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.

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

Fig. 2: Fan performance and resistance curve

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.

The air volume that a fan delivers is influenced by air density and hence by altitude and air temperature.

Figure 3 (below) 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

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

Figure 4: Grid for air flow measurement

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).

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. 

As a last step, the air volume can be calculated by multiplying air velocity with the opening area.
V=air volume in m^3/h
v=air velocity in m/s
A=surface area in m^2

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

Figure 5: Formula to calculate air volume

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 formula as displayed in fig. 5 applies.

Flow coefficient, expansion coefficient, opening of nozzle and air density are combined into one coefficient: the k-factor.

Figure 6: Formula to calculate air volume

The simplified formula to calculate the air volume is displayed in fig. 6.

“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.

Fig. 7: Formula to calculate k factor

Once the fan is installed in the customer unit, the installation situation can cause a change of the k-factor, 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 p1=1.15kg/m3. If the air density in the customer unit is different, the k-factor can be adjusted by applying the formula shown in fig. 7.

Fan differential pressure measurement inlet ring
Figure 8: Differential pressure measurement inlet ring

b) Measurement points to determine differential pressure
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:

3.2 Static back pressure measurement

Figure 9: Pressure measurement points in application

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=p2-p1
See also Figure 9 to differentiate between measurement points for static back pressure of fan and for differential pressure measurement in inlet ring to determine the air volume.

Similar to the pressure measurement as described in 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 3.1 Air flow measurement presents methods to determine the air volume that a fan delivers have been discussed. Chapter 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.