Let's assume it's 30 celsius inside the room and <20 celsius outside.
Enjoy your Monday my thermodynamics friends!
Let's say you have three fans in a computer case, one at the back, one at the front, and one on top of the CPU. If they all blow out of the case, then you've basically created a negative air pressure zone, and you will be sucking air through the vent holes elsewhere, and through any gaps in the case. This may have effects for where the dust is taken in.
If all of them blow into the case from the outside, you've created basically a high pressure zone, and air will be taken from the room and pushed out through the same cracks and vent holes elsewhere.
A fan if it's pushing against high pressure will inherently push less air because it's pushing against air that isn't moving as well. A fan that is trying to pull air out can also become inefficient if it is working hard to suck air in to eventually push out. Therefore it's best to have matching airflow - what you push out, you should try to also pull in somewhere else in more or less an equal CFM (cubic feet / minute).
For a room, it's best to have one area that is your intake and the other side of the room be the exhaust. This way you aren't just exhausting air you just pulled in. If you just have one fan, I suggest blowing out, which will draw air in from other spaces, such as under the door, because of the negative air pressure by pushing air out.
(I've worked these problems with indoor gardens and HVAC systems, where there's a lot higher CFM fans and spaces, but the theory is basically the same)
The highest performing PC fans create about 10mmH2O pressure while typically you can expect about 1mmH2O pressure compared to 10mH2O of the atmosphere. This translates to densening of the atmosphere around components by 0.1% or 0.01%, respectively.
Correct placement and design is by orders of magnitude more important than the increased efficiency due to higher density of air.
You could see much more effect by making your air more humid (humid air has higher heat capacity), which will be also much more pronuncable than the increased density due to pressure.
I think it is because you can measure it yourself with just a tube and water- open end manometer.
Kidding!
(I have a combination energy recovery ventilator/HEPA filter that has has a restriction filter on its exhaust for this reason. Else it would just suck crappy air in from the basement instead of fresh air through the filter.)
If you have to choose one, choose an intake fan for this reason. Best if you can leave another window slightly ajar.
We also have its accompanying HEPA unit: http://www.purifresh.com/cleanroom.html They can be linked (as http://www.purifresh.com/freshairsystem.html) or used separately. (Apparently this is out-of-stock and being redesigned.)
If you have only one, set it to intake and leave another window open a bit (maybe like half the fan size). You can confirm that air is exiting that way by hanging some strips of tissue paper in it.
Positive pressure (blowing in, from outdoors) is also the right thing to do if you are trying to keep smells from other rooms at bay. Whereas negative pressure (blowing out, to outdoors) is best to keep a smell confined to one room (e.g. this is how bathroom fans work).
Even better, now that I own my own house, is a high powered fan in one window pushing the air out and another (lower-powered) fan in a different room pulling the air in with an additional window open. You can stand in front of the open window and feel the additional cool air streaming in. It's a beautiful thing.
It helps to reduce stale odors, bring in fresh air, reduce indoor allergens...also handy when you accidentally burn something in the oven. :)
Positive pressure is very good at assisting the PSU fan and any radiator exit fans you have, negative pressure is very good at cooling the edges of the MoBo and upper drives in a tower system. Negative is generally cheaper, so its usually negative.
Personally for a single fan in a smaller room where you're near the window, I'd say just point it in. That way you get fresh air right where you are sitting, you get the breeze, etc. In the other direction you won't get the cooling effect on your body from the breeze.
So if sweaty 4th graders are any indication, have the fans blow on you regardless of whatever air movement is supposed to be made.
This is often a key difference between Car and Room AC systems.
There’s also a surprising gradient based on how consistent the temperature is top to bottom (and you get into trouble with this on reverse cycle AC systems)
And lastly it’s easy to ignore radiant heat from both walls/ceilings as well as even large monitors.
It all plays in together.
Really? You'd think windchill is a well-understood concept.
The problem is that the room probably has leaks such that if the fan in the hall is creating pressure, then the air doesn't all escape through the window. It'll escape through the path of least resistance or maybe even just spin inside the room (hot air rises too). If there is two doors for the room, some of it may be escaping under the other door. In that case, then air is just circulating back into the hallway. People also forget to take into consideration the expansion of hot air, which I've seen quotes for being up to 6x the volume. This complicates the pressures in the room.
