Production processes are often accompanied by the release of dust-like elements or gases that pollute the indoor air. The problem will be solved by aspiration systems designed and installed in accordance with regulatory requirements.
Let's figure out how such devices work and where they are used, what types of air purification systems there are. We will designate the main working units, describe the design standards and rules for installing aspiration systems.
Air pollution is an unavoidable part of many industrial processes. To comply with the established sanitary standards air purity, use aspiration processes. With their help, you can effectively remove dust, dirt, fibers and other similar impurities.
Aspiration is suction, which is carried out by creating an area of low pressure in the immediate vicinity of the source of contamination.
To create such systems, serious specialized knowledge and practical experience are required. Although the operation of aspiration equipment is closely related to the operation, not every ventilation specialist can handle the design and installation of this type of equipment.
For achievement maximum efficiency combine ventilation and aspiration methods. Ventilation system the production area must be equipped to ensure a constant supply fresh air outside.
Aspiration is widely used in the following industries:
- crushing production;
- wood processing;
- manufacturing of consumer products;
- other processes that are accompanied by the release of large amounts of substances harmful to inhalation.
It is not always possible to ensure the safety of employees using standard protective equipment, and aspiration may be the only opportunity to establish safe manufacturing process in the workshop.
Aspiration units are designed to effectively and quickly remove various small contaminants from the air that are formed during industrial production.
Removal of contaminants using systems of this type is carried out through special air ducts that have a large angle of inclination. This position helps prevent the appearance of so-called stagnation zones.
Mobile ventilation and aspiration units are easy to install and operate, they are perfect for small businesses or even for a home workshop
An indicator of the effectiveness of such a system is the degree of non-knocking out, i.e. the ratio of the amount of contaminants that were removed to the mass of harmful substances that did not enter the system.
There are two types of aspiration systems:
- modular systems– stationary device;
- monoblocks– mobile installations.
In addition, aspiration systems are classified according to pressure level:
- low-pressure– less than 7.5 kPa;
- medium pressure– 7.5-30 kPa;
- high-pressure– over 30 kPa.
The configuration of modular and monoblock type aspiration systems is different.
In hot shops, heating the air coming from outside is not necessary; it is enough to make an opening in the wall and close it with a damper.
Conclusions and useful video on the topic
Here is an overview of the unpacking and installation of the RIKON DC3000 mobile aspiration system for the wood industry:
This video demonstrates stationary system aspiration used in furniture production:
Aspiration systems – modern and reliable way cleaning the air in industrial premises from hazardous pollutants. If the structure is correctly designed and installed without errors, it will demonstrate high efficiency at minimal cost.
Do you have anything to add or have any questions about aspiration systems? Please leave comments on the post. The contact form is located in the lower block.
The following equipment is combined into one aspiration network:
-working simultaneously;
-closely located;
- with the same dust, or similar in quality and properties;
-with the same or a slight difference in air temperature.
The optimal number of suction points is no more than six, but more are possible.
If in any machine the air flow regime periodically changes, i.e. is adjusted in accordance with the technological process, then a separate ventilation unit is designed for it; or very a small amount additional, “passing” suction points (one or two with low flow rates).
Examples of the layout of aspiration installations are on the page.
Determine the air consumption for aspiration and pressure loss (resistance) for each aspirating machine, container, point. Take the data from the equipment's passport documentation or from the "aspiration standards" in the reference literature. Data from similar projects can be used.
Air flow can be determined by the size of the suction pipe or aspiration hole in the machine body, if the pipe and hole are made by the manufacturer and (or) according to the dimensions of the design organization.
If the incoming product ejects some additional amount of air into the equipment (for example, moving at high speed through a gravity pipe), then this additional volume should be added to the standard volume, determining it also according to the standards or calculation methods applicable to this particular supply device and product.
If a certain amount of air is carried away from the equipment with the product being removed, it should also be determined and subtracted from the air flow for aspiration.
Excessive ejection or entrainment of air can be reduced if elements to reduce the speed of movement of the material or product are included in the circuit of the supply and exhaust devices; increase the degree of filling of the flow section of the device (pipe) with the product.
Ejection and air entrainment are completely insignificant and even absent if:
- the flow area of the feeder and outlet is completely filled with the product;
-the product comes from a constantly filled container;
- a sealing device (sluice gate, valve, etc.) is installed in the inlet and outlet structures.
If any equipment is periodically filled from another with large one-time portions in a short time, then an air duct must be installed between them for the free flow of displaced air and distribution excess pressure, which arise inside cases and containers at the time of unloading and unloading. The transfer air duct is of large diameter, vertical or strongly inclined, without horizontal sections.
Add up all costs and divide by the volume of the room - normal air exchange for different enterprises is different, but is usually within 1 - 3 exchanges per hour. Higher air exchange rates are used when calculating the total exchange rate supply and exhaust ventilation to remove harmful emissions, impurities, odors from indoor air.
