How to calculate flow velocity?

[UKDoug]

How do you calculate flow velocity in a port?

Is it simply CFM / CSA = feet per min velocity?

[coulterracn]

Taken from the Performance Trends website FAQ section

What is a velocity probe
A velocity probe is a Pitot tube device used to measure local gas flow velocities inside a bounded path. A velocity probe creates a differential pressure in relation to the local velocity that can be measured with a manometer or the Flow Performance FP1. By probing a port with a velocity probe, areas of high velocity can be located, and are sometimes good areas to explore for improved flow. Pitot tubes are used on air planes and race cars to measure air speed

What is velocity
Velocity is the speed of an object. For flow bench work, air flow velocity is measured in units of Feet per Second or Feet per Minute. Typical air speeds for a typical automotive cylinder head port with a test pressure of 28"wc is around 200 to 400 Feet per Second. Feet per Minute = 60 * Feet per Second.

This gives some insight but doesn't tell how you get that from the Pitot probe reading which is calculated from inches of water on a vertical or u-tube manometer.

There should be a formula on the Forum that calculates Feet per Seconds from inches of water measurement.

Hey guys help us here.

Ray

Found this posted in Formula section

Here is one equation for velocity in feet per minute from a typical Pitot tube:

V = 1096 * SQR[Pv/D]

V = Velocity in Feet per Minute (fpm)

Pv = Pressure Velocity, Differential pressure of Pitot tube impact port(s) and static pressure port(s).

D = Air Density. Usually assumed to be .0750 lbs/cubic foot for SCFM.

SQR = Square Root.

To get the volume of flow (cfm) from the velocity, multiply velocity times the cross sectional area in, square feet, of the bounded path where the Pitot tube is located.

[86rocco1]

[jfholm]

Ed is right and here is the thread for Larry's calculator -


Now you have two

John

[49-1183904562]

Remember, As stated in the formula detail.

This measurement is done with a Pitot-Static tube, measuring the localized impact pleasure difference so it is a closed system as in the incline manometer on an orifice bench.

Thus the pressure used in the formula is the differential at the measurement point and not the difference between the measurement point and atmosphere.

Rick