How Would Engineers Design A Kitchen Vent?

[jyl]

Three story - can't vent up, have to turn 90 deg, get into stairwell, then turn 90 deg to get to the outside. Airflow and duct sizing calculations are my next step.

[RWebb]

ok - let me know if you want info

salesperson side V-a-H was best & quietest - I find it not so quiet & not all that wonderful

it is well made and spendy...

I keep thinking about putting the actual blower up on my (flat) roof

any HVAC consultants in your area(s)?

[look 171]

Originally Posted by RWebb

ok - let me know if you want info

salesperson side V-a-H was best & quietest - I find it not so quiet & not all that wonderful

it is well made and spendy...

I keep thinking about putting the actual blower up on my (flat) roof

any HVAC consultants in your area(s)?

Do you have the single motor or dual blower in that vent a hood?


the Aqua lung or the actual blower would not work up on the4 roof. YOu have to make an encloser to protect it from rain. and still do the electrical. By the time you go through all that, just buy one thats design to go up on the roof. Viking or any of the high end appliance company make them. If you want this thing to rock, get two of them and hook it up to you vent a hood through two different 10" pipes. it will more then do the trick.

[look 171]

Originally Posted by jyl

Three story - can't vent up, have to turn 90 deg, get into stairwell, then turn 90 deg to get to the outside. Airflow and duct sizing calculations are my next step.

That should work ok with two 90s. what are you cooking that you are so concern with the vent? If you are worry about the 90 degree bends, have a roof motor pull as well as the motor down below working together. I do this to laundry vent all the time. We place a motor to pull the has gases out of the pipe as well as the dryer itself. VEry very effective on longer runs in these big houses. I have even install a fan motor mid way for a client once. It was over 40 feet run in a condo. thankfully it had a craw space

[jyl]

The natural place for the duct to exit the house will be on the side, between house and garage, about 10' above the ground. The total length of duct will be about 10'.

That is where the exhaust duct discharges now, though existing duct is, I think, too small to use. Water heater and clothes dryer also discharge there, though about 3' from ground.

Only window directly above that is the 2d floor bathroom, but those are roughly 6' higher than that location.
There are bedroom windows on that side of the 2d story too.

I thought would be easiest to mount a fan on the outside of the house at that location, blowing either horizontally away from house or vertically up.

I was worried about discharging cooking air there but the existing exhaust doesnt smell up the house via bathroom or bedroom windows.

If I have to - code or common sense - run duct all the way to the roof, it'll be another 90 deg bend then something like 20' vertical. And an eyesore.

[RWebb]

mine has a dual blower

[Hugh R]

Ideally, you probably want a face velocity (V) of 100 feet/minute. In order to capture the "stuff a few feet below the inlet to the exhaust (say 3 feet to the stove top). With a six inch diameter duct (D),

Quantity=Velocity times area, or Q=V*A

Q=100 x pi(D^2)/4= 1,478 cfm. Call it 1,500 cfm. You will be adding in for two elbow losses.

If exhaust air is 100F and makeup air is 75F, then you will need 0.95 cubic feet of 75F MUA for every 1.0 cubic feet of 100F exhaust air, at Standard Temperature and Pressure (75F + 460 Rankine/100+460 Rankine).

[jyl]

Thanks, Hugh!

[jyl]

Okay, I am trying to understand how/why these formulas work. Conceptually.

Here's what I am coming up with. Suppose I have a hood that is round with radius r (feet) and flat, with a duct in the center. The fan sucks a certain volume of air into the duct, express as airflow A (cubic feet/sec). Assume that the air gets sucked in equally around the duct, so that there is a hemisphere of air under the round duct whose volume shrinks at the same rate as air is sucked into the duct. I want a molecule of air at the edge of the hood, that is located on a hemisphere of r radius or r feet from the center of the hood, to move at speed v (feet/sec). That means the hemisphere's radius has to shrink at speed v. After 1 second, the hemisphere's radius is v feet less than the original r. We can calculate the change in the volume of the hemisphere, which is airflow A.

Example: r = 2.4 feet (the hood has area 17.5 square feet), v = 1.67 feet/sec (or 100 feet/min). At time t = 0 sec, the hemisphere has radius 2.4 feet and volume 0.5*4/3*pi*2.4^3 cubic feet, or 27.53 cubic feet. At time t = 1 sec, the hemisphere has radius 2.4 - 1.67 = 0.7 feet and volume 0.5*4/3*pi*0.7^3 = 0.70 cubic feet. To do this, the fan has to suck A = 27.53 - 0.70 = 26.83 cubic feet/sec of air. That is A = 1609 cubic feet per minute. Which is close to what Hugh's formula gives.

Now, what I am unsure about is, how do you account for the hood being rectangular, not round? And for the fact that the back of the hood is against the wall, and the hood has side panels? I used to think that the wall placement and side panels should massively reduce the airflow needed, because (I thought) more of the suckage is directed toward the open front of the hood. But now I'm thinking, the hood has no mechanism to direct more suckage to the front of the hood. The fan sucks equally from the back and sides and center of the hood (even though the air is already contained there) as from the front of the hood (where the air really needs to be captured.) If that is right, I am now (maybe) understanding why some hoods have underplates covering the center of the hood, with all the airflow being directed to the edges of the hood. I used to think that was just for a sleek look.

[RWebb]

how do you account for....??

this is all done via look-up tables; the phrase is "semi-empirical"

with a fluid flow textbook you can get deeper into it; also a heat transfer text will have a decent fluid flow section(s) in it - Karlekar & Desmond is more readable; Kreith is more "workmanlike"

are you sure you want to do this??

can you post a sketch ??

if I understand your query, you start with a round hood and then want to understand a rectangular on - right?
- you are also assuming you can neglect pressure variation & boundary layer effects...

