Airflow through an air extraction grill

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Discussion Overview

The discussion revolves around the effectiveness of using a grille to facilitate uniform airflow for drying high humidity air passing through a fabric in a plenum system. Participants explore the relationship between grille design, airflow resistance, and moisture removal from the fabric.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether a simple grille can achieve uniform airflow between two plenums, suggesting that the geometry of the system plays a significant role.
  • Another participant argues that the grille does not affect airflow uniformity if the fabric covers the entire opening, as airflow depends on the fabric's resistance to airflow.
  • It is noted that if the fabric has varying porosity, airflow will be uneven, leading to faster drying in more porous areas.
  • A later reply emphasizes that uniform flow through the grille is not as critical as uniform flow through the fabric itself, suggesting that the pressure drop across the fabric will dominate the airflow characteristics.
  • One participant references drying processes in paper manufacturing to illustrate that drying involves more complexities than simply placing a fan in front of wet material.

Areas of Agreement / Disagreement

Participants express differing views on the impact of the grille on airflow uniformity, with some asserting it makes no difference while others suggest that system geometry is crucial. The discussion remains unresolved regarding the optimal approach for achieving uniform drying.

Contextual Notes

Participants acknowledge that the assumptions about fabric porosity and airflow resistance are critical to understanding the drying process, but these assumptions are not fully explored or defined.

Hedgehope
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TL;DR
Two spaces separated by an open grille, does reducing pressure in one space draw air through the grille from the other space in a uniform manner ? (volume, pressure & velocity)
Hello,

New member here, brainstorming some ideas for my 'invention'

I need to draw the high humidity air contained in plenum 1 into plenum 2 in a uniform manner to assist with evenly removing the moisture from a single layer of fabric, suspended parallel in plenum 1

My question is does placing a 'simple' (evenly spaced, consistent thickness) grille between the two plenums achieve this ? Or is there more to this than meets the eye ?
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Welcome, @Hedgehope !

Air tends to follow the path of less resistance.
The effect of the grille on the uniformity with which the air will flow through the cloth is debatable and depends much on the geometry of the whole system.
 
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The added grille makes no difference.

If the fabric covers the entire opening, and the air is drawn through the fabric, then the airflow through the fabric depends on the resistance to airflow of the fabric. If some portions of the fabric are more porous than other areas, the porous areas will flow more air and dry faster. If the fabric is evenly porous over the entire area, then it will dry evenly.

If the fabric is smaller than the opening, then the air will flow around the fabric and dry the edges faster than the middle.

Hedgehope said:
Or is there more to this than meets the eye ?
There certainly is more to drying than meets the eye. You might find it interesting to read about drying large amounts of paper by searching paper machine dryer section. It's not directly applicable to your situation, but will give you some idea that there is more to drying than just setting a fan in front of something wet.
 
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jrmichler said:
The added grille makes no difference.

If the fabric covers the entire opening, and the air is drawn through the fabric, then the airflow through the fabric depends on the resistance to airflow of the fabric. If some portions of the fabric are more porous than other areas, the porous areas will flow more air and dry faster. If the fabric is evenly porous over the entire area, then it will dry evenly.
I agree with this, though my first reaction was to disagree due to the wording of the OP:

Hedgehope said:
I need to draw the high humidity air contained in plenum 1 into plenum 2 in a uniform manner to assist with evenly removing the moisture from a single layer of fabric, suspended parallel in plenum 1

My question is does placing a 'simple' (evenly spaced, consistent thickness) grille between the two plenums achieve this ? Or is there more to this than meets the eye ?
I'm not sure of the thought process here, but uniform flow through the grille isn't important, just uniform flow through the fabric. Perhaps you're envisioning something like flow straighteners on a wind tunnel, but resistance vs a plenum is what creates uniform flow for your type of situation. You can achieve uniform flow through the grille with a high pressure drop/low free area, but the fabric pressure drop is likely to be pretty high, so that won't have any impact since it will already be uniform coming through the fabric (assuming the fabric is uniform).
 

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