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The large number of perforations in the collector tubing of Pitot arrays often is associated with the number of velocity measurements necessary to satisfy ISO 3966:2008; ANSI/ASHRAE Standard 111-2008, Measurement, Testing, Adjusting, and Balancing of Building HVAC Systems; AMCA Publication 203-90, Field Performance Measurement of Fan Systems; and any TAB requirements. The comparison, however, is physically and mathematically invalid.

Theoretically, a Pitot array averages a velocity profile; in practice, pressure equalization occurs along the length of a common collector tube, before airflow rate is determined by a single sensing element (the pressure transducer). The output of a Pitot array can represent an average reading only for a duct with equal pressure distribution throughout. Because Pitot arrays are sensitive to placement, significant lengths of ductwork are required between disturbances to develop the necessary pressure profile.

Differences between single- and independent-sensor technologies have been confirmed through laboratory testing (Figure 2).

A normal single-point velocity-pressure-to-velocity calculation assumes no difference if velocity pressure is averaged before velocity is determined at multiple points. Mathematically, a significant error (8 to 18 percent of reading) is introduced to the result, compared with independent velocity measurements (Figure 3).

In a laboratory, with professionally selected research equipment and test setup (as in ANSI/AMCA Standard 610-06, Laboratory Methods of Testing Airflow Measurement Stations for Performance Rating), Pitot arrays can produce a measurement uncertainty of +/-2 percent from a reference (the laboratory). In the field, against field references, using unknown-quality P/E conversion equipment and unknown linearization methods, Pitot arrays cannot provide that level of combined total uncertainty with consistency.

Velocity-pressure devices physically sample air through many tiny ports engineered to specific dimensions. Regular maintenance must be performed to prevent clogging of the ports and ensure proper performance. Most Pitot-array suppliers offer the option of a pressurized purge system intended to reduce the manual labor associated with regular cleaning.

In some applications, repeatability, linearity, and turndown are more important than absolute accuracy. In the case of volumetric fan tracking, for example, repeatability is the only measurement attribute of importance. Fan-inlet conditions are unpredictable and extreme to the point of being undesirable. While fan-inlet airflow-measurement placement eliminates some design issues for the engineer, troublesome installation issues (e.g., access to the reverse side of dual-inlet fans) arise, as do fan-sound and fan-performance issues. These issues can be overcome by mounting to the face of the inlet, rather than the inlet throat, which reduces the potential impact on plenum-fan performance to less than 1 percent of rated flow.

The difficulty in determining a true baseline volumetric calculation and the resulting impact on control accuracy is compounded by the difficulty in determining the area of the plane in an inlet cone where a measurement device is to be installed. This is true of any airflow-measurement technology applied at fan inlets. However, when an instrument produces measurements that are repeatable, it can be set up in the field to produce reliable results. This is especially important when accessibility for maintenance or replacement is limited.

CONCLUSION

The information provided in this article is intended to enable better decisions regarding instrument selection, application, and placement. It is hoped this additional knowledge will increase the probability of your next project operating efficiently and reliably.

REFERENCE

1. Dougan, D.S. (2003). Airflow measurement for HVAC systems - Technology comparison. Retrieved from http://www.airflowmeasurement.com/Web_Pdfs/AirflowMeasurement_Comparison.pdf

For past HPAC Engineering feature articles, visit www.hpac.com.

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The vice president of sales and marketing for Ebtron Inc., Leonard A. Damiano is active in the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), serving as a voting member of three technical committees (Instruments and Measurements, Control Theory and Application, and Moisture Management in Buildings), two special-project committees (Standard Methods for Laboratory Airflow Measurement and Method of Test for Rating Air Terminal Unit Controls), and one research-project committee (Stability and Accuracy of VAV Box Control at Low Flows). He has contributed to ASHRAE's Fundamentals and HVAC Applications handbooks and written or co-written numerous technical articles. He also is a member of Air Movement and Control Association International and the U.S. Green Building Council.

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