Car A/C Orifice Tube System
There are three principal differences between an orifice tube type system and the expansion valve type described above. First, the receiver-drier is replaced by an accumulator. Second, unlike the receiver-drier, which is located in the high side of the system at the condenser, the accumulator is located in the low side at the evaporator outlet. Third, a fixed orifice tube replaces the conventional expansion valve.
‘The orifice tube system is employed on many General Motors and Ford vehicles. The GIM system is known as a Cycling Clutch Orifice Tube (CCOT). Ford calls its system Ford Fixed Orifice Tube (FFOT).
Earlier orifice tube systems used a standard thermostatic switch which turned the compressor off and on in accordance with evaporator temperature. Most newer systems have a pressure-sensing switch that controls compressor operation in accordance with system pressure. Measuring pressure is more accurate and responsive than measuring temperature.
Small displacement engines have difficulty maintaining a stable idle when the compressor clutch is continually cycling on and off. In 1985, General Motors solved this problem with the introduction of the DA-V5 compressor, which does not cycle at all. Instead, it varies its output in accordance with system demand. Other manufacturers followed suit, and now many compressors have a non-cycling, variable output.
The accumulator, which serves as a reservoir for any liquid refrigerant flowing out of the evaporator, prevents liquid refrigerant from reaching the compressor. Like the receiver-drier, the accumulator stores excessive refrigerant. It also filters and removes moisture from the refrigerant.
In an orifice tube system, the accumulator is located in the low side of the system, at the evaporator outlet (a receiver drier is located in the high side of the system at the condenser) and a fixed orifice tube replaces the expansion valve used on a receiver-drier type system