overcharge

[SL82]

Had a chat today with another mechanic about diagnosing an overcharge. My question is - Is it possible to have a flashing sight glass if an overcharge exists?

[Gary]

Had a chat today with another mechanic about diagnosing an overcharge. My question is - Is it possible to have a flashing sight glass if an overcharge exists?

Yes it is.

The total charge consists of high side charge plus low side charge.

If too much of the charge is in the low side and not enough in the high side, the sight glass will have flashing.

The sight glass only tells you about high side charge. It does not tell you about total charge.

Oh... and if the valves in the compressor are blown out, the sight glass is not going to clear no matter how much refrigerant you stick in there.

[Grizzly]

May I also add that a sight gas is absolutely useless as an charge indication.
Unless the system is at full load!
Then you may get a better indication of system charge.
But as Gary has already explained they are not foolproof!
Grizzly

[hyperion]

If the expansion valve has failed in the fully open position, then the sight glass will be indicating bubbles or not be full as a result of the perceived demands of the system. You can easily grossly overcharge a system in these conditions and still not get a clear sight glass. The sight glass should only be used as an indicator. Full diagnosis of all measurable factors along with taking system pressures and temperatures should be reviewed before arriving at a conclusion.

[Sridhar1312]

Best possible diagnosis for overchatge will be high suction and high discharge,subject to your condensers are normal.

[Gary]

The best indicators of overcharge (assuming good airflow through both coils) are:

High subcooling (too much refrigerant in the high side)

Low superheat (too much refrigerant in the low side)

[SL82]

If there is good air flow through both coils and the system is overcharged, yes you have high subcooling (too much liquid in the condenser) but how would you flash with all this subcooling? I have found many systems overcharged and corrected them but I have never seen flashing with an overcharge. Flashing with a dirty condenser yes but that is another topic.

[Gary]

If there is good air flow through both coils and the system is overcharged, yes you have high subcooling (too much liquid in the condenser) but how would you flash with all this subcooling? I have found many systems overcharged and corrected them but I have never seen flashing with an overcharge. Flashing with a dirty condenser yes but that is another topic.

In the scenario outlined earlier by Hyperion, you could have flashing with overcharge. The subcooling would be low and the superheat would be low, the extra refrigerant being in the low side.

[desA]

The best indicators of overcharge (assuming good airflow through both coils) are:

High subcooling (too much refrigerant in the high side)

Low superheat (too much refrigerant in the low side)

Good points. Not so sure it's always perfectly cut-&-dried, though.

To add to the debate, would an over-sized TX valve also not result in low superheat at evap exit?

If so, how would you know whether the system was really overcharged, or as a consequence of the incorrect TXV selection?

For instance, you can have a reasonable sub-cooling on the condenser, and low evap superheat. Would the system then be overcharged, or correctly charged?

[Gary]

System problems need to be addressed in a particular sequence:

First airflow problems.

Then high subcooling problems.

Then low superheat problems.

Then wait until the refrigerated space is down close to design temp.

Then high superheat problems.

Then low subcooling problems.

The correct question is not whether the system has an overcharge problem. The correct question is whether the system has a problem. And once we have resolved the problem, the next question is whether the system has another problem.

[Andy W]

On more than one occassion I have come across jobs where oversized expansion valves have been fitted and the engineer has tried to charge the system to the sight glass and completely filled the receiver and condenser with so much liquid that there was no room to condense any more vapour which will result in the system cutting out on HP switch (if fitted) or the compressor stalling.

A simple pump down test with your high side gauge on the discharge service valve of the compressor, pump down the system from the receiver service valve and observe the high side gauge as its pumping down, overcharge or non condensables in the system will soon get that high side gauge rising to the point of cutting off on HP switch.

The above is based on a condensing unit with suction, discharge and receiver service valves fitted.

I have also twice come across split air conditioners with sight glasses fitted in the expansion lines and massiveley overcharged to the point of the systems will not run, one was blowing off on the internal PRV of the compressor.