Views:1 Author:Site Editor Publish Time: 2020-10-23 Origin:Site
Plate heat exchanger is a new type of high-efficiency and compact heat exchanger that has been developed and widely used in recent decades. It is formed by stacking a series of parallel thin metal plates with corrugated surfaces, while the spiral plate heat exchanger is more compact and has better heat transfer performance.
In the field of plate heat exchangers, Swedish Alfa Laval, UK (APV)/USA (SPX), Swedish SWEP (SWEP), German GEA (GEA), German Funke (Funke), US Tranter (Tranter), Italy Spree, Japan (Hisaka), South Korea (LHE) are the world's industry-leading plate heat exchanger manufacturers, and most of them have entered the Chinese market. my country now has at least dozens of professional manufacturers, and has formed a certain market share and leading enterprises. Plate heat exchangers are especially suitable for the fields of medicine, food, wine, beverage, synthetic fiber, shipbuilding, chemical industry, etc., and with the improvement of plate type and structure, its application fields are being further expanded.
(2) Structure and working principle
The plate heat exchanger is composed of three main components: heat transfer plates, sealing gaskets, compression devices, and other components, such as clamping studs, guide rods, and pipes, as shown in Figure 1. On the fixed pressing plate, a plate and a washer are alternately placed, and then the movable pressing plate is placed, and the pressing bolts are tightened to form a plate heat exchanger. The heat transfer plates are stacked in a certain order to form a flow channel between the plates. The cold and hot fluids flow in the respective flow channels on both sides of the plates, and heat exchange is carried out through the heat transfer plates.
(1) Heat transfer plate
The heat transfer plate is the key element of the plate heat exchanger. Its design mainly considers two factors: ① Make the fluid generate strong turbulence at low speeds to enhance heat transfer; ② Improve the plate stiffness, which can withstand higher pressure.
The corrugated form of the heat transfer plate has a greater impact on heat transfer efficiency and flow resistance. In order to meet the needs of different heat transfer conditions, many types of corrugated heat transfer plates have been developed. Among them, the herringbone corrugated board and the horizontal straight corrugated board are the most widely used.
The cross-sectional shape of the herringbone corrugated board is usually triangular, and the angle between the herringbone is usually. When the plate heat exchanger is assembled, every two adjacent plates are turned upside down to form a mesh contact and make the fluid in the channel form a mesh flow. According to statistics, after assembly, as many as 2,300 supporting contacts per square meter of projection area can be formed between adjacent plates. The fluid flows in from a corner hole at one end of the plate, and can flow out from the corner hole on the same side of the other end, which is called "unilateral flow", or it flows out from the corner hole on the other side of the other end, called "diagonal flow". According to the comparative test of various plates, the herringbone corrugated plate not only has good rigidity, but also has better heat transfer performance. Generally speaking, the flow resistance of herringbone corrugated plates is relatively large, and it is not suitable for fluids containing particles or fibers.
The horizontal straight corrugated board is a horizontal straight corrugated board with an isosceles triangle in cross section. Its heat transfer and hydrodynamic properties are good, and the heat transfer coefficient between water and water can be reached. Other cross-sectional shapes include pleated triangular corrugations (British APV company) and stepped corrugations (NPH type plate manufactured by the Japanese Distillation Industry), which belong to this category.
The characteristic of the zigzag corrugated board is that the cross-sectional area of the flow channel after assembly is unchanged, so the resistance is small and it can run at high speed. There are many supporting contacts and high rigidity. However, the board surface structure is complicated and two sets of molds are required.
No matter what kind of plate, they all have the following common parts: concave and convex corrugations to enhance heat transfer; sealing grooves around the plate and corner holes; fluid inlet and outlet holes (corner holes, generally round, large condenser plate corners Holes are often triangular); notches for suspension. After the plates are assembled, there are places where the two plates are in contact with each other, which are called contacts. Their function is to maintain the normal gap shape of the flow channel when there is a pressure difference on both sides of the plates, and at the same time make the flow "net". Strengthen heat transfer. Experience has shown that reasonable contact design is an effective way to improve the plate's pressure resistance.
