Regarding Push/Pull and Thicker fans!!

[GordonFreemanInTheFlesh]

☆Let's say we have 2 thin fans (20mm thick) each 8000rpm.

☆And we have another (38mm thick fan that's also 8000rpm)

Would it be better to run two thinner (20mm fans) in push/pull or one thicker fan (38mm thick)?

Something tells me the CFM will remain the same but the Static Pressure will double!

What is your opinion on this matter?

 

[ThrashZone]

Hi,
I use push/ pull on radiators works really well and I'm able to use a little lower rpm with less resistance

Think thin fans are always used for space limitations not a which one is better.

 

[natr0n]

You never know unless you try.

 

[JATownes]

I honestly am very curious about this now. My guess is that a single, thicker fan will be better than two thin fans. My reason for thinking this is about blade design. I think a thicker frame allows for a better angle and depth of the blades to achieve higher pressure in comparison to the space limitations of 20mm.

Great question I've never really thought of before. If you test this please post results.

 

[TheoneandonlyMrK]

☆Let's say we have 2 thin fans (20mm thick) each 8000rpm.

☆And we have another (38mm thick fan that's also 8000rpm)

Would it be better to run two thinner (20mm fans) in push/pull or one thicker fan (38mm thick)?

Something tells me the CFM will remain the same but the Static Pressure will double!

What is your opinion on this matter?

It's going to depend massively on the fans but shallower fan, shallower Hub, less power so I would expect the 38mm thick to possibly beat two 20mm ones, but it really does depend on what fans, do you have any in mind.

 

[freeagent]

Thick fans are the bestestest.

 

[Count von Schwalbe]

Thicker fan will win.

A second fan, unless it is running reversed to the first (really hard to find) will do little to nothing. The reason? Rotating fam blades impart as much or more of their energy into making the output air column rotate, rather than fly straight back.

Contra-rotating fans can overcome this, they can double the SP, with similar unrestricted flow as a single 20mm thick fan. The 38mm thick fan will have superior airflow and SP to stacked identical 20mm fans.

 

[doyll]

Stacking 2 of same fan doesn't do much unless you have a airflow straightener between them. Fan impeller creates a circular airflow out of fan that matches impeller in 2nd fan in stack, so 2nd fan ends up simply spinning like 1st fan without increasing performance.

For stacked fans to work requires one fan spinning opposite direction or airflow straightener between fans. Push / pull on a radiator has radiator as airflow straightener.

In creasing airflow pressure rating does not increase airflow if there is no airflow resistance, but everything (fan grill, filter, cables, etc.) in airflow is creating airflow resistance. Higher pressure rating increases fans' ability to overcome airflow resistance and that means we almost alwasy will have more airflow from fans with higher pressure ratings.

Hope that all makes sense.

 

[80251]

If you put two fans in push pull across a radiator or heatsink wouldn't that make the idea of contra-rotation irrelevant? Wouldn't the radiator or heatsink tend to eliminate any rotation of the fan's output air stream?

 

[ThrashZone]

If you put two fans in push pull across a radiator or heatsink wouldn't that make the idea of contra-rotation irrelevant? Wouldn't the radiator or heatsink tend to eliminate any rotation of the fan's output air stream?

Hi,
If using a splitter it wouldn't be an issue.

 

[freeagent]

If you put two fans in push pull across a radiator or heatsink wouldn't that make the idea of contra-rotation irrelevant? Wouldn't the radiator or heatsink tend to eliminate any rotation of the fan's output air stream?

Yes.

Contra fans are good for push/pulling through loong boy cases packed full of heatsinks that have no fans on them.

 

[doyll]

If you put two fans in push pull across a radiator or heatsink wouldn't that make the idea of contra-rotation irrelevant? Wouldn't the radiator or heatsink tend to eliminate any rotation of the fan's output air stream?

Yes as said, but not as good as contra fans. Rradiator has small rectangular holes that straighten airflow. Fins on cooler do too but not as good because all are in one plane.

 

[GordonFreemanInTheFlesh]

Thicker fan will win.

A second fan, unless it is running reversed to the first (really hard to find) will do little to nothing. The reason? Rotating fam blades impart as much or more of their energy into making the output air column rotate, rather than fly straight back.

Contra-rotating fans can overcome this, they can double the SP, with similar unrestricted flow as a single 20mm thick fan. The 38mm thick fan will have superior airflow and SP to stacked identical 20mm fans.

