Classification of heat exchangers

Classification of heat exchangers A heat exchanger is a device used for heat exchange between two fluids at different temperatures. The function of a heat exchanger can be to facilitate heat exchange. That is, to exchange a certain amount of heat between specified fluids over a certain period of time; it can also be used for waste heat recovery; or it can serve the purpose of ensuring safety, that is, to prevent pressure increases caused by rising temperatures from damaging certain equipment. Depending on their functions, heat exchangers vary in terms of design, selection, and operating conditions. The basic requirements for a heat exchanger are that it must meet the heat exchange needs, that is, it must achieve the required amount of heat transfer and the appropriate temperature of the heat medium; it should have low heat losses and high efficiency; the flow resistance should be low; it must possess sufficient mechanical strength, as well as strong resistance to corrosion and damage, with minimal maintenance requirements; its structure should be rational, ensuring safe and reliable operation, so that thermal stresses resulting from temperature increases between components do not cause the heat exchanger to crack; it should be easy to manufacture, install, and maintain; and it should be cost-effective, with low total costs over its entire lifespan (total costs including the initial investment in equipment and auxiliary devices, as well as operational and maintenance expenses). Heat exchangers used in domestic hot water systems should also be easy to clean of scale. These requirements often conflict with each other and it is difficult to satisfy them all at the same time; therefore, priorities should be set based on specific circumstances during the selection and design of the heat exchanger, in order to meet the main requirements of the project. Since the failure rate of heat exchangers is low, and heating represents a seasonal load with sufficient time for maintenance, stopping heat supply to the domestic hot water system will not have any significant impact; therefore, it is not necessary to install a backup heat exchanger. The selection of the number of heat exchangers and the determination of the capacity of each one should be adapted to the phased increase in heat load, taking into account the reliability of heat supply. Heat exchangers can be classified into water-water heat exchangers and steam-water heat exchangers depending on the heat exchange medium ； Based on their working principles, heat exchangers can be classified into partition-type, direct-contact type, regenerative type, and heat-pipe type. Surface heat exchangers are also known as partitioned heat exchangers. It refers to a heat exchanger that is indirectly heated through a heat transfer surface. Since in surface-type heat exchangers, the hot and cold fluids are separated by a solid wall during heat transfer, and heat is transferred between them through this wall, their heat exchange efficiency is lower compared to direct-contact heat exchangers; they are therefore used in situations where the two fluids cannot mix together. The main types include tubular, positive-displacement, plate, and spiral-plate types. A tubular heat exchanger is a surface-type heat exchanger that utilizes the wall of thin-walled metal tubes for heat exchange. The effect of fouling on the heat exchange surface should be taken into account during calculations, and a fouling correction factor should be used when calculating the heat transfer coefficient. A shell-and-tube heat exchanger is a type of tubular heat exchanger consisting of a cylindrical shell and a tube bundle equipped with a tube sheet, which is mounted inside the shell. It has a simple structure, low cost, a wide flow cross-section, and is easy to clean of scale; however, it has a low heat transfer coefficient and requires a large area. Shell and tube heat exchangers come in several types, including fixed-tube-sheet steam-water heat exchangers, shell and tube steam-water heat exchangers with expansion joints, floating-head steam-water heat exchangers, U-tube shell and tube steam-water heat exchangers, corrugated-shell and tube steam-water heat exchangers, and segmented water-water heat exchangers. A shell-and-tube heat exchanger refers to a tubular heat exchanger composed of components such as tubes and shells. A plate heat exchanger is a surface-type heat exchanger in which fluids at different temperatures flow alternately between multiple layers of closely arranged thin metal plates to exchange heat. It is mainly composed of heat transfer plates, fixed cover plates, movable cover plates, positioning bolts, and compression bolts, with gaskets used to seal between the plates. Due to the special structure of the plate surface, fluid undergoes intense turbulence at low flow rates, thereby enhancing the heat transfer process. Plate heat exchangers are compact in structure, easy to disassemble and clean, have a high heat transfer coefficient and wide adaptability, as well as material savings. However, the flow cross-section between the plates is narrow, which makes it easy for scale and deposits to form and cause blockages; moreover, leaks can occur if the gaskets lack sufficient heat resistance. This type of heat exchanger is commonly used in heating systems. When calculating plate heat exchangers, the effect of fouling on the heat exchange surface must be taken into account, and a fouling correction factor should be used in the calculation of the heat transfer coefficient. A spiral plate heat exchanger is a surface-type heat exchanger consisting of two parallel metal plates rolled into two spiral channels, with the two fluids flowing on opposite sides of the spiral plates to exchange heat. The spiral plate heat