Mechanical Engineering Principles

FLUID FLOW 251

(a) Cup anemometer. An anemometer is an ins-

trument that measures the velocity of moving

gases and is most often used for the mea-

surement of wind speed. The cup anemometer

has three or four cups of hemispherical shape

mounted at the end of arms radiating horizon-

tally from a fixed point. The cup system spins

round the vertical axis with a speed approxi-

mately proportional to the velocity of the wind.

With the aid of a mechanical and/or electri-

cal counter the wind speed can be determined

and the device is easily adapted for automatic

recording.

(b) Rotary vane positive displacement meters
measure the flow rate by indicating the quan-
tity of liquid flowing through the meter in a
given time. A typical such device is shown in
section in Figure 22.8 and consists of a cylin-
drical chamber into which is placed a rotor
containing a number of vanes (six in this case).
Liquid entering the chamber turns the rotor and
a known amount of liquid is trapped and car-
ried round to the outlet. Ifx is the volume
displaced by one blade then for each revolution
of the rotor in Figure 22.8 the total volume dis-
placed is 6x. The rotor shaft may be coupled
to a mechanical counter and electrical devices
which may be calibrated to give flow volume.
This type of meter in its various forms is used
widely for the measurement of domestic and
industrial water consumption, for the accurate
measurement of petrol in petrol pumps and for
the consumption and batch control measure-
ments in the general process and food indus-
tries for measuring flows as varied as solvents,
tar and molasses (i.e. thickish treacle).

Fluid flow

Rotation

Figure 22.8

(c) Aturbine flowmetercontains in its construc-

tion a rotor to which blades are attached which

spin at a velocity proportional to the velocity of

the fluid which flows through the meter. A typ-

ical section through such a meter is shown in

Figure 22.9. The number of revolutions made

by the turbine blades may be determined by

a mechanical or electrical device enabling the

flow rate or total flow to be determined. Advan-

tages of turbine flowmeters include a compact

durable form, high accuracy, wide tempera-

ture and pressure capability and good response

characteristics. Applications include the volu-

metric measurement of both crude and refined

petroleum products in pipelines up to 600 mm

bore, and in the water, power, aerospace, pro-

cess and food industries, and with modification

may be used for natural, industrial and liquid

gas measurements. Turbine flowmeters require

periodic inspection and cleaning of the working

parts.

Rotor

Direction of

fluid flow

Figure 22.9

22.10 Float and tapered-tube meter

Principle of operation

With orifice plates and venturimeters the area of the

opening in the obstruction is fixed and any change

in the flow rate produces a corresponding change

in pressure. With the float and tapered-tube meter

the area of the restriction may be varied so as to

maintain a steady pressure differential. A typical

meter of this type is shown diagrammatically in

Figure 22.10 where a vertical tapered tube contains

a ‘float’ that has a density greater than the fluid.

The float in the tapered tube produces a restriction

to the fluid flow. The fluid can only pass in the

annular area between the float and the walls of the

tube. This reduction in area produces an increase

in velocity and hence a pressure difference, which

causes the float to rise. The greater the flow rate,

the greater is the rise in the float position, and vice

versa. The position of the float is a measure of the

flow rate of the fluid and this is shown on a vertical

scale engraved on a transparent tube of plastic or

glass. For air, a small sphere is used for the float

but for liquids there is a tendency to instability and

the float is then designed with vanes that cause it

to spin and thus stabilize itself as the liquid flows

past. Such meters are often called ‘rotameters’.

Calibration of float and tapered tube flowmeters can