I was not planning on stacking the but rather:

Fan 1. Fan 2.
Back of case[]--{}--------------{}--[] Front of case
<~~~~Airflow~~~~<
If that makes any sense, basically one fan will be in the front and one in the back!

 

[Count von Schwalbe]

I was not planning on stacking the but rather:

Fan 1. Fan 2.
Back of case[]--{}----------------{}--[] Front of case
<~~~~Airflow~~~~<
If that makes any sense, basically one fan will be in the front and one in the back!

Yes, you can add the static pressure ratings of the fans together if they are not straight up stacked. However, in that case I would recommend using a pair of 38mm fans over a pair of 20mm fans. Bigger=better almost always when it comes to fans.

 

[GordonFreemanInTheFlesh]

Stacking 2 of same fan doesn't do much unless you have a airflow straightener between them. Fan impeller creates a circular airflow out of fan that matches impeller in 2nd fan in stack, so 2nd fan ends up simply spinning like 1st fan without increasing performance.

For stacked fans to work requires one fan spinning opposite direction or airflow straightener between fans. Push / pull on a radiator has radiator as airflow straightener.

In creasing airflow pressure rating does not increase airflow if there is no airflow resistance, but everything (fan grill, filter, cables, etc.) in airflow is creating airflow resistance. Higher pressure rating increases fans' ability to overcome airflow resistance and that means we almost alwasy will have more airflow from fans with higher pressure ratings.

Hope that all makes sense.

My original question was not about stacking them but rather:

Example:
You have a pipe and you install one pull fan(20mm thick) in the front of that pipe as intake and another one(20mm thick) in the end of that pipe as exhaust {There is a distance between the two fans approx. 10cm} so would those two fans in such a configuration out perform a single 38mm thick fan that is configured on the same pipe as exhaust?

All three fans have the same rpm rating say 8000.

Would the performance be in favour of the 20mm thick fans or the single 38mm thick one?

It's going to depend massively on the fans but shallower fan, shallower Hub, less power so I would expect the 38mm thick to possibly beat two 20mm ones, but it really does depend on what fans, do you have any in mind.

These are the ones I'm planning to use as a replacement for my jet engine sounding HP1200w Flex ATX PSU fan. The fan that it uses is 40x40x28mm and it is very loud 50+db and I was wanting to replace it with some 30db, 8200rpm 40x40x20mm Sunon Maglev ones!

Part number: MF40201VX-1000U-G99

To be clear I was never going to stack two 20mm thick fans in the first place but rather replace the stock 28mm thick one with a 20mm thick and put another 20mm thick one in the very front of the case acting like an improvised push pull.

I honestly am very curious about this now. My guess is that a single, thicker fan will be better than two thin fans. My reason for thinking this is about blade design. I think a thicker frame allows for a better angle and depth of the blades to achieve higher pressure in comparison to the space limitations of 20mm.

Great question I've never really thought of before. If you test this please post results.

Will do!

 

[ThrashZone]

Hi,
Didn't say anything before but 8000rpm you'd be taking off soon so buckle up
Don't forget ear protection either

 

[Bones]

Thicker fan will win.

A second fan, unless it is running reversed to the first (really hard to find) will do little to nothing. The reason? Rotating fam blades impart as much or more of their energy into making the output air column rotate, rather than fly straight back.

Contra-rotating fans can overcome this, they can double the SP, with similar unrestricted flow as a single 20mm thick fan. The 38mm thick fan will have superior airflow and SP to stacked identical 20mm fans.

There's more than just this to it.

Along with how "Deep" (Thickness) the body/frame of the fan is there is also something that hasn't been mentioned yet in play.

The thickness of the fanblades themselves is another factor and of course the pitch of them matters too - Which is what I'm getting at here.
It's no secret if the blades are thicker they won't flex as much which matters, if the blade is trying to flatten out as it pushes air, that decreases pitch which means less air moved.

The two basic things are how much push-back the blade gets from the effort of pushing air and the fan's centrifugal force as it spins which will also have a limited effect. Centrifugal force is by far the lesser of the two things and only comes into play with high-speed fans, since the blade is curved it will try to straighten out a bit because of how the blade is made with it's pitch but again, this is only a factor with high-speed fans with thin blades running high RPM's and it's not much of a factor, yet it's there.

