Abstract: This paper introduces the research work and achievements of the shell-side heat transfer enhancement of shell-and-tube heat exchangers, and points out the research direction of enhanced heat transfer technology.

introduction

Shell-and-tube heat exchangers are widely used in chemical, petroleum, power, metallurgy, refrigeration, atomic energy, shipbuilding, food and other industrial sectors. In the past 40 years, a lot of researches have been carried out on the heat transfer enhancement in the tube at home and abroad, and the fruitful results have been achieved. At present, there are no more than one hundred kinds of enhanced heat transfer tube technology. In contrast, the study of shell-side heat transfer is far from extensive and in-depth research. It was not until the 1970s that shell-enhanced heat transfer technology began to receive attention and made great progress.

There are two main ways to enhance heat transfer in the shell: one is to change the shape of the tube or to add fins outside the tube, that is, to enhance heat transfer through changes in the shape and surface characteristics of the tube, such as threaded tubes, spiral grooves, and outer fins. Tube and other enhanced heat transfer tube technology; another is to change the shell-side baffle or inter-tube support to reduce or eliminate the residual dead zone of shell flow and heat transfer, so that the heat transfer area is fully utilized. The first approach is consistent with the mechanism of enhanced heat transfer in the tube, and many reports have been reported in the literature. The development of shell-and-tube heat exchanger shell side baffles or tube bundle supports is manifested by a change in baffles, the purpose of which is to maximize pumping power for enhanced heat transfer rather than consumption of inter-tube supports. Existing support forms include plate supports, rod supports, hollow ring supports, and tube self-supporting.

1 plate support structure

The traditional shell-and-tube heat exchanger is supported by a single arcuate baffle. The shell-side fluid is prone to flow dead angles, and the heat transfer area cannot be fully utilized. Therefore, the shell-side heat transfer coefficient is low, easy to scale, and fluid resistance. And when the fluid flows laterally through the tube bundle, it can also cause the tube to induce vibration, destroying the tube and its reliability in connection with the tube sheet. Therefore, in order to eliminate its drawbacks, many new shell-side baffle support structures have appeared in the past 20 years, such as multi-bow baffles, round plates, shaped orifices, mesh plates, bow baffles and parallels. Partition plate, spiral baffle, etc. The common feature of these new baffle support structures is to improve the shell-side fluid flow and heat transfer dead zone as much as possible, reduce the shell-side fluid flow resistance, and the vibration resistance of the tube bundle can be enhanced.

1. 1 double bow and three bow baffles

The double-bow baffle divides the fluid into two parallel bundles on the shell side of the heat exchanger, and the length of the lateral flow (ie, the number of tubes passing through) is approximately half of the single-bow baffle having the same gap, and has the same fold The pressure drop of the double-bow baffle is 30% to 50% of the corresponding value of the comparable equipment, and the heat transfer coefficient is 60% to 80% of the corresponding value of the comparable equipment. Similar to a double-bow baffle, the three-bow and multi-bow baffles divide the fluid into three or more streams on the heat exchanger shell side.

1. 2 full circular baffles

In order to change the lateral flow of the bow baffle support as much as possible parallel to the longitudinal flow of the tube, eliminating the dead zone and increasing the flow rate of the fluid in the shell, in the power station and petrochemical industry, a full circular baffle appears. The full circular baffle that initially appears drills a large round hole in the plate, allowing the tube to pass through and there is enough clearance for the fluid to pass. The fluid inside and outside the tube generally flows longitudinally, the heat transfer temperature difference driving force is large, and the fluid can generate a jet effect due to the annular gap passage between the tube wall and the orifice plate, so that the fluid quickly forms a turbulent flow away from the air interface, so that the wall surface is not easy to knot. Scale, shell heat transfer is enhanced. However, the entire circular baffle increases the diameter of the heat exchanger housing, and the tube bundle of such a heat exchanger is also poor in vibration resistance due to the lack of a tube support structure. In order to improve the shortage of the large-diameter circular baffle, a small hole is opened between the pipe holes, so that the heat transfer medium passes through the small hole through the baffle, so that the diameter of the casing does not need to be increased. The entire circular baffle with small holes is prone to fouling in the gap between the tube hole and the tube, causing corrosion. In order to make up for this defect, a baffle structure of a rectangular hole, a plum hole, and the like is formed. The baffle plate can support the tube and allow the heat transfer medium to flow through the baffle when the medium flows through the tube hole. At the same time, the jet can be generated, and the tube is washed and self-cleaning, thereby eliminating tube fouling and scale corrosion. Although the special-shaped orifice plate has many advantages, it is relatively difficult to manufacture and manufacture, and the cost is high. Therefore, a mesh-shaped full-circular baffle plate appears in the industry. The baffle plate is a full circle, and each of the four adjacent rows of adjacent baffles is grouped in a small bridge, and the shell-side fluid passes through the baffle from the milling port to form a uniform longitudinal flow.