[Hugh R]

I'm a little rusty on my ventilation stuff, but I have a couple of reference books at home that I'll consult tomorrow while I'm "preparing" for my Wednesday colonoscopy. One thing to think about is that you are using the hood to create a lower air pressure (slight vacuum) and the room air is rushing in to fill that void and is also pushing the contaminants into the area of lower air pressure. So the wall does in a sense direct more air flow from the front and sides. The reason for the diffuser is to not have all the vacuum in one small space and only pulling from the center where the duct is located, but rather to have better capture at the edges of the stove top as well.

[RWebb]

now that's ironic Hugh

best wishes for a drama free procedure

[jyl]

I would appreciate that, Hugh. Hopefully a good ventilation textbook will be just the thing to pass the time waiting for the OsmoPrep to kick in, or kick out as the case may be.

Randy, thanks for the references. I'll take a stroll by Powell's Technical Books this week. Yeah, I am trying to learn something new, hence the oddly obsessive approach - I know it would be easier and more accurate to simply call Vent A Hood and say "what cfm and duct do I need for X situation".

Also, I am hoping to discover some way to get the result I want via more design and less dollars. Big cfm venting and conditioned makeup air systems don't come cheap.

[RWebb]

if you really want to be oddly obsessive just drop by UC Davis and audit an HVAC course in the Eng. school...

re cheap - I've entertained the idea of finding an eatery that has gone out of business and buying their bower off of the top or side of their building. I do know a guy who put a commercial unit on a house he owns... his neighbors hate the noise.

You could perhaps get a local sheet metal shop to build a hood that is a lot cheaper than the Vent a Hood - make it out of mild steel and then get it powdercoated or enameled.

Good Luck... and as always... post pics.

[wdfifteen]

Thinking outside the sphere here - A Carter AFB will pass 750 cfm at an inch and a half of vacuum and Chrysler 426 street Hemi has two of them. So if you were to turn on the fan on your range hood and from somewhere on the other side of the wall would come the unique sound of a Chrysler gear reduction starter ...
You may be living alone but you would be doing it with style.

[Hugh R]

Originally Posted by RWebb

now that's ironic Hugh

best wishes for a drama free procedure

Exhaust, ventilation, colonoscopy, yeah I get it. Very good.

[Hugh R]

From D. Jeff Burton, CIH, noted Certified Industrial Hygienist.

"The formula below relates the quantity of air in cfm entering a hood to the capture velocity (v), distance from the hood's face (x) and size of the hood (A). The most common use of this formula is in determining the optimal distance from a emission sources (i.e., generation of vapor, gas or dust) to the ventilation hood face.

Formula:

Q= v*(10x^2 + A)

where: Q = air flow in CFM,
v = capture velocity usually 100 to 150 feet per minute,
x = distance from hood opening in feet, and
A = area of hood opening in sq. ft.

Assuming v=100 ft/min,
x= 1.5 feet, and
A = 2 feet by 2 feet; for example

Then Q=100ft/min * (10*1.5ft^2 + 4 ft/sq)

So Q = 100* [(10 x 2.25) +4]

Q = 2,650 cubic feet per minute. Which is the air flow necessary if you want to "catch" a vapor or dust emission 1.5 feet from the face of a 2 ft by 2 ft plain (unflanged) square hood.

For flanged hoods, either placing an unflanged hood on a table top in from of the workspace (say doing sanding) then you have blocked 1/4 of the area for the flanged to pull air (i.e., the table top, or in your case, the wall behind the stove, so the formula becomes

Q-0.75v (10x^2 +A), thus dropping the air flow requirement from 2,650 cfm to 1988 cfm.

To go further, using a canopy hood, like you might have with your stove, you use the following formula

Q=1.4Pvx,

where Q=Air flow in CFM,
P=Perimeter length of hood in feet,
v= capture velocity (i.e., 100 feet per minute), and
x=distance from hood opening in feet.

So Q=1.4*100*1.5 from above or

Q = 2,100 cfm
The above assumes a free hanging canopy hood, not one against the wall. So with a canopy hood against the back wall of the stove, I would expect that you would also add in the 0.75 at the front of the equation as discussed above or 75% of the 2,100 cfm or 1,575 cfm.

I'll try and do some more tomorrow.

[RWebb]

speakin' o' cheap... do you have room to mount a really big "attic" fan any where? you could lead the duct work to it an neck it down as you get closer

[Hugh R]

An attic fan probably won't do it. You want the pressurized side of the fan as close to the source as you can get it to push the stuff out, not pull it out.

[jyl]

Thanks again, Hugh. I am digesting what you've posted.

Randy, here is a quickie sketch. Basically something like this:
- range is 36" wide by 33" deep, cooking surface is 36" above floor.
- hood is 60" wide by 42" deep, and 24" tall. Bottom edge of hood is 78" above the floor, so 42" above the cooking surface.
- hood overlaps cooking surface by 9" in front, 12" on each side.
- hood is mounted on back wall.
- range is set against back wall.
- hood has side panels that are more or less 42" deep, and extend down to height of cooking surface. Though the bottom edges of the panels are 12" horizontally from the edge of the cooking surface, they don't touch the range.

I'm basically hoping that if I enclose the cooking surface as much as possible, and otherwise optimize, I can reduce the exhaust cfm to the point where stuff like number and size of ducts, cost of fan, supply of make up air, become easily manageable.