Fan Logic: Air flow, blades, venting....


Awl --
Curious as to "fan logic".
I notice that in, say, kitchen hoods, central A/C air handlers, squirrel cage blowers are almost always used. btw, does "blower" imply the squirrel cage config? I will use "SC blower" visavis traditional blade-type "fan".
The SC blowers in A/C seem to be most often single large squirrel cages, whilst the sc blowers in kitchen hoods seem to be smaller dual cages, on either side of a double-shaft motor.
SC blowers seem to be super-efficient air movers. I scavenged two good-sized ones from a Lennox system, the cage about 1 foot long by 1 foot dia, and just turning it gently by *hand* delivers quite a forceful breeze -- which surprises even the mechanically savvy I demonstrate this to.
Otoh, blade-type fans are used in "boosters" in hvac duct, window/minisplit A/C's, central A/C condensers, roof mushrooms/wholehouse fans.
What governs the choice, and why? Ito of raw cfm's per $ or per "unit size" (in cu in), is one style inherently superior to another?
The, uh, thrust of my Q is that I'm cobbling together a kitchen, and am dizzied by prices of venting hoods that I like (*easily* $2,000-4,000 and not super-fancy -- holy shit!!!), and am not particularly impressed with the few I'm able to turn on, noise-wise, flow-wise. Cosmetically, tho, they are super.
HD has a couple of decent-looking ones in the $400-500 range, but they are a little bulky.
In particular, the fact that the dual sc blowers are located *in* the hood-proper bulks many of the units up considerably, which in this application creates visual line-of-sight issues, due to proximity to an entrance door.
So, I could locate the sc blower right up at the exit vent near the ceiling, or, even better from a visual pov, use an in-line booster type fan. And then I would be able to use the old hood etc from the original set up, which everyone kind of likes anyway.
With an inline fan, I realize the motor itself serves as a flow obstacle, which in a 6" or so duct could be a considerable proportion -- which is not the case in say an outside A/C condenser, where the blades are easily 3 feet diameter..
Not to mention the grease/temp issues in a hood, but which I think could be made manageable by the right motor, filters, and periodic maintenance. I figger if I will occasionally have to clean the squirrel cage in a blower, cleaining the fan blades AND the motor in an in-line ditty isn't such a big deal.
But then, being up at the ceiling, line-of-sight issues would be minimal, so mebbe a sc blower as traditionally used in this type of venting would be better. I would lean toward a single cage like in A/C blowers, to totally get the motor out of the ductwork.
More along shop lines, I had scavenged one of these double-squirrel cage hood-type motors, and kluged it into the exhaust for my welding table. Wow, orders of magnitudes better than the inline-fan ditty I kluged earlier, super-quick fume clearance. But, the previous in-line fan could have been just shitty anyway, so I don't know if apples were compared to apples.
But other in-line fan apps have gone well: The shit motor in my mushroom eventually went, so I scavenged a window A/C 3-speed fan unit, thermostat and all, and this works great -- infinitely superior quality to the original mushroom innards, altho a bit of a pita to install.
If I get lucky, I'll find a small 3 ph motor, and put a VFD on it, for infinite speed control -- small VFD's are not that expensive. I'm hoping to luck out with a system that's quieter than some of the expensive noiseboxes I've seen. I'm surprised that with $4,000 price tags, they don't offer something like this.
Opinions, suggestions for the kitchen?
--
EA




Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Existential Angst wrote:

The best kitchen fan I ever had was a explosion proof squirrel cage exhaust fan I got from a friend. He had used it in a chem lab reagent hood. I ran 8" duct about 15 feet across the attic to the fan, which exhausted through a couple 90 bends to the end wall. I had to mount the motor on a board which was sprung to another board with 4 3/4" D x 1.5" long springs set into 1/4" deep holes in the wood. This isolated fan vibration from the house structure, drastically reducing noise.
The only noise in the kitchen was the air rushing out the grate. The suction would easily hold a plate to the grate if you placed one there.
It was probably not something you'd want to run a lot during the heating season, but sure cleared out smoke in a hurry.
I suspect that the choice of blade or cage fans in most units is a mechanical/price choice.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Also diffrent blade designs produce diffrent CFM vs back pressure curves.
SC blowers (centrifugal blowers) work well against back pressure. Blades not so well.
Mark
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Typical bladed fans don't work well when any duct is involved. A computer case fan works well enough when fastened to a panel on a well ventilated case (same with the outdoor A/C unit), but the same fan isn't going to produce much airflow with a duct attached, a fan can spin without moving much air in the desired direction if restrictions are involved. Many fans will still run at about the same speed even if the air stream is choked off.
A different fan that produces much better airflow is generally referred to as an axial fan or blower (what you referred to as inline, I think), and the commercial/industrial versions can be used with limited lengths of duct (of specific size).
Squirrel cage/centrifugal blowers produce positive air displacement through ducts. They can pressurize ducts or large spaces, and deliver great volumes of displacement. They're often filtered on the intake side and still deliver great airflow, even with filters that are partial restrictions to airflow.
The dual shaft blowers are a more compact version of a larger blower, and the smaller cages operate better with a small motor. For many dual shaft types, the motor is cooled by ambient air, since the motor isn't in the air stream. Formed metal blades in cage construction are likely to supply much better airflow than molded plastic cages.
For many direct drive types, the motor is cooled by the airflow.
Centrifugal/SC blowers are vastly superior in airflow performance.
Cooking appliances introduce other issues, such as cooking steam/vapors, oil spatter and potential fires from flareups.
--
WB
.........
metalworking projects
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Thu, 03 Dec 2009 11:23:58 -0500, Existential Angst wrote:

