# Flow Meters by jnyjhtw

VIEWS: 6 PAGES: 25

• pg 1
```									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

l   Use a tank and measure the volume/mass
collected during a known interval
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
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

```
To top