All phase change refrigerant systems require an expansion device which controls the flow of refrigerant in the evaporator. Two principal types of control are used: Thermal Expansion Valves or Capillary Tubes.
Thermal Expansion Valves balance and modulate the refrigerant flow to the heat load by sensing the temperature of the refrigerant leaving the evaporator. There are three major advantages to this refrigerant control method.
- Maximum efficiency over a wide temperature and load range
- Improved refrigerant return to the compressor assures better cooling at high temperatures and reduces the possibility of liquid slugging which can destroy the compressor.
- Variations in refrigerant charge, particularly in smaller units, are less critical
Alternately, fixed expansion devices, such as Capillary Tubes, work at one preset level and have no ability to compensate for load changes. They are more commonly used in unchanging environmental temperatures with refrigerators and freezers. Since most refrigerators are in a temperature controlled space and have a limited temperature set point, they work just fine. Due to their simplicity, capillary tubes are very inexpensive. (However, this rarely translates into a cost saving for the purchaser of electrical enclosure air conditioning.)