Positive pressure to keep out dust, stacking fans a problem?

[Bronney Hui]

Hello.

I have 2 questions concerning fans.

1. I currently have a 120mm low rpm fan blowing out at the back below the
psu, if I add another 120mm fan on top of that blowing in the same
direction, would it speed up the outflow, or would it actually hinder the
flow as now the new fan got an old fan + grill to deal with (more blocking
objects)?

2. If I install more fans blowing in then blowing out so that the pressure
in is higher, provided that the in fans are all filtered, would it keep out
dusts?

Thanks.

 

[Noozer]

Hello.

I have 2 questions concerning fans.

1. I currently have a 120mm low rpm fan blowing out at the back below the
psu, if I add another 120mm fan on top of that blowing in the same
direction, would it speed up the outflow, or would it actually hinder the
flow as now the new fan got an old fan + grill to deal with (more blocking
objects)?

Can't stack fans to get more airflow. If you're top speed on a highway is
80mph, what will your top speed be if you put another car going 80 behind
you? ... same 80mph. THat 120mm should be fine. The PSU is also sucking air
out of the PC... This means you need a 120mm and an 80mm someplace else
blowing in to get close to equal pressure in the machine.

2. If I install more fans blowing in then blowing out so that the pressure
in is higher, provided that the in fans are all filtered, would it keep out
dusts?

Yes... Keeps dust from being sucked in through your floppy/CD drive, etc.

 

[kony]

Hello.

I have 2 questions concerning fans.

1. I currently have a 120mm low rpm fan blowing out at the back below the
psu, if I add another 120mm fan on top of that blowing in the same
direction, would it speed up the outflow, or would it actually hinder the
flow as now the new fan got an old fan + grill to deal with (more blocking
objects)?

Two fans "serially" connected tends to increase pressure
more than flow rate. If your case has poor intake it would
help, but otherwise it's not going to be of that much
benefit, will produce as much noise as just using higher RPM
rear fan which would also increase flow rate.

You mention the grill... is it a stamped-in-metal grill or a
chrome wire grill? If it's stamped-in-metal, the best thing
you can do is cut out the grill, adding a wire grill if you
really need protection over the opening. Wire grills aren't
nearly as much of an impedance to flow.

2. If I install more fans blowing in then blowing out so that the pressure
in is higher, provided that the in fans are all filtered, would it keep out
dusts?

In theory, yes. In practice you'll have to check the
pressurization because filtered intake drastically cuts down
on flow rate, you will end up with less than 50% (possibly
much less) flow though the filter if it's even remotely
dense enough to stop dust. You might want to plan for 3 X
the intake as exhaust flow rate (that you'd have if it
weren't filtered) to leave a little margin, else the case
will still be negatively pressured and draw air in through
drives/gaps/etc.

Forcing/pulling air though a filter(s) is one of the
situations where doubled, serial fans may help, but easier
and probably sufficient is using thick fans instead, for
example a 120 x 38 mm intake fan might be ideal, with the
filter as large as possible, not merely the same size as the
fan housing itself. Supposing you had 16 sq. inches of
filter, you might double the flow rate by going up to 50 sq.
inches of filter (I don't have a formula to calculate it but
that's a rough guess, since the pressure vs flow rate of an
axial fan is not a linear relationship, you need more than
double the filter area to double the flow).

As an example I have a few early pics of a mod i did to a
case to give it best possible front filtering, though I only
have pics of the bezel, there were 3 x 92mm fans behind it
pulling air in then pushing it through HDD bays. If I
didn't want so much HDD cooling capacity I'd have used a
single 120mm fan instead. Also what can't be seen is that
the end result was an intentional air gap between the fan
intake and the filter, so air was being drawn in though the
entire filter, not just a smaller area in front of the fan.

http://69.36.189.159/usr_1034/filtered/index.htm

Having a much larger filter also greatly reduces how often
it needs be cleaned, and allows lower fan RPM per same flow
rate (quieter).

