Question 26 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is B **Why Option B is Correct:** The Thermostatic Expansion Valve (TXV) regulates the flow of refrigerant into the evaporator based on the superheat setting. Superheat is the difference between the actual temperature of the refrigerant vapor leaving the evaporator and the saturation temperature (boiling point) corresponding to the suction pressure. Superheat ensures that all refrigerant exiting the evaporator is fully vaporized gas, protecting the compressor from liquid slugging. If the superheat setting is set **too low** (e.g., set to $4^{\circ} \mathrm{F}$ instead of the recommended $10^{\circ} \mathrm{F}$), the valve will open wider and/or stay open longer to maintain that low temperature differential. This allows **more liquid refrigerant** into the evaporator than necessary for proper heat exchange and full vaporization. As a result: 1. **The Suction Line will be Abnormally Cold:** Excess liquid refrigerant leaving the evaporator will cool the suction line significantly, potentially causing heavy sweating or frosting far past the evaporator outlet. 2. **Liquid Floodback:** Since the refrigerant isn't fully vaporized, liquid refrigerant or very cold, low-superheat vapor will enter the suction line and potentially flood back to the compressor crankcase, leading to lubrication issues, bearing damage, or catastrophic compressor failure (slugging). **Why Other Options are Incorrect:** * **A) the suction line will be abnormally warm due to a reduced amount of refrigerant returning back to the compressor:** An abnormally warm suction line is typically caused by **high** superheat (the valve is closing too much, starving the evaporator) or low charge. A low superheat setting means the valve is open too much, leading to an **abnormally cold** suction line. * **C) the box temperature will be pulled way down below the normal temperature range:** While a slightly oversized or wide-open valve might result in good cooling, the primary consequence of extreme low superheat is operational failure and efficiency loss, not necessarily a huge drop in box temperature (unless the system was already grossly oversized). Low superheat often suggests inefficient use of the evaporator surface (too much liquid passes through without fully boiling), and the safety hazard of liquid floodback is the immediate primary result. The box temperature might stabilize normally or even be slightly higher due to inefficient heat exchange near the end of the coil. * **D) the receiver level will be abnormally high due to a reduced amount of refrigerant returning back to the compressor:** The amount of refrigerant returning to the compressor (mass flow rate) is directly proportional to the amount leaving the condenser and entering the evaporator. A low superheat setting means the valve is allowing **more** refrigerant flow into the evaporator, not less. Therefore, the refrigerant is moving faster through the system, and the receiver level would likely be lower (or normal, depending on system charge), not abnormally high. An abnormally high receiver level is usually associated with overcharging or a blockage/restriction downstream of the receiver.