Direct expansion, or DX, systems use the vapor-compression refrigeration cycle to remove heat from enclosed spaces and transfer it to the outside environment. The term “direct expansion” refers to the fact that the refrigerant expands to produce a cooling effect that is delivered directly to the conditioned air. DX systems are used in a variety of applications, including refrigerators, freezers, split systems, and package AC or rooftop units (RTUs). In the vapor-compression refrigeration cycle, a “refrigerant” alternately absorbs and rejects heat as it circulates through the system. The system has four main components - a compressor, condenser, expansion device, and evaporator - and the refrigerant changes pressure, temperature, and phase as it circulates through these components. This course introduces the four main DX system components, including their purpose and design. It also describes several other common system components, temperature-sensing devices, several types of valves, pressure switches, and defrost controls.
