Heat exchanger selection
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Our company produces ethanol; there are gases on both sides. Which type of heat exchanger would be suitable? This post was last edited by jia717 on 2009-2-16 at 12:40.]
If the pressure is not very high, a plate heat exchanger should work well, with high heat transfer efficiency as well.
Using spiral plate heat exchangers: The advantages of spiral plate heat exchangers are as follows: ① High heat transfer coefficient: Due to the effect of centrifugal inertial force, the fluid in the spiral flow channels reaches a turbulent state at lower Reynolds numbers (turbulence generally occurs at Re=1400–1800), and higher flow velocities can be used (2 m/s for liquids, 20 m/s for gases), resulting in a high heat transfer coefficient. For heat transfer between water and water, the heat transfer coefficient can reach 2000–3000 W/(m2·°C), whereas in shell-and-tube exchangers it is generally 1000–2000 W/(m2·°C). ②Resistant to scaling and blockage: Since it is a single-channel system for each fluid flow, the flow velocity is high; coupled with the effect of centrifugal inertial forces, turbulence is intense. As a result, the particles suspended in the fluid do not tend to settle, so spiral plate heat exchangers are resistant to scaling and blockage, making them suitable for handling suspensions and fluids with high viscosity. ③Ability to utilize low-temperature heat sources: Due to the long length of the flow channels for fluid flow and the complete ability of the two fluids to flow in opposite directions, operation can be carried out at a small temperature difference, allowing for efficient recovery of low-temperature heat sources. According to some available information, the temperature difference between the hot and cold fluid streams at the outlet can be as low as 3°C. ④Compact structure: The heat transfer area per unit volume is approximately 3 times that of the shell-and-tube type. The main disadvantages of spiral plate heat exchangers are: ① The operating pressure and temperature should not be too high: currently, the maximum operating pressure does not exceed 20 atm, and the temperature is below 400°C. ②Difficult to maintain: Since the commonly used spiral plate heat exchangers are welded together, repairs are difficult once they are damaged.
The commonly used heat exchanger is the fixed-tube-sheet heat exchanger. The selection of a heat exchanger involves understanding the characteristics of various heat exchanger designs. It is necessary to take into account factors such as the operating conditions, whether cleaning is required frequently, heat transfer efficiency, and heat transfer requirements, and then make a decision after conducting a cost analysis. Next, I’ll introduce heat exchangers to the original poster; you can then choose the appropriate type after reviewing them: Heat exchangers are common devices used in various industrial sectors such as chemicals, petroleum, power generation, food processing, and many others, and they play a crucial role in production processes. In chemical manufacturing, heat exchangers can function as heaters, coolers, condensers, evaporators, and reboilers, among other roles, making their applications even broader. There are many types of heat exchangers, but based on the principles and methods of heat exchange between cold and hot fluids, they can be broadly classified into three categories: shell-and-tube, mixed-flow, and regenerative. Among these three types, shell-and-tube heat exchangers are the most widely used.
1. Types of Shell-and-Tube Heat Exchangers
- Jacketed Heat Exchangers: These heat exchangers are constructed by installing a jacket around the outer wall of a container; they have a simple structure. However, their heating surface is limited by the container walls, resulting in a relatively low heat transfer coefficient. To improve the heat transfer coefficient and ensure uniform heating of the liquid inside the container, agitators can be installed within it. When cooling water or a heating medium without phase change is used in the jacket, spiral baffles or other measures to enhance turbulence can also be employed to increase the heat transfer coefficient on that side of the jacket. To compensate for the insufficient heating surface, coiled tubes can also be installed inside the container. Jacketed heat exchangers are widely used for heating and cooling during reaction processes.
- Immersed Coiled Tube Heat Exchangers: In these heat exchangers, metal tubes are bent into shapes suitable for the container and then immersed in the liquid inside it. The advantages of this type include a simple structure, the ability to withstand high pressures, and the possibility of using corrosion-resistant materials. However, the disadvantages are low turbulence levels in the liquid inside the container and a low heat transfer coefficient outside the tubes. To improve the heat transfer coefficient, agitators can be installed inside the container.
- Spray-Type Heat Exchangers: These heat exchangers feature heat exchange tubes fixed in rows on a steel frame. The hot fluid flows inside the tubes, while cooling water is sprayed evenly from above; hence they are also known as spray coolers. Outside the tubes, there is a layer of liquid with higher turbulence, resulting in a significantly higher heat transfer coefficient compared to immersed tube heat exchangers. Additionally, since these heat exchangers are usually placed in areas with good air circulation, the evaporation of cooling water helps to remove some heat, thereby reducing the temperature of the cooling water and increasing the driving force for heat transfer. As a result, the heat transfer efficiency of spray-type heat exchangers is much better than that of immersed tube heat exchangers.
