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Thread: Two stage system troubleshooting
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18-12-2010, 08:06 PM #1
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Cascade system troubleshooting
I have been involved recently in one project (as observer and if I could help with practical advice) which is pretty much undiscovered field for me.
Project is to make CASCADE cooling for freezer of medical samples to temperature of -80°C (if possible) with two Danfoss compressors. Details and diagram is in this PDF document.
What we achieved so far is lowest temperature of -65°C in box measured with (i think) Eliwell thermostat and probe.
Now we have some problem and it manifest itself after some time temperature is around -45 to-50°C and than without obvious cause goes up to -25°C and stay there with heavily frosting on low stage compressor.
We contacted constructor of equipment and he said that problem may be in too short capillary.
Also we switched from original R507 refrigerant in high stage to R404 (it was unavailable to us and it will be for few next days).
Constructor also said that R404A has some problem with component separation at that low evaporation temperatures and that also could contribute to our problem.
We also wait for digital gauges for measurements so do not ask me now for detail numbers since they are not availiable.
Refco Digimon is useless in negative relative pressures and I waiting for Monday evening to get hold on Digicool to make accurate measurements.
What is your taught on design and experienced problems on this mater.
For downloading schematic and details, ask me for password on Private Message.
I am interested what is happening with oil on that temperatures and capillary tube diameter in relation to oil viscosity.
Also, does R410 have enough oil carrying capacity at that temperatures and if it need some addition (like propane or something else)to transport oil properly.
Please keep details and drawing private as this is not published or patented yet. Access to details and drawing will be available only to good known members of this great community.
Cheers!Last edited by nike123; 19-12-2010 at 08:12 AM. Reason: Wrong name for system type
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18-12-2010, 09:02 PM #2
Re: Two stage system troubleshooting
to go to -80°C, is it not better to work with ethyleen and propyleen?
To have this temp you will go to -90°C evaporatortemp.
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18-12-2010, 09:56 PM #3
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Re: Two stage system troubleshooting
This is direct expansion system like this one.
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19-12-2010, 12:58 AM #4
Re: Two stage system troubleshooting
Not to much detail.
Have check the cap sizes, have they been sized for split load, not each for full load? (each either to short or to big a diameter)
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19-12-2010, 06:03 AM #5
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Re: Two stage system troubleshooting
That is all what i have! 0 in TXV designation mean orifice size is 0.
If I understand corecty your question, box is made with two identical evaporators of pasive type like in regular fridge (behind inner aluminum wall embeded in PUR) working together and capilary is sized for split load.
What sould be designed superheat of low stage?
Also what superheat I should expect at -50°C?
Also, on high stage, how mutch should liquid be subcooled in liquid subcooler in order to low stage have normal operation?
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21-12-2010, 07:11 AM #6
Re: Two stage system troubleshooting
Also we switched from original R507 refrigerant in high stage to R404 (it was unavailable to us and it will be for few next days).
Constructor also said that R404A has some problem with component separation at that low evaporation temperatures and that also could contribute to our problem.
Typical refrigerants for the high-temperature circuit include R-22, ammonia, R-507, and R-404a.
What's refrigerant on low stage system?
In some places will have to think ...
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21-12-2010, 07:29 AM #7
Re: Two stage system troubleshooting
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21-12-2010, 08:15 AM #8
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21-12-2010, 08:25 AM #9
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21-12-2010, 09:05 AM #10
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21-12-2010, 09:22 AM #11
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21-12-2010, 09:27 AM #12
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21-12-2010, 09:29 AM #13
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Re: Two stage system troubleshooting
Could you explain reasoning?
Becaose of this?
Therefore, when the compressor stops, the condenser empties into the evaporator, which fills with liquid.
If a liquid receiver is introduced, its contents could migrate into the evaporator when the compressor stops and fill it completely, especially if the evaporator is 'cold'. There would then be extensive liquid slugging when the compressor starts up.
