Case Fan Question

[smokey43]

Hello all,

I'm relatively new at this and have a question in regards to a case fan I just installed. It is a 120mm fan that is located at the top of the case. There is room for one more but wanted to stop and ask before installing a second. My question is , I currently have the fan installed to exhaust air from the case. Is this recommended over blowing the air "Into" the case. Also, should I install the 2nd fan in the same manner? Thanks to all who take time to respond. It is much appreciated.

 

[Ubrales]

The general practice is as follows:

Front, Side, Bottom (if any) ---> Intakes

Top, Rear ---> Exhausts

Both top fans must be exhausts. Try to balance the Intakes and Exhausts CFM within 15%. Positive air pressure or negative air pressure inside the case does not matter. (This subject has been debated to death).

 

[smokey43]

The general practice is as follows:

Front, Side, Bottom (if any) ---> Intakes

Top, Rear ---> Exhausts

Both top fans must be exhausts. Try to balance the Intakes and Exhausts CFM within 15%. Positive air pressure or negative air pressure inside the case does not matter. (This subject has been debated to death).

Thanks so much for the quick response. Much Appreciated.

 

[Ubrales]

You are welcome!

 

[westom]

I'm relatively new at this and have a question in regards to a case fan I just installed. It is a 120mm fan that is located at the top of the case. There is room for one more but wanted to stop and ask before installing a second.

So many will debate this. So few will provide numbers. For example, a 40 cfm fan means internal air temperature for a standard computer is single digit degrees higher than outside air. Second fan has an exponentially decreasing effect. In short, the fan exists so that the system internal temperatures are not much above 100 degrees F when room temperature is 100 degrees F.

Far more important are dead air spots. Massive cooling occurs as long as air is moving over the part - at a rate far below what any hand can feel. IOW a ribbon cable might create dead air over one part causing that part to be slightly warmer. That slightly warmer temperature is only a problem when room temperatures exceed 100 degrees F.

Often two fans blow in series so that if one fan fails, the other continues blowing air in a same amount and direction. But most never learn these relevant numbers. Most fully believe what is Tim Allen's famous joke: "More Power".

Your one fan moves more than enough air for most any computer.

 

[madmatt30]

The benefits of positive pressure are not to do with cooling efficiency IMO.
Positive pressure simply keeps dust out of the case if the intake fans have filters fitted & there are any unused fan mounts or the case is a meshed style.

 

[westom]

Positive pressure simply keeps dust out of the case if the intake fans have filters fitted & there are any unused fan mounts or the case is a meshed style.

Moving too much air is the largest reason for dust. Best solution to dust is computers that move no air into a chassis. These are industrial machines. Dust is problematic with machines that have too many fans.

 

[Ubrales]

Some observations and comments:

Positive pressure in simple terms is an excess of ambient air inside the case. This is as clean or as contaminated as the ambient air. No practical value in a PC.

There are ways to create a dust-free positive pressure by pre-filtering the air in a separate system and then pumping that filtered air into the PC. I do not recommend doing this.

Screens, and not filters, are used in computer cases. This is because filters will create a pressure drop across the filter and place an excessive load on the fans. The fans used in computer cases are light duty fans that move air through open spaces; not through filters.

In order to use a filter in a computer case the fans must be replaced by blowers. Blowers have power to move air through a filter much like the system used in home HVAC. I do not recommend doing this.

Do not replace the screens with filters. Instead, clean the inside of the case using a can of compressed air on a monthly basis, or any other frequency depending on the environment. The compressed air will clean out most of the dust but not all of the dust. Even a gale force wind will not clean out all of the dust in any PC. This is because of the “Boundary Layer” effect which you may research on your own.

 

[madmatt30]

the scientific facts are impressive but the fact is that in a home pc application 99% of the time a positive pressure system will minimise dust & particles entering the case as theyre simply unable to against an outwards push of air
Ill stick to saying filters simply because it seems to be the recognised term.
You could argue that the front foam ones installed in the majority of cheap pc case fronts are infact filters though.

I hate side fan mounts with an abhorrance - even without an intake fitted if there are no screen/filters the rotation of the gpu fans will still draw air into the case.
I steer clear of cases with side fan mounts or as you say buy a couple of 120mm screens & fit even if an intake isnt being used - the same with unused top fan mounts.
If its an intake with a fan or any kind of fan mount with no fan fitted spend a few dollars & fit filters anyway is my advice.

I am not a fan of filling every possible fan mount,in most cases 2 x intake,1x exhaust will do the job fine.
With higher end hardware & hot running gpu's 3 x intake ,2 x exhaust absolute max.
any more is overkill & will likely create crossflow problems across the whole case interior.

This knowledge comes from over 2 decades of building ,maintaining & repairing pc's - a lot coming from homes with pets or heavy smokers - some of the things Ive seen would make your skin crawl believe me 😱

 

[westom]

This knowledge comes from over 2 decades of building ,maintaining & repairing pc's - a lot coming from homes with pets or heavy smokers - some of the things Ive seen would make your skin crawl believe me :

Experience without first learning the underlying facts, concepts and numbers is why junk science exists. We all were taught this in junior high science. Maintaining and repairing does not teach anything about the underlying science. necessary to have a hypothesis. We engineers (not technicians) often untrain techs in those popular myths created only by speculaton. Positive pressure is a prefect example. No filter 'purifies' air. That air pressure comes from 'dirty' air through crude filters. Large particles that might short IC pins are filtered. Dust goes right through those filters.

Of course, filters reduce the amount of air moving through a computer. Reduction of air means less dust. Then a tech would ignore that fact. See less dust. And that *know* positive pressure must mean cleaner air. Filters did not 'clean' air. Filters only reduced airflow. Reduced airflow means less dust problems.

Computer is just fine with a 120 mm fan. A second fan might cause a 2 or 3 degree temperature reduction. And would increase dust problems. Dust problems are created by moving too much air even through crude fitlers that do not eliminate dust. These myths are popular when one uses observeration (also called junk science reaoning) to make conclusions.

In 40 some years of design, we would untrain techs of these urban myths created by observation..

 

[Ubrales]

https://news.google.com/newspapers?nid=1683&dat=20060706&id=27koAAAAIBAJ&sjid=MUUEAAAAIBAJ&pg=2232,3072892&hl=en

I told some of my car enthusiasts that even driving at 100 MPH will not take away the dust from the hood of their cars.