Ducting - if have multiple ducts, what will be the cfm in each duct

[fe16260]

Problem Statement:
I have an whole-house dehumidifier (Aprilaire 1770A) that can be used to duct (8" diameter inlet/outlet) to specific locations in a dedicated area. In other words, i am using this dehumidifier for my crawlspace (1,850 sqft) and i need to duct (both for the supply and return) so that i can efficiently dehumidify the entire crawlspace.

Knowns:
- Dehumidifier has 500 cfm at a static pressure of 0.9" w.c. per 100 ft. (total for supply and return). Unit has a maximum static pressure of 1.5" w.c.

- 8" duct inlet and 8" duct outlet built into the unit

Questions:
Assuming that no plenum is used.

Say i come out of the dehumidifier (supply) with an 8" duct and then branch off with (6) individual 8" ducts (with the total of all of them being 50 ft. or less), will each of the (6) ducts have, theoretically, the same cfm as the outlet (main trunk) duct of 500 cfm or will i need to have a plenum coming directly out of the dehumidifier that feeds the (6) individual ducts?

Thanks in advance for your inputs,

Andy

 

[MintJulep]

No.

 

[Waramanga]

errr.... i think your question needs to be re-worded, it doesnt seem to make any sense as it stands. if you are trying to pull air evenly from under the floor then you need to keep duct sizes and lengths as close as possible to one another, but you wlll want to keep a bit of friction in the ducts so 8" will be too big.

 

[fe16260]

Thanks everyone...

Waramanga...
The Aprilaire 1770A comes with an 8" inlet and 8" outlet for 8" ducts. The issue i am having is one area of my crawlspace is not dehumidifying b/c there are obstructions in the way (pillars, tight opening, etc) so i was thinking about ducting it to specific areas.

If i use a plenum for the supply/return, would it not equalize the cfm in each of the ducts (meaning each duct assuming they are all the same diameter will have the same cfm as the supply cfm?

 

[willard3]

Hire someone who does air systems, this is not so simple as you think.

 

[emalsyd]

You would need to install a volume damper in each line and balance the branch lines to get the same cfm though each duct.

 

[boo1]

Andy the simple way to balance the six lines flow rates is to use the same length flex lines.

This is becouse head loss (pressure drop) is a function of Velosity Squared/Diameter. For the 8 inch pipe it is:
11.4 x 10**-9 * V**2/D per foot of pipe

 

[fe16260]

Boo1 and emalsyd,

THANK you so very much! This is what this site is all about - to help other search for solutions so i thank you for giving me a direction on how to balance the system.

Thanks again
Andy

 

[fe16260]

Boo1

WOuld i need a plenum prior to branching off 6 lines? Also, would i need a plenum with returning the 6 lines back to the unit?