I still don't understand static pressure vs air flow fans

[Luddite]

If there is more air flow wouldn't there also, logically, be more air pressure? Or is a static pressure fan more directional than an air flow fan?

Also, while reading up on intake and exhaust fans, I keep reading how dust filters and front grills are quite restrictive for air flow, and one should consider getting a static pressure fan for optimal cooling. But if your intake fan has no obstructions towards the inside of the case (in the direction of where the fan is blowing), and the dust filter is on the outside, wouldn't your fan be trying to suck air in through the dust filter, rather than trying to blow air out through the dust filter? So how would a static pressure fan be of benefit in that situation?

Do static pressure fans make better exhaust fans? (since they have to blow air through the exhaust grill).

What's the difference between the two types of fans on the back side? Does one type suck more air through the front than the other?

 

[Paperdoc]

Let me try to help.

Any fan has a performance "curve" of what airflow it will deliver against a resisting backpressure. The more resistance there is to the flow of air from the fan, the less the flow is, and the higher the pressure in the space between the fan and the resisting material. Free space is zero backpressure. A solid blockage allows NO air to flow through, and at that point you can measure the pressure in the area in front to the fan and that's the maximum air pressure that fan can generate. In terms of air FLOW, "what comes in must go out", so the intake airflow at the back is exactly the same as the discharge flow out the front.

The difference between the two major types of fans is in the design of the blades. Some designs are capable of generating higher pressures against a resistance to air flow and hence can "push" air better through such a restriction. Other designs cannot do this and would force less air flow thorough that same restriction, BUT can actually deliver MORE flow through a very small resistance than the "high pressure" design fan would be able to.

On practical terms, fans for "high pressure" are designed for use with heatsinks and radiators, where the narrow spaces between the fins present significant resistance to air flow. Free space, open intake and exhaust grilles, and even clean dust filters present MUCH lower air flow resistance and are considered "low pressure" applications where you use the fan types designed for max air flow. Note that the dust filters need to be relatively CLEAN, so you do need to ensure they get cleaned out from time to time to maintain good air flow through them.

The specs published for fans can be confusing, and I figured out how to understand them. In particular I'm talking about the Air Flow and Pressure ratings. Each of these is a MAX value for a DIFFERENT set of conditions. The max of AIR FLOW is what is specified if the fan is blowing freely against NO backpressure. At the opposite end, the max of PRESSURE is what the fan can get up to when it is effectively not able to make any air flow through a major resistance. For many fans, the graph of this Air Flow versus Backpressure is nearly a straight line, although technically it is a very gentle curve. So, if you are trying to compare two or more fans, for each fan you could take those two specs and sketch out the Flow versus pressure line and decide which is better for your situation. Of course, you almost never know what actual backpressure your fan will be dealing with so this can't be exact, but you can get a reasonable approximation of the comparisons.

Here's another important point. Do not concentrate on fan speed. What your fan needs to do is deliver AIR FLOW. Certainly the speed of the motor is a major factor, but the spec for max air flow (at max motor speed) is what you want to look at (bearing in mind the effect of flow resistance / backpressure). Fan noise also is related to these two factors. Higher speed and air flow generates more noise for any given fan. But the relationships among these three for one fan are NOT the same as for others. For example, in comparing two fans, one of which will blow more air by running at a higher speed and generating more noise, what you really cannot get a reading on from published specs is how much noise the two fans will create when they both are run at whatever speed each requires to produce exactly the same AIR FLOW through the same resistance. And that's the point. Although we talk about fan speed control, the truth is that a mobo controls the TEMPERATURE of your system as actually measured by a sensor. (There is one sensor built into the CPU chip by its maker, and one or more additional sensors built into the mobo.) So for each control system (the CPU, or the mobo) the automatic fan control will make the fan do whatever it takes to keep its sensor TEMPERATURE on target, and that means it will generate whatever AIR FLOW it takes, no matter what fan you are using. The control system does not really care about the speed - it concentrates totally on temperature.

Rear fans usually are exhaust, and most commonly do no have any dust filters before or after them. They are classic applications for Air Flow fan designs. Most front intake fans, even with dust filters on their intakes, should be Air Flow types also. High Pressure fans are needed for heatsinks and radiators no matter which way the fan is mounted on them.

 

[Cerb]

It is primarily a marketing gimmick. A proper spec sheet would include a a CFM vs. pressure graph, among others, for every fan (if you look at fans you can buy at, say, Mouser, you'll commonly find such things in their datasheets). At the least, a rated static pressure limit, in mmH2O, should be there. It's not some different types of fans, but fans that can move more air at lower RPM in actual use than others, and computer fan brands, for the most part, don't want to provide any more data than they have to, and have been known to fib a little, even then.

 

[Luddite]

Let me try to help.

Any fan has a performance "curve" of what airflow it will deliver against a resisting backpressure. The more resistance there is to the flow of air from the fan, the less the flow is, and the higher the pressure in the space between the fan and the resisting material. Free space is zero backpressure. A solid blockage allows NO air to flow through, and at that point you can measure the pressure in the area in front to the fan and that's the maximum air pressure that fan can generate. In terms of air FLOW, "what comes in must go out", so the intake airflow at the back is exactly the same as the discharge flow out the front.

The difference between the two major types of fans is in the design of the blades. Some designs are capable of generating higher pressures against a resistance to air flow and hence can "push" air better through such a restriction. Other designs cannot do this and would force less air flow thorough that same restriction, BUT can actually deliver MORE flow through a very small resistance than the "high pressure" design fan would be able to.

