Glossary

11.3 Gas refrigeration cycles

6 min read•july 30, 2024

Gas refrigeration cycles use gases like helium or air to move heat from cold to hot areas. These systems have compressors, heat exchangers, and expansion devices that work together to create cooling effects. They're great for super-cold applications but less efficient for everyday use.

Different types of gas cycles, like Brayton and Stirling, have unique pros and cons. Brayton cycles are simple and reliable, while Stirling cycles are more efficient but complex. Performance depends on factors like pressure ratios and heat exchanger effectiveness. These systems shine in specialized industrial and cryogenic applications.

Gas refrigeration cycles

Principles and components

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  • Gas refrigeration cycles operate on the principle of using a gas as the working fluid to transfer heat from a low-temperature source to a high-temperature sink
  • The basic components of a gas refrigeration cycle include:
    1. : Increases the pressure and temperature of the gas
    2. Heat exchanger (): Releases heat from the high-pressure, high-temperature gas to the surroundings
    3. Expansion device: Reduces the pressure and temperature of the gas
    4. Heat exchanger (): Absorbs heat from the low-temperature source into the low-pressure, low-temperature gas
  • The efficiency of a gas refrigeration cycle is influenced by factors such as:
    • Properties of the working fluid (helium, air, nitrogen)
    • Pressure ratio of the compressor
    • Effectiveness of the heat exchangers

Thermodynamic processes

  • Gas refrigeration cycles involve four main thermodynamic processes:
    1. Compression: The gas is compressed adiabatically, increasing its pressure and temperature
    2. Heat rejection: The high-pressure, high-temperature gas rejects heat to the surroundings in the condenser
    3. Expansion: The gas expands adiabatically in the expansion device, reducing its pressure and temperature
    4. : The low-pressure, low-temperature gas absorbs heat from the low-temperature source in the evaporator
  • The compression and expansion processes are typically modeled as isentropic (constant entropy) processes, while the heat rejection and absorption processes are modeled as isobaric (constant pressure) processes
  • The pressure-volume (P-V) and temperature-entropy (T-S) diagrams are used to represent the thermodynamic processes and states of the gas throughout the refrigeration cycle

Types of gas refrigeration cycles

Brayton cycle

  • The Brayton cycle, also known as the gas turbine cycle, is an open cycle that uses a continuous flow of gas as the working fluid
  • It consists of a compressor, a combustion chamber, a turbine, and a heat exchanger
  • The Brayton cycle operates at higher temperatures and pressures compared to other gas refrigeration cycles, making it suitable for applications requiring high cooling capacities (aircraft air conditioning, natural gas liquefaction)
  • Advantages of the Brayton cycle include its simplicity, reliability, and ability to use a variety of working fluids (air, helium, nitrogen)
  • Disadvantages include lower efficiency compared to other cycles and the need for high-temperature heat sources

Stirling cycle

  • The Stirling cycle is a closed cycle that uses a fixed quantity of gas as the working fluid
  • It consists of a compressor, a regenerator, an expander, and two heat exchangers
  • The Stirling cycle has a higher theoretical efficiency than the Brayton cycle due to the use of a regenerator, which minimizes heat losses during the cycle
  • Stirling cycle-based systems find applications in cryocoolers for infrared detectors, superconducting devices, and small-scale refrigeration units
  • Advantages of the Stirling cycle include its high efficiency, low noise, and ability to operate at lower temperatures than the Brayton cycle
  • Disadvantages include the complexity of the regenerator and the need for precise control of the piston motion

Other gas refrigeration cycles

  • Ericsson cycle: A closed cycle similar to the Stirling cycle but with separate compression and expansion processes, allowing for more flexibility in the design and operation of the system
  • Gifford-McMahon cycle: A closed cycle that uses a regenerative heat exchanger and a displacer to achieve low temperatures, commonly used in cryogenic applications (MRI scanners, particle accelerators)
  • Pulse tube refrigerator: A closed cycle that uses oscillating pressure waves to generate cooling, eliminating the need for moving parts in the cold end of the system, making it more reliable and compact than other cycles

Performance of gas refrigeration cycles

Efficiency metrics

  • The is a key metric for evaluating the efficiency of a gas refrigeration cycle, defined as the ratio of the cooling capacity to the input work: COP=QcWnetCOP = \frac{Q_c}{W_{net}}

where QcQ_c is the cooling capacity and WnetW_{net} is the net input work

  • The Carnot COP represents the maximum theoretical efficiency of a refrigeration cycle operating between two temperature limits, serving as a benchmark for actual cycle performance: COPCarnot=TcTh−TcCOP_{Carnot} = \frac{T_c}{T_h - T_c}

where TcT_c is the cold reservoir temperature and ThT_h is the hot reservoir temperature

  • The actual COP of a gas refrigeration cycle is lower than the Carnot COP due to irreversibilities such as friction, heat transfer limitations, and pressure drops in the system

