Centrifugal fan design usually involves conditions such as volumetric flow rate, total pressure, working medium and its density (or the temperature of the working medium), structural requirements, and special requirements.

Most of the requirements for centrifugal fan design are as follows: The operating point that meets the required flow rate and pressure should be near the high-efficiency point; the efficiency value should be as large as possible, and the efficiency curve should be flat; the stable operating range of the pressure curve should be wide; the fan structure should be simple and have good processability; the selection of materials and accessories should be convenient; it should have sufficient strength, high rigidity, and operate safely and reliably; it should operate stably with low noise; it should have good adjustment performance and strong working adaptability; the fan size should be as small as possible and the weight should be light; and it should be easy to operate, maintain, disassemble, and transport.

There is often a contradiction between aerodynamic performance and structure (strength and process), and it is usually necessary to coordinate and solve the main contradiction. This requires designers to select a reasonable design scheme to address the main contradiction. For example, due to different applications of fans, the requirements also vary. For instance, fans used in public buildings are generally for ventilation purposes. Generally speaking, a crucial requirement is low noise, and multi-wing centrifugal fans have this characteristic; centrifugal fans with a large flow rate are generally double-suction fans; for some high-pressure centrifugal fans, the relative proportion of leakage loss is usually large.

The selection of several important schemes in centrifugal fan design:

(1) Reasonable selection of blade type: For common fans at a certain speed, the pressure coefficient Ψt of the backward impeller is relatively small, the impeller diameter is relatively large, and the efficiency is relatively high; the situation of the forward impeller is the opposite.

(2) Selection of fan drive mode: For example, in drive modes A, D, and F, the fan speed is the same as the motor speed; while B, C, and E are all transmission types with variable speed, and the fan speed can be flexibly selected during the design. Generally, for small fans, the drive mode A, which is directly connected to the motor, is widely used. For large fans, sometimes the belt drive is not suitable, and drive modes D and F are more commonly used. Under high-temperature and dusty conditions, the drive mode should also take into account the protection and cooling of the motor and bearings.

(3) Selection of the shape and size of the volute: The shape and size of the volute should be as small as possible. For fans with a high specific speed, a shortened volute shape can be adopted, and for fans with a low specific speed, a standard volute shape is generally used. Sometimes, in order to reduce the size of the volute, a scheme where the volute outlet speed is greater than the fan inlet speed can be selected. In this case, an outlet diffuser is used to increase its static pressure value.

(4) Selection of the blade outlet angle: The blade outlet angle is one of the main geometric parameters that need to be selected first in the design. For the convenience of application, we divide the blades into: strongly backward curved blades (pump type), backward curved arc blades, backward straight blades, backward airfoil blades; radial outlet blades, radial straight blades; forward curved blades, strongly forward curved blades (multi-wing blades). Table 1 lists the approximate ranges of the blade outlet angles of these blade types in centrifugal fans.

(5) Selection of the number of blades: In a centrifugal fan, increasing the number of blades of the impeller can increase the theoretical pressure of the impeller because it can reduce the influence of relative eddy currents (that is, increase the K value). However, increasing the number of blades will increase the friction loss in the impeller passages, reduce the actual pressure of the fan, and increase energy consumption. Therefore, each impeller has an appropriate number of blades. Sometimes, it is necessary to determine the number of blades according to the designer’s experience. According to the current application situation in China, Table 2 recommends the selection range of the number of blades.

(6) Selection of the total pressure coefficient Ψt: When designing a centrifugal fan, the actual pressure is always given in advance. At this time, it is necessary to select the total pressure coefficient Ψt, and the approximate selection range of the total pressure coefficient can refer to Table 3.

(7) Determination of the main geometric dimensions of the inlet and outlet of the centrifugal impeller: The impeller is the only component of the fan that transfers energy to the gas, so its design has a great impact on the fan; whether the main structure of the impeller can be correctly determined plays a key role in the performance parameters of the fan. It includes the key technology in centrifugal fan design – blade design. The key link in blade design is how to determine the blade outlet angle β2A.