Fan selection issue
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Regarding fan selection, there are some issues I haven’t been able to figure out; I would appreciate some guidance from those with more experience. Here is my understanding: 1. The resistance between the air duct inlet and the fan inlet is overcome by the negative pressure at the fan inlet； 2 The resistance between the fan outlet and the duct outlet is overcome by the positive pressure at the fan outlet ； 3 The total pressure of a fan is equal to the sum of the absolute values of the positive pressure at the fan’s outlet and the negative pressure at its inlet. But what is the relationship between the negative pressure at the fan inlet and the positive pressure at the outlet? For supply fans: the inlet is connected to the atmosphere, and the duct resistance is overcome by the positive pressure at the fan outlet. For exhaust fans with an outlet connected to the atmosphere: the duct resistance is overcome by the negative pressure at the fan inlet. In both cases, should the fan be selected based on its total pressure, or only based on the pressures at the inlet and outlet? I would be extremely grateful for the guidance from those with more experience!
Your understanding is correct... but it looks a bit strange :) . Overcoming this absolute value and that one – it’s confusing to me... Based on the air volume required by the system, the pressure drop in the inlet and outlet piping systems (including fittings, valves, etc., based on the actual possible piping layout) is calculated; by taking into account an appropriate safety factor, the total pressure is obtained. The fan is selected based on the total pressure and air volume. It follows the same principle as the calculation for pumps.
I’d really like to know too; if anyone knows, could they explain it? Thank you!
What the original poster said makes a lot of sense; I’ve learned something from it. Is there any teacher who can add more?
As for this, it has now been understood that the static pressure of the fan is always equal to atmospheric pressure plus the duct resistance.
Learning* it is not bad, but our factory doesn’t use fans:P
For information on fans, please refer to AMCA201 Fan and System; for fluid distribution, refer to SMACNA 1958-2006
I think the total pressure you understand refers to the rated pressure of the fan, while the flow rate and pressure of the fan are determined by the load. The pressure and airflow of the fan itself are matters determined by the manufacturer during design; we simply use the fan without needing to worry about such details.