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2018
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06
Current Status and Development Trends of Domestic Removable Plate Heat Exchangers
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The detachable plate heat exchanger is a highly efficient and energy-saving heat exchange device, characterized by high heat transfer efficiency, lightweight, small footprint, and ease of maintenance, among many other advantages. It is mainly used in fields such as petroleum, chemical industry, beverages, pharmaceuticals, urban centralized heating, light industry, textiles, papermaking, oils and fats, shipping, metallurgy, power (including nuclear power plants), and offshore engineering. Since 2000, the design and manufacturing level of detachable plate heat exchangers in China has greatly improved, with a continuous enhancement of varieties and specifications, and an expanding application range. However, compared to the application situation and development trends abroad, there is still a significant gap in Chinese products.
The removable plate heat exchanger is a highly efficient and energy-saving heat exchange device, characterized by high heat transfer efficiency, lightweight, small footprint, and ease of maintenance. It is mainly used in fields such as petroleum, chemical industry, beverages, pharmaceuticals, urban centralized heating, light industry, textiles, papermaking, oils, shipping, metallurgy, power (including nuclear power plants), and offshore engineering. Since 2000, the design and manufacturing level of removable plate heat exchangers in China has greatly improved, with continuous enhancement of varieties and specifications, and an expanding application range. However, compared to the application situation and development trends abroad, there is still a significant gap in Chinese products.
1. Current Situation
1.1 Since entering the 21st century, conventional symmetric, asymmetric, high NTU type (potentially dense corrugated type), non-adhesive type, wide channel type, plate evaporators, plate condensers, and other existing removable plate heat exchangers from abroad have all been localized, and have been successfully applied in different fields.
Currently, 80% of domestic enterprises still mainly produce conventional ordinary products with low technical content. Compared to international advanced levels, Chinese products still lack special varieties, such as advanced performance new plate evaporators, plate condensers with ultra-large processing capacity, and special-purpose laser semi-welded products.
1.2 Performance
Due to the influence of the sealing gasket's performance, the current maximum design temperature for domestic removable plate heat exchangers is 180°C, and the maximum design pressure is 2.5MPA.
The corrugated form of the plates is still mainly in a herringbone shape, and each enterprise's plate type is equipped with soft and hard plates, allowing for thermal mixing design. The maximum NTU is 5 (with domestic levels reaching up to 7), and the achievable minimum temperature difference at the end is 1°C (0.5°C abroad). The heat transfer coefficient under water-water counterflow operation, with a qualitative temperature of 40°C and a flow rate of 0.5m/s between plates, can reach 5000-6500w/m2 K, approaching international advanced levels.
1.3 Materials
Since the plates are formed by cold pressing molds, only materials that can be cold drawn (pressed) can be used for the plates of plate heat exchangers. Currently, the materials used for domestic plate heat exchanger plates are mainly conventional 300 series austenitic stainless steel, primarily 304 and 316L, which account for over 80% of the total annual usage. In contrast, super austenitic stainless steels such as 904L, 254SMO, and 654SMO, which are widely used abroad, have limited application domestically, and are only used in 254SMO or equivalent grades like 185N and AL6XN. Special expensive materials like titanium-palladium alloys, nickel, and its alloys are used very sparingly, and tantalum metal, which has been successfully used abroad, has not yet been utilized domestically.
1.4 Sealing Gaskets
Sealing gaskets are the weakest link among the main components of removable plate heat exchangers, and their quality significantly affects pressure resistance, temperature resistance, reliability, and applicability. Domestic sealing gasket manufacturing enterprises have achieved international advanced levels in the appearance quality and geometric precision of the gaskets through recent years of technological innovation and internal quality control, but there is still a considerable gap, mainly reflected in: 1. Lower physical performance indicators, such as temperature resistance, aging resistance, high-temperature compression permanent deformation, and tear resistance, which need further improvement. 2. Significant quality fluctuations that vary with batches. 3. Limited varieties. For example, the formulas and vulcanization processes for sealing gaskets used in general media like water and steam are relatively mature, but there is a lack of temperature-resistant nitrile rubber (NBR), ethylene-propylene rubber (EPDM), and (expanded) graphite gaskets. There are no corrosion-resistant, creep-resistant coated polytetrafluoroethylene (PTFE) sealing gaskets.
Personnel engaged in the geometric structure design of sealing gaskets are not familiar with the plate structure of plate heat exchangers, and cannot reasonably control the geometric shape and compression of the sealing gaskets, which affects the use and lifespan of the gaskets (too large a shape can cause plate deformation, while too small a shape can shorten the sealing lifespan). Additionally, designers cannot design corresponding structures for gaskets based on different gasket groove shapes, nor do they pay attention to timely tracking of the latest developments in plate types for plate heat exchangers, resulting in changes in plate structures while the gasket structures remain unchanged.
1.5 Manufacturing
The manufacturing quality of plate heat exchangers is mainly reflected in the forming precision of the heat exchange plates, and the level of forming precision severely affects the overall product's pressure-bearing capacity, heat transfer, and flow resistance characteristics. Although literature has specified that the forming deviation of heat exchange plates should be ±0.10mm, ±0.15mm, and ±0.20mm for nominal heat exchange areas of no more than 0.3m2, 0.3-1.0m2, and greater than 1.0m2 respectively, the requirements of ±0.10mm are lower for foreign companies, yet some domestic enterprises still fail to meet this requirement, with serious cases exceeding ±0.50mm. There are also issues such as obvious indentations on the plates, poor appearance quality, and deformation of the corrugations.
