Page 97 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT
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& Design variations include the rotameter (a float in a . What is the normal size ranges of rotameter tubes?
tapered tube), orifice/rotameter combination (bypass & Tube diameters vary from 0.5 to >15 cm.
rotameter), open channel variable gate, tapered plug
. What are the advantages of rotameters?
and vane, or piston designs.
& Relatively cheap.
. Can a rotameter be used to measure different flow rates?
& Somewhat self-cleaning because as the fluid flows
& Yes. Floats are available in a variety of shapes and
between the tube wall and the float, it produces a
materials of construction, with varying densities that
scouring action that tends to prevent the buildup of
can be used to change the range of the meter, as well
foreign matter. Nevertheless, rotameters should be
as to resist corrosion from the measured fluid.
used only on clean fluids that do not coat the float or
Examples of float materials include 316 stainless
the tube. Liquids with fibrous materials, abrasives,
steel, tantalum, Hastelloy C, Monel, PTFE, PVC,
and large particles should also be avoided.
and so on.
& Rotameter accuracy is not affected by the upstream
& If a correlated flow tube is used, different flow rates
piping configuration. The meter also can be installed
can be attained by using different floats, that is,
directly after a pipe elbow without adverse effect on
different designs and materials made of carboloy,
metering accuracy.
stainless steel, glass, or sapphire.
& Available in different materials for chemical
& Figure 4.19 shows designs of different types of
compatibility.
rotameters.
& Used for measuring volumetric flow rates of many
. What are the essential differences in the operating
liquids and gases.
principle of a rotameter from a venturi or orifice meter?
& With a glass tube meter, the main metering elements
& Orifice and venturi meters use constant restriction/
and the fluid are clearly visible. The user can imme-
flow area and a variable pressure differential. Rota-
diately see any change in flow rate.
meters use a variable restriction/flow area and a
& Wide rangeability: From 5:1 to 12:1.
constant pressure differential to measure flow.
& Rangeability of 12:1, as compared to 4:1, for differ-
& Typically, rotameters are used to measure smaller
ential pressure meters that are commonly used with
flows and the reading is usually done locally, al-
orifice plates.
though transmission of the readings is possible.
& Viscosity compensation: The float can be designed to
. What are the forces acting on the float in a rotameter?
compensate for increased viscosities.
& Buoyancy force acting upward.
➢ Most rotameters are relatively insensitive to
& Gravity acting downward.
viscosity variations. The most sensitive are
& Drag force acting downward. very small rotameters with ball floats, while

Rotameter showing different types of float designs.
FIGURE 4.19