What will happen if the expansion valve in a refrigeration system is stuck closed?

A stuck closed expansion valve will result in pressure build-up in the evaporator because the valve's primary function is to regulate the flow of refrigerant into the evaporator. When the valve is closed, refrigerant cannot enter the evaporator, leading to a significant buildup of pressure since the refrigerant that is already present cannot escape. This pressure build-up can cause various operational issues, including inefficient cooling and potential damage to the system components.

The other options present scenarios that do not align with the consequences of a closed valve. Increased cooling efficiency would not occur since the evaporator would not be receiving the necessary refrigerant to absorb heat and provide cooling. Lower suction pressure is also unlikely because a closed valve restricts flow, causing higher pressure instead. Lastly, optimal refrigerant flow is clearly not achievable if the valve is not allowing any refrigerant to pass through. Hence, pressure build-up in the evaporator is the correct outcome of a stuck closed expansion valve.