Fluid handling – Line condition change responsive valves – Pilot or servo controlled
Patent
1997-10-02
1999-05-11
Michalsky, Gerald A.
Fluid handling
Line condition change responsive valves
Pilot or servo controlled
251 11, 251 85, 251303, F16K1/14
Patent
active
059017414
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The field of the invention pertains to flow controllers, parts of flow controllers and methods of making such items.
BACKGROUND OF THE INVENTION
Mass flow controllers for gases sense the mass flow rate of a gas substantially independently of gas temperature or pressure, provide a measurement, and thus meter such flow, and adjust the mass flow rate, as desired, based on such sensing and metering. Forms of such controllers which operate on heat transfer principles have been widely adopted.
A common commercial form of a mass flow sensor for a gas incorporates a small diameter tube which has two coils of wire wound on the outside in close proximity to each other. The coils are formed from a metallic material having a resistance which is temperature-sensitive.
In a bridge-type electrical circuit, incorporated into the sensor, the coils can then be heated by an electrical current to provide equal resistances in the absence of flow of the gas and a balanced condition for the bridge-type circuit--e.g., a null output signal.
Then, with the gas flowing within the tube, within the relevant measuring range of the sensor, the temperature of the upstream coil is decreased by the cooling effect of the gas and the temperature of the downstream coil is increased by the heat from the upstream coil transmitted by the fluid. This difference in temperature is proportional to the number of molecules per unit time flowing through the tube. Therefore, based on the known variation of resistance of the coils with temperature, the output signal of the bridge circuit provides a measure, or a meter signal, of the gas mass flow.
Typically, the small diameter tube is in parallel with the primary fluid flow route through the controller. And this parallel path is set up using some form of partial fluid flow blockage setting up a pressure drop along the primary path.
With the flow measurement at a given level, and a desired flow setting at a different level, the difference can be eliminated, i.e., the mass flow rate can be controlled, through a valve in the primary fluid flow path.
Typically, in mass flow controllers, the primary fluid flow path is somewhat circuitous to allow for the mounting of a valve seat in a valve sleeve, which is opened or closed in a linear fashion. Thus, the member which is moved to increase or decrease the fluid flow through the valve is typically moved through a force applied along the direction of the fluid flow (the direction through the opening of the valve seat). Typically, the member acting against the valve seat is moved by a structure having a position which is controlled by a solenoid or piezo-electric mechanism, and is often attached to a metallic diaphragm. Such circuitous flow path and linear mechanisms tend to require custom-made conduit parts and larger and/or more forceful mechanisms than may otherwise be desirable. They also tend to be sensitive to back-pressure effects in the primary flow path which may complicate the mechanical operation of the valve.
Somewhat related to all this is a manufacturing requirement that can occur, relating to the mounting of a sleeve containing the valve seat in the primary fluid flow path. Specifically, there may be a requirement to avoid scratching along the primary fluid flow path in putting this sleeve in position. Where, for example, a press-fit operation is used, the inevitable scratching, then, should occur along wall surfaces which do not contact the fluid during fluid flow.
Concerning the need for some blockage of the primary fluid flow path in connection with providing a parallel path through the small diameter sensor tube, a variety of blockage elements have been used and attempted. This includes, for example, employing a primary fluid flow path having a taper, and a blockage structure having a taper which can be positioned, and then held in position through a spring-like mounting mechanism. Of concern in all this is some need to maintain rather smooth, typically laminar-type flow, in some vicinity of the location of the sensor-t
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Duong Hai
Mudd Daniel T.
Michalsky Gerald A.
Millipore Corporation
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