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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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
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ASHRAE Journal - September 2019 - 10
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ASHRAE Journal - September 2019 - 14
ASHRAE Journal - September 2019 - 15
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ASHRAE Journal - September 2019 - 37
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ASHRAE Journal - September 2019 - Cover3
ASHRAE Journal - September 2019 - Cover4
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