What valve is used to prevent evaporator pressure from falling below a set limit?

The evaporator pressure regulator (EPR) is utilized to maintain the evaporator pressure within a predetermined range. This valve plays a crucial role in preventing the pressure from dropping below a specified limit, ensuring that the refrigeration system operates efficiently. By regulating the flow of refrigerant into the evaporator based on the pressure present, the EPR helps to maintain optimal performance and prevent issues such as evaporator flooding or insufficient evaporation. This is particularly important in systems where low evaporator pressure could lead to reduced refrigeration capacity or potential system damage.

In contrast, other types of regulators serve different purposes within a refrigeration system. The crankcase pressure regulator, for instance, is designed to keep the pressure in the compressor's crankcase within a certain range to ensure proper lubrication and avoid compressor damage. The oil pressure regulator is focused on managing oil flow and pressure to ensure adequate lubrication of the compressor. Meanwhile, the head pressure regulator is used to control the pressure on the discharge side of the compressor, which is essential for maintaining proper operation but does not directly address low evaporator pressure.

Thus, the purpose and function of the EPR center specifically on maintaining evaporator pressure, making it the correct choice for this question.