 

[ric]

1. I currently have a 120mm low rpm fan blowing out at the back below the
psu, if I add another 120mm fan on top of that blowing in the same
direction, would it speed up the outflow, or would it actually hinder the
flow as now the new fan got an old fan + grill to deal with (more blocking
objects)?

Inasmuch as the second fan would allow the first to operate at closer to
it's zero static pressure CFM, you would get more outflow. How much more
depends on several factors.

2. If I install more fans blowing in then blowing out so that the pressure
in is higher, provided that the in fans are all filtered, would it keep out
dusts?

To a degree, yes.

 

[Rusty]

Stacking fans of the same velocity will not increase the air flow, it will
reduce the back pressure on forward direction fan, which will allow the fans
to move a little more air out. The principle here is like attaching two
water hoses together. If you attach two hoses together they will only
deliver the same amount of water as a single hose but if you attach a second
hose to the facet you will double the output of water. If the fan move air
at different speeds and volumes the lower volume fan will restrict the high
volume fan.

Installing more intake fans than exhaust fan will create a positive pressure
in the case and will help to decrease the amount of dust collected though
the cracks, CD, and Floppy. It's a good ideal to have a SLIGHT positive
case pressure, a high positive case pressure will cause heat build up in the
case. It's also a good ideal not to move the air though the case quickly as
it will not have time to pickup the heat. Move as much volume of air as you
can at the slowest speed possible with a slight positive case pressure.
Across a heatsink you want to move as a high volume of air as fast as you
possibly can. High speed air (fluid)movement is called turbulent air flow
and low speed air (fluid) is called laminar air flow. Air flow that is part
turbulent and laminar is called transition air flow. You want the case in
laminar air flow and the heat sink fan in turbulent air flow. The reasons
for this have to do with heat removal from a solid object (heat sink) and
heat removal from a fluid (case air). A lot of thermal dynamics involved.
Fan placement is important but that depends on your case design and location
of you high heat components. Generally, cool air in from the bottom and hot
air out the top with the cool air being drawn across the high heat areas.

Actually went through the calculation once for my case did not help much
because there are two many variables involve. Ended up with rheostat fan
controllers on all my fans so that I could actually dial in the air flow to
maximize the cooling. Ended up with all my case fans at about 3/4 to 1/2 of
there rated speed and the cpu fan at maximum.

 

[Bronney Hui]

Two fans "serially" connected tends to increase pressure
more than flow rate. If your case has poor intake it would
help, but otherwise it's not going to be of that much
benefit, will produce as much noise as just using higher RPM
rear fan which would also increase flow rate.

You mention the grill... is it a stamped-in-metal grill or a
chrome wire grill? If it's stamped-in-metal, the best thing
you can do is cut out the grill, adding a wire grill if you
really need protection over the opening. Wire grills aren't
nearly as much of an impedance to flow.

Thanks Noozer and Kony

It's a stamped in grill. Here's my case with 2 pages illustrating the
inside and front filter + defualt fans as what I have now:

http://www.anandtech.com/casecooling/showdoc.aspx?i=2187&p=3
http://www.anandtech.com/casecooling/showdoc.aspx?i=2187&p=4

So there's 1 80mm front intake blowing on the hdd's, with space between the
filter and fan. And 1 120mm low rpm out the back. My psu has 3 fans,
front, bottom and back.

Since I am only occupying 1 5.25" bay on top, I have 4 bays of room to work
with in the front. I am planning to do as you suggested by getting a higher
rpm fan intaking on the bays. But that's about all the room I have, 3x
5.25" cuz I need 1 bay area to tug the psu cables in.

 

[Bronney Hui]

Actually went through the calculation once for my case did not help much
because there are two many variables involve. Ended up with rheostat fan
controllers on all my fans so that I could actually dial in the air flow to
maximize the cooling. Ended up with all my case fans at about 3/4 to 1/2 of
there rated speed and the cpu fan at maximum.

Hmm, this should go in sync with Kony's as if I keep my exhaust at the low
rpm 120mm out back, but add a powerful intake on the 5.25" bays, the cool
air would not get out so quick right?

