# How can you determine if a fan is better at blowing or sucking?

Summary:
I am trying to design some systems that involve fans, both regular computer type fans, and centrifugal type fans.
I am trying to determine if they are better placed in positions where they suck or blow. Is there a rule to determining this?

hutchphd
Homework Helper
"Some systems" is just a little bit vague don't you think? There are a variety of fans for a variety of situations. What are you trying to do?

russ_watters
phinds
Gold Member
2021 Award
How can you determine if a fan is better at blowing or sucking?
If it's made in China, it probably sucks.

PhDeezNutz, HAYAO, Hamiltonian and 1 other person
"Some systems" is just a little bit vague don't you think? There are a variety of fans for a variety of situations. What are you trying to do?

I am being vague because its not specific to any one system, I am working on many, and so just looking for a general rule on determining if a fan is better for pulling or pushing air.

anorlunda
Staff Emeritus
One consideration is dirt, dust, and contamination. If you blow air into a dirty container, some of the dirt will get blown out into the surroundings. If you suck air in from a dirty environment to a clean container, that too causes contamination. In many cases, both kinds of fans are used together, one blowing in and one out. The room's equilibrium is either positive or negative pressure relative to the surroundings.

Think of a bio-hazard handling room for example. You want it so that any leaks leak into the room, not out. Ditto when the door opens, you want the air flow to go into the room. Any exhaust from the room goes through a filter.

phinds and russ_watters
russ_watters
Mentor
I am being vague because its not specific to any one system, I am working on many, and so just looking for a general rule on determining if a fan is better for pulling or pushing air.
I guess as stated, the answer should be that they have to be equal, per conservation of mass/energy. The real answer depends on the design of the total system. E.G., can I put an inlet cone on my fan?

FactChecker, Dr.D and mess
I see so the main consideration is just contamination effecting performance, and that all fans perform equally well sucking/blowing if their position is reversed in the same system, lets say a long tube. I tested this out with a 7' pipe and i got different results for CFM measurement from sucking and blowing, which is why I asked this, but its very likely my test was flawed due to leaking.

hutchphd
Homework Helper
Turbulence is your enemy if flow is what you need to maximize. And air is a compressible fluid so sucking and blowing are very much not interchangeable. The patterns obtained by turning the fan around can be very different. So the devil is in the detail.
Also there are lots of other design considerations as mentioned.

FactChecker
Lnewqban
Gold Member
Summary:: I am trying to design some systems that involve fans, both regular computer type fans, and centrifugal type fans.

I am trying to determine if they are better placed in positions where they suck or blow. Is there a rule to determining this?
https://www.achrnews.com/articles/96369-minimizing-system-effect

DaveE
tech99
Gold Member
Turbulence is your enemy if flow is what you need to maximize. And air is a compressible fluid so sucking and blowing are very much not interchangeable. The patterns obtained by turning the fan around can be very different. So the devil is in the detail.
Also there are lots of other design considerations as mentioned.
I remember watching the flow of water arriving at an outlet in a lake. I notice that eddys appeared some distance before the water entered the outlet, and it seemed to me that we get the same eddys whether water flows in or out. So I would like to know if there is a fundamental difference between the two cases.

hutchphd
Homework Helper
I would think the fundamental difference is that water is approximately incompressible. The presence of the surface makes the analogy more difficult. I claim no particular expertise here, and perhaps wiser heads need intervene.

DaveE
Gold Member

1) When fans work, they move air molecules and create a pressure difference across the fan. Too cool stuff you want lots of molecules. Usually one side of the fan is at ambient pressure, which won't change. So orient your fan to blow into the enclosure to make higher pressure. The other direction will be trying to make a vacuum inside the enclosure. Fans have a hard time collecting molecules from low pressure than higher pressure. You get (slightly) more mass flow.

2) As others have said, you may want to filter the dust. That dust has to flow through the fan, If the fan is oriented to pressurize the enclosure, you can filter the dust at the fan before it enters. In the "vacuum" configuration the filter needs to be away from the fan (at the inlet). However, often there are multiple paths that air can enter a vacuum (holes, seams, etc.), so it's harder to collect all of the dust if the filter isn't at the fan.

3) Usually I want to located the fan at the front panel of the equipment. This is where the humans are, and they normally have cleaner cooler air than the backside of the equipment. This is especially important if you equipment is located in a rack or with other equipment. Do not collect your air from a hot and dusty place.