1. 3 bow baffles plus parallel partitions

One or several parallel flow dividing plates are inserted in parallel between the two baffle partitions of the single-bow baffle shell-and-tube heat exchanger, and the original channel can be changed into multiple parallel channels to divide the original single-flow flow into Multiple parallel flows. In this way, the vortex scale of the dead zone between the plates can be effectively controlled, so that the flow field between the plates can be homogenized and the flow resistance between the tubes can be reduced. When two parallel flow separation plates are added under the condition that Re is not too high, the flow field of the shell side can be controlled to a certain extent, and the heat transfer effect can be better. The enhanced heat transfer performance evaluation factor of the bow-shaped baffle heat exchanger with parallel flow dividing plate is in the range of measured Re, and its value is about 1.20~1.15, the strengthening factor decreases with the increase of Re number. Small, this enhanced heat transfer technology is suitable for use at low to medium Reynolds numbers.

1. 4 spiral baffles

The spiral baffle is a baffle plate that replaces a conventional vertical bow plate with a spiral or approximately spiral shape. The baffle plate has an oblique angle with the cross section of the heat exchanger casing, so that the fluid flows along the spiral passage in the shell side. According to the flow path, the spiral baffle can be divided into single or double head.
The spiral baffle of the spiral baffle heat exchanger causes the fluid to flow in a continuous plunger-like spiral flow on the shell side (ie, p lug flow) without the flow "dead zone" in the conventional baffle heat exchanger, and Due to the interaction between the vortex generated by the swirling flow and the boundary layer of the heat transfer interface of the tube bundle, the turbulence is greatly enhanced, which is beneficial to increase the coefficient of the heat transfer film on the shell side.

2-bar support structure

In order to solve the problem of vibration and damage caused by the lateral flushing of the tube bundle by the conventional baffle heat exchanger, a baffle rod support structure appears. The baffle support structure is composed of a baffle and a support rod. The baffle is formed by welding a number of round bars (baffles) on a ring (baffle), and each of the four different baffles forms a baffle group. The baffles on the baffles are interleaved between the tubes, the diameter of the baffles being approximately equal to the gap between adjacent tubes, and the tubes being clamped tightly by the baffles. Since the flow of the shell-side fluid of the baffle heat exchanger is longitudinal, there is substantially no flow dead zone. In addition, the fluid flows through the baffle bar and alternately generates and deviates from the vortex on both sides thereof (the card vortex street), and the flow cross section shrinks when the fluid flows through the baffle, and then expands, thereby generating a Venturi effect. Due to the effect of the card vortex street and the Venturi effect, the fluid forms a strong erosion on the pipe wall, thereby thinning the heat transfer boundary layer and strengthening the shell heat transfer. The hollow ring is cut into short sections by a small diameter steel pipe, uniformly distributed on the same section between the pipes, and is in linear contact with the pipe, so that the tube bundle is relatively tightly fixed. The use of the hollow ring-tube support can greatly reduce the fluid transporting work lost by the gas in the shell process, and increase the coefficient of the outer heat transfer film by increasing the gas velocity between the pipes to achieve a good utilization of the fluid transfer work. The second is to use rough heat transfer tubes, such as zoom tubes, low-ribbed tubes, petal tubes, etc., to enhance the convective heat transfer when the fluid is longitudinally flushed.
4 tube self-supporting structure
In order to simplify the bundle support and improve the compactness of the heat exchanger, some self-supporting tubes have been developed in recent years, such as puncture diaphragm tubes, spiral flat tubes and variable cross-section tubes. Such a tube is supported by a part of the tube itself such as a puncture diaphragm, a spiral or a reduced diameter portion to support the tube, and at the same time constitutes a shell-side spoiler element, which increases the turbulence of the fluid itself and destroys the tube wall. The upper fluid boundary layer, so that the shell-side heat transfer is further enhanced.
5 Conclusion
The development of shell-and-tube heat transfer enhancement technology for shell-and-tube heat exchangers has two characteristics: one is that the fluid flow in the shell-side flow changes from the lateral flow to the longitudinal flow parallel to the tube, so that the flow pressure drop is reduced and the heat transfer area is obtained. Make full use of the second; the second is the support form toward low flow resistance, simple structure, easy to manufacture and material development. Through the study of the enhanced heat transfer of the shell side, the composite heat transfer enhancement technology combining the low-flow resistance and high heat transfer efficiency of the shell-side heat transfer support structure with the high-efficiency heat transfer tube makes the whole heat exchanger pass. The improved thermal performance will be the direction of the development of shell and tube heat exchangers.