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7/23/2019 (1.) Flow Measurement Lecture
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CHE
215
ProcessMeasurements
Winter
2014
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Many devices are available to measure flow.
The application depends on the physical properties of the
material to be measured, the precision required, cost,
and the type of signal required from the device.
Fluid
Flow
Measurement
Several types will be considered here:
1. Pitot tube
2. Venturi meter
3. Orifice meter
4. Flow nozzle
5. Variable area meter rotameter
6. Ultrasonic7. Thermal
8. Turbine
9. Magnetic10. Vortex
11.Weirs
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Advantages:
1. Cheap
2. Portable3. Velocity profile determination.
4. Very little power loss.
Disadvantages:
1. Average velocity not read directly.
2. Very low readings for gases.3. Can only be used for clean streams.
4. A computer signal not readily available.
Fluid
Flow
Measurement
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22
22
11
2
22
1 zg
gP
g
Vz
g
gP
g
V
cccc
cg
VVPP
2
21
22
21
=
Bernoulliequationforincompressiblefluid
1
221A
AVV
=
cg
A
AV
PP2
1
2
1
222
21
=
Fluid
Flow
Measurement
Therelationshipbetweenthefluidvelocityatpoints1and2
Fromtheequationofcontinuityforisothermalcondition(constant):
Substitution of Equation (3) in to Equation (2):
(1)
(2)
(3)
(4)
Thefluidvolumetricflowrateis(idealcondition)
21
21c
21
2
12
2 PPg2
A
A1
A=Q
(5)
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Under practical conditions (some friction loss),
a coefficient of discharge (or calibration coefficient), Co,which is a function of the ratio of the venturi throat
diameter to that of the pipe and the Reynolds number,
is added to the equation of the fluid volumetric flowrate
as below:
Fluid
Flow
Measurement
21
21c2
12
1
2
2o
PPg2
A
A1
AC=Q
LetM,velocityapproachfactor,2
1
21
1
=
A
A
21
21c2o
PPg2C=Q
AM
(6)
(7)
(8)
Co : discharge coefficient
Co is a function of the
diameter ratio () andReynolds number
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Fluid
Flow
Measurement
Venturi Tubecompressiblefluids:
Foragasflow(compressiblefluid)passingaventuri flowmeter.
Thevolumetricflowrate canbedetermined
2
1
21c2o
PPg2CY=Q
AM
WherethegasexpansionfactorYcanbeestimatedfromthe
followingequations
50
2
1
2
2
1
2
2
1
2
1
2
1
1
22
1
2
1
1
1
1
1
.
P
P
A
A
A
A
P
P
P
P
P
PY
=
: heat capacity ratio
r: P2/P1
Y: expansion factor
Where
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Fluid
Flow
Measurement
Venturi Tubecompressiblefluids
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