Refrigerant Cycle
Students must understand that the AC system doesn't "create cold" - it moves heat from inside the cab to outside. Every component has a specific role in this heat transfer process.
Function: Pressurizes refrigerant and circulates it through the system
Location: Belt-driven, mounted on engine
Key Points:
- Creates the pressure difference that drives the cycle
- Pumps low-pressure vapor to high-pressure vapor
- Refrigerant leaves compressor hot and high-pressure
Function: Releases heat to outside air
Location: On Bobcat loaders - either with the cooling package or on the loader tailgate
Key Points:
- Hot high-pressure vapor enters, cool liquid leaves
- Airflow across fins removes heat from refrigerant
- Refrigerant changes state from vapor to liquid
Function: Drops pressure and temperature
Location: Between condenser and evaporator
Key Points:
- Creates pressure drop from high to low side
- Refrigerant becomes cold and partially vaporizes
- Controls refrigerant flow into evaporator
Function: Absorbs heat from cab air
Location: In HVAC housing
Key Points:
- Cold low-pressure refrigerant absorbs heat
- Blower moves warm cab air across cold evaporator
- Refrigerant changes from liquid to vapor
Refrigerant has a predictable relationship between pressure and temperature.
At any given pressure, refrigerant will boil (change from liquid to vapor) at a specific temperature.
This relationship is key to diagnosing system problems.
Think about why our engine cooling systems are pressurized - it raises the boiling point of water!
| Refrigerant State | Pressure | Temperature | Location in System |
|---|---|---|---|
| High-Pressure Vapor | High (200-300 PSI) | Hot (180-200°F) | Leaving compressor |
| High-Pressure Liquid | High (200-300 PSI) | Warm (120-140°F) | Leaving condenser |
| Low-Pressure Liquid | Low (30-40 PSI) | Cold (30-40°F) | Leaving expansion device |
| Low-Pressure Vapor | Low (30-40 PSI) | Cold (40-50°F) | Leaving evaporator |
Heat that changes temperature without changing state. Example: Warming refrigerant from 40°F to 50°F while it remains liquid.
Heat that changes state without changing temperature. Example: Refrigerant boiling at 40°F - it absorbs heat but stays at 40°F until fully vaporized.
Low-pressure vapor (30-40 PSI, 40-50°F) enters compressor
Compressor pressurizes vapor to 200-300 PSI
Temperature rises to 180-200°F due to compression
Hot vapor enters condenser
Airflow removes heat, refrigerant cools to 120-140°F
Refrigerant condenses from vapor to liquid (latent heat transfer)
Liquid refrigerant passes through expansion device
Pressure drops to 30-40 PSI
Temperature drops to 30-40°F
Some refrigerant "flashes" to vapor (partial boiling)
Cold refrigerant enters evaporator
Warm cab air (70-80°F) blows across evaporator fins
Refrigerant absorbs heat and boils (latent heat transfer)
Air leaving evaporator is 40-50°F - cold air to cab
Cold vapor (30-40 PSI, 40-50°F) returns to compressor
Cycle repeats continuously
Oil circulates with refrigerant through the entire system. Too little oil = compressor failure. Too much oil = reduced cooling capacity. Always account for oil during recovery and replacement.
- Lubricates compressor moving parts
- Helps seal compressor components
- Carries away heat from compressor
- Provides some cleaning action
- Measure oil recovered from system
- Add same amount of new oil to replacement components
- Never add oil without knowing system capacity
- Use only approved PAG or POE oil types
| Factor | Effect on System | What to Check |
|---|---|---|
| Condenser Airflow | Poor heat rejection = high pressures | Debris, fan operation, fin damage |
| Evaporator Airflow | Poor heat absorption = low cooling | Blower speed, cabin filter, evaporator ice |
| Refrigerant Charge | Too much or too little = poor performance | Pressures, sight glass (if equipped) |
| Compressor Efficiency | Weak compression = low pressures | Pressure differential, noise, temperature |