In the selection and design process of centrifugal fans, many fan parameters are often referred to, such as wind pressure, air volume, efficiency, etc., and there is also a parameter that reflects the basic performance of the fan, that is, the specific speed, the ratio of the fan The number of revolutions is the classification basis and design basis of the fan, so it is generally not common when purchasing a fan, but it is very important. What does the specific revolution of the centrifugal fan mean?

## Definition of centrifugal fan specific speed

The specific speed of the centrifugal fan is an important parameter used to judge whether the fans are similar. It is the comprehensive characteristic number between the performance parameters of similar fans, also known as the specific speed. Fans with a large number of revolutions may have a low speed, and fans with a smaller number of revolutions may have a high speed. The specific speed can reflect the characteristics of the centrifugal fan more comprehensively, and integrates the relationship between the three parameters of the fan flow, total pressure and speed:
The larger the number of revolutions, the lower the wind pressure of the centrifugal fan under the same flow;
The smaller the number of revolutions, the higher the wind pressure of the centrifugal fan at the same flow rate.

## Centrifugal fan specific speed calculation formula

The formula of the specific revolution is derived from the similarity law of the fan. After converting and rewriting it into a constant, it is the specific revolution. Use ns means, ns is calculated as follows:

In the formula:
n is the speed of the fan;
Q is the flow rate of the fan;
H20 is the full pressure of the gas in the standard state.

## The role of centrifugal fan specific speed

### 1. The basis of classification

The size of the specific revolution is closely related to the shape and performance curve of the impeller. Different specific revolutions represent the structure and characteristics of different fans:
(1) The smaller the flow rate, the higher the pressure, the smaller the specific revolution, so the specific revolution of the centrifugal fan is relatively small. For the centrifugal fan, if the specific revolution is smaller, the wind pressure needs to be larger, and the impeller The larger the outer diameter;
(2) The larger the flow rate, the lower the pressure, and the higher the specific speed. Therefore, the specific speed of the axial flow fan is relatively large. For the axial flow fan, if the specific speed is larger, the flow rate of the fan needs to be larger. , the larger the outer diameter of the impeller.

### 2. Basis for compiling the series

When compiling the performance curve series of centrifugal fans, if the series of fans are compiled based on the specific speed, the number of fan models can be greatly reduced, and a lot of manpower and material resources can be saved.

### 3. Design basis

The impeller can be divided into high pressure type and low pressure type according to the size of the specific revolution. The smaller the specific revolution is, the high pressure type is suitable for centrifugal fans with small flow; Centrifugal fan. Therefore, the specific revolution is very suitable as a measure of the shape of the impeller, and the model design is selected based on the specific revolution in the design.