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Old Sep 12th 2018, 11:27 PM   #1
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Pressure difference of a room question

Hi,

I am discussing with my friend how to get air to push out of a room like ventilation, but we are not sure how the pressure difference would work.

If we had one side of the room sealed except for a hole where a fan would be blowing inward into the room, and then a door open on the other side of the room, would it be like a diffuser and Bernoulli's equation? The pressure of the room would increase as the air's velocity rapidly decreases, and thus the air would be forced out of the door at the other end? The room is long and large.

I also understand the pressure of the inside of a building is generally lower than atmospheric due to temperature being warmer, however would the fan blowing fresh air from outside combat this?

We are doing a project and got into a debate about the best way to ventilate. He thinks the air has nowhere to go and so the fan would just be blowing everything around the room, but I think the pressure difference would help 'suck' the air out. Also, would a gust of wind at the door stop anything from going out? I think the wind wouldn't even be able to come in, but if it did, would having the door closed a bit more (hence decreasing cross-sectional area) help?

What would be the equations governing this real world scenario? Cross-sectional area must come into play, as well as the fan's speed/volumetric flow rate.

Thank you!
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Old Sep 13th 2018, 09:17 AM   #2
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You have to be careful to distinguish between static and dynamic pressure.
As the air moves the static pressure goes down (Bernoulli), but the dynamic pressure (0.5 * air density * velocity squared) goes up.

(to a simple approximation the dynamic pressure is the pressure measured facing into the wind, the static pressure is the pressure measured at right angles to the wind).

Also you have to consider the mass flow in and out of the room.
if you blow air into the room faster than it can leave at the other end,
then the pressure will obviously increase.
This depends on the size of the opening at the other end.

Usually, for simple ventilation, the velocities and pressures are too small to really worry about.
As you suggest the natural gusts of wind are often more powerful than the artificial ventilation.
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Old Sep 13th 2018, 01:29 PM   #3
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Thank you for your response Woody! I hope I've understood your explanation.

So the fan on the other end won't really do anything to push air out, unless the door is closed enough such that the mass flow in is greater than the mass flow out? I suppose this would include the factor of a gust of wind from outside, as a more closed door wouldn't 'catch' this gust as easily.

In this case, then the static pressure would be increasing as the velocity drops (and thus dynamic pressure drops) due to the air suddenly entering a larger cross sectional area (the room), and with the greater static pressure in the room the air would be forced out of the only place it can: the partly closed door?

And this would all only be provided the effects of all of these elements were not negligible?

Thank you again! It's much more difficult to apply these principles to real life than problems on a paper.
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Old Sep 13th 2018, 05:08 PM   #4
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This is a lot simpler than you're making it out to be. The fan creates a region of high pressure on the side that it blows toward, and low pressure its other side. So, a fan blowing air from outside into a room means higher pressure right by the fan in the room. Given an open door to atmosphere at the other end of the room, the air moves from high pressure to low pressure, and hence from the fan at one end of the room out the opening at the other end.
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Old Sep 13th 2018, 06:52 PM   #5
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Originally Posted by treyland123 View Post
...I also understand the pressure of the inside of a building is generally lower than atmospheric due to temperature being warmer
That's not correct ... the slightest difference in pressure will cause an air movement to bring equality .

Suppose you have a warm room, if you make a hole to the outside at floor level , even without a fan this will tend to draw cooler denser air in from the outside by the 'chimney effect '.... air will escape through doors , windows to(almost) equalise pressure .

If you want to use a fan to push air out of a room ,which is warmer than the outside , make the hole to the outside as high as possible this will help the fan and move more air ...Warm air is more buoyant so it's always trying to push it's way out from an opening high up .. cool outside air will find a way in.

Certain windows are designed to take advantage of this natural flow ,the effect is not that great , but it's there ...

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