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Old 06-08-2016, 14:25   #1
Jd1
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Looking For Quality High Volume 3" Duct Fan

It is time that I tackle the cooling (or lack thereof) of my 200A alternator. Before I hack a vent hole into the engine cowling, I would like to attempt to increase airflow through the existing ducts. I have two 3" ducts to the engine compartment. One for air intake and one for air exhaust that has an inline extraction fan in it.
I would like to add a fan into the intake duct and possibly replace the exhaust fan with a higher volume unit.
All the fans I have been able to find are in the 140 cfm range which really doesn't do much for me. Has anyone seen or used a 3" duct fan, 12V that is over 200 cfm or maybe even more ?
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Old 06-08-2016, 16:05   #2
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Re: Looking For Quality High Volume 3" Duct Fan

I have a Rule 240 cfm Yellow top 4" fan in a 3" duct and it works ok . These are suction fans, so not sure how it will blow.
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Old 06-08-2016, 22:31   #3
Jd1
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Re: Looking For Quality High Volume 3" Duct Fan

But if I go from 4" to 3", what will I drop down in cfm? I might just end up at the same place as if I had bought a 3" fan to begin with .... ??????????
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Old 07-08-2016, 18:26   #4
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Re: Looking For Quality High Volume 3" Duct Fan

This one: 12 Volt High Output Motor Blower, EV West - Electric Vehicle Parts, Components, EVSE Charging Stations, Electric Car Conversion Kits
$220 - 800 cfm
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Old 07-08-2016, 19:02   #5
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Re: Looking For Quality High Volume 3" Duct Fan

Thanks for that link! It isn't the 3" inline/duct fan I was looking for but at that air volume it might be worth modifying my requirements and/or the blower itself.
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Old 07-08-2016, 19:29   #6
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Re: Looking For Quality High Volume 3" Duct Fan

Centifigal fans require very low pressure drops across the entire inlet to outlet flowpath.

Before chasing a higger flow fan you should baseline your current setup both in terms of inlet / outlet flow and temperature differential.

You don't mention whether you are having an overheat problem or are concerned.

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Old 07-08-2016, 19:34   #7
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Re: Looking For Quality High Volume 3" Duct Fan

caution .... if you push more air in than you are blowing out you could end up forcing engine compartment fumes/gasses/heat into your cabin unless your cowling/hatch and bilge area is vapour tight.
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Old 07-08-2016, 20:11   #8
Jd1
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Re: Looking For Quality High Volume 3" Duct Fan

As I mentioned in the original post, I have an issue with my alternator overheating .... or more to the point, I need to reduce the alternator output in order for it to not overheat. The plan is to a) reduce operating temperature in the engine bay and b) to blow cool outside air onto the alternator to improve it's heat shedding capability.
Not running positive pressure in the engine bay is not a concern even if it did happen. It's a clean diesel engine that does not smell or cause other issues.
Currently both the cool air coming in on one duct and the hot air being vented end up in immediate proximity which is stupid to begin with. I will be extending the fresh air input duct to allow it to blow onto the alternator and I can use the 800 cfm fan to suck the hot air out of the other end of the engine bay and to the exhaust. I would however like to install a decent inline fan to help the fresh air enter the engine bay.
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