2016 ASHRAE Annual Conference

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.

This study develops a method to estimate the energy performance of blowers that are driven by electronically commutated motors (ECM) in residential gas furnaces based on the measurement of blower rotational speeds. As the first step, the airflow and power of six different ECM blowers from four manufacturers were measured over a range of external static pressures (ESPs) from 0.1 to 1.2 in. w.g. (25 to 300 Pa) in a well-instrumented laboratory environment with a calibrated nozzle airflow chamber. Then, the ECM blower energy performance was determined from the airflow and power measurements and characterized in terms of efficacy, which is the ratio of blower power to airflow rate. In addition, the relationship between parameters of blower rotational speed and efficacy was investigated, leading to the linear correlation development for each tested blower by taking the blower rotational speed as the independent variable and the efficacy as the dependent variable.

Results from the linear correlation development show that ECM blower efficacies can be accurately predicted by using blower rotational speeds as evidenced by the high R² values ranging from 0.961 to 0.981. For the six tested ECM blowers, the linear factor for the developed correlations varies from -2.881 to -2.657, and the offset factor is in a range of 3.287 to 3.551. Furthermore, a comparison between the predicted and measured efficacies shows an accuracy of ±15% for the developed correlations.

Results generated from this study provide a method to predict the energy performance in terms of efficacies for ECM blowers based on the knowledge of rotational speed. In addition, the experimental data and correlations produced in this study can be used to model the ECM blower efficacy behaviors at different operating speeds.

Methods like the Log-Tchebycheff and Equal Area are commonly used to define the average air velocity across a traverse section. The testing, adjusting and balancing (TAB) of HVAC systems has been adopting those methods to ensure that the installed system is meeting its design capacity. The flow measurements are compromised when space constraints limit the optimal air handling system design; consequently, inadequate straight ductwork with close upstream and downstream fitting disturbances cause common measurement issues. For example, according to ASHRAE 111-1988 field airflow measurements over CAV systems made by experienced technicians commonly have an error as much as 30% when recommended vane anemometers are used while reading irregular flows. Through in-situ airflow measurements in ten air-handling-units, this paper summarizes the statistical studies of measurement uncertainties to explain why the large errors occur in field measurements even though the standard procedures are strictly followed. As a result, a more accurate and robust in-situ airflow local measurement method is introduced in this paper. The proposed method uses hot-wire as the air velocity measurement device due to its proven accuracy at speeds lower than 800 fpm (1 m/s). In order to overcome additional turbulence in the ductwork that is caused by limited duct space, a holding device was also developed for facilitating the time weighted of local airflow measurements.