Keywords: vacuum processing steam injection work performance
1. Introduction In recent years, vacuum technology has been widely used in light chemical industries such as food, tobacco, leather, pharmaceutical, ceramics, sugar, papermaking, etc. The vacuum obtaining device used usually uses a water ring vacuum pump, a rotary vane vacuum pump, or A series combination of a water ring pump and a steam jet pump. Such vacuum obtaining devices have low pumping capacity and poor working stability, and often cannot meet the requirements of the production process; in particular, the water ring type vacuum pump has a large maintenance workload due to wear and scale formation due to high-speed movement of each other. To this end, the Institute of Vacuum Technology of Northwest Institute of Light Industry carried out the application research of steam jet vacuum technology; developed the CAD application software of jet vacuum pump to improve the working performance of jet pump. The steam jet vacuum pump has a large pumping capacity, and can eliminate various kinds of pumped gases which are easy to be polluted, easy to wear, easy to corrode and easily explosive, and has the advantages of stable working performance, small maintenance and repair work, simple structure and easy manufacture. In the successful development of multiple vacuum equipment, multi-stage jet condensing vacuum system is equipped, which improves the working performance of the equipment and the quality of processed products, and finds good social and economic benefits for the enterprise window.
2. Design of steam jet vacuum pump The flow of steam in the jet pump belongs to the flow problem of the variable cross-section pipe, and it has its inherent regularity. When the working fluid flows through the nozzle, the pressure is reduced, the speed is increased, and the high-speed airflow that is injected can be used to illuminate the low-pressure airflow; the reasonable variation of the cross-sectional area of ​​the pipeline has a great influence on the ability to ignite the bottom-pressure airflow. The jet pump should be designed so that its shape and dimensions are as close as possible to the conditions required for airflow expansion or compression to achieve the highest possible efficiency of the jet pump. However, due to the complicated thermal process of steam flow in the jet pump, the theoretical calculations in this aspect are still not perfect. Some parameters and coefficients in the calculation also lack sufficient reliable data, resulting in a large gap between the theoretical calculation and the actual process. To this end, the author based on years of design practice, using the combination of empirical design and theoretical calculation, optimized the design parameters, developed an application software system integrating computer-aided design and manufacturing.
The jet vacuum pump application software is based on the design idea of ​​hierarchical modularization, integrating analysis, calculation and drawing. It consists of a main module and four sub-modules; through the menu prompts, the man-machine dialogue form realizes the calling of each module.
(1) Main control module: Its function is to connect and manage each module to provide a good interface between the user and the computer. The grading modules are all listed in menu form and can be easily selected with the cursor keys. Each module can be called from each other and independently.
(2) Design calculation module: This module mainly completes the design calculation of single-stage injector or multi-stage steam injection condensing vacuum system.
(3) Graphic generation module: This module mainly completes the parameterized graphic drawing of the main parts of the injector and the condenser and the automatic generation function of the assembly drawing.
(4) Nozzle surface CNC machining module: This module has compiled the NC machining program of the nozzle curve and the drawing of the nozzle surface part drawing.
(5) Dynamic simulation module of jet pump working process: This module mainly simulates the dynamic process of airflow during single-stage jet pump operation.
3. Factors Affecting the Performance of the Jet Pump The factors affecting the performance of the jet pump are institutional and structural factors. The former includes changes in working conditions, operating conditions, industrial measures, etc.; the latter refers to factors affecting geometric size and shape.
(1) Working steam pressure: With the increase of steam pressure, the ultimate back pressure of the injector is increased, the work of the pump is more stable, but the steam consumption is increased and the condenser load is increased.
(2) Moisture content of working steam: Excessive water content of working steam will cause fluctuation of vacuum degree under each flow rate, resulting in unstable operation of the pump. The more the water content fluctuates, the more the pump will not work.
(3) Cooling water quantity: If the amount of cooling water entering the condenser is gradually reduced, the exhaust gas temperature will be gradually increased, so that the amount of uncondensed steam is gradually increased, so that the amount of the mixture to be pumped by the next stage injector is increased, resulting in the suction pressure. Rise; if this pressure exceeds the extreme back pressure of the foreline injector, it will cause the foreline injector to overload and destroy the vacuum system. Too much increase in the amount of condensed water not only increases the amount of cooling water bleed, but also increases the resistance of the condenser, reduces the pumping characteristics of the injector, and even overloads the foreline injector. It can be seen that the condenser cooling water has a reasonable design value.
(4) Concentricity of the nozzle and the diffuser axis: When the coaxiality is less than 0.5mm, the design performance of the pump is not greatly affected, and the excessive coaxiality will deteriorate the vacuum.
(5) Relative position of the nozzle and the diffuser: it has a relatively sensitive influence on the ejector system, the ultimate vacuum and the ultimate back pressure. Usually, the suitable relative position has a length range beyond which the performance deteriorates.
4 Application of Jet Vacuum Processing Technology In recent years, we have developed leather vacuum wet processing equipment, food vacuum drying equipment, burley smoke vacuum processing equipment, concentrated vacuum impregnation equipment, etc., in combination with vertical or horizontal research topics. The technical transformation was carried out by imitation of the introduction of the tobacco leaf vacuum regaining machine. The above equipments are equipped with a jet vacuum system designed and manufactured by ourselves to ensure the good working performance of the equipment.
