What action is triggered by a decrease in superheat in a thermostatic expansion valve?

When discussing the functioning of a thermostatic expansion valve (TXV), it is essential to understand the role of superheat in the refrigeration cycle. Superheat is the additional heat in a vapor above its boiling point at a given pressure. A decrease in superheat signifies that the vapor entering the evaporator is less superheated than optimal.

In response to a drop in superheat, the TXV will close slightly. This action helps reduce the flow of refrigerant into the evaporator. By decreasing the flow, the valve prevents the evaporator from being flooded, which could lead to inefficient operation or compressor damage. Essentially, the TXV is designed to control the amount of refrigerant entering based on the superheat present, ensuring that the system operates within its ideal parameters.

The other options do not accurately reflect the behavior of the TXV in response to a decrease in superheat. Maintaining flow or fully opening would not address the need to control refrigerant flow in response to insufficient superheat. Thus, the correct action triggered by a decrease in superheat is for the valve to close, thereby regulating the system for optimal performance.