Alternative to R508

Hello,

We have an environmental chamber with a cascade system that uses R404 and R508 refrigerants and we need to repair it. We are finding the cost of R508 is US$350 - US$1100 per pound depending on where you get it. And the minimum purchase quantity is 5 pounds, we only need 1 pound.

The system is designed to go down to -70 deg C.
We may consider changing the low temperature setting to -50 deg C if there is a lower cost alternative.
Does anyone know if there are any alternative refrigerants that may be less expensive and more readily available than R508?

Art

Could you use r1150 as an alternative?

R23.

R23 or R170 is your only real viable option.
R170 will be easier to find in low quantities and at tremendously cheap cost, however you will have to properly source and seal that leak. Normally R170 is not used in environmental chambers simply because a leak in the evaporator coil could create a very large space sealed with a flammable gas.

R23 has beter mass than r170, ethan has a litle lower temperature.

I vote for r23!

R23 is cheapest, Isn't R508 a blend of R23 and R116 or something else? And if he is looking for -70C, can't he use R410a or CO2 R744? Or would that require an oil change and complete retrofit? I am still learning so don't rip my head off!

R410a would require a hefty vacuum. R744 has dry ice issues. R23 is the best option.

Okay Just learning thanks Chris!

Thanks everyone for all your advice. Maybe you have some advice on more questions.

Listed below are the suggestions and some of their properties.
To really understand this I imagine we need more data.
Does anyone know of a central location to find data relating to refrigerants?

No. Name Mol Mass BP(F) FP(F) Ct(F) CP (psia)
R-1150 Ethylene 28.05 -155 -272 49 742
R-170 Ethane 30.070 -127 -278 90 710
R116 Hexafluoroethane 138.01 -108.8 -149.1 67.8 441
R23 Trifluoromethane 70.1 -115.8 -247.4 78 701
R-410A R-32 (50%) R125 (50%) 72.6 -55.4 162 690
R508a R23(39%)/R116(61%) (did not find data)

Is there a concern that the existing components will not be compatible with all refrigerants?
For example, will some refrigerants dissolve certain types of seals?
We also have to change the cascade condenser.
And we will be using a different condenser probably with different properties...volume heat transfer etc.
The existing cascade condenser seems to be a tube in tube construction.
We are planning to replace it with a SS brazed plate heat exchanger.
Does anyone have advice on the type of solder and flux to use?
Do we need to worry about cleaning the residual flux?
Does anyone have any advice on charging the system with a new condenser and refrigerant?
Do we need to optimize the charge with the new condenser and refrigerant?
With such a small amount of Ethylene is there a danger of fire or explosion?
Could air leak into the system and create an explosive mix?

Wow Art, thats alot of questions. So here are some of mine.
Are you doing this commercially?
Have you ever worked with cascades before?
Are you in house service or outside contractor?
Where are you located?
Why are you changing out the heat exchanger?
Is your background HVAC or refrigeration?

Any commercial system that contains a flamamble gas has to have burst disk and be routed to outside atmosphere to be UL listed.
Yes fractionation is of a concern in a chamber.
The bigger concern with compatabilty is in the oil in the comp first then the seals.
NO SOLDER, BRAZE ONLY

Yea I have plenty of advice on what to do, I do this for a living and usually get consulting fees for these types of questions.

Thanks very much for your interest and your comments.
We will likely be looking for an outside contractors to do most of the work.
I am trying to become more educated in this area so we know our options.
To answer your questions:

Are you doing this commercially? This is a single unit at our company.
Have you ever worked with cascades before? No.
Are you in house service or outside contractor? In house.
Where are you located? Massachusetts
Why are you changing out the heat exchanger? It is leaking between the the refrigerants.
Is your background HVAC or refrigeration? I do not work in either field. I work in electronics.

I found this Cole Palmer WEB site to be useful for determining chemical compatibility. The material in question is entered, then a list of materials and their compatibility is displayed.

http://www.coleparmer.com/techinfo/ChemComp.asp

Art,
Thank you for the response.
It is very rare for a heat exchanger to leak since there are no moving parts and vibration is limited. How did you determine that it was leaking between stages?
Do you have the manufacturer, model#, and serial# of the unit?

How much of this are you planning on doing in house?

With ultra low temp refrigerants you are going to have to spend some money. There are only two in general use, R23 and R508b.
R1150 is extremly flammable and is not used generally, the explosion potential is not worth it. The same for R170
CO2 starts to make dry ice at temps below -70C. This leads to major problems with blockages within the system.
R116 is very hard to source and is expensive as well.
R410a requires POE oil in the system. I have not seen this in any large system.
R23 is my favorite, it is compatible with AB oil and has a high load carrying capacity. Generally you can substitute R23 for R508b.
Try googling "Trifluoromethane". you should be able to find a speciatly gases supplier that can get this to you in small quantities.

Please get the manufacturer, model# and serial number from the unit. Then we can get to some specifics on what you can do.

Thanks again for your comments and interest. I do not have the model number now.
We had someone come in and charge both refrigerants. A day later the unit stopped working again. They then charged one side with nitrogen and it leaked into the other side.
So we concluded it was a leak in the cascade condenser. They came in and removed the refrigerants and we have been planning to have them come back to do the repair, but they have not had the time. Money has been a little tight so we have been looking for someone who uses the R508, so we do not have to pay for the full 5 lbs minimum purchase. As far as what we have been planning to do in-house, we have been thinking we can change the cascade condenser and then have someone come in and re-charge the system.

