is being controlled.
• thermal siphoning may also cause a calibration shift in the span or dynamic range of the mass flow meter of the mass flow controller.
• a method and system are needed that can prevent or reduce thermal siphoning effects when a thermal mass flow controller is vertically mounted, and that can provide a zero shift thermal mass flow sensor for a vertical flow thermal mass flow meter.
• a thermal mass flow meter for measuring flow rate of a fluid includes a conduit that is configured to receive the fluid and that defines a primary flow path between an inlet and an outlet of the conduit.
• the conduit is bound at least in part by a sensor receiving surface.
• a thermal sensing portion of a thermal sensor tube is mounted relative to the sensor receiving surface in a direction substantially perpendicular to both the primary flow path and the sensor receiving surface.
• a thermal mass flow controller for controlling flow rate of a fluid may include a conduit that is configured to receive the fluid and that defines a primary flow path between an inlet and an outlet of the conduit.
• the conduit may be bounded at least in part by a sensor receiving surface.
• the thermal mass flow controller may further include a thermal sensor tube having a thermal sensing portion mounted relative to the sensor receiving surface in a direction substantially perpendicular to both the primary flow path and the sensor receiving surface.
• the thermal mass flow controller may further include a temperature measuring system configured to measure a temperature differential between at least two locations along the thermal sensing portion of the sensor tube, when the sensor tube has been heated and fluid flows within the heated sensor tube.
• the thermal mass flow controller may further include a control valve configured to regulate flow of the fluid into the inlet and out of the outlet of the conduit, so that the fluid flows from the outlet at a desired flow rate.
• thermal mass flow meter When the thermal mass flow meter is mounted in a substantially vertical direction so that fluid within the conduit flows in the vertical direction along the primary flow path, fluid within the thermal sensing portion of the sensor tube may flow in a horizontal direction so as to substantially prevent thermal siphoning when the sensor tube is heated.
• a method for preventing thermal siphoning in a mass flow controller for controlling flow rate of a fluid includes a thermal sensor tube has a thermal sensing portion, and further includes a conduit configured to receive the fluid.
• the conduit defines a primary flow path between an inlet and an outlet of the conduit, and is bounded at least in part by a sensor receiving surface.
• the method includes mounting the thermal sensing portion of the sensor tube in a direction substantially perpendicular to both the primary flow path and the sensor receiving surface.
• FIG.s IA and IB schematically illustrates the operation of a thermal mass flow controller, and the phenomenon of thermal siphoning.
• FIG. 2 illustrates a thermal MFC in which the thermal sensor tube is mounted in a direction perpendicular to the primary flow path and to a sensor receiving surface of the main flow body of the thermal mass flow meter in the MFC.
• FIG. 3 illustrates the thermal MFC of FIG. 2 that has been mounted vertically so that fluid flows vertically within the flow body, and horizontally through the sensor tube.
• a system and method are described for substantially preventing thermal siphoning in a thermal mass flow controller when the mass flow controller is vertically mounted.
• FIG.s IA and IB schematically illustrate the operation of a typical thermal MFC that measures and controls the mass flow rate of fluids, and also illustrate thermal siphoning that may occur when the MFC is mounted vertically, as shown in FIG. IB.
• FIG. IA illustrates a horizontally mounted thermal MFC
• FIG. IB illustrates a thermal MFC that is the same as the MFC shown in FIG. IA, but that is mounted vertically, hi overview, thermal MFCs may measure the mass flow rate of a fluid by using the thermal properties of fluids and monitoring the temperature change of the heated sensor tube as the fluid flows therethrough.
• a thermal MFC may typically include a thermal mass flow meter which actually measures the mass flow rate of fluids, and a control assembly (including a valve and electronic control circuitry that controls the actuation of the valve), which regulates the flow rate of fluids so that the measured flow equals a desired flow setpoint.
• thermal MFCs may measure the mass flow rate of gases and vapors, although flow rates of fluids other than gases and vapors may also be measured.
• the thermal MFC 100 may include: a thermal mass flow sensor assembly 110; a conduit 120 or flow body configured to receive at an inlet 122 the fluid whose flow rate is being measured/controlled; and a bypass 130 within the conduit 120.
• the thermal MFC 100 may further include a valve 140, and a controller 150 that controls the operation of the valve 140 in a way that provides a controlled flow of the fluid from an outlet 123 of the conduit 120.
• the conduit 120 or flow body may define a primary flow path or channel 124, and is bounded at least in part by a sensor receiving wall or sensor receiving surface 170.
• the sensor receiving surface 170 is shown as being substantially parallel to the primary flow path 124.
• the majority of the fluid that is introduced to the MFC through the inlet 122 of the conduit 120 may proceed through the primary flow path 124.
• a relatively small amount of the fluid may be diverted through the thermal mass flow sensor assembly 110 by the bypass 130, and may re-enter the primary flow path 124 downstream of the bypass 130.
• the bypass 130 may be a pressure dropping bypass that provides a pressure drop across the primary flow channel 124 so as to drive a relatively small portion of the incoming fluid through the thermal mass flow assembly.
• the inlets and outlets of the sensor tube 200 may coincide with the inlets and outlet of the primary flow channel 124, and therefore the pressure drop across the bypass 130 may be the same as the pressure drop across the sensor tube 200.
• the thermal mass flow sensor assembly 110 may be attached to the sensor receiving surface 170 that forms at least a portion of a boundary of the conduit 120.
• the thermal mass flow sensor assembly 110 may include: a thermal sensor tube 200 configured to allow the diverted portion of the incoming fluid to flow within the tube between an inlet 230 and an outlet 240 of the tube 200; a sensor tube heater configured to heat the sensor tube; and a temperature measurement system configured to measure a temperature differential between two or more locations along the tube.