The push-back against the blade from the air that's trying to resist movement is what the real thing is about it, that will naturally try to flatten out the blade against it's pitch. The more pressure the fan is making the more push-back it gets and a thin blade can only stand so much push-back against it's pitch before this pitch-flattening effect starts taking place. That's why for example, server fans such as a Delta are made with really thick blades, to resist it and maintain CFM efficiency at a given RPM - Not to mention we all know Delta's tend to be high RPM fans anyway and can move alot of air.

I've got 4 170 CFM fans and a single 240 CFM fan to have seen it myself in action - No way a standard plastic bladed fan can turn the RPM's these can and still push as much CFM's as these do because of it, they'll flatten out and possibly start breaking off blades before they come close to the CFM's these can push.

I'm sure all of you know how much better a fanblade made of metal does vs one made of plastic with the plastic ones being kinda thin, it will flatten out some as it spins reducing the pitch and push it has but a metal fan can resist this much better so it pushes more air by CFM. Thick plastic blades do the same thing and that's why you'd want one with thick blades too if you can get them that way.
You can run a lower fan RPM with them for less noise yet still have good airflow, depending on the actual pitch of the fan blades of course.

 

[GordonFreemanInTheFlesh]

Hi,
Didn't say anything before but 8000rpm you'd be taking off soon so buckle up

They are maglev fans so really they don't exceed 30db

There's more than just this to it.

Along with how "Deep" (Thickness) the body/frame of the fan is there is also something that hasn't been mentioned yet in play.

The thickness of the fanblades themselves is another factor and of course the pitch of them matters too - Which is what I'm getting at here.
It's no secret if the blades are thicker they won't flex as much which matters, if the blade is trying to flatten out as it pushes air, that decreases pitch which means less air moved.

The two basic things are how much push-back the blade gets from the effort of pushing air and the fan's centrifugal force as it spins which will also have a limited effect. Centrifugal force is by far the lesser of the two things and only comes into play with high-speed fans, since the blade is curved it will try to straighten out a bit because of how the blade is made with it's pitch but again, this is only a factor with high-speed fans with thin blades running high RPM's and it's not much of a factor, yet it's there.

The push-back against the blade from the air that's trying to resist movement is what the real thing is about it, that will naturally try to flatten out the blade against it's pitch. The more pressure the fan is making the more push-back it gets and a thin blade can only stand so much push-back against it's pitch before this pitch-flattening effect starts taking place. That's why for example, server fans such as a Delta are made with really thick blades, to resist it and maintain CFM efficiency at a given RPM - Not to mention we all know Delta's tend to be high RPM fans anyway and can move alot of air.

I've got 4 170 CFM fans and a single 240 CFM fan to have seen it myself in action - No way a standard plastic bladed fan can turn the RPM's these can and still push as much CFM's as these do because of it, they'll flatten out and possibly start breaking off blades before they come close to the CFM's these can push.

I'm sure all of you know how much better a fanblade made of metal does vs one made of plastic with the plastic ones being kinda thin, it will flatten out some as it spins reducing the pitch and push it has but a metal fan can resist this much better so it pushes more air by CFM. Thick plastic blades do the same thing and that's why you'd want one with thick blades too if you can get them that way.
You can run a lower fan RPM with them for less noise yet still have good airflow.

Very interesting ....

 

[Count von Schwalbe]

Depending on what is inside the "pipe" you mention, the air column may still be rotating at considerable speed (due to friction, not centrifugal force) and reduce the efficiency of the second fan. Now, if you have a bunch of stuff inside that tube, breaking up the airflow pattern, you will see the benefit of the second fan.

Also, if you use a single fan, note that almost all fans are better at blowing than sucking. You will see slightly higher airflow if you have it on the intake. However, I do not know it it will make a measurable difference.

 

[Bones]

Depending on what is inside the "pipe" you mention, the air column may still be rotating at considerable speed (due to friction, not centrifugal force) and reduce the efficiency of the second fan. Now, if you have a bunch of stuff inside that tube, breaking up the airflow pattern, you will see the benefit of the second fan.

Also, if you use a single fan, note that almost all fans are better at blowing than sucking. You will see slightly higher airflow if you have it on the intake. However, I do not know it it will make a measurable difference.

