Downstream of the FCV is the cold-air unit (CAU), also referred to as the refrigeration unit. There are many various types of CAUs; however, they all use typical fundamentals. The bleed air enters the primary ram-air heat exchanger, where it is cooled by either ram air, expansion or a combination of both. The cold air then enters the compressor, where it is repressurized, which reheats the air. A pass through the secondary ram-air heat exchanger cools the air while maintaining the high pressure. The air then passes through a turbine which expands the air to further reduce heat.