Recommendation For A Rear Venting Insert Liner for Kitchen Hood
Helen
5 years ago
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Comments (19)
zneret
5 years agoHelen
5 years agoRelated Discussions
Help for the right height and hood insert/vent system in new kitchen
Comments (13)opaone opined. I think there is a fairly direct relationship between the baffle area, CFM's and noise. More CFMs for a given set of baffles (and constant baffle area) will increase baffle turbulence noise. However, baffle noise (generally a hiss sound) is not dominant unless a silencer is used with a remote blower. Otherwise, blower blade turbulence noise can be expected to dominate at the flow rates needed when cooking with hot pans (grease or oil near the smoke point). The larger the baffles the less the noise for any given CFM. I assume though that likewise the larger the baffle/CFM the less grease is removed? Two things are happening here. First, if the flow velocity near the baffles is not high enough, rising effluent will reflect from baffle surfaces and potentially escape from the hood aperture. Second, if the velocity in the baffles is not high enough, grease particles from the larger end of the particle spectrum will not be collected as well as they should be, causing them to pass into the ducting where they may condense on the duct surfaces. @Kaseki, do you know if there are any guidelines for baffle design/size/CFM for proper residue removal? Certainly those in the business of designing hoods and baffles will have design rules they use. I haven't come across any specific baffle design guidelines, but hood guidelines for use of particular baffles exist for those sources that produce baffles for commercial uses. Below are links to two example baffle (systems) with their pressure characteristics. (Links may need to be copied and inserted into one's browser.) http://productadmin.componenthardware.com/pdfRepository/repositoryFiles/20120906_025437_type%20i_product%20info_11-2011.pdf http://productadmin.componenthardware.com/pdfRepository/repositoryFiles/20120906_025211_type%20ii_product%20info_11-2011.pdf Baffles are going to be relatively simple on residential baffle arrays, and may have been experimentally tested and revised before production to yield decent effectiveness at minimal production cost. Are there trade-offs to that 90 CFM per sq ft aperture? EG, what happens if it's only 75 CFM? My simplifications are intended to avoid the extreme complexity of a problem space that can best be analyzed by computational fluid dynamics (CFD). What we have is a plume extent that has a shape, and associated with that shape is a velocity profile. The velocity changes with height and angle. When this plume enters the flow field close to the baffles, a lot of possible flow effects can happen, depending on the plume momentum and baffle details. There are also the tapered sides of the hood to consider as well as flat surfaces around the baffles. Picking the worst case plume velocity (~1.2 m/s) and demanding it be met by the baffle air velocity at the baffle gaps, and treating the plume shape (relatively bi-variate Gaussian) as a 10-degree constant velocity expanding cone for capture analysis seem to be adequate assumptions to avoid having readers later whine that their hoods were inadequate. 90 ft/min averaged over the aperture is just one of those approximations where the reality is that cooler cooking will require less, hence induction will require less than gas cooking due to lack of entrained hot gas combustion products in the plume, and in some cases will require more, except that the hood typically extends beyond some parts of any given plume thereby aiding containment of some momentum-degraded sideways drifting effluent near the baffles. Further, the overall air flow has some small capture influence beyond the edges of the hood entrance aperture, particularly for the lower velocity portions of the higher angle plume portions, and the overall flow over the cooking surface tends to move air toward the hood which also has some influence farther down for low velocity effluent. So, for many cases 75 CFM/sq.ft. may be adequate. Below is a copy of a table providing recommended updraft velocities for different cooking devices taken from one of the most useful of on-line sources, the Greenheck guide available at: http://www.greenheck.com/media/pdf/otherinfo/KVSApplDesign_catalog.pdf Note that this is for a 4-ft x 9-ft commercial hood probably 7 ft above the floor. Also note that there are various factors involved in using these numbers that may be found in the accompanying text. And of particular interest to me is that "wok" is listed with the "extra-heavy" category. This is probably applicable to Asian restaurant wok burners providing 100k - 200k BTU/hr gas jets. For residential wok cooking, the plume effects from the burner will be much less, but the temperature