Heat Exchangers Appication For Marine
Chapter One – What Are Heat Exchangers?
Read further and learn more about:
- What are heat exchangers?
- Thermodynamics of heat exchangers
- Flow configuration of heat exchangers
- And much more…
Heat exchangers are pieces of equipment used to transfer heat between two or more fluids for marine application. This process usually involves abundant working or utility media such as water or air that rejects or absorbs heat from a more valuable fluid such as crude oil, petrochemical feedstocks, and fluidized products. There are many types and designs of heat exchangers. The hot and cold fluids may be separated by a wall with high thermal conductivity (usually made of steel or aluminum tube), or they may have direct contact with each other. The fluids may have the same or different phases (e.g., liquid-to-liquid, vapor-to-liquid). Changes in the phases of both fluids are considered by the manufacturer during the design phase.
Heat exchangers differentiate themselves from fuel, electrical, or nuclear-powered heat transfer equipment, such as boilers. The heat source and receiving medium must be both fluids. Fluids are defined as any substance that flows under applied shear stress or external force, which encompasses liquids, gases, and vapors.
Heat exchangers are widely used in many industries such as food, pharmaceutical, bioprocessing, and chemical manufacturing, where heating or cooling is the final or an intermediate step to prepare the fluids for further processing. They can also be used in the sterilization of microorganisms in food and pharmaceutical products. There are many instances where the use of heat exchangers is deemed practical. For instance, high-temperature exhaust gases from power plants and engines contain a large amount of heat which can be recovered by installing a heat exchanger before the smokestack.
Chapter Two – Thermodynamics of Heat Exchangers
All types of heat exchangers operate using the same thermodynamic principles and mechanism of heat transfer. These principles basically describe how thermal energy is transferred at the macroscopic level. Three bodies are interacting in a heat exchanger system: the hot fluid, the cold fluid, and the wall separating the two fluids. Energy flows from the hot fluid, through the wall or barrier, and then into the cold fluid. The following are some thermodynamic principles that are useful to understand how heat exchangers work:
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First Law of Thermodynamics: The first law is referred to as the Law of Conservation of Energy, which states that energy (in the form of heat and work) can neither be created nor destroyed. It can only be transferred to another system or converted to one form or another. In heat exchangers, this statement is translated by the heat balance equation written as:(Heat In) + (Generation of Heat) = (Heat Out) + (Accumulation of Heat)Assuming it operates in a steady-state flow that means that the thermal properties remain constant at all points as time changes, and the system is adiabatic (perfectly insulated), the heat balance equation simplifies to Heat In = Heat Out. This is one of the most basic equations which is used in the design and operation of heat exchangers.
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Second Law of Thermodynamics: The second law introduces the concept of entropy, the degree of disorderliness and randomness of a system. The entropy of the universe is constantly increasing and can never decrease. It tells us the direction of the flow of energy between two interacting systems in which the highest entropy is generated. Heat is always transferred from a body with higher temperatures to lower temperatures which is the natural tendency of all systems. For heat exchangers, the cold fluid gains heat and increases its temperature and the hot fluid loses heat and decreases its temperature.
Mechanism of Heat Transfer
The mechanism involved in the transfer of heat in heat exchangers is a combination of both conduction and convection. The driving force of heat transfer is the temperature difference between two or more regions.
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Conduction: This is the transfer of heat energy by direct collisions of adjacent molecules. A molecule with higher kinetic energy will transfer thermal energy to a molecule with lower kinetic energy. It occurs more readily in solids. For heat exchangers, it takes place on the wall separating the two fluids. Fourier‘s Law of Heat Conduction states that the rate of heat transfer normal to the material‘s cross-section is proportional to the negative temperature gradient. The proportionality constant is the material‘s thermal conductivity.Q = -k AWhere Q is the rate of heat transfer, k is the material‘s thermal conductivity, A is the area normal to the direction of the flow of heat, and dT/dx is the temperature gradient.
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Convection: Convection in heat exchangers occurs through the bulk motion of the fluid against the surface of the wall, thus transferring thermal energy. This phenomenon is represented by Newton‘s Law of Cooling which states the rate of heat loss is proportional of a body to the difference of the temperature of the body and its surroundings (for this instance, the wall and the fluid).Q = h A ?TWhere Q is the rate of heat transfer, A is the area normal to the direction of the flow of heat, and ?T is the temperature difference between the wall and bulk fluid. The convective heat transfer coefficient, denoted by h, is evaluated based on the wall dimensions, physical properties of the fluid, and fluid flow characteristics.
During an operation of a heat exchanger with conductive partition, heat is transferred from the hot fluid to the cold fluid in this sequence:
- From the hot fluid to the adjacent surface of the wall by convection.
- Through the wall surface side by conduction.
- From the wall to the cold fluid by convection.
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