Typical Orifice Tube System
The temperature readings on the system shown above indicate a properly charged A/C system operating at maximum efficiency.
In an HVAC system the refrigerant changes state at the evaporator and at the condenser. At the evaporator the refrigerant liquid changes to a vapor. At the condenser the refrigerant changes from a vapor to a liquid. Each time this state change takes place a tremendous amount of heat energy (BTUs) is transferred with no measurable temperature difference.
If this state change does not take place at both the evaporator and condenser, the efficiency of the HVAC system will decrease significantly. While heat transfer would still take place, the refrigerant cannot remove enough heat from the air in the passenger compartment, and the refrigerant cannot efficiently get rid of the heat at the condenser.
In a properly charged A/C system there will be no difference in temperature across the evaporator.
This indicates that there is still some liquid refrigerant exiting the evaporator and therefore oil circulation is being maintained. Measuring evaporator inlet and outlet temperature is usually fairly easy on an orifice tube system. However on a TXV system, the valve may be inside the HVAC case and taking the evaporator inlet temperature reading after the valve may be impossible.
Typical TXV System
In a TXV system the drier is called the receiver/drier. It is located in the liquid line between the condenser and the evaporator. It serves four functions – it contains a desiccant, which absorbs any moisture from the system. It acts as a liquid vapor separator to ensure that only pure liquid refrigerant reaches the TXV valve, it has a filter to catch any particulate debris and it also acts as storage vessel for refrigerant in the system.
The TXV valve performs the same function as the orifice tube. The key difference is that the TXV valve can vary the refrigerant discharge rate into the evaporator in direct proportion to the heat load on the system. The TXV valve is connected to the evaporator outlet through a capillary tube and sensing bulb filled with refrigerant gas. As the evaporator outlet warms and cools (a reflection of the heat load on the system) the gas trapped in the sensing bulb and capillary tube expands and contracts. This expansion/contraction acts on a diaphragm attached to a metering valve inside the TXV valve to precisely control the flow of refrigerant.
Because TXV systems more precisely control the flow of refrigerant in the system, in general, they are less prone to lubrication issues than orifice tube systems.