Outline of multihead weigher, scheme using basic parameters, calculation and application example

Author: Smartweigh–Multihead Weighter

The multihead weigher (Loss-in-weightfeeder) is a kind of quantitative analysis weighing feeder equipment. From the main purpose, the multihead weigher is used for the whole process of dynamic continuous weighing, which can carry out the raw materials that must be continuously fed. Weighing and quantitative analysis operation, and there are instant total flow of raw materials and total total flow display information. Basically speaking, it is a static data weighing system, which adopts the weighing technology of the static data hopper scale, and uses the weighing sensor to weigh the hopper. However, in the control panel of the multihead weigher, it is necessary to calculate the net weight lost per unit time of the hopper scale to obtain the instantaneous total flow of raw materials.

Figure 1 is a plan view of the principle of the multihead weigher. The brief description of the multihead weigher, the design scheme, measurement and application of the main parameters of the operation and its application case. Figure 1. The principle plan of the multihead weigher. Figure 1 is a schematic diagram of the structure of a multihead weigher. Discharge, when the maximum material level is reached, the discharge valve is closed, and the weighing hopper is supported by a multihead weigher. In order to make the weighing accurate, the upper and lower sides of the weighing hopper are all connected according to the soft channel or the entrance and exit, so that the net weight of the front and rear, left and right machinery and equipment and the raw materials are not used on the weighing hopper.

The right side of Figure 1 is a plan view of the whole process of the continuous feeder. The whole process of the continuous feeder has a cycle system (three cycles are indicated on the figure). Each cycle system consists of two cycle times: when the weighing hopper is empty, the discharge valve is opened to discharge the material, and the net weight of the raw material in the weighing hopper continues to increase. When the maximum material level is reached at t1, the discharge valve is closed. The screw conveyor just started to pour the material, and then the multihead weigher started to work; after a period of time, when the net weight of the raw material in the weighing hopper continued to decrease and reached the minimum material level at t2, the discharge valve was opened again, and the period from t1 to t2 was the function Force feeder cycle time; after a period of time, when the net weight of the raw material in the weighing hopper continues to increase and reaches the maximum material level again at time t3, the discharge valve is closed, and the period from t2 to t3 is the cycle time for re-discharging , and so on. During the cycle time of the force feeder, the speed ratio of the screw conveyor is monitored according to the instantaneous flow rate to achieve a stable feeder; during the re-unloading cycle time, the speed ratio of the screw conveyor will keep the speed ratio just before the start of the cycle time. Change the feeder to the constant volume flow monitoring method.

Because the multihead weigher integrates dynamic weighing and static data weighing, and integrates interrupted feeder and continuous feeding, the structure is easy to seal, and it is suitable for weighing of ultra-fine raw materials such as concrete, quicklime powder, pulverized coal, food, medicine, etc. Weight and seasoning control, can achieve high weighing precision and linearity. 2. Necessity of the design scheme of the main parameters of the operation of the multihead weigher When designing the scheme of the multihead weigher, the main parameters of the operation such as the frequency of discharge, the volume of re-discharge, the capacity of the weighing hopper, and the rate of re-discharge must be considered, otherwise the multihead weigher will not work properly at work. A customer purchased a multihead weigher from the manufacturer for on-site equipment maintenance for feature analysis. Only 3 100kg weighing sensors were purchased. After being put into use, it was found that the zero point was unstable, and the total flow sometimes did not display information and other common faults.

After the manufacturer sent someone to the scene, they realized that the raw material of the customer is boric acid, the relative density is 1510kg/m3, the maximum total flow is only 36kg/h, and the common total flow is 21~24kg/h. The total flow is so small, the hopper adopts three 100kg weighing sensor support points, and the capacity of the analysis hopper is quite large. One can follow the strongly recommended work experience rules below“When the amount of ash is large, the re-discharging frequency is selected as 15 to 20 times/h”To carry over, the net weight of each re-discharging is 36/15~36/20, that is, 1.9kg~2.4kg. The net weight of raw materials borne by each weighing sensor is less than 1 kg, and the reasonable measurement range is about 0.5~1%.

Generally, the reasonable measurement range of the weighing sensor should be at least 10~30% or more, so as to ensure a more accurate weighing. According to the raw material weight of 2.4kg plus the net weight of the hopper and feeding equipment (such as screw conveyor), the total weight is about 10kg. If three load cells are used, the measurement range of each load cell can be selected from 5kg~ 10kg. That is, the measuring range of the originally ordered 100kg sensor becomes 10-20 times larger, resulting in poor reliability of the multihead weigher and low weighing precision.

