USCG Exam PracticeRefrigeration cycle and components
A thermostatic expansion valve (TXV) senses the temperature at the evaporator outlet in order to accomplish which of the following?
- A. Hold a constant evaporator pressure regardless of load
- ✓ Maintain a constant superheat at the evaporator outlet, ensuring only vapor reaches the compressorCorrect
- C. Keep the evaporator coil filled with liquid refrigerant to a set level
- D. Cycle the compressor on and off in response to space temperature
Why B is correct
The TXV sensing bulb measures the temperature of vapor leaving the evaporator and modulates the valve to hold a constant superheat, admitting more refrigerant under high load and less under low load. This ensures the evaporator is fully utilized while preventing liquid from reaching the compressor. Constant evaporator pressure is the function of an automatic expansion valve; maintaining liquid level is a float valve function; compressor cycling is controlled by pressure or thermostat controls.
Cited:NAVEDTRA 14075 §6-4