To reduce high vacuum in indoors an influx of outside air should be provided to the equipment being ventilated or into this room.
Reliably transporting air speed for various types dust and bulk materials accepted according to the recommendations of industry guidelines. You can use information from relevant literature, data from similar projects, and parameters of existing aspiration and pneumatic transport installations of the enterprise.
Air speed in pneumatic transport material pipelines:
V = k(10.5 + 0.57·V vit) m/sec, where V vit is the soaring speed of product particles, k is the safety factor, takes into account fluctuations in the load on the pneumatic conveyor. The calculation of a pneumatic conveying installation is discussed on the page. If we assume that the load in the aspiration duct is constant, then the safety factor should be equal to 1. For some materials, airflow and pneumatic transportation are given in the “Calculation of Aspiration” section of the “Drawings, diagrams, site pictures” catalogue.
Select the type of dust separator taking into account the characteristics of the dust, the planned (desired) efficiency of air purification, operational reliability, and design complexity. The throughput capacity of the dust separator is determined by adding up the costs of all aspirated points and adding 5%. If there are points in the network that are temporarily switched off (closed) by valves, add another 100 m³/hour of suction to the total flow rate for each.
The pressure loss (resistance) in the dust separator is taken from its technical characteristics.
Select the installation location of the fan and air cleaner taking into account their dimensions and the dimensions of the shaped parts of the air ducts attached to them. Provide for the possibility of removing dust and waste, compactness of the air duct network, ease of maintenance and repair. Take into account recommendations for their location on the network. For example, the suction filter is installed further from the machine with the greatest resistance in order to create the necessary vacuum in it to backflush the fabric. Before entering the cyclone, especially a battery one, there must be a straight section at least twice the diameter of the air duct. The fan location is preferable after the dust separator along the network, i.e. in purified air.
When planning the route of air ducts, preference should be given to vertical or strongly inclined ones, if they do not violate industrial aesthetics. If possible, reduce the length of horizontal sections and the number of turns (bends). Avoid areas with dusty air on the discharge side of the fan, especially indoors.
Draw a design diagram of the aspiration network. Divide the network into sections:
-from machines to merging points including tee;
- from the point of union to the next tee inclusive;
-from the point of last union to the dust separator (or fan);
- the area between the dust separator and the fan;
-exhaust section with exhaust.
Indicate the air flow and pressure loss in the aspirating equipment on the diagram. Calculate and indicate air flow rates in each area. Indicate the length of each section of ductwork, including the length of all its fittings. Specify the pressure loss (resistance) of the dust separator.
Select the diameters of the air ducts for each section according to the accepted speed v (m/sec) and air flow Q (m³/hour) in the “data table for calculating round steel air ducts”, which is in the reference literature on aspiration. One of the options is given in the “Calculation of Aspiration” section of the “Drawings, Schemes, Site Pictures” catalogue. From the same “table” take dynamic pressure Nd (Pa) and R - pressure loss per 1 meter of length(Pa/m) for this area. Plot this data on a diagram or in a special calculation table. To select diameters and air duct calculationsyou can use special.
As a rule, technological and transport equipment is supplied complete with a suction pipe. The equipment passport provides data on the aspiration mode.
Sizes and configuration of suction pipes recommended input speeds For various materials are given in reference books on aspiration and pneumatic transport.
The cross-sectional area of the inlet of the pipe (confuser, “transition”) is calculated by dividing air flow on input speed.
To reduce the entrainment of product and dust, to prevent explosive concentrations in air ducts, to reduce the dust load on the filter, the input speed is taken to be the minimum possible and depends on the type of dust and the properties of the main product. Open sources of dust emission are aspirated using top or side suction. Optimal angle narrowing of the confuser 45 degrees.
Determine at each site sum of coefficients his local resistance
(fitting parts): suction pipe (confuser), bends, expansion-constrictions, tee, etc. The coefficients of all types of resistance are known and can be easily found in the standard tables.
Calculate the pressure loss when air passes through local resistances: multiplying dynamic pressure on sum of coefficients plot.
Calculate the pressure loss due to air friction along the length of the section: multiplying loss of 1 meter for the whole length plot.
ADD: pressure loss in the suction machine + losses due to local resistance + losses along the length of the section. The resulting SUM of losses for each section should be plotted on the diagram and in the calculation table.
The pressure loss in the areas between the tees is calculated from the point of union (not including the tee) to the next union including the tee.
Pressure equalization.
Take the sequence of sections that create the greatest pressure losses along the path of air movement as the main line.
To the pressure losses of each section of the main line, add the losses of all previous sections of the main line (only the main line) and indicate this amount at the point of combining with the side one.
At each connection point (tees), compare the pressure loss of the main line with the losses in the connected side section. For proper air distribution, these losses must be made equal. The permissible difference is 10%. For large discrepancies, the diameter of the section with less resistance (usually the side) should be reduced, this will increase the speed in it (at the same consumption!), dynamic pressure and all losses. Recalculate the new resistance of the side section and compare it again with the main one at the point of integration. The diameter cannot be reduced below 80 mm.
