PDA

View Full Version : charging a system



shaun26
22-03-2008, 11:29 PM
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:confused::confused::confused:

cheers
shaun

The MG Pony
22-03-2008, 11:36 PM
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.

shaun26
22-03-2008, 11:40 PM
hi mg pony, thanks for reply:)


i took the pressure and temp just before the txv

The MG Pony
23-03-2008, 02:29 AM
Sounds like the correct procedure to me.

SteinarN
23-03-2008, 08:04 AM
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.

nike123
23-03-2008, 08:59 AM
It is important to charge to certain minimum amount of subcooling when you experience highest load on evaporator and highest outdoor (condensation) temperature.

shaun26
23-03-2008, 12:36 PM
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 :o will have to do some reading into it i think.

thanks again for the advice, time to pay google a visit:D:D

shaun

philfridge
23-03-2008, 01:36 PM
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 :rolleyes: . 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

BigJon3475
23-03-2008, 03:27 PM
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?

shaun26
23-03-2008, 07:28 PM
thanks bigjon and phil and all who have replied, will certainly be using your advice in the future,very helpful :D:D:D

Gary
23-03-2008, 07:35 PM
hi mg pony, thanks for reply:)


i took the pressure and temp just before the txv

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.

shaun26
23-03-2008, 11:21 PM
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 ???

Gary
24-03-2008, 01:21 PM
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 ???

No, that isn't necessary.

praveen
24-03-2008, 01:37 PM
sight glass should clear at full load.that's the final refrigerant top up

shaun26
24-03-2008, 02:58 PM
ok guys, thanks for all the advice :D:D:D