Introduction to Refrigeration

Definition of Refrigeration

• Refrigeration: Process of maintaining a temperature of a confined space lower than its surroundings.
• Cooling vs. Refrigeration:
  • Cooling: Heat transfer from higher temperature to lower temperature.
  • Refrigeration: Actively maintaining lower temperature by extracting heat.

Units of Refrigeration

• Ton of Refrigeration:
  • Two types: Ton (2000 pounds ≈ 907.2 kgs) and Tonne (metric ton, 1000 kgs).
  • One Ton of Refrigeration: Ability to produce one ton of ice from 32°F water to 32°F ice in 24 hours.
  • Equivalent to 12,000 BTU per hour or approximately 3.5 kW.

British Thermal Unit (BTU)

• BTU: Heat required to increase the temperature of water by 1°F.
  • 1 BTU = 0.2522 kilocalories
  • In SI: 1 BTU ≈ 1055 joules

Conversion and Calculation

• 12,000 BTU/hr in SI: ≈ 3.5 kW
• MKS System: 50 kilocalories/minute or 211 kilojoules/minute

Carnot Cycle

• Power Generation:

  • Consists of four processes (adiabatic compression, isothermal heat addition, etc.).
  • Efficiency benchmark for power generating cycles.

• Refrigeration:

  • Reverse Carnot cycle: Transfers heat from lower to higher temperature.
  • Comparison standard for refrigeration cycles.

Coefficient of Performance (COP)

• Measure of efficiency for refrigeration systems.
• COP = Cooling effect / Work spent
• For Carnot Cycle: COP = TL / (TH - TL)

Heat Pump

• Device for pumping heat.
• Used for heating spaces by transferring heat from outside to inside.
• Efficiency: Energy Performance Ratio (EPR) = QH / (QH - QL) = 1 + COP

Practical Application of Carnot Cycle

• Limitations in practical applications due to constant temperature heat addition/rejection.
• Vapor used instead of gas for better heat transfer during phase change.

Methods of Refrigeration

• Ice Refrigeration: Using ice in cabinets for cooling.
• Dry Ice Refrigeration: Solid CO2, useful for transporting food.
• Evaporative Refrigeration: Cooling through evaporation (e.g., earthen pots).
• Expansion of Air: Cooling through expansion after compression.
• Throttling of Gases: Using real gases for cooling through throttling.
• Vapor Compression Refrigeration: To be discussed in detail in future lectures.