An orifice tube is similar to an expansion valve, with both performing an important function in a vehicle’s A/C system. The valve helps control the flow of refrigerant through the evaporator, which is the main cooling component of the system. An expansion valve controls this flow directly through a modulating rod, which opens and closes to various levels depending on cooling demand. It also contains a sensing bulb which detects temperature within the evaporator. An orifice tube has no moving parts and contains a permanent restriction within the system. The orifice tube systems control the refrigerant flow through other means, such as cycling the compressor on and off, or the use of a refrigerant regulating valve within the compressor itself.
The orifice tube allows for a higher flow of R-134 refrigerant during times when more cooling is needed, and a smaller flow of refrigerant when less cooling is needed. This balance of cooling flow is critical for the A/C to operate effectively.
Too much refrigerant flow results in the evaporator running too cold, which causes moisture to freeze on the evaporator coils. This freezing restricts airflow through the coils and eventually results in a loss of cooling for the vehicle. The evaporator and refrigerant lines should be cold to the touch when running properly, but they should not be frozen. If freezing does occur, it may indicate a problem with the orifice tube, or even too much refrigerant in the system. Many vehicles come with a “sight glass” which can be used to see if there are air bubbles present in the refrigerant stream. A sight glass can help determine whether the problem is within the orifice tube, or the refrigerant itself.
Faulty orifice tubes can also result in too little refrigerant flow through the evaporator. If this is the case, the flow of refrigerant will not be enough to absorb the heat that is coming from the condenser. When this occurs, the A/C simply will not be performing well enough to allow the system to cool. If either the evaporator or the refrigerant hose coming from the evaporator are warm or hot to the touch, this could indicate that the orifice tube isn’t doing its job. However, there could be many other causes for this problem, as well.
The diameter of the passage inside the orifice tube is very small compared to those inside the evaporator. This dynamic allows the refrigerant to expand when it passes from the orifice tube into the evaporator. An orifice tube and expansion valve both function as the dividing point between the high pressure and low pressure sections of the A/C system. The small diameter of the passage, while critical for the cooling system, makes an orifice tube susceptible to becoming clogged from debris within the system. An orifice tube becoming stopped up is just about the only way that it can ever fail, as it is a relatively simple component by design.