As mentioned elsewhere in the discussion, the kids probably wanted to feel cool via evaporation since neither of the fans was moving nearly enough air to cool the entire room enough.
I run a couple large crypto mining farms in a pretty hot country. The 'kids' generate the heat and we have to move large amounts of air through them in order to keep them relatively cool. I've had to learn a lot about airflow design and it sounds very similar to what your teacher was trying to achieve.
That would mean the kids near the outlet fan were getting air that was just pulled in from outside via the window seams.
I can see how a single fan blowing right into someone's face could be more efficient at cooling that person than the same fan working for the whole room; but frankly you also forget about all of the other pupils who aren't under the direct "sunlight".
Air conditioning -> cool human by cooling environment
It only accelerates the speed at which your skin temperature moves to match the air temperature through convection.
If you can move air elsewhere to lower the temperature of the room, that is better long term.
For people like me, who use a cheap fan in the summer. Put it in front of you facing towards you. You will feel cooler.
If you are still interested in CFD I can give you a hand, but this is answer we are going to get from it anyway.
The only issue was that it also sucked rain and pollen in as well.
Extract hot air in one location, preferably up high, and let cold air in with a window that is as far away as possible - it doesn't really matter high or low.
Oh great, there goes my Monday. Thanks buddy.
To heat winter rooms that have an underpowered radiator, blow the fan onto the radiator. The job of heating the room is the same as cooling the radiator. This works very well but the room downstream (on the steam line) will be getting less heat. In practice, that was the neighbor's apartment. The freezing neighbors complained to the landlord who eventually tracked it down to my place. Vigorous discussions ensued.
I also discovered that a HEPA filtration unit, even sitting on the floor, helped my allergies a lot. It was not cheap, but when I find my nose running again, it means that something had turned off the filter. For years I had thought that HEPA was a scam. Not so.
Cooling the entire house is an expensive proposition if you're just trying to get to sleep in one room.
It's the same with air - there has to be a good way to get cold air in and hot air out with minimal turbulence from the two flows.
I think you're better off blowing cold air in (there's no such thing really as 'sucking' in physics), as attempting to blow hot air out doesn't necessarily mean it's going to be replaced with cold air from the outside, rather than warmer air elsewhere in the building.
If you have fan in the window blowing into the room it has less potential to multiply the air it takes, it will typically be also already IN the room itself which will greatly decrease its efficiency.
On the other hand if your goal is to cool yourself when sitting at the des it is generally better to place your fan blowing in, this means you can direct the flow of air to wherever you want. This means you can direct it at your desk to cool yourself by just the air movement alone. When you place your fan blowing outside you loose that ability. Blowing the air at you means you can cool yourself even if the air temperature outside is higher than inside!
If your goal is to move as much air through your room then you need to take into consideration the following:
- do you have other openings in the room (like doors... huh)? - can you have flow of air through the building -- typically there will be pressure difference between two sides of building unless the air is completely still
If you can have your door and the window open, observe the direction of air already naturally flowing. Then put your fan so that it blows with already moving air, for maximum efficiency.
Open your bedroom windows on a cool spring/summer night. On the opposite end of the house, put a fan in a window blowing outwards. This cools the entire house, giving your bedroom the freshest air. In the morning, close all the windows. This leaves the house cooled and drastically reduces your A/C usage.
Blowing inward doesn't have the same effect of pulling air through the whole house.
Recently, I bought a whole-house attic fan. Now, I open a window on the upwind side of the house and turn on the attic fan, which is enough to suck a toupee off someone's head. The attic fan both circulates cooler air and also clears out the hot air built up in the attic, which helps a lot.
I believe I'd pull the cooler air into the room by having the air blow in. I might also point the fan up and away from the other side of the window (up because I want to disturb/displace the warm air higher in the room; away from the non-fan side because I don't want my cool air to be pushed right back out.)
I don't want to cool the warmer air, I want to replace it with cooler air.
