June 25, 2018
Airflow measurements can be challenging and costly to obtain with physical meters installed in air handling units (AHUs). In addition, variable air volume (VAV) systems present a number of additional complexities for airflow measurements if compared to simpler constant air volume (CAV) systems.
The varying flow in VAV systems allows airflow velocities to drop at the transition Reynolds number regime during the partial load operation. This causes the velocity profile across traverse locations for airflow measurements to be non-uniform and out of compliance with ASHRAE Standard 111-2008 provisions for accurate airflow measurement.
Typical pitot-tube arrangements are proven to be inaccurate at low flow velocities. Additionally, a large number of sampling points are required if Log-T or Equal Area methods for traverse airflow measurement are followed. Moreover, industry common practice is to record each sampling point for 2 to 3 seconds to obtain a weighted average. All these factors contribute to the addition of uncertainties that will make it difficult to achieve the ASHRAE Std. 111-2008 requirement of +/-5% total uncertainty.
VIRTUAL AIRFLOW SENSORS
Research projects have been developed to show how the indirect measurement of airflow is possible. The intent of virtual airflow sensor technology is to rely on measured variables like fan power, head, and speed to obtain accurate airflow measurements. This method intends to take advantage of existing building automated systems (BAS) to calculate airflows based on already existing data outputs. Then, with programmed equations, airflow measurements can be obtained in real-time, bringing with them many benefits like real-time fault detection, energy-efficient operation, constant pressurization monitoring, etc.
Nonetheless, the equation to be programmed into the BAS needs some calibration coefficients due to the uniqueness of each system operation. In consequence, a calibration procedure needs to be developed to physically measure airflow for the entire operational range of a VAV system. This can be a challenging task as current testing, adjusting, and balancing (TAB) techniques are aimed at CAV systems. Taking a large number of points for the complete operating range of a VAV system can be impractical.
1. Increasing sampling time to reduce oscillation impacts on velocity measurements
To ensure that the measuring device is well fixed in its position and that measurements can be recorded for an extended period of time are a necessity. In order to perform these actions, a holding bracket for velocity probes is invented and an airflow measurement device kit with data recording capability is assembled.
Extended periods of measurement tests showed that 60 seconds is the ideal time sampling period for airflow velocity measurements. With a 60 seconds interval, the 99.7% confidence interval of the weighted average for airflow velocity readings showed to narrow down 7.6 times when compared to 1-second measurements.
2. Reducing sampling locations yet maintaining acceptable accuracy of airflow measurement
Having ways to ensure fixed sampling points during 60 seconds and to override the VAV system operation for fixed fan speed allowed us to perform full Equal Area traverse airflow measurements over a wide operational range for the VAV AHU. However, it is not feasible to record multiple points along a single line with a single-point hot-wire probe anemometer. In consequence, ways of reducing the number of sampling points below the minimum of 25 per ASHRAE Std. 111-2008 are tested. As results showed, having a complete set of data with representative points over a wide range of operational airflow velocities allowed us to select 3-points from the Equal Area defined set of points. These 3-points can be picked in such a way that their average accurately represents the mean airflow velocity across the traverse section for the full operational range. The +/-5% criteria of ASHRAE Std. 111-2008 was used as a guide for validation of the airflow measurement.
3. Synchronizing airflow measurements for both return air fan and supply air fan
The presence of both supply and return fans in AHUs increases the complexity of the airflow measurement calibration due to the varying pressures within the equipment chambers. However, a calibration procedure was developed, varying speeds for supply and return fans to survey the full range of pressures achievable in the AHU. Moreover, since a gap was observed between supply and return airflows, due to the air leakage on the positive pressure side (supply air ductwork), a correction factor for the supply air measurement was obtained by linear regression of sample points for zero pressure in the mixed air chamber of the AHU (i.e. the point where supply and return airflows for each fan are equal).
In-situ fan performance curves were obtained and compared to the manufacturer’s curves, remarking the need for this calibration to be performed. The resulting head vs airflow fan curves showed reduced performance for both fans. The large discrepancy between curves supports the necessity of the calibration procedure to obtain fan curve coefficients to be used for virtual airflow sensors implementation.
ABOUT THIS WORK
This work was presented as a Technical Paper1 for the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) in Las Vegas, Nevada at the Winter Conference in January of 2017. It is included in ASHRAE Transactions 2017 and it achieved the recognition as 2017 ASHRAE Technical Paper Award.
- Rivas Prieto, A., Thomas, W., Song, L., Wang, G. 2017. In-Situ Fan Curve Calibration for Virtual Airflow Sensor Implementation in VAV Systems. ASHRAE Transactions - ASHRAE Winter Conference (Vol. 123, pp. 215-229). ASHRAE Inc.
About the Author
Alejandro Rivas, EI, LEEP AP BD+C. Alejandro holds a Master’s Degree in Mechanical Engineering from the University of Oklahoma with a focus on HVAC systems and has over 7 years of experience as a mechanical designer. His portfolio includes healthcare, higher education, commercial, and housing projects, in which he enjoys not only providing solutions but to design building systems with a reliable approach in mind and aligning with architecture’s aesthetic vision.