ASHRAE Journal - September 2019 - 64

COLUMN ENGINEER'S NOTEBOOK

lower cost and respectable accuracy and reliability, making them probably the most common choice for HVAC
hydronics purposes. Accuracy is typically about ±2% of
reading from 0.4 to 30 fps (0.1 to 9 m/s) fluid velocity. The
cost of these devices is significantly lower than electromagnetic flow meters, especially in larger pipe sizes, as
the cost of an insertion turbine device does not increase
nearly as rapidly (it grows in physical size in only one
dimension) compared to the in-line pipe device, which
grows in three dimensions as pipe size increases.
In addition to somewhat less accuracy, another disadvantage of this type of flow meter is a longer straight run
requirement. You will need 10 diameters of upstream
straight pipe and five diameters downstream to achieve
the quoted accuracy. There is some pressure loss in the
conveyed fluid because of the protruding turbine and
shaft, but it is typically less than 1 psig [7 kPa]. Turbine
flow meters provide unidirectional flow measurement.
Since these types of meters have moving parts, they cannot be expected to last as long between recalibrations
and/or maintenance attention as those devices with no
moving parts.
Typical uses for this type of flow meter include chilled
water, hydronic hot water, condenser water, domestic water, process cooling, and brine. This is the type I
typically recommend and specify most often in HVAC
hydronic applications where extreme accuracy is usually not necessary, because I find that it offers a good
combination of low initial cost, an acceptable degree of
accuracy, and reasonable longevity. If you specify hightemperature construction materials, a variation on the
turbine flow meter can also be used in steam systems,
pressurized high-temperature hot water systems, and
pumped steam condensate lines that always run full. It
is not suitable for measuring flow in pipes that operate
less than completely full, such as with gravity condensate return systems, because at lower flow rates, the
fluid could miss the turbines completely.

Vortex Shedding Flow Meters
The operating principle for the vortex shedding flow
meter (in-line, similar to Figure 1) involves immersing a
blunt-shaped object in a stream of fluid flow. The fluid
separates and generates small low-pressure vortices that
alternate from side to side and are shed along the length
of, and downstream from, the blunt object. Sensors
located downstream from the blunt object can detect
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the lateral pressure changes, first on one side and then
on the other. The frequency at which another vortex is
shed is proportional to the velocity of the flowing liquid.
A microprocessor can be used to display totalizer flow,
flow rate, temperature, pressure, time, and date; microprocessors can also be used as alarms for high and low
flow rate and temperature.
This type of flow meter includes stainless steel wetted
parts and flange pipe connections for permanent in-line
service. Performance is published at 10:1 turndown with
±1.5% accuracy over full-flow range, including all errors
associated with velocity measurement, temperature and/
or pressure measurement, and density compensation.
One important advantage of the vortex shedding flow
meter is that it can be used for steam and gases in addition to typical HVAC hydronic applications. If specifiable
options are included, it can withstand temperatures as
high as 500°F [260°C] and pressures as high as 1,500 psi
[10 MPa], making it a good choice for process applications.
A key disadvantage is a very long straight pipe length
requirement, ranging from 10 to 50 straight pipe diameters upstream of the device and another five straight pipe
diameters downstream. The inlet-side straight length can
be shortened by installing an insertion flow straightener,
but at added cost and pressure drop. This is the type of
flow meter I typically recommend and specify most often
in HVAC steam applications.

Clamp-On Ultrasonic Flow Meters
The operating principle for the ultrasonic flow meter
involves the transit-time technique. The flow meter uses
a pair of transducers, with each transducer sending and
receiving ultrasonic signals through the fluid (Figure 3).
When the fluid is flowing, signal transit time in the
downstream direction is shorter than in the upstream
direction; the difference between these transit times is
proportional to the flow velocity.
One key advantage of this type of meter is that it can
be used on existing piping that is in service without cutting into the pipe itself, making it the meter of choice in
many retrofit applications, or in troubleshooting during
commissioning or test-and-balance. Since the meter
clamps onto the outside of the pipe, it causes no pressure drop in the fluid being measured, other than the
pressure drop in the piping itself. Because the meter is
never in contact with the fluid being measured, it is useful for metering corrosive, toxic, high-purity or sterile


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ASHRAE Journal - September 2019

Table of Contents for the Digital Edition of ASHRAE Journal - September 2019

Contents
ASHRAE Journal - September 2019 - Intro
ASHRAE Journal - September 2019 - Cover1
ASHRAE Journal - September 2019 - Cover2
ASHRAE Journal - September 2019 - 1
ASHRAE Journal - September 2019 - Contents
ASHRAE Journal - September 2019 - 3
ASHRAE Journal - September 2019 - 4
ASHRAE Journal - September 2019 - 5
ASHRAE Journal - September 2019 - 6
ASHRAE Journal - September 2019 - 7
ASHRAE Journal - September 2019 - 8
ASHRAE Journal - September 2019 - 9
ASHRAE Journal - September 2019 - 10
ASHRAE Journal - September 2019 - 11
ASHRAE Journal - September 2019 - 12
ASHRAE Journal - September 2019 - 13
ASHRAE Journal - September 2019 - 14
ASHRAE Journal - September 2019 - 15
ASHRAE Journal - September 2019 - 16
ASHRAE Journal - September 2019 - 17
ASHRAE Journal - September 2019 - 18
ASHRAE Journal - September 2019 - 19
ASHRAE Journal - September 2019 - 20
ASHRAE Journal - September 2019 - 21
ASHRAE Journal - September 2019 - 22
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ASHRAE Journal - September 2019 - 27
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ASHRAE Journal - September 2019 - 31
ASHRAE Journal - September 2019 - 32
ASHRAE Journal - September 2019 - 33
ASHRAE Journal - September 2019 - 34
ASHRAE Journal - September 2019 - 35
ASHRAE Journal - September 2019 - 36
ASHRAE Journal - September 2019 - 37
ASHRAE Journal - September 2019 - 38
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ASHRAE Journal - September 2019 - Cover3
ASHRAE Journal - September 2019 - Cover4
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