Basic overview of air compressor thermodynamics
21 April, 2022
To better understand the physics of air compressor thermodynamics and heat generation, this article discusses the main principles and two gas laws.
How is Heat Transferred
Heat transfer is a fundamental concept in thermodynamics, essential for understanding various industrial processes, including compressed air systems.
Following up on our introduction to thermodynamics, this article explores the three main types of heat transfer: conduction, convection, and radiation, and their significance in different applications.
Heat transfer can take place in three different ways:
Conduction
Convection
Radiation
In real situations, heat transfer takes place simultaneously but not equally in all three ways.
In each case, heat flows from hot to cold: This is a fundamental principle of thermodynamics where heat naturally flows from a higher temperature region to a lower temperature region.
3 Types of Heat Transfer
Conduction
Conduction is the transfer of heat through a solid material. It occurs when molecules in a substance vibrate and transfer energy to neighboring molecules. This process is crucial in many industrial applications where materials need to conduct heat efficiently.
Example of Conduction: Touching a metal rod that has been heated at one end. The heat travels through the rod to your hand.
Convection
Convection is the transfer of heat by the movement of fluids (liquids or gases). This type of heat transfer is essential in systems where fluid movement is required to distribute heat evenly.
Example of Convection: Boiling water. Heat from the stove causes water at the bottom to become less dense and rise, while cooler water descends.
Radiation
Radiation is the transfer of heat through electromagnetic waves. Unlike conduction and convection, radiation does not require a medium and can occur through empty space.
Example of Radiation: The heat from the sun warming your face or roasting marshmallow by a fire. The sun's energy travels through the vacuum of space and warms objects on Earth.
Thermal Conductivity and Heat Transfer Formulas
Thermal Conductivity Formula
Thermal conductivity is a measure of a material's ability to conduct heat. The formula is:
Q = λ * A * t * (Δ T / Δ x)
where:
- Q is the heat transferred (J),
- λ is the thermal conductivity coefficient (W/m*K),
- A is the heat flow area (m2),
- t is the time (s),
- ΔT is the temperature difference (K),
- Δx is the distance (m).
Heat Transfer Formula for Convection
The formula for convective heat transfer is:
Q = - h * A * t * Δ T
where:
- Q is the heat transfer (J),
- h is the convective heat transfer coefficient (W/m2*K),
- A is the surface area (m2),
- t is the time (s),
- ΔT is the temperature difference between the surface and the fluid (K).
The negative sign indicates that heat transfer occurs from higher to lower temperature.
Radiation Heat Transfer Formula
The formula for convective heat transfer is:
Q = - k * A * t * Δ T
where:
- Q is the total heat transmitted (J),
- k is the heat transfer coefficient (W/m2*K),
- A is the area (m2),
- t is the time (s),
- ΔT is the temperature difference (cold-hot)(K).
Logarithmic mean temperature difference (LTMD) in Heat Exchangers
The heat transmission in a heat exchanger is at each point a function of the prevailing temperature difference and of the total heat transfer coefficient. It requires the use of a logarithmic mean temperature difference Өm instead of a linear arithmetic ΔT.
The formula for Log Mean Temperature Difference (LMTD) is:
Өₘ = Ө₁ - Ө₂ / ln ( Ө₁ / Ө₂ )
where:
- Өₘ is the Log Mean Temperature Difference (K),
- Ө₁ is the temperature difference between the two fluids at one end of the heat exchanger (⁀1),
- Ө₂ is the temperature difference between the two fluids at the other end of the heat exchanger (⁀2),
- ln denotes the natural logarithm.
Applications of Heat Transfer and Heat Recovery
Heat transfer and heat recovery are crucial in various industries, including manufacturing, automotive, and energy. While heat transfer involves the movement of heat from one place to another, heat recovery focuses on capturing and reusing waste heat.
Manufacturing
In manufacturing, efficient heat transfer is essential for metal forging, plastic molding, and chemical production. Proper thermal management ensures product quality and reduces energy consumption. On the other hand, heat recovery systems capture waste heat from these processes and reuse it elsewhere in the facility, such as preheating raw materials or heating spaces.
