Page 40 - Fluid mechanics, heat transfer, and mass transfer

FLUID STATICS 17

device. The higher the liquid density, the smaller
the paddle required.
➢ The cost-effective and low-maintenance electro-
mechanical designs are well suited for solids such
as plastic pellets, carbon black, fertilizers, Styro-
foam, and rubber chips and beads.
➢ The paddle switch can handle bulk densities as
low as 35 g/L and the technology is completely
independentof dielectric properties of thematerial.
& Capacitance: A capacitance probe can be immersed
in the liquid in the tank and the capacitance between
the probe and thevessel wall depends on the level. By
measuring the capacitance of the liquid, the level in
the tank can be determined. Capacitance can be
affected by density variations.
➢ Capacitance technology is widely used in many
industries and can handle a wide range of applica-
FIGURE 1.18 Ultrasonic level measurement.
tions from simple storage of acids in small tanks to
high-temperature and high-pressure characteris-
tics offine chemicals in a turbulent process reactor.
This technology also produces highly accurate and & Thermal methods are based on the difference in
repeatable results. thermal characteristics between the fluids, such as
➢ Because capacitance is a contacting technology, temperature or thermal conductivity.
chemical compatibility with the device must be & Radar: Radar devices transmit an electromagnetic
taken into account, as well as the potential for
wave traveling at the speed of light toward a mate-
buildup problems.
rial. Once the electromagnetic wave comes into
➢ Also, the chemistry of liquids must remain contact with the material, it is reflected back to the
constant or homogeneous in nonconductive or source.
insulating hydrocarbon fluids such as oil and
➢ The total transit time to and from the target is
methane.
calculated and is directly related to the distance.
➢ Any change due to temperature or chemical com- ➢ There are two techniques of transmitting radar,
position causes the dielectric property of the ma-
namely, pulse- or frequency-modulated continu-
terial to change, resulting in errors and the re-
ous wave (FMCW).
quirement to recalibrate.
➢ Pulse radar is similar to ultrasonic noncontact
➢ Capacitance technology requires only one open-
method in that fixed-frequency pulses are
ing in a vessel, making it easy to install, and has no
transmitted to a material and then reflected
moving parts that may wear out over time.
back to the source where the time of flight is
& Ultrasonic Methods: Ultrasonic technology uses a
calculated.
piezoelectric crystal stored inside a transducer to ➢ FMCW radar devices continuously transmit a
convert an electrical signal into sound energy. The
range of frequencies, known as a frequency
sound energy is fired toward the material and is sweep. The receiver continuously monitors
reflected back to the transducer (Figure 1.18). the received frequencies and the difference
➢ The transducer then acts as a receiving device and between the transmitter and receiver frequency
converts the sonic energy back into an electrical is directly proportional to the distance to the
signal. target.
➢ An electronic signal processor analyzes the return ➢ High-frequency radar is suited for low-dielectric
echo and calculates the distance between the media due to the narrower, more focused signal,
transducer and the target. The time lapse between which improves the reflection from material.
the sound burst and the return echo is proportional
➢ Low-frequency radar has antenna deposit resis-
to the distance between the transducer and the
tance, a longer wavelength, and physical wave
material in a vessel.
reflection properties.