Vapor and Gas Refrigeration System and Heat Pump Cycles

Heat Pump

A heat pump is a device that uses a small amount of energy to move heat from one location to another. It can heat and cool a space or object by transferring heat from the environment or inside a building to the desired location. Heat pumps use a refrigerant that absorbs heat from the environment or inside a building and then releases it into the desired location. The refrigerant is circulated through a closed system of pipes and coils and undergoes phase changes to absorb and release heat.

Heat pumps can operate in either direction, making them useful for heating and cooling applications. In the heating mode, a heat pump absorbs heat from the environment and releases it inside a building. In cooling mode, it absorbs heat inside a building and releases it to the environment. Heat pumps are typically more energy efficient than traditional heating and cooling systems because they do not generate heat or cold but rather move it from one location to another.

Coefficient of Performance:

Here, the desired effect is maintaining the body at a temperature higher than the surrounding.

Thus,

where:

T_H = High-temperature body

T_L = Low-temperature body

Refrigerator

A refrigerator is a household appliance that keeps food and other perishable items fresh by maintaining a temperature below the ambient temperature. It uses a refrigeration cycle to transfer heat from the inside of the refrigerator to the surrounding environment. The refrigeration cycle begins when the refrigerant is in a low-pressure, low-temperature vapour form. It is then drawn into the compressor, which is compressed and heated. The compressed, hot refrigerant then passes through the condenser, giving off its heat to the surrounding environment and condenses into a high-pressure, high-temperature liquid.

The refrigerant flows through an expansion valve, allowing it to expand and cool. As it expands, it absorbs heat from inside the refrigerator and evaporates into a low-pressure, low-temperature vapour. The refrigerant then returns to the beginning of the cycle, where it is drawn back into the compressor to repeat the process. The transfer of heat from the inside of the refrigerator to the surrounding environment is what keeps the inside of the refrigerator cool.

Where T_H = Constant temperature at which heat is to be rejected

T_L = Constant temperature from where heat is to be extracted

(COP)_HP = (COP)_R + 1

where Q_L= heat at a lower temperature,

Q_H = heat at a higher temperature

Unit of Refrigeration: It has a standard unit of TR (Ton or Refrigeration). 1 TR (one ton of refrigeration) means the capacity to freeze one ton of water from and at 0^oC in 24 h.

Key Points

• A refrigerator operates in a cycle that maintains a body at a temperature lower than the surroundings.
• A performance parameter in a refrigerator cycle is called the coefficient of performance.
• A heat pump is a device that operates in a cycle and maintains a body at a temperature higher than the temperature of the surroundings.

Reversed Carnot Cycle

The reversed Carnot cycle is a theoretical process that describes the operation of a heat pump or refrigeration system in reverse. It is a modification of the original Carnot cycle, which describes the ideal thermodynamic cycle of a heat engine. In the reversed Carnot cycle, a refrigerant absorbs heat from a cold reservoir and releases it to a hot reservoir. This process is the opposite of the original Carnot cycle, which involves the transfer of heat from a hot reservoir to a cold reservoir.

The reversed Carnot cycle can describe the operation of a heat pump, which absorbs heat from the environment and releases it inside a building to provide heating. It can also describe the operation of a refrigeration system, which absorbs heat from inside a refrigerator and releases it to the surrounding environment to keep the inside cool. The reversed Carnot cycle is not a practical cycle that can be implemented in real heat pump or refrigeration systems, but it provides a theoretical basis for understanding the principles of these systems.

Where T₂ and T₁ are the temperatures at sections 2 and 1, respectively and S₁, S₂, S₃ and S₄ are the entropy at 1, 2, 3 and 4, respectively.

Vapour Compression Refrigeration System

A vapour compression refrigeration system is a type of refrigeration system that uses a circulating refrigerant to transfer heat from a lower-temperature space or object to a higher-temperature space or object. It is the most commonly used refrigeration system in various applications, including air conditioners, refrigerators, and industrial cooling systems.The vapour compression refrigeration system consists of four components: a compressor, a condenser, an expansion valve, and an evaporator. The refrigerant is circulated through these components in a closed loop, undergoing phase changes to absorb and release heat.

In the vapour compression refrigeration system, the refrigerant absorbs heat in the evaporator and evaporates into a low-pressure, low-temperature vapour. It is then drawn into the compressor, which is compressed and heated. The compressed, hot refrigerant is then passed through the condenser, where it gives off its heat to the surrounding environment and condenses into a high-pressure, high-temperature liquid. The refrigerant then flows through an expansion valve, allowing it to expand and cool, returning to the evaporator to repeat the cycle.

1 → 2 → Isentropic compression process

2 → 3 → condensation process (constant pressure Heat Rejection)

3 → 4 → Isenthalpic Expansion process

4 → 1 → evaporation process (constant pressure Heat Addition)

Where h₁ = Enthalpy at the inlet of the compressor

h₂ = Enthalpy at the exit of the compressor or inlet of the condenser

h₄ =Enthalpy at the exit of the condenser or inlet of the throttle valve

Key Points

• COP of refrigerator working between two fixed temperatures T_L and T_H (T_H > T_L) will have fixed COP.
• A reversible heat engine can work like a refrigerator or a heat pump.
• Vapour compression refrigeration cycle
• Entropy:
  
  h₂ = h₂‘+C_p (T₂‘ –T₂)
• For isentropic process:
  S₂ = S₁

Vapour Absorption Refrigeration System (VARS)

COP of an Ideal Vapour Absorption System

The coefficient of performance (COP) of an ideal vapour absorption refrigeration system (VARS) measures the system’s efficiency. It is defined as the system’s cooling capacity ratio to the input energy required to operate the system. Below is a schematic diagram of energy transfer in the vapour absorption system.

