Orifice Tube Systems – Normal Operating Characteristics

Orifice tube AC systems are very effective designs that have been used for many years. The one drawback of this system design is that the orifice tube does not change the flow rate of refrigerant into the evaporator core, so the compressor needs to be cycled periodically to keep the evaporator from getting too cold and becoming covered with ice. There are two ways to control the output of the compressor: turn it off for short periods of time or change the internal displacement of the compressor.

The first of these strategies is typically called the cycling clutch orifice tube (CCOT) design. CCOT systems use pressure switches or temperature switches to monitor the conditions of the AC system, which then causes the AC compressor to be turned off to prevent evaporator freeze up. The cycle rate of these systems will vary depending on the ambient temperature and the humidity level. It is common to notice the compressor turning on and off during
operation. A clicking sound will be heard in the engine area as the compressor is cycled, which is often accompanied by a slight change in engine RPM.

In recent years, vehicles that use orifice tubes have used compressors that are a variable-displacement style. Many variable-displacement compressors use mechanical operation to change the internal capacity of the compressor. This action was accomplished by using a valve that sensed the temperature of the suction side of the compressor and changing the angle of the wobble plate, which caused the internal pistons to change the length of the stroke.

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