Alaska 4x4 Network

Go Back   Alaska 4x4 Network > Alaska 4x4 Network Forums > 4x4 Vehicle (car, truck, etc)

Reply
 
Thread Tools Display Modes
Old 01-26-2003, 11:23 AM   #1
Guest
 
Posts: n/a
Default

I want to mount my radiator behind the cab of the Freak, and have considered sinking it into the bed about half to 2/3 the way up the radiator. (Only half to one third of the radiator would be above the flat bed) I am planning on installing a pusher and puller electric fans. (redundancy incase of failure mostly)

My question is this: Does the narrowest point of any ducting need to have the same or larger crossectional area as the fan? Will I cavitate the fan/fans if the opening of the duct has a smaller area than the fan/fans?

My best guess would be yes.

Got any hints? Suggestions? Should I fab up and exhaust duct as well?
  Reply With Quote
Old 01-26-2003, 11:31 AM   #2
BadCo
Molon Labe
 
BadCo's Avatar
 
Join Date: Nov 2004
Location: Eagle River, Alaska
Posts: 7,492
BadCo has a spectacular aura aboutBadCo has a spectacular aura about
Default

Well seeing as Little Giant is a mechanical engineer working in the field of Heating and ventalation, he should be able to help you.
__________________
Rogue Off Road President

KODIAK OFF ROAD

The chair is against the wall
BadCo is offline   Reply With Quote
Old 01-26-2003, 01:17 PM   #3
imported_Little Giant
Registered Member
 
Join Date: Dec 1969
Location: Anchorage, AK
Posts: 469
imported_Little Giant Default Reputation
Default

Quote:
Originally Posted by [b
Quote[/b] (bgreen @ Jan. 26 2003,12:23)]My question is this: Does the narrowest point of any ducting need to have the same or larger crossectional area as the fan?
Not necessarily. *Its okay for the air velocity in the ductwork to by higher than that at the fan, especially with a propeller fan. *But of course as the cross section decreases the friction will increase (resistance to flow). *Without knowing more about the flows you want, fan specs, etc. it hard to say how much smaller you can go.

Quote:
Originally Posted by [b
Quote[/b] ]Will I cavitate the fan/fans if the opening of the duct has a smaller area than the fan/fans?
No, because cavitation is a term that applies to liquid fluid flow, not gases. *Cavitation is a concern for pumps, not fans. *I can go into more detail on that at a later date if you are curious.

The concern for an electric fan in which the motor is located in the air flow, is the motor. *Typically its designed for the motor to dissipate heat into the air stream itself. *With inadequate flow dues to system design, it could experience the problems any electric motor that runs faster than designed and overly hot would encounter. *The fan itself would be okay, the concern is the motor (assuming your using an electric motor since it'll be remote mounted).

Quote:
Originally Posted by [b
Quote[/b] ]Got any hints? Suggestions?
Well, I can give a quick run down of duct design rules of thumb.

If using rectangular duct, keep your aspect ratio (width vs. height, or vice versa) below 2:1.
Using radiused turns rather than miters if possible. *We typically use a radius equal to the width of the duct located on the centerline. *(I'll draw you a picture if that last sentence doesn't make sense).
Try to keep all your transistions, fittings, and bends fairly smooth (15-30* max for divergence and convergence is typical).

In automotive applications it appears electric fans move alot less air than mechanical fans. *With your motor you are going to want a fairly large fan. *You may actually need both fans in there when the Freak is running hard, not just for redundancy. *We'll just have to see how it works out.

I'm guessing the whole system will be relatively short, compared to what I do. *Your biggest losses will come from your turns and transitions, as opposed to different cross sectional areas, if that makes sense. *I think more so on the positive pressure side (the air being 'pushed').

Quote:
Originally Posted by [b
Quote[/b] ]Should I fab up and exhaust duct as well?
At least to the 'exhaust fan', or your puller fan. *Its not as important after that. *But, make sure the air path for the exhaust is there, weather its ducted or not.

Hope that helps. *If you work up a sketch of what it will actually look like I'd be glad to take a look and comment, maybe even show it to a couple of people in the office over some beers.
imported_Little Giant is offline   Reply With Quote
Old 01-26-2003, 05:34 PM   #4
Guest
 
Posts: n/a
Default

So what term is used when the fan cannot draw enough air?

Example: A friend of mine has 500 sq foot shop that he uses to paint cars. He has an aprox 24" diameter fan mounted at floor level on the rear wall. Its driven by at least a 1.5hp motor. I have no idea what rpm, pully ratios etc.

On the other end of his shop, he has two aprox 18" square vents with filters over them.

If you only open one vent the fan makes a hell of a noise (really noticable *fluttering*) when you open both vents, the noise goes away and the fan seems to run smooth.

I would have called it cavitation, but knowing something about the actual definition of cavitation, I agree with you that its not actually *cavitation*.

Quote:
Originally Posted by [b
Quote[/b] ]cav.i.ta.tion \ kav' i ta' shun \ n [1. the rapid formation and collapse of vapor pockets in a flowing liquid in regions of very low pressure. 2. such a pocket formed.] (Webster)
  Reply With Quote
Old 01-26-2003, 08:31 PM   #5
imported_UtChaz
Registered Member
 
Join Date: Dec 1969
Location: Elmendorf
Posts: 1,525
imported_UtChaz Default Reputation
Post

Without the knowledge of all the quirks and queries I'm going to be kinda dumb and ask a silly question... *Why would you need any ducts?

420 HP V-8 Power Toyota, Made for Mud
I don't see any ducting for this setup. *
Chaz
imported_UtChaz is offline   Reply With Quote
Old 01-26-2003, 08:46 PM   #6
imported_Blackjack
Registered Member
 
Join Date: Dec 1969
Location: Soldotna AK
Posts: 357
imported_Blackjack Default Reputation
Default

By mounting the radiator and fans in a duct you can keep them out of muds way.
imported_Blackjack is offline   Reply With Quote
Old 01-26-2003, 10:28 PM   #7
Guest
 
Posts: n/a
Default

Some reasons are as follows:

I think that would have been damaged in the first few hours of nearly any run I went on last year. There is nothing to protect the fans or the radiator.

Ducting will allow me to mount the fan in a less than choice location, and maintain good air flow. I want it as close to the cab as possible, and as low as possible for storage and COG reasons. Ducting will allow me to do that.

Also, what blackjack said. Mud.

Im thinking about having a duct that draws air from right under the rear window, centered from right to left.
  Reply With Quote
Old 01-26-2003, 11:22 PM   #8
imported_Little Giant
Registered Member
 
Join Date: Dec 1969
Location: Anchorage, AK
Posts: 469
imported_Little Giant Default Reputation
Lightbulb

Quote:
Originally Posted by [b
Quote[/b] (bgreen @ Jan. 26 2003,18:34)]So what term is used when the fan cannot draw enough air?

Example: A friend of mine has 500 sq foot shop that he uses to paint cars. He has an aprox 24" diameter fan mounted at floor level on the rear wall. Its driven by at least a 1.5hp motor. I have no idea what rpm, pully ratios etc.

On the other end of his shop, he has two aprox 18" square vents with filters over them.

If you only open one vent the fan makes a hell of a noise (really noticable *fluttering*) when you open both vents, the noise goes away and the fan seems to run smooth.
Surge maybe? But I don't think that's what you are describing.

I can picture the situation with your example, but the proper term isn't coming to me tonight. I'll flip through some text tomorrow at work and see if I can come up with the right word.
imported_Little Giant is offline   Reply With Quote
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -9. The time now is 02:20 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Copyright Alaska 4x4 Network