Design scheme and calculation of main parameters of multihead weigher in practical operation

Author: Smartweigh–Multihead Weighter

Multihead weigher (Loss-in-weightfeeder) is a quantitative analysis of net weight providing equipment, mainly used, multihead weigher is used for dynamic continuous net weight in the whole process, can carry out net weight and quantitative analysis of raw materials that must be continuously provided, and display information about raw materials. Instantaneous total flow and total total flow. The basic principle is the static data net weight machine and equipment, the net weight technology of the static data warehouse scale is selected, and the load cell net weight warehouse is used. However, in the control panel of the multihead weigher, in order to better obtain the net weight lost per unit time of the raw material warehouse scale, it is necessary to measure the instantaneous total flow of raw materials.

The left side of Figure 1 is the frame diagram of the net weight missing scale. When the raw materials in the net weight warehouse are free of necessary raw materials, the raw material valve can be opened. When the maximum raw material position is reached, the raw material valve is closed, and the net weight warehouse is supported by the net weight missing scale. Point. In order to make the weighing more accurate, the upper and lower sides of the weighing warehouse are connected according to soft passages and entrances and exits, and the front and rear, left and right machinery and equipment and the net weight of the raw materials in it do not add to the weighing warehouse. The right side of Figure 1 is a plan view of the whole process of continuous supply. There is a cycle system for the whole process of continuous supply (the information on the figure shows 3 cycle systems).

Each cycle system consists of 2 cycle times: when the net weight warehouse is to reduce the warehouse, the net weight of the raw materials in the net weight warehouse is increased, and when the maximum raw material position is reached at t1, the raw material valve is closed, and the screw conveyor just begins to discharge the raw materials, and the net weight is lost at this time. After the scale starts to work for a period of time, the net weight of the raw materials in the net weight warehouse will decrease. When the minimum raw material position is reached at t2, the raw material valve will be opened again. The time from t1 to t2 provides the cycle time for the force type. After a period of time, the net weight of the raw materials in the net weight warehouse will increase. When the time t3 reaches the maximum raw material position again, the raw material valve is closed, and the time from t2 to t3 is repeated in the force supply cycle time, and the speed ratio of the screw conveyor is monitored according to the instantaneous flow, so as to achieve a stable supply cycle. During the time, the speed ratio of the screw conveyor maintains the speed ratio just before the beginning of the cycle time and does not change, and is provided by the constant volume flow monitoring method. Because the multihead weigher closely combines dynamic weighing and static data weighing, and closely combines interrupted feeding and continuous feeding, the structure is conducive to sealing, and is suitable for concrete, quicklime powder, pulverized coal, food, medicine and other small raw materials. Weighing and feeding operation can achieve high weighing precision and linearity. The 2multihead weigher operates on the necessity of the main parameter design scheme.

When designing a scale with missing net weight, be sure to take into account the main operating parameters such as feeding frequency, re-feeding volume, re-feeding warehouse capacity, and re-feeding rate. Otherwise, the net weight missing scale will not work properly. For example, a customer purchased a multihead weigher from a manufacturer for on-site equipment maintenance. At the time of purchase, only 3 100kg weight sensors were purchased.

The manufacturer sent someone on the spot to know that the customer's raw material is boric acid solution, the relative density is 1510kg/m3, the maximum total flow is 36kg/h, and the common total flow is 21~24kg/h. The total flow is so small, the hopper uses three 100kg weight sensor support points, and the analysis hopper has a large capacity. It is an unscientific problem in model selection. Another problem is that the hopper is connected to a machine with a vibration source during installation. We can select 15~20 times/h according to the following highly recommended work experience standards when the demand is larger, and the net weight of each additional supply is 36/15~36/20, that is, 1.9kg~2.4kg, The net weight of raw materials borne by each weight sensor is less than 1 kg, and the reasonable measurement range is about 0.5~1%.

Generally, the reasonable measurement range of the weight sensor should be at least 10~30% to ensure more accurate weighing. According to the net weight of raw material 2.4kg and the net weight of raw material warehouse and raw material machinery and equipment (screw conveyor, etc.), the total weight is about 10kg. When three weight sensors are used, the measurement range of each weight sensor can be selected from 5kg to 10kg. In other words, the quantity of the purchased 100kg sensor is increased by 10~20 times, the reliability of the multihead weigher is poor, and the weighing precision is low.

This case shows that the design scheme of the multihead weigher must also comply with the design scheme standard, and the main parameters of the machine equipment and operation of the multihead weigher cannot be decided without measurement. 3multihead weigher operation main parameters of the design scheme calculation. 3.1 Calculation of feeding frequency.

