Outside Air Intake Velocity

[BronYrAur]

What the real story with OA intake velocity? I see anywhere for 400 FPM to 1000 FPM of "free area" as acceptable velocities to prevent snow/rain from being sucked in. I have used 500 FPM as my standard for many years, but I am trying to push the envelop a little. I have 18,500 CFM of OA that I will be pulling in from a roof. This is to replace an existing louver in a sub-grade area well. The building has issues with diesel fumes from idling vehicles getting into the area well. I'm running duct up to the roof and sizing it for a 0.03" pressure drop, which I'm guessing matches my existing louver. That gives me a duct around a 50"x50", but the velocity is almost 1,200 FPM. I will transition my last piece of duct to drop the velocity, but what velocity is acceptable? Using 500 FPM, the duct size approaches 75"x75". I plan to terminate by cutting the duct off at a 45 deg angle and putting a birdscreen on it.

Thanks for you help.

 

[lowedogg]

water penetration is totally dependent on the individual louver that you spec. I usually use Ruskin, they have all of that performance data on their website for download. Be careful with doing a transition at the louver, if it isn't deep enough you will get uneven velocity across the face of the louver resulting in regions of high velocity. In extreme climates I have seen moisture eliminators (like on a cooling tower) installed in intake ducts with drains.

 

[BronYrAur]

Are you saying that using just a birdscreen is a bad idea?

 

[Zesti]

Getting moisture (rain, snow) on the inside of your louvre is not a problem if there are provisions that will let the water drain away before it gets to the AHU.

Coating of the ductwork behind the louvre and having a drain to the sewersystem will be needed.

Anyway, 100% "water"-free louvres do not exist.
You might be able to keep out rain but snow blown about by wind can get up any air-intake no matter how it is made.

Don't know if I understand your situation correctly: you have a horizontal duct running for some length over a roof, right?

Have the last couple of metres of the duct running slightly downwards towards the intake and any water sucked or blown in will drain out again onto the roof (coating!).

If you want to be sure that no droplets are carried along by the airstream, put in a "droplet-catcher" (don't know what it would be called English).

 

[willard3]

I live where it snows a lot and use 400 fpm min and 550 fpm max if the intake is out of the prevailing winds or in a sheltered spot.

You should always drain the louver plenum, water will get in.

 

[AbbyNormal]

The wind is going to drive in the rain long before an intake velocity will hoover it in.

I size them for about 700 FPM tops through the free area myself mainly for pressure drop reasons and I use the vertical blade type, they seem to have the best driving rain resistance

Once a storm has a name all bets are off and the water gets in.

The trick is to limit how far it gets in and give it a way to drain back out.

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[AbbyNormal]

If you are worried about a duct riser size, consider a fan.

http://www.greenheck.com/media/pdf/catalogs/HoodedProp_catalog.pdf

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[marcoh]

Instead of cutting the intake back at 45o maybe consider installing a bend down so the intake in the horizontal plane and air is drawn up. This also negates wind pressure on the intake.

 

[ChasBean1]

Or a penthouse (aka doghouse) option could be considered:

http://www.ruskin.com/catalog/servefile.aspx?id=2263

 

[KiwiMace]

I try to use 500fpm (2.5m/s) to size OA louvers also because of the rapid increase in pressure drop as you increase the velocity. Going from 500fpm to 600fpm will add close to another 50% on the pressure drop.

This will also be apparant in poor transition design, that might reduce the effective area of the louver.