Heat exchanger section: [Weekly topic] How do heat exchangers transfer heat? (2011.6.27-7.3)

This post was last edited by ray1513 on 2011-6-27 at 12:19. A heat exchanger is a specialized device that uses the principle of heat conduction to transfer part of the heat from a hot fluid to a cold fluid; it is also known as a heat transfer device. Heat exchangers are classified into three types based on their heat transfer mechanism: mixed-type heat exchangers, regenerative heat exchangers, and shell-and-tube heat exchangers. A mixed-flow heat exchanger, also known as a contact heat exchanger, operates by exchanging heat through the direct contact and mixing of cold and hot fluids. Heat exchange is achieved by taking advantage of the timely separation of the two fluids after they mix and exchange heat. This type of heat exchanger is suitable for heat exchange between gas and liquid fluids. The operation of a regenerative heat exchanger takes place through the surface of the heat storage material located in the heat storage chamber; cold and hot fluids flow alternately over this surface, thereby enabling heat exchange. It is used in various industries, such as regenerators for preheating air beneath coke ovens. These types of heat exchangers are mainly used to recover and utilize the heat from high-temperature waste gases. Devices of a similar type designed to recover cold energy are called heat exchangers, and they are commonly used in air separation units. In a partitioned heat exchanger, the cold and hot fluids are separated by a solid partition, and heat exchange takes place through this partition; hence it is also known as a surface-type heat exchanger. This type of heat exchanger is the most widely used. The relative flow direction of the fluid in a heat exchanger is generally either co-current or counter-current. During flow in the forward direction, the temperature difference between the two fluids is greatest at the inlet and gradually decreases along the heat transfer surface, reaching its minimum at the outlet. During counterflow, the temperature difference distribution between the two fluids along the heat transfer surface is relatively uniform. Under the condition that the inlet and outlet temperatures of the cold and hot fluids remain constant, and when neither fluid undergoes phase change, the average temperature difference is greatest in counterflow and smallest in co-flow. Under the condition of transferring the same amount of heat, using counterflow can increase the average temperature difference and reduce the heat transfer area of the heat exchanger ； If the heat transfer area remains unchanged, using counterflow can reduce the consumption of the heating or cooling fluid. The former can save on equipment costs, while the latter can save on operating costs; therefore, counterflow heat exchange should be used as much as possible in design or production. When there is a phase change (boiling or condensation) in both or one of the cold and hot fluids, since only the latent heat of vaporization is released or absorbed during the phase change, the temperature of the fluid itself does not change; as a result, the inlet and outlet temperatures of the fluid are equal. In this case, the temperature difference between the two fluids is independent of the direction in which the fluid flows. In addition to the two flow directions of co-current and counter-current, there are also flow directions such as cross-flow and mixed-flow. Reducing the thermal resistance of partitioned heat exchangers, saving energy consumption, and thereby improving the heat transfer coefficient is an important issue. Generally speaking, the thermal resistance of metal walls is relatively low.

Reply to 1# w9970165: In the most common type of partitioned heat exchangers, heat transfer occurs mainly through conduction and convection. The hot fluid first transfers heat to one side of the tube wall through convection, then conducts the heat from that side to the other side of the tube wall; finally, the heat on the other side of the tube wall is transferred to the cold fluid via convection, thus completing the heat transfer process in the heat exchanger.

A mixed-flow heat exchanger relies on the direct contact between cold and hot fluids for heat transfer; this method eliminates the thermal resistance caused by the heat transfer surface and the fouling on its surfaces. 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. Based on their different applications, mixed-type heat exchangers can be classified into the following types: (1) Cooling towers (also known as chilled water towers). In such 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 scrubber (or washing tower) This type of equipment is used in industry to wash gases for various purposes, such as absorbing certain components from gas mixtures using liquids, removing dust from gases, and humidifying or drying gases. 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 exchanger: In this type of device, 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 transfer; both fluids then enter a diffuser 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 equipment generally uses direct contact between water and steam to condense the steam. :)

This post was last edited by prt0683 on 2011-6-27 at 15:32. Temperature difference is the driving force for heat transfer, which can be divided into convection, conduction, and radiation

I’m not sure whether solar collectors should be counted as well – they are devices used to absorb solar radiation, convert it into thermal energy, and transfer that energy to a heat medium. Common types include flat-plate, vacuum tube, and concentrator solar collectors

The most common type of partitioned heat exchanger relies mainly on two modes of heat transfer: conduction and convection. The hot fluid first transfers heat to one side of the tube wall through convection, then conducts the heat from that side to the other side of the tube wall; finally, the heat on the other side of the tube wall is transferred to the cold fluid via convection, thus completing the heat transfer process in the heat exchanger.

