Flow Measuring Devices
ENTC-303
Dr. Alvarado
1
Flowmeters
• Devices used to measure flow rate:
– Venturi (based on pressure difference)
– Orifice Plate (based on pressure difference)
– Rotameter (based on drag forces)
– Coriolis (measure mass flow rate)
– Magnetic (magnetic field is used to measure
flow rate)
– Ultrasonic (ultrasonic waves are used to
measure flow rate)
ENTC-303
Dr. Alvarado
2
Manometers
The height of a liquid
column or difference of
height of two columns is
used to measured the
pressure:
p  po  gh
ENTC 303
PROF. ALVARADO
3
Venturi Effect
• The Venturi effect is the reduction in fluid
pressure that results when a fluid flows
through a constricted section of pipe.
- The Venturi effect is a jet effect; as with
an (air) funnel, or a thumb on a garden
hose, the velocity of the fluid increases
as the cross sectional area decreases,
with the static pressure correspondingly
decreasing.
ENTC-303
Dr. Alvarado
4
Venturi Meter
From Bernoulli Equation:
2
2 æ
æ AB ö ö PA - PB
v B,t
ç1- ç ÷ ÷ =
=h
ç
÷
2g è è AA ø ø
rg
in theoretical velocity:
vB,act = Cv v B,t
C v takes into account reduction
Flow mass is given by:
m = Cv r AB vB,t
Substituting with know values we have:
m = 0.962 Cv h1/2
ENTC-303
Dr. Alvarado
5
Orifice Plate
• An orifice plate is a device used for
measuring the volumetric flow rate. It uses
the same principle as a Venturi nozzle,
namely Bernoulli’s principle which states
that there is a relationship between the
pressure of the fluid and the velocity of the
fluid. When the velocity increases, the
pressure decreases and vice versa.
ENTC-303
Dr. Alvarado
6
Orifice Meter
From Bernoulli Equation:
2
2 æ
æ AF ö ö PE - PF
v F,t
ç1- ç ÷ ÷ =
=h
ç
÷
2g è è AE ø ø
rg
in theoretical velocity:
vF,act = Cv vF,t
Flow mass is given by:
m = CC Cv r AF v F,t
CC takes into account the area contraction:
AF
AE
C D takes into account reduction
CD = CC Cv
CC =
Substituting with know values we have:
m = 1.398 CD h1/2
ENTC-303
Dr. Alvarado
7
Rotameter
-Pressure difference is constant in
the rotameter
-Peripheral velocity is constant
-To maintain a constant velocity,
the cross sectional area must vary
-This variation of cross sectional
area occurs as the float moves up
and down the rotameter.
ENTC-303
Dr. Alvarado
8
Rotameter
Time to
Mass
A (mm) B (mm) C (mm) E (mm) F (mm)
(cm)
Fill (s) Flow(kg/s)
Cv
Cd
18
16
14
12
10
Average:
Calculated
Rotameter (cm) Venturi m
(kg/s)
18
16
14
12
10
Calculated
Orifice m
(kg/s)
Error
Venturi
Error Orifice
HL Venturi
(mm)
HL Orifice
(mm)
References
•
•
•
•
en.wikipedia.org/wiki/Venturi_effect
en.wikipedia.org/wiki/Orifice_plate
www.orificeplates.com
www.omega.com
ENTC-303
Dr. Alvarado
10