How to limit throttling/Joule-Thompson effect? 3

[j.doe]

Hi, I've designed a stainless steel cylinder cell to measure the electrical conductivity of R134a in liquid phase (under pressure). I'm a mechanical engineer and not a chemical engineer so i was wondering if you guys can kindly help me out with this. Initially, the procedure we were hoping to implement was to fill from a valve at the bottom (see picture attached) and wait before opening the top valves slightly to see whether liquid is coming out to ensure the refrigerant is in liquid phase before carrying out the experiment (measuring current from electrode inside the cell). The problem is, when opening the top valve, the flash evaporation caused the whole cell to cool significantly (roughly 50C temperature drop, calculated using Clausius–Clapeyron equation) which affected our readings because we want to measure at 25C. The saturation pressure of 134a is 665kPa, it gets released to atmospheric conditions.

I've been thinking about adding a pressure sensor and thermocouple between the top valves (see pic) to use the pressure reading as an indicator of liquid phase, instead of opening it from the top. Then, I would release the liquid from the same bottom valve I filled cell from into an empty cylinder with a relatively long pipe in between, to ensure the cooling effect does not reach the cell. Is there anything you think i can do further to reduce the throttling effect in my system? There must be a way of removing the liquid with a gradual decrease in pressure that stops throttling from occurring when opening a valve right?

I was thinking of covering the valves and pipes at the bottom and top of the cell with heat tapes (set to 25C) as well to limit the throttling. The problem with this though is that a temperature gradient could develop maybe causing bubbles to rise to the top of the cell which would affect our reading (we need uniform liquid conditions). Our cell is within a stainless steel cylinder enclosure to hold it together while being pressured. There is an air gap between the inside of the enclosure and the outside of the cylinder electrode (see pic) so the electrode can't be heated from the outside.

https://files.engineering.com/getfi...99-a95d-a87696065fb8&file=initial_set_up.jpeg

https://files.engineering.com/getfi...6-9c36-6e39175a5a51&file=proposed_set_up.jpeg

 

[IRstuff]

Perhaps you should back up and explain what the general flow of this test process is intended to accomplish and not get into the details of your implementation.

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[j.doe]

@IRstuff I think i did mention that it's intended to measure the electrical conductivity of liquid 134a at a constant temperature. Apologies if i wasn't clear enough.

 

[Latexman]

I'm trying to understand your scope. Do you want to measure R134a electrical conductivity at various pressures or only one pressure? But, always at 25 C, right?



Good Luck,
Latexman

 

[IRstuff]

I got that, but the two containers, transferring the freon, etc."

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[EdStainless]

So you want to be able to adjust the temperature and pressure on a liquid sample?
My first thought is to move the vent valve further away, like on the end of a 50' coil of SS tube. This would minimize the heat that could be transferred. You could also apply heat tape along this long coil in order to reduce the changes.
The other possibility is to let the pressure down in steps. I have not looked at the curves for R134a so I am not sure if this is feasible. If the steps are too small then this wouldn't really help.

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[Compositepro]

Well, it is now clear why there was concern about "liquid locking". That set-up is definitely dangerous and it will result in liquid lock because you are venting all the vapor space just to detect that the cylinder is full. Even worse, as you fill, you are boiling the refrigerant and cooling it significantly, which is your complaint. When the cylinder is full you intend to close the vent valve and the fill valve. So you now have a cylinder of cold liquid refrigerant that is liquid locked and heating up. Something has to give. Your cylinder may be strong enough not to explode. More likely a gasket will leak first Who knows? Put a relief valve on your apparatus.

