Heat Exchanger with Helical Baffles
There are often numerous heat transfer problems involved in the petroleum, chemicals, power, metallurgy, energy and other industrial sectors. The shell and tube heat exchanger is the heat transfer equipment most widely used in the current industrial production. Compared with other types, its main advantages are the large heat transfer area in the unit volume and good heat transfer effect. Combined with a simple structure, wide range of materials required in manufacturing, and greater operation flexibility, it is more and more widely used in the chemical engineering fields.
To increase the speed of shell fluid and intensify the turbulent level to improve the shell film coefficient of heat transfer, the traverse baffle is usually installed in the shell and tube heat exchanger. The most common one is segmental baffle. Fluid winds in the shell outfit with segmental baffle at the continuously changing speed in different directions, and is easy to separation especially in the baffle edge. Due to the flow dead zone between segmental baffles and the shell, fluid undergoes repeated movement of cross streams in the baffles, resulting in the reduction of driving force of heat transfer. For obtaining higher heat transfer performance, only the plate spacing is reduced, which is inevitably accompanied by a higher flow resistance at the cost of higher energy consumption. Therefore, the traditional form change of baffle is badly needed.
Fig. 1 The Shell Flow Diagram of Heat Exchanger with Segmental Baffles
The heat exchanger with helical baffles becomes an ideal alternative because of its unique advantages. It uses the continuous helical backing plate in support of heat exchange tube to make the shell medium do the inclined forward movement along the spiral channel from the shell entrance. Since the traditional horizontal baffling way is changed to the vertical helical baffling means, the heat exchanger with helical baffles greatly enhances its heat transfer effect while reducing the resistance of shell side. Its characteristics are: (1) The continuous smooth spiral flow of medium in the shell side avoids the serious pressure loss caused by horizontal baffling with lower pressure drop. (2) Compared with the segmental baffle, in the state of same pressure drop, it can significantly improve the shell medium flow rate, and thereby increase the turbulence level and the medium heat transfer capability. (3) The spiral movement of the shell medium generates velocity gradient on the radial cross-section and form radial turbulence in favor of the thinned bottom layer of the heat-exchange surface detention and the enhanced film heat transfer coefficient. (4) There is no dead zone. While improving the heat transfer coefficient, the vertical helical baffling way reduces dirt deposition, has stable thermal resistance, and allows the heat exchanger in a highly efficient running state. (5) Thanks to the stronger constraints on the heat exchanger than the segmental baffle, the helical baffle lessens the tube bundle vibration and extends the operational life. (6) When the shell is condensing for heat exchange, the helical baffle can play the role of drainage of the condensate liquid, decrease the coverage of the condensate liquid on the lower rows of tubes, and thereby enhances the effect of heat transfer.
According to StehlikP’s researches, compared with the traditional heat exchanger with segmental baffles, the heat exchanger of helical baffle gets 1.8 times higher of the heat transfer coefficient and 25% lower of flow resistance in the same condition. Chen Shixing concludes: for high-viscosity oil, the heat exchanger with helical baffles has the convection heat transfer coefficient of about 1.5 times in the unit pressure drop that that of the ordinary heat exchanger with segmental baffles; for water, about 2.4 times. Song Xiaoping presents the application of over ten units of heat exchangers with helical baffles in the refinery, and finds that its performance indicators are better than the original segmental baffle heat exchanger. The heat exchanger with helical baffles makes the efficiency of heat transfer greatly improved, the heat transfer area and metal consumption decreased, and thereby the investment in equipment installations reduced.
However, the spiral surface machining is difficult and the coordination between heat exchanger and baffle is hard to achieve. Taking the processing convenience into account, a series of standard sectorial planes (known as helical baffles) are adopted in place of the curved surface alternate connections, to form the similar spiral surface in the shell side and to make the fluid generate continuous spiral flow. See Fig. 2.
