pressure drop measurements in gas flows
pressure drop measurements in gas flows
(OP)
We intend to measure the pressure drop over a length of 1m vertical pipe in a 44mm ID tube.
Air velocities are up to about 20m/sec.
Pressure is just below atmospheric pressure.
We have got the the devices sorted in order to measure the pressure drop. We are only interested in meassuring the static pressure in the tube. We cannot meassure the pressure using a whole in the tube wall because here a liquid film with 2mm film thickness is flowing downwards.
Therefore we intend to use a small tube (1mm ID)to point
perpendicular into the gas flow, in order to meassure the static pressure. (See scetch)
Does it matter how far I point into the gas flow?
Does the shape of the tip has an impact on the measurement?
Does the internal diameter of the tube has an impact?
I want to avoid a pitot tube effect where I meassure also a velocity component.
Sketch:
| |Tube with liquid film at the wall
| |
| |
| ______
| ______ 1mm tube to measure the static pressure
| |
Air velocities are up to about 20m/sec.
Pressure is just below atmospheric pressure.
We have got the the devices sorted in order to measure the pressure drop. We are only interested in meassuring the static pressure in the tube. We cannot meassure the pressure using a whole in the tube wall because here a liquid film with 2mm film thickness is flowing downwards.
Therefore we intend to use a small tube (1mm ID)to point
perpendicular into the gas flow, in order to meassure the static pressure. (See scetch)
Does it matter how far I point into the gas flow?
Does the shape of the tip has an impact on the measurement?
Does the internal diameter of the tube has an impact?
I want to avoid a pitot tube effect where I meassure also a velocity component.
Sketch:
| |Tube with liquid film at the wall
| |
| |
| ______
| ______ 1mm tube to measure the static pressure
| |





RE: pressure drop measurements in gas flows
RE: pressure drop measurements in gas flows
http://virtualpipeline.spaces.msn.com
RE: pressure drop measurements in gas flows
Generally such a measurement would assume a negligible radial pressure gradient. Can the 1 mm measuring tube be inserted so that in does not protrude beyond the 2mm film? Measurement of the film pressure would then indicate the gas static pressure.
Regards
RE: pressure drop measurements in gas flows
http://en.wikipedia.org/wiki/Pitot_tube
These very simple and easy to make devices, work well enough to be able to operate a pressure altimeter in a jet fighter aircraft.
RE: pressure drop measurements in gas flows
One other reason is that as a pressure sensor we use an air dp sensor from sontay which operates for pressures up to 100 Pa. It is calibrated for air, so we don't quiet know how it would react on liquid in the system.
RE: pressure drop measurements in gas flows
When looking up those devices they also seamed to measure the pressure similar to our setup (see stativc pitot device in your link) the pressure openeing is located perpetingular to the flow. Do you think it makes a difference to meassure the static pressure with a single tube as in our proposal?
RE: pressure drop measurements in gas flows
Any flow velocity at all directly across the face of an open pipe end will create an unstable negative pressure.
A pitot static source can be made very small, perhaps with 1/8" od copper capillary tube. Block the open end with a smooth aerodynamic round dome of solder, drill some radial pin holes in the pipe at around ten pipe diameters back from the nose, and polish up the outside of the pipe with very fine emery paper to remove drilling burrs.
Very simple to make, easy to try, and it will work extremely well. This is what everyone uses, and it will give a rock steady static pressure reading up to extremely high airspeeds.
RE: pressure drop measurements in gas flows
Do you know suppliers of such small tubes, or do you recommend to build them on your own?
Ideally I would only need a capillary tube which we bend 90° and than cover the inlet hole. In a next step we would need to drill holes into the tube wall and polish them, Is that right?
Only one Question, do you recommend to drill two holes opposite to each other as shown in the wikipedia article. What is the reason for this?
Do they need to have a certain minimum hole diameter?
RE: pressure drop measurements in gas flows
http:
That is just an indication of the sorts of small sizes of pipe that are available. It is usually cut from a roll, and you can buy short lengths easily enough.
Block the end with soft solder and drill several radial pin holes. By using multiple holes the pressures detected in the boundary layer average out between all the holes. The holes can be as small as you like, as there is zero hole flow once the pressure has stabilized inside the pitot tube. Something like a number 80 drill perhaps ?
Just polish up the outside of the probe tube, so the boundary layer is not pierced with a rough projecting edge at the holes. If it feels smooth to the touch, it will work fine.
Bend the pitot tube into a right angle and solder it into a threaded male pipe fitting. Screw the whole thing into the wall of your process pipe, and you are in business.
Best to mark the threaded fitting, so as it is screwed in, you know when the projecting nose is pointing directly up stream into the flow. It can be pointing slightly out by ten to fifteen degrees without serious errors being introduced. One more reason for having multiple pin holes arranged around the probe.
RE: pressure drop measurements in gas flows
Does it actually matter which way the tube is facing
In the following drawings the gas flow is coming from the top. Is there a difference as long the tube is parallel to the flow? I presume assembly 2) is better because the tube bend does not effect the flow?
1)
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2)
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RE: pressure drop measurements in gas flows
The idea is that the smooth blunt aerodynamic round nose of the probe directs the flow evenly around the outside of the probe tube with minimum induced turbulence. The air then forms a stationary boundary layer that sticks to this probe tube. The very small holes communicate this pressure into the inside of the probe tube.
If you do it the other way around, the supporting bend in the tube will disrupt the flow. Turbulence will then disturb or even sweep away the boundary layer. It is essential that the pitot probe points directly into the oncoming flow.
Your drawing 2 would be the correct one, if the direction of flow is moving downwards.
RE: pressure drop measurements in gas flows
At the film boundary where air and liquid meet how was velocity of film determined-This also relates to film thickness.
Regards
RE: pressure drop measurements in gas flows
This is used in order to calculate film thicknesses in falling film evaporators and vertical condensers. In general film thickness increasses with mass flow and viscosity
RE: pressure drop measurements in gas flows
Regards