• the sensor tube 200 may be a thin- walled, small-diameter capillary tube, and may be made of stainless steel, although different sizes, configurations, and materials may also be used for the sensor tube 200.
• the sensor tube 200 may include a thermal sensing portion 210, which in FIG. IA is shown as being disposed horizontally, parallel to the primary flow path, and two legs 212 which are shown in FIG. IA as being vertical.
• a pair of resistive elements 250 and 251 may be disposed in thermal contact with the thermal sensing portion 210 of the tube 200 at different locations along the thermal sensing portion 210, and may function as both the sensor tube heater and as part of the temperature measurement system.
• the resistive elements 250 and 251 may be resistive coils that are wound around the tube 200 at two locations along the thermal sensing portion 210 of the tube, one upstream (250) and the other downstream (251).
• the sensor tube 200 maybe heated by applying an electric current to the resistive elements, which may thus function as a heater for the tube.
• thermal siphoning may occur, caused by thermal gradients that appear inside the sensor tube as the sensor tube is heated. As explained below, thermal siphoning may occur in the vertically mounted MFC even when the control valve is completely closed as shown in FIG. IB.
• thermal siphoning may cause shifts in zero, i.e. shift the null output to a non-zero signal.
• Thermal siphoning may also cause shifts in the span or dynamic range, i.e. in the flow rates covered by the relevant measuring range of the mass flow meter up to the maximum intended flow rate. As a result, the actual flow measurement may become a function of the inlet pressure and of the nature of the fluid.
• the thermal siphoning effects on zero and span (dynamic range) may increase with increasing inlet pressure and gas density.
• the major factors that affect thermal siphoning may include gas density, the sensor tube diameter, and the attitude of the heated sensor tube.
• Gas density is an intrinsic property of the gas, and thus cannot be manipulated in order to reduce the thermal siphoning effect.
• the small internal diameter of the sensor tube in the MFC may generally reduce the effects of thermal siphoning, manufacturing a tube having such a small diameter may be difficult and impractical, and may limit the dynamic range of the MFC design. Therefore, the attitude of the heated sensor tube maybe a good choice of a criterion to be adjusted in order to reduce thermal siphoning inside the MFC.
• the sensor legs 212 may no longer generate any convective forces. However, the thermal sensing portion 210 containing the heater coils may now generate convective forces, because the thermal sensing portion 210 is now oriented vertically, not horizontally. Since the bypass is unheated, there may be no convective opposition, so that thermal siphoning may occur.
• FIG. 2 illustrates one embodiment of a thermal mass flow controller 300 that is designed to substantially eliminate thermal siphoning when mounted vertically.
• the same reference numerals as in FIG.s IA and IB are used for all the sub-parts of the MFC 300, which is identical to the MFC 100 shown in FIG.s IA and IB, except for the direction in which the thermal sensor tube is mounted or oriented with respect to the primary flow path and with respect to the flow body that contains the bypass.
• the thermal sensor tube is mounted or oriented in a direction that is substantially vertical or perpendicular to the primary flow path and to the sensor receiving surface.
• the thermal sensor tube is mounted relative to the conduit (which defines the primary flow path) in a direction 305 so that the thermal sensing portion 210 is substantially perpendicular to both the primary flow path 124 and the sensor receiving surface 170 (the latter being substantially parallel to the primary flow path 124 in the illustrated embodiment).
• this mounting configuration of the thermal sensor tube minimizes or substantially eliminates the thermal siphoning cause by free convection of the fluid inside the heated sensor tube, when the MFC is mounted vertically.
• a supporting element 310 is provided that supports the sensor tube in the direction 305 substantially perpendicular to the primary flow path 124 and the sensor receiving surface 170.
• the supporting element 310 may be a supporting bracket, for example, and may be configured to secure the sensor tube assembly onto the conduit.
• the supporting element 310 may have apertures to let the fluid pass through the sensor tube and re-enter the primary flow path 124 downstream of the bypass 130.
• FIG. 3 illustrates a thermal mass flow controller 400 that is the same as the one shown in FIG. 2 but is mounted vertically, i.e. illustrates a vertical flow thermal MFC.
• the incoming fluid (whose flow rate is being measured) flows vertically within the flow body or conduit, from an upper portion of the conduit to a lower portion of the conduit.
• thermal sensing portion 210 of the sensor tube may remain horizontal.
• the thermal sensing portion 210 of the sensor tube i.e. the section with the heater coils
• the thermal sensing portion 210 of the sensor tube is now oriented horizontally, and thus generates no convective force, while the convective forces generated by the two legs of the sensor tube cancel. In this way, thermal siphoning caused by convection may be minimized or substantially eliminated.
• the thermal MFC shown in FIG. 3 may have a smaller footprint, compared to MFCs in which the sensor tubes are mounted transverse to the primary flow path and parallel to the sensor receiving surface of the flow body or conduit. For this reason, the conduit 120 of the embodiments shown in FIG.s IA, IB, 2, and 3 are all interchangeable, providing a distinct manufacturing advantage.
• the thermal MFC may have a width of less than or equal to about 1.2 inches.
• the thermal MFC shown in FIG. 3 maybe used to minimize or reduce thermal siphoning effects in large bore flow sensors, which maybe required in order to provide the low pressure drop that is necessary to deliver sub-atmospheric gases.
• gas delivery at less than about 10 Torr bottle pressure, and accurate delivery of the same flow rates at full bottle of greater than about 1000 Torr may typically require wide bore sensors.
• wide bore sensors generally work well, they maybe sensitive to mounting attitude, due to the thermal siphoning effect.
• the vertical mounting of the sensor tube, in a direction perpendicular to both the primary flow path and the sensor receiving surface, may minimize the thermal siphoning problem without requiring an increase in footprint when such thermal MFCs are mounted vertically.

