# Fans... Static Pressure vs Air flow vs RPM



## jambhumalamal (May 10, 2013)

Hello Friends,
I am in bit of a confusion here. I have 2 kinds of fans used for industrial unit cooling. The current fan is nearing its obsolescence and I am looking for a new fan with same or better specifications to replace the older fans.
Following are some specs.

*Old Fan:*
Rated Speed: 4300 RPM
Air Delivery: 19 CFM
Static Pressure: 0.18 inch - H2O
Rated Current: 290 mA

*New Fan:*
Rated Speed: 3900 RPM
Air Delivery: 19.07 CFM
Static Pressure: 0.095 inch - H2O
Rated Current: 260 mA

Below is my opinion about the new fan, please correct me if I am wrong.

Rated speed: Slower than Original fan but doesn't matter as long as CFM ratings are better.
Air Delivery: New fan has better CFM
Static Pressure: : I am not able to figure if the rating for the new fans is worse, same or better than Older fan given other parameters.
Rated current: Better.

ALso, The form, fit and function of both the fans is identical.

Any and all the help would be highly appreciated.

Thanks,
Jambhu


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## erocker (May 10, 2013)

If the fan needs to push air through some sort of tight mesh or something, you will need the higher static pressure. If not, the new fan will be fine. Sounds to me like it is a smaller fan, so as long as there is some airflow, either fan should be fine.


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## jambhumalamal (May 10, 2013)

Thanks for your response erocker.

The fans experience a lot of dust accumulation in the field. So, do you suggest that Higher static pressure fans are always better than the lower ones if all the other parameters are same or better?

Thanks again!


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## de.das.dude (May 10, 2013)

not for dust. If you are using the fans on some sort of heatsink, then higher the staic pressure, the better.

CFM is the speed with which the fan is moving air, and static pressure gives you an idea of with how much force that air is moving.

i think you are using 50mm fans?


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## erocker (May 10, 2013)

jambhumalamal said:


> Thanks for your response erocker.
> 
> The fans experience a lot of dust accumulation in the field. So, do you suggest that Higher static pressure fans are always better than the lower ones if all the other parameters are same or better?
> 
> Thanks again!



I kind of look at it this way:

Higher static pressure = torque

Higher airflow = horsepower

You can have one, the other or both. Higher static pressure means that it will still offer airflow with more resistance. Higher airflow means that the blades/RPM's are designed to be efficient in creating airflow. 

You can have both, and these kinds of fans are generally fast and loud. So that's where the trade-off comes in. Design a fan that provides good airflow at the cost of static pressure but with the benefit of lower noise (and possibly the cost savings of a weaker motor). 

Dust isn't much of an issue unless there's something in front or behind the fan to cause resistance. If there is, you will need a higher static pressure fan, or to do regular maintenance on the fans/filters/heatsink by removing the dust. Maintenance is money, so for an industrial application find something with a high static pressure.


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## lilhasselhoffer (May 10, 2013)

1) Are you sure both are measured in mmH2O?  Static pressure is also measured in mmHg (water versus mercury is the only reasonable assumption for that huge of a difference in pressure).

2) CFM is cubic feet per minute, or the volume of air moved per unit time.  This is a function of the rotational speed, and fan blade area.  You can double the fan blade area, half the rotational speed, and maintain the same flow volume.

3) If you've got a problem with fouling (dust), then you want high static pressure.  As static pressure decreases the fouling layer gets larger.  The fouling layer allows dust to build up.


Yeah, the new fan is probably quieter.  At the same time, quiet is unlikely what you need here.  From your description, this is not the fan you are looking for.


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## jambhumalamal (May 10, 2013)

Hello de.das.dude, lilhasselhoffer and erocker.
Thanks a lot for your responses. After reading your response I got a very clear idea and the correlation between CFM and static pressure..
I will be documenting your responses so that other people in the company could benefit from your responses.

Thanks again !

Jambhu


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## Delta6326 (May 11, 2013)

I suggest you take a look at this page, it helps explain more in depth.

http://martinsliquidlab.org/2013/02...w-specs-are-poor-measures-of-fan-performance/


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## Volkszorn88 (May 11, 2013)

erocker said:


> I kind of look at it this way:
> 
> Higher static pressure = torque
> 
> ...



Deltas


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## tokyoduong (May 13, 2013)

Static Pressure = pressure! duh! the more obstacles in the way of the air, the more static pressure you want.
CFM = air flow. The more air you can move, the more heat you can blow away.

RPM : Moves more air and become louder as it increases in speed

The ideal fan would be a low RPM, high CFM, high static pressure. That would yield low noise and push a lot of air.

Your new fan has less pressure but equal in air flow. It should also be slightly less noisy. However, in a case full of filters, it may not be as good as your old fan.


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