On practical terms, fans for "high pressure" are designed for use with heatsinks and radiators, where the narrow spaces between the fins present significant resistance to air flow. Free space, open intake and exhaust grilles, and even clean dust filters present MUCH lower air flow resistance and are considered "low pressure" applications where you use the fan types designed for max air flow. Note that the dust filters need to be relatively CLEAN, so you do need to ensure they get cleaned out from time to time to maintain good air flow through them.

The specs published for fans can be confusing, and I figured out how to understand them. In particular I'm talking about the Air Flow and Pressure ratings. Each of these is a MAX value for a DIFFERENT set of conditions. The max of AIR FLOW is what is specified if the fan is blowing freely against NO backpressure. At the opposite end, the max of PRESSURE is what the fan can get up to when it is effectively not able to make any air flow through a major resistance. For many fans, the graph of this Air Flow versus Backpressure is nearly a straight line, although technically it is a very gentle curve. So, if you are trying to compare two or more fans, for each fan you could take those two specs and sketch out the Flow versus pressure line and decide which is better for your situation. Of course, you almost never know what actual backpressure your fan will be dealing with so this can't be exact, but you can get a reasonable approximation of the comparisons.

Here's another important point. Do not concentrate on fan speed. What your fan needs to do is deliver AIR FLOW. Certainly the speed of the motor is a major factor, but the spec for max air flow (at max motor speed) is what you want to look at (bearing in mind the effect of flow resistance / backpressure). Fan noise also is related to these two factors. Higher speed and air flow generates more noise for any given fan. But the relationships among these three for one fan are NOT the same as for others. For example, in comparing two fans, one of which will blow more air by running at a higher speed and generating more noise, what you really cannot get a reading on from published specs is how much noise the two fans will create when they both are run at whatever speed each requires to produce exactly the same AIR FLOW through the same resistance. And that's the point. Although we talk about fan speed control, the truth is that a mobo controls the TEMPERATURE of your system as actually measured by a sensor. (There is one sensor built into the CPU chip by its maker, and one or more additional sensors built into the mobo.) So for each control system (the CPU, or the mobo) the automatic fan control will make the fan do whatever it takes to keep its sensor TEMPERATURE on target, and that means it will generate whatever AIR FLOW it takes, no matter what fan you are using. The control system does not really care about the speed - it concentrates totally on temperature.

Rear fans usually are exhaust, and most commonly do no have any dust filters before or after them. They are classic applications for Air Flow fan designs. Most front intake fans, even with dust filters on their intakes, should be Air Flow types also. High Pressure fans are needed for heatsinks and radiators no matter which way the fan is mounted on them.

Thanks!

So if i understand this correctly, fans marketed as airflow fans basically suck in as much air as they push out, but they need to have more free unobstructed space in the front and back (less impedance). Whereas static pressure fans, because of their blade design, can actually push out more air than they draw in, and therefore need less free space in the front and back and can tolerate more impedance. Is this correct?

I also notice most fan companies have segmented their fans into air flow and static pressure. Some companies, like Be Quiet! don't do this and just have one type of fan (e.g. Silent Wings 3).

 

[Billb2]

Thanks!

So if i understand this correctly, fans marketed as airflow fans basically suck in as much air as they push out, but they need to have more free unobstructed space in the front and back (less impedance). Whereas static pressure fans, because of their blade design, can actually push out more air than they draw in, NO, NO, NO. Fans can not create air from nothing! and therefore need less free space in the front and back and can tolerate more impedance. Is this correct?

I also notice most fan companies have segmented their fans into air flow and static pressure. Some companies, like Be Quiet! don't do this and just have one type of fan (e.g. Silent Wings 3).

The curve that describes a fans performance is called a P-Q curve.

Here the best description of P-Q curve in the interwebs.
Read it and you'll know all you need to know.

Personally, I'd never buy a fan if the manufacturer doesn't supply a P-Q curve for it.

As for the distinction of high pressure vs high static, all fans have air flow and pressure. calling a particular fan one or the other is completely arbitrary and total marketing hype.

 

[bfun_x1]

Thanks!

So if i understand this correctly, fans marketed as airflow fans basically suck in as much air as they push out, but they need to have more free unobstructed space in the front and back (less impedance). Whereas static pressure fans, because of their blade design, can actually push out more air than they draw in, and therefore need less free space in the front and back and can tolerate more impedance. Is this correct?

I also notice most fan companies have segmented their fans into air flow and static pressure. Some companies, like Be Quiet! don't do this and just have one type of fan (e.g. Silent Wings 3).

Well another consideration between SP and AF fans is noise. A Corsair SP performance fan will beat a Corsair AF performance fan under pressure and match it in unrestricted airflow. That might make it seem like the best choice for every scenarios but the SP is also 4.6 dB louder than the AF at full speed. So then you got to wonder if an AF fan at 100% can perform as well as a SP fan at 60% and will it be quieter while doing so. Throw in the fact that grills, radiators, and cooling fins can all have different airflow resistances and you have a real mess of an equation.

 

[Paperdoc]

You have one point still wrong. ALL fans, no matter of what design, MUST suck in exactly as much air as they blow out. You cannot make more air out of nothing!

The difference is that "Static Pressure" fans have a blade design optimized for good air flow at higher back pressure (air flow resistance), and often at low back pressures will deliver less air flow than a fan optimized for that latter situation.So if you put the Flow versus Back Pressure lines for both designs on the same graph, they cross over at some intermediate pressure. Recognize also that resistance to air flow can happen on either or both sides of a fan. A fan mounted over a CPU heatsink experiences little air flow resistance on its intake side, and a lot on the discharge side through the fins. So it still need to be a "pressure" type.

 

[DrMrLordX]

My way of thinking is that a fan with a high rating for static pressure is more likely to achieve something close to its airflow rating in actual use. It's also more likely to be noisy relative to its airflow.