Natural ventilation through single opening

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Discussion Overview

The discussion revolves around the feasibility and calculation of natural ventilation through a single opening in a room, particularly focusing on the impact of a ventilation grill. Participants explore the dynamics of airflow, temperature differences, and the implications of using a single vent for cooling a room.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions the effectiveness of natural ventilation through a single opening and seeks clarification on how to account for a ventilation grill.
  • Another participant references the International Mechanical Code regarding minimum opening sizes but notes that the initial problem statement lacks clarity.
  • A participant expresses a desire to cool a room from 40 degrees Celsius to 30 degrees Celsius using only natural ventilation and asks about the calculations involved.
  • It is suggested that hot air must escape from the ceiling for effective cooling, and that a single vent may not suffice unless it is positioned correctly.
  • One participant argues for the use of two vents (one at the top and one at the bottom) to facilitate better airflow, questioning the rationale behind using only one vent.
  • Another participant reiterates the focus on calculating airflow through a single vent due to spatial constraints in the room.
  • A suggestion is made to measure airflow velocity using cigarette smoke as a practical method to gauge airflow, while noting that external factors like wind speed and temperature differences will affect results.
  • Participants discuss the need to adjust calculations for the reduced area of the vent grill, with one noting that a vent with 70% open area would only provide 70% of the airflow of an unobstructed opening.
  • There is a mention of a lack of comprehensive guidelines for this specific problem, with references to existing guidelines being crude and not accounting for hole geometry.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of a single vent versus multiple vents for natural ventilation. While some agree on the need to account for the vent grill's impact on airflow, there is no consensus on the best approach or calculations to use.

Contextual Notes

Participants highlight limitations in existing guidelines and the need for adjustments based on the specific geometry of the vent grill. There is also uncertainty regarding the practical implementation of the discussed methods and calculations.

arc_sd
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Hello,

I am wondering what will happen if I try to ventilate a room by natural ventilation through a single opening. In this opening there will be a ventilation grill.
I already found a paper about counterflow through an open door by temperature differences: http://www.aivc.org/sites/default/files/airbase_4535.pdf
But is this right for my problem? And how do I take the ventilation grill into account?

Thanks in advance!
 
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Welcome to PF!

As far as I can tell you haven't actually stated what the problem is, but the International Mechanical Code specifies the minimum acceptable opening size based on floor area.
 
I want to cool a room down of 40 degrees Celsius with an outside temperature of 30 degrees Celsius, only with natural ventilation (worst case scenario).
I want to know how this can be calculated and if it's possible.
 
You need to allow hot air near the ceiling to escape. That can only happen if cooler air from outside can enter the room lower down.

You cannot ventilate by convection through a single opening unless that opening is close to the height of the room and counterflow is possible.

The grill is a complexity that only arises once you have selected your vents.
 
The opening is from the bottom to the top with a certain spacing between the floor and ceiling. So it is almost the full height.
That's why I showed the example of the door, because this looks the most like my problem.
 
The hottest air pools near the ceiling. That heats the ceiling, then radiates down into the room. If you want to cool the room efficiently you need to vent that uppermost air. That requires a vent in the ceiling or at the very top of a wall.

I cannot understand why you propose one vent when you could have two. Why mix the two flows in one vent unnecessarily. It would be better to have two small vents, one at the top and one at the bottom, than one in the area between them.

What will you have outside the vent(s)? With two vents you can have the top vent with a vertical flue, while the bottom vent can receive cool air from below the building.
 
Yes I know. But I want to know what will be the flow when it is just 1 vent (just because there is no other option in a certain room)
 
arc_sd said:
Yes I know. But I want to know what will be the flow when it is just 1 vent (just because there is no other option in a certain room)
I have no on hand experience with vents and it is not yet clear if it is already built out of the conversations above.
If built, could you measure the airflow velocity by cigarette smoke? Thus you could obtain an airflow by taking the surface area of the vent divided in half (half in half out) and multiply it by the velocity (you could need to average respective to height).
It will vary by a large amount according to outside wind speed and I/O temperature difference.
 
Yes ofcourse that's possible, but I want to calculate it beforehand.
So if the formulas I've found are right I want to use them, but I want to correct them for the vent grill.
Does anybody know?
 
  • #10
Well, I would start by correcting for the reduced open area of the vent grille. If the vent has 70% open area, the best you can get is 70% of the venting of an "open door." If the vent openings are small, say <1 cm, you'll lose more venting ability.
 
  • #11
There are no guidelines for a problem like this?
 
  • #12
The only guidelines I know of are for attic ventilation and these just use "open area" with no input for actual hole geometry...very crude.
 
  • #13
Well ok, so they just take the total area minus the area of the vent grill?
 
  • #14
arc_sd said:
Well ok, so they just take the total area minus the area of the vent grill?
Yep, that's it.
 

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