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hoods - baffles vs. centrifugal and other issues

Posted by cyalexa (My Page) on
Mon, Jul 22, 13 at 21:06

I am replacing my under-cabinet hood. I would like a VAH but they are too tall. My current hood is 6" tall and I frequently bump my head. The VAHs that are 6" tall have less than 300cfm. To get up to 600cfm, the hood has to be 9" tall.

The Kobe brand seems to be a good value. There are several choices that are 6" or less tall, some with baffles, some with centrifugal fans. What are the advantages/disadvantages of the different filtration types? One model, RA2830SQB-1 30 inch, is only 2" tall. Is this too good to be true?

I would appreciate any and all input.


Follow-Up Postings:

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RE: hoods - baffles vs. centrifugal and other issues

Search for kaseki's replies in this forum; he really is knowledgeable about the physics of hood operation.

A 2" tall hood will be far less efficient than a pyramid-type one; it all depends on how the air flows.


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RE: hoods - baffles vs. centrifugal and other issues

As a general rule the ones with centrifugal fans are better at low cfm operation, say under 200 cfm while the baffles need more airflow to be effective.

For the baffle setup all you do is pop them out and put in dishwasher. The fans type you clean out the grease cups from time to time and the channel that leads to the cups.

That super thin Kobe , I think, is a discontinued model that did not last long on the market.


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RE: hoods - baffles vs. centrifugal and other issues

Reading all the ventilation posts on this forum, (and I wouldn't suggest limiting yourself to mine) would take many hours. Selective reading may be more cost-effective. The posts may provide a sort of intuition about what makes sense and what doesn't.

With respect to the questions above, here are some stream of consciousness thoughts.

If possible, consider changing cabinets above the unit so a taller unit can be utilized, particularly for baffle systems. It will provide more room for baffle tilt, and will allow better uniformity of the air flow through the baffles.

Using an external fan or a fan in-line with the duct will reduce the required hood height.

For centrifugal fan systems, such as the VAH, the air flow is determined by the fan layout, but they cannot easily be thin and still get a fan and squirrel cage in the space. They may be non-uniform across the aperture no matter how much height is provided.

Persons owning VAH systems on this forum have complained about the annoyance of cleaning the units. I don't have one so I cannot qualify just where in the annoyance range they might be placed. My guess from what I've read is that they are more annoying than standing in line for an hour, but less annoying than repairing a door lock inside an Audi door. They would seem to be more annoying than cleaning baffle or mesh hoods.

For baffle systems such as my Wolf Island Pro, it is true that the baffle assemblies and the strips they drain to can be cleaned externally, such as in a dishwasher. However it is still necessary in my view to reach up in the unit and wipe the remaining sheet metal surfaces to remove any grease that has precipitated on them.

This brings up another related point, working in a hood over a cooktop risks putting undue pressure on the cooktop. If the cooktop is a semi-pro gas range with cast iron to lean on, this won't be a problem, except perhaps to one's back. For potentially frangible Ceram used with coil and induction units, some care to not weight the glass too much is needed while leaning under the hood.

For general perspective on ventilation, consider what is going on in the ventilation system. A pot or pan on the cooktop emits a thermal plume of effluent, mainly comprising water vapor and grease particulates. The grease particulates have a range of diameters (grease spectrum). The purpose of filtering, whether mesh, baffles (which act centrifugally), or centrifugal fan designs, is to remove the larger particles so they won't coat the ducting farther up.

All these filter schemes are only partially effective, and the amount of each part of the grease spectrum that they collect varies. Spectrum efficiency information is published for many commercial filters. The particulates not filtered rise with the moving air in the duct, and depending on duct configuration, air velocity, and duct wall temperature, either are transported outside or coat the duct. Properly designed and functioning residential systems tend to have very slow build up of grease on their ducting.

The moral of this story is that when generating a lot of greasy effluent, one wants to run the fan on a high enough speed to make the baffles or squirrel cage effective in centrifugal slinging, and keep the velocity of the air in the [properly sized] duct above 1000 ft/min.

If the air flow at high fan speed is low due to a fan too small for the aperture area collecting the effluent, a properly designed [clean] mesh filter nay be more effective. Impingement by particles is more certain with overlapping meshes. These have to be cleaned more often than baffles, for which cleaning is an aesthetic issue rather than grease collection efficiency issue.

While a too small centrifugal fan design will still sling a portion of the grease spectrum passing through it onto its collecting wall, too little air flow will likely result in the hood not containing the [initially] captured effluent, with the result that the plume enters and then curls out of the hood, or just impinges on the mesh and flows sideways without fully flowing through it.

