Chemical Engineering February 2014 - 44

Feature Report
Advanced Thermal
Dispersion Mass Flowmeters
A look at the principles
of operation, installation
and calibration
John G. Olin
Sierra Instruments, Inc.
T
hermal dispersion (TD) mass
flowmeters measure the mass
flowrate of fluids (primarily
gases) flowing through a closed
conduit, such as a pipe. This article
describes the operation and installation
of TD mass flowmeters, and
gives the reader information about
what applications these meters are
most suited for.
Background
The first general description of TD
mass flowmeters is attributed to L.V.
King who, in 1914 [1], published his
famous King's Law revealing how
a heated wire immersed in a fluid
flow measures the mass velocity at
a point in the flow. He called his instrument
a " hot-wire anemometer. "
The first application of this technology
was hot-wire and hot-film
anemometers and other light-duty
TD flow sensors used in fluid mechanics
research and as light-duty
mass flowmeters and point velocity
instruments. This class of TD mass
flowmeters is described in Ref. 2.
It was not until the 1960s and
1970s when industrial-grade TD
mass flowmeters emerged that
could solve the wide range of general
industry's more rugged needs for directly
measuring the mass flowrate
of air, natural gas and other gases
in pipes and ducts. That is the class
of instruments described here.
TD mass flowmeters measure
the heat convected into the boundary
layer of the gas flowing over
Flow
sensor
probe
qm
Flow body
Transmitter
Flow sensor probe
qm
Proccess
fluid
Flow conditioner
Flow sensor
(a) In-line configuration
Flow sensor
probe
Shoulder
Compression
fitting
Fluid
Proccess
fluid
conduit
Flow sensor
(b) Insertion configuration,
installed
qm
Fluid
temperature
sensor
Velocity
sensor
Figure 1. This diagram shows the
major components of in-line (a) and
insertion (b) meter configurations of
thermal dispersion mass flowmeters
the surface of a heated velocity
sensor immersed in the flow. Since
it is the molecules of the gas, which
bear its mass, that carry away the
heat, TD mass flowmeters directly
measure mass flowrate. Capillarytube
thermal mass flowmeters constitute
a second type of thermal
mass flow technology, but their
principle of operation and their applications
are sufficiently different
from TD mass flowmeters that the
American Society of Mechanical
44 ChemiCal engineering www.Che.Com february 2014
(c) Detail of flow sensor,
insertion meter shown
Engineers (ASME) has published
separate national standards for
each type [3, 4].
Typical gases monitored by industrial
TD mass flowmeters include:
air, methane, natural gas, carbon
dioxide, nitrogen, oxygen, argon,
helium, hydrogen, propane and
stack gases, as well as mixtures of
these gases and mixtures of hydrocarbon
gases. Common applications
are: combustion air; preheated air;
compressed air; fluid power; boilers;
temperature
sensor
Open-ended
probe
Velocity
sensor
conduit
http://www.Che.Com
Chemical Engineering February 2014

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