CFM loss through ducting


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Old 02-28-11, 12:54 PM
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CFM loss through ducting

I'm trying to design an enclosure around a generator and need to allow for cooling.

I plan on channeling the outside air into the flywheel van which pulls 777 CFM and adding a few smaller fans totaling around 600 CFM.

To reduce noise...I'm after 3 90 deg bends but total duct length for each duct will be about 2 feet. How large must the duct be to keep inflow at these rates?

The flywheel van is 8" and the other fans will be likely in a 4x20 or 5x20.
 
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Old 02-28-11, 01:09 PM
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What you are asking is not clear.

What is a "flywheel van"?
How exactly will you use "a few smaller fans"?
What is a "4x20 or 5x20"?

I suggest you sketch out what you are planning then post them on Photo Bucket or some other free host and provide some details on what generator you have.

Just so you know, most equipment mounted fans have no ability to pull or push air through any restriction whatsoever.
You would likely have to add an equivalent cfm fan to what is in the generator that can operate at higher static pressures than the original.
 
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Old 02-28-11, 01:24 PM
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Sorry....dumb thumbs on my cellphone.

Flywheel fan....on the front of the motor.

4x20 is the approx size of the ducking I will be trying to channel air with a series of small fans which are rated at 130 CFM each.
I will get a sketch together later tonight.
 
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Old 02-28-11, 06:34 PM
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The engine is a Honda GX620, fan diameter is roughly 9". Here's a rough sketch of a side-view of the right side, not really to scale but gives an idea. The narrow shaded areas will be either 1/8" MLV or a thin layer of sound pad and has a somewhat smooth surface, almost like carpet padding.



On the same side of the generator, I want to add some additional cooling just to keep the air moving over the unit. The flywheel provides a good blast of air but I want to add an additional 600 CFM or so from the side.

Here's a graph of the 5" fan I was looking at, rated at 130 CFM in free air.




The side duct will be roughly the same design but more longitudinal. I want the inlet to enter near the bottom of the enclosure and exit on the opposite side near the top. That exit will be for both the flywheel fan and the extra fans. Here's a view of the right side, the duct will enter just above the battery. I plan to surround the unit in Hardiebacker board and and the tubular frame will be set with 2x4's clamped to the tubes and will give a couple inches of room on all sides, wheels will be removed also.




Perhaps I could have the exit duct on the opposite side of the engine and have the airflow continue in one direction. I could also mount fans inside the unit drawing air towards the opposite end and closing the sides....not sure though. The muffler will not be in the enclosure, nor will the fuel tank. The exhaust will be wrapped in header tape and will exit through the left side then the muffler will be attached to keep heat down inside the enclosure.

I understand airflow is affected greatly with a design like this so I want to get this addressed before I start on the enclosure.
 

Last edited by i6pwr; 02-28-11 at 06:50 PM. Reason: Additional content
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Old 02-28-11, 10:24 PM
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Just to interject a different thought, is the noise you are trying to reduce the exhaust noise? If so, wouldn't an improved muffler configuration be the place to start? If the exhaust noise were eliminated, then a much more open approach for cooling might be acceptable.

I hope I'm reading the thread correctly

Bud
 
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Old 03-01-11, 04:03 AM
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You are correct, actually I have an exhaust can from my previous Honda CBR which muffles the note very well, the noise from the intake fan is also loud...think of a lawn tractor wide open.

The unit in itself is quieter than most generators for what it is....i've heard some kohlers and briggs motors that will run off a heard of elephants. I may not even need the ducting to take that upwards bend over the center baffle...I could remove that and have a larger cover just blocking the fan to reduce perceived noise in a straight line, line it with sound deadening and should not interfere with airflow.

My main goal is just to tone it down so my neighbors don't have to worry about it at 3AM, my house blocks most of the noise but still needs more, 80db at 7M is still alot.

Would I not need the 3 90 deg bends for noise absorbtion and just one large panel covering the ends to eliminate line-of-sight noise, still allowing adequate airflow, set about 12" from the ends of the unit? So a baffle at the unit with a large opening, then another panel about 12" away, so the air will be mainly directed at the ground...intake and exhaust (not hot exhaust)?
 
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Old 03-01-11, 05:04 AM
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I will have my camp up soon and be considering noise issues as you are. My first objective will be to eliminate as much electric as possible and use a battery back-up for much of the little things. But a generator is hard to avoid. I'm on a lake with one camp, so far, beside me and having noise at night would echo everywhere.

Enclosures walls can be treated to absorb most of the noise. If any energy bounces off of them, it can be directed to another sound absorbing wall. If directed down to solid earth, it may just reflect back up, so add material or varied structure to break up the sound.

Search on "anechoic chamber" to see how they absorb and break up noise.

Bud
 
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Old 03-01-11, 07:58 AM
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I have some roxul safe n sound I will be lining the panels with. That or closed cell foam mat is the ticket.....keeping everything cool is the other part.

I'll keep you updated once I finish....for you I would recommend one of the 2-3kw Honda inverted generators.....pricey but very quiet.
 
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Old 03-01-11, 07:34 PM
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Finally uploaded my pics..here's the unit...wheels will be removed and fuel tank will be mounted above enclosure. I basically need to remove line-of-sight sound transmission, three right-angle bends will reduce noise by more than half....I just need to maintain airflow.

Again, perhaps a panel about 8" from the fan and another 8" beyond that, similiar to sketch but with a much larger gap...will that restrict the airflow being that far from the fan?

 
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Old 03-02-11, 06:34 PM
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I have 2 6" booster fans laying around new in box that I never got around to installing....been hearing using fans on the exhaust side rather than having it push through the bends and out the other side is much more beneficial.

I assume having 2 or 3 90 deg bends and having the air pulled through rather than pushed, results in more airflow if pulling correct?
 
 

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