Overview of existing heat exchangers

Heat exchangers are an integral part of many industrial processes, providing efficient heat transfer between different environments. They are used in a wide variety of areas – from the chemical and oil industries to heating and cooling systems in housing and utilities. The variety of designs and operating principles of heat exchangers allows you to choose the best solution for each specific task, be it heating, cooling or heat recovery. This material was prepared by us, a research team from the Moscow Power Engineering Institute. In this article, we will consider the main types of heat exchangers, their operating principle, as well as key application features.

Water-water heater is the main and most expensive element of the heating station. Shell-and-tube and plate heat exchangers are the most widely used in heating and hot water supply systems.

In shell-and-tube heat exchangers, the main structural elements are a cylindrical body, a tube sheet, covers, a tube bundle, and segmented partitions (if necessary). The unit is equipped with nozzles for supplying and removing working media.

Heat is transferred from the hot coolant to the cold one by convective heat exchange through the dividing wall. The flow section of the intertube space in such heat exchangers exceeds the flow section of the pipes by 2.5 – 3 times, as a result of which the heat transfer coefficients in the intertube space due to the difference in the speeds of the coolants can be significantly lower than in the pipe space. In this case, to improve heat exchange, the units are made multi-pass. The organization of passes through the pipe space can be carried out using a longitudinal partition in the distribution chamber. For this purpose, transverse partitions are installed in the intertube space, which additionally act as supports for the tube bundle, fixing the pipes at a given distance from each other and reducing vibrations.

Constructions of transverse partitions:

• with a sector cutout – the coolant, with the help of an additionally installed longitudinal partition, performs a constant rotational movement, turbulizing the flow;

• solid – the movement of the liquid is carried out through an annular gap between the pipe and the partition, which increases the speed of the coolant and as a result the thickness of the laminar boundary layer decreases;

• with a segmental cutout – most often used due to their greater efficiency.

Due to the difference in temperatures of the heat carriers used, the following types of compensators are used to compensate for stresses that arise due to different temperature expansions of pipes and casings:

• lens – consist of one or more lenses that are small in size and light in weight, but have low compensating capacity and high thrust forces, and have a small working pressure range. They reduce temperature stresses by deforming their flexible elements;

• stuffing box – used at pressures over 1.6 MPa, characterized by high compensating capacity, small overall dimensions, low hydraulic resistance. Used for devices with small diameters, as they can pass the working medium, which requires their periodic adjustment. Compensation in them is carried out by slipping the ends of the pipes;

• U-pipe heat exchangers – such devices provide free extension of pipes, which eliminates the occurrence of temperature stresses. Mechanical cleaning of the inner surface of the pipes is almost impossible, so it is necessary to use a heat carrier that does not form deposits. They have significant bending stresses, increased metal consumption due to the free space formed by the bending of the pipes. In addition, they do not have the ability to replace pipes when they fail, and with a large number of pipes, difficulties arise in their placement;

• heat exchangers with a floating head – in such units one of the tube sheets is fixedly connected to the body, and the other has the ability to move freely axially. When heating and extending the tubes, they can move inside the casing from the side of the moving sheet. At increased pressure of the media (5-10 MPa), it is possible to install an additional compensator located in an extended nozzle.

Fastening of pipes in tube sheets is most often carried out by flaring. With this method of fastening, a pipe with a gap is installed in the tube sheet, then its diameter is expanded to ensure complete sealing and reliable adhesion. Welding of pipes is performed in case of circulation of aggressive media in the heat exchanger, which when combined with air can form explosive mixtures, in case of unacceptable mixing of heat carriers, in case of small thickness of the tube sheet to increase the mechanical strength of the connection, in case of risk of corrosion on the surface of the pipe at the point of its contact with the hole in the sheet, in difficult working conditions due to high temperatures and pressures or their sharp fluctuations. Also, with a large number of pipes with a small diameter (12-16 mm), soldering can be used – the organization of a permanent connection of two metals with the help of a third, called solder. In rare cases, a gland connection of pipes is possible, allowing them to move freely in the longitudinal direction and reduce temperature deformations to a minimum. Also, with this type of fastening, quick replacement of pipes is possible, if necessary. However, the significant disadvantages of such a connection are its high cost, complexity and insufficient reliability.

