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External blower size

Posted by SkindyLu (My Page) on
Sun, Oct 13, 13 at 13:00

I'm looking into an Abbaka external blower. My run is 16 ft, with a 45 and 90 elbow required. I'm going to buy a bluestar 36" 6 burner rangetop with 98K BTU's. I'll use a 42" hood. That comes to 1230 cfm if all burners are on high at the same time. I know I will rarely cook like that, but will run the two high burners at the same time sometimes, I'm sure.

The decision I have to make is whether to buy the 1000cfm blower and save myself about $800 or go with the 1400cfm blower. If I use 10" duct with the 1000cfm blower, would that bump me up to an acceptable system? I know the requirements indicate the larger blower, but cost is a consideration and I just don't want overkill.

I'm sure it's subjective, but opinions will help make the decision. Would also like reviews of the Abbaka blowers and MonderAire liners. Thanks much for your time.


Follow-Up Postings:

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RE: External blower size

I have a 36" Bluestar with griddle, and have a 36"x27" Prizer hood with a 1000cfm remote blower on 8" duct. I was limited to 8" duct size due to roof framing constraints, or I could have gone with 1400, but this set up works fine as it is!


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RE: External blower size

  • Posted by ctycdm 10b/Sunset 24 (My Page) on
    Sun, Oct 13, 13 at 14:54

double post delete

This post was edited by ctycdm on Sun, Oct 13, 13 at 16:05


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RE: External blower size

I have the 1000 cfm Abbaka, a four-burner Culinarian rangetop, a similar hood and a straight, 4-foot duct run.

I have enough ventilation. I would not say I have too much ventilation.

Depends also on makeup air.


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RE: External blower size

ctycdm..would you get the griddle again, had you the chance to do it over? I think I'd like it, but my husband likes his Teflon griddle for pancakes. colin3..appreciate the response. It helps.


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RE: External blower size

I absolutely love the built in griddle, and would get it again in a heartbeat! I use it five or six days a week, and sometimes two or three times a day! I'm kind of a short order cook type, so for me it is a joy to use. Once seasoned, it is non stick, and I never need to clean with soap or water. Just scrape and wipe down, and it's ready for the next meal...


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RE: External blower size

I did the 1400cfm abakka with the quiet kit. My cook top is 36" induction and a 12" single 22K btu gas cooktop. My main reason was that I wanted the suck power to be powerful even on low to avoid the noise. My hood overhangs the cooking area by 6inches. This has worked out well. I am glad I upsized.


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RE: External blower size

The airflow required is determined by the temperature of the surface doing the cooking (which sets upward velocity of the effluent), and the aperture area over which the air velocity has to be maintained, and a factor that depends on the hood shape and aerodynamics of the baffles.

The blower rating depends on the above and all the pressure losses in the "circuit" comprising baffles, transitions, ducts, bends, roof cap, and MUA path losses back to the range. (The latter can be a gain if an active MUA is used.)

Lots of guesswork is involved because nothing is very well specified. (I suggest Marconi was better informed about the parts he experimented with than we are about losses across a hood.)

If one has a restrictive MUA and exhaust path, the advantage of 1400 cfm (at zero static pressure) over 1000 cfm (at zero static pressure) may be quite modest. It depends on the restrictions and the respective blowers' fan curves.

Sufficiency may depend more on cooking style and degree of smoky "accidents" than the esoteric calculations will imply. For detail, the references at my My Clippings may be helpful.

kas


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RE: External blower size

@GWlolo..The idea of getting a larger blower so I can run quieter, but still efficiently on low speed is a good consideration. @Kaseki...I really appreciate the technical explanation, even though it's a bit over my head. I have a Lennox gas furnace and my MUA has 6" duct, one elbow, and spans 16 feet from the outside to a large interior cold air return. I have two wall grills for circulation in locations just off my kitchen. Do I understand correctly that if the MUA is ample, there is less pressure loss, thereby I may not need the larger a fan?


