R-508B oil return problems

[Hank Hill]

We have a cascade with 508B(Suva 95) as the low stage and a helical oil separator. This separator does not seem to work with this refrigerant, can't keep oil in it. Does anyone have experience with this new refrigerant and these new separators.

We have an evaparator temp of -82C (-115F) and it seems that the oil separator just does nothing at low temps, and we run at this temp for days at a time, sometimes with no load.

This is on production pharmaceutical freeze dryers and most time the production folk are in no hurry to unload when cycle is complete, but the powers that be insist on refrigeration running.

Seems to us technicians there should be better perfomance with the oil return, could it be that the helicals do not perform well at ultra low temps.

[chemi-cool]

Hi Hank.

Never heard about R508B, what type oil runs in the system?

Isn't the oil separator located between the compressor and condenser?

Is the condensing unit far from the evaporator? Higher? lower?

Give some more details, please.

Chemi

[Hank Hill]

We run POE oil in the R508B low stage and in the R22 high stage. The oil separator is about two feet from the compressor discharge and there is a check valve between the separator and the cascade condenser. The evaporator is only a few feet from the condenser and is higher up.

We used to have R23 in the low stage and standard separators (not the helical). But alot has been changed on the skids including the compressors, receivers, suction accumulators and the low stage condenser and evaporator. This change was about a year ago and we've been fighting the oil migration ever since.

Just wondering if there was anyone out there with experience with this refrigerant or someone who has encountered this problem. Thought about possibly adding another separator in series with the first but I don't know if this is feasible. Any help would be appreciated.

[chemi-cool]

Hi Hank,

Read here about low temp oil separation
http://www.temprite.com/main_catalog...s&category=288

More information here,
http://www.dupont.com/suva/na/usa/li...f/h65923-3.pdf

You will need to change the oil a few times to clean the system from MO leftovers and sludge.

What made you change to helical?

From experiance, a trick I have learnt here, add a little bit of R12 for better returning of oil, I've used it in R134a system and it solved all the oil problem.
Chemi

[Hank Hill]

We switched to helical when they redesigned the system and switched to 508B, at the design engineers recommendation. We have used POE oil from that point. We have since had to add 1.5 gallons of oil to one unit to keep level up.

We are worried about fouling evaparator, it is a plate and frame, and the ***** flow is from bottom to top, with counter fluid flow. I wonder if it would be better to reverse and have ***** go from top to bottom and fluid go from bottom to top.

Please remember this is a pharmaceutical application so any changes would require validation/QC/QA involvement.

We have close to a hundred engineers here, mech. EE, chem, process, etc. but not one with refrigeration experience.

[chemi-cool]

Hi Hank,

The system work with flooded evaporator, I don't know how R508B behaves with oil.

If the evaporator contains all the missing oil from the compressor, then cooling capacity is lower.

It looks to me that there is a problem with original design and I understand you being worried about QA and QC.

I would suggest that you contact DuPont to solve the problem of oil return to the compressor. They should have the answer.

Chemi

[Hank Hill]

We've done some research (from both info posted on the board and other sources) and we think the main problem is low velocity of the discharge gas. These skids are under low load conditions the majority of the time (discharge pressures of about 150 psi). The helical are dependent on high velocity of the discharge gas to function efficiently. We've talked to some people at Temprite about their coalescent oil separators and they sound very promising for fixing our oil migration problems and possibly even increasing our capacity. Now if we can just convince management to try one! Will keep everyone posted on our experiences but it may be awhile as things move rather slowly around here. Thanks for everything. H.H.

[chemi-cool]

Hi Hank,

Please let us know how you progress, it will be of high interest for some.

Good luck.

Chemi

[Peter_1]

If low discharge velocity is the problem, why then not make an artificial load with bypassing - controlled of course - some hot discharged gas back into the suction line and cool it then again with refrigerant injection.
Is relative simple to do.
Every brand has a special valve for this purpose. Not very energy efficient but if it solves your problem, who cares.

