Static pressure loss thru sidewalk type grate.

[Jabba007]

None of the manufactures that I can find have any data on it. I am designing multiple tall shafts to exhaust large volumes of air from an underground parking garage. The shafts will be between 95' and 150' tall. The owner wants sidewalk type grates installed in multiple levels of this shaft with access to them thru doors on various levels of the building.

If I was doing this ONCE I'd make a conservative estimate and go on down the road. But since there may be 4-7 grates in the shaft in series... I thought I'd see what ya'll thought about it.

The shafts are 60-90 sqft each and will have air velocity between 1000 and 1500 fpm.

Jabba

 

[MintJulep]

Test it.

Easy to do.

Then you'll know.

 

[Jabba007]

Love to.

I don't access to facilities like that.

Can you turn me on to where I could do something like that?

Jabba

 

[trashcanman]

Sidewalk grates? Do you mean they are on the walking/driving surface? Are they traffic rated? How do you collect runoff water that will collect in them?

As for pressure drop, the most it could be is the velocity pressure in the duct. At 1500 fpm, Vp=(1500/4005)^2=0.14"W.G.

You say they are in series. I think you really mean they are operating in parallel, all connecting to a common exhaust shaft. Right?

 

[Jabba007]

No... they are not outside at all. There will be a tall shaft exhausting parking garage exhaust. Fans at the top of the shaft. At intermittant points up the shaft... there will be an access door into the shaft and a sidewalk TYPE grate to walk out on. So no... not in parallel... in series. And there should be no water to worry about at all, nor traffic.

Jabba

 

[MintJulep]

Google for "airflow testing" will return a bunch of labs with the capability.

ETL-Intertek (or one of their assoicated companies) has the cabaility.

http://www.intertek-etlsemko.com/

As does Quest apprently.

http://www.qes.com/airflow2.pdf

 

[ChasBean1]

I think you can estimate this with the help of the ASHRAE leakage equation from the 1999 Applications manual, Chapter 51.5.

Say you have 93,750 cfm (based on 1,250 feet per minute velocity and a 75 ft^2 cross section). It's important to know the free area of the grate. If we assume 70%, plug & chug using the ASHRAE equation:

Q = 2610 A (dP^.5)

(Q is 93,750 cfm)
(A is open area in square feet = 52.5)
(dP is resulting pressure loss in inches w.c.)

You get about 0.5 in. w.c. per grate.

I don't know if this is the best estimation for this job (the result seems a bit high to me) but the method is traceable to a known source...

Good luck,
CB

 

[JStephen]

I'd treat it as a screen, using ASHRAE equations based on open area.

 

[MintJulep]

Just an alternate thought:

With 4 to 7 grates in series the total resistance will not be negligable, and it follows that the additional fan costs associated with blowing air through this resistance continually will be non-trivial over the life of the facility. Have you thought about a life-cycle cost analysis yet?

Consider hinging the grates at the access door floor level. When you open the door the grate is staring you in the face, blocking access. Fold the grate down only when necessary to access the interior of the shaft.

 

[lilliput1]

I think you should direct duct your exhaust and supply. You can not guaranty the integrity of plenums. You will have leakages and stack effect to contend with. Someone may not close the shaft access door properly. In fire/smoke evacuation use, you would need the exhaust system to function as designed.

 

[Jabba007]

MintJulep, yeah I had actually thought of that and have already pitched the idea. It's cool to get some parallel development of ideas from a peer though.

Jabba