at the wok surface may be similar -- meaning that some parts of the plume velocities may be similar. So where should a residential wok be located in the table? I have induction for both cooktop and wok hob, and roughly 90 CFM/sq. ft. maximum air flow rate into my Wolf Pro Island hood, mounted at 34-inches over a peninsula, and it does a pretty good capture and containment job. Wall mount instead of island mount, gentle cooking instead of grilling or wokking, induction instead of gas, and other factors such as no cross drafts or an oversized hood may allow lower specific flow rates to be adequate relative to the table or others' experiences. Further, the reality is that for the most affordable MUA schemes, the inherent pressure loss will degrade the hood flow rate beyond that due to just duct and baffle pressure losses. So the renovator's task when contemplating 90 CFM/sq.ft. is to divine the degree to which my conservatism is overkill in the context of the entire ventilation system, kitchen layout, and cooking style, and design accordingly. kas...See MoreRecommend Vent Hood Insert for 48" 1200CFM
Comments (9)I think it really depends. Nothing that moves that much air is going to be quiet. If you can have remote blowers and inline mufflers, then you can reduce some of the sound; and in that case, a VAH might very well the obviously inferior choice. But even then, you still have the noise at the baffles. So, don't expect the hood to be whisper quiet. We have a 20 odd year old VAH. As long as the vent pipe is adequately sized, it works great and noise level is IMHO in the expected range. We also have a new ModernAire hood with internal blowers. It has more power than our old VAH, but it can be really really loud. And part of that is simply the whistling sound that the air makes when it passes the baffles at full air velocity. If I turn down fan speed, it's about as loud as the old VAH. In other words, I don't really perceive a big difference between either designs. Pick what works best considering all your other design constraints. And absolutely make sure that your vent pipe is as large as you can make it....See Morehood insert/range hood liner HELP
Comments (6)It might be easier to think of this hood as being comprised of two assemblies. The outer assembly is made of wood parts rigidly bonded or otherwise attached together to generally resemble a cabinet without a base. The inner assembly (called a liner) is a metal shell that incorporates (hopefully) a set of baffles, light and motor controls, lights, and, optionally, a blower. The blower subassembly comprises a fan blade sub-assembly and a motor sub-assembly. Alternatively, if an external blower is used, then there is also a cover and damper as part of the blower assembly. I would not expect a typical cabinet maker to supply the liner assembly, but may want it on hand to fit to it. As I don't have one of these, I welcome any refinements of my description that may be needed....See MoreKitchen Remodel- Vent Insert Recommendation
Comments (7)Start with performance requirements -- end up with performance. Let's assume that the insert/hood-liner cover/cabinet/hood will be designed to fit the insert once defined (and preferably once delivered so that the dimensions are certain). You need an liner entrance aperture that extends about 3 inches past the sides of the cooktop, or more technically, about 10-degrees from vertical past the edge of the largest pan to be used on either side. Front-to-back distance should be maximized to the same overlap if tolerable aesthetically. Given the intake aperture, find its area in square feet and multiply by 90 ft/min if hot cooking is to be performed, by 60 ft/min if you intend more sedate cooking. (Hot cooking means that the pan is hot enough that grease or peanut oil will vaporize into the cooking plume.) It would typically occur with wok cooking or meat searing. This is the required/desired and hopefully actual CFM. Multiply the resulting value by 1.5 to correct for all the pressure losses that you don't know about yet. This is the blower zero static pressure CFM that won't be achieved in situ, but by which blowers are designated. Use an exterior blower. Use an intermediate Fantech silencer if it will fit. Use a baffled insert. Duct size should cause the full blower power required CFM to induce an air speed between 1000 and 2000 ft/min, with the lower limit preferable to minimize noise and to minimize impingement condensation if the duct is very cold. You will need to match the required CFM with a make-up air (MUA) source. Depending on your house architecture and combustion appliances, if any, this MUA system may be simple or complex. Review MUA threads here and then ask questions....See MoreHelen
5 years agoHelen
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5 years agozneret
5 years agoHelen
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