A liquid receiver is therefore never installed in a system fitted with a capillary expansion device.Last edited by nike123; 21-12-2010 at 09:34 AM.
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21-12-2010, 09:46 AM #14
Re: Two stage system troubleshooting
A cap system relies on genuine sub cooling during the process, a reciever which is fed from a free draining condensor does NOT have liquid sub cooling (excluding the liquid head in the reciever).
Also the cap pressure drop is reliant on the flow, pressure drop and sub cooling, the reciever muffles the balance, as the working conditions change.
(stated in practical sense)
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21-12-2010, 09:47 AM #15
Re: Two stage system troubleshooting
And that as well.
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21-12-2010, 09:54 AM #16
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Re: Two stage system troubleshooting
Thanks mad fridgie!
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21-12-2010, 09:57 AM #17
Re: Two stage system troubleshooting
Pleased i could help, and thanks for all the assistance that you give to others, on the this furom
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21-12-2010, 10:30 AM #18
Re: Two stage system troubleshooting
It's very funny, evaporating pressure below than atmosphere pressure...it's wrong.
A cap system relies on genuine sub cooling during the process, a reciever which is fed from a free draining condensor does NOT have liquid sub cooling (excluding the liquid head in the reciever).
http://www.sporlanonline.com/Februar...Cold%20WAR.pdf
Could you explain reasoning?
Becaose of this?In some places will have to think ...
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21-12-2010, 10:37 AM #19
Re: Two stage system troubleshooting
Acording to ASHRAE 2006 (Ultralow-Temperature) in cascade systems:
For the low-temperature circuit, a high-pressure refrigerant
with a high vapor density (even at low temperatures) is chosen. For many years, R-503, an azeotropic mixture of R-13 and R-23, was a popular choice, but R-503 is no longer available because R-13 is an ozone-depleting chlorofluorocarbon (CFC). R-23 could be and has been used alone, but R-508b, an azeotrope of R-23 and R-116, as superior properties, as discussed in the section on Refrigerants for Low-Temperature Circuit.In some places will have to think ...
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21-12-2010, 10:49 AM #20
Re: Two stage system troubleshooting
That article puts in a condensed fashion the arguments for subcooling in receivers, including free-draining receivers, that I have been pushing for years on these discussion forums - especially alt.hvac and sci.eng.heat-vent-ac - Andy participated in the some of them. I wonder if it is a coincidence that he calls the engineer in that article "Mark"?
Only the dogmatist says he will never change his mind. We all know that some of our opinions are wrong but none of us know which they are for if we did then they just wouldn't be our opinions. - JS Mill.
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21-12-2010, 10:51 AM #21
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Re: Two stage system troubleshooting
Last edited by nike123; 21-12-2010 at 12:22 PM.
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21-12-2010, 10:53 AM #22
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21-12-2010, 10:56 AM #23
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21-12-2010, 11:11 AM #24
Re: Two stage system troubleshooting
Last edited by Aik; 21-12-2010 at 01:02 PM.
In some places will have to think ...
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21-12-2010, 11:23 AM #25
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21-12-2010, 08:06 PM #26
Re: Two stage system troubleshooting
The general argument abot sub-cooling is more related to where you measure the pressure. The common method is to measure compressure discharge pressure, then measure liquid temp, this would give the impression of the amount of liquid sub cooling, what is shown in the article is that the discharge pressure and liquid pressure are the same, on free draining air cooled condensors and recievers, that there is likely to be a significant pressure drop, so the actual sub cooling is significantly less than thought. This can be seen with many of the "R4" based refrigerants, how many of you have struggled to get a clear sight glass, with glide small pressure changes allow for slight evaporation, with being able to recondense.
Also the type and style of the reciever is important, how does it flow, is the boundary affected.
This can be a very complex area.
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21-12-2010, 09:00 PM #27
Re: Two stage system troubleshooting
If we look at the drawing in the above attachment,
we can see a number flaws with the articles argument.
