DIY : Poor Mans/woman's Ductulator !

[meds4me]

Okay, first some basic concepts. Air is like water, we can feel it , it has wieght / volume. Restricting water thru your garden hose causes it to go under high pressure and low volume ( Pinching the end restricts volume; restriction causes high pressure...okay ?). Remove the restriction ( larger pipe / hose) and you can move more water easily as well as more volume.


Ventilation 101


Static pressures; this is a reference to the "restrictions" of the duct design and capabilities of the duct work to "carry" the load.

Most duct work is designed around a concept that its removing "moisture" and other "odors". Since most of the systems at work with in this forum are 4" and 6" dia., I'll be using those as examples.


Wye's -14 to18 cfm Supply or return "box" -26 cfm

45* -5 cfm supply boot (termination ) straight -3 cfm

90* -10 cfm supply or return 90* boot ; -17 cfm

offset -20 cfm ******** ALL NUMBERS ARE "GIVEN" averages, youre actual #'s may vary....




4" is capable of carrying 60 cfm @ .04 static per 100' : Now factor in fitings and length of run .....

Example: Dryer ducting. code for venting a stnd efficentcy dryer is ; 2- elbows and 10' of ducting total. This equates to a loss of 20 cfm for the 90*s elbows. actual capacity is : 40 cfm


If you try to enlarge the duct work for more cfm, you will lose the needed velocity for doing the "task". The reverse is also true: making the duct work smaller will increase the velocity only the total volume of air to be moved will be decreased as well.

 

[DirtySouth]

No sence

 

[meds4me]

If you are wondering how to "design" a system to move air for cooling of light fixtures or the venting your rooms for proper air exchange to keep them "girls" happy.

 

[DirtySouth]

Edit again,still no sence.R u tryin to say every elbo(bend)in ducting takes away from cfm of fan?

 

[meds4me]

correct ! every bend counts against the total volume to be moved. Its called "static pressure". Read the water hose reference again...

 

[BBFan]

Good info meds4me, but your descriptions are a bit technical.

If I need to supply frsh air to my grow room and I use the following:
50' of 4" pipe
1- 90 deg elbows

Do I end up with 50 cfm's? If I have a fan that operates at 75 cfm's, am I wasting energy with no increased flow or is there some marginal improvement?

Thanks again.

 

[koryshea]

Does gravity come in to play? I feed to an elbow and go straight up 2 ft to a T continuing up 1 ft to another elbow.

 

[meds4me]

Good info meds4me, but your descriptions are a bit technical.

If I need to supply frsh air to my grow room and I use the following:
50' of 4" pipe
1- 90 deg elbows

Do I end up with 50 cfm's? If I have a fan that operates at 75 cfm's, am I wasting energy with no increased flow or is there some marginal improvement?

Thanks again.

No, you end up with 40 plus cfm ( You would need to use a :Velometer to record actual airflow). Using the 50 cfm fan. Now up-grading to the 75 cfm fan you will pull/ push air at a higher cfm rating also a higher static pressure (resistance).The bottom line is ude the 75 cfm which should yield 65 cfm which makes the work being accomplished easier on the motor.
Think of it this way : a under -sized motor in a truck "luggs" itself up the hill wasting fuel and time.
The truck with a big block motor (lots of horse power) runs up the hill under the same load conditions. Motor last longer and uses less "fuel" to accomplish the task.

 

[meds4me]

Does gravity come in to play? I feed to an elbow and go straight up 2 ft to a T continuing up 1 ft to another elbow.

No, gravity has no effect other than whats already at work ( keeping us walking around instaed of floating...okay well some of us.....lol).
The issue here is the "T" fitting and multiple uses of elbows. This creates more restriction and thus loss of volume and velocity ( speed and amount of air to be moved). The best rule of thumb to offer here is to NOT combine systems. Keep fresh or make-up air separte from exhaust air ( code says 15' min. to not recirculate same exhausted air).


I'm always open for PM's to talk more about your situation if you dont feel comfortable here...It's just so many have the same issues.
*side note: The use of "flex" duct work is allowed. Just NOT next to a combustable product- heat from an exhaust/ vent pipe, etc. This product has more restriction but is easier to use in those "hard to reach" spots....

 

[DonJones]

"If you try to enlarge the duct work for more cfm, you will lose the needed velocity for doing the "task". "

You do NOT need high velocity to ventilate an area if you have large openings. If your target is say 50 cfm, the easiest way to accomplish it is to use a larger duct rather than trying to increase the velocity.

An example is trying to provide 10 CFM to an air tool. If you try to use a 1/4" air hose, then you have to jack the pressure way up, but if you increase the diameter of the hose to 1/2', then the required pressure drops way way down as does the required velocity of the air flow through the hose, yet the velocity of air out of the tool will stay the same either way because the velocity will jump way up when it hits the smaller diameter opening in the air tool.

Don Jones

 

[zipflip]

^^^ that seems to make more sense to me tahn all the said above

 

[meds4me]

"If you try to enlarge the duct work for more cfm, you will lose the needed velocity for doing the "task". "

You do NOT need high velocity to ventilate an area if you have large openings. If your target is say 50 cfm, the easiest way to accomplish it is to use a larger duct rather than trying to increase the velocity.

[An example is trying to provide 10 CFM to an air tool. If you try to use a 1/4" air hose, then you have to jack the pressure way up, but if you increase the diameter of the hose to 1/2', then the required pressure drops way way down as does the required velocity of the air flow through the hose, yet the velocity of air out of the tool will stay the same either way because the velocity will jump way up when it hits the smaller diameter opening in the air tool. ]


****** same diff as the anaology of water to air. same concept and yes you dont want to loose the velocity for the task at hand. Most "over-design" the ducting and thus the reason for said anaolgy <sp?>.

Thanks DonJones for that....some understand better under diff "formats".




Don Jones

READ ABOVE for answer

 

[4u2sm0ke]

Edit again,still no sence.R u tryin to say every elbo(bend)in ducting takes away from cfm of fan?

Yes he is..Call any HVAC outfit and ask any Questions about air control and movement...meds4me..sounds as though would be on other end of line..anytime you add an elbo or even a wye/tee you are loseing cfm..and as he also stated a booster fan is required if traveling Longer then 20 feet..and that a staight pipe..


great info meds4me..sorry i missed the start. :aok:

 

[meds4me]

Youre welcome 4U...yes 16 yrs of field work with 2 yrs in school for load calc's ect.....Now you can call me : Chief Oneleg...lol