You want the air from the fan to accelerate other air on the way out/in. This is the same principle as turbofan engines, in which a higher bypass ratio (bigger fan) attached to the same fuel burning jet core is more efficient.
Why? Because the relationship between velocity and volume is not linear. Takes more energy to move the same amount of air the faster you accelerate it.
So ideally you want to position the fan in a large opening far enough back that the fast moving air can get a bunch of other air moving as well. Trade the high velocity fan air for a much higher volume.
Being a Swedish summer, maybe around july august I'll need to take the fan out of storage and then it's just to get a draft on myself. Not to move any air around. The draft between the two sides of my home does that better.
Open the window in the room and open the door to the room. Ideally open a window somewhere else in the building and make sure there is a potential airflow from that other window to the one in your room.
Place the fan in the window of the room you want to cool down and let it blow outwards. Alternatively you can place the fan in the same way in that other window you opened.
I'm not the top floor in the corner room which is the hottest room. The entire unit is per floor is much warmer than any other level. The best non-AC cooling I've been able to achieve is to run the bathroom fans with the bathroom windows closed. It pushes out warm air near the ceiling and draws in cooler air from windows and lower parts.
There are much more complex cooling setups that can be achieved that utilise outward-blowing fans though, but a single fan can't will cool _you_ faster if it is blowing onto you.
http://www.thegoodsurvivalist.com/youll-be-incredibly-surpri...
On the other hand, if the purpose is to express, say, smell, blowing the smell out is the better solution.
If there's lots of humidity inside the room you'd probably prefer negative pressure in the room.
Reason: positive pressure can pressure moist air through walls and ceilings and when it hits the cold air on the other side of the insulation it condenses in thr construction and causes rot (or corrosion).
Ignoring that, In to the room. Fan can not create much vacuum but can create much pressure.
Let me give you my most handy example. Imagine a queue. A push can cascade from last person to first person, but a pull can only be apply to last person.
Hence, pressurise the cabin.
In practise it didn't work that well - the temperature difference in a couple of metres of air space (in a single average-height room) is pretty minimal. We left the fan blowing downwards to use the windchill effect in summer - that worked well enough.
“Hacker News trips over themselves to invent HVAC basics from first principles”
Overpressures tend to prduce airstreamss or turbulence downstream, but underpressures don't induce much flow beyond the immediate orifice. Unless convection, ducting and venting can reliably create flow patterns, blowing air in to a conditioned space is a better option thansucking ot out, though if you want to control for emissions (say, filtering exhaust), you might want to combile both.
There's a classic physics experiment showing what happens when you run a water sprinkler, the kind with a spinning arm and one or more jets, backwards (sucking rather than blowing), and hhow it doesn't spin. Feynman discusses this in a story.
The fan(s) themselves contribut some additional heat to the space, but this is fairly negligible, especially relative to circulation effects.
If the heat source is external (e.g., incident sunlight), then natural airflow patterns which might draw air through a hotter chamber (say, an attic or other roof void) could have the opposit of the intended effect. It's key to check overall circulatory flow.
And if internal systems, or people, have significant evaaporative or similar cooling mechanisms, increased internal circulation will greatly improve cooling capacity.
Forced inflow may increase dust or other factors, if not filtered.
TL;DR: If you've only one option, I'd blow in rather than suck out. You might want to consider overall airflow, circulation, and heat distribution or souces (and sinks) within the void. Blowing onto rather than away from the primary heat source (in a cooling scenario) is likely best. If heat sources are external, exhaust from the top of the void is optimal.
Intakes should generally be low, exhausts high (for coolng).
For a large enough and complex enough (multiple chambers, zone, multiple heat sources, significant convection effects, dead zones), you would likely want a combination of intake, circulation, and exhaust fans, though I'd prioritise them in that order.
Disclaimer: I'm a space alien cat, not a fluid thermodynamics engineer.
This questtion has also appeared elsewhere:
https://superuser.com/questions/192920/should-case-fans-blow...
https://physics.stackexchange.com/questions/71620/on-a-hot-d...
Do you want to:
1) Cool down your room?
2) Warm up the outdoors?
3) Warm your room further?