Does the sight glass really need to be full?

A common misconception among refrigeration and air conditioning technicians involves one of the most likely misleading components of a cooling system; the sight glass. As we begin our experiences in the world of troubleshooting and servicing refrigeration and air conditioning systems, a question frequently asked by ourselves and the anonymous helpful voices over the radio or more experienced technician is "Is the sight glass full?" Although most of the time a full sight glass is acceptable, there are many instances where a full sight glass is not only not necessary, but also detrimental to the compressor and/or the efficiency of the system.

For example, in an average residential
split-type system, a full sight glass is probably favorable due to the
inconsistent ambient operating temperatures of both the condensing unit and evaporator and the generally high amount of superheat removed through it's operation. However, in an office building maintaining a somewhat constant return air temperature and a moderate heat load, a full sight glass is not always necessary. In fact, the system may be operating less efficiently than is possible if it is full. The reason being that a building of this sort simply doesn't generate enough heat to cause the desired superheat within the system to fluctuate enough to use any excess liquid refrigerant. With a
steady heat load, the expansion valve will allow a steady amount of liquid refrigerant into the evaporator. As the condenser air temperature decreases at night or in cooler weather, the amount of liquid generated through the condenser increases as well as the sub cooling effect. Meanwhile, the expansion valve or fixed metering device will maintain flow as usual to the unchanging return air temperature. The compressor is then working as hard in a low-load time as it is in a high-load time and the system continues to remove the same degree of superheat.

In many larger package roof-top-units,
there may be 30 feet between the sight glass and the metering device. A large split system may have 100 feet or more between the two. A full sight glass may indicate an excess amount of refrigerant. Now this is not to say that the system won't operate or that the compressor life will shorten. It is simply less efficient.

In a low temp. system such as in kitchen
equipment, especially multi-evaporator no-fan coil systems, a full sight glass may not only be inefficient, but may also endanger the compressor's valves by slugging it with liquid refrigerant. In such a case, an indoor fan-cooled condenser might be in use. The ambient air around it is likely to be a fairly constant temperature. The heat load is probably minimal and unchanging. As one or two or more expansion valves begin to meter open and closed, the amount of liquid refrigerant required to maintain the desired superheat will fluctuate, though slightly. And when the load approaches the desired temperature, the TXV's range may be exceeded on the low end of the scale.
Liquid refrigerant will then pass through to the compressor, slugging it.

Because there is no fan to force the load through the evaporator, the degree of superheat will be minimal: around 3-5 degrees. Under these circumstances, I would recommend reading the manufacturers' preset superheat adjustment and the amount of adjustment open or closed that is necessary to approximately set the TXV to the desired individual evaporator superheat. Most expansion valves change 1-2 degrees per full turn of the adjusting mechanism. Then begin to charge the system monitoring each evaporators' superheat and the superheat of
the system as a whole. Determining the ideal superheats for each can be tricky and it is always application sensitive. But as long as the superheat of the system is not lower than the lowest evaporator's superheat, and is never a negative number, the compressor will not be slugged with liquid. The sight glass generally won't fill over approximately 1/3 full at any time and will operate fully to it's expectations as long as everything is properly sized. I have found that in this type of system, the sight glass serves only to indicate the presence of moisture contamination and confusion if used as a charging tool.


Mark Yarbrough
http://hvacwebtech.com