Measuring and testing – Volume or rate of flow – By measuring thrust or drag forces
Reexamination Certificate
2000-02-16
2003-06-10
Williams, Hezron (Department: 2855)
Measuring and testing
Volume or rate of flow
By measuring thrust or drag forces
C073S861000
Reexamination Certificate
active
06575046
ABSTRACT:
TECHNICAL FIELD
The present invention relates to heating, ventilation and air-conditioning (HVAC) systems. More particularly, the present invention relates to variable air volume HVAC systems.
BACKGROUND OF THE INVENTION
Increased emphasis is being placed in the quality of air within occupied buildings, and therefore increased emphasis is being placed on introducing the correct amount of outdoor air into those buildings. The trend toward “tighter” buildings has resulted in less outdoor air infiltrating into buildings, making it more important that the mechanical ventilation systems introduce the specified amount of outdoor air.
The task of consistently introducing the specified amount of outdoor air into a building is complicated by the fact that many mechanical ventilation systems are the variable air volume type (VAV). On VAV systems, the air delivery volume drawn in by the fans changes. Fan speed is varied and therefore the negative pressure those fans create at the inlet to the outdoor air intake damper also changes. The amount of outdoor air that will be drawn in through the outdoor air damper is dependent on two things: how far open the damper is, and the negative pressure generated by the fans at the damper inlet. If a consistent volume of outdoor air is to be drawn into the building, the damper open position must change whenever the negative pressure at that damper intake changes.
A proposed draft of ASHRAE Standard 62-1989R has included Section 5.6.9.1 to deal with the problem of bringing in the specified cubic feet per minute (CFM) of outdoor air with a VAV system. That section stated, “Variable air volume systems (except those supplying 100 percent outdoor air) shall include controls and devices to measure outdoor air mass flow at the air handler and designed to maintain outdoor air mass flow not less than 90 percent of required level over the expected supply air operating range.” Although this revised standard is still in the proposal stage, the requirement for direct measurement of outdoor air is showing up in project specifications. Accordingly, there is a present need in the industry for an accurate air mass flow measuring device, especially being accurate at relatively low air mass flow rates.
Products are now on the market that attempt to measure and control outdoor air CFM using a calculation based on a measurement of velocity, velocity pressure, or louver pressure drop static pressure. Outdoor air mass flow volume is calculated by using that measured air velocity or velocity pressure and an equivalent duct area, or by using the measured static pressure drop across the outdoor air intake louver and typical louver pressure drop characteristics. In reality, the velocity pressures or static pressures encountered at the outdoor air intake are so low at the minimum outdoor air mass flows that need to be measured, that it is not reasonable to use them for what is supposed to be an accurate measurement. To accurately measure air velocities, ideal conditions must exist, such as long, straight duct runs and uniform air velocities throughout that.duct and known air densities. The outdoor air intake on a typical roof-mounted air handling unit will have a tortuous, turbulent outdoor air mass flow path, widely varying temperatures, changing barometric pressures, and varying wind conditions, which cumulatively make it unsuitable for the aforementioned existing type of measurement techniques. It becomes increasingly difficult to accurately measure air mass flow rates as that air mass flow rate is reduced. The purpose of measuring the air mass flow rate is to be sure that the flow rate does not get below the specified minimum at that low end of its range, typically 10 to 20 percent of the maximum air mass flow rate.
SUMMARY OF THE INVENTION
CONTROLLING FLOW OF OUTDOOR AIR: To deliver the specified volume of outdoor air to the building, the present invention measures outdoor air CFM flow rate, and controls the position of the outdoor air damper to maintain the specified CFM flow rate.
The present invention is a sensing vane that is rotatably displaced by the impingement and flow of an air stream across it. That vane repeatedly and accurately assumes a position according to the mass of air flowing across it. The “vane positions” are translated into air mass flow readings of “standard air” (0.075 lbs./cu. ft.). Therefore, vane position readings always indicate “standard air” because the vane is responding to air mass flow that is, air weight (mass) rather than air volume.
On this illustrative version of the device, the air mass flow that causes displacement of the vane is opposed by a combination of two springs and gravity. Alternative models can use other combinations of springs and gravity, or only gravity, or only springs.
There are many ways to translate the vane position to an air mass flow reading. A simple way would be to have the vane align with a suitable marked scale and directly read air mass flow. Another way is to have the vane position control an electrical signal such that the electrical signal can be translated into an air mass flow reading. Connecting the vane to a potentiometer is one way to accomplish that result.
Through tests we have determined that the displacement of the vane of the present invention accurately and repeatedly indicates the air mass flow.
SENSING VANE: The sensing vane of the present invention functions according to the principals described here but it is adapted to meet the requirements for specific applications. The application will influence the vane size, location, and orientation. The vane adaptation in this illustration is tall and narrow, with a vertical pivot axis Alternatively, a vane could be long and narrow on a horizontal axis to be compatible with horizontal ductwork or arranged for vertical air mass flow.
It is important that the blade rotational friction be minimized. It must be free-swinging to respond to small forces. In this case, virtually all of the weight of the vane is on the lower hinge. The lower hinge or pivot is essentially a conical recess that rests atop and pivots on a fixed, sharp point in order to minimize rotational friction. The upper bearing is a nylon or other minimal friction bushing that keeps the pivot axis in alignment but has little static force on it.
This sensing vane does not add measurable pressure drop to the outside air intake path. At minimum air mass flow, the vane is somewhat perpendicular to air mass flow, but because of the low flow rate there is not a measurable pressure drop. As air mass flow increases, the vane rotates, becoming increasingly more parallel with the air mass flow path and eventually reaching a position where it has swung parallel with and proximate the backwall of the outdoor air intake, essentially out of the air path.
The vane is mounted to a vertical support that is attached to the backwall of the outdoor air intake. That vertical support includes a leading edge lip that overlaps and protects the leading edge of the vane from air mass flow impingement. That lip prevents a turbulent, high-velocity air stream from getting behind the vane (between the vane and the air intake backwall) and causing the vane to flutter.
SPRINGS: Optimum accuracy will result if the Vane Position vs CFM relationship is characterized such that similar changes in CFM will result in similar changes in the vane position, indicating a linear relationship. Generally, very light air mass flow forces must move the vane near the minimum air mass flow position, building up to heavy force near mid-rotation, and then dropping back as the maximum air mass flow position is approached. On spring versions of the present invention, the desired linearity is accomplished by the selection and levering of the springs to act against the forces at various points of vane rotation.
The exemplary embodiment uses springs to oppose the movement of the vane. Two extension springs oppose the force of the air mass flow against the vane. The springs and link arms are such that, at very low air mass flows, only a light sprin
AAF-McQuay Inc.
Allen Andre
Patterson Thuente Skaar & Christensen P.A.
Williams Hezron
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