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How to limit throttling/Joule-Thompson effect?
3

How to limit throttling/Joule-Thompson effect?

How to limit throttling/Joule-Thompson effect?

(OP)
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.

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RE: How to limit throttling/Joule-Thompson effect?

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.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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RE: How to limit throttling/Joule-Thompson effect?

(OP)
@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.

RE: How to limit throttling/Joule-Thompson effect?

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

RE: How to limit throttling/Joule-Thompson effect?

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.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

RE: How to limit throttling/Joule-Thompson effect?

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.

RE: How to limit throttling/Joule-Thompson effect?

Quote (j.doe)

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

RE: How to limit throttling/Joule-Thompson effect?

(OP)
@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 cry?? 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.

RE: How to limit throttling/Joule-Thompson effect?

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.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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RE: How to limit throttling/Joule-Thompson effect?

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.

RE: How to limit throttling/Joule-Thompson effect?

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.

RE: How to limit throttling/Joule-Thompson effect?

(OP)
@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.

RE: How to limit throttling/Joule-Thompson effect?

This may help us:

Good Luck,
Latexman

RE: How to limit throttling/Joule-Thompson effect?

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

RE: How to limit throttling/Joule-Thompson effect?

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

RE: How to limit throttling/Joule-Thompson effect?

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.

RE: How to limit throttling/Joule-Thompson effect?

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.

RE: How to limit throttling/Joule-Thompson effect?

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.

RE: How to limit throttling/Joule-Thompson effect?

(OP)
@Compositepro Thanks a lot for clearing that up. Sorry, didn't understand what the term meant from your previous posts.

@georgeverghese Yeah i agree, will probably adopt that approach.

@chicopee How do you propose to reduce the rate of discharge from the top valve? I've got two ball valves at the top which i basically manually open and close, do you think i need a different type of valve or something? You're right, i studied biomedical engineering in my undergrad but have come across a lot of ppl who don't know what that is, so didn't want to confuse ppl here when reading my post and claimed to be a ME cos i thought that was the next closest subject. We studied bio instead of thermodynamics, but that was literally the only difference in my uni. Sorry.

RE: How to limit throttling/Joule-Thompson effect?

A ball valve is NOT the greatest throttling valve. Try a globe valve or a needle valve.

Good Luck,
Latexman

RE: How to limit throttling/Joule-Thompson effect?

(OP)
Thank you all so much. You guys are the best!!
thanks

RE: How to limit throttling/Joule-Thompson effect?

The additional kit you will need to enable subcooling is nothing to be scoffed at - you can create a tear refrigerant stream from this R134a and let it down to a much lower pressure to chill / subcool the test stream. Talk to a senior process or mechanical engineer to help with configuring the subcooling section.

RE: How to limit throttling/Joule-Thompson effect?

"You need to pressurize the cell to at least 665 kPa with air or nitrogen before introducing liquid R134a into the cell."

What's the point of that? As you say, the pure component vapor pressure of R134A from the diagram is ~665 kPa at 25 C. If you put liquid R134A into a closed vessel that has 665 kPa due to N2 or air pressure, the total cell pressure will equilibrate to ~1,330 kPa at 25C. Liquid refrigerant will still vaporize until the partial pressure of the refrigerant is 665 kPa in the headspace. Liquid vaporization is controlled solely by the partial pressure of that component - injecting an inert gas to increase total system pressure will have NO effect on the vaporization of the refrigerant. If you try to bleed the vessel while simultaneously injecting liquid in order to maintain pressure at 665 kPa, you will be dropping the partial pressure of R134a below 665kPa because the purge stream will be comprised of air/N2 and R134a. This will continue until all the air/N2 is purged - pressurizing the vessel beforehand will actually lengthen the bleed process and induce more vessel cooling, meaning the OP will still have the same cooling issue he experienced when trying to fill the vessel when starting at ambient conditions.

If you were to take the vessel at atmospheric conditions and pump liquid R134A into it until it is 80% full, you vessel would be approximately 665 kPa + 500 kPa = 1165 kPa (compressing air isothermally from 100% volume at 100kPa to 20% volume). OP, if your vessel has an MAWP greater than 1,200 kPa, that is the easiest way to go.

