Pumps – Condition responsive control of pump drive motor – By control of electric or magnetic drive motor
Reexamination Certificate
1998-06-15
2001-02-13
Walberg, Teresa (Department: 3742)
Pumps
Condition responsive control of pump drive motor
By control of electric or magnetic drive motor
C417S044300
Reexamination Certificate
active
06186744
ABSTRACT:
TECHNICAL FIELD
This invention relates to a device for measuring, calculating, displaying, and controlling the volumetric rate of an air delivered to a clean room.
BACKGROUND INFORMATION
Semiconductor fabrication facilities utilize clean rooms for various chip manufacturing operations. Because of the nature of the manufacturing operations, it is usually a requirement that a very precise and constant rate of purified air be supplied to any given clean room. The purified air is fan-driven and purified by one or more filters (generally HEPA filters are used, which are well-known). The manufacturing operations may be adversely affected if a constant volumetric air rate is not maintained.
Over time, the filters become increasingly clogged or loaded with particulate matter which impedes the air rate delivered to the clean room. Filter loading creates a need to make compensating adjustments in order to maintain a constant air rate.
In the past, it was commonplace to use fixed-speed fans to deliver the air. Air adjustments were made by moving or repositioning barriers in the duct work or air path leading into the clean room. These barriers were usually in the form of screens or dampers and functioned to alter the cross-sectional area of the air path. The result was a change in air velocity and rate. With this adjustment method, however, it is difficult to precisely adjust the air.
Alternatively, the air may be adjusted by using a variable frequency drive (“VFD”) to control the fan. A VFD adjusts the frequency of the input power to the fan motor, which adjusts the fan speed, thereby adjusting the volumetric air rate. The higher or lower the fan speed, the higher or lower the air rate, respectively.
When VFDs are used in conjunction with clean room air, the clean room operator must repeatedly measure how much the actual volumetric air rate has deviated from a pre-set standard and make corresponding adjustments to fan speed. In the past, the operator made the air rate measurement by using a hand-held measuring device. Then, depending on the measurement, the operator gradually adjusted fan speed until the air rate returned to the pre-set standard. This was, of course, labor-intensive, but it also had a significant drawback in that the air rate could drop out of adjustment a significant amount before the operator detected it via a manual measurement. If the air rate changed too much before detection, it could have an adverse impact on clean room manufacturing operations. The present invention solves these problems.
SUMMARY OF THE INVENTION
The invention is an automated system for controlling the volumetric rate of air delivered to a clean room. The automated system includes the use of at least one air duct for supplying conditioned air to the clean room. Air is driven through the duct by a fan. Incoming air is directed into the fan through an inlet section. In one embodiment, the system has a first pressure sensor located outside the fan inlet, and a second pressure sensor located in the inlet section, to thereby create a measurable pressure differential that can be correlated to air rate. In a second embodiment, the pressure differential is taken from the difference between the total pressure and the static pressure inside the inlet section.
A system operator inputs a desired rate into a programmable controller that is connected to both the fan and the pressure sensors. The controller is programmed to correlate the desired air rate with a set-point pressure differential between the pressure sensors. In other words, a certain set-point pressure differential must be maintained in order to maintain the desired air rate at a constant level. The controller incrementally adjusts fan speed on an ongoing basis so that the set-point pressure differential will always be maintained, until a different air rate is desired and selected by the operator.
The system must be calibrated before it can be used to supply clean room air. calibration is accomplished by measuring known volumetric air rates by independent means and correlating each air rate to a set-point pressure differential that is also measured at the time the air rate is independently measured. By taking a sufficient number of calibration data points, it is possible to create a mathematical relationship between air rate and pressure differential for the controller to use for maintaining selected rates at a constant.
In one form, the fan inlet is a symmetric convergent/divergent cone having a throat region. The first sensor is located outside and away from the inlet to the cone and the second sensor is located in the throat region. In this embodiment, the first and second sensors are conventional “pressure tap” sensors.
In a second embodiment of the invention, the first pressure sensor is an L-shaped tube sensor located inside the cone at the throat region. A first tube section of the tube lies parallel to the direction of the incoming air that travels through the cone and extends a short distance towards the inlet of the cone. An orifice at the end of this first tube section faces into the incoming air flow. In this manner, the tube is able to sense the total air pressure at the throat of the cone. At the throat region of the cone, the tube bends 90 degrees to form a second tube section. This second tube section passes through the cone's wall at that point, and the first pressure reading is taken from this point. Additionally, one or more conventional pressure tap sensors remain at the throat for sensing the static air pressure at the throat region. As with the first embodiment, air rate is determined by measuring the difference in air pressure between the two sensors. The first air pressure reading (total pressure) is taken from the tube, and the second pressure reading (static pressure) is taken from the pressure tap sensors at the throat.
The invention as summarized above will become better understood upon review of the following description which is to be taken in conjunction with the accompanying drawings.
REFERENCES:
patent: 4552059 (1985-11-01), Potter
patent: 4719791 (1988-01-01), Greiner et al.
patent: 5000052 (1991-03-01), Sipin
patent: 5410920 (1995-05-01), Westwick
patent: 5505763 (1996-04-01), Reighard et al.
patent: 5586861 (1996-12-01), Berger
patent: 5672050 (1997-09-01), Webber et al.
patent: 5988149 (1999-11-01), Gates
Miller Nash LLP
Robinson Daniel
Synetics Solutions Inc.
Walberg Teresa
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