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Thread: charging a system
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22-03-2008, 10:29 PM #1
charging a system
hi all
seeking advice please
i recently commisioned a small fridge unit (txv) and charged it to the subcooling, i noticed the sight glass was slightly bubbling but left it as thought system had been charged properly, problem is one of my technical supervisors checked the unit not long after and complained i had left it short of gas and that he had to charge it to full sight glass.
would just like to know if i did it wrong and should have charged to clear sight glass
cheers
shaun
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22-03-2008, 10:36 PM #2
Re: charging a system
this seems to be debatable depending on the technition, here they simply insist on filling to a clear glass.
Where did you messure your sub-cooling would be the prime question here.
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22-03-2008, 10:40 PM #3
Re: charging a system
hi mg pony, thanks for reply
i took the pressure and temp just before the txv
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23-03-2008, 01:29 AM #4
Re: charging a system
Sounds like the correct procedure to me.
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23-03-2008, 07:04 AM #5
Re: charging a system
Whether you have filled in the correct charge of refrigerant depends on several things.
Has the system a receiver? If it has, then a slightly bubbling sightglas may indicate a slightly undercharged system, but not so much it could influence the cooling capasityat any measurable amount.
When I'm charging a smal system with a TEV and a receiver which hasn't any liquid level sight glass in the bottom, I pay close attention to the changing or reduction of bubbling in the sight glass as the system nears full charge. I will continue to charge gradually as long as I see the amount of bubbling decreased by every little additional charge I put into the system, and at the end acheive a clear sight glass or only an occasional bubble. If I, on the other hand, at stable running conditions and observing a slightly bubbling sight glass, and then adding 10 to 20% additional refrigerant to the system without observing any reduction in the amount of bubbling, then I would assume a slight restriction causing a slight pressure drop between the receiver outlet and the sight glass to be responsible for the bubbling. And I would assume the system to be sufficiently charged.
If the system don't have a receiver and absolutely no non-condensables in it, then it should be possible to get a clear sight glass also. But if the TEV is hunting ever so slightly, then you could observe a clear sight glass for some time, lets say one minute, followed by a slightly bubbling sight glas for anothe minute and so on when correctly charged.
If it is non condensables in the system, then it would be very hard to get rid of the bubbling in a non receiver system. Also pay attention to the condensing pressure. If it starts to increase considerably as you charge the last refrigerant into the system, then it is a sign of overfilled condenser in a non receiver system.
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23-03-2008, 07:59 AM #6
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Re: charging a system
It is important to charge to certain minimum amount of subcooling when you experience highest load on evaporator and highest outdoor (condensation) temperature.
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23-03-2008, 11:36 AM #7
Re: charging a system
hi guys
thanks a lot for replies, helped loads
the system does have a reviever on it, but guess that correctly charging a system depends on a lot more than just subcooling will have to do some reading into it i think.
thanks again for the advice, time to pay google a visit
shaun
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23-03-2008, 12:36 PM #8
Re: charging a system
If the fridge gets to temp ok a few bubbles in sight glass is not a problem , remember the sight glass is a visual aid and your technical guy should have explained this to you if indeed he knows himself . As its not always correct in saying a full sightglass needs to be attained every time a system is gassed up . Depend on who last messed about with the tev lol
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23-03-2008, 02:27 PM #9
Re: charging a system
The only thing that I find useful in a sight glass is when they come with a moisture indicator. Otherwise use sat. press./temps. and superheat and sub cooling/amp draw.....yes take all recordings every time....one tells you only one thing...taking both tells you the whole system....even with a TEV...how do you know the TEV is operating correctly without superheat also?
Last edited by BigJon3475; 23-03-2008 at 03:02 PM.
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23-03-2008, 06:28 PM #10
Re: charging a system
thanks bigjon and phil and all who have replied, will certainly be using your advice in the future,very helpful
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23-03-2008, 06:35 PM #11
Re: charging a system
The subcooling should be checked at the receiver outlet, not the TXV inlet. If the system has no receiver then the bottom of the condenser is the receiver, so you would check at the condenser outlet.
As BigJon indicated, both superheat and subcooling should be checked while charging.
Specifically, you are checking for low superheat and/or floodback. If a TXV is wide open, this could cause you to overcharge the system.
And if the compressor valves are broken, the subcooling is not going to come up and the sight glass is not going to clear.
Assuming the superheat is not low, add refrigerant until the receiver outlet subcooling is 15F/8.5K or the sight glass clears, whichever comes first. A clear sight glass usually comes first.Last edited by Gary; 23-03-2008 at 06:39 PM.
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23-03-2008, 10:21 PM #12
Re: charging a system
thanks gary
another thing, lets say you have a 4 fan condenser and 3 fans are controlled by hp switches, would you have to force all fans on whist unit under full load to get correct subcooling readings ???
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24-03-2008, 12:21 PM #13
Re: charging a system
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24-03-2008, 12:37 PM #14
Re: charging a system
sight glass should clear at full load.that's the final refrigerant top up
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24-03-2008, 01:58 PM #15
Re: charging a system
ok guys, thanks for all the advice
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