Page 108 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT 85

& Units are available using either permanent magnets, heated in a coil and then measured downstream. The
called DC units, or variable flux strength electro- temperature differential is directly related to the mass
magnets, called AC units. flow.
. State the advantages and disadvantages of a magnetic . Can a mass flow meter give a total accumulation of gas?
flow meter. & Yes.
& In general, magnetic flow meters are accurate, reli- . Why is it necessary to have a filter upstream of a mass
able, measurement devices that do not intrude into flow meter?
the system. & Mass flow meters require clean gases. In general, any
& Range of velocity (flow rate) can be increased beyond
particle larger than 50 mm require a filter upstream of
10:1 by increasing sensitivity for voltage detection. the meter.
& The electromagnetic flow meter has a very low . What are the advantages of using a mass flow meter?
pressure drop associated due to its open tube, no & It measures mass directly.
obstruction design, and it is suitable for installations
& It can handle applications whose stream temperature
that can tolerate only a small pressure drop.
and line pressures fluctuate.
& This absence of internal parts is very attractive for
. What are the limitations of using a mass flow meter?
metering corrosive and unclean fluids.
& A mass flow meter calibrated for air will not work on
& The operating principle is independent of fluid den-
other gases or gas mixtures without factory recali-
sity and viscosity, responding only to average veloc-
bration. When the gas is changed, the calibration
ity and there is no difficulty with measurements in
must be updated.
either laminar or turbulent flows, provided that the
& The gas must be dry.
velocity profile is reasonably symmetrical.
. Give typical applications of mass flow meters.
& Magnetic flow meters are widely used to measure the
flow rate of conductive liquids and slurries in process & Nitrogen delivery and control for tank blanketing.
applications. & Control of methane or argon to gas burners.
& Magnetic flow meters can detect the flow of conduc- & Gas delivery and control for fermenters and
tive fluids only. They cannot be used to measure flows bioreactors.
of hydrocarbons, which are nonconductive. As a & Leak testing.
result, they are not widely used in the petroleum & Regulating CO 2 injected into bottles during beverage
industry.
production.
& Since gases and steam are nonconductive, these
& Monitoring and controlling air flow during gas
meters cannot be used to measure them.
chromatography.
& The pipe has to be full of liquid, since these meters
. What is a thermal mass flow meter?
compute flow rate based on velocity times area.
& Thermal mass flow meter includes a heated sensor
& Liner damage and electrode coating can affect the
element that is cooled by the gas flow. The result of
accuracy of magnetic flow meters.
the temperature difference is in proportion with the
& These meters are expensive. While their initial pur-
gas mass flow and therefore a standard for the flow
chase cost is relatively high, most of these meters are rate. A flow-proportional frequency or analog signal
priced lower than equivalent Coriolis meters. provides the flow rate.
& Small meters are bulky in size.
4.1.6.1 Coriolis Flow Meters
4.1.6 Mass Flow Meters
. What is a Coriolis flow meter? What is the principle
. How does a mass flow meter work? involved in its operation?
& A volume of gas has a known mass at standard & Coriolis flow meters measure mass flow by taking
conditions. As pressure and temperature are applied, advantage of the Coriolis effect, which states that if a
the volume will change, but the mass remains con- particle inside a rotating body moves in a direction
stant. Mass flow meters measure flow based on the toward or away from the center of rotation, the
molecular mass of the gas. This measurement is particle generates internal forces that act on the body.
independent of temperature and pressure. One tech- & Simply stated, the inertial effects that arise as a fluid
nique to measure mass flow is to send a part of the flows through a tube are directly proportional to the
flow through a sensor tube. In the tube, the gas is mass flow of the fluid.