4) Filters should be cleaned. If you hide the filter at the back, it will never be seen and never cleaned. On the front panel it is easy to clean and inspect.

5) Don't blow air in the operators face, they won't like it.

7) Try to provide a low impedance path for the air flow before the inlet and after the exhaust. Design equipment so that it is unlikely for the user to do a stupid installation. Say this in the installation/operators manual. Put labels that say "do not block exhaust", etc.

8) Design your cooling to operate at high altitude (like Denver, not the stratosphere). Air cooling is a lot worse in Denver than in LA.

9) Select a fan technology that suits your airflow needs. Some fans are designed to operate well with high flow and low pressure drop, others are the opposite. Match the fan type to your configuration.

10) Did I say think about high altitude performance? Worth repeating... Just because it seems to work in Boston, doesn't mean it will be reliable in Denver; and you probably won't know why your products die early there. On the plus side, most of your market is at sea level.

11) Honestly, some low power products really don't need significant airflow. Sometimes you just need to avoid a sealed up oven. Some don't even really need a fan if you can arrange good convective cooling. None of this applies to them.

12) If you need good lifetime, or if your fan is big and works hard, then ball bearings are good. Expensive, but good.

13) For extra credit, in complex systems that monitor the tachometer on your fan. Test for really high speed too. That may mean you have no airflow because the inlet is blocked. Or not, it all depends on the details.

13) Your average axial fan's datasheet will have a curve like the one below (that's two fans, we'll focus on the good one). Of course what you want is maximum flow. What you have to deal with in the real world is the pressure drop needed to get that flow. Look for that kink in the flow pressure curve. That's where this fan will be working in a good design. If you need 80 CFM, this fan is scary, as soon as your filter gets dirty your transistor will fry. If you need 10 CFM, this fan is probably too expensive (not just cost; front panel space, enclosure size, etc.), unless you have a very restrictive system. You're enclosure has a pressure vs. flow curve too (probably a quadratic?) you can think of superimposing that enclosure curve over the fan curve to get the operating point. A nice theoretical idea, isn't it? We never actually have that data in the commercial world. But you could try a couple of different fans (really different) to get a couple of data points, then you invent the enclosure curve from that with some creative thinking and lab work. It's not a great design process but it's what we actually end up doing.

russ_watters
DaveE
Gold Member
Turbulence is your enemy if flow is what you need to maximize.
Yes, but all of these systems end up with turbulent flow anyway. Laminar flow is really difficult to maintain. Even the wings on a plane or the sails on a boat can't really use truly laminar flow. They are actually using "attached turbulent flow". I have personally never seen or heard of an electronic system that is designed for anything but turbulent flow; given the odd shapes inside the electronic enclosure, laminar flow is essentially impossible anywhere but maybe the fan blades.

In fact, laminar flow will give you the best airflow through the system, but usually not the best cooling. This is because of the thermal gradient through the laminar airstream perpendicular to the hot surface. Air is a pretty poor thermal conductor. You don't want heat to travel through air. What you want is heat transfer by constantly changing cool molecules in the place of the ones that you just heated up. Think heat transfer by mass flow, not conduction.

I have done experiments to verify this with water cooled heatsinks that intentionally create turbulent flow with something like water jets and found a dramatic improvement in local heat flow (flux from a surface). This is one reason (of many) in water cooled systems that you often see flow restriction and increased velocity in the areas where you need good cooling. Of course in air cooling you probably never want extra restriction, there you just want to maximize the mass flow of the whole system.

russ_watters
hutchphd
Homework Helper
I have done experiments to verify this with water cooled heatsinks that intentionally create turbulent flow with something like water jets and found a dramatic improvement in local heat flow (flux from a surface)
That's really interesting because I long ago did a series of experiments on the converse. The question was how fast could we temperature equilibrate cartridges on a carousel/centrifuge in a medical analyzer. I found that if the carousel was fast enough to create turbulence in the chamber, the air conduction was essentially equivalent to dunking in water at temperature. This result must clearly depend upon cartridge materials but I found it nonetheless very surprising.

