Page 96 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT 73

➢ Specially designed pitot probes have been devel-
oped for use with pulsating flows.
➢ One design uses a pitot probe filled with silicone
oil to transmit the process pressures to the DP cell.
At high-frequency pulsating applications, the oil
serves as a pulsation dampening and pressure
averaging medium.
& Errors in detecting static pressure arise from fluid
viscosity, velocity, and fluid compressibility.
& The key to accurate static pressure detection is to
minimize the kinetic component in the pressure
measurement.
. What is a pitot-venturi flow element? What is its
advantage?
& A pitot-venturi element consists of a pair of concen-
tric venturi elements in place of a pitot tube.
& Low-pressure tap is connected to throat of inner
venturi, which in turn discharges into throat of outer
venturi. FIGURE 4.18 Annubar.
& It is capable of developing a pressure differential of
five to ten times that of a standard pitot tube.
. What is a reverse pitot tube or pitometer? is proportional to the flow rate. The flowing fluid
enters the bottom of the meter, passes upward
& Pitot tube with one pressure opening facing upstream
through a tapered metering tube, and around a float,
and the other downstream is called reverse tube/
exiting at the top. The float is forced upward until the
pitometer.
force is balanced by gravitational forces.
. What is the advantage of reverse pitot tube over standard
➢ Either the force of gravity or a spring is used to
pitot tube?
return the flow element to its resting position when
& Coefficient C for this type is of the order of 0.85. This
the flow decreases.
gives about 40% increase in pressure differential as
➢ Gravity operated meters (rotameters) must
compared with standard pitot tubes and is an advan-
be installed in a vertical position, whereas spring
tage at low velocities. Compact types of pitometers
operated ones can be mounted in any position.
are available, which require comparatively small
openings for their insertion into a duct. & Flow is through the annular area between float and
tapered tube whose area is variable depending on
. What is an annubar?
flow rate.
& An annubar consists of several pitot tubes placed
across a pipe to provide an approximation to the & The height of rise of the float in the tube is calibrated
velocity profile and the total flow can be determined with the flow rate, and the relation between the meter
based on the multiple measurements (Figure 4.18). reading, that is, position of the float, and the flow rate
is approximately linear.
. “Averaging pitot tubes are a good choice for the mea-
& Floats have a sharp edge at the point where the
surement of high flows.” True/False?
reading should be observed on the tube-mounted
& True.
scale.
& The reading is taken at the center of the float. It is
recommended that the float be at eye level to min-
4.1.3 Variable Area Flow Meters
imize reading errors.
4.1.3.1 Rotameters
& The scale can be calibrated for direct reading of air or
water, or can read percentage of range.
. What is a rotameter? Has the glass tube comprising the
rotameter is of constant cross section? & Most rotameters are made of glass with etched scale
& Rotameters, or variable area flow meters, operate on on the outside so that flow readings can be taken
the principle that the variation in area, of flow stream visually.
required to produce a constant pressure differential, & Used to measure the flow of liquids, gases, and steam.