Orifice Tubes Fixed and Variable
We often classify automotive air conditioning systems by the type of metering device used. There is the TXV or thermostatic expansion valve and the FOT or fixed orifice tube along with it's close cousin the VOV variable orifice valve type systems. A VOV is an improvement in performance over the fixed orifice valve because like the TXV it can change its output based on operating conditions. A metering device lowers the pressure and temperature of the refrigerant and provides this liquid refrigerant to the evaporator.
A fixed orifice tube is a small brass tube incased on a plastic case with a filter at each end to protect it from any debris in the system. It is located between the condenser and the evaporator core. It's usually easy to locate because the manufacturers put a crimp in the line to keep the orifice tube from traveling to far down the tube towards the evaporator core.
There is also a change in temperature at the orifice tube that is noticeable to the touch. This is because the orifice tube acts as a restriction in the system and any time there is a restriction in an A/C system, the temperature will drop. For instance, if there is a crease in a condenser the technician would likely see ice or frost forming at that location.
A VOV variable orifice valve looks the same but is able to change it's output during different operating conditions. A fixed orifice tube is really a compromise between highway performance, stop and go, and idle performance. This is because at idle the condenser has less air flow and re-circulated heated engine compartment air cooling its surface. This causes the head pressure to increase but also causes vaporized refrigerant leaving the condenser to enter the orifice tube.
This is because 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. This improves performance at low speeds and idle while still providing a high level of performance at highway speeds.
