The direct expansion (DX) system comprises a direct expansion coil and a condensing unit (CDU), typically with the CDU containing the compressor, condenser and expansion device. In certain applications, the expansion device is located in the indoor unit, which also includes the DX coil and fan.
Refrigerant coil or DX coil
Direct = transfer heat to/from the refrigerant directly
Expansion = refrigerant will evaporate and expand to produce
a cooling effect.
Note: the DX coil should be selected at a SST (Saturated Suction Temperature) that is suitable for application. This SST will enable acceptable system modulation as indoor and outdoor conditions change.
- A lower design SST will limit the
- minimum mixed air temperature (coil entering temperature),
- minimum airflow modulation, and/or
- minimum outdoor operating temperature.
- The maximum operating SST should not exceed the level detrimental to the compressor.
- Consult manufacturer’s data, as this will vary between compressors.
- Generally, SST below 12.8 °C (55 °F) is considered safe for most scroll compressors.
Related notes:
Vapor compression refrigeration cycle key component
How the vapor compression refrigeration cycle work?
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