Air-pumps (aka fans) - series or parallel better

Cathar · 07-06-2003, 12:56 AM

Haven't much time to think on this, but the thought did occur to me.

It's been proven pretty much time and again that sticking water pumps in series is better than sticking them in parallel, when pushing against any form of back-pressure.

I was wondering if the same applied to axial fans?

An axial fan will push maybe 1/4 of its rated flow through a radiator. Rather than sticking two fans in parallel has anyone crunched through the math yet to find whether or not that in series would be better than in parallel?

I'd do it myself but I'm feeling lazy today and just thought I'd ask if it had been discussed before?

FuzzyFace · 07-06-2003, 01:40 AM

Comair Rotron discusses this in one of their engineering notes.
http://www.comairrotron.com/Engineering/airflow.htm
Series for high impedance, parallel for low impedance.

Cathar · 07-06-2003, 08:16 AM

Thanks for the link FuzzyFace. It is pretty much as I expected.

Gulp35 · 07-06-2003, 01:05 PM

I've read on some of the less wise forums (OC.com, HardOCP.com) that when running fans in series that they had to be spinning in separate directions.

I know this is false (ex. Jet engines all turbines turn the same way) but would using a adapter, reducing the fan size and increaseing the velocity of the air (similar to what happens in jets and turbofan engines) increase performance in radiators? and thus reducing the size of radiator for a given cooling capability (ex. Cathar you could use only one of your Big Arse radiators instead of two).

sevisehda · 07-06-2003, 07:03 PM

In actualaty someone would have to find either the curve or at least some data points for the fan. Fans are just like pumps when it comes to the interaction between reistance and flow.
2 fans in series will up the pressure gradient which should increase airflow, by how much is anyones guess.

As for increasing air through a radiaor. I've read sucking air through is better than pushing, however I'm sure fan-rad-fan would be even better.

The problem with 2 fans is they make alot of noise because of the interaction of the rotating fan blades. There is alot of turbulence which mkaes plenty of noise and hurts flow. By going fan-rad-fan the radiators fins will straigten the airs flow and reduce the noise, compared to 2 fans on top of eachother.

Boli · 07-07-2003, 05:32 AM

May sound like a dumb question but I'll ask t anyway:

Are you talking about the fan WIRING (i.e. a electrical circuit with two fans on in parallel/series)

or the PLACEMENT of the fans (i.e. two fans side by side on a radiator or two fans one at either side of the rad.)

Pritorian · 07-07-2003, 09:02 AM

Quote:

Originally posted by Boli
May sound like a dumb question but I'll ask t anyway:

Are you talking about the fan WIRING (i.e. a electrical circuit with two fans on in parallel/series)

or the PLACEMENT of the fans (i.e. two fans side by side on a radiator or two fans one at either side of the rad.)

Placement

-P-

Boli · 07-07-2003, 10:42 AM

Then a fan on either side of the radiator works best but it is noisier.

I have both sides on 5v so they are silent but it is better than one 7v fan, though if I want O can REALLY get the air flowing and take it to within a dgree of amibiant (as apposed to 10*C)... only do that on hot days.

Cathar · 07-07-2003, 06:40 PM

Okay, the deal is this.

I presently have a large 25cm house exhaust fan driving 2 x Camry heater-cores. I've discovered that house exhaust fans don't like running 24/7, and after about 6 weeks the fan is giving up the ghost slowly (spinning slower and slower).

So I have 2 x 15cm Comair-Rotron 210CFM beasties on the way.

The two heater-cores sit side-by-side in a panel in a large radiator/pump combo box that the computer sits on top of, but I made the box fairly modular so I can change its configuration fairly easily.

So the question specifically pertains to my scenario. Will it be better to do:

1) Create separate plenum chambers for each radiator and have 1 fan acting on each radiator.

2) Put both fans in parallel sucking (or blowing) through both radiators. Basically the same as configuration 1) but without the plenum chamber separator

3) Stick the radiators in the middle of the box and have 1 fan at each end of the box and do a push-pull job.

Pritorian · 07-07-2003, 07:06 PM

Now your point 2 and 3 should be very similar inn performance if the fans are powerfull. ( like yours )

But for slower moving fans and fans that are not that powerfull, i would think that the push pull config would do the best job.

Just guessing here, so correct me if i´m wrong

Reading the linky now.. maybe i´ll change my mind, hehe..

edit: spelling.

Cathar · 07-07-2003, 09:42 PM

Okay, looking at the various radiators from BillA's tests, and the finnage and surface area, and a bit of hand-waving, and I'd roughly project that the two Camry cores in parallel would have an air-flow pressure drop of around 0.05inH2O at 100CFM.

