Views:0 Author:Site Editor Publish Time: 2020-05-27 Origin:Site
With the development of society and the acceleration of urbanization, the central heating method is gradually mature, and as a highly efficient heat exchange equipment, plate heat exchangers have high heat exchange efficiency, small footprint, light weight, assembly and disassembly The characteristics of convenience, variety of plates and small fouling coefficient are widely used in heating systems. At present, plate heat exchangers have become the core components of indirect heat exchange units for central heating. However, in actual use, the hardness of the circulating water of the secondary network is not up to standard, the suspended matter is high, and the corrosion of chlorine or oxygen is likely to cause scaling, blockage, corrosion or perforation of the heat exchanger plate, which will affect the normal function of the heat exchanger. It will bring huge cost pressure or economic loss to the enterprise. Therefore, it is necessary for us to do a good job in the selection, scale inhibition, anti-corrosion and safety cleaning of the heat exchanger plates to ensure the normal production of the enterprise.
At present, the temperature of the primary heat medium used in urban central heating is generally 130 ° C / 70 ° C, and the temperature of the secondary heat medium is 95 ° C / 70 ° C or 85 ° C / 60 ° C. In this case, the plate heat exchanger The flow rate in the heat medium flow path is only about half of the flow rate in the secondary heat medium flow path. For symmetrical flow channels, the primary heat medium flow rate is about 50% of the secondary heat medium flow rate, then the convective heat transfer coefficient between the fluid in the primary heat medium flow channel and the plates is about 70% in the secondary heat medium flow channel Left and right, therefore, when the medium and secondary side medium flow is large, it is appropriate to use an asymmetrical plate heat exchanger. Generally speaking, the convective heat transfer coefficient of the cold fluid and the hot fluid at the interface of the two sides of the plate determines the size of the surface heat exchange coefficient of the plate heat exchanger, and is smaller than the smaller of the cold and hot convection heat transfer coefficients. The heat transfer performance of the plate heat exchanger is fully exerted, and the convective heat transfer coefficient between the cold and hot fluids and the interface between the two sides of the plate should be appropriately increased. The selection calculation of the plate heat exchanger is very complicated. To ensure the reasonable completion of the selection calculation of the heat exchanger, the influence of the flow rate between the heat exchanger plates, the matching of the flow rate between the cold and heat medium plates, etc. must be considered comprehensively. , Temperature difference, medium and physical properties of the process are within a reasonable range, to ensure the full play of the advantages of plate heat exchangers, so that the equipment can obtain economic and reliable operation.
The following are the calculation steps for the selection of plate heat exchangers: list the physical and thermal parameters of the cold and hot media → select the plate type → find the fluid flow rate → calculate the Reynolds number of the cold and hot fluids → calculate the cold and heat Calculate the heat transfer coefficient of the fluid → calculate the heat transfer coefficient → calculate the theoretical heat exchange area of the heat exchanger → calculate the number of flow channels of a process of the heat exchanger → calculate the number of heat exchanger processes → calculate the pressure drop of the heat exchanger → check the exchange Thermal area → Check pressure drop.
In the central heating system, the circulating water in the heating network is mainly tap water and deep well water, which has a high hardness. When the water reaches the boiling point, sediments will be generated in the pipe network. Due to the small circulation interface of the plate heat exchanger, the flow rate between the plates is also On the lower side, the precipitates easily form scale on the hot side or suspend in the circulating water, and then deposit on the surface of the heat exchanger, forming secondary scale, affecting water quality, and causing scaling and blockage of the heat exchanger. Due to the low thermal conductivity of the dirt layer, the heat transfer resistance is greatly increased, which reduces the heat transfer efficiency of the heat exchanger to a certain extent; when a scale layer is formed on the surface of the plate heat exchanger, it will reduce the flow area of the equipment , Increase the resistance of the fluid flowing through the equipment, and then consume more pump power, bringing unnecessary cost waste. Generally, according to the different deposition mechanism of the scale layer, the scale can be divided into crystal, corrosion, chemical, biological and other scales. In addition, during the construction of the heating pipe network, due to improper management or environmental impact, some impurities will inevitably enter the pipe network, such as welding slag, some debris, garbage, mud, stone, braid, etc. that remain when welding the pipe It is left during the construction of the pipeline. During the long-term use, the rust mud formed on the inner wall of the pipeline enters the heat exchanger with the circulating water and causes the heat exchanger to foul and block.