exchanger has a compact structure, and its heat transfer coefficient is generally higher than that of shell-and-tube heat exchangers. Compared with plate heat exchangers, it has a wider flow cross-section and is less prone to clogging. Its main drawback is that it cannot be disassembled for cleaning. Mixed-type heat exchangers are also known as direct-contact heat exchangers. It refers to heat and mass exchange that occurs when two fluids at different temperatures come into direct contact. They have high heat transfer efficiency, and mainly include wetted-type and nozzle-type heat exchangers. A wetted-area heat exchanger is a type of mixed-flow heat exchanger in which water flows in a dispersed manner through the small holes in several levels of wetting trays, coming into direct contact with steam. It mainly consists of a housing and a water spraying tray; the water to be heated enters from the top and flows downward in fine streams through the sieve holes in the spraying tray. Steam enters from the top or bottom of the housing, comes into contact with the water to be heated, condenses and releases heat, and the heated water is discharged from the bottom of the heat exchanger. Compared to surface-type heat exchangers, wetted-tube heat exchangers have higher heat transfer efficiency. Under the same designed heat load conditions, they require a smaller heat transfer area and are more compact in design. However, they cannot recover pure condensate water, which means that this increases the capacity of the heat source water treatment equipment required, and it is also necessary to take into account the issue of making use of any excess condensate water in the system. In addition to its heat exchange function, this type of heat exchanger also serves as a water storage tank (it can replace the expansion tank in heating systems) and performs pressure regulation (the steam pressure inside the casing can be used to maintain a constant pressure in the system). A nozzle-type heat exchanger is a type of heat exchanger in which the water to be heated flows through a nozzle, where it comes into direct contact with steam that is injected from numerous inclined small holes in the wall of the nozzle. It is mainly composed of a housing, nozzle, drain plug, mesh cover, and packing. As the heated water passes through the Laval-shaped nozzle, steam is injected into the water from outside the nozzle through small holes in its wall; the two mix rapidly due to their high-speed flow, thereby heating the water. Nozzle-type heat exchangers are compact in size, simple to manufacture, easy to install, have sensitive control, allow for a large temperature difference during heating, and operate smoothly. However, their heat transfer capacity is limited; they are generally used only for hot water supply and small hot water heating systems. When used in heating systems, they are usually installed on the outlet side of the circulation water pump. A steam-water heat exchanger is a heat exchanger in which the heating medium is steam and the medium to be heated is water. A water-water heat exchanger refers to a heat exchanger in which both the heating medium and the medium being heated are water. A volumetric heat exchanger is one in which the flow cross-section for the water to be heated is large and the flow velocity is low; in addition to heat exchange, it also serves to store hot water. It is mainly composed of a shell and a bundle of U-shaped bends connected in parallel; steam or hot water flows through these tubes, and the size of the shell is determined by the water storage capacity. The effect of fouling on the heat exchange surface should be taken into account during calculations, and the heat transfer coefficient should be calculated using a method that accounts for the thermal resistance caused by scale. Its main feature is that it also serves as a water storage tank. It is easy to remove scale, but its heat transfer coefficient is much lower than that of shell-and-tube heat exchangers; it is mainly used in hot water supply systems. A rapid heat exchanger is one in which both the heating medium and the medium to be heated flow at high speeds, in order to achieve intense heat exchange. It has high thermal efficiency, a compact structure, and occupies little space, but it suffers from high head loss and cannot store hot water for use in regulation. When the water distribution is uneven or the pressure of the heat medium is unstable, it is difficult to regulate the water temperature; when the quality of the feed water is poor, scaling occurs severely in the heat exchanger, making it hard to clean. It is commonly used in hot water supply systems. A heat pipe heat exchanger is a heat exchanger that utilizes the principle of heat pipes to achieve heat exchange. A heat exchange tube bundle composed of several heat pipes is arranged within the shell via partition plates; a heat pipe-type heat exchanger is one that utilizes the principle of heat pipes to carry out heat exchange. A heat exchange tube bundle composed of several heat pipes is arranged within the shell via partition plates; these partition plates create hot and cold fluid channels respectively with the heating and cooling sections of the heat pipes as well as the corresponding inner cavities of the shell. The hot and cold fluids flow continuously through these channels alongside the heat pipe bundle to facilitate heat transfer. It is commonly used in waste heat recovery projects. The following are respectively: air cooler, fully welded heat exchanger, plate heat exchanger. This post was last edited by dreamxy2008 on 2008-1-12 09:53.]

U-tube heat exchanger Last edited by wsts00100 on 2009-1-21 14:40]

Floating-head heat exchanger Last edited by wsts00100 on 2009-1-21 15:07]

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