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The role of a thermal expansion valve (TXV) is to precisely control the flow of refrigerant into the evaporator, ensuring that the system operates efficiently. By maintaining a constant superheat at the evaporator outlet, the TXV allows for optimal heat exchange, which is crucial for effective cooling.

In an air conditioning system, superheat refers to the temperature of the refrigerant vapor above its boiling point at a given pressure. By continuously adjusting the amount of refrigerant that enters the evaporator based on the temperature and pressure conditions, the TXV ensures that the refrigerant is fully vaporized before it exits the evaporator. This helps prevent liquid refrigerant from returning to the compressor, which can cause damage and reduce the system's performance.

This function is essential for achieving the desired cooling effect and maintaining system efficiency, making the TXV a critical component in an air conditioning setup. The other choices do not accurately describe the function of a TXV. For instance, increasing the system’s overall size or changing the refrigerant type does not relate to the responsibilities of a TXV, and regulating airflow is typically handled by other components in the system.