♨️Thermodynamics of Fluids Unit 14 – Refrigeration and Liquefaction

Key Concepts and Definitions

• Refrigeration process of removing heat from a low-temperature reservoir and transferring it to a high-temperature reservoir
• Liquefaction process of converting a gas into a liquid by cooling it below its critical temperature
• Refrigerant working fluid in a refrigeration system that absorbs and releases heat
  • Common refrigerants include ammonia (R-717), hydrofluorocarbons (HFCs), and carbon dioxide (R-744)
• Coefficient of Performance (COP) ratio of the desired cooling effect to the required work input
• Carnot cycle ideal thermodynamic cycle that sets the upper limit for the efficiency of any refrigeration system
• Critical point specific temperature and pressure at which the liquid and vapor phases of a substance become indistinguishable
• Enthalpy measure of the total heat content of a system
• Latent heat energy absorbed or released during a phase change at constant temperature

Thermodynamic Principles Behind Refrigeration

• First Law of Thermodynamics energy cannot be created or destroyed, only transferred or converted between different forms
  • In refrigeration, heat is transferred from a low-temperature reservoir to a high-temperature reservoir
• Second Law of Thermodynamics heat naturally flows from a high-temperature reservoir to a low-temperature reservoir
  • Refrigeration requires work input to move heat against its natural flow direction
• Clausius Statement of the Second Law heat cannot spontaneously flow from a colder body to a hotter body without external work
• Kelvin-Planck Statement of the Second Law no heat engine can convert all its input heat into work in a cyclic process
• Entropy measure of the unavailability of a system's thermal energy for conversion into mechanical work
  • Refrigeration aims to minimize the increase in entropy during the heat transfer process
• Isentropic process thermodynamic process in which the entropy of the system remains constant
• Isothermal process thermodynamic process that occurs at a constant temperature

Refrigeration Cycles Explained

• Vapor-compression cycle most common refrigeration cycle, consisting of four main processes: compression, condensation, expansion, and evaporation
  1. Compression: Refrigerant vapor is compressed to a high pressure and temperature
  2. Condensation: High-pressure refrigerant vapor condenses into a liquid, releasing heat to the environment
  3. Expansion: Liquid refrigerant expands through a throttling device, reducing its pressure and temperature
  4. Evaporation: Low-pressure refrigerant absorbs heat from the cooled space and evaporates back into a vapor
• Absorption refrigeration cycle uses a binary mixture of refrigerant and absorbent to achieve cooling
  • Common working pairs include ammonia-water and lithium bromide-water
• Thermoelectric refrigeration uses the Peltier effect to create a temperature difference between two electrical junctions
• Stirling cycle closed-cycle regenerative heat engine that can be used for refrigeration
• Joule-Thomson effect change in temperature of a gas when it is forced through a valve or porous plug while kept insulated
• Cascade refrigeration system uses multiple refrigeration cycles with different refrigerants to achieve lower temperatures

Liquefaction Process and Methods

• Linde-Hampson cycle uses isenthalpic expansion and regenerative cooling to liquefy gases
  • Commonly used for liquefying air and its components (nitrogen, oxygen, and argon)
• Claude cycle modification of the Linde-Hampson cycle that incorporates an expander to perform work and improve efficiency
• Kapitza cycle uses a combination of regenerative cooling and isentropic expansion to liquefy helium
• Magnetic refrigeration uses the magnetocaloric effect to achieve very low temperatures near absolute zero
• Joule-Thomson valve throttling device that causes a gas to expand and cool without performing external work
• Cryogenic liquids liquids with extremely low boiling points, such as liquid nitrogen (77 K) and liquid helium (4.2 K)
• Ortho-para conversion process by which the spin isomers of hydrogen (ortho and para) interconvert during liquefaction

Components of Refrigeration Systems

• Compressor increases the pressure and temperature of the refrigerant vapor
  • Types include reciprocating, scroll, screw, and centrifugal compressors
• Condenser heat exchanger that removes heat from the high-pressure refrigerant vapor, causing it to condense into a liquid
  • Can be air-cooled or water-cooled
• Expansion device reduces the pressure and temperature of the liquid refrigerant
  • Examples include capillary tubes, thermostatic expansion valves (TXV), and electronic expansion valves (EEV)
• Evaporator heat exchanger that absorbs heat from the cooled space, causing the low-pressure refrigerant to evaporate
• Refrigerant lines connect the components of the refrigeration system and carry the refrigerant
• Accumulator prevents liquid refrigerant from entering the compressor
• Receiver stores excess liquid refrigerant and ensures a steady supply to the expansion device
• Filter-drier removes moisture and contaminants from the refrigerant

Performance Metrics and Efficiency

• Coefficient of Performance (COP) ratio of the cooling capacity to the power input
  • Higher COP indicates better efficiency
  • $COP = \frac{Q_c}{W}$, where $Q_c$ is the cooling capacity and $W$ is the work input
• Carnot COP theoretical maximum COP for a refrigeration cycle operating between two temperatures
  • $COP_{Carnot} = \frac{T_c}{T_h - T_c}$, where $T_c$ and $T_h$ are the absolute temperatures of the cold and hot reservoirs
• Volumetric efficiency ratio of the actual volume of refrigerant discharged by the compressor to the theoretical maximum
• Isentropic efficiency compares the actual compressor performance to an ideal isentropic compression process
• Exergy efficiency measures the ratio of the actual COP to the Carnot COP
• Capacity the amount of heat removed by the refrigeration system per unit time
  • Measured in watts (W) or British thermal units per hour (BTU/h)
• Energy Efficiency Ratio (EER) ratio of the cooling capacity (in BTU/h) to the power input (in watts) under specific test conditions

Real-World Applications

• Domestic refrigerators and freezers used for food storage and preservation in households
• Air conditioning systems provide comfort cooling in buildings and vehicles
  • Can be window units, split systems, or central air conditioning systems
• Industrial refrigeration used in food processing, chemical plants, and manufacturing facilities
  • Examples include blast freezers, cold storage warehouses, and process chillers
• Cryogenic applications involve the use of very low temperatures for various purposes
  • Liquid nitrogen is used for cryopreservation of biological samples and superconductivity research
  • Liquid helium is used in magnetic resonance imaging (MRI) and particle accelerators
• Heat pumps use refrigeration principles to provide space heating and water heating
  • Can extract heat from air, ground, or water sources
• Refrigerated transport used to transport perishable goods over long distances
  • Includes refrigerated trucks, containers, and railway cars
• Liquefied natural gas (LNG) production involves the liquefaction of natural gas for storage and transportation

Challenges and Future Developments

• Phaseout of ozone-depleting refrigerants (CFCs and HCFCs) under the Montreal Protocol
  • Transition to alternative refrigerants with lower global warming potential (GWP)
• Improving energy efficiency to reduce power consumption and environmental impact
  • Development of advanced compressors, heat exchangers, and control systems
• Adoption of natural refrigerants (CO2, ammonia, hydrocarbons) with low GWP and zero ozone depletion potential
• Integration of renewable energy sources (solar, geothermal) with refrigeration systems
• Waste heat recovery using absorption refrigeration or heat pumps
• Miniaturization of refrigeration systems for portable and wearable applications
• Development of advanced materials (magnetocaloric, electrocaloric) for solid-state refrigeration
• Optimization of refrigeration systems using machine learning and artificial intelligence techniques