Low Discharge Superheat

[pbcd]

I have a Carrier Centrifugal chiller model 19XR-4747333CEH64S. I am getting a Low Discharge Superheat protective limit alarm. The manual states the primary cause is "oil in refrigerant" or overcharged. Could someone pls. explain the theory of how the oil can cause this alarm. Thanks.

[union 189]

All chillers have oil loss when running that is why they all have some kind of oil recovery system on them. With say that oil will float on top of the refrigerant and carry over and cuase the discharge sensor to sense carry over.Check the oil skimmer sight glass this will tell you alot of what's going on.

[pbcd]

Union 189

So the carryover is "cold" oil and that is what the discharge thermister is sensing?

[RichardL]

As 189 correctly stated "All" chillers will have a certain % of oil in the refrigerant, this aids in heat transfer as long as the percentage is low and hinders heat transfer when high oil concentrations are encountered.
A little is good, too much is verry bad. Too much oil will cut the blades from the impeller and severely reduce the compressor effeciency.
It will also trigger a "Low-Superheat" warning as this foamy mix is sucked out of the evaporator.

[pbcd]

Thanks for the info. I'll checkout the oil recovery system.

[pbcd]

If the compressor oil level has been maintaining the same level would it be safe to say that the cause of the low discharge superheat is NOT the oil in the refrigerant? The unit has never been recharged (refrigerant). The discharge thermister reading is accurate. Any other ideas what could be causing this problem? Thanks ..

[GT Jets]

If the compressor oil level has been maintaining the same level would it be safe to say that the cause of the low discharge superheat is NOT the oil in the refrigerant? The unit has never been recharged (refrigerant). The discharge thermister reading is accurate. Any other ideas what could be causing this problem? Thanks ..

Do you understand where the discharge superheat comes into play and how it is monitored?

Something is causing the discharge temperature to read "low", that is, lower than the expected setpoint minus the saturated temp/pressure of the condenser.

That being said, the condenser temp sensor may be out of whack as well...

Also too, if you have excess/inadequate evaporator water flow you could flash too much refrigerant.

Although it is rare, you may be flooding the evaporator with refrigerant due to low condenser water flow..

To point you in the right direction, the machine needs to be totally logged and that information analyzed.

You will need;

Both condenser and evaporator approaches, which use most of the following...
Saturated temps of both sides (refrigerant).
water flows and temps of both sides

Load
oil pressures and temperatures (extremely cold oil does bizarre stuff...)


You can do this from the LID, but i would recommend doing it "the old fashioned way" to check for sensor and transducer accuracy.

Good hunting!

GT

[milkyway]

I have a Carrier Centrifugal chiller model 19XR-4747333CEH64S. I am getting a Low Discharge Superheat protective limit alarm. The manual states the primary cause is "oil in refrigerant" or overcharged. Could someone pls. explain the theory of how the oil can cause this alarm. Thanks.

i believe the discharge super heat should be around 25 i think., do u know how to check the discharge superheat.,

[txhvac]

134a loves to migrate oil, Carrier XL's (R-22) & XR's if they get too much oil in the evap, the oil will ride right on top of the refrigerant, it can cause the machine to carryover the machine will not load up. Oil & refrig. foam hitting the vane & impeller loads up motor, forcing an unload based on amp draws, and settings. I think I might check all the sensors, verify condenser/chilled water temps and flows, what load is the machine running at? An overfeeding float valve (metering device) could cause your problem too. Hope this can help.

[Dilp172]

Heres a link to a really good article that explains this subject in detail,they dumbed it down so even i could get it.
http://www.efftec.com/resources/stacking-carryover.php