Axial innen engl - TLT Turbo GmbH
Axial innen engl - TLT Turbo GmbH
Axial innen engl - TLT Turbo GmbH
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Am Weinberg 68 · D-36251 Bad Hersfeld/Germany<br />
Phone: +49.6621.950-0 · Fax: +49.6621.950-100<br />
Application example for<br />
axial-flow fans<br />
The volume flow measuring system<br />
determines the operating volume flow<br />
of an axial fan installed in an air-handling<br />
installation. The system greatly<br />
facilitates the determination of fan volume<br />
flow. As a result, the cost and effort<br />
required for in-duct measurements<br />
on the system side, as often<br />
carried out during commissioning,<br />
can be significantly reduced. Moreover,<br />
adjustments are faster and easier<br />
to carry out.<br />
The measuring accuracy of the volume-flow<br />
measuring system is approx.<br />
± 5%.<br />
Description of the measuring<br />
system<br />
The volume flow measuring system is<br />
firmly mounted on the fan to determine<br />
the volume flow in its installed condition.<br />
The reading is obtained by a<br />
simple differential-pressure measurement<br />
between the sockets of the<br />
pressure measuring lines connected<br />
to the measuring system. The calibration<br />
curve is prepared individually, taking<br />
into account fan-specific parameters.<br />
Key influencing variables include<br />
fan type, inlet density, rpm and<br />
fan design size. Being established individually<br />
with reference to the above<br />
parameters, the calibration curve is<br />
not transferable to other fan types<br />
Options<br />
Adirect automatic volume flow measurement<br />
can be implemented by<br />
means of root-extracting differential<br />
pressure transducers or suitable<br />
electronic evaluation units. The following<br />
equation may be used for this<br />
purpose:<br />
Volume flow calculation:<br />
V = k x A x<br />
� 2 x d<br />
p<br />
VOLUME FLOW MEASURING SYSTEM VDE<br />
FOR CENTRIFUGAL ROOF FANS<br />
AND AXIAL FLOW FANS<br />
and sizes. At fan operating speeds<br />
deviating from the rated rpm level,<br />
major inaccuracies will be obtained,<br />
especially when the fan is running at<br />
slower speeds.<br />
The measuring system consists of an<br />
additional casing extension made of<br />
sheet steel, pre-installed measuring<br />
lines for the pressure measurement,<br />
and a sheet aluminium hub body attached<br />
to, and rotating with, the fan impeller.<br />
The pressure measurement<br />
ring line consists of plastic hose as<br />
employed in pneumatic applications,<br />
as well as brass screw-in adapters.<br />
The four measuring points, spaced<br />
evenly over the circumference, are<br />
connected into a ring line by means of<br />
T-fittings. The connection to a pressure<br />
measuring instrument is likewise<br />
made via a T-fitting.<br />
Technical description<br />
The calibration curves can be computed<br />
for flow medium throughput conditions<br />
as habitually employed in ventilation<br />
technology, and apply to air only.<br />
The measuring system is standardized<br />
for AXN series fans in the 315<br />
to 2000 size range. It is recommended<br />
to use the volume flow measuring<br />
device only at blade angles equal to,<br />
or exceeding, 10 degrees so as to to<br />
maximize the accuracy of the reading.<br />
At blade angles below 10 deg.,<br />
the magnitude of the differential pressure<br />
indication will not suffice for an<br />
Calculation of k:<br />
Correction factor:<br />
k = f (Re, NG, n)<br />
Reynolds number:<br />
c x Dh Re = x<br />
�<br />
Hydraulic diameter:<br />
Dh = A· 4<br />
�<br />
�<br />
Rotational Speed:<br />
n [rpm]<br />
Reference cross section:<br />
NG [mm]<br />
022<br />
unambiguous volume flow measurement.<br />
The accuracy of the system depends<br />
essentially on its installation<br />
conditions and on the inlet-side approach<br />
flow situation. An irregular or<br />
swirl-impaired inlet airflow will reduce<br />
the accuracy of the volume flow measuring<br />
system. The differential pressure<br />
measurement can be carried out<br />
with the aid of electronic differential<br />
pressure sensors or other standard<br />
commercial transducers, whether<br />
mechanical or hydrostatic. The measuring<br />
range of the differential pressure<br />
instrument should be 0 - 2000<br />
Pa, at a resolution of 1 Pa. The measuring<br />
tolerance of the instrument<br />
employed should be better than ± 2.5<br />
of the measured value.<br />
Uses:<br />
The system may be used for the following<br />
<strong>TLT</strong> axial-flow fans:<br />
Series AXN 12/56<br />
AXN 9/56<br />
AXN 6/56<br />
BVAXN 12/56<br />
BVAXN 9/56<br />
BVAXN 6/56<br />
Flow velocity in the annulus area:<br />
c [m/s]<br />
Kinematic viscosity:<br />
� [kg/ms]<br />
Inlet density:<br />
[kg/m3] �<br />
Reference surface area for the volume<br />
flow determination:<br />
A= [m2 ] = annulus area of the blading<br />
d P= Differential pressure [Pa] at the<br />
volume flow measuring point
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