ASHRAE Journal - September 2019 - 66

COLUMN ENGINEER'S NOTEBOOK

fluids. Ultrasonic flow meters can also be used on large
diameter piping (up to NPS 24 or DN 600), whereas
a full-body in-line type meter may be prohibitively
expensive in those large sizes.
Since no tapping or cutting of the pipe is required, the
cost of labor to install is lower. Materials of construction
do not need to be compatible with the fluid being measured. Performance is published at ±1% accuracy for liquids and ±2% for gases over a very wide range of velocities.
A disadvantage of the ultrasonic meter it that it must
be individually configured and programmed for each
specific application. The customer must specify data,
including pipe material and pipe wall thickness, and
the exact fluid being metered. Once it is configured for
a given application (for example, natural gas in a Type L
copper pipe), it cannot be easily installed for a different
service (e.g., steam in a Schedule 40 steel pipe) without
somewhat complex field reprogramming. The cost of the
electronics and clamp-on transducers is comparatively
high when used on small-diameter piping, versus the
insertion turbine type, but the relative cost differential
is more favorable on larger piping.
Straight pipe length requirement is typically 10 straight
pipe diameters upstream of the device and another five
straight pipe diameters downstream. This may defeat the
previously mentioned advantage of installation in a retrofit if suitable straight lengths of piping cannot be found in
the run where flow measurement is desired.
I typically apply the permanent version of this meter
on natural gas piping for non-custody transfer convenience metering, not for utility company billing, but
for purposes internal to a building, such as metering
natural gas to each laboratory in a university research
building. It is also the meter I choose for gravity steam
condensate return lines where flow may be less than
full pipe volume. And, it is sometimes the only practical
option in existing systems operating 24/7/365 when it is
not allowable for system shutdown to install in-line or
insertion style flow meters.

Thermal Energy Meters
Finally, I want to mention thermal energy meters
(often known as "Btu meters" in the United States).
Thermal energy meters are found in some multi-tenant
buildings that have a central heating and/or central
chilled water plant, or at each building in a campus
environment with a central heating/chilled water plant.
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FIGURE 3 Clamp-on ultrasonic flow meter.

Electronics Enclosure

Clamp-On Transducers

Piping, Any Size
Mounting Hardware

It gives the building or campus owner a way to track
individual building use or even to bill individual tenants
for their chilled water and heating water usage based on
energy used.
This meter combines any of the flow meter technologies discussed herein with a pair of temperature sensors in the supply and return piping. In my experience,
the insertion turbine flow meter is the most common
choice for routine hydronic system thermal energy
metering. By measuring flow rate of either chilled
water or hydronic heating water, in combination with
temperature sensors measuring the rise (or drop) in
temperature entering and leaving a building, a Btu
(kW) measurement is an easy calculation. Advantages
and disadvantages match those of the flow meter itself,
plus the additional inaccuracy of temperature measurement-now flow rate inaccuracy and temperature measurement inaccuracy could compound each other. To
minimize that inaccuracy, the two temperature sensors
are usually bath-calibrated and matched for the specific
temperature range for each application. The calculated
differential temperature used in the energy calculation
is typically accurate to within 0.15°F (0.08°C), including
the error from individual temperature sensors, sensor
matching, input offsets, and calculations.

Conclusion
This column offers an overview of several available
technologies for selecting and specifying pipe flow
metering in building HVAC systems, in an attempt to
add to and expand upon the information provided in
the ASHRAE Handbook-Fundamentals.1 While there is not
one correct answer for all flow measurement applications, I most often choose the following:

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ASHRAE Journal - September 2019

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