Method and device for measuring airflows through HVAC grilles

[0001] This application claims priority from provisional application Ser. No. 60/358,641 filed Feb. 21, 2002.

[0002] The present invention relates generally to a method and device for measuring the airflow exiting the duct system of an HVAC system with a fan powered by a controlled external energy source and a flow measurement device.

[0003] Airflow through an HVAC system flows into the system through return grilles or outdoor air intakes, is pushed or pulled by the fan, and then is forced through a duct system, exiting through a supply register or grille into a room to provide cooling, heating or ventilation. It is often desirable to measure the airflow out of or into the duct system to determine if the desired flow is being provided to or removed from each room or zone.

[0004] Airflows entering or exiting grilles are commonly measured using either a pressure-measurement grid that attempts to determine average velocity or a flow measurement device including an impeller in which the speed of the impeller is an indication of the speed of the airflow.

[0005] There are several drawbacks to both of these techniques. Pressure measurement grids suffer from inaccuracies that are caused by spatially non-uniform velocities. Empirical register dependent correction factors have been employed to account for non-uniform velocity biases with pressure measurement grids, however, the use of these correction factors is cumbersome and unreliable in the field applications. Flow straightening elements for pressure-measurement grids have also been employed to eliminate the need for correction factors and improve accuracy, but create excessive flow resistance.

[0006] The process of measuring the airflow with the impeller-based flow measurement device also creates flow resistance, impeding the airflow being measured. With conventional flow measurement devices, the impeller is spun by energy in the airflow that is being measured, which means that the impellear changes the flow being measured. Another problem sometimes associated with the prior art impeller-based flow measurement devices is that the measurement of the airflow is biased if the airflow exiting the duct system has a swirled profile. Flows have also been measured in research settings using a flow capture hood and a remote calibrated fan.

[0007] Hence, it would be beneficial to employ a flow measurement device that does not impede the airflow being measured and that accommodates for non-uniform or swirled airflow profiles.

[0008] A fan powered by an external energy source arid a flow measurement device measure the airflow exiting a supply register or grille of an HVAC system without impeding the airflow being measured. As the fan is powered by an external energy source rather than being powered by energy in the airflow, there is less flow resistance and impedance to the airflow. Preferably, the fan is insensitive to pressure differentials. or a pressure-sensitive fan is submitted to appropriate pressure differentials.

[0009] Preferably, a tachometer is employed to measure the RPM of the fan. As airflow is a function of the RPM of the fan, the RPM of the fan is an indication of the airflow through the supply register or grille. For fans whose RPM/flow calibrations are sensitive to pressure differentials, the flow-straightening element is designed to keep the pressure differential across the fan within pressure differential calibration limits over the full range of flows measured, so that flow measurement is not impacted by a pressure drop or rise across the fan. This is especially important at low RPMs. The pressure drop across the fan should be very low, such as between 0-3 Pa.

[0010] Alternatively, a voltage or current meter is employed to measure the voltage or current supplied to the external energy source that drives the fan, and thereby determine the airflow. As the voltage or current increases, the airflow through the supply register or grille increases.

[0011] Finally, a pressure sensor can employed to measure the pressure differential across the flow-straightening element and therefore determine the airflow. Pressure sensors are positioned in the hood and between the straightening element and the fan, respectively, to determine the pressure differential and therefore the airflow.

[0012] Independent of the chosen technique for sensing the flowrate, a control circuit can be employed to automatically adjust the voltage or current supplied to the external energy source to maintain the desired zero pressure differential between the pressure in the hood and pressure in the room.

[0013] These and other features of the present invention will be best understood from the following specification and drawings.

[0021] FIG. 1 schematically illustrates the flow measurement system 20 of the present invention. Heated or cooled air from a furnace or air conditioner 22 flows through the duct system 24 and exits through a supply register or grille 26 into a room for heating or cooling. A fan 32 powered by an external energy source 30 and a flow measurement device 28a, 28b or 28c measures the airflow exiting the duct system 24. As the fan 32 is powered by an external energy source 30 rather than being powered by energy in the airflow, there is less flow resistance and impedance to the airflow being measured. Preferably, the RPM/flow calibration of the fan 32 is insensitive to pressure differentials or the fan is a pressure-sensitive fan, such as a muffin fan, submitted to appropriate pressure differentials.

[0022] The fan 32 is positioned in front of the supply register or grille 26 and under a capture hood 36. Because the fan 32 is powered by an external energy source 30, a flow-straightening element 34 is preferably employed to overcome any effects of a non-uniform or swirled airflow profile on the fan 32. Preferably, the RPM/flow calibration of the fan 32 is insensitive to swirl and the flow straightening element 34 is not required. If a flow straightening element 34 is employed, the airflow first passes through the flow-straightening element 34 and is straightened prior to approaching the fan 32. The flow-straightening element 34 is a honeycomb, a plurality of straws, or a perforated plate. FIG. 2 illustrates the flow-straightening element 34 including a plurality of straws.

