WO1991012496A1

WO1991012496A1 - Air flow sensor

Info

Publication number: WO1991012496A1
Application number: PCT/US1991/000840
Authority: WO
Inventors: David S. Dougan; Michael J. Urbaniak
Original assignee: Ebtron, Inc.
Priority date: 1990-02-12
Filing date: 1991-02-12
Publication date: 1991-08-22
Also published as: AU7252191A

Abstract

An airflow sensor (100) for an HVAC system adapted to measure a flow rate of outside air has one end located in the mixed air plenum. The device has a first flow conditioner (220) formed of a plurality of tubes of diameter less than 1/8 inch. On the downstream side of the first airflow conditioner is a sensor grid (232, 234) which measures the airflow. On the downstream side of the sensor grid is a second airflow conditioner (240) which is formed of a plurality of tubes similar to the first airflow conditioner and which isolates the grid from the turbulence in the plenum. The sensor grid is calibrated for this situation, and because the first airflow conditioner produces a fixed level of airflow and turbulence for any given airflow. The first and second airflow conditioners are also structural, and the entire assembly is located within a sleeve (200) which is preferably formed from a second of ductwork. The first and second airflow conditioners form the structural support which keeps the ductwork integral.

Description

AIR FLOW SENSOR

1. Field of the Invention

The present invention relates to an airflow measuring device which includes structure that enables it to accurately operate even when located in a turbulent airflow location. More specifically, the present invention includes an airflow sensor with flow conditioners on both sides of the sensor.

2. Background of the Invention

As energy costs rise, so too rise the demands for ways to reduce energy costs for the building owners. The architects and engineers have responded to the demand by designing "tighter" buildings.

By design, less air is allowed to enter the building (fixed windows, etc.); therefore, by reducing the amount of air entering the building, the energy costs are reduced because it takes less energy to heat/cool the inside air than it does to heat outside air.

When less outside air enters a building to mix with the recirculated air within the building, however the impurities that may be present in the air will have a tendency to increase.

It is possible that through neglect or human error, in some cases, no outside air is entering some buildings to dilute the contaminants within the air. The main constituent of the contaminants is carbon dioxide, the by-product of human breathing. However, gases (such as CFCs) from machines such as refrigerators and chemical fumes (such as formaldehyde) from materials such as carpet, furniture etc.^" will also accumulate. It has been found that the quality of the indoor air in a building is a function of the exchange rate of the outdoor air within the building. The principal society in the heating, ventilating and air conditioning (HVAC) industry is ASHRAE. The recent ASHRAE standard 62-1989 is entitled "Ventilation for Acceptable Indoor Air Quality," and states that airflow measurement and control of outside air should be provided in many applications, to provide for the proper exchange of outdoor air within a building.

Until the advent of the present invention, no satisfactory way has ever been provided for accurate measurements of outdoor air entering a building. ASHRAE standard 62-1989 recommends airflow measurement for this control of outside air. However, since no direct way has been known for doing this, it has been done by maintaining a differential between the supply air pump and return air pump by measuring their volumes with air measuring stations, and controlling according to the difference between the two. However, many errors may occur in such a calculation, and there is no way of really assessing the amount of these errors. When the differential of the supply and return flow is a low intake value, the errors in outdoor air intake are even worse.

However, the design of most outdoor air intakes have made it impossible for a standard airflow measuring device to be used. The reason for this is that the louvers and dampers are generally in a very confined space. Often, there is little or no ductwork, and the louvers and dampers are attached directly to the mixed air plenum. This provides a very turbulent environment, and one in which conventional airflow sensors cannot accurately operate.

Another problem related to the above is that wind from the outside, and turbulence from the fan and damper of the existing HVAC system influences the accuracy of conventional flow measuring devices. Finally, the location where the device is located typically has velocities of 50-500 feet per minute. The combination of space limitations, low velocities, and affect of wind and air turbulence under it impossible to measure using a conventional system.