[Hank Hill]

We have HG bypass and liquid injection, maybe we should have mentioned this earlier, but that is one reason we've been so shocked to see the oil keep leaving.

Like we said this pharmaceutical production, IV products, 10K to 14K bottles at once, around 2 mil per cycle, and we have very little "extra" capacity with our cooling, so we are worried about fouling the evap.

The problem is almost definetly with original design, we've added 6 litres or so of oil and still running on edge, no oil leaks so that oil is some darn where.

[Peter_1]

You're using the ***** HE from bottom to top.
If you look at www.aia.se , you will see that they recommend it the opposite way.
I think that the oil remains in the HE and that the compressors sucks only the evaporating gas whereby the oil remains in the compressor.
This is +/- the same problem occuring in a flooded evaporator.
Be carefull that the added oil not sudden comes back all at once to the compressor destroying the pistons.

[D. Sergius]

Hank,

Did you pre-charge the separator with oil when it was being installed? This is a common over-sight in oil separator installations.

I have worked on lots of cascade systems and have used 508 on a few occasions.

Helical separators are generally pretty good, but as you know they are not 100% eff. in separating oil.

At -80 F. evap. temp. oil gets pretty thick.
I bet this system operates continuosly, and the evap. never gets a chance to warm up and allow the oil to flow back.

Maybe you customer needs to know this.

[Andy]

Hi Hank
Helical oil separation depends on inpingment, velocity is important. If the compressor unloads you may end up lower than the minimun velocity.
Having said that any new system will need oil added to wet the system, 6 litres seems excessive.

What about the suction superheat, you wouldn't have liquid carryover by any chance, this would cause the oil to carryover at a high rate. Look out for low discharge temperature, foaming oil in the sump and low suction superheats
Kind Regards. Andy

[phil68]

Hi Hank
Helical oil separation depends on inpingment, velocity is important. If the compressor unloads you may end up lower than the minimun velocity.
Having said that any new system will need oil added to wet the system, 6 litres seems excessive.

What about the suction superheat, you wouldn't have liquid carryover by any chance, this would cause the oil to carryover at a high rate. Look out for low discharge temperature, foaming oil in the sump and low suction superheats
Kind Regards. Andy

Helical oil seperators use the swirling action of the discharge gas through a helix to promote seperation of the heavier oil droplets from the gas by centrifugal force, similar to a Dyson vacuum cleaner. The older type of oil seperator uses a system of baffles so that the oil droplets impinge on them & drop out of the gas Agree with the gas velocity bit, but the main problem with cascade systems is oil migration to the evaporator because of the oil becoming too viscous at the extreme low temps.
Some environmental test chamber manufacturers add pentane to the 2nd stage gas charge to keep the oil viscosity low at the lower temps., when I worked for Thermotron UK they had their own product called 'Thermotec' which we were sure was just a can of pentane with their brand name on it.
Working for other chamber manufacturers who didn't add pentane I would quite often be called to them tripped on their oil pressure switch. Checking the comp. oil sightglass would find it completely empty, but all that was needed was to ramp the chamber up to say +100 deg. C, hold it there for 10 mins. & then do a flat-out pull down to ambient. That would return all the oil back to the compressor. In the end I wrote a controller program to do this & instructed customers to run it once a week to get the oil back into the compressor.
On a similar note I was involved with a huge test chamber for the MOD that required -54 deg C for 1 month. It used 2 Bitzer 25 hp compressors on R22 & R23 & we had to fit an oil management system to stop oil starvation due to migration. The oil reservoir volumes were 10 times that of the compressor crankcase capacity, but incredibly the migration would consume that amount in the 1 month at -54.

[Andy]

Hi Hank
If I was having these problems I would check the basics first, superheat, subcool ect. You mentioned fouling of the evaporator, so it is possible that you have liquid carryover

Failing that, change the oil to say a polyalphaolifin PAO and fit a coalescent oil filter after the helical oil filter, dominic hunter or such like.