Must be a coaxial tube in tube style heat exchanger that broke somehow? That's odd. Unless it's a brazed plate that isn't rated for the temperatures it was put into. And the plates cracked or the braze cracked internally.

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Originally Posted by art_h

We had someone come in and charge both refrigerants. A day later the unit stopped working again. They then charged one side with nitrogen and it leaked into the other side.So we concluded it was a leak in the cascade condenser.

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Honestly, sounds like they did not know what they were doing. Where both stages empty of refrigerants when they charged the one with nitrogen? Did they isolate the HX to see if it truely did have a leak? Again its very unusal to have a HX leak from one stage to the other.

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but they have not had the time

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Sound more like they dont want to deal with it.

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Money has been a little tight so we have been looking for someone who uses the R508, so we do not have to pay for the full 5 lbs minimum purchase

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It is highly doubtful that you are going to find someone who just happens to have it sitting around, plus it is only used in cascade systems. As you have found out it is cost prohibative to have and not use. You dont have EPA certs so no one can just sell it to you. Also it is considered a High pressure gas (approx 630 psi at room temp) so you have to have special cannisters to transport it in.

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As far as what we have been planning to do in-house, we have been thinking we can change the cascade condenser

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You should be able to do that IF That is the problem.

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then have someone come in and re-charge the system.

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A cascade is not like a conventional system, there are special procedures that need to be done to ensure proper operation and a long life.


When they removed the refrigerant, did they put a nitrogen holding charge in the system or did they let air into it?

Here is someone qualified to do cascade work.

http://www.alertscientific.com/index.html

Thanks everyone for your help.
This is not a brazed plate condenser.
It looks like some type of tube in tube.
The material seems to be too robust to be leaking.
But this seems to be the only place it could be leaking.
We have been planning to replace it with a brazed plate.
The system is filled with Nitrogen now.

If you want a definitive answer if the interstage HX is leaking between stages remove refrigerant from both stages and fill one with helium and use a helium detector on the other.

If they just used nitrogen in one stage and seen a lose in pressure in that stage ,but a small gain in pressure in the other stage ,that COULD just be caused from refrigerant coming out of solution with the oil in the lower pressure(non-nitrogen filled) stage.
Just a example how a interstage leak could be mis-diagnosed.

Thanks, I was thinking this may not be a definitive test.
I was planning ro re-charge one side with nitrogen again and see what happens.
Can you recommend a good leak detector?

art_h can you post some picts its hard to visualize all that you are talking about...

Bacharach & Inficon are ones I'm familiar with. They use the principles used in Mass spectrometry but are only designed to detect the molecule of helium ,with is not normally present. They are very expensive 3 to 5 grand +++,not something you would buy for a one time use.

In the field we isolate both the first stage side and the second stage side so that you are only pressurizing the HX itself, not the rest of the system. I install hand valves on all four lines, 1st stage in and out of HX, 2nd stage in and out of the HX. Then install 2 pressure gauges, one on the 1st stage side and one on the 2nd stage side. Pressurize one side with 200psi dry nitrogen and let sit. Come back and compare readings. If the pressurized side has fallen and the non pressurized side has risen then add both readings together and you should get 200 psi. If it dosent add up start checking for leaks in other parts of the circuit, and start the test over once they are found.

Walts (wdrzal) is the best way but is usually cost prohibative due to the cost of the test equipment. The helium molecule is much smaller than the nitrogen molecule, therefore helium can find much smaller leaks than the nitrogen test.

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Originally Posted by art_h

Hello,

We have an environmental chamber with a cascade system that uses R404 and R508 refrigerants and we need to repair it. We are finding the cost of R508 is US$350 - US$1100 per pound depending on where you get it. And the minimum purchase quantity is 5 pounds, we only need 1 pound.

The system is designed to go down to -70 deg C.
We may consider changing the low temperature setting to -50 deg C if there is a lower cost alternative.
Does anyone know if there are any alternative refrigerants that may be less expensive and more readily available than R508?

Art

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I have R503, R23, R1150, Iceon89, R116, R14, and about 15 others Now I can mix up some R508 for you but I can only get the blend within a few percent of correct. Costs per pound is astronomical the 116 was 650.00 for 15 lbs, the R23 was 285.00 for 15 lbs.

your not in Vegas are you????

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Originally Posted by chilly1

I have R503, R23, R1150, Iceon89, R116, R14, and about 15 others Now I can mix up some R508 for you but I can only get the blend within a few percent of correct. Costs per pound is astronomical the 116 was 650.00 for 15 lbs, the R23 was 285.00 for 15 lbs.

your not in Vegas are you????

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Thanks very much for your interest. I am in Massachusetts.
That is quite a price difference.
I don't think the ratio is critical.
I understand R508B is a revised version of R508A.
And I found a reference to R508A 61/39 and R508B 46/54.
I would guess anything in between would be ok.
I wonder why it is so expensive.
Do you blend refrigerants a lot?
How is it done?
Can you also separate the refrigerants after they are mixed?
Do you know if this is blend azeotropic?