Heat Transfer Research Technology enhanced shell and tube heat exchanger shell
http://china.toocle.com 2009 年 12 月 03 日 10:04 Chinese heat exchanger network
Social business December 3 hearing
Ouyang Ti Kenny Wong
Abstract: This paper presents the results of shell and tube heat exchanger shell heat transfer enhancement with regard to research and get to start, pointed out the enhanced heat transfer technology research.
Keywords: shell and tube heat exchanger; shell; enhanced heat transfer
INTRODUCTION
Shell and tube heat exchanger has been widely used in the chemical, petroleum, power, metallurgy, refrigeration, atomic energy, shipbuilding, food and other industrial sectors. Nearly 40 years, the enhanced heat transfer tube at home and abroad have done a lot of research, and achieved fruitful results, there are no less than the enhanced heat transfer tube technology hundred species. In contrast, the shell Heat and respect is far broader and deeper tube studies. Until the 1970s, shell heat transfer enhancement technology began to be seriously and has made great progress.
Ways shell heat transfer enhancement are mainly two kinds: one is to change the shape of a tube or pipe plus fins, that is, by changing the shape and surface properties of the tubes to enhance heat transfer, such as threaded pipe, spiral groove tubes, external fins enhanced heat transfer tubes tube technology; another one is to change the baffle between the shell or tube supports, to reduce or eliminate the flow and heat transfer shell stranded dead zone, so that the heat transfer area can be fully utilized. The first way to strengthen the tube is consistent with the mechanism of heat transfer technology, there are a lot of literature. Shell and tube heat exchanger shell baffle or control support for the development of performance baffles change its purpose is to maximize the pumping power for enhanced heat transfer, rather than consume between pipe supports. Existing support in the form of a support plate, rod bearings, rings hollow tube support and self-supporting.
A plate support structure
The traditional shell and tube heat exchanger with a single segmental baffle support, shell fluid flow dead easy to produce, the heat transfer area can not be fully utilized, and thus lower shell heat transfer coefficient, easy to scale, large fluid resistance. And when the fluid flows across the tube bundle, the tubes can also generate induced vibration and damage the reliability of the tube and the tube plate connection. Therefore, in order to eliminate its drawbacks, the past 20 years there have been many new shell baffle support structure, such as multi-segmental baffles, the entire circular plate, shaped orifice, mesh panels, plus parallel segmental baffles dividers, spiral baffle plates. The common feature of these new baffle plate supporting structure as much as possible to improve fluid flow and heat transfer shell dead zone, lower shell fluid flow resistance and vibration resistance tubes can be enhanced.
1.1 pairs of arcuate and three segmental baffles
Double segmental baffle heat exchanger shell side fluid into two parallel beams, the length of the lateral flow (ie, the number of cross-flow through the tube) is substantially the same as having a half notch single segmental baffles, and has the same fold flow plate gap spacing and single segmental baffles compared drop double segmental baffles corresponding values for comparable equipment 30% to 50%, and the heat transfer coefficient is 60% ~ 80% than the corresponding values of the device [ 1]. Similar to the double-segmental baffles, three arched and multi-segmental baffles in the shell side of the heat exchanger will be divided into three or more strands of fluid flow.
1.2 full circular baffle
To change the segmental baffles supporting lateral flow as much as possible parallel to the longitudinal flow tube, eliminating stranded dead zone, improve fluid flow in the shell of the power plants and the petrochemical industry, the emergence of the entire circular baffle. Entire circular baffle plate drilled originally appeared in the big hole, both to allow the tube through, there are enough clearance to allow fluid through. Inner and outer tubes were generally longitudinal flow of fluid, large temperature difference driving force, and because the channel wall and the orifice ring gap between the role of the fluid jet can produce the fluid leaving empty soon turbulence, so that the wall is not easy to knot dirt, shell heat strengthened. But the whole circular baffles increases the diameter of the heat exchanger housing, and because of the lack of pipe support structure, anti-vibration performance of such a heat exchanger tube bundle is very poor. Large enough to improve the entire hole of the circular baffle plate, the opening between the hole hole, the heat transfer medium through the baffle orifice, so without increasing the diameter of the housing.
With a full circular orifice of the baffle hole in the gap between the tubes and fouling can easily cause corrosion. To compensate for this deficiency, the emergence of a rectangular hole, plum blossom hole baffle structure, such as Hole, which baffles both support tubes, while still allowing the heat transfer medium flows through the baffles, when the medium flows through the holes , results in a jet, the tube has a self-cleaning and erosion, thereby eliminating the under-deposit corrosion and fouling of the tube.
Alien Orifice While many advantages, but manufacturing is relatively difficult, high cost, and therefore appeared mesh circular baffle entire industry. It baffles for the entire round, with each of the four holes adjacent baffle the same line as a group, at the bridge of the milling pass, shell-side fluid baffles from milling through the mouth, perfecting area uniform longitudinal flow [2] .
1.3 segmental baffles plus parallel plates separated
In a single bow bulkhead baffle shell heat exchanger tube between two parallel baffle baffle insert a piece or several pieces of parallel flow dividers, can be changed to the original channel multi-strand parallel channels, the original single-stream into multi-strand parallel flow. In this way, you can effectively control panel reflux eddy scale between the dead zone, so that the flow field between the plates can be homogenized and flow resistance between the pipe can be reduced. In the case of Re is not too high flow plus two parallel plates separated after a certain extent, can control all of the shell side vortex flow field, and can play a better heat transfer enhancement effect. Arcuate baffle heat exchanger with parallel flow separator plate heat transfer enhancement comprehensive performance evaluation factor in the Re range of measurement, and its value is about 1.20 ~ 1.15, enhanced with the increase of Re number of factors and less small, this technique is suitable for heat transfer enhancement at low Reynolds number using [3].
1.4 helical baffle
Helical baffle is to replace traditional vertical bow plate or similar helical spiral baffles, baffle and heat exchanger shell cross-section has a tilt angle so that the shell fluid flow along a spiral path. By Helical flow channel can be divided into the number of single or double. Figure 1 is a schematic diagram of the fluid in the segmental baffles and helical baffle heat exchanger shell side flow of contrast.
Contrast Figure 1 segmental baffle heat exchanger with helical baffle heat exchanger shell side fluid flow schematic
Helical baffle heat exchanger with helical baffles on the shell side fluid plunger in a continuous spiral flow (ie, p lug flow), does not appear traditional baffle heat flow within the "dead zone", and As the boundary layer interaction and control of heat transfer interface swirling vortex generated so greatly enhance the degree of turbulence, will help improve the shell-side heat transfer film coefficient.
2 rod support structure
In order to solve the traditional baffle heat exchanger tube bundles due to fluid lateral erosion problems caused by vibration and damage appeared rod baffle support structure. Baffled by the baffle and the support rods rod support structure. Baffle is made in a circle (baffle ring) on a number of welding rod (rod baffle) constitute, not exactly the same every four constitute a baffle and baffle group (Figure 2). Baffle baffle on the pole staggered tubes interspersed between the baffle rod diameter approximately equal to the gap between adjacent tubes, the tube is tightly clamped rod baffle.
As the rod baffle heat exchanger shell longitudinal fluid flow, basically there is no flow dead zone. Further, the fluid flow through the baffle rod (round bar) in the both sides thereof alternately into and out of the vortex generating (Kaman vortex), and fluid flow through the baffle and the reduction of area, and then expanded to produce a venturi effect . As the role of Kaman vortex venturi effect of the fluid on the wall to form a more intense erosion, thus thinning the boundary layer heat transfer, heat transfer will strengthen the shell.
2 baffle grid structure diagram
3 hollow ring support structure
Supporting structure is hollow ring hollow ring to replace baffle plate made between pipe supports. The hollow ring cut into smaller diameter pipe stub, evenly distributed on the same section of the tube between the tube linear contact is fixed relatively close together so that the tube bundle. Schematic hollow ring bearing the heat transfer tubes shown in Figure 3. Using a hollow ring between the tube support can significantly reduce the gas in the shell as a result of repeatedly baffled fluid delivery lost power, relying on inter-tube gas velocity increases to improve the film coefficient of heat pipe rumor, to achieve the best use of fluid delivery success. Second, the use of rough-type heat exchanger tube, such as zoom control, low fin tube, petal tube, enhanced convection heat transfer fluid when vertical erosion.
Figure 3 Schematic hollow ring bearing the heat transfer tubes
4 from the pipe support structure
To simplify the bundle support and improve the compactness of the heat exchanger, in recent years, developed a number of self-supporting tubes, such as puncture the diaphragm tube, spiral flat tubes and tapered tubes. Such tubes by itself as a part of the tube puncture the diaphragm, spiral point contact or adjustable parts to support the pipe, while the composition of the shell spoiler element, increasing the turbulence of the fluid itself, undermining the wall fluid boundary layer, so that the shell heat further enhanced.
5 Conclusion
Shell and tube heat exchanger shell strengthen the development of heat transfer technology exhibit two characteristics: First, the shell fluid flow from the lateral flow becomes parallel to the longitudinal flow of the pipe, the flow decreases the pressure drop, heat transfer area to get leverage; Second, low flow resistance toward support in the form of simple structure, easy to manufacture and provincial timber direction. By strengthening investigate heat transfer on the shell, and the low flow resistance and high heat transfer efficiency of heat transfer enhancement shell support structure and efficient combination of enhanced heat transfer tubes of composite heat transfer enhancement technology, the entire heat exchanger pass Thermal overall performance is improved, will be the direction of shell and tube heat exchanger development.
Reference links:
China HVAC Network: http://www.cnhvacrnet.com/detail/4897524.html
Chinese heat exchanger network: http://www.31hrq.com/detail/4897524.html
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