If it is not possible to equalize the pressure in this way, then take the option with the closest values, and install additional local resistance in the area with lower pressure losses: a diaphragm between the two flanges, but better - an adjusting valve. - according to tables of local resistances or by calculation.
Fan selection.
The performance of the fan is equal to the performance of the dust separator plus air suction in the sealing device of the dust separator. The suction in the suction filters takes 15% of the net flow rate of the network, or according to the norms. The suction in cyclones is taken into account if they are installed on the suction side of the fan: for TsOL, 4BTssh, single-row CC take 150 m³/hour, for double-row CC - 250 m³/hour.
The pressure that the fan must develop is equal to the total network resistance along the main line plus 10% reserve.
The total network resistance is the sum of the pressure losses of the sections main highway only, including: resistance of the first aspirating machine, pressure loss in the air ducts of each section, Ch. lines, resistance of the dust separator, pressure loss in the area between the dust separator and the fan, pressure loss in the exhaust section and exhaust resistance.
Based on pressure and flow rate, from all numbers and types of dust fans, select the one whose aerodynamic characteristics, the intersection of these parameters, gives the point of greatest efficiency. You can choose from catalogs and recommendations of manufacturers and trading organizations of ventilation equipment and equipment.
The rotation speed of the fan impeller is determined by its aerodynamic characteristics. Fan shaft power (kW): Nv. = (QH)/1000 efficiency where Q is the fan performance in m³/sec, i.e. m³/hour must be divided by 3600; H - fan pressure in Pa; efficiency - fan efficiency.
Electric motor power, kW: Ne = (k·Nв)/n·п where n = 0.98 - bearing efficiency; n - transmission efficiency: when the fan impeller is mounted on the electric motor shaft n = 1, when transmitted through a coupling n = 0.98, when V-belt drive n = 0.95. Electric motor power reserve factor k = 1.15 for electric motors with power up to 5 kW; k = 1.1 for electric motors with a power of more than 5 kW. Case Study The selection of a fan for a specific aspiration network is given on the page “Selection and calculation of a fan.”
In this way, it is possible to calculate a ventilation unit for aspiration or pneumatic transport of dusty, fine-grained materials in a low concentration of air mixture at enterprises for storing and processing grain, for cleaning from impurities and enriching cereals, in flour milling and feed milling, in woodworking for removing sawdust and shavings from machine tools, in food, textile industry and others where there are sources of dust emission. Low concentration is considered to be a dust or waste content of no more than 0.01 kg per 1 kg of air. Pressure losses in air ducts with more dust are calculated.
Separate pages are devoted to the aspiration of receiving, storing and cleaning grain: calculation of the aspiration installation of the grain cleaning department, tower or point of the grain receiving enterprise, the aspiration system of the floors of the working building and the silo building of the elevator.
Aspiration systems are used in a wide variety of industries, where the air is polluted with debris, dust and harmful substances. Modern woodworking, food, chemical production It is impossible to imagine without such equipment as an effective, modern and reliable aspiration system.
She is also mandatory element in metalworking, metallurgy, mining. Requirements for the environmental condition of production are constantly increasing, so more and more advanced aspiration systems are required. Without the use of this equipment, it would be impossible not only to be inside the production premises, but also on the street near many industrial enterprises.
Types of systems
Currently, enterprises carry out the calculation and installation of monoblock or modular type aspiration systems.
- Monoblock design. The monoblock system is completely autonomous and mobile. It is installed next to equipment that needs waste collection. The components of a monoblock system are a fan, a filter, and a waste container.
- Modular design. Modular aspiration systems - complex designs, manufactured according to individual order to specific customer requirements. They may include air ducts for aspiration systems, fans low pressure, separators. Such designs can work both within one workshop and be designed for a large plant.
Aspiration systems are also divided into direct-flow and recirculation. The difference is that the former, after capturing dirty air, purify it and release it into the atmosphere, while the latter, after cleaning, return the air back to the workshop.
Before installing aspiration complexes, they are developed, which necessarily includes drawing up a planar diagram based on the required power. With proper calculation, the system can not only clean the workshop of dust and harmful substances, but also return warm and fresh air, thereby reducing heating costs.
Main system components
- Cyclone. Uses centrifugal force to remove solid dust particles from the air. The particles are pressed against the walls, then settle in the discharge hole.
- Roof filters. They consist of a filter block and a receiving chamber. The air is purified and then returned indoors. These nozzles are placed on external bunkers and used instead of outdoor cyclones.
- Dust and chip catchers. They are used in enterprises engaged in wood processing.
- Filtered sleeves. Inside these sleeves, the solid component of the air-dust mass is released, in other words, the air is separated from contaminants.
