The purpose of a direct expansion, or DX, system is to remove unwanted heat from a space or substance and transfer it to another space or substance. DX refrigeration systems are sometimes called mechanical refrigeration systems because they use mechanical components to remove and transfer heat. They are widely used in refrigerators, freezers, split systems, and package AC or rooftop units (RTUs). DX systems use the vapor-compression refrigeration cycle in which a “refrigerant” alternately absorbs and rejects heat as it circulates through four main components – the compressor, condenser, expansion device, and evaporator. The temperature and pressure of the refrigerant can be checked at several locations to monitor system operations and help with troubleshooting efforts. This course discusses how system temperatures, pressures, superheat, and subcooling can be used to monitor the condition of a DX system, help troubleshoot problems, and charge refrigerant. When troubleshooting a DX system, make sure any troubleshooting chart you are using is appropriate for the expansion device, as symptoms differ for fixed orifice devices and TXVs.
