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10-09-2008, 04:50 AM #1
Minimum vacum pressure for refrigeration system
Dear All,
I want to know the standard vacuum pressure for refrigeration system, as better you can give what standard reference (DIN, ISO,.etc). Because we’ve ever experience fracture U-bend tube in evaporator application in -25 deg C (in connection between U-bend and straight tube). Our prediction, It is caused there is excessive water content inside tube. I think there is minimum recommended vacuum pressure for system. In my opinion, minimum vacuum pressure have correlation with permissible water content inside system.
Please your advice
Best Regards
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10-09-2008, 09:55 AM #2
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Re: Minimum vacum pressure for refrigeration system
Check this thread!
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10-09-2008, 10:13 AM #3
Re: Minimum vacum pressure for refrigeration system
Hi Bro
More detail required for a good answer. Like system pressures and temperatures, which water, and in what pipe?, application?
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10-09-2008, 10:23 AM #4
Re: Minimum vacum pressure for refrigeration system
Hello Nike123. Thanks for your information. What standard does it refer to ? Iso, DIN, or...etc ?
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10-09-2008, 11:51 AM #5
Re: Minimum vacum pressure for refrigeration system
I'd say 500 microns at least. I have installed refrigeration equipment which specifies in the warranty/installation that 500 microns is required so that's my only reference.
With a freezer I'd at least want to get it around there, preferably 250 microns. But 500microns with nitrogen purges would be the minimum.Last edited by paul_h; 10-09-2008 at 11:58 AM.
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10-09-2008, 12:50 PM #6
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Re: Minimum vacum pressure for refrigeration system
I am not aware of any standard addressing that matter.
Maybe someone will come up with some standard. Until then i am only aware of this CPS recommendation.
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11-09-2008, 06:33 AM #7
Re: Minimum vacum pressure for refrigeration system
EN378 states minimum 290 or 295 microns. something like that.
Anyhow, you must go below vapor pressure of the water at the temperature you will use the refrigerant.
But if tubes cracks, I doubt that this is due to water in the refrigerant. You then lust have a lot of water.
I should search more for material stress.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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11-09-2008, 07:58 AM #8
Re: Minimum vacum pressure for refrigeration system
Hi All,
I've tested in laboratory, the water content in oil until 1000 ppm water. I think maximum recomended is only 50 ppm water. I can't measure water content in refrigerant. But in my opinion water content in oil can represent this case. FYI, this unit is applied in room temp -40C using R-22, so evap temp about -45C (below atm pressure). Almost ten units get same problems. Please see attached file. Is there anybody that experienced like this case ?
Peter, do you have the copy of EN378?
Thanks
Best Regards
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11-09-2008, 08:28 AM #9
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Re: Minimum vacum pressure for refrigeration system
I have worked once at construction (only automation part) of one stage unit (blast freezer) working with R22 and -45°C suction temperatures. From what I witnessed and learned then, I could tell that is very challenging for construction, because there is low refrigerant amount to cool the compressor at that evaporation temperature (low density) and he (compressor) must be mainly cooled with oil cooled in separate oil cooler. If cooling of compressor is not sufficient, then high discharge temperatures could lead to oil disintegration and forming of water in circuit. Therefore high water content maybe is not related to vacuuming of unit, it could be because of oil breakup.
In my case these compressors are Copeland screw type.
What is your discharge temperatures at that compressors?Last edited by nike123; 11-09-2008 at 08:41 AM.
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12-09-2008, 06:13 AM #10
Re: Minimum vacum pressure for refrigeration system
You have indeed a problem which is almost the same as this one http://www.refrigeration-engineer.co...ange+phenomena post 7
I think you have a material problem of the copper used in the system, perhaps attacked by something inside the system.Last edited by Peter_1; 12-09-2008 at 06:20 AM.
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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12-09-2008, 09:01 AM #11
Re: Minimum vacum pressure for refrigeration system
The weld looks overheated and oxidised. I have found pin prick leaks on bends before where too much stress is induced to the cu tube whilst forming the bend. Also @ -40 there would be grunty fans creating vibration which could fatigue the cu tube. Not sure how water in system could create these fractures. one last thing - if you evacuate the system too low the oil will boil and begin to deteriorate or break down, when this happens @ lo press or hi temp it can create water, acid and other nasties
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12-09-2008, 10:40 AM #12
Re: Minimum vacum pressure for refrigeration system
They seemed not overheated at all.
