The expansion device in a window air conditioner is responsible for regulating the flow of refrigerant into the evaporator coil. It is a critical component that controls the refrigeration cycle by allowing the high-pressure liquid refrigerant to expand and convert into a low-pressure gas.


There are several types of expansion devices that can be used in air conditioning systems, including window air conditioners. The options include:

1. Capillary Tube:
The correct answer to this question is the capillary tube. A capillary tube is a small, narrow tube made of copper or aluminum. It is a simple and inexpensive expansion device that does not require any moving parts. The capillary tube acts as a fixed-length, fixed-diameter restriction in the refrigerant flow path. It creates a pressure drop, causing the liquid refrigerant to expand and vaporize as it passes through the tube. The length and diameter of the capillary tube are carefully selected to match the refrigerant and system requirements.

2. Thermostatic Expansion Valve (TXV):
A thermostatic expansion valve is another commonly used expansion device in air conditioning systems. It is a more advanced and adjustable device compared to the capillary tube. The TXV has a sensing bulb that senses the temperature at the evaporator outlet. Based on this temperature, the valve modulates the refrigerant flow to maintain optimal evaporator pressure and superheat. However, window air conditioners typically do not use TXVs due to their higher cost and complexity.

3. Automatic Expansion Valve (AXV):
The automatic expansion valve is a variation of the thermostatic expansion valve that does not require an external sensing bulb. It uses internal pressure and temperature sensors to regulate the refrigerant flow. Similar to the TXV, AXVs offer better control over the system, but they are not commonly used in window air conditioners.

4. Float Valve:
A float valve is used in some refrigeration systems, such as ice machines, to control the liquid refrigerant level in the evaporator. It opens and closes based on the liquid level, allowing the refrigerant to enter the evaporator when needed. However, float valves are not typically used in window air conditioners.


In conclusion, the expansion device used in window air conditioners is the capillary tube. It is a simple and cost-effective device that creates a pressure drop and allows the liquid refrigerant to expand and vaporize. Other options, such as thermostatic expansion valves, automatic expansion valves, and float valves, are less commonly used in window air conditioners due to their higher cost or complexity.