Q_g = heat supply to the refrigerator in the generator

Q_e = heat is absorbed by the refrigerator in the evaporator

Q_c = heat is rejected by condenser or atmosphere

Coefficient of Performance:

Q_g + Q_e = Q_c

For reversible cycles, the net entropy change is zero.

COP of Carnot refrigerator

The efficiency of the Carnot engine

Here, T_g = generator temperature

T_c = T_o = absorber temperature (Generally surrounding temperature)

T_e = evaporator temperature.

The COP of an ideal VARS is typically lower than the COP of an ideal vapour compression refrigeration system (VCRS) because the VARS relies on a heat exchanger rather than a mechanical compressor to circulate the refrigerant. However, VARS systems have the advantage of not requiring a mechanical compressor, which makes them simpler and less expensive to operate.

Refrigerant

A refrigerant is a substance used in a refrigeration cycle to transfer heat from a lower-temperature space or object to a higher-temperature space or object. It is a vital component of refrigeration systems and heat pumps, and it plays a critical role in the operation of these systems. Refrigerants are chosen for their ability to easily evaporate and condense and their low boiling points. This allows them to absorb heat when they evaporate and release heat when they condense, making them useful for cooling applications.

Many different refrigerants are used in various refrigeration and heat pump systems. Some common refrigerants include chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs). The choice of refrigerant depends on its application and desired properties, such as its cooling capacity, environmental impact, and stability.

Designation of Refrigerant

In International Standards, Refrigerants are designated as R, followed by some numeric values.

For a hydrocarbon Chemical formula: C_r H_s F_t Cl_y

If S+t+y=2r+2, designation of refrigerant is R(r-1)(S+1)t

It S+t+y=2r, designation of refrigerant is R1(r-1)(S+1)t

For an Inorganic Refrigerant

Designation is R(700 + molecular weight)

e.g.

Desirable Properties of a Good Refrigerant

A good refrigerant should have a low boiling point, high latent heat of vaporization, high thermal conductivity, low toxicity and flammability, low environmental impact, chemical stability, and compatibility with system materials. It should also be able to effectively transfer heat from a lower-temperature space or object to a higher-temperature space or object to provide efficient cooling. There are several desirable properties that a good refrigerant should possess:

1. Low boiling point: A refrigerant should have a low boiling point so it can easily evaporate and absorb heat at relatively low temperatures. This is important for efficient heat transfer and cooling performance.

2. High latent heat of vaporization: A refrigerant should have a high latent heat of vaporization, the amount of heat required to vaporize the refrigerant. This allows the refrigerant to absorb a large amount of heat when it evaporates, which is essential for effective cooling.

3. High thermal conductivity: A refrigerant should have a high thermal conductivity, which is the ability of the refrigerant to conduct heat. This allows the refrigerant to effectively transfer heat from one location to another.

4. Low toxicity and flammability: A refrigerant should be non-toxic and non-flammable to ensure safety in handling and use.

5. Low environmental impact: A refrigerant should have a low potential to contribute to global warming or ozone depletion. This is important because refrigerants can be released into the environment through refrigeration and air conditioning systems.

6. Chemical stability: A refrigerant should be chemically stable and not prone to decomposition or breakdown, which can reduce its effectiveness and cause damage to the refrigeration system.

7. Compatibility with system materials: To prevent corrosion and deterioration, a refrigerant should be compatible with the materials used in the refrigeration system, such as seals and hoses.

Uses and Properties of Some Refrigerant

Many different types of refrigerants are used in various refrigeration and air conditioning systems, each with its own unique properties and characteristics. Here are a few examples:

1. Chlorofluorocarbons (CFCs): CFCs were once widely used as refrigerants, but their use has been significantly reduced due to their negative impact on the ozone layer. They are highly stable and non-toxic but potent greenhouse gases and contribute to global warming.

2. Hydrochlorofluorocarbons (HCFCs): HCFCs were developed as a replacement for CFCs and have a lower ozone depletion potential. However, they are still potent greenhouse gases and contribute to global warming.

3. Hydrofluorocarbons (HFCs): HFCs are newer refrigerants with a low ozone depletion potential and are not as harmful to the environment as CFCs and HCFCs. However, they are still greenhouse gases and contribute to global warming.

4. Carbon dioxide (CO2): CO2 is a natural refrigerant with low global warming potential and is not harmful to the ozone layer. It has high thermal conductivity and pressure but a low latent heat of vaporization and requires specialized equipment to handle it.

5. Ammonia (NH3): Ammonia is a natural refrigerant that is non-toxic and has a high thermal conductivity and a low boiling point. It is often used in industrial and commercial refrigeration systems, but it can be corrosive and requires careful handling.

6. Water (H2O): Water is a natural refrigerant with a high latent heat of vaporization and high thermal conductivity. It is non-toxic and non-flammable, but it has a relatively high boiling point and requires a large volume to transfer a significant amount of heat.

Important GATE Notes
Free and Forced Vibration	Simple Harmonic Motion
Zeroth and First Laws of Thermodynamics	Universal Joint
First Law of Thermodynamics	Helical Gear
Triangle Law of Forces	Connecting Rod
Types of Support	Constant Acceleration
Mechanical Energy	Constant Velocity

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