Tag: sight glass

Often in commercial HVAC and refrigeration, you will either find or install sight glass/moisture indicators. The sight glass portion is simple, it’s just there to show if the liquid line has a full line of liquid or if it has bubbles which shows it’s a liquid/vapor mix.

A clear glass on a running system generally means a full line of liquid (or totally flat but you would know that already if you have gauges attached). Reading subcooling essentially does the same thing as a sight glass, it simply proves that the system has a full line of liquid. In HVAC Subcooling actually gives you more data that a full sight glass in that it tells you the actual amount of heat that the refrigerant has lost past the condensing temperature.

In refrigeration systems with receivers, a sight glass is an excellent tool and can be relied upon as an indicator of liquid refrigerant to the metering device.

The moisture indicator shows you if the system is dry or if it has moisture content.

First, make sure you are aware that older sight glasses may not be sensitive enough to pick up wet conditions with HFC refrigerants that contain POE oil.

Second, when installing a sight glass keep it sealed as long as possible before installing. If you open the indicator to air prematurely it may change color due to moisture in the air. If that does happen most indicators will change back after being installed, a proper vacuum pulled and the system run for several hours. If it still reads wet after that time the system likely is wet and new line driers should be installed and deep vacuum pulled.

You best defense against a wet system is fresh line driers, good installation practices that prevent moisture entry and proper evacuation confirmed by an accurate micron gauge.

— Bryan

Because some have expressed confusion, this article pertains to refrigeration systems that have a Receiver.

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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.

 

Why?

 

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 refrigerant and fill the liquid line and condenser, the metering device begins restricts flow somewhat 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. Ok, 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 as it did in the simple system we looked at
first. This is why, when you have a receiver, you need to use either a sightglass or some form of receiver level monitoring to determine if you’re charge is correct and not just use subcooling.

 

— Jeremy Smith

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