Introduction to Compressors in HVAC Systems

Types of Compressors

• Reciprocating Compressors: Traditionally common in HVAC systems.
• Rotary Compressors: Used for generally smaller units.
• Scroll Compressors: Increasingly common in refrigeration and air conditioning.
• Screw Compressors and Centrifugal Compressors: Used in very large applications.

Function of Compressors

• Compressors are designed to compress refrigerant vapor (not liquid).
• The process involves decreasing the volume of the refrigerant to increase its pressure.
• High pressure moves to low pressure, which facilitates the flow of refrigerant.

Types of Compressor Construction

• Hermetic Compressors: Completely sealed and not accessible without cutting open.
• Semi-Hermetic Compressors: Can be accessed by removing a series of bolts.
• Open Drive Compressors: Energy is introduced via a shaft, separate from the motor; these are rare due to leakage issues.

Critical Considerations for Compressor Operation

• Ensure Vapor Only: Feed compressors only with vapor, not liquid, to avoid damage.
• Temperature and Mass Flow Rate: Control the temperature and flow rate of the refrigerant to keep the compressor cool.
  • Refrigerant cooled compressors are most common, as the refrigerant cools the compressor and motor.
• Compression Ratio: This is the discharge pressure divided by the suction pressure.
  • A higher compression ratio means more work and increased temperature, affecting efficiency.

Oil Control in Compressors

• Purpose: Lubricates bearings inside the compressor.
• Risk of Foaming/Viscosity Loss: Caused by liquid refrigerant in the suction line or overheating.
• Flooded Starts: Liquid refrigerant migrating into the compressor during the off cycle can cause oil loss when the compressor starts.
  • Mitigated by using solenoids, pump down solenoids, crankcase heaters, and hard shut-off TXVs.

Basic Compressor Functionality

• Flow of Refrigerant:
  • Vapor enters through the suction line, is compressed, and exits through the discharge line.
  • Suction line has a low temperature; discharge line has a high temperature.
• Temperature Increase:
  • Compression increases molecular velocity, raising temperature.
  • Not a significant increase in heat content, just temperature.

Condensation

• Compressors cannot turn vapor directly into liquid due to increased temperature.
• Condensation occurs in the condenser where heat is rejected.

Conclusion

• This was an introductory overview of compressors.
• Further exploration of compressor failures and diagnostics will be addressed in future content.