With a squirrel cage, you can keep the motor out of the air flow with just a straight shaft. To keep the motor out of the airflow with a blade-type, you'd need a belt, which implies more parts and more failure modes.
Hope This Helps! Rich
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Existential Angst wrote:

There is a difference in squirrel cage design for kitchen ventilation. If I remember correctly, the blades are straight instead of curved to help prevent grease buildup. It's what I've seen in commercial kitchen exhaust fans but the blades in an inexpensive home unit may be curved. I do seem to remember taking apart a cheap home range hood and it had curved blades. I don't know about the ones them rich folks buy.
TDD
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Everyone is correct-- fans are more efficient from a power usage standpoint, but only if there is little/no resistance to airflow. Then squirrel cage type blowers are used. I have a blower that I rigged up for a kitchen vent-- the blower is under the house and piped to a vent in the wall. There really wasn't any way to put a hood over the stove, but what I have works fairly well. Originally there was a vent in the same spot with a tiny fan in it that went straight outside. It moved almost no air, and dripped grease down the outside wall. TDD is right about commercial units having straight vanes, likely to collect less grease. On a resi unit, it would probably be less of a problem. The average household is not going to be dumping frozen French fries into hot oil every few minutes 24/7, as restaurants do. FWIW, there were some furnaces that used dual squirrel cages on a double shaft motor on a few certain models. I have replaced motors on a Janitrol, Payne, Fedders, and Ruud that used that setup. They were fairly old units then, and it was at least ten years ago when I did the last one. Larry
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Lp1331 1p1331 wrote:

The last dual squirrel cage blower I dealt with was on a 15 ton unit. I think it had 1" shaft going through the darn thing. I despise air handlers for homes that have bolt in instead of the slides out blower housings. GRRRRRR! I'm not as agile as I used to be, especially in a cramped attic.
TDD
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Thu, 3 Dec 2009 11:23:58 -0500, "Existential Angst"

<snip>
Go study commercial kitchen hoods, and duplicate the design features in a small size - or call a local manufacturer and have one made, restaurant hoods are commonly made to size and aren't that expensive... And if it's a new house you probably have to go this way to satisfy the building inspector and get your Certificate Of Occupancy... Go meet with the City or County authorities and find out what your local inspector will buy off on BEFORE you start.
They use a standard design to keep the prices down, and just dial in the sizes needed and bang them out of sheet stainless, using standardized pieces. They have standard size grease catch baffle panels that lift out of a slot for easy cleaning - for a home kitchen sized hood you might only need one or two panels. There is a drain gutter for the grease that collects in a pre-made stainless1/8 size steam pan.
The fan is on the roof - commercial style "Upblast ventilator" made with a grease collection cup to catch the gack that collects in the fan and keep it off your roof. There are also wall mount versions. They make small sizes in the 1/4 - 1/3 HP range which is huge compared to a 'residential' unit, so run a speed control on it. Easy to fix (full access) or just replace with new.
You build a hard curb on the roof with lumber, the duct comes up to the fan with a flat flange and the roofer runs the roofing up the sides of the curb. The fan has a flat flange on the bottom with a rain lip, and the whole thing lifts right off for service or replacement.
Residential calls for simple 6" or 7" round galvanized snap duct to a simple vent jack, but overkill is allowed - commercial use ducting is double wall welded mild steel, with an air gap - in case of a stove or hood grease fire, this keeps the heat contained so you don't light the building rafters. You might end up with a hybrid with spiral-lock heavier round steel duct, but not the full double-wall - residential stove fires won't have the heat load of a commercial double-vat 60-pound deep fat fryer.
Oh, and they do make small automatic fire extinguisher systems specifically for residential stove hoods. Talk to your insurance company, you might be able to get a serious discount.
--<< Bruce >>--
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

    HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.