Which evaporator design must include the 4-pipe defrost if using hot gas defrost?

The correct answer for the requirement of a 4-pipe defrost system when using hot gas defrost is based on the characteristics of Direct Expansion (DX) evaporators.

DX evaporators utilize a refrigerant that expands directly within the evaporator coil. When a hot gas defrost is employed, it is essential to use a 4-pipe configuration to effectively manage the flow of refrigerant and heat. In this setup, one set of pipes carries the refrigerant to cool the space, while another set allows for the hot gas to circulate through the evaporator coil for defrosting purposes. This design ensures that the defrost process can be efficiently separated from the operation of the cooling function, maintaining the integrity of the system during both cooling and defrost cycles.

The requirement for a 4-pipe system is particularly important in DX systems because it allows for better control of the temperature and prevents cross-contamination of the refrigerant supply during the defrost process, which is crucial for maintaining efficiency and performance during operation.