Fig. 2 Helical Baffle Arrangement in the Shell Side
To reach the spiral flow stability of the shell medium, the helical baffles are required to have the consistent spacing (called baffle spacing F), the same installation angle α, and generally be placed in the lower part of the top entrance and exit axes or the upper part of the bottom axes. See Fig. 3.
Fig. 3 Helical Baffle Arrangement in the Entrance and Exit Ports
Since 1997 its first domestic application in Fushun Petroleum Factory, thousands of heat exchangers with helical baffles have been rapidly promoted in the chemical & refining devices in more than 20 companies. Application results show that the helical baffle in the shell side imposes the impact on reducing the fluid pressure drop compared to the vertical segmental baffle. But it’s not fairly obvious in the heat exchanger efficiency. Some heat exchangers, in particular, the large-diameter heat exchangers with helical baffles are inferior to those of segmental baffles. Through many simulation experiments of different diameter shells and different angles of heat exchangers with helical baffles, it is proved the main reason are the mechanical processing restriction and the extremely difficult achievement of complete helical continuous baffle processing. Traditional helical baffle is overlapped by two or four panels and arranged into a similar spiral surface. The segmental baffles of 360°/x in projection are placed in correct order with the shell axis in the end-to-end binding at an angle. The straight flanges of the adjacent two baffles intertwine at the top and butt joint. The triangular space formed between two adjacent baffles is likely to lead to the medium flow along the baffles, form the short-circuit leakage current and depart the spiral flow.
Fig. 4 Traditional Structural Arrangement of Helical Baffle
Short-circuit leakage current decreases the flow of an ideal channel. Especially in the large-diameter helical baffle shell and tube heat exchanger, because numerous media flow along the triangle space and gap formed by two adjacent baffles, the spiral flow of main sprue is reduced, the medium flow slowed down, and the efficiency of heat transfer severely affected. To ensure the efficiency rate of heat exchange, Dalian Haite Heat Transfer Technology Co., Ltd. developed a new anti-short circuit helical baffle to make the medium in the shell side flow through in the almost ideal spiral flow pattern.
Fluid Flow Analysis
On the basis of the segmental baffle, the new anti-short-circuit helical baffle is widen with one or two row(s) of tube spacing on both side of straight flanges, and overlapped in the two adjacent straight flanges, and then penetrated by one or two row(s) of heat exchange tubes. The flow simulation experiments show that the short circuit still exists when both sides of the straight flanges of the segmental baffle are widen to 5~10mm more simultaneously. The shell medium flows through in the almost ideal helical flow pattern and short circuit is put to an end when one row or two rows of tube spacing is added. The structural arrangement of anti-short-circuit helical baffle is shown as Fig.5
Fig. 5 Structural Arrangement of New Anti-short-circuit Helical Baffle
The overlapping connection exerts good guiding effects on the media of the tube bank, reducing short circuit of the triangle space formed by the crossing of two adjacent straight flanges and ensuring the heat transfer efficiency. The penetration of one row or two rows of two adjacent heat exchange tubes into the adjacent two segmental baffles reinforces the tube bundle rigidity and avoids the quadrant separation trends. The helical baffle structure in the overlapping part has a good anti-vibration effect.
In a word, the heat exchanger with helical baffles proves its advantages in the combination property as long as it is properly designed. However, in comparison with the traditional segmental baffle type, there are still much more researches needed to do, for example: the studies on shell and tube heat exchangers are mostly focus on the segmental baffle until now, and TEMA standard is also directed against it. Regarding to the standardized design of helical baffle, it’s needed to conduct detailed analyses and studies on the flow and heat transfer mechanism. Besides the factors to affect the traditional segmental baffle, there are geometrical factors (layout pattern, spiral angle, thread pitch), phase-change cases, and physical properties of different media to be considered. Along with the constant development of computer technologies and in-depth studies of the mechanism, the helical baffle heat exchanger is believed to have wider applications.