4.1 Technical transformation of the tobacco leaf vacuum regaining machine The vacuum re-moisture machine of the tobacco leaf uses the vacuum principle to eliminate the air in the gap between the tobacco leaves in the cigarette pack, so that the humidified airflow enters it, humidifies and warms the tobacco leaf, and improves the water content and toughness of the tobacco leaf without causing the temperature to pass. High and damage the physical and chemical quality of tobacco leaves, remove tobacco green gas, sterilization, reduce the loss of tobacco leaves, improve the quality of tobacco leaf burning. At present, both domestically produced and imported vacuum resurgence machines generally have problems such as complicated structure, long working time, large maintenance workload, large energy consumption, and poor resurgence quality. The main reason is that the vacuum system and the humidification system have poor working performance. Caused by. For example, the Italian Comas resurgence machine consists of a Roots pump, a rotary vane pump and a refrigeration unit; the UK Leger vacuum system is a series of water ring pumps and jet pumps. During the working process, due to the presence of a large amount of steam, corrosive gases and dust, the mechanical pump rotor and pump chamber are rapidly corroded and worn, resulting in a decrease in the working performance of the vacuum system, and the maintenance workload of the pump is increased. The vacuum system and humidification system of the introduced tobacco vacuum resurgence machine were rebuilt, and the two- or three-stage steam-jet condensing vacuum system designed and developed by ourselves and the high-speed air-flow self-absorbent atomizing tobacco leaf humidifying device were respectively adopted. The vacuum system of the vacuum regaining machine after the transformation has stable performance, the humidification system has remarkable effects, and the long-term operation under the condition of basically no maintenance has reached the requirements of the production process, shortened the resurgence working time, and achieved great social and economic benefits. The project won the second prize of Shaanxi Science and Technology Progress Award, and the modified vacuum resurgence machine has spread over more than 60 cigarette factories and tobacco re-baking plants across the country.
4.2 Food vacuum drying equipment In view of the problems existing in the existing drying methods at home and abroad and the characteristics of vacuum drying process, in the process of equipment development, some modern scientific research results are absorbed and adopted, and the vacuum mechanism, temperature mechanism and automatic control technology are applied. In the drying industrial process. A multi-stage jet condensing vacuum system with high working vacuum and large displacement is designed to enhance the dewatering of materials and the rapidity of the working process. The exhaust gas heating by the jet vacuum pump enables the energy to be fully utilized and the production cost is reduced, and is a novel vacuum drying device. The quality of its dried products is comparable to that of freeze-dried products, and the crispy products are better. Under the same production capacity, the equipment investment cost is only 1/6 of that of cold drying, and the energy consumption is only 1/2-1/3 of the cold drying, and the production cost is reduced. The project has won the second prize of Xianyang Science and Technology Progress Award and the first prize of Shaanxi Provincial Patent Technology. It is a high-tech of deep processing of agricultural and sideline products.
4.3 Leather vacuum wet processing equipment The leather industry has formed a set of traditional wet processing techniques, including water immersion, liming hair removal, deliming, softening, pickling, tanning, retanning, dyeing, refueling and other processes. The above-mentioned conventional processes have long been incapable of overcoming shortcomings such as long production cycles and large pollution levels. To this end, the Institute of Vacuum Technology of Northwest Institute of Light Industry introduced the vacuum mechanism into the leather wet processing process for the first time, developed a leather vacuum drum, and designed a multi-stage jet condensation type with high vacuum, large displacement, and corrosive gas. Vacuum system. The vacuum mechanism is used to make the tincture penetrate into the inside of the skin evenly and quickly, and the collagen is combined with the skin collagen faster and better. The process conditions compatible with the vacuum mechanism are applied to carry out the wet processing of the leather, and the expansion of the skin tissue is strengthened. And the speed of the work process. After a lot of experimental research, we found a fast and less polluted leather wet processing method. This topic is the China Light Industry Association's science and technology project "New Technology and Equipment for Wet Processing of Less Polluted Leather", which is a major change in the leather wet processing technology.
4.4 Concentrated vacuum impregnation equipment Concentrated vacuum impregnation equipment has the dual functions of impregnation and concentration. The impregnation liquid can be automatically changed step by step according to the process requirements and the concentration is increased to enhance the impregnation effect. The impregnation process is treated once in the same equipment and the impregnation solution can be reused. The evacuation system uses a multi-stage steam jet vacuum pump with a vacuum of 133-1330 Pa. In a vacuum environment, the permeability of the impregnation liquid is enhanced and the impregnation process is accelerated. The equipment can be used for pickling, sauce and syrup of agricultural and sideline products or for pre-treatment of vacuum drying of fruits and vegetables. It is a high-tech with low investment, quick effect and good economic benefit, and has obtained national patent.
5 Conclusion Compared with other types of vacuum pumps, water vapor jet vacuum pumps are not only simple and reliable in structure, stable in performance, convenient in operation and maintenance, large in pumping capacity, but also have no strict requirements on the pumped medium, and can eliminate toxicity and flammability. Explosive, corrosive and various gases containing impurities; widely used in different processes in the light industry, such as vacuum impregnation, vacuum concentration, vacuum drying, vacuum evaporation crystallization, vacuum distillation, etc. Accelerate the speed of the process and effectively improve the quality of the processed products. Therefore, it is of great significance to develop and research high-efficiency, low-consumption steam-jet vacuum processing technology for the upgrading and upgrading of equipment in the light chemical industry and to improve the market competitiveness of products.
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