Munching through the PQ curves for the fans (at 12V) it would appear that a ~200CFM total air-flow would be predicted for the two fans in parallel (~0.25inH2O pressure drop matched off against each fan's PQ curve).

Since 200CFM is about what even a single fan can push in free-flow mode, the prediction would be that in this scenario that the two fans in parallel would be the better option than in series.

If the fans and radiators are split into their own separate plenum chambers, the problem becomes a simpler one of predicting air-flow for a single fan against a 0.10inH2O at 100CFM scenario. Here we can roughly determine that each fan would push ~140CFM through a single radiator, so clearly the separate plenum chamber operation is much better, since we have now boosted per-radiator flow from 100CFM to 140CFM just by separating the two parallel fans acting on two parallel radiators into a pairs of single fans acting on single radiators.

In series the total air-flow works out to around 180CFM.

So I can somewhat predict that for my particular scenario with fairly large free-flowing radiators, that a separated pairing is better than a parallel pairing, which is again better than in series.

For a more air-flow restrictive radiator setup (say if I were only using a single radiator), then the in-series option would work out better.

Time to do some more wood-work...

Rufus210 · 07-08-2003, 12:53 AM

I was actually just thinking about this. The setup I'm going to be using is a large (11" x 7") Chevette-style heater core with 2 x 120mm fans (these to be exact). The heatercore seems to have similar resistance to those in Joe's oc.com article, but my fans are relatively weak (especially since I will most likely 7V them). With that considered, would a push-pull make more sense, or 2 x pull?

[yes, I know 2xpush, 2xpull would be the best...but that's an extra $25 and I'm cheap

]

Razor6 · 07-09-2003, 12:48 AM

Assuming you can obtain the performance curves for your fan and you can figure out how much resistance to flow your radiator presents, figuring out whether parallel or serial is best should be pretty easy I would think. Making a graph like the one below would allow you to see how much air flow you get for series and parallel fans.

07-06-2003, 12:56 AM	#1
Cathar Thermophile Join Date: Sep 2002 Location: Melbourne, Australia Posts: 2,538	Air-pumps (aka fans) - series or parallel better Haven't much time to think on this, but the thought did occur to me. It's been proven pretty much time and again that sticking water pumps in series is better than sticking them in parallel, when pushing against any form of back-pressure. I was wondering if the same applied to axial fans? An axial fan will push maybe 1/4 of its rated flow through a radiator. Rather than sticking two fans in parallel has anyone crunched through the math yet to find whether or not that in series would be better than in parallel? I'd do it myself but I'm feeling lazy today and just thought I'd ask if it had been discussed before?

07-06-2003, 01:40 AM	#2
FuzzyFace Cooling Neophyte Join Date: Dec 2001 Location: California high desert Posts: 52	Comair Rotron discusses this in one of their engineering notes. http://www.comairrotron.com/Engineering/airflow.htm Series for high impedance, parallel for low impedance. __________________ Have fun, FuzzyFace

07-06-2003, 01:05 PM	#4
Gulp35 Cooling Savant Join Date: Sep 2002 Location: Atlanta, GA Posts: 135	I've read on some of the less wise forums (OC.com, HardOCP.com) that when running fans in series that they had to be spinning in separate directions. I know this is false (ex. Jet engines all turbines turn the same way) but would using a adapter, reducing the fan size and increaseing the velocity of the air (similar to what happens in jets and turbofan engines) increase performance in radiators? and thus reducing the size of radiator for a given cooling capability (ex. Cathar you could use only one of your Big Arse radiators instead of two). __________________ Epox 8RDA+ v1.1----------Tyan K8SDPro 1700+ JUIHB 0310XPMW (12.5x200Mhz@1.792v)--------2xOpteron246 (2Ghz 1MB L2) 2x256MB Kingston PC3000 (BH5?) 2-2-2-8 2.7v---2x512MB Corsair Reg.&ECC PC3200 ATi Radeon 9800Pro (stock, too hot)-----ATI RageXL WD800JB+WD2500JB-------WD1600JD Sony DVD/CDRW-------NEC 3520A Black Forton350W--------Antec TruePower2.0 550W EPS12v

07-07-2003, 07:06 PM	#10
Pritorian Cooling Savant Join Date: Sep 2002 Location: Norway Posts: 201	Now your point 2 and 3 should be very similar inn performance if the fans are powerfull. ( like yours ) But for slower moving fans and fans that are not that powerfull, i would think that the push pull config would do the best job. Just guessing here, so correct me if i´m wrong Reading the linky now.. maybe i´ll change my mind, hehe.. edit: spelling. __________________ Boo!

07-06-2003, 08:16 AM	#3
Cathar Thermophile Join Date: Sep 2002 Location: Melbourne, Australia Posts: 2,538	Thanks for the link FuzzyFace. It is pretty much as I expected.

