Commercial Kitchen Vent Fan Noise

Most vent fans seem to be installed right above the stove. You can install the fan at the other end of the duct and suck the air from the roof which will reduce the noise a lot.

There should be metal filters installed at the inlet end of the exhaust system and these will muffle a fair amount of the noise. If they aren't installing filters then the need to as they are an important safety feature. Without them any airborne fat, oil or dust will be sucked up into the duct where they will build up and ultimately be a fire hazard.

W.A.G. : It's duct noise that'l bother ya, - if the installation even works.

1250 CFM Thru 3/4 Sq. Ft. Duct = 1660+ FPM. Air Vel. in the duct.

It's a long duct with four turns.

Thats a bit fast for good flow or noise attenuation.

How much power drives the fan?

At what static pressure is the fan rated 1250 CFM?

An 8" by 12" duct is ²/₃ square feet not ¾ square feet but that makes the velocity of the air in the duct even worse at 1,875 feet per minute or a bit over 21mph, 34Km/h or 9.5ms^-1. While this is not unachievable it dose seem a bit high to me and will certainly make a lot of noisy. The only way to suppress the duct noise would be external insulation and the filters I mentioned earlier at the inlet. Even so it will be noisy.

The fan and vent are not yet installed. The framing is just now being put up. As a future resident and owner of the place I am trying to make damn sure the thing isn't too loud before it is built. I am so concerned because the last kitchen vent hood this developer had installed is very loud, the duct work is too small, there is a lot of wind noise from it, and the fan, only 8-10 ft up a straight shaft transmits a lot of noise into the kitchen.

On the drawing board the fan is rated at

1 hp 208 V and it is mounted outside at the top of the elevator shaft.

Again, on paper, the static pressure is to be 2.2 esp in.

I, too, had been concerned about the velocity and possibility of wind whistling through the duct though I had calculated the cross section as even less, at 2/3 of a square foot and that put the speed somewhat higher. 8 X 12 = 96 and 96/144 = .6666. 1250 /.6666 = 1875 fpm or about 21 mph. I did not know if this was fast enough to produce a whistle or not?

There is an outside supply duct that feeds directly to the hood. This supply duct is some larger that the vent duct and it does not look like its powered by its own fan.

It is my understanding that the duct work is to be welded at all its joints to seal the grease inside, in case of fire, and that both ducts are to be supported by the wooden framing using rubber sound and vibration isolators and stand off legs. Since this duct goes up through one of my bedrooms I have asked about it mounting in some detail.

The fan, hood, etc. are manufactured by Greenhack. I can get more information on Monday.

It certainly seems that it is going to be a noisy installation. You could always tie a piece of the duct to the roof a car and see how much noise it makes at 21mph. As I mentioned earlier there needs to be filters at the inlet end. This is to stop the grease, fat, oil and dust from entering the duct, fouling the duct and fan up and ultimately causing a fire. If the contractor plans to install it without these filters he is a corner cutting idiot and I suggest you find another contractor.

This is the sort of filter I am talking about

http://www.airfiltrationprod.com/al-panel.html

they are fire proof, will stop the build up of contaminants in the duct and on the fan and can easily be washed. I put the ones I use in my kitchen through the dishwasher every so often. They will also reduce the noise coming from the duct and fan.

Sir,

you know what, the duct size of 8"x12" is sufficient and the duct velocity (9.8 m/sec) is acceptable to Fire Code (7.5 m/sec). I see that its not the fan or the duct the problem. Since you are using hood try some commercially available hood with carbon filter its washable, or fabricate your hood with deflectors, if the stove is positioned to the wall then have the deflector away from the occupied zone so that noise will not resonate directly to user but will bounce off to the wall.

"you know what, the duct size of 8"x12" is sufficient and the duct velocity (9.8 m/sec) is acceptable to Fire Code (7.5 m/sec)."

I wouldn't be absolutely sure that it complies with fire standards unless you have documentation from the appropriate authority. Even then there are times that the standard isn't up to scratch and any kitchen exhaust system that has a fan and duct will if it hasn't got a filter end up covered with grease, fat, oil and dust. In the event of a fire on a stove top this can then ignite and rapidly spread the fire. Regardless of what the regulation is about there being a filter it's a bad idea not to have one.

Sir,

My first statement is to point out that there is no need to increase the duct size to reduce the noise, and as you can see on my statement that i suggest to used filter (Carbon that is washable)

Regardless of what the regulation is about there being a filter it's a bad idea not to have one.

There should be grease filters located in the hood. Any additional filters are not usually installed, but can be located downstream and are used to control kitchen cooking odors only. Carbon filters as well as electrostatic parcipatators and sprays are used in these cases. Adding a filter in this situation is a bad idea. This creates increases static, less flow, plus an additional maintenence issue. The fan sounds like it is existing, so he is going to have to replace it if re-shimming can't work. The need to look at the manufacturer's fan performance curve first will answer that question if the fan has anything left in it. I agree if the installation was permitted properly, and knowing the fire chief is god in this situation, you need to get his approval first before altering anything or adding an obstruction in the exhaust duct.

This whole point is moot anyway, the velocity is proper and is also the noise culprut. The most he can do is ensure to wrap the duct with the fire blanket as in the link. http://www.fireretardantsinc.com/3m/images/15a_duct_wrap.pdf

Your kitchen exhaust duct is sized properly at 2200 feet per min. with 1250 cfm through an 8"x12" duct. This is the industry standard. The duct is inherent to noise due to that velocity and there is nothing you can do to change it. I don't find two 90 degree turns and 2 45 turns excessive, although an accessible grease clean-out is required every 10'-0", or change in direction. This duct should be wrapped with a 1" clearence from combustibles type insulation. A fire board or fire wrapped insulation that is good for 2000 degrees for 2 hours is required around the kitchen exhaust duct, unless your duct is running in a non-combustible masonry enclosure. The kitchen exhaust fan may be quiet, but your duct velocity (that is correct) for a kitchen hood is the cause. If you add anything in the kitchen exhaust duct, it will need approval of the local fire chief, and ask him if he will accept it before you install anything in the duct for liability reasons overall. I would think that he will be against it. In addition, increasing the duct static pressure will cause the fan cfm to decrease, so you would need to replace or re-shim the fan to make up for the larger air pressure drop. I would say if it isn't wrapped, wrap it with the fire rated insulation. That may help your noise. Other than that, there is not much more you can do.