 

[Rusty]

That's correct. After looking at your case there are three 80x80mm fans on
the PSU and one 120mm for exhaust. If you remove the stamped grills with a
dremel and estimate of the total exhaust flow (30cfm at the PSU and 50cfm at
the 120mm) is approximately ­±80 cfm, which means that you need ±80 cfm on
the intake to match the exhaust. If you are up to it you could cut two 80mm
holes in the side panel at the bottom of the panel for two 80x80mm intake
fans. You could also place the intake fans one directly across from the cpu
and another adjacent to the hard drive bay at the bottom or across from your
video card. This would give you ~90 cfm on the intake and ~80 cfm on the
exhaust. A good template to use for 80mm fans is a small CD (the half size
one). My son has a similar case design and we did this to his case placing
two UV LED fans across the bottom of the side panel and this dropped the
case temperature about 8°C. He lives in a dorm and noise was an issue and
the UV fans looked good in the side panel. If you use a high volume 80mm
intake fan such as a thermaltake smart fan or a vantec tornado the dba on
those types of fans is quite high (they make lots of high pitch noise).
Three standard ±30 cfm 80mm intake fans are allot less noisy. Good luck.

I have a excel data base the will estimate cfm from the current (amps)
rating on the fan if you are interested. It will work on all 80x80mm
nonvariable speed fans.

 

[ric]

Stacking fans of the same velocity will not increase the air flow, it will
reduce the back pressure on forward direction fan, which will allow the fans
to move a little more air out.

Kinda contradictory statements, no?

The principle here is like attaching two
water hoses together.

Completely different principles. The addition of the second fan will reduce
static pressure (as you mention above), thereby allow the fans to operate at
closer to zero static pressure (specified) CFM.

 

[Rusty]

Ric,

Kinda contradictory statements, no?

To explain the theorical principals involved here as you already know would
require a lot of knowledge about fluid mechanics and radial fans. I assume
that you fully understand the PVT properties of a compressable fluid and the
mechical principles involved in radial fan mechanics. Explain, to everyone
what happens to the air flow in a fan stacking situation because I not sure
I can without going into alot of mechanical and fluid mechanics. Zero
static pressure is what? Engineers in understand most people do not.

A fluid is a fluid weather it be air or water the dynamics of the fluid will
change but the mass balance equation that govern the flow of a fluid will
not. You can not put more fluid mass in than you can take out. If you put
two pumps in a series of the same type and specifications they will not move
more fluid than the rating on any one of the individual pumps although the
energy associated with the system will increase and it is nonlinear (not
double the system energy Gibbs Free Enery Law). The energy needed to move
the fluid remains the same but now with two pumps the energy is shared
nonlinearly between the two pumps reducing the load between them. Most
liquids are for the most part incomprssable where gases are compressable
(air). The slight volume increase will come from the properties of the
fluid (air) being compressable. The principle of mass balance remains
vailid but due to the changing compressable fluid properties of air volume
changes. These are called pressure, temperture, volume diagrams (PVT
diagrams) and the interreaction between the fan motor, blades, and the air
properties determine the fans performance curve. Due to the PVT properties
involved in a stacked fan combination volume increases due to zero static
pressure. Same principle different dynamics. Now, could you please defend
your statement "completely different principles" because pressure as far as
I know only affect the fluid properties (volume) not the principle mass
involved.

Completely different principles. The addition of the second fan will
reduce
static pressure (as you mention above), thereby allow the fans to operate
at
closer to zero static pressure (specified) CFM.

I really don't see how you get close enough to a zero static pressure in a
stacked fan arrangement due to frictional effects between the two fans to
make a big difference in volume. If the fans turned in opposite directions
and moved air in the same direction (like a turbine) I could see your point.
Explain.

 

[Bronney Hui]

Rusty,

Thanks for your advice, however I am not that keen on cutting things and I
think I'd add 1 80mm fan on the 5.25" bay for now, see how it goes, then add
another one if needed. I got 4 bays to use and they might just fit.

and I failed my 1st year Chem btw hehe hated the PVT thing.