Automotive
In the automotive industry, heat transfer is critical for engine cooling, climate control, and electric vehicle battery management. Effective thermal systems enhance vehicle performance and longevity by efficiently moving heat away from critical components. Meanwhile, heat recovery systems can capture the heat generated by engines and exhaust systems to improve fuel efficiency or power additional vehicle systems.
Energy
In the energy sector, heat transfer is key to power generation, whether through traditional fossil fuels or renewable sources like solar and wind. Efficient heat exchange systems maximize energy output and minimize waste. Heat recovery, on the other hand, involves capturing waste heat from power generation processes and using it for other purposes, such as district heating or additional power generation through combined heat and power (CHP) systems.
Air and gas solutions for EV charger manufacturers
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Air and gas solutions for EV charger manufacturers
Heat Transfer in Compressed Air Systems
Heat Generation During Compression
During the compression process, air pressure and temperature increase due to work done on the air. This heat must be managed to maintain system efficiency and prevent damage.
Importance of Cooling Compressed Air
Uncontrolled heat in compressed air systems can lead to equipment damage, reduced efficiency, and moisture condensation. Cooling the compressed air is crucial to prevent these issues.
Utilizing Heat for Ancillary Equipment
Heat generated in compressed air systems can be utilized in ancillary equipment such as dryers and drying tanks, enhancing overall system efficiency.
Heat recovery
Heat recovery involves capturing and reusing waste heat generated during compression. This process reduces energy consumption, lowers operating costs, and promotes sustainability by minimizing wasted energy.
Benefits of Heat Recovery:
Reduced Energy Consumption: Utilizing waste heat reduces reliance on external heating sources.
Lower Operating Costs: Energy savings translate to lower operational expenses.
In this video, we explore compressor heat recovery. Did you know a significant portion of the electrical energy consumed by compressors turns into heat is often wasted?
With energy recovery systems, we can repurpose this excess heat for heating, drying, or generating hot water for tasks like showering and industrial processes like bottle cleaning and chocolate tempering.
Want to know more about energy recovery for your business? Understanding the different types of heat transfer is crucial for various industrial applications, including compressed air systems. By managing and utilizing heat effectively, industries can improve efficiency and reduce costs. Learn more in our ebook.
For more information on heat recovery and other energy-efficient solutions, visit our heat recovery page.
FAQs and examples of heat transfer in daily life
What are the three types of heat transfer?
The three types of heat transfer are conduction, convection, and radiation. Conduction is the transfer of heat through direct contact between materials, as seen when a metal rod heats up along its length.
Why does heat flow from hot to cold?
Heat flow from hot to cold is driven by the second law of thermodynamics, which states that entropy, or disorder, tends to increase in an isolated system.
How is heat transferred by conduction?
Heat is transferred by conduction through direct contact between molecules in a solid material.
How is heat transferred by convection?
Heat is transferred by convection through the movement of fluids, distributing heat evenly.
How is heat transferred by radiation?
Heat is transferred by radiation through electromagnetic waves, without the need for a medium.
What is an example of conduction heat transfer?
An example of conduction is a metal spoon getting hot from its handle to its tip when placed in a hot liquid.
What is an example of convection heat transfer?
An example of convection is the circulation of warm air in a heated room.
What are some examples of radiation heat transfer?
Examples of radiation include the warmth from a fireplace and the heat from the sun.
What type of heat transfer is boiling water?
Boiling water involves convection, where heat causes the water to circulate and transfer heat.
Which type of heat transfer can happen through empty space?
Radiation can occur through empty space, as it does not require a medium.
What type of heat transfer is the sun?
The sun transfers heat to Earth through radiation.
What type of heat transfer do dryers use?
Dryers typically use convection to transfer heat and dry materials.
What type of heat transfer is a microwave?
A microwave uses radiation to transfer heat and cook food.
What is the chemistry behind roasting marshmallows?
When marshmallows are roasted, heat transfer through radiation causes a chemical change. The sugar undergoes caramelization and Maillard reactions, producing water molecules that evaporate and leave carbon behind, resulting in a blackened, crispy exterior. This combination of chemical reactions gives roasted marshmallows their distinctive flavor and texture.
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