As a commonly used ventilation equipment in workshops, the main principle is to increase the pressure of indoor gas, and then send it out of the room through pipes. It is a kind of fluid machinery, which leads to whether the centrifugal fan is selected. , or when designing a ventilation system, it is inseparable from the calculation and measurement of the wind pressure of the fan. The commonly used pressure parameter is the total pressure. What does the total pressure of the centrifugal fan mean? What does it have to do with static and dynamic pressure?
1. The full pressure of the centrifugal fan
The total pressure of the centrifugal fan is actually the algebraic sum of the static pressure and the dynamic pressure. It is the comprehensive reflection of the macroscopic flow of the fluid and the thermal motion of the molecules. It represents the total energy of the unit gas. Generally, it reflects the working power of the centrifugal fan. The dynamic pressure and static pressure of the fan are both relative occasions and have specific conditions.
In addition, when selecting a centrifugal fan, there is also a parameter called residual pressure outside the machine. This parameter mainly refers to the total pressure that the unit itself can provide after the pressure loss. It can also be understood as the total pressure outside the machine. It is not It is a standard concept, but it should also be considered in the full selection of centrifugal fans, otherwise the centrifugal fan pressure in actual use may be insufficient.
2. Static pressure and dynamic pressure of centrifugal fan
1. Static pressure of centrifugal fan
The static pressure is the pressure energy on the wall of the centrifugal fan pipe due to the molecular motion force, and the size of each point in the flow field is the same; It is still static, and its molecules are always in thermal motion. The existence of static pressure energy only depends on the thermal motion of the molecules, and has nothing to do with the macroscopic flow or not. In other words, whether it is a static or flowing fluid, there is an inherent static pressure generated by the thermal motion of its molecules.
2. Dynamic pressure of centrifugal fan
Dynamic pressure refers to the pressure generated when the air flows. As long as the air flows in the air duct of the centrifugal fan, there will be a certain dynamic pressure. Dynamic pressure is actually the energy generated by the macroscopic flow of the fluid. Therefore, if there is no macroscopic flow of the fluid, there will be no dynamic pressure. The centrifugal fan has high wind speed, high dynamic pressure, and relatively low static pressure. In the process of fluid flow, after deducting resistance loss, static pressure and dynamic pressure will transform each other, which is not constant.
3. The relationship between total pressure and static pressure and dynamic pressure
(1) The static pressure of the centrifugal fan only exists in the inlet and outlet pipe network and must be equal to the resistance of the pipe network. The relative static pressure of the fan at the outlet at both ends of the pipe network is equal to “zero”.
(2) The dynamic pressure of the centrifugal fan is not consumed in the inlet and outlet pipe network. When the cross-sectional area of the inlet and outlet pipe network is the same, the dynamic pressure at the inlet and outlet of the fan is equal. If the cross-sectional area of the pipe network is different, the dynamic pressure in the pipe network is also different.
(3) In the inlet and outlet pipe network of the centrifugal fan, the total pressure is rising and falling, the static pressure is all consumed in the resistance of the pipe network, and the total pressure at both ends of the pipe is equal to the dynamic pressure.
(4) When the centrifugal fan only has an inlet pipe and no outlet pipe, the static pressure value measured at the measuring point at the inlet flange is the inlet static pressure of the fan. When the inlet and outlet areas of the fan are equal, this static pressure is equal to the full pressure of the fan.
(5) When calculating the static pressure of the centrifugal fan, it is necessary to subtract the dynamic pressure contained in the static pressure from the static pressure. That is to say, the value of the inlet static pressure of the fan is greater than the static pressure of the fan.