DaveE
Gold Member
That's really interesting because I long ago did a series of experiments on the converse. The question was how fast could we temperature equilibrate cartridges on a carousel/centrifuge in a medical analyzer. I found that if the carousel was fast enough to create turbulence in the chamber, the air conduction was essentially equivalent to dunking in water at temperature. This result must clearly depend upon cartridge materials but I found it nonetheless very surprising.
I think water has crappy thermal conductivity too. I guess it's all about perspective. In my world metal>liquid>gas (although I hear He is really good!) and Cu, Al are the normal materials, LOL.

Frodo
Gold Member
There is no such thing as "suck" when applied to a fluid or gas.

Say a fan is set up to blow air to the right. How can the fan blade possibly affect air molecules to the left of it? It cannot attract them by gravity, electrostatic, electromagnetic means or by any other force.

What a fan does is physically hit the air molecules in front of the blades from behind, just like a bat hits a ball. These air molecules therefore move to the right. This creates a region of reduced pressure in the region they have been moved from.

Air molecules to the left of the fan at the higher original pressure now expand into this region of reduced pressure because, on average, they get more thermal hits from behind them (the left) than they do from in front of them (the right). These air molecules therefore move to the right until they get as many hits from in front as they do from behind.

Notice an air molecule cannot be attracted by the fan - it can only be hit from behind, either by another air molecule, or by the fan blade.

So you now have a difference: You can only reduce the air pressure behind the fan to a vacuum at absolute maximum best case, or 14psi below atmospheric pressure. But you can increase the pressure in front of the fan to whatever pressure you like.

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DaveE
Gold Member
There is no such thing as "suck" when applied to a fluid or gas.

Say a fan is set up to blow air to the right. How can the fan blade possibly affect air molecules to the left of it? It cannot attract them by gravity, electrostatic, electromagnetic means or by any other force.

What a fan does is physically hit the air molecules in front of the blades from behind, just like a bat hits a ball. These air molecules therefore move to the right. This creates a region of reduced pressure in the region they have been moved from.

Air molecules to the left of the fan at the higher original pressure now expand into this region of reduced pressure because, on average, they get more thermal hits from behind them (the left) than they do from in front of them (the right). These air molecules therefore move to the right until they get as many hits from in front as they do from behind.

Notice an air molecule cannot be attracted by the fan - it can only be hit from behind, either by another air molecule, or by the fan blade.

So you now have a difference: You can only reduce the air pressure behind the fan to a vacuum at absolute maximum best case, or 14psi below atmospheric pressure. But you can increase the pressure in front of the fan to whatever pressure you like.

This is absolutely not at all how fans work. It's even self-contradictory. You say fans can't affect air upstream and the claim the same fan creates a low pressure zone tonto which upstream air moves. So which is it?

Let's restrict our discussion to a continuous medium (so let's not worry about individual molecules for the moment). This is reasonable unless we are operating in near vacuum. Fan blades effectively function similarly to airfoils. They draw air in on one side exactly as fast as they push it away from the other side and they do it by creating low and high pressure regions respectively. This is sustained by the continuous input of energy by the fans motor.

mess, jrmichler, DaveE and 1 other person
Keith_McClary
Gold Member

russ_watters
Frodo
Gold Member
You say fans can't affect air upstream and the claim the same fan creates a low pressure zone tonto which upstream air moves. So which is it?
May I suggest you carefully read what I said before contradicting me. I said " How can the fan blade possibly affect air molecules to the left of it? It cannot attract them by gravity, electrostatic, electromagnetic means or by any other force. ".

Can you perhaps tell us all how a molecule upstream of a fan blade moves to wards the fan. I state that it does so because more air molecules behind it than in front of it hit it, so it is the air molecules behind it which cause it to move forward. The fan doesn't "suck" - it pushes air molecules from behind causing a low pressure region into which the upstream air expands because the air molecules are being hit from behind.

I use air molecules because, the last time I checked, I found air was is comprised of discrete molecules and not as continuous matter. It seems sensible to describe what is actually there rather than some figment of someone's imagination.

This forum really fascinates me! When people are introduced to an idea they haven't heard of before they immediately close their minds and say "That's wrong" without spending a moment's thought as to whether it may be correct.

May I therefore ask you to spend a few minutes actually thinking about what I said and then come back. And, please don't digress by bringing aerofoils into the matter as the fan could be flat bladed.

We can digress further to centrifugal fans later - let's stick to conventional fans for the present.

berkeman
Mentor
I use air molecules because, the last time I checked, I found air was is comprised of discrete molecules and not as continuous matter. It seems sensible to describe what is actually there rather than some figment of someone's imagination.

