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Auto AC orifice tube system

Auto AC Orifice Tube Operation

How does an automotive AC orifice tube system work

Some car makers now use automotive AC orifice tube systems in their AC. An orifice tube is simply a refrigerant metering device to control the rate of flow of refrigerant into the evaporator located in your dash. In the illustration below, you’ll see that low pressure gas flows out of the evaporator coil and into the accumulator. A low pressure switch is often located right on the accumulator or in the low pressure line before the compressor. The low pressure switch is used to control when the compressor clutch engages and disengages. As the temperature of the refrigerant gas drops, so does it’s pressure and the switch turns off the compressor clutch. As cabin air blows across the evaporator, the gas heats and raises temperature and pressure and the switch then operates the compressor clutch again.  It’s really that simple.

Refrigerant flow through the system

When the AC compressor engages, it sucks low pressure refrigerant gas from the evaporator and accumulator.  The compressor then compresses the gas into a higher pressure and forces it into the condenser. During the compression cycle, the gas rises in temperature. Airflow across the condenser cools the high pressure gas, causing it to condense into a high pressure liquid.

The high pressure refrigerant flows to the screen and port in the orifice tube. Once the high pressure liquid passes through the orifice, the liquid pressure drops and the liquid begins to fill the evaporator. Airflow across the evaporator causes the liquid refrigerant to boil and turn into a gas again. The amount of heat it takes to change the refrigerant from liquid to gas is what an AC system removes from your car.

Where is the orifice tube located?

Orifice tubes are always located in the refrigerant tubing that connects to the high pressure port on the evaporator. However, the tube may be located in the evaporator tubing itself.

In either designed, you must evacuate the system and disconnect the high pressure line from the evaporator to replace the orifice tube. If the screen is clogged, the entire system must be flushed and components replaced. The accumulation of debris caused by Black death, or the failure of a major components, cannot be flushed out of the condenser or evaporator. Those components must be replaced to restore proper AC operation.

AC orifice tube

AC orifice tube system

What goes wrong with an orifice tube?

Orifice tubes fail due to a clogged screen. The clog is caused by oil deterioration due to a low oil condition, acid formation in the system due to excess moisture or metal particles from component failure (also known as black death).

AC orifice tube

New and clogged AC orifice tube. Clogged tube shows evidence of “Black Death.”

© 2018 Rick Muscoplat

Posted on by Rick Muscoplat

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