Fixed and Variable Orifice Tubes
An air conditioning system contains a metering device that lowers the pressure and temperature of the refrigerant to the evaporator core. Automotive air conditioning systems can be classified by the type of metering device used. There is the (TXV) thermostatic expansion valve, the (FOT) fixed orifice tube, and the (VOV) variable orifice valve. The VOV is an improvement in performance over the fixed orifice valve, because like a TXV, it can change its output based on operating conditions.
A fixed orifice tube contains a small brass tube encased in a plastic case with a filter at each end to protect it from any debris in the system. It's located on the high side between the condenser and the evaporator core. It is usually easy to locate because the manufacturers' put a crimp in the line to keep the orifice tube from traveling down the tube towards the evaporator core.
The tube is sometimes tricky to remove with just needle-nose pliers. A special tool is available to pull stubborn orifice tubes. Often there is debris in the inlet filter. Metal debris is typically from compressor wear, and gooey debris is usually from a ruptured accumulator desiccant bag.
A variable orifice valve looks the same as a fixed orifice tube but can change output depending on different operating conditions. A fixed orifice tube is a compromise between highway performance, stop and go, and idle performance.
At idle, the condenser has less air flow across its surface. It causes head pressures to increase but also causes vaporized refrigerant leaving the condenser to enter the orifice tube.
At low speed and idle conditions, the refrigerant is moving through the condenser very rapidly, sending high-temperature vapor and liquid to the orifice tube. Subcooled liquid entering the orifice tube is ideal for flooding the evaporator core of a CCOT, cycling clutch orifice tube system. Vapor entering the orifice tube will reduce the effectiveness of the refrigerant.
With a VOV at idle, a bimetal control spring blocks off one of two ports, reducing the flow of refrigerant and allowing more heat to be removed in the condenser. As speeds increase, the valve opens back up, allowing maximum flow through the system again. It improves performance over a fixed orifice tube at low speeds and idle, while still providing a high level of performance at highway speeds.