July 2015
TTM-02: Determination of duty point of fan
1
Introduction
The duty point or operating point of a fan in a device is crucial information for selecting the correct fan
product. This applies for new developments as well as for retrofitting of existing units. Not knowing the
correct duty point can result in selecting the incorrect fan product and issues like the unit not performing
properly, the unit being too noisy or power consumption being higher than expected can occur.
Figure 1: ebm-papst EC plug fan mounted on test rig in Melbourne
The ideal way to establish the duty point of a device is by attaching it to an air performance test rig.
This is the most accurate method but might not always be possible due to logistical issues, availability
or size restrictions. With a few simple steps the duty point can be easily determined with sufficient
accuracy that allows selection of the correct fan product.
2
Fan performance curve and duty point
The fan performance curve is the performance characteristic of a fan. When installed in a device, the
fan will deliver a certain volume of air and will overcome a certain static back pressure that is caused by
the device in which the fan is installed, for example by filters, grilles, coils or ducts. Air volume and
static pressure at which the fan operates in a device is known as duty point or operating point of a fan.
With increasing air flow through a device, the pressure also increases. This relationship is known as the
resistance curve of a device. Each device that incorporates a fan has its own resistance curve.
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Figure 2: Fan performance and resistance curve
The air volume that a fan delivers is influenced by air density and hence by altitude and air
temperature.
Below Figure 3: Static pressure in application shows a fan installed in an application and the static
pressure losses of this application, caused for example by the inlet louvres, the filter, the heater, and so
on. The total static back pressure that the fan needs to overcome is the differential pressure before and
after the fan. It’s called “Pressure Increase in the fan” in the graph.
Figure 3: Static pressure in application
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3
Performance measurement
3.1 Air flow measurement
To measure the air flow, various test equipment can be used, for example vane anemometer, hot wire
anemometer or differential pressure method over the inlet ring using a manometer.
3.1.1 Vane anemometer and hot wire anemometer
Vane anemometer and hot wire anemometer allow measurement of air velocity. The airflow needs to be
measured in a grid like fashion and is usually done on the suction side of a system. The number of
testing points depends on the surface area. Once all measurements are completed, the average air
speed needs to be calculated.
Fig. 4 illustrates how such a grid can be set up. In this example the air flow direction is from right to left,
with the application being the grey cube. The fan would sit inside the grey cube drawing the air in. In
this example, a grid with 16 testing points has been selected (red lines).
Figure 4: Grid for air flow measurement
In a second step, the opening area of the system needs to be calculated. Any guard grilles or other
obstructions of the opening area must be accounted for as they reduce the size of the opening area. In
the above example, the opening area is the light grey coloured area of the cube. It is calculated with the
following formula: 𝐴 = 𝑎×𝑏.
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As a last step, the air volume can be calculated by multiplying air velocity with the opening area.
V = v×A×3600
m!
V = air volume in h
m
v = air velocity in s
A = surface area in m!
In the field, this test method can very often be used quite easily. However, it is also a fairly inaccurate
measurement method. The following factors influence the accuracy:
-
Number of test points in the grid. Generally speaking, the higher the number of test points,
the higher the accuracy.
Position of vane anemometer or hot wire anemometer in the air stream. Slight tilting might
not allow you to “catch” the maximum velocity at a certain test point.
Distance of vane or hot wire anemometer from opening area. Generally speaking, the
anemometer needs to be held as close to the opening area as possible. However, if the
opening area is obstructed, for example by the copper tubes of a heat exchanger, it is
advised to keep a small distance.
3.1.2 Differential pressure method using a manometer
a) Formula to calculate air volume
When a backward curved fan is used in the application, inlet rings with pressure tap can be used to
determine the air volume. The following formula applies:
2 × Δp
V! = α × ε × Ad ×
ρ1
-
V! = air flow in m3 / s
α = dimension-less flow coefficient, describes friction and geometry
ε = dimension-less expansion coefficient, describes the gas characteristic of the air
Π
Ad = × d 2 = opening of the nozzle
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Δp = p1 − p2 = differential pressure
kg
ρ1 = density of the air in 3
m
Flow coefficient, expansion coefficient, opening of nozzle and air density are combined into one
coefficient: the k-factor.