Although the plate of the plate heat exchanger is made of stainless steel, it has strong corrosion resistance, but if the chloride ion content in the water is high, it will cause corrosion; water and oxygen are also necessary for the heat exchanger plate to be corroded Conditions, if the corrosion is carried out uniformly on the entire metal surface, the corrosion rate will not be accelerated, and the harmfulness will not be too great, and if the corrosion is concentrated on certain parts of the metal surface, the corrosion rate will be accelerated, easy The phenomenon of rusting and wearing is very harmful, which will not only reduce the working efficiency of the equipment, but also greatly shorten the service life of the equipment and increase the maintenance cost. In addition, due to the occurrence of corrosion, the metal materials of the heating system will be damaged, causing serious erosion and leakage of pipes and terminal equipment, and damage to the room decoration materials. In serious cases, the plate heat exchanger and boiler will be scrapped in advance. Therefore, the water quality index of the circulating water of the plate heat exchanger is very important.
3. Measures to deal with the problems existing in the use of plate heat exchangers
(1) Prevent scale and solid impurities
If a sodium ion exchanger is used in the treatment of supplementary water, the hardness of the supplementary water must meet the water quality standards, and the hardness, suspended solids, PH value and other technical indicators of the circulating water in the heating system must be strictly tested to effectively prevent calcium The production of magnesium scale. If tap water is used for supplementary water, scale inhibitors should be added to the water during the treatment process to ensure that the corrosion rate and scale inhibition rate of the scale inhibitors and corrosion inhibitors can meet the requirements. In the actual production process, the The amount of scale inhibitor added should be regularly checked and tested to avoid scale formation caused by insufficient amount of addition or waste or corrosion caused by excessive amount of addition. The solid impurities in the circulating water are mainly rust mud, mud sand, residue, copper oxide, biological slime, etc. In order to exclude or reduce the content of impurities such as solid suspended solids, the content of suspended solids in the heating system should be controlled at 2-5mg / L, for a well-treated circulating water system, the suspended matter content of the supplementary water should not be greater than 0.5-1.0 mg / L, and the suspended water content of the circulating water of the plate heat exchange system should be less than 10 mg / L; in the return water and supplementary water Add filters to the pipeline to filter some suspended particles with larger particles; strengthen construction supervision and management. After installing each section of the large-diameter pipeline, organize the construction personnel to clean the welding slag and welding rod in time, and also after the construction is completed. Carry out thorough cleaning; strengthen management during daily operation and discharge sewage regularly to reduce suspended impurities.
(2). Antiseptic and safe cleaning
If the chloride ion content in the water exceeds 200ppm, it is not appropriate to use ordinary stainless steel plates when selecting the plate heat exchanger. A series of effective measures can be taken to reduce the chloride ion content in the primary and secondary network media to keep it at Safe concentration range, but for the entire central heating system, this method is costly and cumbersome to manage. Generally speaking, for systems using more stainless steel heat exchangers, the chloride ion content should be controlled below 300 mg / L, or strictly below 50 mg / L. Caustic soda can be added evenly during the operation of the heating system to improve circulating water PH value, control the PH value at a stable level, after this treatment, the effect of scale and corrosion inhibitor will also be greatly improved. Correctly clean the plate heat exchanger and determine the cleaning method. For stainless steel materials, strong acids are generally used as cleaning agents. Hydrochloric acid cannot be used. During the cleaning process, try to reduce the cleaning time and soaking time, preferably less than 2h. During the soaking period, The corrosion rate and scale inhibition rate of the cleaning mother liquor are tested in stages to avoid corrosion of the heat exchanger; no matter which cleaning fluid is used, the heat exchanger must be thoroughly rinsed with clean water after cleaning to remove residual cleaning fluid to avoid secondary corrosion.
In short, in order to give full play to the function and superiority of the plate heat exchanger, the reasonable calculation and selection should be based on the characteristics of the central heating system. At the same time, the problems of scaling, blockage, corrosion, etc. In the daily operation and management of the heat exchanger, practical and effective measures are taken to reduce the adverse effects of scaling, clogging, and corrosion on the heat transfer efficiency and heat supply effect of the heat exchanger. Of course, we must continue to explore and innovate in practice to effectively solve the problems in the operation of the plate heat exchanger, minimize the incidence of equipment failure, extend the service life of the equipment, and make the plate heat exchanger in the central heating system.