Informatie
March 23, 2023

# Determining the efficiency of a centrifugal fan.

As you know, there are specific fans available for almost all applications. In this information article, we are going to give you an example of centrifugal fan efficiency. If you want to know more about the operating point or efficiency of your fan, contact us at sales@slingerlandtechniek.nl

## Centrifugal fans for drying systems

This week while prospecting, I phoned a potential customer who supplies manure drying systems to poultry farms.

From the customer’s story, it was clear that in autumn and winter, it is very important to dry manure properly because dry manure means less weight, so less cost. They dry this manure using centrifugal fans. These are used to blow in air. After our conversation, the customer was actually immediately interested to get acquainted because the centrifugal fan was an important part of his drying system.

After the pleasant conversation, I sent him an email with a link of our centrifugal fans. The customer then started searching our selection tables himself and sent me the following message not much later;

## Email communication

Hi Steven,

Scrolling through the sheets, I don’t actually come across a 7.5KW centrifugal fan that gives 15,000m3h at 2500 pascal pressure.

Do you know which one I should have then?

## Response

Hello Sir,

I have briefly studied your application, but we think this is indeed impossible. If anyone else can supply these fans, then I am very curious about the fan curves. Because it seems that these fans generate energy….

To determine the efficiency of a centrifugal fan, we need the following:
Absorbed power is calculated using the following formula:
Air volume x total pressure : 3600 : (efficiency x 10) = power consumption

If we fill this in with the air and pressure data you provide and with the assumption that this fan has 87% efficiency, we arrive at the following formula:

15,000 m3/h x 2500 Pa : 3600 : 870 = 11.97 kW….

So a fan with 87% efficiency has a power consumption of 11.97kW. This means it will never achieve this with a 7.5kW motor.

If we fill in the same formula with 110% efficiency (this is impossible as the machine would generate energy), we arrive at the following data:

15,000 m3/h x 2500 Pa : 3600 : 1100 = 9.46 kW….

So even with this efficiency, the fan does not make it below 7.5kW.

With this explanation, the customer was naturally willing to send the curves of the colleagues.

## Conclusion

After studying the curves above, we saw that the numbers were slightly different. From the curve, you can see that the fan can move 12,000 m3/h of air at 1900 pascal pressure.

12,000 m3/h x 1900 Pa :3600 : 870 = 7.2kW

This is already a pretty efficient fan. But we still don’t actually know whether they are talking about dynamic pressure or total pressure. So, very curious, we googled some more and finally found drawings of the fan. This fan has an inlet of 285mm.

If 12,000 m3/h is sucked through this inlet, you have a speed of 55 metres per second and a dynamic pressure loss of 1800 Pa, so there is almost nothing left for the system. So it is very likely that this fan has its operating point somewhere to the left of the curve. So moving less air and building up more pressure.

Since even this data did not really match what the customer wanted, they gave us permission to measure the fan through.

We took measurements and created the actual fan curve, see tables below (the blue dotted line is what the colleagues claim the fan does, and the red line is what we measured).

After these measurements, we were able to show that the fan does not do what the supplier promises, and so we have now supplied alternative ones with 5.5kW motor power. These are going to give the same result.