1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Natural ventilation through single opening

  1. Jun 23, 2015 #1

    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!
  2. jcsd
  3. Jun 23, 2015 #2


    User Avatar

    Staff: Mentor

    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.
  4. Jun 24, 2015 #3
    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.
  5. Jun 24, 2015 #4


    User Avatar
    Science Advisor

    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.
  6. Jun 24, 2015 #5
    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.
  7. Jun 24, 2015 #6


    User Avatar
    Science Advisor

    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.
  8. Jun 24, 2015 #7
    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)
  9. Jun 26, 2015 #8


    User Avatar

    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.
  10. Jun 26, 2015 #9
    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?
  11. Jun 26, 2015 #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.
  12. Jun 29, 2015 #11
    There are no guidelines for a problem like this?
  13. Jun 29, 2015 #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.
  14. Jun 30, 2015 #13
    Well ok, so they just take the total area minus the area of the vent grill?
  15. Jun 30, 2015 #14
    Yep, that's it.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook