6 ms·
Like others have joked about, it's really about where you want the air and temperature to go. Let's say you have three fans in a computer case, one at the back
by cbanek 8y ago
Like others have joked about, it's really about where you want the air and temperature to go.
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)
- kenhwang 8y agoJust to add, for computer cases, you want it to be a high pressure zone because intake fans generally have filters in front to catch the dust before it gets into the case. Negative pressure means dust gets sucked in from the cracks.
- jjcm 8y agoAlso while extremely trivial here, higher pressure air has a higher capacity to transfer heat.
- lmilcin 8y agoWhile this is technically true, the difference is very minor at the typical pressure ranges being used. 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.
- mberger 8y agoTIL about mmH2O. Is this common in HVAC? I thought pressure was mostly in mmHg or bars(millibars).
- Cerium 8y agoYes, fans usually have a specification on the pressure they can create in inches of water or millimeters of water. I think it is because you can measure it yourself with just a tube and water- open end manometer.
- lightedman 8y ago"The highest performing PC fans create about 10mmH2O pressure" I'm guessing you've never worked with a Delta fan. Those things produce enough pressure to literally suspend themselves in mid-air.
- phkahler 8y agoThanks for mentioning filters. I'm just in the process of designing an ITX case specific to a board/CPU and plan to partition it so the CPU fan is also the case fan. I hadn't thought about dust. Good point.
- FullyFunctional 8y agoDryer sheets :) I don't remember where I found this tip, but recycling used dryer sheets as air filters works fairly well (definitely beat having nothing).
- mark212 8y agoOr use the new ones for a delightful fresh scent! Kidding!
- colanderman 8y agoSame applies for house fans -- you want the intake fan to be slightly stronger than the exhaust fan. Else the negative pressure will draw outside air in through cracks and all the fun smells that come from those places. (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.
- toomuchgov 8y agoWhat is the make/model of your ERV?
- colanderman 8y agoIt's a window unit: http://www.purifresh.com/erv.html http://www.purifresh.com/erv.html It's the only such kind I could find when we were in the market a few years ago. (Was renting a place with mold issues.) Works well for a single room. It's wonderful to pull fresh air in during the winter. We also have its accompanying HEPA unit: http://www.purifresh.com/cleanroom.html http://www.purifresh.com/cleanroom.html They can be linked (as http://www.purifresh.com/freshairsystem.html http://www.purifresh.com/freshairsystem.html) or used separately. (Apparently this is out-of-stock and being redesigned.)
- saalweachter 8y agoMy vague understanding is that negative pressure in a house is also how you get radon gas.
- sureaboutthis 8y agoAs one who designed medical equipment (though not the mechanical engineer) this is the answer.
- colanderman 8y agoI strongly disagree on the setup with only one fan. Having only an exhaust fan in the room will draw nasty air in through cracks in the house's structure, possibly stuff with e.g. mold spores. My wife could never remember which way was which for our window fan; I could tell by smell when she had left it exhausting for more than an hour. 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).
- m0rose 8y agoAgreed. In my own experience, it's much more effective when cooling a room to pull the cool air in than to try to exhaust the hot air from the entire house so that cool air will eventually trickle into the room you're trying to cool. Changing from pushing the air out to pulling it in directly was an epiphany for me when I was younger and lead to many nights of better sleep. 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.
- spdustin 8y agoIn some zones, whole-house ventilation fans (which are installed in the ceiling below the attic) are excellent and efficient ways to cool and/or ventilate a house quickly—by exhausting the air in the living area of the house up and out through the passive vents in the attic. Open some windows, turn on the whole-house fan, and the air in the house is fully exchanged with fresh air from outside very quickly. It helps to reduce stale odors, bring in fresh air, reduce indoor allergens...also handy when you accidentally burn something in the oven. :)
- mark212 8y ago
- cortic 8y agoYou never want matching airflow in a PC case, it creates pockets of heat, frying components, usually the top disc drive or sometimes the PSU. 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.
- neltnerb 8y agoHeight is also important if you've got a multi-floor house. In upper stories, make the fans face outward and on the ground floor make the fans face inward and you'll set up a continuous air path throughout the whole house favoring cooler air. 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.
- Steel_Phoenix 8y agoI also like to take prevailing wind into account. Even if it's a small breeze, fighting it can really drop effectiveness.
- pbhjpbhj 8y agoAnd consider shadow - let in colder air from shaded areas rather than warm air heated by surfaces in direct sunlight.