Heat transfer

Thermodynamics is the branch of physics that studies thermal energy and is based on the so-called laws of thermodynamics that describe its properties and how energy is transferred and converted into other forms.

The laws of thermodynamics are as follows:

  • First Law of Thermodynamics (Principle of Conservation of Energy): The total energy of a physical system is constant, and the difference between the initial and final energy is equal to the amount of energy that is transferred or which becomes work.
  • Second Law of Thermodynamics (Principle of Entropy): In a thermodynamic transaction, the total amount of entropy of a system and its surroundings always increases or remains constant.
  • Third Law of Thermodynamics (Zero Entropy Principle): At absolute zero temperature, the entropy of a perfect pure system is zero.

Some of the main factors we have to keep in mind when we talk about thermodynamics are temperature, pressure, amount of substance and entropy.

In this section we talk about thermal equilibrium, temperature, the kinetic theory of gases, heat capacity, sensible heat, latent heat, phase changes, the heat capacity of a solid, the heat capacity of a gas, adiabatic systems, types of heat exchange, convection, conduction, radiation, psychrometric diagram, etc.

Types of heat exchangers

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TYPES OF HEAT EXCHANGERS There are many types of heat exchangers and various ways to classify them. In this article, we will classify them based on: 1. Classification by Construction Direct contact Indirect contact Tube heat exchangers Plate heat exchangers 2. Classification by Operation Liquid-liquid heat exchangers Liquid-gas heat exchangers Gas-gas heat exchangers Bulk solid heat exchangers Classification by Construction Heat exchangers can transfer energy via direct contact, that is, by fully mixing the fluids, with cooling towers being one of their main examples. However, this system can lead to the transmission of contaminants between the two fluids, making it unsuitable for most cooling systems, energy recovery, and treatment of gases, liquids, and bulk solids. In these cases, where it is necessary to keep the two fluids separate, an indirect contact system is used. This construction involves an element, usually plates or tubes, which act as a wall and keep the two fluids separate. Within the category of indirect contact exchangers, there is a special case: rotary heat exchangers, where both fluids travel through the same space but alternately, which could cause a slight mixing, but this is considered almost negligible. Focusing on the two main families of indirect contact, those of plates and tubes, it can be said that for the same power, plates achieve a high heat transfer coefficient in a very compact space, but they reduce the fluid flow area, making them more prone to fouling. On the other hand, tubes provide a larger surface area for fluid flow, making them highly recommended in dirty, dusty environments, or with sticky, viscous fluids, or even with sediments. They are less likely to become clogged and thus also reduce maintenance and cleaning costs. Tube heat exchanger Tube heat exchangers consist of cylindrical, flat, or oval tubes, and their design is chosen based on the specific characteristics of each system. Within this family, we find: Smooth tube heat exchangers. Since they have a similar exchange surface both inside and outside the tubes, this is a very common design when working with fluids that have similar specific heat values. Thus, in applications between two air flows, we can refer to classic smooth tube exchangers, while in applications involving water, sludge, milk, or juices, we can refer to tubular, multitubular, pyrotubular, coaxial, or double-tube exchangers, as well as shell and tube exchangers. Tube and fin heat exchangers. These are specifically designed to compensate for the energy transfer between two fluids with different specific heat values. This is a common situation in systems where gas flows are in contact with other fluids such as superheated water, thermal oil, refrigerants (ammonia, R134, R410a, etc.), or steam. For example, the specific heat of gas is around 1,214 kJ/m³·K, while the specific heat of water is 4,186 kJ/m³·K. This means that water can release almost four times more heat than the air can absorb, and the way to correct this is by increasing the exchange surface on the air side using elements called fins, which can be continuous plates perpendicular to the tubes or helical plates wrapped around the tubes. Plate heat exchanger Plate heat exchangers consist of flat or corrugated plates. Among them, we find different designs suited to various applications: Pillow plate heat exchanger. Emerging technology, very versatile and efficient, with a surface in the shape of a pillow, giving it the name “pillow.” Its design allows it to handle not only viscous, sticky, and sediment-laden fluids, but also to transfer energy to granular solids. This makes it an excellent alternative to fluidized beds, reducing energy consumption, minimizing waste, lowering environmental pollution, and improving the final product quality by applying energy uniformly. Cross-flow plate heat exchanger. A plate system widely used in energy recovery for applications such as air conditioning, directly integrated into air handling units. It is an excellent system for achieving high efficiency, but it requires advanced air filters, as its compact form makes cleaning difficult. Welded plate heat exchanger. The plates are joined by welding, which prevents internal cleaning and limits their use to installations free from contamination. Plate and gasket heat exchanger. The gasket system allows plates to be disassembled, cleaned, and replaced. This makes it more versatile than the welded system, but the channels through which the fluids pass remain small and can easily become blocked, making them unsuitable for viscous, sticky, or sediment-laden fluids. Classification by Operation Heat exchangers are designed to transfer energy optimally. To maximize their efficiency, it is essential to consider the type of fluids and their properties. An example of this is the previous case, where heat exchange occurs between a gas with a specific heat of 1,214 kJ/m³·K and water with a specific heat of 4,186 kJ/m³·K. Similarly, we find: Liquid-liquid heat exchangers. These include pillow plates, welded plates, plate and gasket exchangers, concentric tubes, coaxial tubes, and pyrotubular exchangers. Liquid-gas heat exchangers. These include smooth tubes, tubes with continuous fins, and tubes with helical fins. Gas-gas heat exchangers. These include multitubular exchangers, smooth tubes, and cross-flow exchangers. Bulk solid heat exchangers. These use the Pillow Plate technology. Small design details can increase or decrease turbulence, enhancing the exchange coefficients and leading to substantial differences between one supplier and another. That is why investment in R&D is a key factor in the evolution of this sector, which is increasingly recognized for its contribution in terms of efficiency, savings, and sustainability.

Conduction, convection & radiation

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CONDUCTION, CONVECTION & RADIATION HEAT TRANSFER IN NATURE In nature, we find fascinating examples of heat transfer through conduction, convection, and radiation—three fundamental mechanisms in thermodynamics. For example, imagine a summer morning at the beach. Early in the morning, the air remains calm because there is a thermal equilibrium between the temperature of the air mass over the sea and the air mass over the land. As the Sun heats the Earth’s surface, the temperature of the air over the land rises more quickly than that of the air over the sea. This creates a thermal contrast: the warm air over the land rises, while the cooler air from the sea moves toward the land to take its place. This movement of air masses is a clear example of thermal convection, the same principle that allows hot air balloons to rise. The more the Sun heats the land, the stronger this temperature difference becomes, increasing the speed of the sea breeze. This rising warm air favors the formation of small cumulus clouds, and if the temperature difference is significant enough, cumulonimbus clouds may appear, which are responsible for sudden summer storms. Unlike radiation, which transfers energy without direct contact (such as the Sun’s rays heating the sand), convection depends on the movement of fluids like air or water. On the other hand, thermal conduction occurs when two objects at different temperatures come into contact—for example, when we walk barefoot on hot sand at noon and feel the heat transferring to our feet. So, the next time you’re at the beach and notice the sea breeze picking up at midday, think of BOIXAC, the specialists in thermal exchange for the industry.

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