Which type of evaporator requires the greatest refrigerant charge?

The flooded evaporator requires the greatest refrigerant charge among the options provided. This is because a flooded evaporator operates with the evaporator coils filled with liquid refrigerant, meaning it maintains a reservoir of refrigerant in the evaporator at all times. This design is specifically intended to maximize heat transfer efficiency and maintain optimal evaporation conditions, particularly in applications with high cooling loads.

In a flooded evaporator system, the liquid refrigerant is typically fed into the evaporator using a metering device, allowing it to occupy the evaporator's entire volume. As a result, this method necessitates a larger initial charge of refrigerant compared to other evaporator types, which may only require enough refrigerant to achieve efficient vaporization without having it completely fill the coil.

In contrast, direct expansion evaporators utilize refrigerant in a more dynamic manner, where the refrigerant enters in both liquid and vapor phases and does not require as much of a charge. Bare tube evaporators primarily rely on surface area for heat exchange rather than a high volume of refrigerant. Plate surface evaporators, often used for smaller refrigeration applications, typically also have a lower refrigerant charge due to their compact design and effective heat transfer capabilities without needing a large volume of refrigerant.