Result of Stacking 2 fans 2

[MDGroup]

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

 

[telecomguy]

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>

 

[bithkits]

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.

http://www.martin-electronics.co.za

 

[rb1957]

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.

 

[ione]

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

 

[MDGroup]

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?

 

[ione]

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

 

[tgrimley]

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

 

[sreid]

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.

 

[rb1957]

(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).

 

[TheTick]

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.)

 

[electricpete]

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

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

 

[MiketheEngineer]

And you might get some weird whistling noises!!

 

[drawoh]

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

 

[Compositepro]

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.

 

[dgallup]

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.

 

[KENAT]

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

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[Walterke]

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

NX 7.5
Teamcenter 8

 

[zekeman]

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.

 

[MDGroup]

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.