The use of bag filters is very effective method purification, thanks to which up to 99.9% of particles larger than 1 micron are captured. And due to the use of pulsed filter cleaning, it works as efficiently as possible, which saves energy.
Installation of aspiration units does not require modifications technological processes. Since the cleaning structures are made to order, they adapt to existing technical processes and fit into existing technological equipment used, for example, in woodworking. It is thanks to accurate calculations and reference to specific conditions that high operational efficiency is achieved.
Waste is removed from special bins using containers, bags or pneumatic transport.
Many companies are involved in the development and installation of treatment systems. When choosing a company, carefully study the offers, based not only on advertising materials. Only a detailed conversation with specialists about the characteristics of the equipment can help draw a conclusion about the integrity of the supplier.
System calculation
In order for the aspiration system to work effectively, it is necessary to make its correct calculation. Since this is not an easy matter, this should be done by specialists with extensive experience.
If the calculations are made incorrectly, the system will not work normally, and a lot of money will be spent on rework. Therefore, in order not to risk time and money, it is better to entrust this matter to specialists, for whom designing aspiration and pneumatic transport systems is their main job.
When making calculations, it is necessary to take into account a lot of factors. Let's look at just a few of them.
- We determine the air flow and pressure loss at each aspiration point. All this can be found in the reference literature. After determining all the costs, a calculation is carried out - you need to sum them up and divide them by the volume of the room.
- From the reference literature you need to take information about the air speed in the aspiration system for different materials.
- The type of dust collector is determined. This can be done by having data on the throughput performance of a particular dust collection device. To calculate productivity, you need to add up the air flow at all aspiration points and increase the resulting value by 5 percent.
- Calculate the diameters of the air ducts. This is done using a table taking into account the speed of air movement and its consumption. The diameter is determined individually for each section.
Even this small list of factors indicates the complexity of calculating the aspiration system. There are also more complex indicators, which only a person with specialized knowledge can calculate. higher education and work experience.
Aspiration is simply necessary in conditions modern production. It allows you to meet environmental requirements and preserve the health of your personnel.
When developing the technological part of the project, the issues of aspiration and dust removal must be comprehensively addressed technological equipment ensuring appropriate sanitary standards.
When designing dust collection installations for cleaning waste gases and aspiration air emitted into the atmosphere, it is necessary to take into account the speed of air or gas in the devices; physicochemical characteristics and particle size distribution of dust, initial dust content of gas or air, type of fabric for bag filters, temperature and humidity of dust. The amount of exhaust gases and aspiration air from technological installations is determined by calculation during design.
Thus, for the mill aspiration system:
Q = 3600·S·V m = 3600··V m, (5)
where Q is the amount of air passing through the mill in 1 hour S is the cross-sectional area of the mill; V m is the speed of air movement inside the mill, taking into account suction in the system; D is the diameter of the mill.
Temperature of exhaust gases and aspiration air (not less) - 150ºС. V m = 3.5 – 6.0 m/s. Then:
Dust content of 1 m3 of exhaust gases and aspiration air is 131 g. Permissible dust concentrations in purified gases and air should not exceed 50 mg/m3.
To clean the aspiration air leaving the ball mill, we adopt a two-stage cleaning system:
1. Cyclone TsN-15, purification degree 80-90%:
¾ 1 battery: 262 - 262·0.8 = 52.4 g/m3;
¾ 2nd battery: 52.4 - 52.4·0.8 = 10.48 g/m3;
¾ 3rd battery: 10.48 - 10.48·0.8 = 2.096 g/m3;
¾ 4 battery: 2.096 - 2.096·0.8 = 0.419 g/m3.
2. Electric precipitator Ts-7.5SK, purification degree 85-99%:
0.419 - 0.419·0.99 = 0.00419 g/m3.
Dust settling device. Cyclone TsN-15
Cyclones are designed to clean dusty air from suspended solid particles (dust) and operate at temperatures not exceeding 400°C.
Figure 8 – Group of two cyclones TsN-15
Selecting a dust settling device for product supply:
Q = 3600 · ·V m = 3600 · ·5 = 127170/4 = 31792.5 m 3 /h.
Technological calculation can be made using the formula:
M = Q/q = 31792.5/20000 = 1.59 (accept 2 pieces)
Then the actual equipment load factor over time: K in = 1.59/2 = 0.795.
Table 19 - Technical specifications groups of two cyclones TsN-15
Electrostatic precipitator
The Ts-7.5SK electric precipitator is designed for dust removal of gases and waste from drying drums, as well as for removal of dust from air and gases sucked out of mills.
To remove dust settled on the electrodes located in the electrostatic precipitator, they are shaken using a shaking mechanism. Dust separated from the electrodes enters collecting hoppers and is removed through sluice gates.
The electrostatic precipitator reduces the concentration of dust in the air by 33.35%, while releasing 1.75 grams per cubic meter into the atmosphere. meter.
