Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Ask HN: Do I place the fan so that it blows air inwards or outwards?

news.ycombinator.com

Post reply on HN
Print view View original

61–70 of 120 posts

Re: Ask HN: Do I place the fan so that it blows air inwards or outwards?

#61

Post by misnome »

kosma wrote: post #52

There is a far bigger problem than thermodynamics in play, namely: dust. Not just in computer cases: it also applies to places like photography darkrooms. The usual pattern of placing a single blowing-out fan in a ventilation duct is all wrong: it pulls air in from the cracks in doors and windows. The dust-proof way is to make the fan pump the air into the room, but also install a HEPA filter on the air intake pipe/f…

This isn’t exactly wrong, but ignores the fact that the primary purpose of a fan in a darkroom isn’t to clear dust, but extract chemical fumes. You probably want fumes pumped directly outside, not back through the gaps in the door into the general building.

Re: Ask HN: Do I place the fan so that it blows air inwards or outwards?

#62

Post by shmerl »

For computer cases, take in account dust. You can use dust filters on the intake, and create positive pressure in the case. So you'd need more air blowing in, and less blowing out in order for dust not to accumulate inside (with negative pressure, dust will start actively sneaking in through holes in the case).

Re: Ask HN: Do I place the fan so that it blows air inwards or outwards?

#63

Post by phkahler »

kenhwang wrote: post #28
cbanek wrote: post #9

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 fo…

Just 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.

Thanks 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.

Re: Ask HN: Do I place the fan so that it blows air inwards or outwards?

#64

Post by reitanqild »

Another design input:

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).

Re: Ask HN: Do I place the fan so that it blows air inwards or outwards?

#66

Post by whatsstolat »

fastbeef wrote: post #47

I say this with love, but... Man, I cannot _wait_ until this guy[1] tears into this thread. “Hacker News trips over themselves to invent HVAC basics from first principles” [1] - http://n-gate.com/hackernews/

Fun site. I had to open in incognito as it seems to block HN traffic.

Re: Ask HN: Do I place the fan so that it blows air inwards or outwards?

#68

Post by segmondy »

palad1n wrote: post #33

Growing up, we had no air conditioning. Instead, we had 2 fans upstairs, blowing out two different windows. In this way, 3 rooms all got air sucked in the windows. (We shut the downstairs windows, since no one slept there.)

Actually the way to do it is to have the upstairs fans blowing out the windows, then the downstairs window open. This is more effective at night when the outside temperature has dropped and the house is still hot. Hot air rises, so you will get cool air pulled in. This is how whole house/attic fans work.

Re: Ask HN: Do I place the fan so that it blows air inwards or outwards?

#69

Post by bkanber »

namibj wrote: post #58
adrianN wrote: post #6

I assume you want to move the cold air in and the warm air out. How to best achieve that completely depends on the shape of your room and the holes to the outside. You should probably do some computational fluid dynamics on a model of your place to see which blowing direction works better. It probably works best if you have separate inlets and outlets for the air that are maximally far apart, like on the ceiling and…

Can you recommend any specific software? I'd finally have a reason for the 3d-scanner in my phone... (getting the model at a suitable resolution of <3cm voxel-cube size to not have to model it all from hand)

Overkill for the hobbyist, but ANSYS products like Fluent and CFX are what the professionals use. Quite pricey though.

Re: Ask HN: Do I place the fan so that it blows air inwards or outwards?

#70

Post by zAy0LfpBZLC8mAC »

tomohawk wrote: post #43
realpeopleio wrote: post #37

My 4rd grade teacher was convinced of the science of using two fans to cool off the classroom: one blowing cool air into the classroom from the hallway and one fan blowing hot air out of the classroom through a window. The kids that sat near the fan blowing in the room from the hallway were quite content, but the kids by the window were always complaining and they would secretly turn the fan in the window around so i…

If the goal is to make the room cooler, your teacher is right. If the goal is to make the students cooler, then the 4th graders are right. 4th graders sweat, and the fan will cause convective cooling of the 4th graders.

s/convective/evaporative/
Post reply on HN