Static pressure in HVAC?

Question

how does the static pressure effect the duct design or equipment selection? how does it effect the cfm ? is it a good thing or bad thing?

thanks

Answer 1

knowing the static pressure you can have a better design

normally the static pressure between inside and outside the building is 0.03 to 0.13 inch water

1st select the CFM fan size based on air change per hour (ACH) and cuft of the room

CFM = (CUFT)(ACH) / 60

ACH = 8 for WR

ACH = 15 for KIT

ACH = 0.35 for whole house

etc

next select fan HP

HP = (CFM)(inch water) / (6355.8 * eff)

use a manometer to measure the intake and exhaust of duct system. the diff pressure is your static pressure in inch water,

if you exhaust the air to outside the house, the static pressure outdoor is cosidered as zero.

Answer 2

Hs=static pressure, the fluid is at rest

Hv=velocity pressure

Ht=total pressure = Hs+Hv

Manometer or pitot tube gives readings for Ht and Hs, so we have Hv and can find the fluid velocity V=sqrt(2gHv)

so at any given CFM, we will know the duct size or vice versa.  CFM=Area*V

of course there are other concerns like pressure loss when picking the fittings and equipments.