Blocked pipe = increased air velocity?

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Discussion Overview

The discussion revolves around the relationship between air velocity and vacuum in a blocked pipe scenario, specifically in the context of an air flow station. Participants explore the implications of blocking the intake on air velocity and vacuum measurements, seeking to understand the underlying principles and calculations involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that blocking the intake of their air flow station leads to an increase in both vacuum and air velocity, questioning the expected behavior of airflow in this scenario.
  • Another participant asks for clarification on whether the inlet is partially or completely blocked and inquires about the locations of velocity and vacuum measurements.
  • A participant describes their setup, indicating that measurements increase with the degree of blockage, and expresses a desire for a conversion factor to account for the increased vacuum readings in their calculations.
  • One participant suggests that the velocity calculation should incorporate velocity pressure, defined as total pressure minus static pressure.
  • A participant queries the type of velocity measurement instrument being used, indicating a need for more information on the measurement methods employed.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus, as there are varying interpretations of the measurements and calculations involved, as well as differing opinions on the factors that should be considered in the analysis.

Contextual Notes

There are limitations regarding the assumptions made about the flow characteristics, the definitions of pressure types, and the specific measurement techniques used, which may affect the interpretations of the results.

radonbc
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Blocking the intake of an air flow station I built results in the FPM shooting up along with the vacuum. ie: wide open both ends 4" pipe with inline fan reads . 5" wc / -190 pa / 3700 fpm on instruments monitoring intake pipe. Blocking the intake increases both the vacuum to -470 pa and increases the velocity to 5400 FPM.
MY QUESTION: Is their a conversion factor to plug in at the beginning of a CFM equation to compensate for vacuum on the intake side. ie: .0873(4"pipe) x 3700 fpm = 323 cfm This answer seems like it should be smaller using the same equation with blocked intake but it actually increases unless some allowance for the blocked intake is factored. ?
 
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Are you talking about a partially or a completely blocked inlet; and, how and where are you measuring your velocity and where are you measuring your vacuum?
 
The flow station was between the damper/closure and the fan on the intake side. The measurements increase as the rate of blockage increases from partial to near full. ie: increasing damper angle, then blocking intake. It is a lot of work to change things around to measure velocity on the exhaust side so I was wondering if there was a conversion that factored in the higher vacuum readings.
 
If I'm understanding your post correctly, you should be using the velocity pressure in the velocity calculation. It's total pressure minus static pressure.
 
What type of velocity measurement instrument are you using?
 

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