ASHRAE Journal - September 2019 - 63

COLUMN ENGINEER'S NOTEBOOK

FIGURE 1 In-line electromagnetic ﬂow meter.

FIGURE 2 Insertion turbine ﬂow meter.

Electronics Enclosure
Electronics Enclosure
Insertion Rod (Inside Assembly)
Ball Valve
Piping
Branch Outlet Fitting
Turbines
Piping

In-Line Flow Meter

meter comes from Faraday's Law. We know that a voltage will be induced in a conductive ﬂuid when it passes
through a magnetic ﬁeld. Furthermore, that voltage
will be directly proportional to the velocity of the conductive ﬂuid. So, this ﬂow meter works by generating a
magnetic ﬁeld (meaning it needs an external source of
electric power) then measuring the induced voltage via
electrodes in the ﬂow tube. Fluids to be measured must
have an electrical conductivity of at least 5 µS/cm, which
covers most HVAC ﬂuids. It would not work with, for
example, deionized water.
In-line electromagnetic ﬂow meters tend to be both very
expensive (especially in larger pipe diameters) and very
accurate-usually much more expensive and more accurate than we typically need in the HVAC world. Accuracy
for the brands I've reviewed are about ±0.2% of reading
from 1.6 to 33 fps (0.5 to 10 m/s) ﬂuid velocity, and repeatability is ±0.5% of reading. They can even be used at low
ﬂow rates, below 1 fps (0.3 m/s), with reasonable accuracy.
In addition to excellent accuracy, another advantage is
that you only need three diameters of upstream straight
pipe and two diameters downstream in order to achieve
that accuracy. Pressure loss in the conveyed ﬂuid is usually negligible because there are no restrictive devices,
and no parts of the ﬂow meter protrude into the ﬂow
area. And since these types of meters have no moving
parts, they offer long-term reliability with little maintenance and minimal recalibration. Potential applications
include chilled water, hydronic hot water, condenser
water, water/glycol/brine solutions, and bidirectional
ﬂow detection for primary/secondary decouplers, and
domestic water. They are not appropriate for steam
systems, as they are rated for ﬂuids up to 212°F [100°C]
only. Because of the high cost, I typically specify these

only for very critical measurement applications.
Incidentally, there are a few manufacturers of insertion electromagnetic ﬂow meters, which offer a signiﬁcant improvement on cost while sacriﬁcing just a little of
the accuracy of the in-line type. It may be worth keeping
an eye on this technology as more vendors begin to offer
such a product.

Insertion Turbine Liquid Flow Meters

Another type of permanent ﬂow meter in HVAC systems is the turbine ﬂow meter, which is more common.
These are not mounted in-line as valve or ﬁtting would
be, but instead are inserted into a standard length of
pipe via an oriﬁce designed for that purpose (Figure 2).
Some are even designed to permit installation in an
active pipeline via "hot tap" and are therefore good for
retroﬁt installations without pipeline shutdown. As
with the previous meter, wetted parts are typically constructed of stainless steel or synthetic materials for good
corrosion resistance.
The operating principle for the turbine ﬂow meter is
fairly simple. The device includes one or two small turbines inserted directly into the ﬂowing ﬂuid. The ﬂow of
passing liquid spins the turbine, and a small magnetic
counter mounted on the turbine's shaft counts the number of revolutions in a minute (rpm). Simple calibration
converts the rpm to a ﬂuid ﬂow rate (ft/min or m/s) and
then to a volumetric ﬂow rate (gpm or L/s) based on a
known pipe diameter. A single turbine may be used in
small-diameter piping (NPS 2 or DN 50 and smaller) and
dual independent turbines with an averaging feature
are recommended in larger pipes.
Turbine ﬂow meters are a good compromise between
S E P T E M B E R 2 0 19

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