[0023] Several characteristics of the flow measurement system 20 can be used to measure the flow entering or exiting the supply register or grille 26, including flow measurement devices 28a, 28b or 28c. Preferably, a tachometer 28a is employed to measure the RPM of the fan 32. As shown in FIG. 3, there is a linear relationship between the RPM of the fan 32 and the airflow as long as the pressure drop across the fan 32 is kept below 0-3 Pa. Preferably, the pressure drop across the fan 32 is maintained to prevent a change in the RPM/flow calibration of the fan by more than 5%. The RPM of the fan 32 is used to determine the airflow through the supply register or grille 26 as the airflow is a function of the RPM of the fan 32.

[0024] When using the preferred RPM method for measuring airflow for fans 32 whose RPM/flow calibrations are sensitive to pressure differentials, such as muffin fans, the flow-straightening element 34 must be designed to keep the pressure differential across the fan 32 within pressure differential calibration limits over the full range of flows measured, so that the flow measurement is not impacted by a pressure drop or rise across the fan 32. This is especially important at low RPMs.

[0025] The pressure drop across the pressure-sensitive fan 32 should be very low, such as between 0-3 Pa. Higher pressure drops across the fan 32 would influence the RPM-flow calibration of the fan 32, and therefore complicate the measurement of the airflow, requiring both RPM and pressure sensing for the calibration. For some pressure sensitive fans 32, the RPM-flow calibration only works when the pressure drop or rise across the fan 32 is kept below 1 Pa at lower flows and below 2-3 Pa at higher flows. The flow range over which a pressure sensitive fans 32 are extra sensitive to the pressure differential across the fans is determined by the open area of the fans 32. The impact of pressure differential can also be mitigated through fan 32 design. Fans 32 that are insensitive to pressure differentials are not impacted by the pressure drop or rise across the fan 32.

[0026] An adequately small pressure differential across the fan 32 can also be maintained by choosing an appropriate diameter for the fan 32 relative to the flow resistance of the flow straightening element 34. A worker skilled in the art would determine the appropriate diameter for the fan 32 for use in the system 20 to control the pressure differential by reducing the diameter of the fan 32 until the RPM of the fan 32 is high enough that the calibration is not impacted over the full range of flow straightening element 34-pressure drop combinations.

[0027] FIG. 4 illustrates the relationship between fan 32 sensitivity to pressure differential at different air flow rates and the importance of keeping the pressure differential across pressure-sensitive fans 32 within small pressure differential calibration limits. As shown, at low flow rates, such as 53 cubic feet per minute, the calibration of the fan 32 decreases as the pressure change increases. At high flow rates, such as 209 cubic feet per minute, the calibration of the fan 32 stays relatively constant as the pressure change increases. At all flow rates, the calibration of the fan 32 is constant at about 14 RPM/cfm when the pressure change is zero. Therefore, it is important to maintain the pressure differential across the fan 32 close to zero to maintain a constant RPM calibration of the fan 32.

[0028] Alternatively, as shown in FIG. 5, a volt or current meter 28b is employed to measure the voltage or current supplied to the external energy source 30 which drives the tan 32 and used to determine the airflow. As the voltage or current increases, the airflow through the supply register or grille 26 increases. The plot of FIG. 5 was generated employing a 10 inch diameter supply register or grille 26 and employing a flow-straightening element 34 of straws and a 8 inch diameter supply register 26 employing a flow-straightening element 34 having only half the number of straws.

[0029] Finally, a pressure sensor 28c can be employed to measure the pressure differential across the flow-straightening element 34 (pressure difference between the capture hood 36 and tho region between the fan 32 and the flow straightening element 34) and therefore determine the airflow. Pressure sensors 38a and 38b positioned before and after the flow-straightening element 34 determine the pressure differential across the flow-straightening element 34 and therefore the airflow. The relationship between the pressure differential and the air flow is determined by the type of flow-straightening element 34 employed. In one example, the flow-straightening element 34 is a plurality of straws, and the air flow has a linear relationship with the pressure differential. If the flow-straightening element 34 is a perforated plate, the air flow is related to the square root of the pressure differential. However, it is to be understood that other types of flow straightening elements 34 can be employed to determine the pressure differential and therefore the air flow.

[0030] FIG. 6 illustrates a graph relating airflow to DC voltage (at constant pressure) for two different types of registers. As shown, as the DC voltage increases, the airflow increases. In both cases, the DC voltage to flow ratio is the same.

[0031] By employing a fan 32 with an external energy source 30 to measure the airflow, the fan 32 does not impede or add resistance to the airflow being measured. Additionally, by employing a flow-straightening element 34 to straighten the airflow, the airflow measurement is not biased by a non-uniform or swirled flow profile. To assure that the fan 32 does not influence the airflow being measured, it is also possible to automatically control the air. The system 20 measures airflow without significant bias and with a high degree of reproducibility without requiring exotic sensors, electronics or other components. The same equipment can be used to measure the airflow and control the pressure. By controlling the energy source so as to make the pressure seen by the grille or register 26 the same as it would be without the fan 32 in place, the fan 32 does not impede nor augment the airflow to or from the grille or register 26.

[0032] A control circuit 40 (shown in FIG. 1) can be employed to automatically adjust the voltage or current supplied to the external energy source 30 to maintain the desired zero pressure differential between the capture hood 36 and the room. Alternatively, the pressure differential can be zeroed manually.

[0033] The fan 32, a pressure transducer, and control circuitry can be run on AC or DC power. The preferred example uses direct current or batteries.

[0034] The foregoing description is only exemplary of the principles of the invention. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed, however, so that one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention.