【 Weak current College 】 heat exchanger design selection and use

Energy is currently the most important problems facing mankind, energy development and transformations use has become an important topic of States, and the heat exchanger is energy utilization process essential equipment, almost all industrial fields to use, chemical industry, metallurgy, power, transportation, aviation and aerospace sectors are widely applied.

In recent years because of new technology and new energy development, various types of heat exchanger and the ever-increasing importance of the industrial sector, and the heat exchangers and energy-saving measures in more critical equipment, therefore, either from the industry's development, or from the efficient use of energy, heat exchanger of reasonable design, manufacture, and operation is a very important significance.

1 classification of heat exchanger

1.1 direct heat-exchanger. without heat transfer fluid by the wall, cold and hot fluids in direct contact with a heat transfer operation of heat exchanger, such heat exchanger used in industrial production.

1.2 heat transfer between the wall of the heat exchanger. cold and hot fluid through the tubes, plates, wall thermal exchange of heat transfer operation of heat exchangers, is the most common and most commonly used heat exchangers, cooling, thermal fluid are fluid, can be air, smoke, steam, water, the focus of this article is for the discussion of the heat exchanger type.

1.3 regenerative heat exchanger, heat transfer of a batch-in waste heat regenerator is practical and effective way of recovering waste heat, often being used for recovery of waste heat from the combustion gases and steam consumption inequality as regulation means.

2 several characteristics of the heat exchanger and use in practical design selection, often known high-temperature fluid and the sides of the low-temperature fluid temperatures, doing crafts design selection, you need to consider is that there is minimal heat transfer area, there is as much of the heat transfer rate and lower equipment cost and construction fees. Furthermore, in the operation and maintenance cleaning more convenient premise considering heat exchanger design and selection of basic equations of heat transfer: from on-as you can see, at a certain speed, the larger the Reynolds number, the larger the heat transfer coefficient, while at the same time, pressure drop is also bigger.

2.1 tube heat exchangers-heat exchangers are the most frequently used common structure, it includes: fixed tube heat exchanger, the U-tube heat exchangers, with inflation Festival heat exchangers, floating head heat exchanger, segmenting type heat exchanger, a tube heat exchanger, etc. fixed tube heat exchanger has the structure is simple, light weight, low cost; disadvantage is due to thermal expansion and cause pipe bends .U-tube heat exchangers is to overcome this disadvantage will pipe made out "U" type, Plug one end fixed, make the heat exchanger is not affected by the expansion of the impact of the structure is relatively simple, light weight, its shortcomings are not mechanical cleaning, removal of the pipe inconvenience, unit capacity and quality of heat low, applicable to the large temperature difference, tube media comparison clean occasions with expansion joints of the heat exchanger can solve the problem, use the expansion expansion joint structure, so it is suitable for large temperature difference between the fluid and high pressure fluid, because the connector can be removed for cleaning, so you can handle easily scaling fluid, which is not suitable for low pressure of gas, but the drawback is the manufacturing complex. floating head tube-shell heat exchangers, floating head not connected with housing, free Flex, this will solve the problems of thermal expansion, but also facilitate cleaning, maintenance can take the tube to tube sheet for fixed, pipes, tube heat exchanger volume for each casings, its 1m3 heat transfer area of about 30 ~ 40m3. U-tube heat exchangers, floating head heat exchanger volume for each casings, its 1m3 heat transfer area around for 70m2.

2.2 Plate Exchanger. due to the heat exchanger for heat transfer surface can press out embossing line liquid, in low Reynolds number condition can appear turbulent state, so the higher heat transfer coefficient, generally up to 3000 ~ 5000Kcal/m2.h. ° c, and the same velocity of shell and tube heat exchanger, this value is about Shell and tube heat exchanger for heat transfer coefficient for 3 ~ 5 times, though, when the plate heat exchanger resistance will be larger, such as reactive power consumption in the same condition than the plate heat exchanger tube-shell heat coefficient ratio of twice around. due to the heat exchanger of compact structure, the gap is small, thus the heat transfer surface area per unit volume increases, its installation about Shell and tube of 1/2 ~ 1/3, to savings in occupies and construction costs, the volume of each casings, for 1m3 heat area around, in addition, 80m2 heat exchangers or heat transfer area easily, for shell and tube heat exchanger in the need to increase the capacity of the liquid, the original heat exchanger for heat transfer area nearly impossible, but the plate heat exchanger heat transfer area is very easy to increase, thus increasing the processing capacity, in addition, the heat exchanger plate only heat the shell plate when exposed to the atmosphere, the heat loss is negligible, nor need insulation measures.

Plate heat exchanger in operation and maintenance of the characteristics of the Assembly and disassembly is one of the more convenient, you can even do not have to be completely disassembled, only to release the clamp bolts can be taken out the plate clean, replace washers, and replace the plate heat exchanger this for media materials prone deposition has become particularly important. allowing use of temperature and pressure side: for heat exchanger plates on each of the two washers between, to prevent the leakage, and thus it the material of the sealing around the total length is very long, prevent gasket leak is the plate heat exchanger an important link, washers can withstand temperature, pressure and chemical stability is also often a plate heat exchanger with temperature and pressure limit, and allows the use of the material scope., because the gap between the heat transfer surface, heat transfer surfaces have a bump, than the traditional smooth pressure losses, in addition, the plate heat exchanger on the working pressure is low.

2.3 plate-fin heat exchanger. due to the special structure of the fin, make the fluid to the channels formed strong turbulence, the thermal resistance of boundary layer constantly destroyed, thereby effectively reduces thermal resistance, improve the heat to boiling water; General heat transfer coefficient is 1500 ~ 30000Kcal/m2.h. ¡æ. plate-fin heat exchanger structure is compact, the heat transfer surface area per unit volume, General than tubular heat exchanger for thermal efficiency is much more than five times, for each casings volume, its 1m3 heat transfer area-160m2 leftRight-plate-fin heat exchangers are lightweight and strong, due to the fin is very thin, generally 0.2 ~ 0.3mm, due to compact structure, small size, generally useful aluminium manufacturing, thus the weight very light, at the same time, the fin is both a major heat transfer surfaces, and the support of two divisions, the intensity high-plate-fin heat exchanger adaptable, on the same device may allow a 2 ~ 9 medium heat, and can be used for gas-gas, gas-liquid, liquid-liquid, heat exchange between the main drawback: because-runner is small, easy to cause blockage and increase resistance drop, when the heat exchanger fouling, cleaning very difficult, but because of the heat exchanger plate and fin is very thin aluminum plates (foil) requests made, so the media on aluminum does not produce corrosion, once the internal corrosion of leaking, it can be difficult to repair.

3. heat pipe heat exchanger tube is packaged in sealed containers of liquid evaporation and condensation, and effectively transfer heat of a heat-transfer device. some heat pipe as heat exchanger components while the assembled heat exchanger called heat-pipe heat exchangers-heat pipe heat exchanger used in gas-gas heat exchanger, heat pipe, when both ends of the heating and heat release section equipped with finned heat effect to enhance the-thermal heat exchanger heat transfer surface lies essentially isothermal, per unit volume of the heat transfer area larger, select different refrigerants, allows the heat pipes in different temperature conditions of use, the structure is relatively simple, from thermal expansion caused by problems of smaller. However, heat pipe heat exchanger is the most important issue, as the heat-transfer element of heat tubes of refrigerants for the use of the work environment is a critical amount of heat transmission and operating temperature limit, if you exceed this limit, to continue its work, the evaporation segment will "burn" and stop working.

4. concluding remarks

In the associated with the operation of the heating or cooling, there is always a wide variety of heat transfer, narrow heat exchanger only note the significance of reclaimed heat, just see the thermal heat transfer equipment, recycling and recovery of different degree of effectiveness of importance, but through a heat exchanger to heat recovery, cooling fluid deserves to improve enthalpy, thus further the effective use of these waste heat, which makes the heat exchanger has a new value, while on the other also has a new awareness. General heat exchanger always want cold fluid outlet temperature higher only beneficial, but also does not require infinite heat transfer area. at the same time, also because of the cooling fluid two-fluid flow ratio, fluid state, there is no heat transfer wall, equipment, structures, methods of operation and other factors that affect not only heat transfer equipment, but also affect the heat transfer process, so that these factors not only a clear distinction is more difficult, but also by heat exchanger selection range.