BUBBLES IN SIGHT GLASS

[Tool-Slinger]

There are many circumstances to consider when properly charging a system as everyone knows. The reasons for proper refrigerant charge are important and many factors need to be considered for a properly designed and operating system.

A common design for air conditioning & heat pump:
Evaporator Temperature 40° to 50° F
Suggested Superheat: 8° to 12° F

The TXV should be set to maintain proper superheat for the evaporator, not for the compressor. TXV’s usually require the sub cooled method for charging. Liquid sub cooling is required so that you have only liquid entering the expansion valve with no bubbles present. However the position of the TXV will change based on the load condition and if not consistent the TXV will hunt trying to maintain the super heat set point. The design conditions need to be established first for any system such as tonnage, proper air flow, sensible and latent loads along with refrigerant piping. There will be times when the system has been pushed beyond it's design criteria and this will need to be considered.

The basics as most know are as follows:
Liquid line saturation temperature minus actual liquid line temperature is called sub cooling. As the liquid travels through the condenser, its temperature begins to drop below the temperature at which it condensed. (I.e. R-22 @ 250 Lbs = 117°) When the liquid is at a temperature that is cooler than the condensing temperature, the liquid enters a state called sub cooled. When a refrigerant liquid is sub cooled, the refrigerant is pure liquid.

Note: The liquid can be no cooler than the temperature surrounding the condensing coil. If the refrigerant liquid leaves the condenser coil at a temperature that is colder than the air passing through the coil, there is a pressure drop in the coil which is causing the temperature of the refrigerant to fall. For example if a drier were installed and it became plugged the temperature at its outlet would be cold.

The refrigerant level will vary by how much refrigerant charge is in the system. At high charge levels the sub cooling will be high and at low charge levels the sub cooling will be lower.

Importance of sub cooling: The liquid that exits the condenser coil must travel down the liquid line to the evaporator metering device. As the pressure keeps dropping due to friction loss between the refrigerant and piping. As the liquid pressure falls, the corresponding saturation temperature also drops.

This reduces the temperature difference between the saturation temperature and the liquid line temperature. In other words, the sub cooling at the condenser coil outlet is greater than it is 50 Ft down the liquid line in the case of a split system.

Flash Gas: When the pressure loss in the liquid line is great, the liquid may reach a pressure where the saturation temperature for the pressure equals the temperature of the liquid. At that point, vapor will form along the liquid. This is called flash gas.

To prevent this condition from occurring, it is important to charge the system so there is enough sub-cooling to overcome the pressure loss that occurs in liquid lines. The amount of sub cooling will be determined by liquid line length, liquid lift, fittings, liquid line size and overall condenser size.

This is not a complete scenario as there is much more to consider such as total super heat which will also indicate proper charge and if set correctly will help reduce flood back to the compressor in a part load condition. This subject is long and there are many opinions here which make for an interesting discussion.

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Trouble shooting is not part of the repair……understand the symptom’s and you will find a solution.

Yes, and it is important to note here for the casual reader, we are trying to measure the quality of the refrigerant at one point in the system,... As inaccurate as that becomes, you cannot check pressure on the discharge and the temperature on the liquid-line, due to pressure drop. Same goes for checking pressure and temp across a filter-drier. The place where one has to check pressure MUST be where the temp is also checked for the most accurate reading.

Still, it is common to have 10 degrees subcooling reading and bubbles in the sight-glass. There are many factors to consider, custom application to each jobsite.