Which evaporator design must utilize a four-pipe system when employing gas defrost?

The choice of a direct expansion (DX) evaporator requiring a four-pipe system for gas defrosting is rooted in its design and operational mechanism. A DX evaporator circulates refrigerant directly through the evaporator coils, which allows for efficient heat transfer and effective cooling of the surrounding air.

In a gas defrost system, hot gas is routed through the coils to elevate their temperature, melting any frost or ice buildup. A four-pipe system facilitates this process by providing separate lines for the hot gas and the refrigerant, allowing the system to perform defrosting without any cross-contamination or loss of efficiency. This separation is crucial because the hot gas must be managed distinctly from the cooling refrigerant to maintain proper operation and efficiency during the defrost cycle.

Conversely, other evaporator designs, such as finned-tube, shell-and-tube, and plate evaporators, typically do not necessitate a four-pipe system for gas defrosting, either because of their inherent designs or the way they handle defrosting processes differently. Therefore, the requirement for a four-pipe system in a DX evaporator is a critical factor for the effective implementation of gas defrosting, making it the correct answer.