4ä¹Introduction: Yu Xiumin (943 male, Zhejiang Dongyang, Bo, teach, E-mail: huaike for this purpose, this paper specializes in the two-way internal circulation three-phase fluidized online cleaning technology of heat exchanger tubes.
1 pipe outer two-way internal circulation three-phase fluidization online cleaning technical solution as shown. A quantity of sand that can circulate in the equipment housing is added to the fluid outside the heat exchanger tubes. An inert gas of a certain pressure and flow rate is introduced into the bottom of the casing, and a large number of bubbles are continuously floated through the first air pipe system, so that the density of the two-phase mixture in the ventilated area is smaller than that of the adjacent area that has not been ventilated. many. Therefore, the gas-water mixture in the venting area flows upwards rapidly, and its speed is significantly greater than the free sedimentation speed of the cleaning sand, and then flows upwards together to form a fluidized upward violent movement of the gas-liquid-solid three-phase mixture, on the outer wall of the tube. The dirt produces strong scouring and frequent impacts for rapid cleaning of the dirt. After the three-phase mixture flows to the top of the unit, the gas escapes to the surface and is discharged from the exhaust port. The fluid in the adjacent area that has not been vented rapidly flows down, forming an internal circulation flow outside the tube. The liquid-solid two-phase mixture separated at the top is also returned to the bottom of the equipment and continues to participate in the cycle cleaning. After a certain period of cleaning, it is changed to the second air duct system air jet, forming an internal circulation fluidized flow cleaning which is exactly opposite to the previous one, achieving the purpose of uniform, clean and rapid cleaning.
2 Two-way internal circulation three-phase fluidized cleaning kinetics test, due to the polymerization, fragmentation of the bubble, the escape and separation of the liquid surface, the change of the bubble volume with the change of pressure during the upward flow, etc., the three-phase fluidization flow is non- In the steady state process, the outer tube structure is a mutually connected and very complicated flow path, so the velocity field of the inner loop cannot be theoretically calculated. However, the engineering design must ensure that the upward velocity of the venting zone is greater than the sedimentation velocity of the sand, so the simulation test shown is designed. The test equipment consists of 40 032mmX3mmX2200mm tubes with a 40mm equilateral square arrangement with 10 tubes per row. In order to facilitate direct observation of the three-phase fluidized circulating flow condition and measure the flow velocity, the inert gas introduced into the bottom of the device is supplied by the Roots blower during the test, and the gas volume is adjusted by the valve and measured by the flow meter. The pressure of the cleaning gas is measured by a differential pressure gauge. The flow velocity field outside the tube is displayed and measured by the color water method. Considering the symmetry of the arrangement of the tubes on both sides of the gas pipe, only the velocity field of the three-phase mixture flowing on the side of the gas pipe is measured. The cleaning gas enters 2m below the liquid level, because the volume of the bubble gradually becomes larger in the upward floating motion, so the velocity fields are measured separately for the two sections with the height of 300mm and 1080mm above the gas pipe mouth to study different depths (exact It is said to be the effect of static pressure on the velocity field of the fluidized mixture.
3 Velocity field test of internal circulation fluidized cleaning During the test, it can be directly observed that the internal circulation fluidized cleaning movement is very intense, the sand is cleaned to achieve the desired internal circulation flow, and the tube can be cleaned quickly, uniformly, cleanly and efficiently. Artificial artificial scale outside. And the vertical velocity field curve for the cyclic flow at different depths. It can be seen from the figure that the gas flowing through a certain pressure and flow rate can make the upward flow velocity of the fluid outside the pipe reach 0.4 m/s or more, which is greater than the sedimentation velocity of the cleaning sand by 0.243 m/s, so the three-phase fluidization can drive the cleaning. The sand is cleaned; under the conditions of ensuring cleaning, the flow rates at different positions on the same section are quite different, varying from about 0.3 to 0.6 m/s. This easily causes uneven fluidization cleaning. The fluidization of the two-way circulating flow is formed by the alternate jets of two independent air duct systems, not only in the opposite flow direction, but also in the opposite flow rate: the original upward flow region becomes the downward flow region, and the original lower flow region becomes The higher flow rate area achieves the purpose of uniform, rapid, thorough and efficient cleaning of dirt.
The flow velocity curve of the adjacent pipe in the vicinity of the 300 mm section of the air pipe is 1 080 mm. The flow velocity curve of the adjacent section of the pipe is C. The minimum air supply pressure is approximately the hydrostatic pressure of the pure liquid at the air supply port. For example, when the air supply port is 2 000 mm below the liquid, the minimum air supply pressure is 19459 Pa. When the air supply pressure is lower, the gas flow rate is too small to meet the three-phase fluidization speed requirement. Considering that the maximum pressure is at the beginning of the supply of all the liquid outside the tube, the supply pressure of the cleaning gas is slightly larger than the static pressure of the liquid outside the tube, plus a 15% margin to overcome the flow resistance.
