View Full Version : Sight glass location

22-09-2001, 12:20 PM
Recently there was a discussion(arguement) among our techs as to the correct location of the liquid line sight glass.Some say it should be before,some say after, others say it does not matter if its before or after the drier. I would like know what other techs think on this subject.

Prof Sporlan
22-09-2001, 02:57 PM
Installing sight glass downstream of the liquid line filter-drier:

Pro: one can see the condition of the refrigerant as it enters the expansion device. If the expansion device is starving, checking for the presence of flash gas ahead of it is normally one of the first things to consider.

Con: if the sight glass shows flash gas, one does not know if it is the result of a low refrigerant charge, or a restricted filter-drier.

Installing tight glass upstream of the liquid line filter-drier:

Pro: if flash gas is present in the sight glass, it will not be the result of a restricted filter-drier. One can have a high confidence level that the system has a low refrigerant charge.

Con: one cannot see the condition of the refrigerant as it enters the expansion device. If the evaporator is starving, and the sight glass is clear, one can only speculate the filter-drier is restricted. One can sometimes detect a restricted filter-drier by measuring a temperature drop across it. But this technique is often not reliable. Significant pressure drop can occur across a liquid line filter-drier at low delta T.

Of course, the Prof has a solution the above debate: install two sight glasses.... one upstream and one downstream of the filter-drier.... :p

Assuming one didn't want to go thru the trouble and expense of installing two sight glasses, the Prof opts for placing it downstream of the filter-drier. If a system is not properly cooling, it is important for the technician to determine if the expansion device is operating properly. And that can be more easily accomplished by knowing the condition of the refrigerant entering it.

23-09-2001, 07:03 PM
Hey Prof's, thanks for the response. I am going to share this info with the rest of the techs. Hope it doesn't start another "debate" :p

11-12-2001, 10:01 PM
All the replys are good ones but I would be aware that they only relate to large airhandlers. On mini splits expect to see bubbles in the sight glass otherwise the system is probably overcharged.

Me I would probably purt the site glass before the filter drier but I think we can all agree before or after they are both valid.

18-04-2002, 11:16 PM
I always install the sight glass downstream of the
filter drier. I like to see the condition of the liquid going to the TXV. If there is flash gas present and a plugged drier is the cause, you will usually be able to detect a noticable temperature
drop from the inlet to the outlet.

21-04-2002, 09:35 PM
Your discussion was very usefull .
But I want to know if it is necessary installing a flat sight glass at the middle of receivers body .and what level of liquid is satisfied in it (at normal operation)?

23-05-2002, 09:26 PM
I say a sightglass has one real use: as a moisture indicator. You should know the moisture content after the drier. Plus what the prof said able the valve getting a solid column of liquid.

23-05-2002, 09:42 PM
Hi,Pirestanti, I think you are asking about the liquid level in a liquid receiver. The glass shows a level from 10% to 80% aproximately. Define normal operation, if you are feeding one source (valve or vessel) or muliple. On a receiver feeding one device the level will drop at peak loading(due to a shortage or an inadequate receiver size) or when the evaporator is at it's lowest temperature. A range of levels is permissible for normal operation.
Regards. Andy.