A/C Metering: CCOT, TXV
This reminds us of the difference in metering devices and methods of the two most common types of automotive air conditioning systems in use today. There's a lot of things that are the same about these systems, but there's also a lot of things that are different.
At first glance, a (CCOT) cycling clutch orifice tube system will have an accumulator and a cycling switch attached. This accumulator is located on the low side of the system, after the orifice tube. This is different than a (TXV) thermostatic expansion valve system that has a receiver drier located on the high side of the system, between the condenser and the expansion valve. Both of these devices are dryers containing a desiccant to clean and absorb moisture from the refrigerant.
They also use a different method of metering refrigerant into the evaporator core. A (FOT) fixed orifice tube meters the same fixed amount of refrigerant based on the diameter of the tube inside the FOT's casing. A cycling clutch switch is used to engage and disengage the compressor clutch according to the system’s low side pressure. It's important to replace the old tube with the correct one. Orifice tubes can become clogged and severely hamper A/C performance. Follow manufacturer’s recommendations. Some aftermarket companies offer a variable orifice tube that claims to improve performance, especially at red lights and stop and go traffic. Always check and follow manufacturer’s recommendations.
The expansion valve above contains a small needle valve and diaphragm that’s attached to a sealed gas chamber. Temperature causes the gas in this chamber to expand and contract resulting in movement of the valve. This needle valve opens and closes to allow refrigerant into the evaporator core. As temperatures rise, the valve opens allowing more refrigerant into the evaporator. As it cools, the valve closes restricting the flow of refrigerant into the core. This provides optimum cooling while preventing evaporator core freeze up.