Positive or Negative Case Pressure ?

I admit to not understanding 'negative pressure' or 'positive pressure' in the case.

Once a pressure has been established, with constant fan speeds, air in = air out. Changing the fan speeds will result in a change in :average) pressure (in the case), either up or down until equilibrium has once again been established. However this does tell you what you are interested in.

It's possible to have average positive or negative air pressure in the case, and no airflow, and thus minimal cooling.

In any case, whether negative or positive pressure, I would have thought what matters is the cooling power, and that means airflow. And air in still = air out under stable conditions. So the average pressure in the case must be equal to the average pressure outside the case whatever. It's the pressure gradients and resistance to airflow thought the case that matter.

Dumb_Question
6.February.2015

 

Good to see thinking. Not quite accurate though, you can maintain negative or positive pressure in a case.
http://www.silverstonetek.com/techtalk_cont.php?tid=wh_positive&area=en
I have cases in dusty environments I keep under positive pressure with the inlets filtered. That way less dust gets in.

 

Thank you for the reference to the Silverstone article DOA. It needs to be read very carefully. In the article, a distinction is made between airflow though through the fans, and other vents/holes in the case. In regard to the article, case air pressure is defined by the pressure of the fans, ignoring other airflow pathways. This is convenient to help determine the general airflow direction through no-fan apertures - thus the idea that positive pressure (as defined in the article) is 'good' (helps prevent dust getting in). I was not considering fan apertures as separate apertures (or was I ? this may have lead to a degree of my seemingly not being able to think clearly about the issue). However, the silverstone article goes on for the most part to stress the importance of airflow.

It's possible to have positive or negative air pressure in your case, depending on the fan settings in your case.

This does not necessarily define the airflow though. One may have positive air pressure in the case, with no airflow.

It might be thought that with positive air pressure in the case, there shouldn't be any air going into your case; in that situation, why do you need air filters to keep the dust out ? (rhet). I think that is because at some places within the case the air pressure is lower than that outside the case; near the exhaust, where air comes out, the air pressure inside the case must be higher then outside. It is difficult to say what the average air pressure in the case is under these circumstances, at least I can't think of it clearly

That you have dust accumulation in tour filters is a good sign (?) because it implies you are getting airflow through your case, even better if all the dust is at the outside of the in-filter and none is at either face of the out-filter !


A Digression:

The only way I can I can think of to see airflow inside a computer case is by introducing coloured smoke or vapour into the air around the case and, under certain illumination conditions watching its path, and perhaps measuring the speed of the particles caught in the airflow inside the case. (I doubt whether anyone would do this for fear of polluting the inside of their case, with possible damage implications). This does not determine the air pressures in the case of course. I speculate that the air pressure gradient required to create significant airflow is immeasurably small, let alone measuring the absolute air pressures, although near a hurricane such pressures can be measured easily and are of the order of 10^5 Pa with gradients 3 Pa/m (I can't rely on my mental arithmetic though). Now, a pressure difference of 3 Pa, I would think, is very difficult to measure

Dumb_Question
7.Februry.2015

 

Coloured smoke is a standard way of determining airflow experimentally to see what's happening around cars, aircraft. I suggested 'vapour' as alternative because it tends not to have particles which can settle on your PCs components, in it, vapours potentially have other problems, eg some may be in danger of condensing.

I have seen all-perpex/plastic cases, as well as one that have a whole single side-panel transparent (so that users might see the wonder of coloured light from LEDs. They seem to readily available but are a minority of cases) - but I expect most people know this

Dumb_Question
7.February.2015