Landscapes

• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Fluid Mechanics (AREA)
• Engineering & Computer Science (AREA)
• Automation & Control Theory (AREA)
• Measuring Volume Flow (AREA)
• Flow Control (AREA)

Priority Applications (4)

Applications Claiming Priority (2)

Publications (3)

Family

ID=37963959

Family Applications (1)

Country Status (7)

Families Citing this family (20)

Family Cites Families (21)

• 2005
  • 2005-11-22 US US11/284,452 patent/US7296465B2/en active Active
• 2006
  • 2006-11-03 DE DE112006002995.9T patent/DE112006002995B4/de not_active Expired - Fee Related
  • 2006-11-03 GB GB0811056A patent/GB2446116B/en not_active Expired - Fee Related
  • 2006-11-03 WO PCT/US2006/043013 patent/WO2007061608A2/en not_active Ceased
  • 2006-11-03 JP JP2008542329A patent/JP2009516853A/ja active Pending
  • 2006-11-03 KR KR1020087013536A patent/KR101380800B1/ko active Active
  • 2006-11-21 TW TW095142904A patent/TWI408344B/zh not_active IP Right Cessation
• 2014
  • 2014-12-24 JP JP2014261316A patent/JP2015057618A/ja active Pending
• 2016
  • 2016-12-02 JP JP2016234888A patent/JP6163245B2/ja not_active Expired - Fee Related

Also Published As

Similar Documents

Legal Events