Factors affecting performance

  • The efficiency of a gas refrigeration cycle can be improved by:
    • Optimizing the pressure ratio of the compressor
    • Minimizing pressure losses in the system
    • Enhancing heat exchanger effectiveness
    • Selecting appropriate working fluids with desirable thermodynamic properties
  • Exergy analysis can be used to identify the sources and magnitudes of irreversibilities in a gas refrigeration cycle, helping to pinpoint areas for efficiency improvements
  • Exergy destruction occurs due to factors such as:
    • Friction in the compressor and expander
    • Heat transfer across finite temperature differences in the heat exchangers
    • Mixing of streams with different temperatures or pressures
    • Pressure drops in the system components
  • Minimizing exergy destruction is crucial for improving the overall performance and efficiency of gas refrigeration cycles

Applications of gas refrigeration systems

Low-temperature applications

  • Gas refrigeration systems are commonly used in applications requiring very low temperatures, such as:
    • Cryogenic cooling (liquid nitrogen production, superconductor cooling)
    • Liquefaction of gases (natural gas, hydrogen, helium)
    • Space cooling (satellite infrared detectors, space telescopes)
  • These applications leverage the ability of gas refrigeration cycles to achieve temperatures well below those attainable by conventional vapor compression systems
  • Examples of low-temperature applications include:
    • Liquid nitrogen production for industrial processes, medical use, and cryopreservation
    • Liquefaction of natural gas for storage and transportation
    • Cooling of infrared detectors in satellites for remote sensing and astronomy

Industrial and commercial applications

  • Brayton cycle-based systems are employed in various industrial and commercial applications, such as:
    • Aircraft air conditioning
    • Natural gas liquefaction and processing
    • Industrial process cooling (chemical plants, refineries)
  • Stirling cycle-based systems find applications in:
    • Cryocoolers for infrared detectors and superconducting devices
    • Small-scale refrigeration units for medical and scientific equipment
  • Gas refrigeration systems offer advantages such as high reliability, low maintenance, and the ability to use environmentally friendly working fluids (air, helium) in these applications

Limitations and challenges

  • Gas refrigeration systems are limited by their relatively low cooling capacities compared to vapor compression systems, making them less suitable for large-scale commercial and residential cooling applications
  • The high operating pressures and temperatures in gas refrigeration systems require specialized materials and components, which can increase the cost and complexity of the system
  • The efficiency of gas refrigeration systems decreases significantly when operating at very low temperatures due to increased irreversibilities and heat transfer limitations
  • Challenges in gas refrigeration system design and operation include:
    • Optimizing the system components for specific operating conditions and working fluids
    • Minimizing leakage and ensuring proper sealing of the system
    • Developing advanced materials and manufacturing techniques to withstand extreme operating conditions
    • Integrating gas refrigeration systems with other energy systems for improved overall efficiency and performance

Key Terms to Review (20)

