Air Velocity in Parking Exhaust Duct 1

[EnOm]

Hi
What is the maximum acceptable design velocity for air inside the ducts in a small parking about 6 ft below ground level?
I'm talking with respect to acceptable noise levels/acceptable pressure drop.

The main duct riser will then rise up to the roof of the building through a shaft between apartments.
The shaft is surrounded by kitchens and toilets.
The walls of the shaft are made of 200mm hollow concrete blocks. No sound attenuating materials are to going to be added to the walls.
Do I need to reduce the velocity from that used in the parking lot to reduce the noise further since now the shaft is passing in close proximity with the habitable parts of the building? Or will the walls provide enough sound attenuation?

Best Regards

 

[Drazen]

yes, you need to reduce it if it is too high.

the noise, however, does not come from air flow only, it also comes from fan motor.

there are some ari excel calculation sheets you can find by googling.

 

[EnOm]

Drazen,
Thank you for the response. You mention exactly what I am looking for.
How much is "too high" a velocity for the parking lot?
And how much is "too high" a velocity for the shaft?
Do the Excel sheets you mentioned calculate noise levels generated in ducts? That's my main concern here.

Best Regards

 

[willard3]

You will have to do some of your own search for this in Engineering manuals. It's not a simple problem.

 

[Drazen]

enom, i was little sarcastic using "too high" instead of exact figure.

as willard said... there is no silver bullet for that. you cannot just take velocity recommendation and relieve yourself responsibility to do proper engineering work.

for very simple residential calcs, i use 5 m/s as maximum velocity, but only as a rough guide.

it depends much on available space, needed air flow, fan noise... how to decide on it. in general, you should find sleeping room closest to your vertical shaft, than calculate/estimate noise that reaches such room.

 

[317069]

800[fpm] maximum

 

[EnOm]

willard3,
Thank you for your response. I understand, I thought it was a situation where a rough estimation would work. I'm guessing I'd have to go a bit into sound engineering for this? If you could kindly suggest some relevant Engineering manuals on the topic, I'll be grateful.

Drazen,
Thank you for your response. This project is a small building so I guess a simple rule of thumb would do, but you are right, I should learn more on the topic since I might be faced with more complex problems in the future. Are there any good references that you know?
I'm just guessing but I assume the procedure would be to estimate the noise levels generated by the air flowing inside the ducts, then calculating the attenuation provided by the sheet metal ducts, then that of the block wall, to arrive at the noise level inside the rooms adjacent to the shaft?

317069
Thank you for your response.

 

[cdxx139]

As a mechancial engineer, I have no idea how to calculate the noise generated in the duct, then account for attenuation of the sheet metal, insulation (if provided) and then block wall.

But with duct and a block wall (in the vertical) it sounds similar to a mechancial room, which I design for 1200 fpm and up to 1500 fpm (if not abbutting sound sensitive areas).

800 fpm would definately solve any sound issues you may have (especially where the rooms surrounding the shaft is not bedrooms). Start at 800 fpm, and see if you can have the duct that big. If the architect wants it any smaller, let them know a sound consultant should review if faster velocities can be used (they are notoriusly conservative and may not allow faster velocities). Let them take resposibility, do NOT take acoustic responsibility if velocities are greater than 800 fpm.

knowledge is power

 

[cdxx139]

"they are notorious" meaning sound consultants.

knowledge is power

 

[willard3]

Trane software estimates sound power.

I'm sure there are others.

 

[317069]

But still have a question about your project,
you say it is a 6 feet below the ground parking area.
do you mean the parking height is 6 feet only, I doubt, there should be a part of parking wall above the ground and in this case why don't you exhaust it directly to the outside without a duct, does it work this way?

 

[EnOm]

cdxx139,
Thank you for your response, and for the advice. I did it for 850 fpm. The architect did not have any objection since space is not an issue in the area the duct comes up since its located towards the back of the building. As for your advice to request for revision by a sound consultant; that is a luxury I can only dream of. They don't are not operating with that kind of budget on this project. It's a small building an economical solution.I guess some extra sheet metal would do the job here.

willard3,
Thank you for your response. I will look into those, but first I better do some reading on HVAC Acoustics.

317069,
Thank you for your response. You are right, about 4 ft of the basement wall extends above the road level on the front facade only, which I cannot exhaust to. On the 3 other sides the underground basement extends all the way to the property boundary line. So exhausting through the part of the wall above the ground on any of those 3 sides would mean exhausting to the neighboring property.

 

[cdxx139]

At 800-850 fpm, i dont see any need for a sound consultant, anything over, yes. Good luck!

knowledge is power

 

[willard3]

If you and your client are concerned about air noise, a budget solution is not the way to fly.

Your client should review his budget.

 

[EnOm]

cdxx139,
Thank you.

willard3,
You are right, but I don't make these decisions, or liaise directly with clients that much. I'm still at the bottom of the food chain

 

[willard3]

Bottom of the food chain does nor relieve you of professional responsibility. Ignoring the problem will not make it go away.

 

[SAK9]

Enom,

The question is why you should bear the risk if some one does not want to spend the money.If I were you and had a similar situation on my hands,I would put in sufficeint caveat clauses in the design brief so that the client can not go after me if anything goes wrong.For eg the car park fans will operate only during certain hours during the day(no operation in the night to disturb anyone's sleep.Provide a generous estimated noise level which would be best verfied by an acoustic consultant etc.In addition to the above specify ultra quiet fans operating at low speeds.

 

[cry22]

where do you guys get this 800 FPM number from?
SMACNA rates ductwork by pressure class, normally, you're in the 2" SP category for such ductwork, with a 2"SP you are essentially in the "low pressure" category as it used to be called - i.e up to 2000 FPM.

 

[cdxx139]

That classification of duct can handle 2000 fpm, but without a sound consultant, it would be prudent to reduce the airflow to conservative levels to almost gurantee there is no noise from the velocity of the airflow of that shaft going through the building. You dont agree?

knowledge is power

 

[sprinkler1000]

I believe you can go as far as 1500 fpm in the shaft without having to worry too much. Critical is the position and noise level of the fan, and sizing grilles correctly.