1; The sub cooled liquid from the condensor pass direct through the saturated vapour, "what an excellent energy transfer method" the liwuid would warm and the vapour would condense.
2; If the thermal length is too short for the above to occur fully, the liquid stream will cause a wave/ripple on the top of the liquid held within the reciever, breaking the surface boundary layer, enhanceing the heat transfer between the liquid and the vapour, whilst also reducing the natural stratification.
3; Even at this point, assume the liquid is highly sub cooled (which i believe it would not), the sub cooled liquid has to pass through the body of the liquid in the reciever, which at the top is at saturation, mixing and temperature equalibrium will occur.
Hence little actual sub cooling at the reciever outlet. (as stated in my earlier post about pressure drop)
In an ideal world, your reciever would be vertical with a reasonable head of liquid. The sub cooled liquid would enter the bottom of the reciever and would be sized that no mixing occurs. The naturally stratisfied liquid/vaour would insulate the sub cooled liquid from picking up large amounts of energy from the vapour. And you then would indeed have genuine sub cooled liquid leaving the reciever
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21-12-2010, 09:06 PM #28
Re: Two stage system troubleshooting
Only the dogmatist says he will never change his mind. We all know that some of our opinions are wrong but none of us know which they are for if we did then they just wouldn't be our opinions. - JS Mill.
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21-12-2010, 09:16 PM #29
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21-12-2010, 09:54 PM #30
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Re: Two stage system troubleshooting
Mark and Mad, I love to see your argumentations!
Especially if they are in connection with my system problems.Last edited by nike123; 21-12-2010 at 10:02 PM.
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21-12-2010, 09:58 PM #31
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21-12-2010, 10:01 PM #32
Re: Two stage system troubleshooting
Does the low stage have an oil separator? What kind of oil? Does it have some oil carrying fluid (propane, pentane, R12, etc.) added?
Generally, as a substitute for low stage subcooling, I measure the interstage temp difference (low stage SCT minus high stage SST).
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21-12-2010, 10:14 PM #33
Re: Two stage system troubleshooting
I think your check valve, might be on your second stage compressor discharge, to stop migration into your comp head.
"Have you not read what Nike wrote?" "Well I thought I did" "Read again" 'I have and I see your point, The check valve is between the reciever and condenser"
How ever this would not stop migration to the evap, and would reduce ever so slightly migration to the head.
Mad and Mad (having an argument)Last edited by mad fridgie; 22-12-2010 at 12:44 AM.
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21-12-2010, 11:02 PM #34
Re: Two stage system troubleshooting
normally r290 or r600 for cascade units to reach -80
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22-12-2010, 06:53 AM #35
Re: Two stage system troubleshooting
From the physical point of view I totally agree with you, but this article has other opinion and I don't undestand it's explanation...I don't know where is the truth...
I think your arguments are good arguments for your position. But I believe I have better arguments for my position. I have added the issue to the queue in my list of future articlesIn some places will have to think ...
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22-12-2010, 07:03 AM #36
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22-12-2010, 06:24 PM #37
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22-12-2010, 06:32 PM #38
Re: Cascade system troubleshooting
This would seem to indicate oil logging problems, oil coating and insulating the inner surfaces of the evaporator. If the evaporator is insulated, then we might expect floodback to the compressor. Unless/until the oil return problem is resolved, other problems cannot be properly identified.
Last edited by Gary; 22-12-2010 at 06:46 PM.
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22-12-2010, 06:56 PM #39
Re: Two stage system troubleshooting
Even the best of oil separators will allow some tiny percentage to pass through. If that oil remains in the evaporator, then over a period of time sufficient oil will accumulate to cause logging problems. For this reason some small amount of oil carrying fluid must be added to ensure oil return, even with a good separator.
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22-12-2010, 08:45 PM #40
Re: Two stage system troubleshooting
If we look at the article, and having an open mind, I will attempt to explain what they mean.
