formula to calculate static pressure in ducts

[balaji]

hi friends,
can anyone tell me whats the key role of static pressure in ducts how its calculated using formula what will happen if it increase or decrease how to suggest this duct size should have this much static pressure

[B G Scott]

Duct work pressure is combination of static and velocity pressure.
Velocity pressure Pv is produced by speed of air movement
Static pressure s.w.g which maintains flow against restistance.
Total pressure is the sum of the veloicty pressure and the static pressure.

Example
If for a given duct size the if air flow is reduced then velocity pressure will also fall as the air speed will decrease, assuming that all other factors/ parameters dont change.

To measure static presure you need Pitot-static tube connected to a suitable guage which will give you a reading of static pressure and total pressure and you will see from above that if you subtract the static pressure from the total pressure you will then know the velocity pressure.

Dont know if this helps.

Try and think of it like this. If you have a hose pipe connected to a water supply with the end sealed off then as no water is flowing there can be no velocity pressure an no flow is taking place, but if you drilled a hole in the side of the tube water would come out (static pressure) then as flow starts velocity pressure increases with speed of flow, lots of other factors influence the flow of air, system resistance, condition of components in the system etc.

[balaji]

To measure static presure you need Pitot-static tube connected to a suitable guage which will give you a reading of static pressure and total pressure and you will see from above that if you subtract the static pressure from the total pressure you will then know the velocity pressure.
this is the information i got from your reply thanks for it but what i need is how you will say there should be 200pa,100pa static pressure for the ducts how to calculate this pls i need in depth

[balaji]

to calculate dynamic pressure there is a formula iam asking for static pressure

[nevgee]

Your static pressure will be a function of the compressor / Fan that produces it.

You can measure your static using a pitot tube and

[Peter_1]

There's again not such a formula tot calculate static pressure for ducts.
You must look to the whole circuit and every reduction, bend, turn, airturn will increase your resistance and increase pressure. even increasing speed will increase static pressure.
Your ducts must withstand the static pressure which is inside the ducts.
http://www.mece.ualberta.ca/courses/mec463/2001w/463_sol08B.pdf#search

[balaji]

thanks for the pdf file peter it is difficult for me to understand due to units conversions anyway ill hope it will be useful for me

[balaji]

can you pls provide me the tables peter

[wambat]

hi friends,
can anyone tell me whats the key role of static pressure in ducts how its calculated using formula what will happen if it increase or decrease how to suggest this duct size should have this much static pressure


Static pressure is the measure of the potential energy of a unit of air in the particular cross section of a duct. Air pressure on the duct wall is considered static. Imagine a fan blowing into a completely closed duct; it will create only static pressure because there is no air flow through the duct. A balloon blown up with air is a similar case in which there is only static pressure. If you increase the static pressure in a duct such as with a reducer, the staic pressure will decrease and the velocity pressure will increase.

Total Pressure

Consists of the pressure the air exerts in the direction of flow (Velocity Pressure) plus the pressure air exerts perpendicular to the plenum or container through which the air moves. (static pressure) In other words:

PT = PV + PS

PT = Total Pressure

PV = Velocity Pressure

PS = Static Pressure