Your problem is not due to the Joules-Thompson effect. The refrigerant is boiling in the cylinder while you are filling it and venting it. You will always get some cooling when you fill with liquid because some refrigerant must flash when entering an unpressurized container. You are performing exactly the same procedure that that is done when filling a propane tank. These have spit valves on them that vent vapor from a dip tube in the tank. The end of the dip tube is located at the 80% fill level so that liquid starts venting to inform the the operator to stop filling at that point. People would still commonly overfill the tanks, so the law was changed about 20 years ago to require these tanks to be equipped with OFPD's (over-fill protection device), which are float-valves that stop the fill when the correct level is exceeded. These are not 100% reliable and are not supposed to be used to actually control the filling operation. The spit valve is still supposed to be used. The spit valve also vents non-condensible gasses that otherwise tend to accumulate in the tank. However, in my experience, most tank fill operators do not use the spit valve anymore. OFPD equipped tanks have triangular valve handle; the old ones were round.

Propane tanks have always been required by law to have relief valves, so that overfilled tanks will vent rather that explode. This is why these tanks are not supposed to be stored inside closed spaces where vented vapors could accumulate or ignite. I hope that you are aware of the asphyxiation hazard of refrigerants.

If you must control temperature of your experiment, you need to put your cylinder in a lab water bath and let the temperature stabilize before your experiment.

 

[Latexman]

There must be a way of removing the liquid with a gradual decrease in pressure that stops throttling from occurring when opening a valve right?

No, not on a system with liquid and vapor R134a only. If P decreases, liquid will flash and chill the remaining liquid. Temperature is fixed by pressure, and vice versa. You need a system that can be pressurized with another component like nitrogen or air. Adding another component gives you one more degree of freedom. Then you can control pressure and temperature. Study up on the "phase rule".

To see liquid, use a pressure vessel with a sight glass. All rated for the pressures they will see, of course.

Good Luck,
Latexman

 

[IRstuff]

Looks like someone already solved this problem and made measurements:
Measurement_of_electrical_properties_of_refrigerants_and_refrigerant-oil_mixtures

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[j.doe]

@IRstuff yeah i've seen that study before, there are several studies like it, i'm basically doing the same experiment with an alternative design and slightly different process to measure higher resistivity values. The other studies don't mention anything about throttling though. They've all apparently failed to limit impurities to get an accurate (much higher) resistivity reading. The company sponsoring my research, based on their experience and previous unpublished research many years ago (which apparently got burned in a building fire) believe the resistivity is at least 3-4 order of magnitude higher, which my cell seems to confirm based on preliminary tests.

@Latexman yes i'm trying to measure the conductivity at a constant pressure (a value above saturation pressure to ensure liquid phase) and temperature of 25C. Thanks for the "phase rule" suggestion, seems really useful, gonna take some time to understand it. Would you mind explaining briefly how i could incorporate, say nitrogen, in such a system to control the pressure so that i can reduce it gradually when releasing the liquid to limit throttling? Yeah we did try to implement a sight glass in our cell when designing it but we couldn't get it to fit the dimensions we required, well that's at least according to our technician.

@EdStainless the problem with a long coil though is that there's a higher likelihood of accumulating impurities, where we especially want to limit moisture impurities which has been found to limit high resistivity measurements. Your point about reducing pressure in steps sounds interesting, how would that work exactly?

@Compositepro Thank you dude, that's really enlightening information, never even considered this stuff. What does "liquid lock" actually mean though ?? I'll definitely talk to the company people about installing a relief valve. Did tell them about the water bath, they were adamant to keep water away from the device to prevent moisture somehow getting in and corrupting the measurement, will tell them again though since that seems unlikely.

 

[IRstuff]

One option would be to close off the test container and pump it down to vacuum, which is probably necessary anyway, to minimize contamination. If the test container is relative small compared to storage container, there would be a smaller pressure change. Alternately, one could imagine having an internal bladder that's evacuated that the R134a goes into in the test container. That might mitigate any JT effects.

After which, you basically have wait until the temperature stabilizes, if there is any JT effect going on.

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[georgeverghese]

Measuring at atmospheric conditions will always result in the refrigerant temp going down to the corresponding sat vap temp, that is a thermodynamic brick wall you cannot break through. To me you will have to operate the measurement at a pressure which will maintain the refrigerant subcooled by a few degC to ensure liquid phase measurement. As far as I know, electrical or thermal conductivity of a liquid shows no sensitivity to operating pressure, for most practical purposes.

