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Result of Stacking 2 fans
2

Result of Stacking 2 fans

Result of Stacking 2 fans

(OP)
What happens if you stack 2 cooling fans on the back of an electronic enclosure?

There isn't room to put them side-by-side, so they are piggybacked one on top of the other.

Does it double the amount of airflow? or does it just push the same air twice?

The outlet opening is still the same size, so I assume you don't have to double the size of the intake?

Is the CFM doubled, or does it stay the same?


Ultimate question: Does stacking 2 fans increase the cooling across the electronics inside of the enclosure?

Any thoughts on this are appreciated? Thanks
 

RE: Result of Stacking 2 fans

You essentially double the pressure axis of the fan curve.  You've increased the power to overcome resistance but as you noted, you are still moving the same ultimate volume of air.

<tg>

RE: Result of Stacking 2 fans

If you can accommodate two fans on top of one another, then just use a stronger fan with a larger motor!

For example, for computer fans, the vantec tornado fan is 92x92x38mm instead of the usual 92x92x25mm to accommodate the larger (thicker) motor

Adriaan.
I am a Mechatronics Engineer from South Africa.
www.martin-electronics.co.za

RE: Result of Stacking 2 fans

my 2c ... yes, but not by much ... i expect each fan will have a pressure drop = a flow speed increase, that each fan will draw power.  that would imply that the 2nd fan should exhaust air faster than a single fan.  

my sense is that the doubled fans are not going to be as efficient, so the result of doubling the fans is less than 2* a single fan, and would be noisier.

RE: Result of Stacking 2 fans

The mass flow rate remains constant, but the volumetric flow rate decreases for each stage you add as the air density increases.

RE: Result of Stacking 2 fans

(OP)
A few quick comments:

We can't use a single larger fan. The way the circuitry is set up, each fan is tied to a different power supply. So if one power supply comes on, one fan turns on. If the other comes on, the other fan turns on. If both are on, both fans are on.
I can't change anything on the electronics side.

So, if the 2 stacked fans increase pressure, does that mean that velocity is increased. So, if velocity is increased, doesn't that mean that airflow is increased.

So, cooling is increased, but not doubled? Is that correct?

RE: Result of Stacking 2 fans

No. Increased pressure doesn't mean increased velocity. Increased flow rate, with constant duct, implies increased velocity.

RE: Result of Stacking 2 fans

Consider adding an OR gate to allow either power supply to switch on the fan?

RE: Result of Stacking 2 fans

How much air a fan will flow depends on the resistance to air flow the fan sees.  If there is high resistance to air flow, stacking fans will increase the air flow.  If the resistance to air flow is low, fans in parallel will increase the air flow.

 

RE: Result of Stacking 2 fans

(apologies for repeating myself) ...

if each fan is consuming power, then each fan will have a pressure drop  and so each fan will be increasing the exhaust velocity (ie the double pack should have a higher exhaust velocity than a single fan) but less than double (i'd expect).

something else to consider ...
with a double fan stack, the single fans will be less efficient, as their inlet or outlet (depending on which individual of the pair we're talking about) are obstructed (by the other fan).

RE: Result of Stacking 2 fans

It's like batteries.  Fans in series double pressure w/ same airflow.  Fans in parallel double airflow w/ same pressure.

(Yes, there are some differences due to compressibility, efficiency, etc.)

RE: Result of Stacking 2 fans

"etc" would also include system flow-resistance characteristics as others mentioned.  

=====================================
(2B)+(2B)'  ?

RE: Result of Stacking 2 fans

And you might get some weird whistling noises!!

RE: Result of Stacking 2 fans

MDGroup,

   Two fans in series, have the ability to double pressure.  The actual pressure is a function of airflow and system resistance.  

   If your system has insignificant air resistance, the two fans will run at close to zero pressure drop, and they will run together at maximum airflow.  Effectively, the second fan will have no effect on the airflow.  

   If your system has significant air resistance, each fan will run approximately at half the pressure drop.  Your total airflow will increase.  You can look at the fan curves to see the actual amount.

   In other words, two fans in series are a solution to a system with a lot of back pressure.  Two fans in parallel are a solution to a system with low back pressure.   

               JHG

RE: Result of Stacking 2 fans

If there are no restrictions to air flow, fans develop zero pressure. So if you put two fans in series, you will decrease air flow because you will add more resistance but cannot add more velocity.

If the air flow is mainly controlled by the restrictions to the fan (i.e., the flow is much lower than a free-flowing fan will produce) then putting the fans in series will double the air flow. And putting fans in parallel, in this case, would not increase the air flow.

The way to think of fans, blowers, and centrifugal pumps is that they impart momentum (velocity) to a fluid within the device. A fan is designed to impart relatively low velocity to relatively high volumes of air. When low velocity air encounters a restriction it will generate only a small pressure. When a fan outlet is blocked the air just recirculates in the fan. A blower will generate higher velocity and therefore more pressure when restricted. In applications where there is much flow restriction, a blower should be used.