Last, I would argue that to get relatively uniform flow, the height of a hood assembly needs to be similar to its shorter aperture dimension, at least with baffles. If a mesh is used to choke the aperture, then the irregular flow of a thin height design can be evened out. However, actual flow in this case will not be very high unless a fan designed for significant pressure loss is used.

kas


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RE: hoods - baffles vs. centrifugal and other issues

Kaseki wrote, "Reading all the ventilation posts on this forum, (and I wouldn't suggest limiting yourself to mine) would take many hours. Selective reading may be more cost-effective."

Now you tell me! Actually, I don't regret the hours spent reading old threads. I learned that I should get a 36" hood to put over my 30" cook top if at all possible. I will probably bump my head more often but it seems like the benefits still outweigh the risks. There is no cabinet on either side of the hood but there is 3" of wall available on either side. The cabinet under which my current hood resides is only 30" wide but if I can find a hood that is finished at least 3" around its top perimeter I guess the hood can stick out past the bottom of the cabinet. I think this would look better than what I would end up with if I tried to change the cabinet (cabinets are custom built and stained so a change would not be likely to match). I do use the space in the cabinet and would hate to lose it but may consider removing it altogether and going to a chimney style hood. I'm more likely to find someone that can match paint than I am to find someone to match a new cabinet to the rest of the cabinets.

In spite of my reading today, I do not understand the meaning and importance of baffle tilt.

I will consider an in-line or external fan, if I can find a competent installer (I live about an hour from a big city). Are these used in addition to or in place of the internal fan?

In my advancing years I am highly unlikely to stand in line for an hour for anything and while I am not capable of fixing an Audi door lock, I kind of enjoy simple DIY projects. That annoyance analogy aside, VAH is out of consideration unless I remove the cabinet. I looked at a VAH in a showroom and it looked easier to clean than what I have currently. My cook top is induction so I do not and will not lean on it while cleaning the hood. I've got a nifty ladder that has a platform I can stand on yet still be close to to what I am trying to reach.

Lastly, I don't understand this - "Last, I would argue that to get relatively uniform flow, the height of a hood assembly needs to be similar to its shorter aperture dimension, at least with baffles. If a mesh is used to choke the aperture, then the irregular flow of a thin height design can be evened out. However, actual flow in this case will not be very high unless a fan designed for significant pressure loss is used." Is the aperture dimension just the dimension of the baffle or the dimension of the bottom of the hood? Is 6" considered a thin height design? And, how does one determine if a fan is designed for significant pressure loss?

Thanks to everyone that commented and thanks in advance for replies to this post!


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RE: hoods - baffles vs. centrifugal and other issues

Baffles are best installed at an angle so the collected grease drains to one end. NFPA 90-something requires more than 45 degrees for commercial hoods. My Wolf hood angle is probably somewhere in the 20 - 30 degree range. (Sorry, don't have time this morning to measure.)

Normally, an external or in-line fan would be in place of an internal fan.

Air flow follows the easiest path, like electrical current, but as more flows in the easiest path it becomes less easy and the flow spreads out. (With apologies to Maxwell and Faraday.)

If one were to attach a vacuum cleaner to the bottom of an open large but low height box, and turned it upside-down, the flow just below the hose attachment point would be higher than out near the edges of the box. Now cover the open top of the box (which is down) with cloth reinforced by a 1/4 inch screen. The restriction below the vacuum hose attachment would force it to pull air from other parts of the box (top) aperture. The more the cloth chokes the flow, the more even it becomes, but the choking lowers the total cfm moved.

Fan flow rate performance (cfm) can be plotted against the pressure difference (inches of water column or psi or pascals) across the fan. This is often called the fan curve. Fan curves are available from Broan on-line for their hood fans, and at Rotron for industrial fans, and tabulated forms of the fan curves from Fantech from their web site.

Typically, fans are specified by cfm at zero static pressure, but the actual pressure drop in a ventilation system will be higher. One could have 0.1 inches across the baffles, another tenth through all the ducting and transitions, and some unknown and variable amount due to closing up the house and not having a "proper" make-up air system to keep the pressure (relative to the outside) near zero.

As a result, a fan specified at X cfm hanging in free air might only move 2/3 X cfm in the home (or much less if the make-up air is being pulled through cracks). There are fan designs that are more tolerant of pressure drop, but may have other undesirable properties, such as non-monotonic fan curves causing them to "hunt" under some conditions.

Advancing years provides the experience to have the extended vocabulary needed to deal with the door locks.

kas


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RE: hoods - baffles vs. centrifugal and other issues

>>>"Advancing years provides the experience to have the extended vocabulary needed to deal with the door locks."<<<

Also. with advancing years, the use of an "extended vocabulary" no longer leads to a soapy mouthwash.:>)


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