Table 20 - Technical characteristics of the electrostatic precipitator Ts-7.5SK
| Indicators | Dimensions and parameters |
| Degree of air and gas purification from dust in % | 95 – 98 |
| Maximum gas velocity in m/sec | |
| Temperature of gases at the inlet to the electrostatic precipitator in °C | 60-150 |
| Gas temperature at the outlet of the electrostatic precipitator | No more than 25 °C above their dew point |
| Resistance of the electrostatic precipitator in mm water. Art. | No more than 20 |
| Allowable pressure or vacuum in the electrostatic precipitator in mm of water. Art. | |
| Initial dust content of gas in g/m 3 no more | |
| Active cross-sectional area of the electrostatic precipitator in m3 | 7,5 |
| Number of electrodes in two fields: | |
| precipitative | |
| crowning | |
| Shaking motor: | |
| type | AOL41-6 |
| power in kW | |
| End of table 20 | |
| Indicators | Dimensions and parameters |
| number of revolutions per minute | |
| Sluice gate motor: | |
| type | AO41-6 |
| power in kW | 1,7 |
| number of revolutions per minute | |
| Power heating elements for 8 insulators in kW | 3,36 |
| The electrodes are powered with high voltage current from an electrical unit of the type | AFA-90-200 |
| Rated power of the transformer in kVA | |
| Rated rectified current in ma | |
| Rated rectified voltage in kV | |
| dimensions in mm: | |
| length | |
| width (without shaking mechanism drive) | |
| height (without sluice gate) | |
| Weight in t | 22,7 |
| Manufacturing plant | Pavshinsky Mechanical Plant of the Moscow Regional Economic Council |
Fan
Centrifugal fans high pressure VVD type are designed to move air in supply and exhaust ventilation systems industrial buildings with a total loss of total pressure of up to 500 sec/m2. Fans are manufactured in both right and left rotation and are supplied complete with electric motors.
Introduction
Local exhaust ventilation plays the most active role in the complex of engineering means for normalizing sanitary and hygienic working conditions in production premises. At enterprises associated with the processing of bulk materials, this role is played by aspiration systems (AS), ensuring the localization of dust in places of its formation. Until now, general ventilation has played an auxiliary role - it provided compensation for the air removed by the AS. Research by the Department of MOPE BelGTASM has shown that general ventilation is integral part a complex of dust removal systems (aspiration, systems to combat secondary dust formation - hydraulic flushing or dry vacuum dust collection, general ventilation).
Despite the long history of development, aspiration has received a fundamental scientific and technical basis only in recent decades. This was facilitated by the development of fan manufacturing and the improvement of air purification techniques from dust. The need for aspiration from the rapidly developing metallurgical industries also grew. construction industry. A number of scientific schools have emerged aimed at solving emerging environmental problems. In the field of aspiration, the Ural (Butikov S.E., Gervasyev A.M., Glushkov L.A., Kamyshenko M.T., Olifer V.D., etc.), Krivoy Rog (Afanasyev I.I., Boshnyakov E.N., etc.) became famous . , Neykov O.D., Logachev I.N., Minko V.A., Serenko A.S., Sheleketin A.V. and the American (Hemeon V., Pring R.) schools that created modern fundamentals of design and methodology calculating the localization of dust emissions using aspiration. Developed on their basis technical solutions in the field of design of aspiration systems are enshrined in a number of regulatory and scientific and methodological materials.
These methodological materials summarize the accumulated knowledge in the field of designing aspiration systems and centralized vacuum dust collection (CVA) systems. The use of the latter is expanding especially in production, where hydraulic flushing is unacceptable for technological and construction reasons. The methodological materials intended for the training of environmental engineers complement the course “Industrial Ventilation” and provide for the development of practical skills among senior students of the specialty 05/17/09. These materials are aimed at ensuring that students are able to:
Determine the required performance of local suction pumps and CPU nozzles;
Choose rational and reliable systems pipelines with minimal energy losses;
Determine the required power of the aspiration unit and select the appropriate draft means
And they knew:
Physical basis calculating the performance of local suction stations;
Fundamental difference hydraulic calculation CPU systems and AC air duct networks;
Structural design of shelters for reloading units and CPU nozzles;
Principles for ensuring the reliability of AS and CPU operation;
Principles for selecting a fan and features of its operation for a specific pipeline system.
Guidelines are focused on solving two practical problems: “Calculation and selection of aspiration equipment (practical task No. 1), “Calculation and selection of equipment for a vacuum system for collecting dust and spills (practical task No. 2).”
The testing of these tasks was carried out in the autumn semester of 1994 in practical classes of groups AG-41 and AG-42, to whose students the compilers express gratitude for the inaccuracies and technical errors they identified. Careful study of materials by students Titov V.A., Seroshtan G.N., Eremina G.V. gave us grounds to make changes to the content and edition of the guidelines.
