Clippings by kaseki

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RE: HELP! On Custom Vent Hood (Follow-Up #7)

posted by: kaseki on 11.22.2011 at 10:10 am in Appliances Forum

The paper at this link provided by another member in a different thread may be a helpful summary.

kas

Here is a link that might be useful: CA energy board paper


NOTES:

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clipped on: 09.12.2012 at 04:42 pm    last updated on: 09.12.2012 at 04:43 pm

RE: Blower Recommendation for Wolf Pro Island Hood (Follow-Up #7)

posted by: kaseki on 11.20.2007 at 12:03 am in Appliances Forum

Reference articles:

Thermal plumes of kitchen appliances: part 1 idle mode

Thermal plumes of kitchen appliances: part 2 cooking

NOTES:

http://lib.tkk.fi/Diss/2006/isbn951228233X/article1.pdf

http://lib.tkk.fi/Diss/2006/isbn951228233X/article2.pdf

clipped on: 08.30.2012 at 12:33 pm    last updated on: 08.30.2012 at 12:45 pm

RE: Blower Recommendation for Wolf Pro Island Hood (Follow-Up #12)

posted by: kaseki on 11.25.2007 at 06:03 pm in Appliances Forum

PART 2

We next consider using a 34-inch by 48-inch Wolf hood to provide adequate overlap of the plume to handle expansion in the hood and plume deviations due to air disturbances in the kitchen. The diameter of the plume at one meter is shown in the papers to be under 34 inches.

The actual entrance area of the Wolf Pro Island Hood of these dimensions is unknown to this author. There appears to be a slight return on the edges of the hood, and there are switches inside one edge. We guess that the actual entrance area is 10 square feet. (32 in by 46 in is 10.22 sq.ft.)

300 cfm divided by 10 sq. ft. leads to an average air velocity at the hood entrance of 30 ft/min. However, interpolating the results for the induction range in the part 2 article, the peak velocity of the plume studied was 0.66 m/s or 130 ft/min. This will vary with cooking temperature, but we are using a average over the cooktop so it shouldn't be too far off.

What happens when a plume with a nominal Gaussian velocity distribution and central velocity of 130 ft/min enters a hood with an average intake velocity of 30 ft/min? The plume reaches to top of the hood and the higher velocity part spills downward. It is unlikely it will all reach 30 ft/min and be perfectly captured before some of it spills out.

If the entire hood entrance were provided an average airflow of 130 ft/min to ensure capture without any roiling, then the cfm required would be 1300 at the hood entrance. The roof fan that could achieve this would likely be rated 2000 cfm at zero static pressure. Probably (we hope), 1300 cfm is overkill because much of the plume is well under 130 ft/min.

[handwaving begins]
If we assume that the baffle entrances are 40% of the hood aperture, and consider that the roiling plume has to pass close to baffle entrances with airflows thus 2.5x that of the average, then a 600 cfm average airflow would yield baffle edge air velocities of 2.5 x 60 or 150 ft/min. This might be enough for sufficient capture. Here we are hoping that spilling effluent is slowed down and is drawn into the higher speed baffle entrances before any can leak out of the hood and contribute to kitchen odor.
[end handwaving]

Wolf recommends for a 36-inch induction cook-top a 600 cfm fan. This is fan cfm at zero static pressure, so they are likely actually expecting somewhat less at the hood, maybe 400.

We would choose 600 real cfm for this case to be safe and use a nominal 900 cfm roof fan to obtain it (pending analysis of all the pressure losses involved). This rate will be higher than needed for many cooking conditions, but the roof fan can always be turned down. Running at reduced power reduces noise, and beats kicking oneself for not having enough power when the need arises.

Schlieren photography of this hood and cook-top combination (or a very elaborate computational fluid dynamics analysis) would be needed to determine the true minimum cfm that ensures full capture and containment.

kas

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clipped on: 08.30.2012 at 12:27 pm    last updated on: 08.30.2012 at 12:27 pm

RE: Blower Recommendation for Wolf Pro Island Hood (Follow-Up #11)

posted by: kaseki on 11.25.2007 at 05:17 pm in Appliances Forum

The following is an example calculation pitting a nominal Diva induction cook-top against a Wolf Pro Island Hood. As the reader will find, some hand-waving is needed to close on a required cfm.

PART 1

We need two conversion factors:

To get ft/min from m/s, multiply by 197
To get cfm from m^3/s, multiply by 2119

First, we look at what the induction cook-top effluent plume characteristics are.

We assume that the entire 9.6 kW maximum power output of the Diva 36-inch cook-top is being used, and that the pan configuratin is such that the heated surfaces have the same area as the cook-top. The first assumption is unlikely and hance conservative, and the second has only a second order effect on the result.

Using the methods of the referenced papers, we can find that the hydraulic diamter D_h is 0.64m. We want a hood that is over our head, so we choose 1-m height about the cook-top. (Higher means larger cfm.) Of the 9.6 kW output by the Diva, only 6% heats the air per the referenced experiments. Hence the convective heating is 0.576 kW.

We use the equation:

q_v = 0.05*((z + a D_h)^5/3 * phi^1/3)

where a is 1.7, z is 1m, and phi is 0.576 kW

(The papers use a coefficient of 0.05 in one place, and 5 in another, and claim phi is in watts but their tabulated results require phi to be in kW.)

Solving the equation and converting to cfm leads to 300 cfm in the convective plume from all 5 hobs.

continued below

kas

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clipped on: 08.30.2012 at 12:26 pm    last updated on: 08.30.2012 at 12:26 pm

supplemental (Follow-Up #23)

posted by: kaseki on 12.13.2011 at 09:14 pm in Appliances Forum

For the effects of velocity on duct deposition, there is the link below.

I believe this analysis lead to some changes in NFPA 96 [?] allowing lower duct velocities in some circumstances.

kas

Here is a link that might be useful: ASHRAE analysis

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clipped on: 08.30.2012 at 12:24 pm    last updated on: 08.30.2012 at 12:24 pm

Greenheck guide (Follow-Up #6)

posted by: kaseki on 08.28.2012 at 08:33 pm in Appliances Forum

A tour de force that is worth viewing.

kas

Here is a link that might be useful: Greenheck Guide

NOTES:

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clipped on: 08.30.2012 at 11:32 am    last updated on: 08.30.2012 at 11:32 am