Calculating External Static Pressure for DUCT 1

[moeg]

Hey,

a very quick question.

When you have a straight duct going out from an Exhaust Fan. Do you consider pressure loss at the branches that are feeding to the main line. ( index run)

if there is a grill at the duct run, do you consider any pressure loss for it?

please,

 

[willard3]

Yes.

 

[dbill74]

Whenever there is a branch take-off or grille, you are adding/reducing the amount of air flowing through the main line. This will change the pressure drop per linear foot of duct. Also anytime there is a change in direction there is an associated pressure drop. the amount of pressure drop will vary based on fitting sizes and air flows. I believe it is SMACNA's manual that has charts/tables and diagrams to help you determine the appropriate drop for your situation.

 

[317069]

I am trying to imaging your system, you have an exhaust fan with exhaust duct, there are branches feeding to exhaust duct after the fan outlet, is it right?

 

[moeg]

I have an exhaust fan, sucking from a straight line through the corridor.
in the line, there are branches coming from the rooms.
1 corridor, 3 rooms, i am collecting air from each room, in the same duct.

do i consider pressure drop at the branches? or consider it as a straight duct till the final exhaust grill?

 

[317069]

each fitting in the index run is considered as a resistance, you should take it in account.
for you application, you consider suction and discharge parts of the index run, it is the external pressure that the fan has to face.

 

[nabeel3]

No need to consider all the branches feeding the main. Consider only the index run . All the fitting losses in the index run should be considered in calculating the S.P

 

[paco131]

For your external static pressure loss use the furthest and most restrictive run of duct from the fan inlet to the last air device. Just to make sure, you do not add all of the fitting losses from every branch, add the external static for the furthest or most restrictive run of duct. Take into consideration dbill74 statement above about airflows in the main duct.