- Double-Shell Heat Exchangers: These heat exchangers consist of concentric tubes of different diameters connected by U-shaped bends. In this type of heat exchanger, one fluid flows inside the tubes, while the other flows in the annular space between them. Both fluids can achieve high flow velocities, resulting in a high heat transfer coefficient. Moreover, in double-shell heat exchangers, the two fluids can flow in pure counterflow, resulting in a large logarithmic mean driving force. These heat exchangers have a simple structure, can withstand high pressures, and are easy to use (as the number of tube sections can be adjusted as needed). Due to their advantages of high heat transfer coefficient, strong driving force for heat transfer, and ability to withstand high pressures, double-shell heat exchangers are almost exclusively used in ultra-high pressure production processes, such as those involved in the production of high-pressure polyethylene at pressures of 3000 atmospheres.
- Tubular Shell Heat Exchangers: Tubular shell heat exchangers are the most typical example of shell-and-tube heat exchangers. They have a long history of industrial use and still remain the most common type of heat exchanger today. These heat exchangers consist of a shell, tube bundles, tube sheets, and end caps. The shell is usually circular in shape, with parallel tube bundles inside, with both ends of the tube bundles fixed to the tube sheets. In these heat exchangers, one fluid flows inside the tubes, and this flow path is called the tube side, while the other fluid flows outside the tubes, and this flow path is called the shell side. The wall of the tube bundle serves as the heat transfer surface. To improve the heat transfer coefficient of the fluid outside the tubes, a certain number of horizontal baffles are typically installed inside the shell. These baffles not only prevent short-circuiting of the fluid and increase its velocity but also force the fluid to flow through the tube bundle multiple times along specified paths, thereby increasing turbulence. The most common types of baffles are semi-circular and disc-shaped, with the former being more widely used. Each time the fluid passes through the tube bundle once, it constitutes one tube side pass, while each time it passes through the shell once, it constitutes one shell side pass. To increase the velocity of the fluid inside the tubes, appropriate partitions can be installed in the end caps to divide all the tubes into several groups. This way, the fluid can pass through only part of the tubes each time, traveling back and forth through the tube bundle multiple times, which is known as multi-tube side operation. Similarly, to increase the velocity of the fluid outside the tubes, vertical baffles can be installed inside the shell to force the fluid to pass through the shell space multiple times, known as multi-shell side operation. In tubular shell heat exchangers, due to the different temperatures of the fluids inside and outside the tubes, the temperatures of the shell and the tube bundle also differ. If the temperature difference between them is too large, significant thermal stress can occur inside the heat exchanger, potentially causing the tubes to bend, break, or become loose from the tube sheets. Therefore, when the temperature difference between the tube bundle and the shell exceeds 50°C, appropriate measures should be taken to compensate for this temperature difference and reduce thermal stress.
2. Mixed-Flow Heat Exchangers: Mixed-flow heat exchangers rely on direct contact between cold and hot fluids for heat transfer. This method eliminates the thermal resistance caused by the heat transfer surface and any fouling on either side of it. As long as the contact between the fluids is good, a high heat transfer rate can be achieved. Therefore, in any situation where fluids are allowed to mix with each other, mixed-type heat exchangers can be used, such as for the washing and cooling of gases, the cooling of circulating water, mixed heating between steam and water, the condensation of steam, and so on. Its applications are widespread in chemical and metallurgical enterprises, power engineering, air conditioning engineering, and many other production sectors. Types of mixed heat exchangers: Depending on their application, mixed heat exchangers can be classified into the following types: (1) Cooling towers (also known as chillers). In these devices, water that has been heated during production is cooled using natural or mechanical ventilation before being reused, thereby improving the economic efficiency of the system. For example, the circulating water in thermal power plants or nuclear power stations, as well as the cooling water used in ammonia synthesis, is cooled in water cooling towers before being reused; this method is widely employed in practical engineering applications. (2) Gas scrubbers (or washing towers) are used in industry to wash gases for various purposes, such as using a liquid to absorb certain components in gas mixtures, removing dust from gases, humidifying or drying gases, etc. But its most widespread use is for cooling gases, with water being the most common liquid used for this purpose. The spray chamber, which is widely used in air conditioning projects, can be considered a special form of it. The spray chamber can not only cool the air like a gas scrubber, but it can also heat it. However, it also has disadvantages such as high requirements for water quality, large floor space, and high energy consumption for water pumps; therefore, in ordinary buildings, spray rooms are no longer commonly used or are only employed as humidification devices. However, it is still widely used in textile factories, cigarette factories, and other facilities where regulating humidity is the main purpose! (3) Jet heat exchangers: In such devices, a fluid at higher pressure is ejected through nozzles, achieving very high speeds; the fluid at lower pressure is introduced into a mixing chamber where it comes into direct contact with the jet for heat and mass transfer. Both fluids then enter a diffusion tube, and after reaching the same pressure and temperature at its outlet, they are delivered to the user. (4) Mixed-type condenser: This type of device uses direct contact between water and steam to condense the steam. 3. Regenerative heat exchanger: A regenerative heat exchanger is a device used for regenerative heat exchange. It contains a solid filler to store heat. Fire grids are generally built using refractory bricks, etc. (sometimes metal corrugated strips, etc., are used). Heat exchange takes place in two stages. In the first stage, hot gas passes through the fire grid, transferring heat to it and storing it there. In the second stage, the cold gas passes through the fire grid and is heated by the heat stored in it. These two phases alternate. Usually, two regenerators are used alternately; that is, when hot gas enters one regenerator, cold gas enters the other. It is commonly used in the metallurgical industry, such as in the regenerative chambers of steelmaking open-hearth furnaces. It is also used in the chemical industry, such as air preheaters or combustion chambers in gas furnaces, and regenerative cracking furnaces in synthetic oil plants. Regenerative heat exchangers are generally used in applications where the mixing of media is not a major requirement.
