Quiz Corner: HVAC system flowmeters

Q: Which of the following flowmeters could reasonably be applied to measure air flow in a heating, ventilation and air conditioning (HVAC) system?

A. Pitot tube
B. Orifice plate
C. Thermal
D. Magnetic
E. Flume

Magnetic flowmeters (Answer D) and flumes (Answer E) only measure liquid flow so they are not correct. Pitot tube flowmeters (Answer A), orifice plate flowmeters (Answer B) and thermal flowmeters (Answer C) could be applied in this service.

That said, the question is not clear because not all HVAC systems are created equal. Pitot tubes (Answer A) and thermal flowmeters (Answer C) typically exhibit negligible pressure drops and are often applied to air pipes and ducts operating at low pressure.  In many such applications, insufficient pressure may be available to operate orifice plates (Answer B) that exhibit higher pressure drops. Further, even if sufficient pressure were available, the higher pressure drop across the flowmeter can increase energy consumption that can be saved when the air fan incorporates a variable speed drive.  That said, orifice plate flowmeters are available for these applications (typically in round pipes).

Additional complicating factors

The HVAC industry is notoriously known for being sensitive to cost. As such, the initial cost of the flowmeters may override the cited technical evaluation.

David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes LLC, which offers engineering, seminars, strategic, marketing consulting, distribution consulting and expert witness services for manufacturing and automation companies. Spitzer and Boyes is also the publisher of the Industrial Automation INSIDER. He has more than 40 years of experience and has written more than 10 books and 350 articles about flow measurement, instrumentation and process control.

Spitzer can be reached at 845-623-1830 or via spitzerandboyes.com. Click on the "Products" tab to find his Consumer Guides to various flow and level measurement technologies.