pitot tube calculations 1

[rwelk]

I am trying to spec some pitot tubes for an application where air is moving through a duct. My data values are as follows:

velocity of air = 14.1 m/s
static pressure (SP) = 4.5 in
water is the medium in the tube

I'm trying to find the differential pressure (DP), and i am not a mechanical engineer, but i did the following:

((density of water)*g*H) + SP = dynamic pressure

dynamic pressure = 1/2*(density of air)*(vel. of air)^2

I have never done a fluids calculation in anything but metric (i am from canada), so i had to convert my pressures....so here's what i got.

((1000kg/m^3)*9.81*h) = 0.5*(1.2kg/m^3)*(14.1)^2 + 1120Pa

h = 0.126m = 5"

I was expecting an answer less then 4".

Can anyone tell me where i might have made an error with this. I feel like I overlooked something simple and it is effecting my calculations.

 

[friartuck]

System static pressure + Dynamic ( or velocity pressure)=Total Pressure


Pitot tubes can measure static, total or velocity pressure depending on how the hoses are connected.

The nose of the tube which faces the airstream measures the total component. The side holes around the nose measure the static.

When both tubes are connected together, the total pressure opposes the static pressure to give the difference which is the velocity pressure.

VEL PRESSURE = .5 X RHO x VxV

0.5 X 1.2 X 14.1 X 14.1= 119.286 Pa

(249.17Pa = 1"wg so 119Pa = 0.478"wg) :note always positive values

You also have a static pressure of 4.5" so total pressure is 4.5+0.478=4.978" wg. (assumed positive static i.e. on outlet of fan)

Your calcs are spot on.

I assume that the static pressure you are talking about is a positive one i.e. +5"wg, otherwise you will get a negative pressure i.e. -5+0.4978=4.50"wg.i.e. if on suction side of fan.



Friar Tuck of Sherwood

 

[rwelk]

Well I am glad to hear that.

I was told to that I should get a pitot tube that ranges from 0-3.77inwc DP.

Would I be correct in saying that this range should actually be 0-5inwc DP? What I am asking is the answer I get for total pressure considered to be the differential pressure?

I apologize if I sound uneducated on this topic, it is not exactly my strong point.

any further help would be appreciated.

 

[friartuck]

I assume u are trying to monitor velocity??

I apologise for being rude, but i find it easier to explain if i compare the pitot tube to a willy, however the 'willy' not only has a hole in the normal place, but a number of holes around the sides.

The end of the pitot (single hole) points into the airstream and measures the total force of air which includes the velocity and static. This hole is connected to a pipe on one side of the manometer (the bottom end).(Assuming you are using an inclined manometer) note the VP values are always positive.

This pushes the water (or other suitable fluid) up the tube and if you like, tries to push it out of the tube.

The holes around the end are also connected via a simple annulus to another connection. Because the holes are at 90 degrees to the airstream, they measure only static pressure.

This could be a positive or a negative value depending on which side of the fan you are on. (actually, technically, you could have varying static pressures in the duct depending on the changes in section of the duct)

The end of the static pipe goes into the other end of the inclined manometer. In effect, the static pressure acts against the total pressure pipe and the difference measured is by simple deduction the velocity pressure.

You then use this reading to assess the duct velocity.

When you use the inclined tube manometer (Inclined to gain more accuracy), then you can connect or disconnect the pipe connections to it and gain the various pressure readings such as total (disconnect the static pipe), or velocity, (both connected)




hope this is as clear as mud

Friar Tuck of Sherwood

 

[imok2]

Friertuck is quite correct, so in an effort to add a visual to his description please go here:

http://www.dwyer-inst.com/htdocs/airvelocity/AirVelocityIntroduction.cfm

 

[rwelk]

Thanks for the help everyone.

This has definitely cleared the mud.

Cheers

 

[rwelk]

Actually, I have another related question I think you guys can answer. If you are on the suction end of a duct, would the static pressure be negative? Is that possible?

 

[quark]

Yes. That is the reason why we provide fresh air opening in the suction duct(if the pressure is positive then the air will flow from system to the atmosphere).

 

[rwelk]

makes sense....that's incredible

thanks for the help everyone