The air column itself doesn't spin, unless you have something inside the body of the fan or just beyond it to induce spin, it simply moves along. If you create something that will induce spin, that can help if your fan has the push that will help drive the air through the system but there's always resistance to any force the fan tries to impart to the air being moved.
Said resistance will naturally increase due to the fact you're trying to move and spin air at the same time.

Ain't no free lunch here.

Centrifugal force itself is not cancelled out and always has the same effect - It's a natural force you can't just eliminate but at the same time it's effect is extremely limited too so it's fair to say it's essentially a non-factor when you get down to it.

True, fans are better at trying to push air in than trying to draw it out so an ideal setup has more than one fan pushing air in.
Note that if you have a fan on the outlet side that could be detrimental depending on how much CFM of air the exhaust fan itself can move, basically causing it to be a restriction rather than helping. I'd have to say you want a little more CFM capability on the exhaust side than the inlet side, such as a 40 CFM fan for intake and maybe a 50-60 CFM fan at the exhaust to offset this potential problem. The intake fan pushes the air in and the exhaust fan helps it to move along through the system and also to where it needs to go.
Even though the air goes in at a given rate of CFM at the exhaust side of the intake fan, as the air travels through and expands throughout the case it's velocity slows down too. An exhaust fan(s) will do a couple of things, one is to help direct the airflow where you want to go for exiting the system and also helps to maintain air velocity in the system as it passes through.

 

[doyll]

Mounting a front intake and a rear exhaust fans will not double pressure rating!!!
The open space between fans allows air from other area in case to mix / equalize pressure in rest of case.

Keep in mind our fans make very little pressure.
For example, the difference in barometric pressure at sea level and 30 feet above sea level is 11.013 mm H20.
That means the difference in pressure between sea level and 10 feet above sea level of 3.671 mm H2O,
Divide that to 5 feet difference in altitude and we have 1.836 mm H2O less pressure on chest than on our feet .. same amount of static pressure a fan rated 1.836 mm H2O greater pressure than on intake side.
1.836 mm H2O more in sealed container on exhaust side of fan than is on intake side of fan.
1.836 mm H2O more on feet than on chest 5 feet above feet.

Fans we use in our systems rarely have static pressure rating as high as 1.836 mm H2O .. and that is at full speed .. which most of us never use.
8000rpm fans will be making more, but I suspect the pain and hammering at that noise level will completely overpower anything else happening in the room.

 

[GordonFreemanInTheFlesh]

Mounting a front intake and a rear exhaust fans will not double pressure rating!!!
The open space between fans allows air from other area in case to mix / equalize pressure in rest of case.

Keep in mind our fans make very little pressure.
For example, the difference in barometric pressure at sea level and 30 feet above sea level is 11.013 mm H20.
That means the difference in pressure between sea level and 10 feet above sea level of 3.671 mm H2O,
Divide that to 5 feet difference in altitude and we have 1.836 mm H2O less pressure on chest than on our feet .. same amount of static pressure a fan rated 1.836 mm H2O greater pressure than on intake side.
1.836 mm H2O more in sealed container on exhaust side of fan than is on intake side of fan.
1.836 mm H2O more on feet than on chest 5 feet above feet.

Fans we use in our systems rarely have static pressure rating as high as 1.836 mm H2O .. and that is at full speed .. which most of us never use.
8000rpm fans will be making more, but I suspect the pain and hammering at that noise level will completely overpower anything else happening in the room.

These are Maglev fans and are rated at 27.5dba at full speed!

Yes, you can add the static pressure ratings of the fans together if they are not straight up stacked. However, in that case I would recommend using a pair of 38mm fans over a pair of 20mm fans. Bigger=better almost always when it comes to fans.

My application is complicated but basically there is no way I can use 38mm fans because they are noisy!

 

[80251]

I believe industrial fan manufacturers make fans even thicker than 38mm, for example, I have a Nidec 127x50mm and a Delta 150x50mm. Purpose built contra-rotating fans are even thicker than this.

 

[GordonFreemanInTheFlesh]

I believe industrial fan manufacturers make fans even thicker than 38mm, for example, I have a Nidec 127x50mm and a Delta 150x50mm. Purpose built contra-rotating fans are even thicker than this.

My original requirement is 1U fans ....seems like this thread turned into a wild goose chase.

 

[claes]

That’s on you my friend. If you had just said you’re looking for a replacement for your PSU fan we wouldn’t have spent a dozen posts figuring out what you mean by pipe and the other dozen discussing physics