The approach described by IRstuff is the correct one. Pull you vessel down to as low of a vacuum as you can. Then inject R134A until 80% full. No need to bleed the system, and depending on the level of vacuum you obtained, you should be between 665-800 kPa in the vessel. You can estimate the % full of the vessel by the final vessel pressure at 25C. For an isothermal compression of an ideal gas, P1V1 = P2V2. If you initial vessel is 5 kPa, your vessel will be ~80% full at 665 kPa + (5 kPa *100%Volume/20%Volume) = 690 kPa. Or, you could just weigh the vessel before and after adding to determine the amount added.

If air contamination is an issue, then you will need to bleed pressure down to 665 kPa at 25 C. There is no way to avoid the cooling effect during this bleed - you will need to keep it in a water bath and wait for temperature to recover to 25 C before taking measurements.

Edit: Corrected bad terminology Re Latexman's and Compositepro's corrections.

RE: How to limit throttling/Joule-Thompson effect?

Adding nitrogen pressure is pointless. It does nothing to affect the equilibrium vapor pressure of the liquid. It does prevent boiling, however. Boiling occurs when the liquid vapor pressure is greater than the ambient pressure. When vapor pressure is less than ambient pressure, evaporation is what happens.

RE: How to limit throttling/Joule-Thompson effect?

Quote (TiCl4)

Liquid boiling is controlled solely by the partial vapor pressure of that component - injecting an inert gas to increase total system pressure will have NO effect on the boiling point of the refrigerant.

So, water boils at the exact same point (temperature) no matter the altitude?

What is a "partial vapor pressure"? Vapor pressure of a component is a function of temperature only. There's no concentration term in the Antoine equation. I think you are confused with partial pressure.



Good Luck,
Latexman

RE: How to limit throttling/Joule-Thompson effect?

Adding nitrogen adds one degree of freedom. As long as the total pressure is more than the saturation pressure, one can control temperature and pressure. With only R134a present, those are forever tied together. It adds controllability.

Good Luck,
Latexman

RE: How to limit throttling/Joule-Thompson effect?

What would be the point of controlling or changing total pressure independently of temperature for this experiment?

RE: How to limit throttling/Joule-Thompson effect?

The issue is nitrogen contamination of the R134 test sample. At the pressures required to keep the R134 liquid, there's going to be a high percentage of nitrogen dissolved in the liquid R134. That potentially messes with the bulk electrical conductivity measurements.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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RE: How to limit throttling/Joule-Thompson effect?

True, I hadn't thought much about dissolved nitrogen in the R134a. Anyone know the solubility at around 1 Mpa? I didn't find any free data using Google on this.

Good Luck,
Latexman

RE: How to limit throttling/Joule-Thompson effect?

btw, j.doe

How are you dealing with the overall question of R134 purity? Seems odd to me that you'd purify R134 into one container and then have to transfer it to another, risking additional contamination at several interfaces. How do you even know you're not contaminating the R134 in the primary container already? Every valve is a potential source of organic contamination.

There may even be contaminants that aren't even tested for. We had the misfortune to discover that stainless steel typically contains trace amounts of boron, which they don't test for, but wreaks havoc in certain semiconductor materials. Since almost all process piping is stainless steel, that undetectable amount of boron in the steel essentially tanked our development project.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: How to limit throttling/Joule-Thompson effect?

I thought the OP was having difficulty controlling the R134a in the cell. He said it would flash off and not be at 25 C where it was needed. That's why I suggested to add nitrogen or air; to disconnect the temperature/pressure relationship of the vapor/liquid R134a in the cell.

Good Luck,
Latexman

RE: How to limit throttling/Joule-Thompson effect?

Thank you both for the corrections. "As iron sharpens iron"...

As to N2 solubility, I've found the following reference for the Henry's Law parameter as being 0.0013 for N2 in R134a (Table 3).

Solubility of nitrogen in one-component refrigerants: Prediction by PC-SAFT EoS and a correlation of Henry’s law constants November 2011International Journal of Refrigeration 34(8):2109-2117

https://www.researchgate.net/publication/251508745...

Latexman: Even if N2 were added, opening the bleed valve would bleed of a N2/R134a mixture, causing additional R134a to vaporize to maintain the vapor pressure. He would still have the same cooling issue, no?


RE: How to limit throttling/Joule-Thompson effect?

Seems to me that the OP has several problems

> JT expansion of R134 into 2nd container -- that may be completely unavoidable, given the high pressures involved; BUT, it's a transient thing, so the question is whether the gas will condense back into liquid after equilibration. It seems to me that in the steady state, if there was sufficient pressure in the 1st container and the 2nd container is small compared to the 1st, then you'd be back to the saturated gas and liquid in the second container.

