Chapter 3: The Refrigeration Cycle (Vapor Compression Cycle)

 

Chapter 3: The Refrigeration Cycle (Vapor Compression Cycle)

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🔹 3.1 What is Refrigeration?

Refrigeration is the process of removing heat from a space or substance to lower its temperature and maintain it below ambient.

➡️ Used in HVAC for:

Air conditioning rooms

Preserving food and medicine
Cooling industrial machines/data centers

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🔹 3.2 Types of Refrigeration Systems (Overview)

Type	Used in
Vapor Compression System	Most common in homes, offices
Absorption System	Industrial applications (uses heat)
Thermoelectric Cooling	Small electronics, car coolers

We focus here on the Vapor Compression Refrigeration Cycle (VCRC), which is the core of HVAC cooling.

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🔹 3.3 Main Components of Vapor Compression Cycle

Compressor

Sucks low-pressure refrigerant vapor
Compresses it into high-pressure, high-temperature vapor
🔧 Heart of the system

Condenser 

Releases heat from refrigerant to surroundings

• Converts vapor → liquid
  🔧 Placed outside the room

Expansion Valve (Capillary Tube/TXV)
Drops pressure of refrigerant
Cools liquid refrigerant
🔧 Acts like a nozzle
Evaporator
Absorbs heat from room air
Refrigerant evaporates into vapor
🔧 Located inside the room (cooling coil)

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🔄 3.4 Cycle Process (Step-by-Step)

1. Compressor → 2. Condenser → 3. Expansion Valve → 4. Evaporator → Back to Compressor

🧊 Detailed Flow:

Evaporator: Refrigerant absorbs heat from indoor air → becomes gas

Compressor: Compresses gas to high pressure
Condenser: Releases heat to outside → refrigerant turns liquid
Expansion Valve: Drops pressure → liquid becomes cold and re-enters evaporator

📌 Refrigerant (e.g., R-22, R-410A, R-32) is the fluid that cycles and transfers heat.

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📊 3.5 Pressure-Temperature Graph (P-H Diagram)

A Pressure-Enthalpy diagram helps HVAC engineers analyze energy use in each stage:

• Compression → Condensation → Expansion → Evaporation
  (Diagram can be provided upon request.)

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🛠️ 3.6 Practical Examples

Split AC:

Indoor unit = Evaporator
Outdoor unit = Compressor + Condenser

Refrigerator:

Back coil = Condenser
Inside coil = Evaporator

Chillers in buildings:

Same cycle but uses chilled water instead of air directly

❗ 3.7 Common Refrigeration Terms

Term	Meaning
Refrigerant	Heat-carrying fluid in the cycle
Subcooling	Liquid refrigerant cooled below condensing temp
Superheating	Vapor refrigerant heated above boiling point
COP	Efficiency ratio (Cooling Output / Input)

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🧠 Why Refrigeration Cycle Matters in HVAC?

✅ Controls temperature
✅ Maintains humidity
✅ Allows air conditioning, refrigeration, and process cooling
✅ Helps in energy-efficient cooling designs

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📄 Assignment Questions

1. Draw and label the vapor compression refrigeration cycle.

2. Explain the role of each component in the cycle.

3. What is the function of a refrigerant?

4. Define subcooling and superheating.

5. Compare compression and absorption refrigeration systems.

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💼 Interview Questions

1. How does a vapor compression cycle work?

2. What is the function of the expansion valve?

3. Why is refrigerant used in HVAC systems?

4. What happens if a compressor fails?

5. Name three types of refrigerants and their applications.