Expansion Block/Orifice Tube: Description and Operation

Expansion (Orifice) Tube
The expansion tube restricts the flow of high-pressure, medium temperature liquid refrigerant from the condenser. As the high-pressure, medium temperature liquid refrigerant passes through the expansion tube, it expands to become a low-pressure, low temperature liquid/vapor mixture.

The expansion (orifice) tube consists of a small brass tube molded into a plastic holder, with fine mesh screening protecting both ends of the tube. The expansion tube is installed inside the evaporator inlet refrigerant line where it connects to the condenser outlet. The expansion tube is serviced as an assembly by replacement only.

When the engine is turned off during A/C system operation, refrigerant in the system flows from the high side of the system to the low side through the expansion tube until pressures equalize. This may be detected as a hissing sound for 30 to 60 seconds after shut down, and is normal.