How Would Engineers Design A Kitchen Vent?

[jyl]

Just an idea, maybe a dead-end, but I'll ask.

I have what I suppose you engineers would call a "fluid dynamics" problem.

The task is to design a range hood exhaust system with make-up air (MUA) such that
(1) the smoke, grease, vapors, effluent etc from the range when cooking, grilling, stirfrying at full blast is all captured in the air exhausted
(2) the air exhausted is the exact same air that entered through the MUA. Repeat - the same air that comes in, goes out.
(3) the location and size of ducts, registers, etc works in the existing structure
(4) [bonus points] the MUA is passive, i.e. not fan-driven.

As you can probably guess, the goal is to have an exhaust system that is
- very effective at removing yuckies
- does not backdraft fireplace, water heater, etc
- relatively low-cost (existing structure, no interlocked MUA fan, no heating for MUA air)
- does not freeze [overheat] the house in winter [summer] with the exhaust is pulling 700 cubic feet per minute of air out of the kitchen and thus pulling 700 cfm of 25F [100F] air into the house. This is enough air to fill a box 7 feet x 10 feet x 10 feet, every minute, so you can see how the house could quickly turn into an icebox [oven].

So, I've found surprisingly little information about this. In the residential context, range and hood manufacturers mostly ignore MUA, some local codes require it, but there is nothing about how to design the systems other than a simple air balance. In the commercial context, I've found some helpful information (for restaurant kitchens) but it is at commercial scale/cost - 5,000 cfm systems, heated fan-driven MUA, etc.

I have come up with some schemes, but have no reason to think I have any useful intuition about airflow. So, I was thinking, how would you engineers handle this? And assume you didn't have a multi-thousand dollar software package.

Would you buy a student edition of some modeling software and set up the room and system?

Would you build a little scale model of the room - from clear plastic, say - and blow/suck smoke around?

Would you just pick a scheme and build it, sort of a $5K experiment?

[peppy]

The noise from some of these systems is terrible, and the grease that is removed is not all caught in the filters.


In the restaurant we have hoods that are short and do not have forced make up air. Just a vent that allows them to pull in fresh air (they are old and would not pass codes now)

I would vote for a scale model.

[billybek]

The make up should keep the space just slightly on the positive pressure side.
It can be done, but remember that every cubic foot of air that you exhaust, you will need to condition slightly more than a cubic foot to replace it. When it is minus 20 or over 100 degrees F, that gets pricey for a home kitchen.

[turbo6bar]

Residential duct sizing is covered in ACCA's Manual D. Using recommended maximums for return ductwork (your make-up air), the minimum duct size would be 14" round for 700 CFM. However, this is with an HVAC blower fan pulling air. If you want the make-up air passive flow, the duct would need to be huge so that MUA is not pulled from the house.. It might be preferable to do as suggested, and make the area positive pressure with an axial fan. Fantech and Elicent make axial fans that would work for this application.

I don't do this stuff for a living, but I don't see why your make up air duct could not be sufficiently placed so that your vent hood pulls most flow from the MUA and little from the house.

Also, you should test system performance as related to hood height. As the hood moves closer to the range, you'll need fewer CFMs.

My intuition (could be bass ackwards) thinks MUA ducts in the wall on either side of the range would create a crossdraft carrying smoke over and up.

The other concern is if you have fan providing MUA, you now have a great source of air for combustion. This would not be a good place for a grease fire.

I would search the forums at greenbuildingadvisor.com and jlconline.com. Gotta get to work on the fixer-upper now. Will do some hunting later if you don't find a solution quickly.

[speedracing944]

I would do an oversized velocity stack much like what you find on the top of a Weber carb with it going through a steam driven turbine powered by a boiler on the stove which then exits via a converging diverging cone which propels the exhaust at supersonic speeds.

Speedy

[jyl]

I looked at this article - a lot to think about:

http://www.fishnick.com/ventilation/designguides/CKV_Design_Guide_2_031504.pdf

I started wondering where/how 700 cfm can exit a 14" duct (1 sq ft) at 12 ft/sec, that is close enough to the range to satisfy the goal (2) without disrupting the airflow required for the hood to capture all smoke/grease per goal (1). I think you're right, goal (4) isn't going to happen.

I was originally thinking a duct under the floor (full basement) to a floor vent under the range, which would be a space about 6" tall. If the range is free-standing (no cabinetry on either side) then the air will have about 4 sq ft to come out (front and sides), so should slow to 3 ft/sec. Will it then turn upward and flow smoothly up the sides and front of the range, past the cooking surface, and up into the vent hood - sort of an air curtain idea? Or will it simply make the floor really cold, unless deliberately directed upwards?

The "backwall" design shown in the article seems interesting, though I'm not sure why the duct seems to point down not up.

[jyl]

Suppose - panels on either side of the range plus a couple inch space between range and backwall, to direct MUA up through cooking area and into vent hood? If gap between panels/wall and range is 2", and range back is 36" and sides are 33", that is about 2 sq ft, so air can slow to 6 ft/sec, hmm still sounds too high - high enough to blow out pilot lights, distort burner flame patterns, etc.

[HardDrive]

In terms of design software, don't buy anything. Download Google Sketchup. I use it all the time. The basic version is free.

Google SketchUp

[Superman]

Engineers did invent vents. You guys are simply having another look.

[HardDrive]

Why MUA?