1. Calculation and selection of aspiration equipment
Purpose of the work: determining the required performance of the aspiration unit, serving the system aspiration shelters for loading areas of belt conveyors, selection of an air duct system, dust collector and fan.
The task includes:
A. Calculation of the productivity of local suction (aspiration volumes).
B. Calculation of the dispersed composition and concentration of dust in the aspirated air.
B. Selecting a dust collector.
D. Hydraulic calculation of the aspiration system.
D. Selection of a fan and an electric motor for it.
Initial data
(Numerical values initial values are determined by the number of option N. Values for option N = 25 are indicated in parentheses.
1. Consumption of transported material
G m =143.5 – 4.3N, (G m =36 kg/s)
2. Particle density of bulk material
2700 + 40N, (=3700 kg/m 3).
3. Initial moisture content of the material
4.5 – 0.1 N, (%)
4. Geometric parameters of the transfer chute, (Figure 1):
h 1 =0.5+0.02N, ()
h 3 =1–0.02N,
5. Types of shelters for the loading area of the conveyor belt:
0 – shelters with single walls (for even N),
D – shelters with double walls (for odd N),
Conveyor belt width B, mm;
1200 (for N=1...5); 1000 (for N= 6…10); 800 (for N= 11…15),
650 (for N = 16…20); 500 (for N= 21…26).
Sf – cross-sectional area of the gutter.
Rice. 1. Aspiration of the transfer unit: 1 – upper conveyor; 2 – upper cover; 3 – transfer chute; 4 – lower shelter; 5 – aspiration funnel; 6 – side outer walls; 7 – side internal walls; 8 – hard internal partition; 9 – conveyor belt; 10 – end outer walls; 11 – end inner wall; 12 – lower conveyor
Table 1. Geometric dimensions of the lower shelter, m
| Conveyor belt width B, m |
|||||||
Table 2. Particle size distribution of the transported material
| Faction number j, |
|||||||||
| Size of openings of adjacent sieves, mm |
|||||||||
| Average fraction diameter d j, mm |
* z =100(1 – 0.15).
Table 3. Length of sections of the aspiration network
| Length of aspiration network sections |
|||
| for odd N |
for even N |
||
Rice. 2. Axonometric diagrams of the aspiration system of transfer units: 1 – transfer unit; 2 – aspiration pipes (local suction); 3 – dust collector (cyclone); 4 – fan
2. Calculation of the productivity of local suction
The basis for calculating the required volume of air removed from the shelter is the air balance equation:
The air flow rate entering the shelter through the leaks (Q n; m 3 / s) depends on the area of the leaks (F n, m 2) and the optimal vacuum value in the shelter (P y, Pa):
(2)
where is the density of the surrounding air (at t 0 =20 °C; =1.213 kg/m3).
To cover the loading area of the conveyor, leaks are concentrated in the area of contact of the outer walls with the moving conveyor belt (see Fig. 1):
where: P – perimeter of the shelter in plan, m; L 0 – shelter length, m; b – shelter width, m; – height of the conventional gap in the contact zone, m.
Table 4. The magnitude of the vacuum in the shelter (P y) and the width of the gap ()
| Type of transported material |
Median diameter, mm |
Shelter type "0" |
Shelter type "D" |
||
| Lumpy |
|||||
| Grainy |
|||||
| Powdery |
|||||
Air flow entering the shelter through the chute, m 3 /s
(4)
where S is the cross-sectional area of the gutter, m2; – the flow rate of the reloaded material at the exit from the chute (the final speed of falling particles) is determined sequentially by calculation:
a) speed at the beginning of the chute, m/s (at the end of the first section, see Fig. 1)
, G=9.81 m/s 2 (5)
b) speed at the end of the second section, m/s
(6)
c) speed at the end of the third section, m/s
– coefficient of sliding of components (“ejection coefficient”) u – air speed in the chute, m/s.
The slip coefficient of components depends on the Butakov–Neikov number*
(8)
and Euler's criterion
(9)
where d is the average particle diameter of the material being handled, mm,
(10)
(if it turns out that , should be taken as the calculated average diameter; - the sum of the local resistance coefficients (k.m.c.) of the gutter and shelters
(11)
ζ in – k.m.s, air entry into the upper shelter, related to the dynamic air pressure at the end of the chute.
; (12)
F in – area of leaks in the upper cover, m 2 ;
* Butakov–Neykov and Euler numbers are the essence of the parameters M and N widely used in regulatory and educational materials.
– Ph.D. gutters (=1.5 for vertical gutters, = 90°; =2.5 if available inclined section, i.e. 90°); –k.m.s. rigid partition (for shelter type “D”; in shelter type “0” there is no rigid partition, in this case lane = 0);
Table 5. Values for type “D” shelter
Ψ – particle drag coefficient
(13)
β – volumetric concentration of particles in the gutter, m 3 / m 3
(14)
– the ratio of the particle flow velocity at the beginning of the chute to the final flow velocity.