SUMMARY OF THE INVENTION The present invention has been provided in order to obviate all of these problems. This device can be placed against an outdoor air intake damper and directly penetrate into the mixed air plenum. There is no ductwork needed because of a self-contained housing of this device. According to the present invention, the device includes a sensor array which is isolated from all air turbulence on both sides by airflow conditioners. The airflow conditioner is preferably a honeycomb-shaped structure formed of a plurality of tubes which are connected together in a way to provide structural integrity. A sensor assembly is located between the two flow conditioners, and is calibrated for this situation. The honeycomb structure used according to the present invention preferably includes openings allowing airflow which are less than 1/8 inch in diameter.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be described in detail herein with respect to the accompanying drawings, in which:

FIGURE 1 shows the air flow sensor of the present invention mounted in its preferred location penetrating a mixed air planum;

FIGURE 2 shows a cross-sectional view of the air flow sensor along the plane 2-2 in Figure

_> • FIGURE 3 shows a portional cut away view of the air flow sensor, showing its three main constituent layers; and

FIGURE 4 shows a detail of the honeycomb structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A presently preferred embodiment will now be described in detail herein.

The present invention is intended to operate in conjunction with airflow sensors of the grid array type. Examples of such devices are those described in U.S. Patents 4,494,406 and/or 4,581,930. It should also be understood that any similar flow sensor array could be used, but the preferred embodiment intends use with one of these flow sensor arrays.

One important thing which the prior art has taught about placement of such flow sensor arrays, is that they must be mounted on the trailing edge of a non-lifting airflow stream. The inventors of the present invention have recognized that this is less important than consistency of the airflow for any given speed. The flow conditioners of^'the present invention ensure this consistent operation.

Figure 1 shows the airflow sensor 100 of the present invention mounted in its preferred location. This preferred location is within the outside air intake 102 which leads outside air into the mixed air plenum 104. The mixed air plenum 104 mixes the outside air from the outside air intake 102 with recirculated air from return duct 106. The mixed air (supply air) is blown by fan 108 into ductwork 110 which is circulated throughout the building after appropriate temperature conditioning.

Return duct 106 is shown with damper 112, and can also, or alternately, have a fan therein.

The airflow sensor 100 is shown mounted in its preferred position, in the outside air intake 102, and extending into the mixed air plenum 104. A detailed layout of the preferred embodiment of airflow sensor 100 is shown in Figures 2, 3 and 4. The device of the present invention is intended to be a self-contained unit formed within a short section of ductwork which forms sleeve 200. The sleeve 200 has a cross-sectional shape which is substantially rectangular, defining a hollow inner portion 202 inside. Air flows in the direction of arrow 204 through the hollow inner portion, from upstream end 203 toward downstream end 206.

Figure 2 shows a cross-sectional view of the device taken along the plane 2-2-2 (Figure 3) . Figure 2 shows the component structures making up the airflow sensor of the present invention as mounted within sleeve 200.

The first element within sleeve 200 in the upstream direction is first flow conditioner 220. This first flow conditioner stabilizes the characteristics of the incoming air by creating a pressure drop. The purpose of this flow conditioner is not to remove all turbulence, but rather to produce a turbulence for any given flow rate which is consistent and reproducible for that given flow rate.

Figure 3 shows a partially cut-away view showing all three layers 220, 230 and 240 of Figure 2. Element 220 is shown in Figure 3 as being made up of a number of tubes, which are cemented together to form a honeycomb-shaped structure. This is shown in further detail in Figure 4. Figure 4 shows the arrangement of flow conditioner 220, which is the same as that of flow conditioner 240, to be discussed later. The flow conditioner is made up of a number of tubes 222, all of which are cemented to one another at locations 224. The tubes are preferably formed of polycarbonate or aluminum, and are cemented together such that each tube is mounted between the two adjacent tubes so that it contacts these two adjacent tubes at tangent points therebetween, and its lowest point sits at a lowest point possible in a gap between the two tubes. The cementing together of the various tubes 222 is done in such a way that the resultant structure 220 has top-to-bottom and side-to-side mechanical stability and strength. This mechanical strength is the reinforcement for the sleeve 200, which otherwise would have little top-to-bottom strength, and virtually no side-to-side strength.

Of course, any desired shape could be used for the tubes 222, including cylindrical and hexagonal shape. The term "tubes" is intended to cover all such shapes.

Flow conditioner 220 preconditions the air such that a degree"of turbulence of the air for any given airflow is repeatable. Therefore, no matter what kind of turbulence or airflow is upstream of the unit, the airflow in area 224, between flow conditioner 220 and sensor array 230 will be the repeatable. This yields a condition such that the flow conditions in area 224, being repeatable, can accurately gauge the flow at position 204.

