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The metering device is a purposeful restriction in the liquid line prior to the evaporator. The metering device is needed to lower the pressure of the liquid refrigerant prior to the evaporator. This pressure reduction allows the refrigerant to expand and reduce in temperature while in the evaporator. This allows the refrigerant to be at a lower temperature than that of the air crossing the evaporator coil. The amount of refrigerant that the metering device is supposed to allow into the evaporator must be large enough to absorb the heat of the air crossing the coil but be small enough to have an effective pressure drop across the metering device.
A liquid line restriction problem reduces the amount of liquid refrigerant even further than the design of the metering device. This results in a reduced ability for the system to remove heat from the building. Because there is less refrigerant entering the evaporator, the refrigerant pressure in the evaporator will be too low. Since the temperature and pressure are related while the refrigerant is in the saturated state, the saturated refrigerant will also have a temperature that is too low. Often, the saturated temperature ends up being below 32° F due to the liquid line restriction problem. This causes the humidity in the air crossing the coil to freeze onto the outside of the coil further blocking the heat transfer from the air to the refrigerant.
Before the evaporator coil freezes, the refrigerant will have a high superheat because the heat from the air crossing the coil will be more than the refrigerant can absorb. When there is less saturated refrigerant in the evaporator coil than the system is designed for, the refrigerant moves quickly out of the saturated state and becomes excessively superheated before exiting the evaporator coil. Because of this, the beginning of the coil is very low in temperature and likely freezing while the end of the coil is at a high temperature. In the case of a bad liquid line restriction problem, the sat temperature will be below freezing (32° F) while the temperature on the vapor line exiting the evaporator coil may match the temperature of the air in the return duct. This results in a very high superheat.
A high superheat is an indication of either a low refrigerant charge or a liquid line restriction problem. To tell the difference between the two problems, we look at subcooling. A system with a low refrigerant charge will have a low subcooling. A system with a liquid line restriction will have a normal to high subcooling. The high subcooling is due to less refrigerant being in the evaporator and more refrigerant being in the condenser.
Low Refrigerant Charge: Low Vapor Saturated Temp, High Superheat, Low Subcooling,
Low Delta T
Liquid Line Restriction: Low Vapor Saturated Temp, High Superheat, Normal to High Subcooling, Low Delta T
The saturated temperatures, total superheat and subcooling must be measured before the evaporator coil is frozen while the system has proper airflow crossing the coil.
If you notice a high superheat and an excessively high subcooling, this is likely a system with a liquid line restriction that is overcharged. A previous technician has likely added pounds of refrigerant into the system in an attempt to raise the low side saturated temperature to a level above freezing.
Causes of a Liquid Line Restriction:
· Clogged filter drier
· Partially closed liquid line service valve
· Jammed solenoid valve
· Clogged screen before and/or after the metering device
· Undersized metering device
· Clogged metering device
· Stuck TXV
· TXV that has lost bulb pressure
Stay tuned for our next article on the individual diagnosis of each component that can cause a liquid line restriction!
Check out our book “Refrigerant Charging and Service Procedures for Air Conditioning”.
The full outline is available at https://www.acservicetech.com/the-book
If you have already purchased our book, be sure to tell local HVACR Instructors about our book and what you think of it. We would love to get the book into the hands of the next generation of HVACR Technicians!
Published: 8/27/2019
Author: Craig Migliaccio
About the Author:
Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 15 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel
