Which of these is an automatic metering device?

The thermal expansion valve (TXV) is an automatic metering device because it regulates the flow of refrigerant into the evaporator based on the cooling demand of the system and the pressures within the system. The TXV adjusts the amount of refrigerant that enters the evaporator by responding to changes in temperature and pressure, making it highly effective for managing the heat exchange process.

As the evaporator cools down and the temperature changes, the TXV senses these changes and modulates the flow accordingly. This automatic response helps maintain optimal system performance, enhances efficiency, and ensures that the evaporator operates under the best conditions for heat absorption.

In contrast, other options like the fixed piston and capillary tube are considered fixed metering devices. The fixed piston does not adjust for changing system conditions, meaning the flow of refrigerant is constant and can lead to inefficiencies. The capillary tube has a predetermined flow rate and does not adapt to varying loads. Meanwhile, the electric expansion valve, while also automatic, can provide additional features like electronic control, but the TXV is the classic example of an automatic metering device.