Reply to 1# w9970165: I. Increasing the heat transfer coefficient – To raise the heat transfer coefficient of a heat exchanger, it is necessary to increase the surface heat transfer coefficients on both the cold and hot sides of the plates, reduce the thermal resistance caused by fouling layers, use plates with high thermal conductivity, and decrease the thickness of the plates. Only by doing so can the heat transfer coefficient of the heat exchanger be effectively increased. a. Increasing the surface heat transfer coefficient of the plates: The corrugations in plate heat exchangers enable turbulence to occur at lower flow velocities (at a Reynolds number of 150), thereby allowing for a higher surface heat transfer coefficient. This coefficient is dependent on the geometric structure of the plate corrugations as well as the flow conditions of the fluid. The wave shapes of the plates include herringbone, straight, spherical, etc. Years of research and experimentation have shown that corrugated sections with a triangular shape exhibit the highest surface heat transfer coefficients, lower pressure drops, and a more uniform stress distribution under compression. On the other hand, herringbone-shaped plates, although difficult to manufacture, also have high surface heat transfer coefficients; moreover, the larger the angle of the corrugations, the higher the flow velocity of the fluid within the channels between the plates, and thus the higher the surface heat transfer coefficient. b. Reducing the thermal resistance of the fouling layer: The key to reducing the thermal resistance of the fouling layer in heat exchangers is to prevent scaling on the plates. When the fouling thickness of the plate is 1 mm, the heat transfer coefficient decreases by about 10%. Therefore, it is necessary to pay attention to monitoring the water quality on both the cold and hot sides of the heat exchanger, in order to prevent scaling on the plates and to stop debris in the water from adhering to them. Some heating supply companies add chemicals to the heating medium to prevent water theft and corrosion of steel components; therefore, it is necessary to pay attention to water quality and the issue of impurities accumulating on the heat exchanger plates caused by viscous chemicals. If there are sticky impurities in the water, a specialized filter should be used for treatment. When selecting a pesticide, it is advisable to choose one that is non-sticky. c. Use plates with high thermal conductivity; the material for these plates can be austenitic stainless steel, titanium alloys, copper alloys, etc. Stainless steel has good thermal conductivity, with a thermal conductivity of about 14.4 W/(m·K). It is strong, has good formability, and is resistant to oxidation. Its price is lower than that of titanium alloys and copper alloys, which is why it is widely used in heating systems; however, it has poor resistance to chloride ion corrosion. d. Reducing the thickness of the plates: The designed thickness of the plates has no relation to their corrosion resistance; it is related to the pressure-bearing capacity of the heat exchanger. Thickening the plates can improve the pressure resistance of the heat exchanger. When herringbone plate assemblies are used, adjacent plates are inverted relative to each other, with their corrugations in contact with one another, thus creating support points that are dense and evenly distributed. The corner holes of the plates as well as the edge sealing structures have been continuously improved, enabling the heat exchanger to possess excellent pressure resistance. The maximum pressure resistance of domestically produced detachable plate heat exchangers has reached 2.5 MPa. The plate thickness has a significant impact on the heat transfer coefficient; a reduction of 0.1 mm in thickness increases the overall heat transfer coefficient of symmetric plate heat exchangers by approximately 600 W/(m·K), while it increases that of asymmetric plate heat exchangers by about 500 W/(m·K). While ensuring that the heat exchanger can withstand the required pressure, a smaller plate thickness should be chosen as much as possible. II. High logarithmic mean temperature difference: The flow patterns of plate heat exchangers include counterflow, co-flow, and mixed flow. Under the same operating conditions, the logarithmic mean temperature difference is greatest in counterflow and smallest in co-flow, with the mixed flow pattern falling between the two. Methods to increase the logarithmic mean temperature difference of a heat exchanger include using a counterflow or a mixed flow pattern that is close to counterflow as much as possible, raising the temperature of the fluid on the hot side, and lowering the temperature of the fluid on the cold side. III. Determination of the location of inlet and outlet pipes: For plate heat exchangers with a single-flow arrangement, to facilitate maintenance, the fluid inlet and outlet pipes should be positioned as close as possible on the side of the fixed end plate of the heat exchanger. The greater the temperature difference of the medium, the stronger the natural convection of the fluid, and the more pronounced the effect of the resulting stagnation zone. Therefore, the inlet and outlet positions of the medium should be arranged such that hot fluids enter from above and exit from below, while cold fluids enter from below and exit from above, in order to reduce the impact of the stagnation zone and improve the heat transfer efficiency of the heat exchanger.

Reply 6# penryn

This post was last edited by string_0 on 2011-6-28 04:22. The heat transfer within a heat exchanger occurs through the following processes. 1. Heat is transferred from the center of the high-temperature medium to its edge in the form of convection (a mode of heat transfer; the rate is primarily determined by the turbulence level of the medium). 2. Heat is transferred from the high-temperature medium to the pipe wall (primarily determined by the thermal conductivity; there are many factors that influence this, including scale formation and the properties of the high-temperature medium). 3. Heat is transferred from the outside of the pipe wall to its inside (a mode of heat transfer; it is primarily determined by the properties of the metal). 4. Heat is transferred from the inside of the pipe wall to the low-temperature medium layer (primarily determined by the thermal conductivity; there are many factors that influence this, including scale formation and the properties of the low-temperature medium). 5. Heat is transferred from the low-temperature medium layer to the turbulent region of that medium (primarily determined by the turbulence conditions and the properties of the low-temperature medium)

The modes of heat transfer are essentially conduction, convection, and radiation, all of which are utilized in heat exchangers.