The role of intermittent asphalt mixing equipment in China's highway construction has been widely recognized by the industry. In order to ensure good asphalt mixture quality, batch equipment is very consistent with the current situation of China's material market, so as to ensure the rapid and good development of China's highway construction.
The quality of asphalt mixture is related to many factors. Without talking about the influence of many other factors, this paper focuses on the relationship between asphalt mixing cycle process and quality, efficiency and energy consumption.
The product quality of asphalt mixture has quality standard requirements. In order to achieve this product quality, the production enterprise has formulated the production process guidance document with reference to relevant materials and experimental data. So as to ensure the continuous supply of qualified asphalt mixture in China's highway construction.
It is generally recognized in the industry that the mixing cycle of intermittent asphalt mixing equipment is usually 45 seconds. Although it is not a mandatory standard, it basically guides relevant departments (engineering design, equipment design, supervision, etc.) to become the basis for compliance.
The mixing cycle includes various process steps related to it, which can also be said to be a mixing sequence. Some processes overlap in time sequence and some are independent. Independent process sequence must be guaranteed in the mixing cycle, so it is the main factor limiting the shortening of mixing cycle. When designing products, each equipment manufacturer designs, optimizes and organizes their products more or less by analyzing the sequence diagram and related elements and extending the process.
Next, the mixing process is analyzed by simulation. The mixture gradation shall be subject to the standard working condition.
The mixing cycle includes the following processes:
1. Batching - sequential measurement of 6 kinds of aggregates; Sequential measurement of 3 kinds of powders; Measurement of asphalt;
2. Numerical stability time and opening and closing time of each silo;
3. Three scales open the door and discharge materials to the empty scale, and the sending time of door closing signal;
4. Mixing --- after all three materials are put into the mixing pot, start timing mixing;
5. The opening and closing time from the discharge valve of the mixing pot to the emptying of the mixing pot and the completion of closing the door to send a signal.
In Item 1 above, powder, asphalt and aggregate can be measured at the same time, that is, they can be overlapped in time. Take the longest time. From the above analysis and actual production, it can be seen that the proportion of aggregate in the grading is more than 90%, and the measurement time of six kinds of aggregate is the longest. Therefore, each manufacturer has developed different improvement schemes: one is to increase the area of aggregate bin door. The discharging speed is accelerated and the metering time of various aggregates is shortened. However, the fly material error at the end of aggregate measurement is very large. The measures taken by the existing equipment in the market are different door size schemes. However, another way to find the balance point between fast and minimum error is to measure multiple bone scales at the same time. Equivalent to the overlap of time, the aggregate metering time can also be reduced. Similarly, the above method can also be used to deal with the error, or the material falling amount can be limited directly through the small door. However, the latter method is difficult to realize when the material and gradation allow.
Item 2: a weighing scale is required for the measurement of various materials to feed back signals to the control system through weighing to open and close the door. This process takes time. Shortening the opening and closing time, shortening the stability time of metering signal and shortening the transmission time of signal after computer calculation are the research topics of various manufacturers. It is also the embodiment of measurement accuracy and stability, and the control system patents of various manufacturers are also reflected here.
Item 3: the action speed of each door is the embodiment of the performance of various mechanical components, including the installation position of cylinder, solenoid valve and signal sensor. This involves the designer's idea of type selection, the technical level of on-site installation and commissioning personnel and the understanding of the process. This time should reach the ideal minimum time.
Item 4, mixing time. When all materials are put into the mixing pot, start mixing and timing until the empirical time data and experimental results tell you that they have been mixed evenly and meet the quality requirements of asphalt. This is also a time item without specific values. Now, the mixing time is basically set according to the requirements of engineering designers and supervisors and the time given by them. Generally, the laboratory does not verify this time, which accounts for a very large proportion in the actual production cycle. Whether it is reasonable is debatable. It will be discussed in detail later.
Item 5: dwell time when the discharge valve of the mixing pot is opened. This is directly related to the design of the mixing pot, which is simply the size of the door. However, how to determine that the material in the mixing pot has been emptied is related to the cycle time, and it is also the key to determine this time.
Through the above analysis, we have clearly analyzed the mixing production cycle sequence of an asphalt mixture.
After this sequence is clear, it can play a guiding role in structural design, component selection, installation and commissioning, production process parameter setting, and produce asphalt mixture quickly and well. The shortening of time can bring the effect of energy conservation and emission reduction.
The mixing time must be guaranteed in the production process. In terms of mechanical structure design, it is mainly the design of mixing pot, mixing arm and mixing blade. From the types of existing mixing pots, the inclination angle of mixing arm and mixing blade, the rotating speed of mixing shaft and the linear speed of blade are relatively close, so there is no comparative analysis here. There are two schools of mixing pots: one is a mixing pot specially used for asphalt mixture mixing, and the other is an improved cement concrete mixing pot. Asphalt mixture mixing pot is mainly reflected in the number of mixing arms and blades, and the most representative is the early Marini small pot quick mixing. In fact, the pot is not small, and quick mixing is his characteristic. By comparing the two kinds of mixing pots, it is found that there is a lot of difference in the number of mixing arms between cement mixing pot and asphalt mixing pot, and the mixing arms and blades act on the materials in the mixing pot a lot more times in the same mixing time. It is not difficult to understand that under this quick mixing mode, the time required for the two pots to achieve uniform mixing will be significantly shortened (for example, when mixing a pile of cement, two people and two shovels are compared with one person and one shovel, which is faster is obvious). Some scholars believe that the large blades of the cement pot will also be beneficial to uniform mixing. Each has its own characteristics! From the perspective of energy conservation, for the mixing pot of the same specification, the fast mixing is larger in terms of its actual energy consumption. These energy consumption are consumed for the mixing of materials in a short time. In other words, the output will increase at the same time. The increase of productivity means saving energy consumption for the production of complete equipment.
Unfortunately, in the production management of the industry, the inertia of cognition leads to the customary requirements of mixing time and process parameters. Even the quality supervision of some engineering projects requires that the mixing time must reach how many seconds, rather than taking the product quality as the standard. This is a reflection of conservatism. This practice is not conducive to equipment technology innovation and market competition.
This is an analysis of the mixing time. For the design of the mixing pot door, the key point should be to reduce the venting time. For the venting time, the increase of the number of mixing arms and blades is also conducive to shorten the venting time.
The operating speed of the opening and closing process of various doors is subject to the performance of components. How to achieve a balance point of fast, stable and long life in the selection of components is the starting point for designers to consider shortening the time to the greatest extent.
Finally, installation and debugging. Installation and commissioning is very important. The professional level of commissioning engineers directly affects the setting of production process parameters. Only after fully understanding the role of various production process parameters in shortening auxiliary production time and the scientific setting of mixing pot characteristics can the production equipment give full play to its due production capacity. A stirring cycle is calculated in seconds. The output will increase by about 3% every second. Comprehensive energy consumption will be significantly reduced.
Therefore, taking the mixing cycle as the main design line of intermittent asphalt mixing equipment, through the analysis of various influencing factors, mutual relations and extension, continuous analysis and research, from reverse design to forward design, is the key to improve the production efficiency of asphalt mixing equipment.