Plate materials include carbon steel, stainless steel, aluminum and its alloys, brass, Monel, nickel, aluminum, titanium, etc. Stainless steel is currently the most widely used. Because titanium has good corrosion resistance, especially in chlorine-containing media, although titanium is very expensive, titanium plate heat exchangers are still widely used in more corrosive occasions. The thickness of the plate is very thin, ranging from 0.5 to 1.5 mm, usually about 1 mm. The key to the manufacture of plate heat exchangers is the forming of the plates, which are currently almost all stamped plates.
(2) Sealing gasket
In order to prevent fluid leakage and internal leakage between the two fluids, a sealing gasket is necessary. It is installed in a sealed groove, is subjected to pressure and temperature during operation, and is corroded by working fluid. In addition, it is required to have good elasticity after multiple disassembly and assembly. In addition to corrosion resistance and temperature resistance, the quality of rubber requires other physical properties to meet the following requirements: According to different operating pressures, the hardness should generally be 65 to 90 Shore hardness, the compression set is not more than 10%, and the tensile strength , Elongation ≥200%. The failures that occur in the operation of plate heat exchangers are rarely the damage of the plates or other mechanical parts, and most of them are the problems of gaskets, such as gasketing, elongation, aging, breakage, etc. Therefore, there are special requirements for the material of the sealing gasket of the plate heat exchanger. Currently, the widely used materials are: natural rubber, nitrile butadiene rubber, styrene butadiene rubber, neoprene, EPDM, silicone rubber, etc. Their use temperature is below 80～150℃. In recent years, compressed asbestos mats and asbestos rubber mats have been developed, and the operating temperature has increased to 260-300°C. As the process puts forward higher requirements on operating pressure and temperature, the improvement of the sealing structure of the plate heat exchanger has also become a focus of attention. The sealing problem is so prominent because of its long sealing periphery. For example, a plate heat exchanger with 200 plates and an area of 0.5 each has a total gasket length of about 900m. Considering its frequent disassembly and cleaning, it is not easy to keep it leak-free. In order to better prevent internal leakage, a double seal is used on the gasket. At the same time, in order to detect the internal leakage in time, many manufacturers have opened grooves on the sealing gaskets, which are usually called "signal holes". Once there is a leakage, the fluid will leak out first.
(3) Compression device
It includes fixed and movable compression plates and compression bolts. It is used to compress the gasket to produce sufficient sealing force so that the heat exchanger does not leak during operation, and the compression force is generated by tightening the bolt. For large-scale plate heat exchangers, the sealing pressure even exceeds 98×104 N, so a strong frame is required. In the manufacturing cost, the compression device occupies a considerable proportion. Therefore, attention should be paid to the relationship between the size of the plate and the load. If conditions permit, a larger number of small-sized plates should be used to reduce the cost of the compression device.
The main problem of the plate heat exchanger is that its operating pressure and temperature increase are restricted by the structure. General domestic plate heat exchangers can only be used for pressures below 0.6 MPa and temperatures between 120 and 150°C. At present, in China, through the improvement of plate type and frame structure, new type of sealing gasket and plate material are used, and the pressure and temperature resistance have been greatly improved. At present, the largest single-piece area of domestically produced plate heat exchangers has reached 2.0, the highest working pressure is 1.6MPa, and the highest working temperature is 200°C. For many industrial thermal processes, especially when the fluid is corrosive and must be made of precious metal materials, under the conditions of pressure 1.5MPa and temperature below 150°C, there is a tendency for plate heat exchangers to gradually replace the shell and tube type. As for the problem that the plate heat exchanger is difficult to achieve large flow operation due to the narrow flow passage and the restriction of the corner holes, due to the emergence of large plate heat exchangers in recent years and the use of multi-stage parallel operation methods, it has not become a major problem.