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.

Fixed orifice tube

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.

A/C evaporator core sensing bulb location.

An expansion valve is a variable metering device, meaning that it changes refrigerant output according to temperature or pressure. The question mentions placing the TXV's bulb on the inlet tube of the evaporator core; it should be placed on the outlet tube. The bulb contains a similar refrigerant as the system. The valve opens to allow refrigerant to flow into the evaporator core as the bulb warms. It then closes to restrict refrigerant flow into core as the bulb cools. It cycles open and closed to provide optimum cooling while preventing evaporator core freeze up.