Auto AC orifice tube system

Auto AC Orifice Tube Operation

How does an automotive AC orifice tube system work

Some car makers use 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.

How Refrigerant flows through the system

Compressor engages and sucks refrigerant from the evaporator

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 gas and forces it into the condenser. During the compression cycle, the gas rises in temperature, heating it to around 150°F.

The condenser cools the hot refrigerant, causing it to condense back into a liquid

Airflow across the condenser cools the high pressure gas, causing it to condense into a high pressure liquid.

The refrigerant flows into the orifice tube

The high pressure refrigerant flows out of the condenser and into 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 heat that an AC system removes from your car.

Where is the orifice tube located?

Orifice tubes are always located in the refrigerant tubing that runs from the condenser to the evaporator. It may be located in the tubing or at the inlet to the evaporator.

In either design, 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 system

What goes wrong with an orifice tube?

Orifice tubes fail due to a clogged screen or clogged orifice.

The clog is caused by oil deterioration from overheating or running when low on refrigerant.

Or, it can clog due to debris from hose deterioration

If the system has had a leak and air an moisture have entered the system, the moisture can react with the oil to cause sludge and that can clog the orificee  tube.

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

© 2018 Rick Muscoplat