Fluid handling – Processes
Reexamination Certificate
1999-05-18
2001-06-05
Lee, Kevin (Department: 3753)
Fluid handling
Processes
C137S606000, C137S496000, C137S510000, C137S613000
Reexamination Certificate
active
06240943
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the mixing of gases, and more particularly relates to a gas-mixing device which receives gases from elevated pressures, combines those gases to form a mixture of predetermined ratios, maintains constant gas ratios under steady state conditions, maintains constant gas ratios under intermittent flow conditions associated with starting and stopping the flow of the mixture, and delivers the mixture to a receiver.
BACKGROUND ART
Accurately produced non-explosive gas mixtures are required in applications as diverse as welding, lasers, commercial and industrial processing, and medical surgery, to name just a few examples. These applications require that gases drawn from high-pressure sources be combined in specified proportions before being discharged at lower pressures for immediate use. It is necessary that the specified proportions of the gases in the mixture be constantly maintained. In addition, the typical use requires intermittent flow, that is, the gas flow is often stopped and started as required by the process.
Presently, when the flow of a mixed gas is interrupted, there is an increased probability that the mixture ratio of the gases is measurably altered for several reasons. For example, intermittent flow adversely affects mixture ratio, the result of incremental errors caused by pressure changes in volumes of gas between upstream control valves and their respective flow control orifices, or differences in individual valve response times, when the flow is started or stopped. In addition, separate gauges are typically used to monitor each inlet port. With multiple gauges, mis-calibration, or the differences between working tolerances of the gauges, often accounts for mixing errors. More exacting and delicate applications have a low tolerance for mixture errors associated with intermittent flow or multiple gauges.
Attempts have been made to overcome the problem of maintaining a mixture ratio under interrupted flow and changes in operating pressures. For example, it is known to construct a mixing device which closes independent of the pressure in the feeding lines, as the pressure differential between the mixing chamber and gas discharge chamber drops below a set value. Also, it is known to absorb pressure fluctuations within a gas-mixing device by providing surge dampers and back pressure valves. In addition, it is known to provide a bi-stable diaphragm which allows the gas from a high pressure source to replenish the pressure within a receiver, by opening and closing in response to the receiver's lower pressure level. However, the existing devices do not maintain constant gas ratios under both sonic and subsonic flow; do not perform all functions within a self-contained single mechanical device; do not maintain constant gas ratios when a part of the gas-mixer is depressurized; and generally employ separate pressure gauges for each supply line.
U.S. Pat. No. 4,699,173 (Röhling) describes a mixing device which seeks to maintain a constant ratio between gases during interrupted flow. Röhling includes a gas discharge chamber separated from a mixing chamber by a pressure-sensitive, spring-mounted assembly that includes a connecting channel. There, the connecting channel is held in the open position by the assembly until the pressure in the discharge chamber rises. The assembly then closes the connecting channel to stop the flow of mixed gases. After the connecting channel is closed, increased pressure in the upstream supply lines causes upstream pressure reducing valves to close. The assembly opens the connecting channel when the pressure in the discharge chamber drops below a predetermined level.
The mixing device disclosed in U.S. Pat. No. 4,576,159 (Hahn) describes a mixing chamber which operates together with pulsed control valves to produce a regulated flow of mixed gases. In addressing the problem of mixture ratio errors, Hahn teaches that the outlet orifices of the pulsed valves should be mounted so as to be in good thermal communication with one another. As explained, this is important to minimize the effect of relative thermal expansion or contraction of the orifices which can render the relative flow rates of two gases inaccurate.
U.S. Pat. No. 5,411,051 (Olney et al.) is directed to a means for automatically replenishing a receiver, such as a deflated tire, from a high pressure reservoir. To that end, Olney teaches a bi-stable diaphragm which moves between two stable positions, respectively opening and closing the air flow passage from the high pressure reservoir. When the tire pressure falls below a threshold level, the increased pressure differential across the diaphragm causes it to flex and lift, thereby opening an air passage between the tire valve stem and high pressure reservoir. By way of alternative embodiments, the bi-stable diaphragm may contain a magnetic element, be constructed of pre-stressed metal, or be configured to include bellows.
The primary thrust of the prior art devices is to supply air or combine gases drawn from elevated pressure sources, and discharge the mixture at a lower pressure. Because conventional mixers usually start and stop flow with a control valve upstream or downstream from the control orifices, intermittent flow adversely affects the mixture ratio as a result of the pressure changes between the control valves and their respective flow control orifices. Likewise, because conventional mixers use separate gauges to monitor each inlet port, mis-calibration or the differences between working tolerances of the gauges may create mixing errors. Finally, the configuration of the prior art devices do not lend themselves to a single, self-contained and compact device.
Thus, there is a need in the art for a gas mixing device which maintains mixture ratios under both steady state and intermittent flow conditions; eliminates mixture errors under both sonic and subsonic flow conditions; uses only a single pressure gauge to determine all inlet port pressures; is easy to dismantle and repair; lends itself to a compact and unobtrusive configuration; is simple in design; and is rugged enough to be compatible with a variety of environments.
SUMMARY OF THE INVENTION
The present invention seeks to provide a gas mixing device which maintains constant gas ratios under steady state flow conditions and maintains constant gas ratios under intermittent flow conditions associated with starting and stopping the flow of gases. The present invention also seeks to produce the same gas mixture regardless of whether flow is in the sonic or subsonic region. Furthermore, the present invention seeks to reduce mixture errors inherent with multiple gauges by providing only one pressure gauge to read the inlet pressure at each inlet port. Finally, the present invention also seeks to maintain constant gas ratios by producing gas mixtures of greater than normal accuracy with equipment having lower than normal costs.
In accordance with the present invention, these objectives are accomplished by providing equal inlet pressures to the flow control orifices by means of upstream pressure controls, and maintaining equal pressure downstream of the flow control orifices by means of a single control valve and a mixed gas passage which allows all of the flow control orifices to discharge into a common chamber.
In accordance with the present invention, the above objectives are further accomplished by providing a device, configured to combine flowing gases received from elevated pressure sources, comprised of: a housing including a plurality of inlet ports; a chamber within the housing in communication with the inlet ports via a plurality of corresponding flow control orifices; a control valve, connecting the chamber to an outlet port and which opens and closes in response to upper and lower limits of pressure at the outlet port; a delay valve, connecting the inlet ports to the chamber and which opens abruptly after the control valve has opened sufficiently; and, a pressure measuring assembly rotatably mounted to the housing an
Kilpatrick & Stockton LLP
Lee Kevin
LandOfFree
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