Absorption refrigeration: Absorption refrigeration is a cooling process that utilizes a heat source to drive a refrigeration cycle, where heat is absorbed from a low-temperature environment and rejected to a high-temperature environment. This method typically employs a refrigerant and an absorbent in a system that allows the refrigerant to be absorbed by the absorbent, creating a solution that can be heated to release the refrigerant vapor. Unlike mechanical refrigeration that uses a compressor, absorption systems rely on thermal energy to achieve cooling, which makes them useful in various applications, including those where waste heat is available.
Adiabatic process: An adiabatic process is a thermodynamic process in which no heat is transferred into or out of the system. During this type of process, any change in the internal energy of the system is solely due to work done on or by the system, making it essential in understanding how systems behave under different conditions.
Ammonia: Ammonia is a colorless gas with a distinct odor, consisting of one nitrogen atom and three hydrogen atoms (NH₃). It is commonly used as a refrigerant and is a crucial component in various refrigeration cycles, particularly in gas refrigeration and absorption refrigeration systems. Its properties, including high latent heat of vaporization, make it an effective medium for heat transfer in cooling applications.
Carnot Equation: The Carnot Equation is a mathematical representation that defines the maximum possible efficiency of a heat engine operating between two temperature reservoirs. It connects the concepts of thermodynamic efficiency, absolute temperature, and heat transfer, illustrating how efficiency can never reach 100% due to inherent energy losses in real processes.
Clausius Inequality: The Clausius Inequality is a fundamental principle in thermodynamics that states that for any real process, the change in entropy of a system is greater than or equal to the heat transferred into the system divided by the temperature at which the transfer occurs. This inequality helps establish the direction of thermodynamic processes and emphasizes that real processes are irreversible, highlighting the importance of entropy in understanding energy transformations.
Coefficient of performance (COP): The coefficient of performance (COP) is a measure of the efficiency of a heating or cooling system, defined as the ratio of useful heating or cooling provided to the work input required to produce that effect. A higher COP indicates a more efficient system, whether it be for refrigeration, heat pumping, or air conditioning applications. Understanding COP is crucial in evaluating the performance of different thermodynamic cycles and systems used for temperature control.
Compressor: A compressor is a mechanical device that increases the pressure of a gas by reducing its volume, commonly used in refrigeration and air conditioning systems to circulate refrigerants. This essential component enhances the efficiency of thermodynamic cycles by compressing low-pressure gas into high-pressure gas, facilitating heat transfer and energy conversion processes.
Condenser: A condenser is a heat exchanger used to condense vapor into liquid by removing heat from the vapor, usually through contact with a cooler surface or fluid. This process is crucial in various thermodynamic cycles and systems, enabling efficient heat transfer and phase change essential for energy conversion, refrigeration, and air conditioning.
Evaporator: An evaporator is a heat exchanger that facilitates the phase change of a refrigerant from liquid to vapor, absorbing heat from the surroundings in the process. This essential component plays a critical role in refrigeration and air conditioning systems by removing heat from a space or substance, allowing for effective temperature control and cooling. The evaporator's operation directly influences the efficiency of refrigeration cycles and heat pumps.
First Law of Thermodynamics: The First Law of Thermodynamics states that energy cannot be created or destroyed, only transformed from one form to another, which means the total energy of an isolated system remains constant. This principle underlies various processes, cycles, and energy interactions that involve heat, work, and mass transfer in different systems.
Freon: Freon refers to a class of chemical compounds known as chlorofluorocarbons (CFCs) and hydrofluorocarbons (HFCs) that are primarily used as refrigerants in various cooling systems. Freon is essential in gas refrigeration cycles due to its effective heat absorption properties, which facilitate the cooling process in appliances like refrigerators and air conditioners while also influencing the design and efficiency of these systems.
Heat absorption: Heat absorption refers to the process by which a substance takes in thermal energy from its surroundings, causing an increase in its internal energy and often leading to a change in temperature or phase. In gas refrigeration cycles, this concept is crucial as it enables the system to extract heat from a low-temperature reservoir, facilitating the cooling effect needed for refrigeration applications. This heat transfer process plays a vital role in the efficiency and effectiveness of refrigeration systems.
Ideal gas refrigeration cycle: The ideal gas refrigeration cycle is a thermodynamic cycle that uses an ideal gas as the refrigerant to transfer heat from a low-temperature reservoir to a high-temperature reservoir. This cycle typically involves four key processes: isentropic compression, isobaric heat rejection, isenthalpic expansion, and isobaric heat absorption. Understanding this cycle provides insight into how refrigeration systems operate using the principles of thermodynamics to achieve cooling.
Isothermal process: An isothermal process is a thermodynamic process in which the temperature of a system remains constant while the system undergoes a change in volume or pressure. This type of process is crucial for understanding how systems interact with their surroundings and how energy is exchanged in various thermodynamic cycles.
Real gas refrigeration cycle: The real gas refrigeration cycle is a thermodynamic cycle that describes the process of refrigeration using real gases instead of ideal gases. Unlike ideal gases, real gases exhibit behaviors like non-linear pressure-volume relationships and changes in specific heat capacities, which affect their performance in refrigeration applications. Understanding this cycle is crucial for optimizing the efficiency and effectiveness of refrigeration systems in practical settings.
Refrigerants: Refrigerants are substances used in cooling systems that undergo phase changes to absorb and release heat, thereby providing refrigeration. They play a crucial role in various thermodynamic cycles, including the Rankine cycle and gas refrigeration cycles, where they facilitate the transfer of heat from one location to another to achieve desired temperature reductions.
Refrigeration Effect: The refrigeration effect is the amount of heat absorbed from a low-temperature reservoir by a refrigerating system to achieve cooling. It is a crucial concept in understanding how refrigeration cycles operate, particularly in the context of transferring heat from inside a space to the outside environment, making it essential for both vapor-compression and gas refrigeration processes.
Second Law of Thermodynamics: The Second Law of Thermodynamics states that the total entropy of an isolated system can never decrease over time, and it tends to increase, leading to the concept that energy transformations are not 100% efficient. This law establishes the directionality of processes, implying that certain processes are irreversible and energy has a quality that degrades over time, connecting tightly to concepts of heat transfer, work, and system analysis.
Thermal efficiency: Thermal efficiency is a measure of how well an energy conversion system, such as a heat engine, converts heat energy into useful work. It is defined as the ratio of the useful work output to the heat input, typically expressed as a percentage. This concept is crucial for evaluating and optimizing the performance of various thermodynamic cycles and systems.
Vapor-compression cycle: The vapor-compression cycle is a thermodynamic process used in refrigeration and air conditioning systems that involves the compression, condensation, expansion, and evaporation of a refrigerant to transfer heat from one area to another. This cycle is crucial for the operation of refrigerators and heat pumps, as it efficiently removes heat from a low-temperature environment and releases it to a higher temperature environment, making it possible to maintain cooler temperatures indoors.
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