If you should solder it this way for your Belgium certification, you should fail.
Even for the Dutch STEK, you should fail.
We as examiners saw open every solder and the filling must be all over the gap. That's regulation.
If you don't see a brown/black color on the outside, then 100% sure it's not OK.
You can clearly see that the solder hasn't filled completely the inner gap.
Most solders I see on this picture are only glued - sorry - instead of soldered.
The lower tube left on the picture is a very bad soldering.
I think that some liquid was trapped between the gap and this liquid expanded and created this rupture.
Best you can do is cut this piece away, open it and look what you see inside it.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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17-09-2008, 07:32 PM #13
Re: Minimum vacum pressure for refrigeration system
in answer to the question of vacuum,it is not only the amount of vacuum pressure , but also the time of vacuuming.
you can reach the perfect vacuum at say 30minutes of pumping,but that dos not tell you that all the moisture or water is out and the tubes are clean.
in fact we should have hydro-stats on the pumps so we can tell if there is still water or moisture in the lines,and if then reached a acceptable point we can say the installation is vacuum.
Ice
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17-09-2008, 08:53 PM #14
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Re: Minimum vacum pressure for refrigeration system
If you look at graph I posted here you will know by pressure rise after vacuum pump isolation if there is still moisture in system.
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17-09-2008, 09:13 PM #15
Re: Minimum vacum pressure for refrigeration system
Reading this thread it becomes clear that Belgium/Netherlands seems to be having a very good standard in the quality of servicing refrigeration units!!
Indeed, the STEK in Holland and the exams in Belgium proof to be very good for the trade!!
About Vacuum:
Very simple, look at what the coldest surrounding temperature is of your system to be evacuated.
Look at the evaporation pressure of water on that coldest temperature at that moment (it will be harder in winter)
For example: @ 15,5 °C water evaporates at a pressure of 17,5 mbar (abs)
To make sure all moisture is gone, go at least 10 mbar deeper and keep it for at least a couple of hours.
I always try to achieve what the STEK writes: 2.7 mbar (270 Pa)
If you can (with vacuum pump valve closed ) stay under 10 mbar after 2 hours than you have:
1: A dry system
2: A leak tight system
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17-09-2008, 09:39 PM #16
Re: Minimum vacum pressure for refrigeration system
nice chart Nike123, there is some truth in this, but i am not convinced about the moisture in the system,sorry.
Ice
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17-09-2008, 09:58 PM #17
Re: Minimum vacum pressure for refrigeration system
Both nike 123 and icecube51 are right!
The chart of nike123 is made for a surrounding temperature of the system of around 25°C.
And as icecube51 said, take long enough time to reassure that pressure is levelling up and staying under the necessary vacuum pressure.
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17-09-2008, 10:06 PM #18
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Re: Minimum vacum pressure for refrigeration system
Convinced or not, that is simply matter of laws of physics.
If moisture is in system, it will evaporate at adequate pressure-temperature combination and raise pressure in system by that action. That will be visible by accurate vacuum meter (electronic).
No triple evacuation or vacuuming of few days, without checking that behavior of pressure in system is like that in graph, will not ensure that you really have dry system.
Triple evacuation could shorten vacuum drying. And, vacuum drying by time without checking pressure rise after isolation is not action governed by actual data. It is simple guessing.
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17-09-2008, 10:15 PM #19
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Re: Minimum vacum pressure for refrigeration system
Water at 25°C evaporates at 31,7 mbar. That is 27752 microns. If you consider pressure drop from vacuum pump to point where moisture is, it still has lot of low pressure to water boiling. This chart is for broad spectrum of surrounding temperatures.
If you have water in system, and it still evaporates after vacuum pump isolation, you will have evaporation pressure for that temperature of water (saturation pressure/temperature of water).
If you have leak, pressure will rise until it is equal with surrounding.
If you have acceptable moisture content for application of your system, pressure of system, after pump isolation, will rise to that acceptable level and stay at that level. Now you are sure that your system is dry enough.Last edited by nike123; 17-09-2008 at 10:37 PM.