Sensor grid array 230 is downstream of first flow conditioner 220, and is preferably a grid array such as those shown in the Patents 4,494,406; and 4,581,930, and includes a number of sensor elements shown as 232 and 234. These sensor elements are connected to wires on which is produced an indication of the air flowing through the device. Finally, second flow conditioner 240 is downstream of both first flow conditioner 220, and sensor array 230, and located at the exit end 206 of sleeve 200, between the first flow conditioner 220 and the mixed air plenum 104. This second flow conditioner 240 serves as an isolator, and isolates the sensor assembly 230 from the substantial turbulence which exists in the mixed air plenum. This is necessary and desirable for many reasons. Conventional airflow sensors could not be used in such a situation. Conventional airflow sensors need to be located several duct diameters away from any disturbance. This has meant that such airflow sensors could only be used 20-30 feet away from the dampers and supply and return fans. Since the outside air duct is relatively short or non-existent, this has left no place where such an airflow sensor could be located. The second airflow conditioner 240 is structurally similar to the first airflow conditioner 220, and is formed the same (as shown in Figure 4) . However, the function of the second airflow conditioner is to isolate the sensor array 230 from the mixed air plenum. It has been found that the sensors 232 and 234 are very sensitive, and that air flowing past the airflow sensor such, as shown by arrow 120 in Figure 1, can actually affect the airflow measurement. Moreover, air may actually, in some situations, flow from the mixed air plenum 104 to the outside in a direction opposite arrow 204, depending on the pressure in the plenum. The second conditioner acts as an isolator, and isolates the sensor array 230 from any of the conditions occurring in the plenum.

According to this preferred embodiment, the sleeve is 24 inches wide (dimension A shown in Figure 2) . The first airflow conditioner 220 is located 15-1/2 inches from the first end 203. The first and second flow conditioners are 3/4 inch wide.

The present invention is also intended to be used in a flow situation of between 50 feet per minute and 500 feet per minute. Since the device is used in the path itself, it has been found by the inventor of the present invention that the size of the cells 222 in the flow conditioner should be smaller than any used according to the prior art. Accordingly, the present invention preferably uses cells which are less than 1/8 of an inch in diameter, and the preferred size of these cells is 3/32 of an inch in diameter.

The preferred embodiment uses a first flow conditioner 220 which has 3/32 inch diameter cells, and is 3/4 of an inch in width. The second flow conditioner is identical.

The array of sensors 230 is preferably located five inches from the second flow conditioner 240 and two inches from the first flow conditioner 220. This has been found by the inventors to produce an optimum measuring situation.

In some situations, it may be also desirable to further isolate the sensors from the outside air. For instance, when there is a high pressure in the plenum, further flow conditioning may be necessary. An option area 250 is also provided in which options may be added to the sleeve. The first of these options is a third flow conditioner intended for the situation where there is a high negative pressure in the plenum. This third flow conditioner is intended to be located six inches from end 203, and to be three inches in width. The third flow conditioner preferably uses 1/8 inch cells.

Alternately, an air filter assembly can be used at location 250. This air filter may or may not be associated with a movable damper. A differential pressure switch will be installed across conditioner 220 to determine the direction of airflow.

In order to operate properly, the sensor grid 230 must be calibrated for the specific situation. Normally sensor grids are intended to be operated with a flow conditioner at the front end only. They are calibrated for this situation.

The present invention requires the sensor 230 to be calibrated in the environment with flow sensors 220 and 240. The flow conditions are greatly altered by the use of the 3/32 inch tubes of the flow conditioner assembly at both ends of the sensor grid. Therefore, the sensor grid is calibrated by using known amounts of airflow and measuring sensor outputs. For instance, airflow amounts from 50 feet per minute to 500 feet per minute flow rates are used. The device is put into a chamber having a known flow rate, and voltages from the sensors are measured. These voltages are used to form a calibration, which calibrates the voltage produced with the flow rate. This relation is not necessarily linear, since turbulences may in fact be formed in the area 224 and the area 236. However, these turbulences are equalized, such that they are repeatable in any situation for a given flow rate. Although only a few embodiments have been described in detail above, those of ordinary skill in the art will certainly understand that many modifications are possible in the preferred embodiments without departing materially from the teachings thereof. Accordingly, all such embodiments are intended to be encompassed within the following claims.