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 must be determined after calculation. 3. Calculation of the design scheme of the main parameters of the operation of the multihead weigher 3.1 Calculation of the discharge frequency Figure 1 details the operation of the multihead weigher. Each cycle system includes the whole process of discharge, so what is the appropriate discharge frequency? For the multihead weigher, the larger the cycle occupancy ratio of the force feeder in each cycle system (time occupancy = the cycle of the force feeder / the re-discharging cycle), the better, generally it should exceed 10:1. This is because the precision of the cycle time of the force feeder far exceeds the cycle time of re-unloading. The greater the cycle occupancy of the force feeder, the higher the overall precision of the multihead weigher.

The frequency of the circulatory system per unit time of the multihead weigher is generally expressed as the frequency of the circulatory system per hour when the amount of ash is larger, that is, times/h. Because the precondition is based on the larger amount of ash feeding per hour, the ash feeding per unit time (for example, per second) is a time constant. The less the frequency of the circulation system, the greater the amount of material discharged each time, the larger the capacity and net weight of the weighing hopper, and the lower the precision of the weight loss and calculation using the multi-range weighing sensor; the more the frequency of the circulation system, The lower the amount of each discharge, the smaller the capacity and net weight of the weighing hopper, and the higher the precision of weight loss and calculation using a weighing sensor with a small measuring range.

However, the frequency of the circulation system is too high, the feeding machine equipment starts and stops frequently, and the control board of the multihead weigher often switches between the cycle time of the force feeder and the cycle time of the re-feeding, which is not very good. The highly recommended re-discharging frequencies are shown in Table 1, but the most important and highly recommended are the three discharging frequencies in the middle. As a rule of work experience, most of the loss-in-weight feeder system software is very suitable for powdery materials and granular materials with poor fluidity. times/hour.

When the ash feeding amount is lower than the larger ash feeding amount, the frequency of re-feeding is reduced, so that the cycle occupancy rate of the force feeder is larger, which is more beneficial to improve the precision. As a rule of work experience, some applications with a very low total flow rate of the feeder, although the hopper capacity is very small, can still store raw materials for one hour or longer feeding, and the time for re-feeding exceeds 1 hour. The following example: The total flow of the larger feeder is 2kg/h. The ratio of the raw material pile is 803kg/m3. The total flow of the larger volume feeder is 2/803=0.0025m3/h. If the hopper capacity is 0.01m3 (roughly equal to 25b250m×25b250m×The size of a cube hopper like 25b250m), sufficient raw material usage for 2h~3h, and each feeding amount is below 10kg, so there is no need for automatic feeding, manual service feeding can be considered production and manufacturing regulations, but its total flow is linear slightly lower.

3.2 The formula for calculating the volume of re-discharging has selected the frequency of re-discharging, and then the volume of re-discharging and the total volume of the feeder can be calculated. According to the characteristic analysis of a multihead weigher: the total flow rate of the larger feeder is 275kg/h, the bulk density of the raw material is 485kg/m3, and the total flow rate of the larger volume feeder is 270/480=0.561m3/h. The frequency of the material is selected as 15 times/h. The calculation method of the volume of the re-discharge is: the volume of the re-discharge = the larger amount of ash (kg/h)÷Density (kg/m3)÷Re-discharge frequency (re-discharge frequency/h) In this example, re-discharge volume = 270÷480÷15=0.0375m33.3 Calculation of weighing hopper capacity The capacity of the weighing hopper in the design scheme will undoubtedly exceed the calculated re-discharging volume. This is because it is necessary to consider that the weighing hopper is unavoidable when re-discharging is started. There are also some“Residual raw materials”and the top of the hopper has storage that is unlikely to be full“free space”, if each accounts for 20%, then the re-discharging volume is divided by 0.6, and the necessary hopper capacity can be obtained, and the final weighing silo capacity should be glossy according to the finalized silo capacity. Calculation method of re-discharging volume: weighing hopper capacity = re-discharging volume÷Where k: k is the calculated capacity index of the hopper, which can be 0.4~0.7, and 0.6 is strongly recommended.

In this example, Weigh Hopper Capacity = 0.0375÷0.6=0.0625m3 If the capacity of the shaping silo has specifications such as 0.6m3, 0.2m3, 1.b2503, etc., it should be shiny up to 0.08m3, and the capacity of the weighing hopper should be 0.08m3. 3.4 The discharge rate is calculated again because of the multihead weigher In the re-discharging cycle time, the low-precision constant-capacity method feeder is selected, so the re-discharging speed of the vibrating feeder is specified to be faster (generally, it should be operated within 5s~20s). Calculation method of re-discharge rate: re-discharge rate = [re-discharge volume (m3)÷Discharging time again (s)×60(s/min)]+[Total flow of larger volume feeder (m3/h)÷60 (min/h)] In formula 2, the discharge rate again consists of two items.

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