With the found numbers B u and E u, the slip coefficient of the components is determined for a uniformly accelerated particle flow according to the formula:
(15)
The solution to equation (15)* can be found by the method of successive approximations, assuming as a first approximation
(16)
If it turns out that φ 1 Let's look at the calculation procedure using an example. 1. Based on the given particle size distribution, we construct an integral graph of particle size distribution (using the previously found integral sum m i) and find the median diameter (Fig. 3) d m = 3.4 mm > 3 mm, i.e. we have the case of overloading lumpy material and, therefore, =0.03 m; P y =7 Pa (Table 4). In accordance with formula (10), the average particle diameter 2. Using formula (3), we determine the area of the leaks of the lower shelter (bearing in mind that L 0 = 1.5 m; b = 0.6 m, at B = 0.5 m (see Table 1) F n =2 (1.5 + 0.6) 0.03 = 0.126 m 2 3. Using formula (2), we determine the flow of air entering through the leaks of the shelter There are other formulas for determining the coefficient, including: for a flow of small particles, the speed of which is affected by air resistance. Rice. 3. Integral graph of particle size distribution 4. Using formulas (5)… (7) we find the particle flow rates in the chute: hence n = 4.43 / 5.87 = 0.754. 5. Using formula (11), we determine the amount of k.m.s. gutters taking into account the resistance of shelters. When F in =0.2 m 2, according to formula (12) we have With h/H = 0.12/0.4 = 0.3, according to table 5 we find ζ n ep =6.5; 6. Using formula (14) we find the volumetric concentration of particles in the gutter 7. Using formula (13), we determine the drag coefficient 8. Using formulas (8) and (9), we find the Butakov–Neikov number and the Euler number, respectively: 9. We determine the “ejection” coefficient in accordance with formula (16): And, therefore, you can use formula (17) taking into account (18)… (20): 10. Using formula (4), we determine the air flow entering the lower shelter of the first transfer unit: In order to reduce calculations, let us set the flow rate for the second, third and fourth reloading nodes K 2 =0.9; k 3 =0.8; to 4 =0.7 We enter the calculation results in the first row of the table. 7, assuming that all reloading nodes are equipped with the same shelter, the air flow rate entering through the leaks of the i -th reloading node is Q n i = Q n = 0.278 m 3 /s. We enter the result in the second row of the table. 7, and the amount of expenses Q f i + Q n i – in the third. The amount of expenses , - represents the total productivity of the aspiration installation (air flow entering the dust collector - Q n) and is entered in the eighth column of this line. Calculation of dispersed composition and dust concentration in aspirated air Dust Density The flow rate of air entering the exit through the chute is Q liquid (through leaks for the “O” type shelter – Q Нi = Q H), removed from the shelter – Q ai (see Table 7). Geometric parameters of the shelter (see Fig. 1), m: length – L 0 ; width – b; height – N. Cross-sectional area, m: a) aspiration pipe F in = bc.; b) shelters between the outer walls (for departure type “O”) c) shelters between the inner walls (for shelter type “D”) where b is the distance between the outer walls, m; b 1 – distance between the internal walls, m; H – shelter height, m; с – length of the inlet section of the aspiration pipe, m. In our case, with B = 500 mm, for a shelter with double walls (shelter type “D”) b = 0.6 m; b 1 =0.4 m; C =0.25 m; H =0.4 m; F inx =0.25 0.6 =0.15 m2; F 1 =0.4 0.4 =0.16 m2. Removing the aspiration funnel from the gutter: a) for shelter type “0” L y = L; b) for “D” type shelter L y = L –0.2. In our case, L y =0.6 – 0.2 =0.4 m. Average air speed inside the shelter, m/s: a) for type “D” shelter b) for shelter type “0” =(Q f +0.5Q H)/F 2 . (22) Air entry speed into the aspiration funnel, m/s: Q a /F in (23) Diameter of the largest particle in the aspirated air, microns: Using formula (21) or formula (22), we determine the air speed in the shelter and enter the result in line 4 of the table. 7. Using formula (23), we determine the speed of air entry into the aspiration funnel and enter the result in line 5 of the table. 7. Using formula (24), we determine and enter the result in line 6 of the table. 7. Table 6. Mass content of dust particles, depending on Fraction number j Fraction size, microns Mass fraction jth particles fractions (, %) at , µm Values corresponding to the calculated value (or closest value) we write out from column 6 of table and enter the results (in shares) in lines 11...16 of columns 4...7 of table. 7. You can also use linear interpolation of the table values, but you should keep in mind that the result will be obtained, as a rule, and therefore you need to adjust the maximum value (to ensure ). Determination of dust concentration Material consumption – , kg/s (36), Density of material particles – , kg/m 3 (3700). Initial moisture content of the material –, % (2). The percentage of particles in the reloaded material is smaller - , % (at = 149...137 microns, = 2 + 1.5 = 3.5%. Consumption of dust reloaded with the material - Aspiration volumes – , m 3 /s ( Maximum concentration of dust in the air removed by local suction from the i-th shelter (, g/m 3), Actual dust concentration in the aspirated air where is the correction factor determined by the formula wherein for shelters of type “D”, for shelters of type “O”; in our case (at kg/m3) Or with W=W 0 =2% 1. In accordance with formula (25), we calculate .and enter the results in the 7th line of the summary table. 