Claims

WHAT IS CLAIMED IS;

1. An airflow sensor assembly comprising: means for housing the airflow assembly; a first airflow conditioner, located within said housing means at an upstream position with respect to a direction of air flow, to face a flow of air, for conditioning an airflow passing therethrough to produce a conditioned air flow at a location downstream thereof; airflow sensing means, located at said location downstream of said first airflow conditioner with respect to a direction of airflow, for sensing an amount of airflow and producing a signal indicative thereof; and a second airflow conditioner, located downstream of said first airflow conditioner and said airflow sensing means with respect to a direction of airflow, for isolating a downstream side of said airflow sensing means from airflow conditions downstream of said airflow sensing means.

2. An assembly as in claim 1 wherein said second airflow conditioner is located at a downstream edge of said housing means.

3. An assembly as in claim 1 wherein said airflow sensing means is disposed in a heating ventilation and air conditioning system, and is disposed to receive outside air at its upstream end, a downstream end thereof being located in a mixed air plenum of said heating, ventilation and air conditioning system.

4. An assembly as in claim 3 wherein said housing means is a substantially rectangular sleeve of ductwork.

5. An assembly as in claim 4 wherein at least one of said first airflow conditioner and said second airflow conditioner are mounted in said sleeve in such a way as to provide structural integrity thereto.

6. An assembly as in claim 3 wherein at least one of said first and second airflow conditioners comprise an array of tubes, structurally coupled to one another to provide a system which provides structural integrity to said housing means.

7. An assembly as in claim 3 wherein said first and second airflow conditioner are each comprised of a plurality of connected tubes, extending in an upstream to downstream direction, and wherein a diameter of each of said tubes is less than or equal to 1/8 of an inch.

8. An assembly as in claim 7 wherein said tubes are cylindrical and a diameter of each of said tubes is less than or equal to 3/32 of an inch.

9. An assembly as in claim 8 wherein at least one of said first airflow conditioner and said second airflow conditioner are mounted in said housing means in such a way as to provide structural integrity thereto.

10. An assembly as in claim 3 wherein said sensing means is a calibrated sensor which is calibrated for operation in its location between said first and second airflow conditioners.

11. An airflow sensor assembly which is disposed in a heating, ventilation and air conditioning system, and is disposed to receive outside air at its upstream end, a downstream end thereof being located in a mixed air plenum of said heating, ventilation and air conditioning system comprising: means for housing the airflow assembly; a first airflow conditioner, located within said housing means at said upstream position with respect to a direction of air flow, to face a flow of air, for conditioning an airflow passing therethrough to produce a conditioned air flow at a location downstream thereof; airflow sensing means, located at said location downstream of said first airflow conditioner with respect to a direction of airflow, for sensing an amount of airflow and producing a signal indicative thereof; and a second airflow conditioner, located downstream of said first airflow conditioner and said airflow sensing means with respect to a direction of airflow, and adjacent said mixed air plenum, for isolating a downstream side of said airflow sensing means from airflow conditions downstream of said airflow sensing means.

12. An assembly as in claim 11 wherein said first and second airflow conditioners are formed of a plurality of tubes extending in an upstream to downstream direction, each of said tubes having a diameter which is 3/32 of an inch or less.

13. An airflow sensor assembly for use in an HVAC system for measuring a flow rate of air in the system, comprising: means for housing the assembly, said means comprising a section of ductwork; a first flow conditioner, located within said housing means, and formed of a plurality of connected tubes extending in an upstream to downstream direction, each tube being less than or equal to 1/8 of an inch in diameter; a sensor grid assembly, located downstream with respect to a direction of airflow of said first flow conditioner for sensing a flow of air thereacross; and a second flow conditioner, located downstream of said sensor grid assembly, and between said second grid assembly and a mixed air plenum of said HVAC system, said second flow conditioner assembly comprised of a plurality of tubes extending in an upstream to downstream direction each less than or equal to 1/8 inch in diameter.

14. An assembly as in claim 13 wherein each of said tubes of said first flow conditioner assembly and said second flow conditioner assembly are all less than 3/32 of an inch in diameter.

15. An assembly as in claim 14 where at least one of said first airflow conditioner and said second airflow conditioner is mounted in said housing means in such a way as to provide structural integrity thereto.