For the multihead weigher, the larger the ratio of the force supply cycle (time ratio = the force supply cycle/re-supply cycle) in each circulatory system, the better, generally it must exceed 10:1. This is due to the fact that the force type provides cycle time with a much higher precision than resupply cycle time, and the longer the force type provides cycle time, the higher the overall precision of the multihead weigher. The circulatory system frequency per unit time of the multihead weigher is generally expressed as the circulatory system frequency per hour when the demand is larger, that is, times/h.

Taking the larger demand per hour as the standard, the demand time constant per unit time (for example, per second) is the precondition. The less the frequency of the circulation system, the greater the amount of each feeding, the greater the capacity and net weight of the net weight warehouse, the lower the precision of measuring the weightless state by applying many levels of load cells, the more the frequency of the circulation system, the greater the amount of feeding each time. The lower it is, the smaller the capacity and net weight of the net weight warehouse, and the higher the precision of applying a small amount of load cells to measure the weightless state. However, the frequency of the cycle system is too high, the feeding machine equipment is often terminated, and the control panel of the multihead weigher often switches between the force feeding cycle time and the refeeding cycle time, and it is not very good.

As the regularity of work experience, most of the weightless state provides system software, especially for powdery and poorly fluidized particles, the frequency of resupply is selected as 15~20 times/hour when the demand is large. When the demand is lower than the larger demand, the frequency of re-supply is reduced, and the force-type supply cycle accounts for a larger proportion, which is conducive to improving the precision. Except for the regularity of work experience, some applications that provide a particularly low total flow, although the warehouse capacity is very small, they can still store the raw materials provided for more than 1 hour, and the time to provide more than 1 hour.

The following cases: Larger provides a total flow of 2kg/h. The raw material deposition ratio is 803kg/m3. The total flow of larger volume feeding is 2/803=0.0025m3/h.

If the warehouse capacity is 0.01m3 (approximately equivalent to 250Mm×250Mm×The size of the cubic meter warehouse like 250Mm) is sufficient for 2h~3h of raw material usage, and each raw material usage does not exceed 10kg, so automatic raw materials are not required, and manpower raw materials can be considered. Manufacturing regulations, but the total flow linearity is slightly lower . 3.2 Calculate the feeding volume again. After selecting the refill frequency, it is possible to measure the refill volume and the total supply volume.

Take a multihead weigher as an example: the larger one provides a total flow of 270KG/hour. The bulk density of the raw material is 485kg/m3. The total flow of larger volume feeding is 270/480=0.561m3/h.

At the larger delivery rate, the frequency of re-delivery was chosen to be 15 times/h. Calculation method of reloading volume: reloading volume = larger addition amount (kg/h)÷Density (kg/m3)÷Re-entry frequency (re-entry frequency/h) In this case, the re-installation volume = 270÷480÷15=0.0375m3. 3.3 Net weight warehouse capacity calculation.

The net weight warehouse capacity of the design plan must exceed the calculated net weight warehouse capacity. This is because the net weight warehouse will inevitably have remaining raw materials and free space at the top of the warehouse when the net weight warehouse starts the net weight warehouse. If each accounts for 20%, and the net weight warehouse capacity is divided by 0.6, the necessary warehouse capacity can be obtained.

The final selected net weight warehouse volume must be glossy according to the fixed warehouse volume. Calculation method of reloading volume: net weight warehouse capacity = net weight volume÷k. In the formula: k is the estimated surplus index of the raw material warehouse, which can be 0.4~0.7, and 0.6 is proposed.

In this example, net weight warehouse capacity = 0.0375÷0.6=0.0625m3. When the volume of the shaped warehouse is 0.6m3, 0.8m3, 1.0M3 and other specifications and models, the gloss should be 0.08m3 upward, and the capacity of the weighing warehouse should be 0.08m3. 3.4 Measure the re-installation rate.

The multihead weigher is provided by a low-precision fixed capacity method in the reloading cycle time, so the reloading rate of the reloading machine is specified to be fast (generally, it should be controlled within 5s~20s). Calculation method of reloading rate: recapitalization rate = [recapitalization volume (m3)÷Reinvestment time (s)×60(s/min)]+[large volume capital investment total flow (m3/h)÷60(min/h)] In Equation 2, the re-adding rate consists of 2 new items, the first new item is the addition rate based on the re-adding volume, and the second new item is often ignored by many people, indicating that the same The rate of addition of time, the raw material that must fill this part when added again. According to the value, the re-feed rate is about 30 times the larger addition rate. In other words, according to this value, when estimating the re-feed rate of other net weight missing scales, it can be estimated at 25-40 times the larger addition rate. .

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