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RE: External blower size

It is possible that a larger fan with more restrictive MUA will flow less air per unit time than a smaller fan with a more ample flow. Usually, though, the MUA duct size is established to meet the actual needs of the system, and possibly make-up air from other exhaust vents also.

Your 6-inch MUA is intended to deal with modest exhaust, and a 1400 cfm rated fan with 10-inch duct can pull a lot more than your MUA path can easily replace.

What happens, though, is that there is always MUA matching exhaust, it just might be low cfm for each. In such cases the house pressure will be significantly negative, possibly as high (negative) as an inch of water column depending on many factors

The risk MUA is intended to avoid, beyond poor bacon effluent removal, is backdrafting combustion appliances (or fireplaces) by having insufficient MUA. If you have no such appliances, or they have their own MUA, or they are isolated from the kitchen, then you can always open windows to get the air flow up. Some communities may require MUA that meets some specified requirements.

kas


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RE: External blower size

I do have a gas fireplace, kiseki, but I don't know if it has it's own MUA. Good explanation of the situation. Most helpful. Sounds like I should stick with smaller blower and/or look into more MUA. Would it help to change the 6" duct to 10"? The ceiling area around the duct is open right now and it could be done fairly easily. Maybe I should do that AND get the smaller blower? I have a window I can easily open, so perhaps that is sufficient? You are really being a big help on this. It's pretty difficult to sort through when so many seem to know so little. Thanks much.


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RE: External blower size

Whether a 10-inch MUA duct will help will depend on the restrictions getting the air into the rooms of the house. For example, what sizes are the furnace to room ducts? Is there a furnace filter in place? Is the MUA flow going against the flow that the furnace and/or air conditioning move the air in the relevant ducts.

Perhaps you could have a 10-inch duct to a Broan or other MUA powered damper that dumps into the house in some way before the duct continues toward connecting to the furnace. The damper opens when the flow in the hood duct is high enough.

Generally, not liking cooking odors in high concentration or grease on the walls, I will always vote for more hood ventilation flow rate, within the bounds of good enough.

Good enough for full containment (very approximately and very condensed) is a blower zero static pressure rated flow rate equal to the hood aperture area (sq. ft.) x 90 ft/min* x (3/2) for pressure losses. This assumes not too much restriction on the necessary MUA flow. Full capture requires the hood significantly overlap the hot surfaces from which grease and smoke effluent is being generated. Lower flow rates may be fine if only frying eggs and not bacon.

Time searching this forum on MUA will provide more information and insight.

kas

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* 90 ft/s comes from the rising effluent velocity of 3 ft/s for hot cooking surfaces per references in my My Clippings, 60 seconds per minute, and an advantage of 2 for the flow dynamics of baffle hoods of nominal configuration (somewhere between a guess and an observation).


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RE: External blower size

I'm going to continue to read about MUA as suggested and I'm already smarter than I was, thanks to you, Kiseki. From what I gather, I'm looking at the powered damper, no matter what I choose for a blower. Think I'll like that better than always opening the window. It gets cold here! My hood will overhang the rangetop by 6 inches, so my capture area should be sufficient. I can probably run a second duct, this one 10", in another joist pocket for the new MUA powered damper that would dump into my cold air return. It would be shared by my furnace MUA interior end run. Is that OK? (I need discussion ideas for my HVAC guy :))


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RE: External blower size

Yes, talk to your HVAC guy. If the MUA is heading toward the return registers, it is traveling opposite to the normal furnace flow. This may be ok as then the furnace pushes air to diffusers, and the MUA supplies air to the registers, and both emit air when the hood is on high power. The net restriction through all these parallel paths may be passible. Of course the cold MUA and warm furnace air all eventually go out the hood duct.

kas


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RE: External blower size

It's nice having you as my wingman on this, Kiseki. I still need to decide what size blower to get, so I can size the damper appropriately. And that appears to come down to my style of cooking. Hopefully my day will produce some answers.


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RE: External blower size

HVAC suggested that I could use a non-powered 'butterfly' damper (with another duct run) rather than a powered damper. I understand the principle, but would there be enough 'un-forced' intake for an external blower?


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