07-06-2003, 07:03 PM	#5
sevisehda Cooling Savant Join Date: May 2003 Location: NY Posts: 234	In actualaty someone would have to find either the curve or at least some data points for the fan. Fans are just like pumps when it comes to the interaction between reistance and flow. 2 fans in series will up the pressure gradient which should increase airflow, by how much is anyones guess. As for increasing air through a radiaor. I've read sucking air through is better than pushing, however I'm sure fan-rad-fan would be even better. The problem with 2 fans is they make alot of noise because of the interaction of the rotating fan blades. There is alot of turbulence which mkaes plenty of noise and hurts flow. By going fan-rad-fan the radiators fins will straigten the airs flow and reduce the noise, compared to 2 fans on top of eachother.

07-07-2003, 05:32 AM	#6
Boli Cooling Savant Join Date: Apr 2003 Location: Blackburn / Dundee Posts: 451	May sound like a dumb question but I'll ask t anyway: Are you talking about the fan WIRING (i.e. a electrical circuit with two fans on in parallel/series) or the PLACEMENT of the fans (i.e. two fans side by side on a radiator or two fans one at either side of the rad.)

07-07-2003, 10:42 AM	#8
Boli Cooling Savant Join Date: Apr 2003 Location: Blackburn / Dundee Posts: 451	Then a fan on either side of the radiator works best but it is noisier. I have both sides on 5v so they are silent but it is better than one 7v fan, though if I want O can REALLY get the air flowing and take it to within a dgree of amibiant (as apposed to 10*C)... only do that on hot days.

07-07-2003, 06:40 PM	#9
Cathar Thermophile Join Date: Sep 2002 Location: Melbourne, Australia Posts: 2,538	Okay, the deal is this. I presently have a large 25cm house exhaust fan driving 2 x Camry heater-cores. I've discovered that house exhaust fans don't like running 24/7, and after about 6 weeks the fan is giving up the ghost slowly (spinning slower and slower). So I have 2 x 15cm Comair-Rotron 210CFM beasties on the way. The two heater-cores sit side-by-side in a panel in a large radiator/pump combo box that the computer sits on top of, but I made the box fairly modular so I can change its configuration fairly easily. So the question specifically pertains to my scenario. Will it be better to do: 1) Create separate plenum chambers for each radiator and have 1 fan acting on each radiator. 2) Put both fans in parallel sucking (or blowing) through both radiators. Basically the same as configuration 1) but without the plenum chamber separator 3) Stick the radiators in the middle of the box and have 1 fan at each end of the box and do a push-pull job.

07-07-2003, 09:42 PM	#11
Cathar Thermophile Join Date: Sep 2002 Location: Melbourne, Australia Posts: 2,538	Okay, looking at the various radiators from BillA's tests, and the finnage and surface area, and a bit of hand-waving, and I'd roughly project that the two Camry cores in parallel would have an air-flow pressure drop of around 0.05inH2O at 100CFM. Munching through the PQ curves for the fans (at 12V) it would appear that a ~200CFM total air-flow would be predicted for the two fans in parallel (~0.25inH2O pressure drop matched off against each fan's PQ curve). Since 200CFM is about what even a single fan can push in free-flow mode, the prediction would be that in this scenario that the two fans in parallel would be the better option than in series. If the fans and radiators are split into their own separate plenum chambers, the problem becomes a simpler one of predicting air-flow for a single fan against a 0.10inH2O at 100CFM scenario. Here we can roughly determine that each fan would push ~140CFM through a single radiator, so clearly the separate plenum chamber operation is much better, since we have now boosted per-radiator flow from 100CFM to 140CFM just by separating the two parallel fans acting on two parallel radiators into a pairs of single fans acting on single radiators. In series the total air-flow works out to around 180CFM. So I can somewhat predict that for my particular scenario with fairly large free-flowing radiators, that a separated pairing is better than a parallel pairing, which is again better than in series. For a more air-flow restrictive radiator setup (say if I were only using a single radiator), then the in-series option would work out better. Time to do some more wood-work...

07-08-2003, 12:53 AM	#12
Rufus210 Cooling Neophyte Join Date: May 2003 Location: Houston Posts: 15	I was actually just thinking about this. The setup I'm going to be using is a large (11" x 7") Chevette-style heater core with 2 x 120mm fans (these to be exact). The heatercore seems to have similar resistance to those in Joe's oc.com article, but my fans are relatively weak (especially since I will most likely 7V them). With that considered, would a push-pull make more sense, or 2 x pull? [yes, I know 2xpush, 2xpull would be the best...but that's an extra $25 and I'm cheap ]

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)