What is the purpose of distinguishing between dynamic pressure and static pressure in fans? !

The explanation on Baidu is very detailed. 【What is the difference between static pressure and dynamic pressure of a fan? 1. Static pressure of a gas: The pressure Pst exerted by a gas on a surface that is parallel to the direction of the gas flow. 2. Dynamic pressure of a gas: The pressure increase that occurs when the energy associated with the gas’s flow velocity C (m/s) is converted into pressure without any loss. Pd = ρ * C * C / 2, where ρ is the density of the gas flow, in kg/m^3. 3. Total pressure P of a gas: The sum of the static pressure and dynamic pressure of the gas at the same location. P = Pst + Pd. 4. Total pressure of a fan: This refers to the difference in total pressure between the outlet and inlet sections of the fan. Static pressure: The pressure generated as a result of air molecules colliding with the pipe walls due to their random movements.】 The method of measurement is as follows: a small hole is made in the wall of the fluid pipeline, and a pressure gauge tube is connected to it; the pressure gauge tube is perpendicular to the horizontal plane. The height of the liquid column in the gauge tube represents the pressure at that point in the pipeline relative to the atmosphere, that is, the gauge pressure. Dynamic pressure: Dynamic pressure is the pressure generated due to the movement of a fluid, and its value is never less than zero. The calculation method is ρν²/2, where ρ is the fluid density and ν is the fluid velocity. When it comes to ventilation equipment, the term \"static pressure\" is not a scientifically accurate expression; however, it becomes acceptable once one gets used to it. Static pressure and residual pressure refer to the same physical quantity. Static pressure is the pressure measured when the fan is turned on and the outlet is closed (at this point, there is no dynamic pressure), and it is equal to the total pressure. Residual pressure refers to the total pressure remaining after accounting for the resistance caused by components other than the fan, such as coils and filters; this value helps in selecting the appropriate piping. Regarding the connections of fan-coil units, the pipe resistance is not high (not exceeding 1 Pa/m); it is sufficient to consider only the local resistance at the supply and return air outlets. Static pressure refers to the static pressure measured when the fan is turned on and the outlet is closed (with no dynamic pressure at this time). Dynamic pressure refers to the pressure generated by air flow when the outlet is opened; dynamic pressure = 0.5*q*v2 = 0.5*air density*wind speed squared ； In engineering, wind speed is usually designed as a constant value; therefore, the dynamic pressure remains constant. Thus, what is needed to overcome the resistance in the pipelines is actually static pressure. That’s why reputable manufacturers generally refer to static pressure rather than outlet pressure. Ducts and pipes are not the same. In fluid mechanics, air is a compressible fluid while water is an incompressible fluid; the models established in fluid mechanics theory are based on different principles. \"Static pressure is the pressure energy exerted on the wall surfaces due to molecular motion, and its value remains constant at all points within the flow field.\" ； Dynamic pressure is the pressure energy resulting from fluid momentum, and it exists only in the direction facing the incoming flow. These are a pair of theoretical categories. Total pressure is the sum of static pressure and dynamic pressure, reflecting the level of the fluid’s capacity to do work. During fluid flow, after accounting for frictional losses, static pressure and dynamic pressure convert into one another. It is not immutable. ” Most people understand that total pressure equals dynamic pressure plus static pressure, but they don’t have a very deep understanding of the fact that static pressure and dynamic pressure can transform into each other. Total pressure = dynamic pressure + static pressure. In other words, once the fan is selected, its total pressure can be considered a constant value (although it is also affected by factors such as the power supply voltage). However, due to the resistance at various points in the system, both local and along the flow path, the total pressure at those different points varies. Regarding kinetic energy, many people have looked up information in textbooks, and what they said is correct! However, there are differing opinions on how the static pressure is determined. Hehe, actually, it’s very simple. Since total pressure = dynamic pressure + static pressure, then doesn’t that mean static pressure = total pressure – dynamic pressure? ? ? Some people might think it’s incorrect to compare love to the hydrostatic pressure of water, and indeed the formula used is not wrong; however, there is a significant difference between air and water. Although in most practical engineering applications, air can be considered incompressible. By comparing the dynamic viscosity values of air and water, one can see how large the difference between them is! ! ! Can the kinetic energy of air molecules be compared to that of water? Water flows downhill, but what about air? Its mode of movement is even more complex, and the complexity lies in the fact that \"static pressure and dynamic pressure convert into each other\", with this conversion occurring almost all the time. In other words, static pressure is difficult to measure, and it is also measured very rarely. But dynamic pressure is easy to measure, haha; therefore, static pressure = total pressure – dynamic pressure. Speaking of residual pressure, conceptually, it refers to the pressure that remains. The excess pressure outside the machine is the pressure that remains after accounting for the losses within the fan itself. Residual pressure is a relative value; in other words, it changes within the system, and the residual pressure at each point is different from one another. Its calculation formula is: residual pressure = total pressure – pressure loss (local and frictional). For example: if a pipe section is divided into two parts, labeled A (start) – B – C (end), then the residual pressure at point B becomes the total pressure of the B-C section. The same principle applies to the external pressure: External pressure = Total pressure of the fan – Pressure losses within the fan (local and along the flow path). Of course, if the fan and the unit (together with accessories such as filters and surface coolers) are considered together, this is what was referred to earlier as the \"excess pressure of the air conditioning unit\". In short, in a sense, the residual pressure = total pressure (with respect to the remaining system) = dynamic pressure + static pressure. 】

Thank you, expert, for your answer. . I have a question for the experts: if a fan has three identical air outlets, could there be a significant difference in the air volume delivered by them?

By adjusting the resistance of the three air inlets to be the same, it becomes uniform.