> Not even sure how big a deal this is anyway. My generation of EEs, at least, has taken something like Dust-off and inverted the can to spray liquid freon onto ICs to cool them down, so getting liquid Freon out of a pressurized container is achievable in the transient case. This suggests that an internal bladder (another contamination source) might be the best approach, or super-cooling the 2nd container with LN2, or somesuch, to keep the Freon from flashing.

> Why not put the R134 into the 2nd container from wherever came from to get into the 1st container; that would eliminate a truckload of potential contaminants.

> Contamination -- possibly unavoidable as well; the OP has not stated how the R134 came into being in the first place and is attempting to make measurements that are ostensibly better than previous. One would think that would put contamination at the top of the list, even ahead of an JT considerations. High purity R134 is only around 99.5%. Some claims of 99.99%, but who knows... I mea, if it's just to be used as a refrigerant, why does one even need 99.99%?
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: How to limit throttling/Joule-Thompson effect?

Good find, but that's not Henry's Law parameters. It's a standard deviation of Henry's Law parameters between an EOS and a correlation.

I don't think that article lists any Henry's Law parameters; not of R134a anyway. It does give parameters to an equation that allows one to calculate R134a's Henry's Law parameters.

Good Luck,
Latexman

RE: How to limit throttling/Joule-Thompson effect?

I'll concede adding nitrogen may affect the measurement, not sure how much, but let's focus on doing this with only R134a. Agreed?

Good Luck,
Latexman

RE: How to limit throttling/Joule-Thompson effect?

The OP is essentially duplicating the test setup of the paper I referenced earlier, but, in the paper, the calculation of the bulk resistivity of the R134 is predicated on complete filling of the test (2nd) container to a known volume, so having any gas phase in the 2nd container is a non-starter. This would mean that the OP needs to either bleed the gas off the second container, or allow the gas to get back to the 1st container.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: How to limit throttling/Joule-Thompson effect?

(OP)
yeah my supervisor just told me the same thing about not using nitrogen, so i'll just be using 134a like you guys said. Thanks for the detailed process explanation @TiCl4. To answer your questions about purity and contaminants @IRstuff, the first container containing the 134a is too large to keep inside my lab, it's a pharmaceutical grade cylinder with ≤6ppm moisture content (moisture is the main impurity we're trying to limit). The company supplies the 134a to pharma companies for use as an inhaler propellant, and this whole project is to do with understanding the electrostatic properties of formulations within inhalers (not sure i should be disclosing this stuff lol). I guess we are risking additional contamination when transferring to the smaller cylinder but they taught me how to limit such contamination when doing the transfer (by opening the valves first to purge the valves before connecting the 2nd cylinder). You're right contamination is at the top of my list, and i think that's the main reason why i got such higher values for resistivity in the preliminary tests compared to previous studies.

RE: How to limit throttling/Joule-Thompson effect?

Why do you need to measure its conductivity? Surly someone had already done it and the data is available?

RE: How to limit throttling/Joule-Thompson effect?

(OP)
@thatguy1988 there have been 3 academic studies trying to determine its conductivity but the results they've got doesn't correspond to the behavior of 134a observed in industry. They think the academic studies are at least 4 orders of magnitude off. So they sponsored me to get better results. It's difficult to do properly as a few ppms of moisture increases the conductivity by an order of magnitude and commercial cells don't allow you to test liquids under pressure.

RE: How to limit throttling/Joule-Thompson effect?

This might be a bit left field, but can you use something like a constant pressure sample cylinder?
Precharge the non-product end with whatever pre-charge gas takes your fancy @ 200bar, then slowly fill the product end of the cylinder with your 134a.
You could adapt the product end cap of the cylinder to fit whatever sensor/cells are required for your analysis.
The piston/piston seal in the cylinder seperates the 134a from the pre-charge gas and having gas on the pre-charge side means you can have 100% liquid in the product side.
You wont need to vent the 134a as there should be no vapour on the product side.

RE: How to limit throttling/Joule-Thompson effect?

True, I hadn't idea a good deal approximately dissolved nitrogen in the R134a from shoes graph. Anyone recognize the solubility at around 1 Mpa? I failed to locate any loose statistics the use of Google in this.

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