Deep vacum problems

Hi Guys don’t know if you are able to help me at all, I have built a three stage cascade which is running R507a in the first stage then R1150 in the second stage and R14 in the third stage compressor.

The compressors are 1st stage 24000BTU, second stage 17000BTU, third stage 17000BTU.

Oil Sep on both the second and third stage so there should be no oil getting past.

The problem I am having is the third stage is pulling a very deep vacuum of 20” with liquid line pressures of 15 bar when running, this is with a very short cap tube of 2.3mtrs of 0.036 cap tube, I have checked to make sure there are no blockages and have replaced cap tube many times now also with new filter driers, the third stage has always been vacuumed to 250 microns so there should be no moisture in the system at all, i just can not understand why this is happerning or what is going on.

If anyone can help I would be most grateful as I am pulling my hair out at the moment and cannot see why I have this problem.
When the system was first run up it ran fine for one day then the problem started with this deep vacuum
Pictures of the build

Wooooowwwwww, what a nice machine you have made!!
What's the application if I may ask?

What can happen is that you go that low where oil blocks the capillary, even if you use oil separators. There's always passing some oil through any separator, especially at the conditions you're working.

You could add some R134a in the R14 which will remain in a liquid state at these conditions. This R134a will flush the capillary. R134a is also known for its good rinsing capabilities. It's a common used practice in gas mixtures, used in autocascade.
How much you will have to use will be a trial and error. But I'm eager to know your results.

Another thing, you pulled till 250 microns but this isn't enough for the temperatures you're working. 250 microns is good for 'normal' refrigeration applications.
I don't have here a watervapor pressure chart with me but I should try to reach at least 40 microns and flush at least twice with real dry nitrogen. Install a good dryer.

Apart from this, my idea was to build a 3 stage as a school project during the evening classes I'm giving.
I now have a very good example.

I now see the application, CPU cooling.

Thanks for the info peter, yes the unit is for cooling a computer CPU for some extreme cooling, it is so much better to be able to switch on a unit rather than having to buy LN2 to keep things cool and hopefully when tuned corectly it should be able to hold around -130c with 250watts of load on the system so only the power bill to worry about L.O.L

Thanks again will give it a try.

Wow Very Shmick Installation!!

Well guys still having all the same problems as before, I have removed the filter drier as well now but this R14 refrigerant seems to be blocking the cap tube, the temperature starts to come down to around -130c then it slowly starts to rise again back up into the + fingers, there is very little cooling on the suction line and is ain a deep vacuum all the time with high pressures from the liquid line.
I have tried all that has been stated but still am having these problems, is there any way I can get over this problem or is this typical of the R14 refrigerant, would changing the cap tube I have from .036 to .042 help at all as the cap tube I have and the refrigerant used seems to just be blocking in the cap tube.

Any help would be gratefully welcomed.

It's not the refrigerant but the oil that blocks your capillary.
Refrigerant can't block, and especially not R14 at these temperatures.
Did you added already some R134a in it like I recommended?

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Originally Posted by Peter_1

It's not the refrigerant but the oil that blocks your capillary.
Refrigerant can't block, and especially not R14 at these temperatures.
Did you added already some R134a in it like I recommended?

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[FONT='Verdana','sans-serif']Hi there peter
Yes mate I added some R134a but still am having the same problems, I have even taken out the drier as well and re-vac and added but the cap is still blocking when the temp starts to come down.

Pulling my hair out now as I have seen the temps on this system and now it is playing up.

I have blown nitrogen though the system and there are no blockages at all, it is only when the temperature starts to come down that the cap tube is blocking.[/FONT]

I suggest you add some more R134a. The cooler it comes, the more the oil will become sticky.
Or you need an oil with a lower flock point (temperature at which wax crystals forms)
It certainly can't come from the refrigerant itself.
The moment you flush it with nitrogen, till then the wax crystals are already become liquid again and you don't notice a blockage. The blockage is only there when the refrigerant is really cold.

The pour point is the lowest temperature at which an oil will flow. This property is crucial for oils that must flow at low temperatures.

Quote:

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Originally Posted by Peter_1

I suggest you add some more R134a. The cooler it comes, the more the oil will become sticky.
Or you need an oil with a lower flock point (temperature at which wax crystals forms)
It certainly can't come from the refrigerant itself.
The moment you flush it with nitrogen, till then the wax crystals are already become liquid again and you don't notice a blockage. The blockage is only there when the refrigerant is really cold.

The pour point is the lowest temperature at which an oil will flow. This property is crucial for oils that must flow at low temperatures.

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Hi there
I have tried some more R134a but still having the same problems with oil freezing in the cap tube.

I have taken down some info as this might help to see what I have done.

The temperature of the liquid line before cap tube on the 3rd stage is -89c when the problems start.


The oil I am using is Emkarate Synthetic polyol ester RL 32H

I have applied heat to the cap tube with a heat gun, and the temp starts to drop again until the cap tube blocks again in a different place so it is an oil freezing problem I am seeing in the cap tube as you have said.

would putting in a larger diameter cap tube help this problem as it will have more space to flow, I would need to make it longer but don’t know if I would still have the same problem with oil freezing in a larger cap tube.

Thanks again for all your help.

Regards
Paul

Well guys recoverd the R14 today and charged with some R1150 in the third stage and everything is fine, no high pressures or deep vacuums so I don’t no if it is an oil problem as I am seeing -115c at the evap, it seems to be a problem with using R14 refrigerant that is causing high pressures and deep vacuum.

I thought I would let you guys know this as you may be able to pin point the problem that I am having with using the R14 refrigerant in the 3rd stage of this cascade.

thanks again for all the help guys I need it.

Whats the 2nd stage inlet evap temp?