1. Types of Shell-and-Tube Heat Exchangers
- Jacketed Heat Exchangers: These heat exchangers are constructed by installing a jacket around the outer wall of a container; they have a simple structure. However, their heating surface is limited by the container walls, resulting in a relatively low heat transfer coefficient. To improve the heat transfer coefficient and ensure uniform heating of the liquid inside the container, agitators can be installed within it. When cooling water or a heating medium without phase change is used in the jacket, spiral baffles or other measures to enhance turbulence can also be employed to increase the heat transfer coefficient on that side of the jacket. To compensate for the insufficient heating surface, coiled tubes can also be installed inside the container. Jacketed heat exchangers are widely used for heating and cooling during reaction processes.
- Immersed Coiled Tube Heat Exchangers: In these heat exchangers, metal tubes are bent into shapes suitable for the container and then immersed in the liquid inside it. The advantages of this type include a simple structure, the ability to withstand high pressures, and the possibility of using corrosion-resistant materials. However, the disadvantages are low turbulence levels in the liquid inside the container and a low heat transfer coefficient outside the tubes. To improve the heat transfer coefficient, agitators can be installed inside the container.
- Spray-Type Heat Exchangers: These heat exchangers feature heat exchange tubes fixed in rows on a steel frame. The hot fluid flows inside the tubes, while cooling water is sprayed evenly from above; hence they are also known as spray coolers. Outside the tubes, there is a layer of liquid with higher turbulence, resulting in a significantly higher heat transfer coefficient compared to immersed tube heat exchangers. Additionally, since these heat exchangers are usually placed in areas with good air circulation, the evaporation of cooling water helps to remove some heat, thereby reducing the temperature of the cooling water and increasing the driving force for heat transfer. As a result, the heat transfer efficiency of spray-type heat exchangers is much better than that of immersed tube heat exchangers.
- Double-Shell Heat Exchangers: These heat exchangers consist of concentric tubes of different diameters connected by U-shaped bends. In this type of heat exchanger, one fluid flows inside the tubes, while the other flows in the annular space between them. Both fluids can achieve high flow velocities, resulting in a high heat transfer coefficient. Moreover, in double-shell heat exchangers, the two fluids can flow in pure counterflow, resulting in a large logarithmic mean driving force. These heat exchangers have a simple structure, can withstand high pressures, and are easy to use (as the number of tube sections can be adjusted as needed). Due to their advantages of high heat transfer coefficient, strong driving force for heat transfer, and ability to withstand high pressures, double-shell heat exchangers are almost exclusively used in ultra-high pressure production processes, such as those involved in the production of high-pressure polyethylene at pressures of 3000 atmospheres.
