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Flow Meters

VIEWS: 6 PAGES: 25

									MER 160 - Design of Thermal Fluid Systems

               Flowmeters

            Professor Anderson
            Winter Term 2005


                                        1
Flow Measurement

 The choice of a flow meter is influenced
 by:
    ¡accuracy
    ¡range
    ¡cost complication
    ¡service life


 The simplest cheapest device that gives
 the desired accuracy should be chosen

                                            2
Flow Measurement Methods

 l Direct Methods
 l Restriction Flow Meters

 l Linear Flow Meters

 l Others

 l Traversing Methods




                             3
Direct Methods


¡   Steady liquid flows
    l   Use a tank and measure the volume/mass
        collected during a known interval
¡   Steady Gas flows
    l   Compressibility effects usually make this
        impossible
¡   Major Advantage - no calibration is
    required


                                                    4
   Restriction Flow Meters
 An effective way to
 measure flowrate
 through a pipe is to
 place a restriction in the
 pipe and measure
 pressure drop from:

(1) High P-Low V upstream to
(2) High V-Low P downstream



                               5
Restriction Flow Meters




¡   Correlation of the pressure difference
    with velocity provides a means of
    measuring flow rate.
¡   From Bernoulli’s (for ideal flow)




                                         6
Restriction Flow Meters

¡   There is a head loss from point 1 to 2:



¡   The ideal situation has hL = 0.
¡   There is no accurate expression for hL.
¡   Need to use empirical coefficients.




                                              7
RFM -Orifice Meter


Typical Orifice
meter is
constructed by
inserting a plate
with a hole
between two
flanges of a pipe.



                     8
   RFM - Orifice Meter




Co = Orifice Discharge
Coefficient
Value depends on
geometry.
                         9
RFM - Nozzle Meter


¡   Uses a contoured nozzle placed between
    flanges of a pipe.
¡   Flow pattern is closer to ideal.
¡   More expensive.




                                        10
RFM -Nozzle Meter
¡   Losses are accounted for by a Nozzle
    Discharge Coefficient:




                                       11
RFM - Venturi Meter

 ¡   Most precise and most expensive.
 ¡   Geometry is designed to reduce head losses to
     a minimum.
 ¡   Streamlined contraction and gradual expansion.
 ¡   Most head loss caused by friction.




                                               12
RFM - Venturi Meter
¡   Losses are accounted for by a Venturi
    Discharge Coefficient:




                                        13
RFM - Laminar Flow Meters

The LFE contains small sub passages where
the flow is fully developed and laminar.
Produces a pressure drop directly proportional
to flow rate.
High Range of Flow Rate




                                                 14
RFM - Example Problem

What is the pressure drop across an 8 in
 orifice in a 12 in water pipe for a flow
 rate of 5 ft3/s? The water temperature is
 60 oF.




                                        15
 RFM - Example Problem

First calculate Re:




From Figure: Co ~ 0.61
What would it be for a nozzle meter (~0.99)?
How about a Venturi Meter (~0.99)


                                               16
RFM - Example Problem

Calculate Pressure Drop:




                           17
RFM -Summary




               18
Linear Flow Meters
Rotameters: A float
 rises through the
 meter as the fluid
 flows. The float
 reaches an
 equilibrium height as
 a function of flow
 rate. (net forces add
 to zero - buoyancy,
 weight, drag)


                         19
 Linear Flow Meters




Turbine Meter: Angular
 velocity of the freely
 rotating turbine is
 proportional to fluid
 velocity. The angular
 velocity is picked up
 magnetically.
                          20
   Volume Flow Meters
Measure the volume of fluid that has passed
through a pipe during a given time instead
of the instantaneous rate. (i.e. gasoline
pumps)
Nutating Disk Meter:
measures the flow of a
liquid by counting the
number of times a disk
in the path of the liquid
is caused to wobble back and forth.

                                              21
Volume Flow Meters




                     22
 Volume Flow Meters


Bellows-type Flow
Meter:The bellows
are alternately
filled and emptied
as the gas flows
through the meter,
and their action is
used as a counting
mechanism.

                      23
Others
Electromagnetic - a conductor moving in
  a magnetic field produces as
  electromagnetic force - this is
  proportional to velocity.
Ultrasonic - measures the difference in
  travel time for a sound wave to travel
  upstream and downstream between 2
  measuring stations.
Vortex Flow Meter - Vortex shedding
  from a cylinder is detected. The shedding
  frequency is proportional to flow rate.
                                         24
Traversing Methods
It is sometimes impractical to install
fixed flow meters and instead the flow
rate is found by traversing the pipe with
a velocity probe and integrating the
profile.
 l   Pitot Probe
 l   Hot Wire Anemometer
 l   Laser Doppler Anemometer
 l   Particle Image Velocimeter.




                                        25

								
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