Kind Regards. Andy

[phil68]

Hi Hank
If I was having these problems I would check the basics first, superheat, subcool ect. You mentioned fouling of the evaporator, so it is possible that you have liquid carryover

Failing that, change the oil to say a polyalphaolifin PAO and fit a coalescent oil filter after the helical oil filter, dominic hunter or such like.

Kind Regards. Andy

We're talking about the 2nd stage of a cascade system here, where condensing temps. are in the order of -40deg.C, so subcooling isn't an issue. Or liquid carry over for that matter. Sorry Andy, but you're looking at this from the perspective of a normal fridge system

[dogma]

HFC - R508B

A blend of R-23 and R-116 intended for very-low-temperature refrigeration systems, typically the low stage of a multi-stage system. R-508B has properties similar to R-503 and can be used to replace either R-13 or R-503 in an existing system. The discharge temperature does not increase significantly compared to R-13, in contrast to the increase seen with R-23. POE lubricants should be used, and it may still be necessary to charge hydrocarbon oil additives to ensure proper oil return. Check with OEM for specific oil and charging recommendations.

[Andy]

We're talking about the 2nd stage of a cascade system here, where condensing temps. are in the order of -40deg.C, so subcooling isn't an issue. Or liquid carry over for that matter. Sorry Andy, but you're looking at this from the perspective of a normal fridge system

The normal rules of refrigeration have not been suspended. If you have too much subcooling the expansion valve/metering device will not operate correctly, too little and the discharge superheat will rise, effecting the oil separator operation. Too little suction superheat will allow droplets of refrigerant into the compressor oil causing oil scrub out.

This is a normal fridge system, vapour compression as I recall. If we get the basics correct the plant will run correctly, ignore the basics at our peril

Kind Regards. Andy

[Andy]

HFC - R508B

R-23. POE lubricants should be used, and it may still be necessary to charge hydrocarbon oil additives to ensure proper oil return. Check with OEM for specific oil and charging recommendations.

I suggested PAO oil, probably has to be some sort of POE oil, but I would check with the oil manufacturer about the PAO option as it will work with some HFC refrigerant.
Kind Regards. Andy

[dogma]

Hi Hank
change the oil to say a polyalphaolifin PAO

I am not familiar with this type of oil Andy. I just checked my notes from TAFE with no mention.

Will have to research. Cheers

http://www.gardnerdenver.com/GDCorpD...-P5-15_MAT.pdf

Well what do you know. there tis'.

[phil68]

The normal rules of refrigeration have not been suspended. If you have too much subcooling the expansion valve/metering device will not operate correctly, too little and the discharge superheat will rise, effecting the oil separator operation. Too little suction superheat will allow droplets of refrigerant into the compressor oil causing oil scrub out.

This is a normal fridge system, vapour compression as I recall. If we get the basics correct the plant will run correctly, ignore the basics at our peril

Kind Regards. Andy

Trust me, liquid carry-over doesn't happen on the 2nd stage of a cascade system, the liquid can't physically exist in the suction as it's super-super-heated by the time it reaches the compressor. The whole temp. range is moved -40 deg. C, condensing & evaporating, apart from the discharge which is much hotter & has to be passed through a de-superheater to bring the discharge down to say room temp. before being condensed at -40. The compressor windings rely on the temp. of the gas coming back, which is usaully in the same range of a normal fridge.