Where liquid and vapour meet "boundry layer" we have some surface tension "imagine a thin layer of another material" the surface is a bit stronger than the liquid below and the vapour above. So we need a liitle move force (breaking the layer) to get transfer of energy.
Also this layer attracts non moving particles of both liquid and vapour refrigerant. The combination acts as an insulator (same as insulation in cold room) and aids stratification
Hence we could have quite a big difference in temperature between the bottom of the reciever and the vapour at the top. In the example I do not believe this happens. It is upto you to decide which will actually happen.
Being practical, I would suspect that both are possible and/or a bit of both, All I say is that if you want to know genuine sub cooling, you measure your liquid pressure at the point you measure your temperature, and do not rely on your compressor discharge pressure as being the liquid pressure.
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22-12-2010, 08:57 PM #41
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22-12-2010, 09:01 PM #42
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22-12-2010, 09:04 PM #43
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22-12-2010, 09:29 PM #44
Re: Two stage system troubleshooting
I have come across very low footprint air cooled water chillers - also very noisy - that have a condenser refrigerant side pressure drop - at full load - of 1.2bar. That's like say 17 bar at the compressor discharge and 15.8 at the condenser outlet. Talking R22 here you can see easily the degree to which an apparent subcool could mistaken a young tech measuring pressure at the compressor discharge.
On such chillers the best place to connect the fan speed control pressure reference is on the liquid line. Not just because ultimately that is the pressure we are trying to maintain by controlling the fan speeds - the TEV capacity maintaining liquid line pressure - but because if controlled via the discharge then when the compressor unloads to say 25% the condenser pressure drop will have dropped to 1/16th of 1.2bar meaning that to maintain the discharge pressure at the compressor the fans would have to slow down enough to raise the liquid line by 1.125bar as a consequence. Thus the available TEV pressure drop would have increased at the time when we do not need as much capacity out of it and would prefer it to have reduced in capacity so as to have better TEV low superheat stability.Last edited by DTLarca; 22-12-2010 at 09:32 PM.
Only the dogmatist says he will never change his mind. We all know that some of our opinions are wrong but none of us know which they are for if we did then they just wouldn't be our opinions. - JS Mill.
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22-12-2010, 09:34 PM #45
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22-12-2010, 09:46 PM #46
Re: Two stage system troubleshooting
It's not related - the moderator should package all these off topic posts and move them into a newly created thread with a matching title. This is rarely done - at least since 1995 this is how it happens - people come along and start discussions within discussions - I tried to prevent such occurrences myself back in 1995 and 1996 but gave up in the end.
If your compressors and cap-tubes are right then your problem is as Gary says - oil type and return provisions. This very topic has been discussed before either here on this discussion board or on one of the old Usenet groups - I forget where - but changing the oil type solved the problem.
I do want to discuss the principle of subcool occurrence with Mad Fridge - but I will start another thread for that - good luckOnly the dogmatist says he will never change his mind. We all know that some of our opinions are wrong but none of us know which they are for if we did then they just wouldn't be our opinions. - JS Mill.
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22-12-2010, 09:54 PM #47
Re: Two stage system troubleshooting
It would seem that you are between a rock and a hard place, on this project.
When doing this type of R&D, it is the quantity and quality of data that you require (more the better) measure pressure and temp at each point in the system.
To start with do not worry about the end result.
Get the system to balance, and know your upper and lower limits.
Only make "one" change at a time.
As gary has clearly stated Oil will become a problem, but for R&D purposes can be flushed from the system after each system modification.
Use base principles,
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22-12-2010, 10:17 PM #48
Re: Two stage system troubleshooting
Nike, add a little bit R134a into the system to rinse the cap tubes. The R134a remains liquid and flushes it.
Why R410a for the LP?It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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22-12-2010, 10:28 PM #49
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22-12-2010, 10:32 PM #50
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