This forum really fascinates me! When people are introduced to an idea they haven't heard of before they immediately close their minds and say "That's wrong" without spending a moment's thought as to whether it may be correct.

May I therefore ask you to spend a few minutes actually thinking about what I said and then come back. And, please don't digress by bringing aerofoils into the matter as the fan could be flat bladed.
Please lose the attitude. When you earn your PhD in Aero/Fluid Dynamics, you can push back a bit if you have a valid point. Right now you don't have either.

Can you perhaps tell us all how a molecule upstream of a fan blade moves to wards the fan. I state that it does so because more air molecules behind it than in front of it hit it, so it is the air molecules behind it which cause it to move forward.
And what causes the low pressure area on the fan side of those upstream molecules? If the fan were not causing the lower pressure area, there would not be a pressure differential to generate that "push".

Frodo
Gold Member
When you earn your PhD in Aero/Fluid Dynamics, you can push back a bit if you have a valid point. Right now you don't have either.
Please withdraw that comment before I report you to a moderator. You have no knowledge of my academic qualifications nor of my expertise. Neither is relevant here. I seem to recall Einstein was a patent clerk when he upended the scientific world.

DaveE
Gold Member
There is no such thing as "suck" when applied to a fluid or gas.

Say a fan is set up to blow air to the right. How can the fan blade possibly affect air molecules to the left of it? It cannot attract them by gravity, electrostatic, electromagnetic means or by any other force.

What a fan does is physically hit the air molecules in front of the blades from behind, just like a bat hits a ball. These air molecules therefore move to the right. This creates a region of reduced pressure in the region they have been moved from.

Air molecules to the left of the fan at the higher original pressure now expand into this region of reduced pressure because, on average, they get more thermal hits from behind them (the left) than they do from in front of them (the right). These air molecules therefore move to the right until they get as many hits from in front as they do from behind.

Notice an air molecule cannot be attracted by the fan - it can only be hit from behind, either by another air molecule, or by the fan blade.

So you now have a difference: You can only reduce the air pressure behind the fan to a vacuum at absolute maximum best case, or 14psi below atmospheric pressure. But you can increase the pressure in front of the fan to whatever pressure you like.
Seems like "suck" is a reasonably useful word to describe the process of air moving in a pressure gradient. It is shorter, after all. Some things are just a bit to complex to always analyze at the microscopic level. The processes you describe happen at roughly the speed of sound with an uncountably large number of particles. So it does seem to us that sucking is real.

It really helped me understand how my sailboat worked when I learned that the airflow was altered ahead of the boat, well before any air reached the luff of the jib.

Maybe sometime I'll write a post about how nothing can "fall", what with GR and such.

russ_watters
Frodo
Gold Member
Seems like "suck" is a reasonably useful word to describe the process of air moving in a pressure gradient.
I agree - "suck" is a useful concept to work with.

But it is as well to understand what is actually happening whenever one resorts to a concept which isn't actually totally true. Archimedes Principle is a useful concept but the unwary will fall l into trap after trap if she uses it blindly. It is much better to go back to the underlying principle that the body experiences an upthrust calculated as the surface integral of the pressure.

Note how I think I helped the OP by showing why there is a difference between "sucking" and "blowing", something I don't think others brought out. His question was:
I am trying to determine if they are better placed in positions where they suck or blow. Is there a rule to determining this?

DaveE
Gold Member
This forum really fascinates me! When people are introduced to an idea they haven't heard of before they immediately close their minds and say "That's wrong" without spending a moment's thought as to whether it may be correct.

May I therefore ask you to spend a few minutes actually thinking about what I said and then come back.

OK, I think I understood what you said. But I didn't understand why you thought we needed that explanation, or what point you wanted to make beyond fans can't suck air out of a vacuum, which we probably already knew.

The thing that fascinates me about the forums is how much the readers know. Thanks for the reminder that air moves by bumping molecules.

russ_watters
Frodo
Gold Member
Dave

Pax! My "spend a few minutes actually thinking about what I said " was addressed to berkeman after his put down. It was not addressed to you!

Your comment was reasonable and, as you see, I agree with you. I have no disagreement with you - my disagreement is with berkeman not with you - ok? You agree with my explanation - he doesn't (which I guess means he does not agree with you either).

I too sail. Have you ever sailed when it is snowing? You can actually see the wind!