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18-09-2008, 08:23 PM #20
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18-09-2008, 08:43 PM #21
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19-09-2008, 03:35 PM #22
Re: Minimum vacum pressure for refrigeration system
Hi,
maybe those articles can bring more light...
http://www.robinair.com/acsolutions/...m.htm#factors2
http://www.aircondition.com/tech/que...-to-Boil-Water
http://yarchive.net/ac/vacuum_pumps.html
Best regards, Josip
It's impossible to make anything foolproof because fools are so ingenious...
Don't ever underestimate the power of stupid people when they are in large groups.
Please, don't teach me how to be stupid....
No job is as important as to jeopardize the safety of you or those that you work with.
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20-09-2008, 03:16 PM #23
Re: Minimum vacum pressure for refrigeration system
Thanks Josip.
A pitty it is not in the terms we use in the BeNeLux.
We use mbar/Pa and Degrees celsuis.
Are there any charts with those units?
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20-09-2008, 04:01 PM #24
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20-09-2008, 04:11 PM #25
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20-09-2008, 04:28 PM #26
Re: Minimum vacum pressure for refrigeration system
KoelGoeroe see once
or http://en.wikipedia.org/wiki/Vapour_pressure_of_water
or http://fds-smv.googlecode.com/issues...rmulations.docIt'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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20-09-2008, 05:44 PM #27
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Re: Minimum vacum pressure for refrigeration system
So, it doesn't matter what is pump capacity, ambient temperature, vacuum pipe diameter and length, whether you removed valve core or not, amount of moisture in circuit, quality of oil in pump, you still make evacuation 2 hours per ton of capacity?
Isnt that prety much blind process?
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20-09-2008, 06:00 PM #28
Re: Minimum vacum pressure for refrigeration system
is it not so,the bigger the system,the bigger the tubes and so on ??
i have and old Alcatel vacuum pump whit a capacity of 125ltr / hour.
and i now very well how see sounds when see starts in vacuum. from that point on i let her go for 2 hours / ton.
never had any problems yet. still always interested in how someone dos it.
Ice
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20-09-2008, 06:39 PM #29
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20-09-2008, 06:52 PM #30
Re: Minimum vacum pressure for refrigeration system
Uiteraard zag ik dat je van België daarom ook mijn vraag van waar ergens in de Vloanders als ik vragen mag?
Het is zelden dat er hier een Belg arriveert alhoewel ik relatief veel publicteit maak voor dit forum.
Ice, for the new EU certification, you must also vacuum the system.
Every session I've noticed that peoples are surprised discovering that there's a huge difference between the so well known sound of a pump reaching almost it's lower vacuum level and the digital reading on a vacuummeter.
I too listened in the past to the sound of the pump and especially when closing and opening the manifold valves.
Anyhow, a digital vacuummeter is from now on one of the obligated tools you must have in your van if you want to be certified. Vincotte or Veritas will come to control this.
Did you know you have to be certified (theoretically) at least end October?
So I should say, do it the old way and crosscheck with the vacuummeter on a point as far as possible from the pump.
I often use it one new installs to see if the copper is lekafre before pressuring. The smallest leak makes that the pump levels at a high level, even with the smallest leak. This saves me nitrogen.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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20-09-2008, 06:56 PM #31
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Re: Minimum vacum pressure for refrigeration system
That is the way I was instructed by older technicians when I started in this trade, and it is always bothered me how I will know when I am done and is it done properly. That was my reason to by my first electronic vacuum gauge with my own money. Try buy one, they are cheap and reliable today and you will be amazed how many difference have these factors I told above.
Last edited by nike123; 20-09-2008 at 06:58 PM.
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20-09-2008, 06:57 PM #32
Re: Minimum vacum pressure for refrigeration system
Forgot to say, I mostly flush once with OFN and repeat the vacuumproces once again.
Like in the old days but we then flushed with refrigerant.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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21-09-2008, 01:58 PM #33
Re: Minimum vacum pressure for refrigeration system
ho Peter,
just started independence in march this year. still have to now allot of the necessary paperwork. but hurt of it. trying to figure out how to do this. and Nike123,the digital vacuum meter is on his way. my baby is getting old and i want to be sure see still doing a good job.may by no more like see is joust to,but see never let me down yet, so.
i am working on it.
Ice
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06-03-2009, 11:02 PM #34
Re: Minimum vacum pressure for refrigeration system
En Ice, reeds gecertificeerd?
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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