The heat carrier with a higher flow rate and higher viscosity is directed into the inter-pipe space to ensure a lower speed of movement. At heating points, horizontally located heat exchangers are most often installed for ease of maintenance.

The main advantages of shell and tube heat exchangers:

• high efficiency due to large heat exchange surface;

• the ability to work in a wide range of pressures and temperatures;

• ability to work with aggressive environments;

• increased resistance to water hammer;

• ease of maintenance, which consists in the convenience of mechanical cleaning of both the internal pipes and the casing;

• reduced requirements for water purity;

• maintainability.

Flaws:

• high metal content;

• high price;

• dimensions, which makes their use in confined spaces difficult.

The main structural element of plate water heaters is a plate made corrugated to intensify heat exchange. The unit consists of a group of plates suspended from the upper guide 3. The ends of the guides are fixed on one side in a fixed plate 1 and on the other side on a rack 7. Using a movable plate 2 and tie rods 6, the plates are compressed into one package during assembly.

The plates are stamped from thin sheet steel or other metals and alloys. To increase compactness and increase the heat transfer coefficient, the profiles are made in various shapes. Currently, heat exchanger manufacturers have their own patented plate surface profiles, among which several main types can be distinguished:

• flat and channel plates – characterized by zigzag or spiral channels, usually used together with smooth plates. Paired connection of such plates allows to create two systems of channels, isolated from each other by a heat-transfer wall;

• tape-flow type plates – are periodically repeating corrugations of various shapes, located parallel to the smaller side of the plate, and are distinguished by a design diversity in the shapes and sizes of parts;

• mesh-flow type plates – in them, the turbulent elements of the profile simultaneously create a grid of mutual supports between the plates, which allows increasing the rigidity of the package and ensuring its normal operating conditions at higher pressures.

The channels are sealed using rubber gaskets of various shapes. Each plate has a rubber ring gasket that limits the channel for one of the moving media and covers two corner openings through which the coolant can flow, as well as two small rubber gaskets that isolate the other two openings and provide free passage for the second working medium. The hot and cold coolants move in gaps formed between two adjacent parallel plates. The movement can be carried out in a direct flow, counter-flow, or a mixed pattern. The following designs of devices are possible:

• dismountable – made of separate plates with gaskets, adapted for cleaning the entire heat exchange surface, can be used to work with viscous media requiring an increased channel width, allow relatively easy change of heat exchanger capacity by changing the type and number of plates, easy cleaning of the plates and channels of the device, replacement of plates if necessary for their repair. But due to the use of interplate seals, a limitation is imposed on the temperatures and pressures of heat carriers, their contamination with mechanical impurities;

• soldered – in this design, the connection of the plates to each other is carried out by soldering using nickel, used for more aggressive environments, or copper, due to which greater compactness is ensured. Such units have a more durable design, therefore they have improved technical performance characteristics, allow working with aggressive environments, have a lower cost compared to other types of plate devices. Their use is difficult when working with contaminated media, since cleaning in such units can only be carried out chemically by stopping its operation and without the ability to check the result of washing;

• semi-welded – in such devices the plate pack is made in a combined way. The plates are welded together in pairs, to the outer side of which seals are attached, and then the next such pair. Such a design allows using an aggressive medium as one of the heat carriers to eliminate the possibility of its leakage;

• welded – in this design, the plates are welded together without the use of seals. They can be used in processes with high temperature and pressure parameters, as well as with aggressive environments, since the welded connection eliminates the possibility of leakage and mixing of environments. They have a long service life and lower maintenance costs, can be designed with more complex and non-standard configurations depending on the requirements of the technical process.

Advantages of plate heat exchangers:

• compactness, due to the small thickness of the plates and their parallel arrangement with small gaps for the flow of the coolant;

• low metal consumption;

• the ability to change thermal power by increasing or decreasing the heat exchange surface by adding or removing plates, adjusting the flow pattern of coolants.

Flaws:

• low thermal inertia;

• high cost of maintenance;

• high hydraulic resistance, which requires more powerful pumps;

• the need to use purified water;

• instability of the heat transfer coefficient;

• dependence of hydraulic losses on water hardness;

• poor maintainability;

• limited coolant temperature due to the physical properties of the gasket material;

• the need for strict assembly conditions to ensure tightness.

Authors of the material: Temrina D.N., Guzhov S.V.