Where to place a fan along a duct?

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Discussion Overview

The discussion centers on the placement of a fan within a circular horizontal duct and its impact on airflow and operating conditions. Participants explore factors such as duct length, friction, pressure differentials, and specific applications involving leaks and particulate transport.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the fan's position affects its operating point or if it is solely determined by the total duct length and friction.
  • Another participant inquires about the pressure differential (ΔP) values relevant to the scenario.
  • A participant provides typical ΔP values of 1-2 Pa/m based on their experience.
  • One response suggests that, under certain conditions, the fan's position may not matter, but hints at the possibility of other influencing factors.
  • A later reply discusses the implications of duct leaks, emphasizing that the fan's placement must consider the direction of pressure to manage leakage effectively, particularly in specific applications like aircraft cooling systems and particulate transport.

Areas of Agreement / Disagreement

Participants express differing views on the significance of fan placement, with some suggesting it may not matter under certain conditions, while others highlight specific scenarios where placement is crucial. The discussion remains unresolved regarding the general applicability of these claims.

Contextual Notes

Participants reference specific applications and conditions that may affect the fan's placement, indicating that assumptions about duct integrity and operational context are critical to the discussion.

TSN79
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Picture a circular horizontal piece of duct, open at each end. If a fan is to be placed somewhere along the duct and move a certain amount of air, does it make a difference for the fan and it's operating point where it is positioned? Or does it only come down to the total length of duct and the friction it creates?
 
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What are the ΔP values?
 
Bystander said:
What are the ΔP values?
It's just a pictured scenario, but typically 1-2 Pa/m in the situations I deal with.
 
For what you described, no, it does not matter. If there are other factors you aren't telling us about though, it might.
 
If there are any leaks in the duct you will want to arrange the pressure inside so that the leakage is in an acceptable direction. For example in the duct-work for an airplane shore cooling system the fan must be at the inlet (positive pressure) even if rigid ducting were used as the leaked ambient air is polluted by engine exhaust and is also the wrong temperature.

If the duct is used for transporting particulates then the fan must be at the outlet (negative pressure) otherwise the transported materiel will be sprayed out the leak.

BoB
 

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