I guess I'm missing something. If you want MUA from outside, its going to enter the home at whatever the ambient outdoor temperature happens to be. We have a powerful Viking hood, and if we run it on high on a cold day, no question is sucks in cold air from every little crack in the houses envelope.

Ya can't get something for nuthin' (that is the extent of my engineering knowledge )

[look 171]

Originally Posted by jyl

Suppose - panels on either side of the range plus a couple inch space between range and backwall, to direct MUA up through cooking area and into vent hood? If gap between panels/wall and range is 2", and range back is 36" and sides are 33", that is about 2 sq ft, so air can slow to 6 ft/sec, hmm still sounds too high - high enough to blow out pilot lights, distort burner flame patterns, etc.

What makes you think the air coming out of the basement will flow upward and get suck out. the hood will draw air from the easiest path which is right outside of the hood. I think you will have to have it blown forced in from the bottom. I would add a filter at the inlet.

[look 171]

Originally Posted by peppy

The noise from some of these systems is terrible, and the grease that is removed is not all caught in the filters.


In the restaurant we have hoods that are short and do not have forced make up air. Just a vent that allows them to pull in fresh air (they are old and would not pass codes now)

I would vote for a scale model.

Two way to quiet air flow. Have a exterior mounted motor to pull air and install a muffler between the filter or hood area and the outlet. We do this all the time in Kitchens we build (not restaurant because they couldn't care less about noise but cost). it works really well.

[red-beard]

Look has it right. To create flow, you need energy. There isn't enough in the cooking to do it.

Put the fan on the outside, far from the intake and muffle the noise.

[john70t]

If the MUA comes from another source, it would be unheated.

To be practical and a direct-replacement, the input and output air would have to use the existing ductwork that has been framed in.

This could be achieved by using a split vent which also provides some heat exchange, however:
-the effective conduite would be cut in half,
-the inlet air would be heated in the summertime,
-on the exterior, the two sides would probably need to be located far away from each other to conform to code. This may require an ugly and large attachment.

[jyl]

Yeah, I'm resigned to using a fan for the MUA. So won't achieve goal (4).

I'm trying to avoid having to condition the MUA, so don't want to tie it to the existing central heat/ac system. I'm trying to achieve this by having the MUA enter the room, pick up the grease/smoke, and exit the room, with a minimum of involvement by existing room air.

There is a basement under the range location and the basement stairwell behind the range wall, with the exterior 12' away (and not a pretty part of the exterior either) so there is reasonable access for ductwork and I can put fans etc there.

[oldE]

Lets have a look at this from another perspective.

Rather than use outside air, how about incorporating the following basic parameters:

As you stated in #2 above, "(2) the air exhausted is the exact same air that entered through the MUA. Repeat - the same air that comes in, goes out. " I'm not entirely sure whether you meant the same air volume or the same air molecules. If it is the latter, then:

If you are using the same air, one extractor fan should be able to remove oil/vapor/particulate laden air from the stove top. Then it should go through some centrifugal separator/extractors to get rid of the liquids/vapors and solids. Then through a charcoal filter system (or maybe an ionizer?) to reduce odors. The air could then be returned to the stove top area in sort of a curtain effect to satisfy parameter 2.

The benefits would be one fan source (as you said above,: kiss #4 goodbye) and no heat differential between hot air off the stove top and outside air. A heat exchanger would reduce some of the energy losses, but would mean the air exiting the house would have to be cleaned before it went through a heat-recovery process to reduce condensates in the heat recovery system.

Or you could abandon the relatively inefficient stove top cooking system and go for a totally enclosed system.

Have fun
Les

[jyl]

So, step one, how much exhaust airflow?

Assuming 36" commercial range w/ salamander broiler, hood 42" on sides and 60" on front (so, about 12" overhang on sides and about 8" on front, both range and hood mounted against the back wall, with full side panels.

I used two methods. One is a generic calculator - for hood of circumference c, mounted at height y above source, to achieve capture air velocity v, what is airflow A. The calculator recommends v >= 0.2 m/sec. With no side panels, using c = 3.7 m (just front and sides), y = 0.69 m, calculator says I need A = 3,310 cfm (!). Not sure how to incorporate the side panels, but I decided to subtract them from circumference, leaving c = 2.6 m (one side panel) or c = 1.5 m (two side panels). Get A = 2,400 cfm (one side panel) or 1,500 cfm (full side panels).

Exhaust Hoods

Another is a commercial kitchen ventilation design guide. This factors in the volume of contaminated air Qc produced by each type of cooking equipment, area A of equipment and of hood, etc. Combining the Qc of a gas range and a salamander, using A = 36" x 33", and hood area 42" x 60", I get 1,400 cfm. Adding the factor for side panels, I get 1,260 cfm.

http://old.greenheck.com/pdf/kitchen/KVSAppl&DesignSeptember2005.pdf

I figure the generic calculator is not designed for cooking equipment (which produces heated contaminated air, which naturally rises), so I'll use the second result.

Jeez, this is already a lot higher than I expected. Hardly any residential exhaust hood/fan goes above 1,000 cfm.

Anyway, on to step two.

[look 171]

I think Vent-A-Hood and a GE monogram unit both have a 1200cfm unit. The Vent a hood is better for clearing grease and smoke because it has no filter and it is a straight run outward. they both have twin motors that pulls 600cfm each. I have a vent a hood at my parents.

[jyl]

Thanks! That helps.

[RWebb]

I have a Vent-A-Hood, BTW -- let me know if you want any info - it exhausts straight up out the roof.