This article is written by RSES CM and excellent market refrigeration tech – Jeremy Smith
I frequently see techs online struggling with charging or troubleshooting refrigeration equipment and using subcooling as a diagnostic or charging method. Please don't do this unless you understand it fully.
Many times, trying to charge a refrigeration system to a specific subcooling value is going to result in a serious overcharge.
Glad you asked.. First, let’s take a look at a simple system and focus on the condenser, liquid line and metering device.
As we condense the refrigerant and fill the liquid line and condenser, the metering device restricts flow somewhat, causing liquid to back up into the condenser.
This ‘stacking’ effect as it’s commonly called, allows more time for the liquid to be in the condenser and to reject heat. That heat rejection is what results in additional subcooling. Adding more gas to this system will simply result in more liquid being stored in the condenser, more heat rejection from that liquid and, consequently an increasing subcooling value. That’s the system that you understand and that subcooling can be effectively used as a diagnostic and charging metric.
Now, let’s put a receiver in the system between the condenser and the metering device.
We’ve got liquid in the condenser and it enters the receiver before the metering device. As the liquid line fills and the metering device starts to restrict as before, where does the liquid wind up? The receiver. It doesn’t wind up in the condenser where heat can be rejected, but rather in a tank to be stored. Now, if you’re measuring subcooling, before OR AFTER the receiver, you’re not going to see a significant change in that value before or after we reach a proper charge.
If you continue to add gas to the system it’s going to continue to fill the receiver until that liquid backs up to the inlet port of the receiver. Now, you’re seriously overcharged because a receiver shouldn’t be more than 80% full, but the system can now back liquid up into the condenser and allow for the subcooling to increase.
This is why, when you have a receiver, you need to use either a sight glass or some form of receiver level monitoring to determine if you’re charge is correct and not just use subcooling.
— Jeremy Smith