hvac fan selection

[Ufadnewbie]

Hi.

could someone please help to point me in the correct direction. I am designing a product that removes air from an underfloor air distribution plenum and moves it to occupants above desk level,at a height of 1.4m Given that the airflow rate is too low through the plenum I am going to insert a fan into the 6inch air pipe sitting on the raised floor. The flow will be diverted through 3 air vents of 60mm radius.
My question regards fan selection.due to the size i am considering a 6in compact tubeaxial fan. i require a flowrate of 360m3/h.

My question is how do i go about calculating the static pressure so i can get the duty point on the fan curve?

any ideas would be great

 

[blutfort]

If I understand your question correctly 6" round duct has a standard pressure drop per 100 feet at a given cfm your ductulator should have that value for you.

The vendor of the diffuser should have a table in their catalog or IOM that gives static drop at various cfms, or m3/h in your case.

Add those two together and you should have the total static.

There will be some positive pressure on the inlet to the fan so dont forget to subtract that from the total static load.

Hope I'm not forgetting something - cause I've never needed to use a booster like that, but that is how I find operation points for series fan VAV boxes. In fact you might want to look at a catalog on series fan vav boxes on trane or york's web site. I think they might have some sizing procedure notes.

Good Luck!

 

[ChasBean1]

Ufadnewbie,

This is a low drop application. If you have a six inch tube, loss is nearly negligible through it because the run is so short. I'd add up the total leakage area of the three outlets (about .365 ft2 total). Flow is known, open area is known, so you can estimate pressure drop across the three openings. For this I get about 0.05" or 13 Pa. For the fan, I'd probably select the required flow at 0.10 in. w.c. Be careful about noise! You'll want a quiet fan here or maybe some sound dampening...

CB

 

[Ufadnewbie]

hi. the air pipe is vertical and material selection will be polyproylene. given that this has an equivalent loss roughness of 0 for new pipes, there will be no static pressure drop?is that correct?
positive pressure at the inlet of the fan is very low, circa 35Pa. We are considering using air vents from scrapped cars, crazy i know, therfore finding the static here may be difficult. would it be fair to assume that the sp is going to be exceptionally low?

 

[Ufadnewbie]

hi chasbean.yes i should have noted that the tube is 6in diameter and 1.4m high .will it still a low drop application.inlet pressure is 50Pa, so i calculated the drop at 1.4m at approx 16pa.
the 3 openings will be at 120 degrees of each other, and the air vents will be placed at the end of a 250mm pipe coming out of the air pipe.
Yeah i was considering mounting the fan on a rubber mount or dampening the pipe to prevent noise.excellent suggestion.

 

[ChasBean1]

50 Pa seems very high for under floor pressure. At that value, I'd suggest mocking one up with just a volume damper in it (no fan) and see what you get for flow.

 

[lowedogg]

I recommend consulting the ASHRAE Underfloor Air Distribution Design Guide. There are a lot of special considerations when designing UFAD systems. If your firm does a lot of UFAD, you should definitely buy this book.

High air velocity near the floor in the occupied space will almost undoubtedly generate complaints. I would be remiss to put in a booser fan like you have described.

Have you tried speeding up the supply fan to increase plenum pressurization? What sort of floor diffusers are being used? What is the static pressure in the plenum? 50 Pa should be plenty to make swirl diffuser work properly. Is the pressure evenly distributed in the plenum? Is the plenum sealed properly? Keep in mind that the plenum is most effective at low velocity. High velocity in the plenum significantly decreases the performance of the floor diffusers. If you are trying to force too much air through a small plenum, the high velocity will actually result in negative pressurization in parts of the plenum. I have seen air being sucked into the floor plenum through perforated tiles in data centers because of this.

In my experience, a lack of airflow from the floor diffusers is almost always due to a pressurization/airflow management problem in the plenum.