16. An assembly as in claim 14 wherein said sensing means is a calibrated sensor which is calibrated for operation in its location between said first and second airflow conditioners.

17. An airflow sensor assembly adapted for use in an HVAC system, between a source of air for said HVAC system, and a mixed air plenum of said HVAC system comprising: a sleeve for housing said assembly, said sleeve having a first end-adapted to face an upstream direction with respect to a flow of air, and a downstream direction, adapted to face downstream with respect to said flow of air, a downstream side of said sleeve adapted to be mounted in a mixed air plenum of said HVAC system; first flow conditioning means, located to receive said airflow at said upstream direction, for conditioning said airflow in a fixed way for any given airflow, to produce an airflow at a first location, downstream of said first airflow conditioning means, said downstream flow being repeatable for any given airflow although not necessarily linear; means for sensing an airflow at said first location; and second airflow conditioning means, located downstream of said first location, for isolating said sensing means from said mixed air plenum.

18. An assembly as in claim 17 wherein said sleeve is a substantially rectangular sleeve of ductwork.

19. An assembly as in claim 18 wherein at least one of said first airflow conditioner and said second airflow conditioner is mounted in said sleeve in such a way as to provide structural integrity thereto.

20. An assembly as in claim 17 wherein at least one of said first and second airflow conditioners comprise an array of tubes, structurally coupled to one another to provide a system which provides structural integrity to said sleeve.

21. An assembly as in claim 17 wherein said first and second airflow conditioners are each comprised of a plurality of connected tubes, extending in an upstream to downstream direction, and wherein a diameter of each of said tubes is less than or equal to 1/8 of an inch.

22. An assembly as in claim 21 wherein a diameter of each of said tubes is less than 3/32 of an inch.

23. An assembly as in claim 17 wherein said sensing means is a calibrated sensor which is calibrated in its location between said first and second airflow conditioners.

24. An airflow sensor assembly comprising: means for housing the airflow assembly; a first airflow conditioner, located within said housing means at an upstream position with respect to a direction of air flow, to face a flow of air, for conditioning an airflow passing therethrough to produce a conditioned air flow at a location downstream thereof; airflow sensing means, located at said location downstream of said first airflow conditioner with respect to a direction of airflow, for sensing an amount of airflow and producing a signal indicative thereof; and a second airflow conditioner, located downstream of said first airflow conditioner and said airflow sensing means with respect to a direction of airflow, for isolating a downstream side of said airflow sensing means from airflow conditions downstream of said airflow sensing means wherein said housing means is a substantially rectangular sleeve of ductwork; where at least one of said first airflow conditioner and said second airflow conditioner is mounted in said sleeve in such a way as to provide structural integrity thereto; wherein said first and second airflow conditioner are each comprised of a plurality of connected tubes, extending in an upstream to downstream direction, and a diameter of each of said tubes is less than 3/32 of an inch.

25. An assembly as in claim 24 wherein said sensing means is a calibrated sensor which is calibrated in its location between said first and second airflow conditioners.

26. An airflow sensor comprising: means for housing the airflow assembly; a first airflow conditioner, located within said housing to face a flow of air, for conditioning airflow passing therethrough; sensing means, located downstream of said first airflow conditioner with respect to a direction of airflow, for sensing an amount of airflow and producing a signal indicative thereof; and a second airflow conditioner, located downstream of said first airflow conditioner and said sensing means with respect to a direction of airflow, for isolating said downstream side of said airflow sensing means; wherein said housing means is a substantially rectangular sleeve of ductwork; where at least one of said first airflow conditioner and said second airflow conditioner is mounted in said sleeve in such a way as to provide structural integrity thereto.

27. An assembly as in claim 26 wherein said airflow sensor is disposed in a heating, ventilation and air conditioning system, and is disposed to receive outside air at its upstream end, a downstream end thereof being located in a mixed air plenum of said heating, ventilation and air conditioning system.

28. An assembly as in claim 27 wherein said first and second airflow conditioner are each comprised of a plurality of connected tubes, extending in an upstream to downstream direction, and wherein a diameter of each of said tubes is less than or equal to 1/8 of an inch.

29. An assembly as in claim 28 wherein a diameter of each of said tubes is less than 3/32 of an inch.

30. An assembly as in claim 29 wherein said sensing means is a calibrated sensor which is calibrated in its location between said first and second airflow conditioners.