[Andy]

[QUOTE=phil68]Trust me, liquid carry-over doesn't happen on the 2nd stage of a cascade system QUOTE]
Less likely than a higher temp system, but still possible, especially with short suction pipe runs as in packaged units. usually during the initial pulldown phase when the evaporation pressure is up. Anyway even with superheated suction vapour liquid can be carried into the compressor, velocity will do this.
What is R508B an blend Blends have different refrigerants at differing boiling points, some parts of the blend may well be at a significantly higher evaporting temperatures at the system low stage evaporating pressure.
Kind Regards Andy

[phil68]

In 18 years of working on cascade systems I've never seen oil wash-out due to liquid carry-over, even with short suction pipe runs. Even with high velocities involved. 100% of oil problems have been due to migration due to the oil becoming overly viscous.
[QUOTE=Andy]

Trust me, liquid carry-over doesn't happen on the 2nd stage of a cascade system QUOTE]
Less likely than a higher temp system, but still possible, especially with short suction pipe runs as in packaged units. usually during the initial pulldown phase when the evaporation pressure is up. Anyway even with superheated suction vapour liquid can be carried into the compressor, velocity will do this.
What is R508B an blend Blends have different refrigerants at differing boiling points, some parts of the blend may well be at a significantly higher evaporting temperatures at the system low stage evaporating pressure.
Kind Regards Andy

[Peter_1]

We're talking about the 2nd stage of a cascade system here, where condensing temps. are in the order of -40deg.C, so subcooling isn't an issue. Or liquid carry over for that matter. Sorry Andy, but you're looking at this from the perspective of a normal fridge system

I have some experience with Meissner coils (R22/R23 and propane/ethylene systems) and autocascade
systems from Polycold in aluminium vaporising vacuum chambers.

What you're saying - if I understood you correct - is that the fact that you're condensing at -40°C that measuring subcool isn't possible/important?

Having R508 in a super-super-heated state - whatever this may be - says that liquid carry over isn't possible while the system is running?
It's not that difficult to let run a compressor in the first stage with some liquid droplets in the suction gasses.
Explain once why you think it's super-super heated.
And it depends also what type of expansion device is used in the 1st stage.

These measurements andy mentioned are basic refrigeration rules, valid for any installation and has nothing to do with the fact that you have a cascade system or an autocascade.

I think you don't know what systems Andy is working on.

1. We don't have oilseparators at all in these machines. You don't need them if the system and lines are proper designed and installed.
2. Feeding evaporator from bottom to top is looking for problems, especially at these conditions.
3. Machines we service are filled with PAO oil.

[dogma]

What is R508B an blend Kind Regards Andy

All 500 blends are azeotropic.

[Mark Sanchez]

I would recommend looking at a coalescing seperator.
Bypassing too much hot gas may have adverse effects.

Mark

[S.M.Gokhale]

Hi Hank,

We used to have R23 in the low stage and standard separators (not the helical) 

Helical Oil separators need to be sized correctly to maintain discharge gas velocity for the oil separation to be effective.
All types of oil separators have to be sized based on the system capacity at the evaporating temperature.
As you have already used conventional OS successfully with R-23, I feel you could try and use a conventional Oil Separator for R-508B refrigerant also. However, the capacity of the system when converted from R-23 to R-508B would increase by about 30% -
Refer :http://refrigerants.dupont.com/Suva/...pdf/h60080.pdf

R-508B will provide 98% capacity as compared to 74% of R-23 (With reference to R-503 as 100%)
However this will also depend on the sizes of other equipments such as evaporator and condenser and the TEV selection.(There was a very long thread on 'System Balance' in the forum)

You can also refer : http://www.henrytech.com/techtips_pdfs/HT-TT4.pdf

for additional information on Oil Separators.

Now, in case you have the model no. of the old OS, you can find out the capacity of the same at maybe -40 Deg. C evaporating (No manufacturer provides ratings below this temperature).Now you have two options - Either instal OS with about 25 to 30% higher capacity - if such a model is available OR instal the same model.

I have used a "Danfoss' (conventional) oil separator for -70 Deg. C evaporating with R-404A and POE oil and it worked perfectly. However, Danfoss has only two models.
I am now trying out Henry OS for a new machine with 15 H.P. 'Bitzer' two stage compressor for same conditions.

