Measuring air pressure @ high temperature 1

[guyguy]

hello,

I am trying to measure static air pressure in a pipe @ 300 deg C and at max. of 150 psi. The air is flowing in the pipe.
It seems quite difficult to find a sensor that survives these temperatures.
I was told that if i will connect a short tube and take the measure few inches away from the pipe port, the temperature in the tube would be lower, and then i can use almost any pressure sensor.
my questions is:
1. is this for real, and if yes why? i can't understand the physics behind. please explain.
2. how do i evaluate the temperature drop, or in other words, what is the minimal length required for reducing the temperaure that the sensor feels to 120 C deg. (from 300)

thanx,
guyguy

 

[25362]

I believe there are plenty of gages (Bourdon type and others) that can measure and withstand easily the P,T conditions above. These are used with steam and other process fluids at conditions even more severe than those listed by guyguy. Just do a Google search.

 

[guyguy]

25362,

I now see that I was not clear enough. I am going to use a pressure transducer. That is why I have a problem.

anyone?

guyguy

 

[Montemayor]

guyguy:

Either way, with/without a transducer, you have no problem to speak of.

Pressures are routinely measured at elevated and cryogenic temperatures without any problems on subjecting the sensing element to the moving fluid's temperature. The trick is: use the little studied fact of natural convection.

Without convection in the fluid, you in effect create a "heat trap". I was taught this technique in operating cryogenic plants and later employed it in steam reformer furnaces with continued success. All you need to know and apply is that if you create a "dead fluid" zone between the actual measured fluid and the sensing element, you have created a virtual static fluid without any natural convection - and, consequently, little heat transfer. In the cryogenic operations, we used a "pig-tail" - a circular loop within the measuring tube that created this relatively static condition. For measuring gas pressure, a simple circular loop was sufficient; however, for measuring liquids there is a fundamental requirement you must fulfill: you must create or have a gas on the sensing device's side of the static liquid seal. In cryo liquids, this is not a problem: the liquid itself vaporizes and forms a gas.

This is why a lot of young engineers had a lot of problems believing that the simple water manometer gauges (indicating liquid Oxygen & liquid Nitrogen levels) in front of my air separation columns were directly connected to their corresponding -350 oF sources without freezing! The heat trap did the trick and enable people like Carl Linde in Germany to build and operate the first air separation columns in the late 1800's with the use of only simple water manometers to measure the liquid cryogens. It is still used today in many applications because of the simplicity and reliability.

This is a good question. It had me dumbfounded also when I first confronted my first application. Simple heat transfer convection principles are the answer.

Measuring pressure is no sweat. However, measuring temperature -- that's a toughie that requires high state of the art thermocouples.

I hope I was able to explain it to you without boring you.



Art Montemayor
Spring, TX

 

[MintJulep]

The reason this works is that there is essentially no flow in the line leading to the transducer assuming that the pressure you are monitoring is reasonably steady-state.

Once the tube fills up heat immediately starts leaving the surface of the tube. With no flow, there is very little heat transfered from the media in the pipe to the media in the tube.

The only heat transfer in is conduction where the tube meets the pipe.

The calculation of the length required is relatively simple, but simpler still is to just look it up somewhere.
Here for example:

http://www.setra.com/tra/app/app_hig.htm

 

[guyguy]

do you know of any other websites but with theory details. (i understand the concept but would like to see some formula, some physics...)

 

[25362]

Allow me to ask why is it that you prefer to have precisely a temperature of 120 deg C in the connector. Is it meant to ensure the avoidance of water condensation ? Thanks.

 

[guyguy]

no particular reason
actually i prefare the temperature as high as i can get the transducer work since i want the tube to be short.
i wanted to see a formula/calculation so i figured i'd give the numbers myself...

i have a transducer that operates at max 150.

and, i guess u right about condensation. thanks!

 

[hacksaw]

gg

the heat loss calculation you ask about is a tedious exercise but entirely possible. but setting up the calculation for representative service conditions cost far more than your transmitter and only confirms that how quickly the temperature reaches equibrium with the ambient condition.

fortunately the length is rarely critical (>10D) given that adequate convection cooling (or heating in the case of cryo-fluids) is allowed. the thermal issues rarely dominate.