Verification of the Accuracy of Air Flow Measurement Using the Multi-Nozzle Chamber Method (ST-16-C031)

The multi-nozzle chamber method for air mass flow measurement has been in use in the HVAC&R industry for decades.  The primary flow element is the elliptical nozzle defined by American Society of Mechanical Engineers (ASME) standards.  The ASME nozzle is a passive, ridged construction element that does not require periodic calibration.

As the HVAC&R industry is subject to greater performance efficiency requirements, measurement accuracy for airflow becomes a critical issue.  The accuracy of many instruments for the measurement of temperature, pressure, humidity, and power has improved over the past couple decades.  New test standards now require the evaluation of the uncertainty of measurements and derived values.  These developments have raised questions about what can be realistically expected for the accuracy of the multi-nozzle chamber air flow meter (AFM), especially due to the lack of open literature test data with multi-nozzle configurations.

To determine the accuracy (or uncertainty) to be expected from typical multi-nozzle chambers, a four-nozzle AFM was constructed in strict accordance with current standards and tested at an independent, multi-industry, gas flow test laboratory.  The test laboratory used their primary National Institute of Standards and Technology (NIST) traceable critical flow Venturi test method with an average uncertainty of +0.3% of the flow.  Six nozzle flow configurations consisting of each of the four nozzles separately, a particular combination of three nozzles and all four nozzles simultaneously, were each subjected to three nozzle throat velocities for a total of 18 different tests.  The velocities included the lowest and highest defined by industry standards and one intermediate velocity.

The test laboratory utilized their NIST traceable, independent mass flow measurement in series with the test AFM and included a measurement of three required parameters: nozzle differential pressure, inlet temperature, and barometric pressure.  Dry air was used to eliminate errors associated with the calculation of moist air properties.  Confirming air mass flow rates were calculated using the nozzle diameters, nozzle flow coefficients, and the measured parameters.  The results of all 18 flow rate tests were compared and shown to be within +0.2 to +0.4%.  This project demonstrates that a typical multi-nozzle AFM, when constructed in accordance with industry standards, can be used for air flow measurements that are accurate to better than +0.4% of reading over the entire flow range.