With a global drive towards advances in energy efficiency and reliability, it is important to remain focused on continual improvements in all components within the system.

Not only is improved energy efficiency beneficial to the environment through a reduction in power related carbon emissions, it also benefits the pocket of the end-user through lower equipment running costs.

ebm-papst – the world leading manufacturer of OEM fan solutions – are at the forefront of fan motor and impeller design, and constantly innovating to enhance and develop fan technologies for the next generation of products.

Backward-curved fans

ebm-papst manufacture a wide range of fan products, including axial, forward-curved, and compact fans, but one of the most popular fan types used is the backward-curved centrifugal fan.

Backward-curved centrifugal fans are most suitable for medium to high pressure applications such as Computer Room Air Conditioning (CRAC) units, Air Handling Units (AHUs) and ducted systems. Thanks to their efficient direct drive and integrated variable speed control capability, they have also become a popular choice for building services upgrade solutions, and are regularly used in Data Centre and AHU upgrade projects.

The Importance of Inlet Rings

It is often overlooked, but with backward-curved centrifugal fans it is important to install an inlet ring with the impeller. The inlet ring guides the air into the impeller, ensuring smooth laminar flow as the air enters the impeller.

It is a simple yet invaluable component which maximises the performance of the fans, and an incorrectly installed inlet ring risks an increase in noise and a loss in performance.

To overcome this, ebm-papst produces a range of factory-built fan modules for customers. These modules ensure that the fan and inlet ring are perfectly aligned within a compact mounting frame. The fan modules also greatly simplify the installation of these fans within applications. If however you wish to provide your own mounting method, we would like to draw your attention to the following guide.

Installation and Performance

The correct installation of inlet rings is key. It is vital that they are installed with the correct alignment and fit to ensure we achieve the most efficient performance.

The inlet ring should be positioned concentrically and there should be a small overlap of the inlet ring into the inlet of the fan.

The following diagrams and graphs show the effect that different inlet ring overlaps and air gaps can have on the performance.

 Effects of changing the overlap

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To ensure the most efficient performance of a backward-curved centrifugal fan the inlet ring overlap needs to be correct. The change in the overlap affects the air performance and the efficiency. As shown on the graph, no overlap or a gap between the impeller and inlet ring is not advised as this provides a lower efficiency. If there is no overlap but no gap, this shows an improvement in both air performance and efficiency. The best installation, and advised installation, is to have an overlap. As seen, this provides the best performance.

As an example, if the impeller diameter was 485mm, and the overlap, as advised in the catalogue, is 10mm then this gives a 2% overlap. The method to calculate this is X/D = Overlap%.

Effects of changing the air gap

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The change in the air gap also affects the air performance and the efficiency. The ideal installation is to have the smallest gap possible but with no interference between inlet ring and impeller. As the graph shows the larger the gap, the worse the performance is.

As an example;

Impeller outer diameter (D) = 716mm

Inside diameter of the impeller = 446mm

Inlet ring diameter = 438.8mm

Air gap (S) is (446-438.8)/2 = 3.7mm

3.7/716 = 0.5% air gap

If the air gap is larger, for example 4.7mm, then the air gap % would be 0.7%, demonstrating that the higher the percentage the worse the performance.

Full Assembly

As mentioned, ebm-papst offer a simple solution which can save us a lot of time and hassle when it comes to inlet rings; in fact it practically takes this whole headache away! To ensure perfect installation, ebm-papst offer their backward-curved fans pre-installed in factory set frames; ensuring that the fan and inlet ring are perfectly aligned.

The RadiPac and RadiCal ranges come with the option of the fully assembled module.

Constant Volume

Inlet rings are an important part of the installation when it comes to backward curved centrifugal fans, but did you know you can provide constant volume by using an inlet ring?

If you’re looking to provide constant volume in a system where the system resistance may change (i.e. AHU where filters can become partially blocked over time) you can install an inlet ring with a pressure tapping.

The inlet rings with the pressure sensors work by comparing the static pressure upstream of the inlet ring with the static pressure within the inlet ring. The airflow can then be calculated using the equation;

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The K factors are calculated for each of our inlet rings. Below are the K factors for our RadiPac inlet rings.

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Do you have inlet rings installed with your backward curved fans? Are your inlet rings installed correctly? These are questions you can now answer.

Now you are an expert in inlet rings you can be sure you are achieving the best performance from your fans. However, if you have any questions, we have not answered yet or if you are interested in making your life easier with the full assembly option then do not hesitate to get in touch! 

You can email me at



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