Comparing the curves, it can be seen that the change of the gas-liquid mixture flow velocity at different depth sections at the same position has little effect, so the influence of the shell depth on the fluidization velocity can be ignored.
To ensure reliable fluidization of the cleaning sand, there must be sufficient air volume to ensure that the fluidized mixture flows upwards. When a cloth gas pipe is supplied with gas, the rising flow velocity of the mixture in the range of 2 adjacent tubes on the left and right sides in the vertical direction is significantly higher than the sand sedimentation velocity of 0.24 m/s, which can ensure the reliable three-phase fluidization of the range. Flow cleaning. Therefore, the structural design may be such that one air duct is arranged in the middle of each of the four rows of tubes, and the two air duct systems are spaced apart from each other.
The amount of cleaning gas is proportional to the cross-sectional area S of the heat exchanger tubes for each cleaning. The consumption of cleaning gas per unit area of ​​the pipe is measured by the test to be 60.6 m3h per square meter. From this data, it is known that the gas consumption per hour is small. The inner diameter of the casing is Dm, the number of heat transfer tubes is N, and the outer diameter of the heat transfer tube is dm. The engineering calculation formula for the consumption of cleaning gas is multi-zone group ventilation and cleaning for large equipment, so as to avoid the use of atmospheric sources. Reduces noise pollution. For example, a large equipment with a diameter of 01 600 mm and 560 diameter 038 mm heat transfer tubes requires only 94.5 m3 / of gas for cleaning outside the tube. In order to improve the uniformity of the cleaning, and also to reduce the capacity of the gas source and the gas supply capacity, four sets of air pipes can be used, and the air is supplied at intervals.
4 Concentration of cleaning sand The premise of fluidization is that the free settling velocity of sand must be less than the rising flow velocity of the gas-liquid mixture. Therefore, a reasonable settling velocity must be determined based on the speed profile of the test. If the sedimentation speed of the sand is selected too much, the range of fluidization of each air supply pipe to drive the adjacent area is small, and the air consumption required for cleaning is large, because the air pipe is arranged between the pipes in the structural design, and the two pipes The distance between them is 0.25D in the conventional design, so the number of air supply pipes is generally required, and the manufacturing cost is also high. Also, too much sand cannot pass through the heat transfer tube area, causing cleaning failure. If the sedimentation speed of the sand is chosen too small, the sand is too fine, the cleaning speed is too slow, and it is not economical. Therefore, the comprehensive analysis and comparison, the choice of 8 ~ 10 mesh sand, its free settling velocity of about 0.24m /. sand concentration is also an important factor affecting the three-phase fluidized cleaning effect. If the concentration is too low, the cleaning speed is too slow; if the concentration is too high, not only the pressure of the cleaning gas is high, but also the compressor is required to supply the gas, the cost rise is uneconomical, and there is a possibility that the sand is partially blocked and deposited between the tubes, resulting in Cleaning failed. According to the test measurement, the total volume of sand should be 2%~5% of the volume between the tubes.
5 The exhaust port of the cleaning gas is to discharge the cleaning inert gas on the liquid surface in time, so that the pressure on the liquid surface is close to the atmospheric pressure, so as to clean the pressure of the inert gas - the lowest value but the gasping 1 is too large 6 pairs The structural strength of the casing is too much affected and it is not economical. If the area of ​​the exhaust port is too small, the outer exhaust body will entrain the liquid. Therefore, the minimum value of the vent area is determined by the maximum allowable speed of the vent. As a result of the test, the maximum speed of the exhaust port is allowed to be 29.5 m/s. In addition, attention should be paid to the arrangement of the position of the exhaust port. First, it should be as close as possible to the upper part of the equipment. Secondly, select the appropriate number of exhaust ports according to the size of the equipment, and pay attention to the surrounding environment.
6 cleaning costs low internal circulation three-phase fluidized cleaning medium-sized equipment, generally only about two hours, only one operator. The pressure of the cleaning gas is low, and the height of the tube plate between most of the relatively high vertical heat exchangers is only 6,000 6600 Pa. It is sufficient to supply the gas without the compressor. For example, a large-scale equipment with a diameter of 1600 mm and a height of 6000 mm is used for cleaning for 2 hours according to the amount of air cleaning required per square meter of pipe area. The electricity cost of the rotary blower is more than 10 yuan, and the staff time is less than 100 yuan. Therefore, the cleaning cost is very low.
7 Two-way internal circulation fluidized cleaning technology Summary Two independent jet systems rotate jet cleaning to form three-phase fluidization in alternating positive and negative directions and internal circulation flow. The cleaning effect is thorough, uniform and fast; the flow field outside the tube The test result is the basis of the design of the gas pipe structure. A gas pipe can drive the fluidized cleaning of the two rows of heat transfer tubes on each side; the consumption of the cleaning gas is calculated as 24. The cleaning method is simple in structure and convenient to operate. Cleaning costs are very low.
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