180
5 Performance Measurement
always somewhat lower than unity and depends on the Reynolds number of the
approaching flow.
5.3.4 Industrial Mass Flow Rate Meters
Industry mostly applies mass flow rate meters with a voltage signal output appropriate to be read into a computer or on a display. Several physical principles may be applied. Turbine meters, acoustic meters, magnetic meters and
vortex meters are commonly used. With a turbine meter, a turbine wheel is
mounted into a casing. The rotational speed is a measure for the flow rate.
Rotational speed mostly is read out magnetically (see Sects. 5.5.1 and 5.5.2).
A Doppler-acoustic meter is possible with a fluid containing solid particles or
bubbles larger than about 30 µm. An acoustic pulse is sent out by a piezoelectric
transducer. The pulse is reflected by the particles or the bubbles and detected by
a second piezoelectric transducer. The Doppler-effect modifies the frequency
of the acoustic signal. Magnetic flow rate measurement is possible if the fluid
is electro-conductive. The voltage generated within a conductive fluid flowing
perpendicularly through a magnetic field is proportional to the flow velocity. An
object is positioned in the flow with a vortex meter. Flow rate determination is
based on the frequency of the vortices shed.
5.3.5 Positioning of Flow Rate Meters in Ducts
In principle, the approaching flow to a pressure drop device or a flow rate meter in a
duct must be fully developed. This means a velocity profile that does not change in
the flow direction. This is extremely difficult to realise in practice. Standards require
approach lengths of 20–30 times the duct diameter. The approach length depends
on the Reynolds number and the kind of upstream flow perturbations (e.g. number
of bends passed). Smaller lengths apply when flow regulating devices, called flow
conditioners, are placed upstream. These are plate systems that parallelise the flow
and generate shear zones with intense turbulent mixing. Even with conditioners, the
required approach lengths remain large. Flow rate measurement by an inlet nozzle,
as in Fig. 5.4 (left), may be more convenient. Suction must be performed from a
sufficiently large space, so that β = 0 applies.
5 Performance Measurement
always somewhat lower than unity and depends on the Reynolds number of the
approaching flow.
5.3.4 Industrial Mass Flow Rate Meters
Industry mostly applies mass flow rate meters with a voltage signal output appropriate to be read into a computer or on a display. Several physical principles may be applied. Turbine meters, acoustic meters, magnetic meters and
vortex meters are commonly used. With a turbine meter, a turbine wheel is
mounted into a casing. The rotational speed is a measure for the flow rate.
Rotational speed mostly is read out magnetically (see Sects. 5.5.1 and 5.5.2).
A Doppler-acoustic meter is possible with a fluid containing solid particles or
bubbles larger than about 30 µm. An acoustic pulse is sent out by a piezoelectric
transducer. The pulse is reflected by the particles or the bubbles and detected by
a second piezoelectric transducer. The Doppler-effect modifies the frequency
of the acoustic signal. Magnetic flow rate measurement is possible if the fluid
is electro-conductive. The voltage generated within a conductive fluid flowing
perpendicularly through a magnetic field is proportional to the flow velocity. An
object is positioned in the flow with a vortex meter. Flow rate determination is
based on the frequency of the vortices shed.
5.3.5 Positioning of Flow Rate Meters in Ducts
In principle, the approaching flow to a pressure drop device or a flow rate meter in a
duct must be fully developed. This means a velocity profile that does not change in
the flow direction. This is extremely difficult to realise in practice. Standards require
approach lengths of 20–30 times the duct diameter. The approach length depends
on the Reynolds number and the kind of upstream flow perturbations (e.g. number
of bends passed). Smaller lengths apply when flow regulating devices, called flow
conditioners, are placed upstream. These are plate systems that parallelise the flow
and generate shear zones with intense turbulent mixing. Even with conditioners, the
required approach lengths remain large. Flow rate measurement by an inlet nozzle,
as in Fig. 5.4 (left), may be more convenient. Suction must be performed from a
sufficiently large space, so that β = 0 applies.