The overall size of the volute with an unpowered fan should be as small as possible.
带有无动力风机的蜗壳整体尺寸应尽可能小。
For high specific speed fans, a shortened worm shape can be used, while for low specific speed fans, a standard worm shape is usually used.
对于高比转速风扇，可以使用缩短的蜗杆形状，而对于低比转速风扇，通常使用标准的蜗杆形状。
Sometimes in order to reduce the size of the volute, you can choose a solution where the volute outlet speed is higher than the fan inlet speed.
有时为了减小蜗壳的尺寸，可以选择蜗壳出口速度高于风机入口速度的方案。
The outlet diffuser is used to increase its static pressure value.
此时使用出口扩压器来提高其静压值。
2, in addition, in unpowered fans, increasing the number of blades of the impeller can increase the theoretical pressure of the impeller, because it can reduce the influence of the relative eddy current (that is, increase the value of k).
2、另外，在无动力风机中，增加叶轮的叶片数可以增加叶轮的理论压力，因为这样可以减少相对涡流的影响(即增加k的值)。
However, the increase of the number of blades will increase the friction loss of the impeller channel, reduce the actual pressure of the fan, and increase the energy consumption.
但叶片数量的增加会增加叶轮通道的摩擦损失，降低风机的实际压力，增加能耗。
Therefore, each impeller has an optimal number of blades.
因此，每个叶轮都有一个最佳叶片数。
The exact number of blades sometimes depends on the experience of the designer.
叶片的确切数量有时取决于设计者的经验。