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The simplified formula to calculate the air volume is:
V! = k × Δp
with k = constant, V! in m3/h, Δp in Pa
“k” is assumed to be constant in order to simplify the formula, however, the flow coefficient and
expansion coefficient that influence the k-factor are not constant. They are influenced by pressures p1
and ∆p, velocity of the air and the conditions of the upstream and downstream flow (e.g. turbulences).
This means they are influenced by duty point and installation situation for the fan.
k-factors in ebm-papst catalogues are determined by tests with the fan attached to the air performance
test rig in a certain installation category with ideal inlet and outlet conditions and for typical air volumes
that the fan is used at.
Once the fan is installed in the customer unit, the installation situation can cause a change of the kfactor, especially if the air intake is very restricted. This can therefore lead to inaccuracy of the test
method. The new k-factor cannot be calculated. The only way to determine the new k-factor is by
attaching the complete unit to an air performance test rig which is in many cases not possible.
Therefore the k-factor in ebm-papst catalogues can be taken to determine the air volume and provides
sufficient accuracy in most cases.
Furthermore, the k-factor is based on an air density of ρ1 =1.15kg/m3. If the air density in the customer
unit is different, the k-factor can be adjusted by applying the following formula:
Knew = K × 1.15 / ρ new
b) Measurement points to determine differential pressure
Figure 5: Differential pressure measurement inlet ring
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The first pressure measurement point p1 for determination of the differential pressure needs to be
located in the plenum around the inlet ring. A point needs to be chosen where air velocity and
turbulences are minimal as they will influence the test result. The opening of the pressure tube needs to
be perpendicular to the air stream so that only static pressure and not the dynamic pressure component
is measured.
The second static pressure measurement point p2 for determination of the differential pressure is in the
inlet ring. This can be either just one pressure tap or a piezometer ring in the inlet ring. If the air flow is
fairly laminar, then one pressure tap is usually sufficient. In all other cases, especially when the air
intake is very restricted, a piezometer ring provides higher accuracy.
If there are no additional pressure losses at the air intake side, e.g. no ducts, filters or coils, then
ambient pressure can be used.
The differential pressure ∆p = p1 – p2 between plenum at air intake side and inlet ring is used to
calculate the air volume by using the previously mentioned formula:
V! = k × Δp
with k = constant, V! in m3/h, Δp in Pa
3.2 Static back pressure measurement
To determine the static back pressure of the fan a manometer needs to be used.
At the fan suction side, the pressure is negative compared to ambient. At the fan pressure side, the
pressure is positive compared to ambient. The static back pressure of the fan can be calculated by
using the following formula:
∆𝑝 = 𝑝! − 𝑝!
See also Figure 6: Pressure measurement points in application to differentiate between measurement
points for static back pressure of fan and for differential pressure measurement in inlet ring to
determine the air volume.
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Figure 6: Pressure measurement points in application
Similar to the pressure measurement as described in 0 3.1.2 Differential pressure method, it needs to
be ensured that the opening of the pressure tube is perpendicular to the air stream and that a point with
the lowest air velocity and turbulence is selected.
If this is not considered, it is very likely that not only static pressure but also the dynamic pressure
component is measured which falsifies the measurement result. If there is high turbulence at the
measurement point, fluctuating values can be measured with the manometer.
3.3 Determination of duty point
Chapter 0 3.1 Air flow measurement presents methods to determine the air volume that a fan delivers
have been discussed. Chapter 0 3.2 Static back pressure measurement shows how to measure the
static back pressure that a fan needs to overcome. Both figures combined give you the duty point of the
fan.
4
Conclusion
Using hot wire anemometer, vane anemometer or differential pressure method over inlet ring allows
determination of the air volume that a fan delivers. Depending on how the fan is installed in the
application, one of these methods might be favourable to the others. Measuring the pressure difference
between plenum on suction side and pressure side of the fan provides the static back pressure that the
fan needs to overcome.
When carrying out measurements with the methods described above, the tolerances can be quite high
depending on the installation situation, the number and position of measurement points and the
occurrence of turbulences. However, in most cases the described methods are accurate enough to get
a good understanding of the approximate duty point that a fan operates at.
ebm-papst A&NZ Pty Ltd
ABN 33 115 927 556
10 Oxford Road · Laverton North VIC 3026 · Phone +61 (03) 9360 6400 · Fax +61 (03) 9360 6464
NSW Phone +61 (02) 9827 6400 · NSW Fax +61 (02) 9827 6464 · NZ Phone +64 (09) 525 0245 · NZ Fax +64 (09) 525 0246
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