The conditions usually given in the design of centrifugal fans include volumetric flow rate, total pressure, working medium and its density (or working medium temperature), and sometimes structural and special requirements.
The requirements for the design of centrifugal fans are mostly: the operating point that meets the required flow rate and pressure should be near the rate point; The rate value should be as high as possible, and the efficiency curve should be flat; The stable working range of the pressure curve should be wide; The fan has a simple structure and good processability; Convenient selection of materials and accessories; Has sufficient strength and rigidity, and can work safely; Stable operation and low noise; Good regulation performance and strong adaptability to work; The fan size should be as small as possible and lightweight; Easy to operate and maintain, easy to disassemble and transport.
However, it is generally impossible to meet all of the above requirements simultaneously. There is often a contradiction between aerodynamic performance and structure (strength, process), and it is usually necessary to focus on coordinating and resolving the main contradiction. This requires designers to choose a reasonable design plan to solve the main contradiction. For example, with different uses of fans, the requirements also vary. For example, fans used in public buildings are generally used for ventilation and air exchange, and the most important requirement is low noise. Multi blade centrifugal fans have this characteristic; Centrifugal fans that require high flow rates are usually of the double suction type; For some high-pressure centrifugal fans, the relative proportion of leakage loss is generally larger when the specific speed is lower.
The selection of several important schemes in the design of centrifugal fans:
(1) Reasonable selection of blade type: For common fans, at a certain speed, if the pressure coefficient of the backward impeller is smaller, then the impeller diameter is larger and its efficiency is higher; The opposite is true for the forward impeller.
(2) Selection of fan transmission mode: If there are three transmission modes A, D, and F, the fan speed and motor speed are the same; B, C, and E are all variable speed, and the fan speed can be flexibly selected during design. Generally, small fans are widely driven by direct connection with the electric motor through transmission A, while for large fans, sometimes belt transmission is not suitable and transmission methods D and F are often used.
Under high temperature and dusty conditions, the transmission method also needs to consider the protection and cooling of the motor and bearings.
(3) The selection of the external dimensions of the snail shell: The external dimensions of the snail shell should be as small as possible. For high specific speed fans, a shortened snail shape can be used, while for low specific speed fans, a standard snail shape is generally selected. Sometimes, in order to reduce the size of the snail shell, a scheme can be adopted where the outlet velocity of the snail shell is greater than the inlet velocity of the fan. In this case, an outlet diffuser is used to increase its static pressure value.
(4) Selection of blade outlet angle: The blade outlet angle is one of the main geometric parameters that must be selected first during design. For ease of application, we classify blades into: strongly curved blades (water pump type), curved blades, straight blades, and wing shaped blades; Radial outlet blades, radial straight blades; Forward curved blades, strongly forward curved blades (multi wing blades). Table 1 lists the approximate range of outlet angles for these blade types in centrifugal fans.
(5) Selection of blade number: In centrifugal fans, increasing the number of blades in the impeller can increase the theoretical pressure of the impeller, as it can reduce the influence of relative eddies (i.e. increase the K value). However, an increase in the number of blades will increase the friction loss in the impeller channel, which will reduce the actual pressure of the fan and increase energy consumption. Therefore, there is an optimal number of blades for each type of impeller. The specific number of blades to be determined sometimes depends on the designer's experience. According to the current application situation in China, the recommended range of blade numbers is shown in Table 2.
(6) The selection of the total pressure coefficient PSI t: When designing a centrifugal fan, the actual pressure is always predetermined. At this point, it is necessary to choose the total pressure coefficient. 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 main dimensions of the impeller are shown in Figure 1. The impeller is the component that transfers energy from the fan to the gas, so its design has a significant impact on the fan; The correct determination of the main structure of the impeller plays a crucial role in the performance parameters of the fan. It includes the key technology of centrifugal fan design - blade design. The key step in blade design is how to determine the blade outlet angle β 2A.
Design analysis of key technologies
When designing a centrifugal fan, the key is to master the determination of the outlet angle β 2A of the impeller blades.
According to the different exit angles β 2A of the blades, they can be divided into three types: backward curved blades (β 2A<90 ℃), radial exit blades (β 2A=90 ℃), and forward curved blades (β 2A>90 ℃).
Three types of blade impellers are currently used in fan design. The characteristics of front curved blade impellers are small size and weight, low price, and rear curved blade impellers can improve efficiency and save energy. Therefore, in modern production of fans, especially large power fans, rear curved blades are mostly used.
Modern front curved blade fans have significantly improved efficiency compared to older products, so they are still widely used in low flow and high pressure situations or low pressure and high flow situations.
Radial outlet blades are no longer commonly used in China. In some fans that require wear and corrosion resistance, radial outlet straight blades are commonly used.
When designing the impeller of a centrifugal fan, it is also necessary to consider the relationship between the specific speed and the blade type (e.g. Table 4). Therefore, when determining the blade outlet angle, it is necessary to comprehensively consider the effects of the three blade types on pressure, radial size, and efficiency, and then determine them based on Table 1 and Table 4.
Correctly determining the outlet angle β 2A of the centrifugal fan impeller blades will lay a solid foundation for determining other major geometric dimensions of the impeller, thus playing a key role in the performance of the entire centrifugal fan.