RE: Result of Stacking 2 fans

I'd be concerned about the case where only one fan is on.  I would expect the fan that is off to create significant resistance and decrease airflow compared to a single fan.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.
 

RE: Result of Stacking 2 fans

Ok, so there isn't room to put both fans into the back panel.

Is there room to mount both fans internally, with ducting (or equivalent) of some kind to an out let on the rear equivalent to the size of at least one fan or perhaps a bit more as you won't lose the bit normally covered by the center of the fan?  There will be losses etc, but it might work.

I did something vaguely similar earlier this year, they haven't finished the testing yet so I'm not sure it worked properly though!  I basically split the enclosure into 2 plenums by putting a baffle in the middle, on which the fans were mounted.  Both plenums had vents in to allow flow.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Result of Stacking 2 fans

Could you make a sketch of the situation? You'd probably get some better advise if you do.  

NX 7.5
Teamcenter 8

RE: Result of Stacking 2 fans

What you have with 2 fans in series is approximately double the static pressure for the same cfm. If the fan curve given by

eq(1) cfm=f(p)

f(p) is the fan function of pressure

then 2  stacked it becomes

eq(2) cfm =f(2p).

Now the system pressure drop is approximately proportional to  cfm^2, so

eq(3 )p=a*cfm^2

The intersection of eq 1 and eq 3 gives the cfm with one fan and eq 2 and 3 with the stacked fans.

I did this with some arbitrary fan curve and found that the cfm gain of stacked over one fan  increases  modestly, showing the most improvement for more restrictive systems.
If you are looking for doubling, don't go here.
    

RE: Result of Stacking 2 fans

(OP)
I appreciate all of the responses. This isn't my design so I can't change it; I am trying to understand it.

So, in summary, when you stack 2 fans:
The CFM stays the same.
The pressure will be increased.

But, if there are a lot of restictions to airflow at the intake and inside of the enclosure, 1 fan may not be able to exhaust the full CFM. But with 2 fans, it is able to better overcome resticted flow and get closer to the CFM that each fan could push.

Does that sound about right? Close?

Thanks.

 

RE: Result of Stacking 2 fans

no, i think you're missing the point that in the doubled set-up, neither fan will work as efficiently as a single fan ('cause they obstruct each other).

RE: Result of Stacking 2 fans

I recommend you read up on 'fans in series' and 'fans in parallel'.  You'll find lots of resources to better understand.  You're getting a little theory here as well as a little practical input.  That's why I hedged my previous statements with 'essentially' and 'ultimate'.  Stacked fans do overcome high resistance.  And in a sense, you should expect higher CFM, but this is only true because the pressure/flow rate curve becomes steeper.  Think of a 100cfm fan that provides 50cfm at your current system resistance.  You might see 65cfm flow by putting the fans in series, but you are not doubling the maximum capacity of the fan from 100cfm to 200cfm for the pair.  The best you can get is still 100cfm at zero system resistance.  But you are also hearing some concerns about the added resistance of the series fan if only one is powered at a time (of if they are running at slightly different speeds, etc).

<tg>

RE: Result of Stacking 2 fans

physics can not be denied.

Qfan1 = Qfan2*(Dfan1/Dfan2)^3 * (RPMfan1/RPMfan2)

Q=volumetric flow rate
D = hydraulic diameter (same for both fans)
RPM = speed of fan (assume its the same for both??)

thus reducing to:
Qfan1=Qfan2    no flow increase

 

RE: Result of Stacking 2 fans

won't the downstream fan create a lower back pressure on the upstream one (if both are working at the same time) ?  I think the two fans working will suck more air than a single fan, and i also expect one fan working alone in this doubled setup will suck less than a single fan (because of the obstruction of the other (non-working) fan).

RE: Result of Stacking 2 fans

snoopnoon, not denying physics.  Your calculation is assuming a constant flowrate rather than one dependent on pressure.  

See the graph on this page: http://www.greenheck.com/library/articles/42

At a pressure of 3 (units unknown), a single fan will provide a flow rate of 13 while two fans will produce about 15.

But here's a little info on the real world application of series fans: http://www.ebmpapst-ad.com/media/content/technical_articles/TA_Using-fans-in-series-and-parallel.pdf

<tg>

RE: Result of Stacking 2 fans

You may want to consider using using counter-rotating fans or stator vanes to straighten the flow before it enters the second fan. Compressors in turbine engines are similar to what you're describing. Turbine engine compressors generally result in a pressure rise of 15%-60% per stage.

I found this paper on the web with a bit of information about compressor design:
http://www.tpe.energy.lth.se/fileadmin/tpe/Axial_Flow_Compressor_Mean_Line_Design.pdf

//signed//
Christopher K. Hubley
Mechanical Engineer
Sunpower Incorporated
Athens, Ohio
--
http://engineeringliberty.wordpress.com

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