Introduction to Refrigeration (Unit-I)

 

Introduction to Refrigeration (Unit-I)

Definition of Refrigeration

Refrigeration is the process of removing heat from a space, substance, or system to lower its temperature below that of its surroundings. It is commonly used in food preservation, air conditioning, and industrial applications.

Refrigerating Effect & Unit of Refrigeration

• Refrigerating Effect: The amount of heat removed from a space or substance to achieve cooling.

• Unit of Refrigeration: The standard unit of refrigeration is 1 Ton of Refrigeration (TR), which equals the heat absorbed to freeze 1 ton (2000 lbs or 907 kg) of water into ice in 24 hours.

  • 1 TR = 3.5 kW = 210 kJ/min

Coefficient of Performance (COP)

COP is a measure of a refrigeration system's efficiency, calculated as:

$$COP = \frac{\text{Refrigerating Effect}}{\text{Work Input}}$$

A higher COP means a more efficient refrigeration system.

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Types of Refrigeration

1. Ice Refrigeration: Uses natural or artificial ice for cooling, historically used before mechanical refrigeration.

2. Dry Ice Refrigeration: Uses solid CO₂ (dry ice), which sublimates directly to gas at −78.5°C, absorbing heat.

3. Steam Jet Refrigeration: Uses steam ejectors to create a vacuum and cool water by rapid evaporation.

4. Throttling Refrigeration: Uses expansion of high-pressure gases to lower temperature (Joule-Thomson effect).

5. Liquid Nitrogen Refrigeration: Uses liquid nitrogen (-196°C) for ultra-low temperature cooling in medical and industrial applications.

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Carnot Refrigeration Cycle (Introduction Only)

• The Carnot cycle is an ideal refrigeration cycle that provides maximum efficiency.

• It consists of four reversible processes:

  1. Isothermal Heat Absorption (at low temperature)

  2. Adiabatic Compression

  3. Isothermal Heat Rejection (at high temperature)

  4. Adiabatic Expansion

• It serves as a benchmark for real refrigeration systems.

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Air Refrigeration – Bell-Coleman Cycle

• Also called the Reverse Brayton Cycle, used in aircraft cooling.

• Uses air as the working fluid.

• Cycle consists of:

  1. Compression (raises air temperature and pressure)

  2. Cooling (removes heat at constant pressure)

  3. Expansion (reduces temperature and pressure)

  4. Heat absorption (air absorbs heat from the surroundings)

Advantages & Disadvantages of Air Refrigeration

✅ Advantages:

• Simple and lightweight (suitable for aircraft).

• No phase change (reduces complexity).

❌ Disadvantages:

• Lower efficiency than vapor compression systems.

• Requires large compressor and expansion devices.

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Simple Problems in Refrigeration

Basic problems involve calculating COP, Refrigerating Effect, and Work Input using thermodynamic formulas.