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Originally Posted by Blaster

Whats the 2nd stage inlet evap temp?

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Hi there the suction temp from the second stage HX is around -95c

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Originally Posted by sacha35

Hi there the suction temp from the second stage HX is around -95c

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Suction temp -95, so inlet on HX will be a lot colder than that. Make the 2nd stage warmer will help with oil management problems on 3rd stage. R14 will condense at very low pressure below -100.
Also would cut cap tube to 1,7 mts of 0,9mm id cap tube.

Have you tried to switch off the 2nd stage compressor while running the 3rd? Try it if you havent tried it before, this will unblock the cap tube. Just make sure you watch the pressure gauges, also is a good way to test the pressure cut off...

Quote:

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Originally Posted by Blaster

Suction temp -95, so inlet on HX will be a lot colder than that. Make the 2nd stage warmer will help with oil management problems on 3rd stage. R14 will condense at very low pressure below -100.
Also would cut cap tube to 1,7 mts of 0,9mm id cap tube.

Have you tried to switch off the 2nd stage compressor while running the 3rd? Try it if you havent tried it before, this will unblock the cap tube. Just make sure you watch the pressure gauges, also is a good way to test the pressure cut off...

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Thanks for the info balster, i have tried what you said about running 3rd stage when second stage was at -40c and all works fine, eveap gets very cold nice and quick but as soon as the second stage hits -70c the cap tube blocks on the 3rd stage.

Will make the second stage warmer to around -80c on 2nd stage and try some 0,9 cap tube for the 3rd stage.

Thanks again for the info.

0.036'' cap tube is the same as 0,9mm i think :p, 2,3mts is not short for the compressors you are using :)

R14 is very harsh on oil management, you can also try adding 5% of a oil carrier, r290 or r600a will improve oil circulation

Hi all
Interesting topic and I might be a little out of my depth here. Could you not simply remove via refrigerated oil seperator? and I thought refrigerants in general contained a certain percentage of oil.

they do, but he's using the worst type for his low side, he really should be using zerol for his final stage with a bit R-290, and an Oil sep would be a good idea.

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Originally Posted by The MG Pony

they do, but he's using the worst type for his low side, he really should be using zerol for his final stage with a bit R-290, and an Oil sep would be a good idea.

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Hi Mate thanks for the info, i have changed the oil sep to a temrite 902, also have some Zerol 150 on order so just awaiting this to be deliverd, the problem is how to get all the old oil out of this compressor so that i can change it out with some Zerol 150?

hello i have the same proble with a similar equipment, i resolver drop the vacuum to 100 micron in the 3rd stage, the ? if how is very easy floch the sistem wiht R-11 if ni have using a "Electra" is a solver and good trap moisture, and using the prosedure triple evacuating using cleand nitogen, remember in this equipment the most litter particular of moisture if the big problems,conect a light bult 100 watt front the evaporator you remember me good luck........

just drain the oil out through the discharge of the compressor, set it up right again for a while then put it back upside down and run about 2-3psi N2 through it. compare oil out to the mfdata then add the zerol tot he discharge and add a tad to the accumulater.

Sorry I forgot one thing about evacuation and I'm not sure if it applies to your system. An oil can be dropped down to a certain vacume limit before it boils and then decomposes. So I guess the proceedure would be to flush out with solvent (refrigerant), evacuate to lowest, prove vacume, add little refrigerant or industrial dry nit to above minimum vacume for oil, add oil. Then re-evacuate using infra red light, and speaker or vibrator to break oil surface tension where moisture can escape to the minimum oil vacume point. Then as you do - charge the system.

Paul,

How much R134a did you add? It's one you would have to play with till it worked for you. However asyou said and other heating the cap tube made temps flow nicely again.

What about putting a heater on the cap, there is plenty of stuff that can be used. It's a random idea but it might work!

Other than that try a larger cap and shorter length, then add R134a if problem still there till results come through, mix with adding heat till you get a best then if it's working with the above you need a heater there!

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Originally Posted by pjalchemist

Paul,

How much R134a did you add? It's one you would have to play with till it worked for you. However asyou said and other heating the cap tube made temps flow nicely again.

What about putting a heater on the cap, there is plenty of stuff that can be used. It's a random idea but it might work!

Other than that try a larger cap and shorter length, then add R134a if problem still there till results come through, mix with adding heat till you get a best then if it's working with the above you need a heater there!

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You don't fix a problem by intentialy adding another one, we call that a hack job, and it is the badge of dishoner to the failed technition!

Seen a fair few cooling units in my time that needed heaters on the meetering stages to stop freeze over that have come out the factory like that!

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Originally Posted by pjalchemist

Seen a fair few cooling units in my time that needed heaters on the meetering stages to stop freeze over that have come out the factory like that!

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Then send it back with a giant "FAIL" stamp on the box!

Adding a heater is negating the whole concept of the goal of sub cooling the refrigerant and rejecting heat from it! might as well choke off the condensor rather then add a heater, same effect!

If it is freezing up there is an fundimental issue with the unit or the design of the unit that must be rectified properly! Does it take more work and thought and time? Yes but once resolved it is truely fixed, and equipment life will be better for it and the quality of repair superior!

In the time of low cost garbage, incompitant & half assed repairs (due to under-trained & overloaded techs) and throw away products, taking some time to do a therough and solid repair doesn't go un-noticed :)

Just some personal thoughts and observations.

thanks...:off topic:

I accept: the unit is for cooling a computer CPU for some extreme cooling, it is so much better to be able to switch on a unit rather than having to buy LN2 to keep things cool and hopefully when tuned corectly it should be able to hold around -130c with 250watts of load on the system.

why you don’t use oil free compressor it is better for you