Some time ago, guide vanes were commonly supplied with axial fans in order to improve fan pressure development. However, low energy costs and a lack of focus on energy efficiency resulted in guide vanes being removed from axial fans, to reduce their cost. Today’s focus and increased emphasis on energy efficiency (through the revisions of the BCA) has made it again more commercially viable to use guide vanes with axial fans.
What does a guide vane do?
When air comes off an axial fan, it travels in a rotating (swirling) pattern. A swirling air flow consists of both a rotating velocity component, and an axial velocity component. The ratio of rotating velocity to axial velocity increases with blade pitch angle.
In simplistic terms, a discharge guide vane is used to straighten the fan discharge air pattern - therefore reducing the swirling velocity component.
As the amount of axial fan rotational velocity is dependent on the pitch angle of the blade, the benefit of using guide vanes becomes more pronounced at higher blade angles. Guide vanes may also be used on different types of fans (such as mixed flow and centrifugal fans), but these fans have a lower rotational velocity component and therefore, the beneficial effect of a guide vane is reduced on these type of products.
When testing a ducted inlet and ducted outlet fan to ISO5801, a star straightener is included in the discharge duct section that simulates the conditions in a normal duct run.
The star straightener removes the swirl from the airstream to:
- Allow accurate pressure measurement of fan discharge pressure.
- Allow the duct designer to use standard fitting losses for pressure drop calculations. This is important as fitting losses are mostly tested with non-swirling air flow patterns.
The net effect of adding a guide vane to a fan is that the reduction in swirl provides a static pressure increase when testing the fan to ISO5801.
The other benefit of using guide vanes is that the fan’s power consumption remains unchanged.
If we increase the pressure development (p f), and the power consumption remains the same (P r), the fan efficiency will improve. An example of this is as follows:
|
Axial Fan |
Axial Fan with Guide Vane |
| q v Air flow (m³/s) |
1.72 |
1.72 |
| Static Pressure (Pa) |
144 |
186 |
| Fan Diameter (mm) |
500 |
500 |
| Velocity Pressure (Pa) |
46 |
46 |
| p f Total Pressure (Pa) |
190 |
232 |
| P r Impeller Power (kW) |
0.53 |
0.53 |
| Impeller Total Efficiency (%) |
61.7% |
75.3% |
On-site pressure measurements.
ISO5801 “Industrial fans – performance testing using standardized airways” is the international standard for testing the performance of fans. If all fan manufacturers correctly test to this standard, it means the results are comparable to a high degree of accuracy.
However pressure measurements on-site rarely follow the conventions of ISO5801 and as a result produce values that are not aligned to tests performed to ISO5801. Some common errors include:
- The air flow must be straightened before measuring fan discharge pressure on site. It is impossible to accurately measure discharge pressure where significant swirl is present.
- A ducted fan performance tested to ISO5801 includes pressure losses due to the star straightener. A normal duct system test does not include a star straightener. Therefore the on-site test result will be different to the ISO5801 test result.
- Fan static pressure is commonly measured by subtracting the inlet static pressure from the outlet static pressure. This is incorrect. The correct measurement of fan static pressure is: P s(outlet) - P s(inlet) - P v(inlet)
This is a very common error and will often suggest a fan is performing correctly as the velocity pressure has not been subtracted.
The above points will both increase and decrease the measured static pressure on site by various amounts depending on the impact of each error. The reality is that they are masking the system effects on the fan, and this topic will be covered in an upcoming Technically Speaking article.
Note: Guide vanes may be used on the inlet or discharge of a fan. When guide vanes are placed upstream (inlet) of a fan, they will increase the power consumption and noise level of a fan. We have only considered the use of downstream (discharge) guide vanes in this technical article.
With the increased efficiency requirements of BCA2019, the use of guide vanes may increase the choice of compliant fan selections. By reducing the swirl, guide vanes will also help the site measured pressure of an installed fan (or discharge fitting) to better align with the catalogued data.