 

[LittleInch]

OK, Compositepro as usual has great advice, but we're missing some data here.

1) what pressure is your liquid R134a at when introducing it into your vessel?
2) Is it in liquid form? At what temperature?
3) What pressure rating s your test vessel?
4) Do you have (and if not why not) a pressure relief valve on this test vessel?
5) How full does this vessel need to be?

Using nitrogen could be used to have a dry gas at more than 6.6 bar before you enter the liquid R134.

Then monitor pressure and vent off the nitrogen maintaining the pressure > 6.6 barg to stop the r134 boiling and lowering the temperature.

Or you could maybe allow a bit of flashing and then use temperature to detect liquid level.
Or weight

But unless your vessel has a high design pressure, you really need either a gas volume or some relief valves to prevent it failing.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[j.doe]

@LittleInch 1) I'm not sure tbh, should probably find that out.
2) I transfer the 134a from a large cylinder stored outside (with 200 bar pressure) into a smaller one, the smaller cylinder is vacuumed and put into liquid nitrogen to cool it so that the transfer of 134a from the larger cylinder to the smaller one is faster. The larger cylinder has a valve for releasing the refrigerant in gas phase and liquid phase, we use the liquid phase to fill the smaller cylinder. We then heat the small cylinder to 25C and transfer it to the cell, which is also at 25C. The cell which the refrigerant is fed into has also been vacuum pumped beforehand. So when transferring the refrigerant into the cell, if it is a liquid in the storage cylinder, it'll inevitably evaporate initially when transferred into the cell. Then, as i said earlier, we fill the cell and then open the top valve after a while to check whether liquid is coming out to make sure it's a liquid.
3) We haven't tested that, but i calculated the maximum pressure a solid SS cylinder of our dimensions could theoretically withstand is 9MPa. However, and this is a big point, our cell is not a solid cylinder but made up of 3 cylinders with gaskets in between, so it would probably withstand a lot less pressure before failing.
4) Honestly, i'm not sure, i don't know anything about valves, it was the people from the company that attached valves onto it, and told us to leave it to them. I was only in charge of designing the cell and now of doing the experiment. So, yh I'll be contacting them about it on Monday.
5) It's got a volume of around 55ml, we only need to fill to 40ml.

Thanks for the suggestion. I did tell them the same thing about filling with nitrogen before inserting the 134a, not because of throttling, didn't know about this before, but to limit cavitation effects that might occur. They disagreed, don't remember why. So yeah, will bring it up again, thank you.

@ georgeverghese I think i understand what you're trying to say, thank you, it's a good idea.

 

[Latexman]

This may help us:

Good Luck,
Latexman

 

[Latexman]

You need to pressurize the cell to at least 665 kPa with air or nitrogen before introducing liquid R134a into the cell. Or put 665 kPa of backpressure on the liquid/vapor R134a in the cell.

Good Luck,
Latexman

 

[j.doe]

Thank you Latexman for the explanation. Anyone know what "liquid locking" means in this context as Compositepro was referring to in his comment above?

 

[Compositepro]

I explained that in your other thread. It is when a closed volume is completely filled with liquid, without any vapor space. When this happens, any expansion of the liquid (usually due to thermal expansion) is severely constrained and will result in very high pressure that can crack steel. The thermal expansion of liquids is much higher than most metals or solids. This is how glass thermometers work.

 

[georgeverghese]

If Psat is 665kpa abs at 25degC, and this is the max temp you want to measure this parameter at, select say 700-720kpa abs as operating press to keep the R134a subcooled during measurement. Then design pressure for the test cell would be say 800kpa abs = 700kpag.

 

[chicopee]

The rate of discharge of the vapor thru the top valve is too fast. You have to slow (very, very slow) down the flow rate so that the heat transfer rate from ambient condition can maintain a uniform temperature within the contained fluid assuming that the initial liquid temperature was at ambient temperature. The problem that you described is a typical one that you encounter in thermodynamics problems so I am a little surprised that as a ME who should have had thermodynamics did not realize the problem.