March 11, 2024
Heat exchanger definition
The heat exchanger is a device that transfers part of the heat of the hot fluid to the cold fluid, that is, a large closed container is filled with water or other media, and there are pipes running through the container. Let hot water flow through the pipes.
Due to the temperature difference between the water in the pipe and the hot and cold water in the container, heat exchange will occur. For the thermal balance of junior high school physics, high-temperature heat is always transported to low temperature, so that the heat of the water in the pipe can be exchanged to the cold water in the container. Heater desert.
Classification and structure of heat exchangers
Heat exchangers can be divided into:
Cooler, condenser, heater, heat exchanger, reboiler, steam generator, waste heat (or waste heat) boiler.
According to the heat exchange method, it can be divided into:
Direct contact heat exchangers (also called hybrid heat exchangers), regenerative heat exchangers and dividing wall heat exchangers.
The following mainly introduces heat exchangers classified according to heat exchange methods:
1) Direct contact heat exchanger
Direct contact exchangers rely on direct contact between cold and hot fluids for heat transfer. This heat transfer method avoids dirt and thermal resistance on the heat transfer partition and both sides. As long as the contact between the fluids is good, there will be a greater heat transfer rate.
Therefore, hybrid heat exchangers can be used wherever fluids are allowed to mix with each other, such as gas washing and cooling, circulating water cooling, steam-water mixed heating, steam condensation, etc. Its applications span chemical and metallurgical enterprises, power engineering, air conditioning engineering and many other production sectors.
Commonly used hybrid heat exchangers include: cooling towers, gas scrubbers, jet heat exchangers and hybrid condensers.
2) Regenerative heat exchanger
Regenerative heat exchanger is a device used for regenerative heat exchange. It is filled with solid filling to store heat.
Generally, the fire grid is built with refractory bricks (sometimes metal corrugated belts, etc.) are used.
Heat exchange takes place in two stages.
In the first stage, the hot gas passes through the fire grid, transfers heat to the fire grid and stores it.
In the second stage, the cold gas passes through the fire grid and is heated by receiving the heat stored in the fire grid.
These two phases occur alternately. Usually two regenerators are used alternately, that is, when hot gas enters one device, cold gas enters the other. It is often used in the metallurgical industry, such as the regenerator of open-hearth steelmaking furnaces.
It is also used in the chemical industry, such as air preheaters or combustion chambers in gas furnaces, and regenerative cracking furnaces in artificial petroleum plants.
3) Partitioning wall heat exchanger
In this type of heat exchanger, the hot and cold fluids are separated by a metal so that the two fluids do not mix and transfer heat.
In chemical production, hot and cold fluids often cannot come into direct contact, so the dividing wall heat exchanger is the most commonly used heat exchanger.
The following mainly introduces the classification of dividing wall heat exchangers:
a）Jacketed heat exchanger
This type of heat exchanger is made by installing a jacket on the outer wall of the container and has a simple structure; however, its heating surface is limited by the container wall and its heat transfer coefficient is not high.
In order to improve the heat transfer coefficient and heat the liquid in the kettle evenly, a stirrer can be installed in the kettle.
When cooling water or heating agent without phase change is introduced into the jacket, spiral partitions or other measures to increase turbulence can also be installed in the jacket to increase the heat transfer coefficient on one side of the jacket.
In order to make up for the lack of heat transfer surface, coiled tubes can also be installed inside the kettle.
Jacketed heat exchangers are widely used for heating and cooling of reaction processes.
b) Snake tube heat exchanger
The coiled tube heat exchanger is further divided into an immersed coiled tube heat exchanger and a sprayed coiled tube heat exchanger.
Snake tubes are mostly made of metal tubes bent into various shapes that are suitable for the container, and are immersed in the liquid in the container.
Its advantages are: simple structure, can withstand high pressure, and can be made of corrosion-resistant materials.
Disadvantages: The liquid turbulence in the container is low, and the heat transfer coefficient outside the tube is small.
Fix the heat exchange tubes in rows on the steel frame.
Hot fluid flows in the tube, and cooling water is poured evenly over the device.
Advantages: The hot fluid flows in the tube, and the cooling water pours down evenly above the device. The heat transfer coefficient is large, so the heat transfer effect of the spray heat exchanger is better than that of the immersed coil heat exchanger.
However, it needs to be placed in the open air. It occupies a large area and water easily splashes into the surrounding environment, making it inconvenient to use.
c)Jacketed heat exchanger
Because the fluid flow velocity inside the tube and outside the tube is larger. Cold and hot fluids can flow in pure countercurrent, so their heat transfer coefficient is large and the heat transfer effect is good. Commonly used water heating is a simple sleeve-type heat exchanger.
d) Shell and tube heat exchanger
Shell-and-tube heat exchangers are the most typical dividing wall heat exchangers. They have a long history of application in industry and still occupy a dominant position among all heat exchangers.
The shell and tube heat exchanger mainly consists of a shell, a tube bundle, a tube plate and a head. The shell is mostly round, with parallel tube bundles inside, and both ends of the tube bundle are fixed on the tube plate.
There are two types of fluids that exchange heat in a shell-and-tube heat exchanger: one flows inside the tube, and its stroke is called the tube side; the other flows outside the tube, and its stroke is called the shell side. The wall surface of the tube bundle is the heat transfer surface.
In order to improve the heat transfer coefficient of the fluid outside the tube, a certain number of transverse baffles are usually installed in the shell.
The baffles not only prevent the fluid from short-circuiting and increase the fluid speed, but also force the fluid to cross-flow through the tube bundle multiple times according to the prescribed path, greatly increasing the degree of turbulence. There are two commonly used baffles: round-shaped and disc-shaped (as shown in the figure below). The former is more widely used.
