Air-to-air heat exchanger

An air-to-air heat exchanger, is a preheating systems used, for instance, in boilers where exhaust fumes are used to increase the temperature of the inlet air flow reducing the thermal difference. So the same boiler can achieve the same result by using less energy.

In these recovery systems we can find different systems such as air-to-air, air-to-gas or air-to-exhaust fumes systems, and also using other methods such as air-to-water or air-to-steam systems. For example in the wood dryers.

These systems are important to take advantage of all our industrial resources and use the exceding industrial energy to reduce our consumption and being more responsible with the natural environment.

At Boixac Tech we have air-to-air heat exchangers that can work with high temperatures, high pressures, dusty and corrosive environments.

Types of heat exchangers

March 14, 2025March 10, 2025 by Oficina Tècnica

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.

Industrial heat recovery

February 17, 2025June 1, 2021 by Oficina Tècnica

INDUSTRIAL HEAT RECOVERY THE GREENEST, OPTIMAL AND SUSTAINABLE ENERGY BOIXAC had the honor of being invited and participate in the podcast Con G de Geo, which aims to bring engineering closer analyzing concepts such as industrial heat recovery, sustainability, through renewable energies, energy optimization and the efficient use of our resources. You can read the trasncription of our contribution below and we encourage you to listen to us through the following link. “In December 2019, the European Green Deal was approved, which aims to achieve climate neutrality by 2050. To do this, a scale was made with the different actions to be carried out and, one of the steps on which we will stop and we will analyze if we have done our job is in 2030. In addition to aspects such as recovering biodiversity, improving animal welfare or promoting sustainable forest management, there are three aspects that directly influence the field of energy: – Establish a minimum share of renewable energies of 40%. – Improve energy efficiency by 36-39%. – Reduce greenhouse gas emissions by 55%. All these aspects are important to find a solution to the climate emergency but, at BOIXAC, we understand that if the world population continues to increase, for example, only in Spain an increase of 2% in the next 15 years is prevented, beyond the use of renewable energies, sustainability goes through the change in consumption habits and the optimization of our resources. In this sense, considering that the Spanish industry consumes about 31% of total energy, its modernization and optimization is one of the keys to our future. When we go along the highway, as far as the eye can see, we see factories that need energy for their processes, for instance to heat wastewater and facilitate the biological digestion of sludge, dry cement for its correct conservation, increase CO2 in greenhouses to increase the rate of photosynthesis, cool foods such as chocolate for modeling, etc. All processes that need to heat or cool require energy, and energy maintains a balance. In fact, heat is the transfer of energy from an area of high temperature to another area of lower temperature. If, for example, we look at what happens in our homes when we turn on the air conditioning, we will see this balance. While the indoor unit blows out cool air, the outdoor unit blows out excess heat. Starting from this energy balance, we see that a certain renewal of the indoor air is needed to maintain its quality. For this renewal we take the outside air and cool or heat it depending on each need. At the same time that we introduce the new air, we must expel the excess air from the interior so that the new one can fit and this is where we come in with heat recovery. If we make a leap from our homes to the industry and imagine, for example, that the outside air is at 20ºC and we want to heat it so that it reaches 80ºC inside, for example, in a dryer where we need to extract moisture . Here we apparently need equipment that is capable of increasing the air temperature by 60ºC, from 20 to 80ºC. However, there is another option that is smarter, cheaper and more sustainable. When we take this air from the outside at 20ºC and we want to heat it to introduce it into a room, the same flow of air that was inside at 80ºC will be expelled. By means of a heat recovery system we make these two air flows cross each other without mixing through a system known as cross flows. We do not mix these flows in order to maintain the quality of the previously filtered air, but we do extract the heat from the outgoing air flow and transfer it to the incoming air flow. With this system we achieve two objectives; 1. The cold air that we are introducing will rise in temperature, so that the equipment we use to heat it, often boilers, will be able to work more relaxed, consuming less energy and, therefore, saving and being more sustainable. 2. The hot air that we are expelling will significantly lower its temperature, resembling the ambient temperature and, therefore, we will be even more sustainable. The technology of heat recovery units may change depending on the application and the manufacturer, but, as we have seen, it is based on perfecting the filters to offer correct air quality, and the fans to obtain air circulation. the lower electricity consumption and the energy recuperators that are the heart that allow the magic of heat exchange. Here you can add other added values such as control or isolation. In our particular case, from BOIXAC, we specialize in industrial heat exchangers and, just as it is important to work to improve ventilation and filtering techniques, exchangers also progress to offer solutions resistant to corrosive environments, high pressures and temperatures. up to 950ºC, with flattened tubes to reduce pressure losses and compact constructions that currently reach efficiency levels of over 80%. In the industrial field, applications have many singularities such as fluids, viscosities, pressures, temperatures, materials, fouling coefficients, etc. That is why each project is studied in detail to optimize its construction and thus achieve the objectives of energy efficiency, sustainability and savings necessary for industrial progress.”