Due to the poor precision of the heat exchange plates and their unreasonable matching with the gaskets, the overall pressure-bearing capacity of the product decreases, or the overall heat exchange area cannot be increased, which also affects the heat transfer and flow resistance characteristics during operation, increasing the limitations and adverse effects of product applications. Domestic enterprises have generally recognized the need to improve and enhance the appearance quality of products, but they do not pay enough attention to controlling internal quality. The main issues are as follows:
(1) Poor performance of the dedicated hydraulic press for plate forming. The key equipment for plate forming, the dedicated hydraulic press, has low precision and reliability, such as unreasonable structure and weak overall rigidity. When pressing the plates, the upper and lower worktables of the hydraulic press first deform, resulting in uneven forming depth of the plates. Due to the poor quality of hydraulic control components, severe oil leakage occurs, preventing the equipment from operating for long periods at rated working pressure. Additionally, domestic presses mainly use those below 20,000t, while the precision and reliability of 30,000t and 40,000t presses used for large or ultra-large plate forming are even lower, limiting the development of large-scale plate types.
(2) Low mold precision. Another reason for the low forming precision of the plates is the unreasonable structure or failure to meet precision requirements of the plate pressing molds. This is due to the disconnect between product design and mold manufacturing in China, where product designers do not have high requirements for mold design and manufacturing, do not control the gap between upper and lower molds reasonably, and do not comprehensively consider the rebound of the plate during pressing, resulting in molds that cannot accurately reflect design intentions and deviate from design requirements. This is reflected in changes in the geometric shape of the corrugations, with severe cases showing trapezoidal corrugations appearing serrated, and excessive thinning of the plates, leading to low qualification rates for pressing special materials like titanium and nickel.
There is a general lack of high-level mold design and processing personnel in the country, and the processing methods are outdated. Some companies still use CNC milling machines to process molds, while only a few processing companies fully utilize CNC machining centers. In contrast, well-known foreign companies use high-speed (20000r/min), high-performance specialized machining centers and high-quality tools to complete mold processing, ensuring the precision and service life of the molds.
1.6 Application
In China, detachable plate heat exchangers are mainly used in urban central heating, petroleum, chemical, light industry, metallurgy, and power industries where the temperature, pressure, and medium are not very harsh. Among them, the number of detachable plate heat exchangers used for urban central heating accounts for about 70% of the total product number each year. Although new plate types have emerged in recent years, the application fields have expanded accordingly. For example, the emergence of high NTU value plates has allowed Chinese products to enter the refrigeration and air conditioning sector with a temperature difference of 1°C. The introduction of gap plates has enabled our products to enter industries dealing with viscous and granular media such as alcohol and sugar production. However, in large products and special plate materials with high demand in fields like nuclear power, metallurgy, electricity, and shipping, applications remain limited. The main reasons are the incomplete variety and specifications of high-tech products, low quality performance levels, low reliability, and inaccurate process selection calculations.
2. Existing Problems
2.1 Low Level of Process Selection
(1) The level of process selection or technical support personnel is low, and they cannot correctly select plate types, plate materials, and sealing gasket materials based on working conditions. Due to a lack of experience in calculation selection, the product selection calculations are often unreasonable, with safety margins being either too large or too small, preventing products from operating at optimal conditions, leading to unsatisfactory usage effects, and causing some users in certain fields to doubt the quality of the products, thus limiting the application range.
(2) Lack of Advanced Calculation Selection Software. Nearly half of the domestic enterprises have used software for selection, but the vast majority are limited to water-water conditions. For some conditions involving phase changes or special media, only empirical estimates can be made, lacking advanced design calculation methods, software, and supporting databases.
2.2 Lack of Emphasis on Experimental Research
The overall performance of plate heat exchangers and whether the designed and manufactured products meet user working conditions are fundamentally based on the accurate equations from experimental testing. Literature 4 stipulates that thermal performance and fluid resistance characteristics should be determined for each type of product. However, domestic enterprises generally do not emphasize experimental research. Some companies conduct thermal testing on products due to project bidding requirements but do not analyze or effectively utilize the test data and standard equations.
Well-known foreign companies such as ALFA LAVAL, APV, and GEA have dedicated product R&D centers, investing 2%-3% of their annual sales revenue in improving existing products and conducting experimental research on new products.
2.3 Poor Innovation Capability
(1) Poor independent development capability, low-level repetitive production, and serious imitation. ① Some companies do not have technical departments or technical personnel. ② The level of plate design is low, and they do not master the core technology of plate design, mostly relying on imitation or copying. ③ The product development of most companies is limited to frame design, and there are too few technical personnel capable of product development and mold design, with high-level R&D talents being even scarcer.
(2) Low Investment in Research and Development. Due to the simplicity of the production equipment for detachable plate heat exchangers and low investment, there are many production enterprises, but they are often too small in scale. This leads to low-level, low-price vicious competition among enterprises, resulting in a serious lack of research and development investment in the entire industry. This not only affects the development of new products but also prevents technical personnel from consistently engaging in the design and development of plate heat exchangers, which is also the fundamental reason for the scarcity of high-level R&D talents.
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