Each time the fluid passes through the tube bundle in the tube is called a tube pass, and each time the fluid passes through the shell is called a shell pass.
In order to increase the velocity of the fluid in the pipe, appropriate partitions can be installed in the heads at both ends to divide all the pipes into several groups evenly.
In this way, the fluid can pass through only part of the tubes and return to the tube bundle multiple times at a time, which is called multi-tube pass.
Similarly, in order to increase the flow rate outside the tube, longitudinal baffles can be installed in the shell to allow the fluid to pass through the shell space multiple times, which is called multi-shell pass.
In a shell and tube heat exchanger, due to the different temperatures of the fluid inside and outside the tube, the temperatures of the shell and the tube bundle are also different. If the temperature difference between the two is large, great thermal stress will occur inside the heat exchanger, which may cause the tubes to bend, break or become loose from the tube sheet.
Therefore, when the temperature difference between the tube bundle and the shell exceeds 50°C, appropriate temperature difference compensation measures should be taken to eliminate or reduce thermal stress.
Compensation method:
Attach an expansion ring to the shell or use U-shaped tube heat exchanger and floating head heat exchanger.
➪ Fixed tube plate heat exchanger
When the temperature difference between hot and cold fluids is not large, a fixed tube plate heat exchanger can be used.
It has a simple structure and low cost, but it is difficult to clean and is not suitable for fluids that are prone to scaling and fluids with large temperature differences.
If the temperature difference is not very large, a fixed tube plate heat exchanger with a compensation ring can be used.
e) Plate heat exchanger
The plate heat exchanger is composed of a set of rectangular thin metal heat transfer plates, which are clamped and assembled on the bracket with a frame.
The edges of two adjacent plates are lined with gaskets (made of various rubbers or compressed asbestos, etc.) for compression. There are round holes in the four corners of the plates to form fluid channels.
The difference between plate heat exchanger and shell and tube heat exchanger:
a.High heat transfer coefficient
Since different corrugated plates are inverted with each other to form a complex flow channel, the fluid flows in a rotating three-dimensional manner in the flow channel between the corrugated plates, which can produce turbulent flow at a low Reynolds number (generally Re=50~200), so heat transfer is The coefficient is high, generally considered to be 3 to 5 times that of the shell and tube type.
b. The logarithmic average temperature difference is large and the terminal temperature difference is small.
In the shell and tube heat exchanger, the two fluids flow in the tube side and the shell side respectively. Generally speaking, the flow is cross-flow, and the logarithmic average temperature difference correction coefficient is small. However, the plate heat exchanger mostly has a co-current or counter-current flow mode. , and its correction coefficient is usually around 0.95. In addition, the flow of cold and hot fluids in the plate heat exchanger is parallel to the heat exchange surface and there is no side flow. Therefore, the temperature difference at the end of the plate heat exchanger is small, and the heat exchange with water can be Below 1℃, while shell and tube heat exchangers are generally 5℃.
c. Small footprint
The plate heat exchanger has a compact structure, and the heat exchange area per unit volume is 2 to 5 times that of the shell and tube type. Unlike the shell and tube type, which requires a reserved maintenance space for extracting the tube bundle, the plate heat exchanger can achieve the same heat transfer. The area occupied by the heat exchanger is about 1/5~1/8 of the shell and tube heat exchanger.
d. Easy to change the heat exchange area or process combination
As long as a few plates are added or removed, the heat exchange area can be increased or reduced; by changing the plate arrangement or replacing a few plates, the required process combination can be achieved and adapted to new heat exchange conditions. It is almost impossible to increase the heat transfer area of shell heat exchangers
e.Light weight
The thickness of the plates of the plate heat exchanger is only 0.4~0.8mm, while the thickness of the heat exchange tubes of the shell and tube heat exchanger is 2.0~2.5mm. The shell of the shell and tube type is much heavier than the frame of the plate heat exchanger. , Plate heat exchangers are generally only about 1/5 of the weight of shell and tube heat exchangers.
f. Low price
Using the same materials and the same heat exchange area, the price of plate heat exchangers is about 40% to 60% lower than that of shell and tube heat exchangers.
g. Easy to make
The heat transfer plates of plate heat exchangers are stamped and processed with a high degree of standardization and can be produced in large quantities. Shell and tube heat exchangers are generally made by hand.
h. Easy to clean
As long as the compression bolts of the frame-type plate heat exchanger are loosened, the plate bundles can be loosened and the plates can be removed for mechanical cleaning. This is very convenient for the heat exchange process that requires frequent cleaning of the equipment.
i. Small heat loss
In plate heat exchangers, only the shell plate of the heat transfer plate is exposed to the atmosphere, so the heat dissipation loss is negligible and no insulation measures are required. Shell and tube heat exchangers have large heat losses and require thermal insulation.
j. Small capacity
It is 10%~20% of the shell and tube heat exchanger. i. The pressure loss per unit length is large. Since the gap between the heat transfer surfaces is small and the heat transfer surfaces are concave and convex, the pressure loss is greater than that of traditional smooth tubes.
k. Not easy to scale
Due to sufficient internal turbulence, it is not easy to scale, and its scaling coefficient is only 1/3~1/10.k of that of a shell-and-tube heat exchanger. The working pressure should not be too large, and the medium temperature should not be too high, as it may leak the plate type heat exchanger. The heat exchanger is sealed with a gasket, the working pressure should generally not exceed 2.5MPa, and the medium temperature should be below 250°C, otherwise leakage may occur.
l.Easily blocked
Since the channels between the plates are very narrow, generally only 2~5mm, when the heat exchange medium contains larger particles or fiber substances, it is easy to block the channels between the plates.
