A refresher for people already in the trade, not a training course. Always follow the equipment manufacturer’s instructions and current codes.
Introduction to Refrigeration
The Basic Refrigeration Cycle
The refrigeration cycle is the foundation of all HVACR work. It consists of four main stages:
Low-pressure gas is compressed to high-pressure, high-temperature gas.
High-pressure gas releases heat and condenses into liquid.
Liquid passes through TXV, pressure and temperature drop.
Low-pressure liquid absorbs heat and evaporates into gas.
Key Components: Compressor, Condenser, Expansion Valve, Evaporator
Check yourself: What happens during the compression stage?
- Gas cools down
- Pressure increases
- Liquid forms
- Heat is released
Answer: B. Pressure increases
Superheat & Subcooling
Understanding Superheat and Subcooling
These measurements are critical for proper system charging and troubleshooting.
Superheat
Temperature above saturation
Suction Line Temp - Saturation Temp
Subcooling
Temperature below saturation
Saturation Temp - Liquid Line Temp
Target Values: Superheat: 10-20°F | Subcooling: 10-15°F
Check yourself: What is the formula for superheat?
- Saturation - Suction
- Suction - Saturation
- Head - Suction
- Ambient - Saturation
Answer: B. Suction - Saturation
TXV vs EEV
Thermostatic Expansion Valve vs Electronic Expansion Valve
| Feature | TXV | EEV |
|---|---|---|
| Control Method | Diaphragm & Bulb | Stepper Motor |
| Precision | Good | Excellent |
| Superheat Adjustment | Fixed or Adjustable | Dynamic |
| Cost | Lower | Higher |
| Applications | Standard Systems | VRF, Precision Cooling |
Best Practice: Always check superheat after TXV installation to verify proper operation.
Check yourself: Which valve uses a stepper motor for control?
- TXV
- EEV
- Both
- Neither
Answer: B. EEV
Refrigerants & Oils
POE vs MOE & Associated Refrigerants
Mineral Oil (MOE)
- Used with CFCs and HCFCs (R-22, R-12)
- Not miscible with HFC refrigerants
- Hygroscopic - absorbs moisture
Polyolester Oil (POE)
- Used with HFCs (R-410A, R-134a)
- Highly hygroscopic - must keep sealed
- Compatible with newer refrigerants
Check yourself: Which oil is used with R-410A?
- Mineral Oil
- POE
- Alkylbenzene
- PAG
Answer: B. POE
Pressure-Temperature Relationships
Reading a P/T Chart
Every refrigerant has a fixed relationship between pressure and its boiling (saturation) point. A pressure-temperature chart lets you convert a gauge reading into a saturation temperature in seconds.
- Higher pressure always corresponds to a higher saturation temperature, for any refrigerant.
- Each refrigerant (R-410A, R-32, R-454B, R-22) has its own chart or dial on a manifold gauge.
- Match the gauge to the refrigerant type printed on the equipment nameplate — using the wrong chart gives false readings.
Check yourself: A pressure-temperature chart is used to convert what into what?
- Voltage into amperage
- Gauge pressure into saturation temperature
- CFM into static pressure
- Superheat into subcooling
Answer: B. Gauge pressure into saturation temperature
Diagnosing Charge Problems
Reading the Symptoms of a Bad Charge
- High superheat
- Low subcooling
- Low suction pressure
- Low superheat
- High subcooling
- High head pressure
Non-condensables: Air trapped in the system raises head pressure without raising subcooling proportionally — a common sign the system needs to be evacuated and recharged, not just topped off.
Check yourself: High superheat with low subcooling most likely points to which condition?
- Overcharge
- Undercharge
- Restricted airflow
- Bad thermostat
Answer: B. Undercharge
Recovery, Evacuation & Reclaim
Handling Refrigerant Responsibly
- Recovery: removing refrigerant from a system into a recovery cylinder without necessarily cleaning it.
- Evacuation: pulling a deep vacuum (typically below 500 microns) to remove air and moisture before recharging.
- Reclaim: refrigerant that has been cleaned to AHRI purity standards so it can be resold or reused.
Check yourself: What is the main purpose of deep evacuation before charging a system?
- To increase superheat
- To remove air and moisture from the system
- To test the compressor
- To pressurize the liquid line
Answer: B. To remove air and moisture from the system
Leak Detection Methods
Finding Refrigerant Leaks
- Electronic leak detector: senses halogen gas concentration in the air - fast and sensitive, the field standard.
- Bubble solution: soapy solution brushed on joints and fittings - visual confirmation, useful for pinpointing a known area.
- UV dye: added to the system, then checked later with a UV light - good for slow, hard-to-find leaks.
- Nitrogen pressure test: pressurize the system with dry nitrogen and monitor for pressure drop before ever charging refrigerant into it.
Check yourself: What is the field-standard tool for quickly scanning a system for a refrigerant leak?
- Bubble solution
- UV dye
- Electronic leak detector
- Manometer
Answer: C. Electronic leak detector
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