Fans resistance and operating point

[NoJosh]

Hi everyone

I was wondering if anyone had any knowledge with respect to fans. If i want a silent system, i would have to use a big fan with low rpm's. I have looked a bit at fans, and have met the terms resistance curve that describes the total pressure loss in the system, that the air has to overcome. And has seen something about that the operating point for a fan is where the system curve intersects with the fan curve. Now with regard to this, i was wondering how the resistance for the system is found, is it with some flow simulation program FEM/FEA analysis? And does the fan have to operate at the operating point, can you deviate from this point as long as you have the pressure needed to move the flow?

Thanks in advance for your help

Regards
NoJosh

[Brian_UK]

The system resistance is found during the design stage of the ductwork installation.

Like all things of this nature the sizing of the fan is the last item on the list.

Assuming that your ductwork system provided a system resistance of 500 Pa at the design air flow rate then you would select the fan that can provide those figures.

With the fan curve / system resistance curve there will be some latitude with regard to pressures and airflow but care needs to be taken to ensure that you do not push the fan so far along it's curve that it falls off it and is unable non produce the pressure required.

Woods Fan Engineering Guide is good bible for these matters

[NoJosh]

Thanks for the response, i will look into the engineering guide. Im just uncertain of how the resistance is calculated with regard to pipes, elbows, coils, filters and so forth. But i'll try and see if the guide will give the information

[nike123]

Every duct fitting has its own equivalent length for calculating pressure drops. You can find these in ASHRAE books.

You can use some software for calculations like McQuay duct sizing.
http://www.daikinmcquay.com/McQuay/DesignSolutions/Software

Better is first to learn how to do it manually, than you can use software, since when using software, rule is "Garbage in = Garbage out".

REVIT MEP has built in very good duct sizing software, you just place equipment and select it to connect, and software does all calculations, routing, sizing etc..., but it costs about 5000$!
If you make lot of ducting, it pays itself quickly.



https://www.youtube.com/watch?v=lX-X6ZKj0LU

[NoJosh]

Is there any difference in regard to ductless split AC systems?

[nike123]

Yes, they don't have ducts, therefore no pressure drop!

[NoJosh]

But there is still resistance from piping, coils, elbows, filters..and so forth?

[nike123]

Certainly!