Page 118 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT 95

Vortex shedding flow meters. Note: Different shapes of shedders are used in the designs for getting the desired flow rate
FIGURE 4.39
measurements.

➢ Its length in the direction of the flow must be a & Used for clean, low-viscosity, and swirl-free fluids
certain multiple of its width. flowing at medium to high speed. Vortex formation in
& The majority of vortex meters use piezoelectric- or high-viscosity fluids may not be dependable.
capacitance-type sensors to detect the pressure os- & Any erosion, corrosion, or deposits that change
cillation around the bluff body. These detectors the shape of the bluff body can shift flow meter
respond to the pressure oscillation with a low voltage calibration.
output signal that has the same frequency as the & The meter has no moving parts and involve
oscillation. Such sensors are modular, inexpensive, relatively low pressure drop compared to obstruction
easily replaced, and can operate over a wide range of meters.
temperature ranges—from cryogenic liquids to su- & Vortex meter accuracy is from medium to high,
perheated steam.
depending on the manufacturer and the model.
& Sensors can be located inside the meter body or & Under low flow conditions, vortices are formed
outside. irregularly, so low flow conditions present a problem
. What are the characteristics and applications of vortex for vortex meters.
shedding flow meters? & The lower flow rate limit on vortex meters
& Vortex meters are among the most versatile of meters, appears to be at Reynolds numbers near 10,000. This
and can be used to measure liquid, gas, and steam can be a problem in metering high-viscosity pipe
flows. flows.