All the Best
Shridhar

[Andy]

[QUOTE=Peter_1]
2. Feeding evaporator from bottom to top is looking for problems, especially at these conditions QUOTE]

Phil this is probably why you have never seen liquid carryover. The coil has been circuited as in a flooded coil precisely to stop liquid carryover. The only way liquid is coming out of this coil is if the system is over charged

Good thread anyway

Kind Regards. Andy

[phil68]

[QUOTE=Andy]

2. Feeding evaporator from bottom to top is looking for problems, especially at these conditions QUOTE]

Phil this is probably why you have never seen liquid carryover. The coil has been circuited as in a flooded coil precisely to stop liquid carryover. The only way liquid is coming out of this coil is if the system is over charged

Good thread anyway

Kind Regards. Andy

Yep, good debate

[Samuel Roh]

I'm cascade refrigeration engineer worked for all brands of environmental test chambers.
1. Adjust discharge pressure to 295psi while -100%cooling demands.
2. ***** should flow from top to bottom on the evaporator.
3. I've never tried, but want to recommend to try. Put the hot gas bypass into evaporator but injection should not be changed.
4. Add the Pentain little by little.
5. Check if Helical oil separator capacity is enough.
Advise me compressor model, oil separator model.

Busy. I'll talk again with your further comments.

[chefid]

pentane would be a good place to start

[alex242]

Working for Thermotron adding THERMOTEC "Pentain" would be a great place to start.
Never ever had a problem.

[Emmett]

Hank,
Did you ever resolve your problem? if so how. Having similar issue with new equipment start up.

[Peter_1]

As happens so many times here, Hank posted twice a problem, members helped him from all over the world but then afterwards a simple thank you or a last post with the solution so that we all can learn....
I call this good manners which he apparently lacks.

[Gary]

I realize that this is an old thread, but as a point of interest the importance of an oil carrying agent such as R12, pentane, etc. should be stressed.

If the oil separator catches 99% of the oil that means 1% is going to the evap where it will try to stay. Given long enough, all of the oil ends up in the evap. If the separator catches 99.9%, it just takes longer for it all to go to the evap.

Cascades need an oil carrying agent, regardless of the separator.

[Emmett]

Gary,
I have yet to see where anyone has diffenitively stated that they have used pentane or R-12 for that matter to resolve oil return issues when using POE oil, I contacted Mobil Artic and their reply was that as far as they know Hydrocarbons are not compatable with their product. I am not sure where to go from here, it seems that I am spending more time and energy on resolving this issue then the manufacturer. If you are aware of anyone who has added pentane to a POE system I would sure be interested in what the results were, esspecially if they have had the system for awhile.

[cool101530]

I agree w/Gary. On a cascade Ultra-low freezer, (-80C) another refrigerant is added to keep the head up, and velocity. 404A seems to be popular with the Thermo brand.

[NoNickName]

Pentane is commonly added as carrying agent, and at Mobil's probably they just don't have an idea of what you're talking about.

[grizwold50]

hi hank, can you tell me what make and manufacturer is then i can tell what oil separtor you need i always use an temprite 900 series oil separtor they are proven makes and work very well. silly question but have you put an oil additive in the system with the r508 like r290 or r170 or r404a. but please get back to me with the manufacturer.

kindest regards graham

[Gary]

There is no oil separator that catches 100% of the oil. No matter how good the oil separator, some tiny amount always gets through and travels to the evaporator. If it stays in the evaporator then it will be added to, a tiny amount at a time, until the evaporator is oil logged. That's why an oil carrying agent is needed.

[grizwold50]

hi, i would recommend use try using r290 pure propane as an oil additive as i use this in ult freezers.

kind regards graham

[jcook1982]

I think it is important to know also what type of metering device is he using?
I would definitely want to add propane if using POE. But with 508 it is possible to switch to AB and use Pentane. The pentane works best with HFC refrigerants like 508 and AB oil. Give it a whirl....

[jcook1982]

Just so you know hank i work almost exclusively with 508 refrigeration system...

[ECAB24]

We have had good success with pentane returning oil on R-508B systems. I work for an environmental chamber manufacturer.