Cooling of electrical equipment often represents the largest cooling load on military aircraft environmental control systems. When this load is not accompanied by a similarly large fresh air ventilation demand, it is appropriate to consider closed-loop cooling systems so as to minimize power consumption. Vapor-cycle refrigeration generally is considered the most efficient. However, there are limitations on sink temperature which can make vapor-cycle refrigeration unusable in specific instances. Further, there are other applications where the available sink is marginal for known phase-change refrigerants. In such circumstances the efficiency or coefficient-of-performance of a closed-loop air-cycle refrigeration system can approach that of the vapor cycle. As the power advantage becomes smaller, the inherent maintenance or operational disadvantage of the vapor cycle tends to become an overriding consideration.