- Tubular Shell Heat Exchangers: Tubular shell heat exchangers are the most typical example of shell-and-tube heat exchangers. They have a long history of industrial use and still remain the most common type of heat exchanger today. These heat exchangers consist of a shell, tube bundles, tube sheets, and end caps. The shell is usually circular in shape, with parallel tube bundles inside, with both ends of the tube bundles fixed to the tube sheets. In these heat exchangers, one fluid flows inside the tubes, and this flow path is called the tube side, while the other fluid flows outside the tubes, and this flow path is called the shell side. The wall of the tube bundle serves as the heat transfer surface. To improve the heat transfer coefficient of the fluid outside the tubes, a certain number of horizontal baffles are typically installed inside the shell. These baffles not only prevent short-circuiting of the fluid and increase its velocity but also force the fluid to flow through the tube bundle multiple times along specified paths, thereby increasing turbulence. The most common types of baffles are semi-circular and disc-shaped, with the former being more widely used. Each time the fluid passes through the tube bundle once, it constitutes one tube side pass, while each time it passes through the shell once, it constitutes one shell side pass. To increase the velocity of the fluid inside the tubes, appropriate partitions can be installed in the end caps to divide all the tubes into several groups. This way, the fluid can pass through only part of the tubes each time, traveling back and forth through the tube bundle multiple times, which is known as multi-tube side operation. Similarly, to increase the velocity of the fluid outside the tubes, vertical baffles can be installed inside the shell to force the fluid to pass through the shell space multiple times, known as multi-shell side operation. In tubular shell heat exchangers, due to the different temperatures of the fluids inside and outside the tubes, the temperatures of the shell and the tube bundle also differ. If the temperature difference between them is too large, significant thermal stress can occur inside the heat exchanger, potentially causing the tubes to bend, break, or become loose from the tube sheets. Therefore, when the temperature difference between the tube bundle and the shell exceeds 50°C, appropriate measures should be taken to compensate for this temperature difference and reduce thermal stress.
2. Mixed-Flow Heat Exchangers: Mixed-flow heat exchangers rely on direct contact between cold and hot fluids for heat transfer. This method eliminates the thermal resistance caused by the heat transfer surface and any fouling on either side of it. As long as the contact between the fluids is good, a high heat transfer rate can be achieved. Therefore, in any situation where fluids are allowed to mix with each other, mixed-type heat exchangers can be used, such as for the washing and cooling of gases, the cooling of circulating water, mixed heating between steam and water, the condensation of steam, and so on. Its applications are widespread in chemical and metallurgical enterprises, power engineering, air conditioning engineering, and many other production sectors. Types of mixed heat exchangers: Depending on their application, mixed heat exchangers can be classified into the following types: (1) Cooling towers (also known as chillers). In these devices, water that has been heated during production is cooled using natural or mechanical ventilation before being reused, thereby improving the economic efficiency of the system. For example, the circulating water in thermal power plants or nuclear power stations, as well as the cooling water used in ammonia synthesis, is cooled in water cooling towers before being reused; this method is widely employed in practical engineering applications. (2) Gas scrubbers (or washing towers) are used in industry to wash gases for various purposes, such as using a liquid to absorb certain components in gas mixtures, removing dust from gases, humidifying or drying gases, etc. But its most widespread use is for cooling gases, with water being the most common liquid used for this purpose. The spray chamber, which is widely used in air conditioning projects, can be considered a special form of it. The spray chamber can not only cool the air like a gas scrubber, but it can also heat it. However, it also has disadvantages such as high requirements for water quality, large floor space, and high energy consumption for water pumps; therefore, in ordinary buildings, spray rooms are no longer commonly used or are only employed as humidification devices. However, it is still widely used in textile factories, cigarette factories, and other facilities where regulating humidity is the main purpose! (3) Jet heat exchangers: In such devices, a fluid at higher pressure is ejected through nozzles, achieving very high speeds; the fluid at lower pressure is introduced into a mixing chamber where it comes into direct contact with the jet for heat and mass transfer. Both fluids then enter a diffusion tube, and after reaching the same pressure and temperature at its outlet, they are delivered to the user. (4) Mixed-type condenser: This type of device uses direct contact between water and steam to condense the steam. 3. Regenerative heat exchanger: A regenerative heat exchanger is a device used for regenerative heat exchange. It contains a solid filler to store heat. Fire grids are generally built using refractory bricks, etc. (sometimes metal corrugated strips, etc., are used). Heat exchange takes place in two stages. In the first stage, hot gas passes through the fire grid, transferring heat to it and storing it there. In the second stage, the cold gas passes through the fire grid and is heated by the heat stored in it. These two phases alternate. Usually, two regenerators are used alternately; that is, when hot gas enters one regenerator, cold gas enters the other. It is commonly used in the metallurgical industry, such as in the regenerative chambers of steelmaking open-hearth furnaces. It is also used in the chemical industry, such as air preheaters or combustion chambers in gas furnaces, and regenerative cracking furnaces in synthetic oil plants. Regenerative heat exchangers are generally used in applications where the mixing of media is not a major requirement.
I think a regular tube bundle will suffice; the heat transfer coefficients on both the gas sides are quite low, so there’s no need for any reinforcement. Those using heat pipes work too, right? They are commonly used in preheaters; I’m not sure if they are applicable here
For this condition, plate heat exchangers are recommended
In the ethanol industry, our company has worked on plate exchangers, namely spiral-plate type ones