Refrigerant Cycle
The AC system doesn't "create cold" - it moves heat from inside the cab to outside. I may interchange the terms 'heat' and 'energy' in this training. They are really the same thing. Also, you will hear me refer to the 'load' of the system. This is the amount of heat (or energy) that is getting put into the evaporator by the cab air.
A small restriction that drops pressure and controls flow. The expansion device works the same way — high-pressure liquid refrigerant passes through a restriction, pressure collapses, and the fluid gets cold.
Moves fluid by raising its pressure. The AC compressor is the refrigerant pump — it pulls in low-pressure vapor and pushes out high-pressure vapor to keep the cycle moving. An important difference - The AC compressor only wants to compress gas, not liquid.
Rejects heat to passing air. The AC condenser does the same job — hot refrigerant vapor flows through fins, airflow strips the heat away, and the refrigerant comes out as cool liquid.
Boiling water absorbs heat without getting hotter while it boils. Refrigerant does the same thing in the evaporator — it boils at 40°F, pulling heat from cab air without the temperature rising until it's fully vaporized.
Raising pressure raises the boiling point — that's why equipment cooling systems are pressurized. Refrigerant works in a similar way: at low pressure in the evaporator it boils easiely at a low temperature, but the compressor raises its pressure so it can condence into a liquid even while at a high temperature.
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
- Similar to the way a hydraulic pump creats the flow necessary for pressure, the compressor creats the flow necessary for the expansion valve to do its job.
- Refrigerant leaves compressor hot and high-pressure
Function: Drops pressure
Location: Very close to the inlet of the evaporator
Key Points:
- A variable orifice that creates pressure drop from high to low side
- Refrigerant becomes cold and partially vaporizes
- Has feedback that controls refrigerant flow into evaporator.
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: 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 state from liquid to vapor
Function: Absorbs moisture and debris from refrigerant
Location: Between condenser and expansion valve
Key Points:
- Removes moisture and contaminants from refrigerant
- Stores a small amount of liquid refrigerant
Moisture that gets into the refrigerant circuit doesn't just get carried along harmlessly - it causes lasting damage. Moisture reacts with refrigerant to form corrosive acids that attack compressor internals and metal lines. At the expansion device, moisture can freeze at the pressure drop and block the orifice, causing intermittent or total loss of cooling. Moisture also breaks down refrigerant oil, reducing its lubricating properties. This is why the system must always be evacuated under a deep vacuum before charging, and why the receiver/dryer's desiccant has a limited capacity - never leave a system open to atmosphere any longer than necessary.
Function: Protects system from lack of refrigerent or excessive pressure
Location: On the receiver dryer or in the high-pressure line
Key Points:
- Monitors system high and low pressure
- Cycles compressor on/off
Function: Controls temperature of evaporator
Location: Near HVAC box
Key Points:
- Monitors and controls evaporator temperature
- Cycles compressor on and off by system load
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 — see the System Oil Accounting reference.
- Lubricates compressor moving parts
- Helps seal compressor components
- Carries away heat from compressor
- Provides some cleaning action
- Typical Bobcat systems hold 150 - 200 ml of oil (5 - 6.7oz)
- Measure oil recovered from system.
- Add same amount of new oil to replacement components
- Never add oil without knowing system capacity
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.
Once refrigerant finishes changing state, any further heat added or removed is sensible heat — this is where superheat and subcooling come from. Superheat is the sensible heat added to refrigerant vapor after it has fully boiled off in the evaporator (its temperature above the boiling/saturation point). Subcooling is the sensible heat removed from refrigerant liquid after it has fully condensed in the condenser (its temperature below the condensing/saturation point). Both confirm the refrigerant has fully changed state at that point in the system.
Low-pressure vapor (Less than 30psi) enters compressor
Compressor pressurizes vapor somewhere close to 8x
Temperature rises due to compression
Hot vapor enters condenser
Airflow removes heat, refrigerant cools by at least 20degF
Refrigerant condenses from vapor to liquid (latent heat transfer)
Liquid refrigerant passes through expansion device
Pressure drops to about 30psi or less
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 45-53°F - cold air to cab
Cold vapor returns to compressor
Cycle repeats continuously