7 ( specified flow rate divide the dust by the corresponding numerical value of line 3, and enter the results in line 7; for convenience in the note, i.e. in column 8, enter the value). 2. In accordance with formulas (27...29), at the established humidity, we construct a calculated relationship of type (30) to determine the correction factor, the values of which are entered in line 8 of the summary table. 7. Example. Using formula (27), we find the correction factor psi and m/s: If the air dust content turns out to be significant (> 6 g/m3), it is necessary to provide engineering methods to reduce the dust concentration, for example: hydroirrigation of the material being transferred, reducing the speed of air entry into the aspiration funnel, installing settling elements in the shelter or using local suction separators. If by means of hydroirrigation it is possible to increase the humidity to 6%, then we will have: At =3.007, 3. Using formula (26), we determine the actual dust concentration in the first local suction and enter the result in line 9 of the table. 7 (the values of line 7 are multiplied by the corresponding i-th suction - the values of line 8). Determination of the concentration and dispersed composition of dust in front of the dust collector To select a dust collection installation for an aspiration system that serves all local exhausts, it is necessary to find the average parameters of the air in front of the dust collector. To determine them, the obvious balance relations of the laws of conservation of the mass transported through the air ducts of dust are used (assuming that the deposition of dust on the walls of the air ducts is negligible): For the concentration of dust in the air entering the dust collector, we have an obvious relationship: Keeping in mind that dust consumption j-and fractions in the i –th local suction It's obvious that 1. Multiplying in accordance with formula (32) the values of line 9 and line 3 of the table. 7, we find the dust consumption in the i –th suction, and enter its values in line 10. We enter the sum of these expenses in column 8. Rice. 4. Distribution of dust particles by size before entering the dust collector Table 7. Results of calculations of the volumes of aspirated air, dispersed composition and dust concentration in local suction and in front of the dust collector Dimension For the i-th suction Note G/s at W=6% 2. Multiplying the values of line 10 by the corresponding values of lines 11...16, we obtain, in accordance with formula (34), the amount of dust consumption of the j-th fraction in i-th local suction. The values of these quantities are entered on lines 17...22. The row-by-line sum of these values, entered in column 8, represents the consumption of the j-th fraction in front of the dust collector, and the ratio of these sums to the total dust consumption in accordance with formula (35) is the mass fraction of the j-th dust fraction entering the dust collector. The values are entered in column 8 of the table. 7. 3. Based on the distribution of dust particles by size calculated as a result of constructing an integral graph (Fig. 4), we find the size of dust particles, smaller than which the original dust contains 15.9% of the total mass of particles (µm), the median diameter (µm) and dispersion particle size distribution: The most widely used for cleaning aspiration emissions from dust are inertial dry dust collectors - cyclones of the TsN type; inertial wet dust collectors - cyclones - SIOT workers, coagulation wet dust collectors KMP and KTSMP, rotoclones; contact filters – bag and granular. For handling unheated dry bulk materials, as a rule, NIOGAZ cyclones are used with dust concentrations of up to 3 g/m 3 and microns, or bag filters with higher dust concentrations and smaller dust sizes. At enterprises with closed water supply cycles, inertial wet dust collectors are used. Purified air flow – , m 3 /s (1.7), Dust concentration in the air in front of the dust collector – g/m3 (2.68). The dispersed composition of dust in the air in front of the dust collector is (see Table 7). The median diameter of dust particles is , µm (35.0). Dispersion of particle size distribution – (0.64), Density of dust particles – , kg/m 3 (3700). When choosing CN type cyclones as a dust collector, the following parameters are used (Table 8). aspiration conveyor hydraulic duct Table 8. Hydraulic resistance and efficiency of cyclones Parameter µm – diameter of particles captured by 50% in a cyclone with a diameter of m at air speed, dynamic air viscosity Pa s and particle density kg/m 3 M/s – optimal speed air in cross section cyclone Dispersion of partial purification coefficients – The coefficient of local resistance of the cyclone, related to the dynamic air pressure in the cross section of the cyclone, ζ c: for one cyclone for a group of 2 cyclones for a group of 4 cyclones Permissible concentration of dust in the air, emitted into the atmosphere, g/m 3 At m 3 /s (37) At m 3 /s (38) Where the coefficient taking into account the fibrogenic activity of dust is determined depending on the value of the maximum permissible concentration (MAC) of dust in the air working area: MPC mg/m 3 Required degree of air purification from dust, % Estimated degree of air purification from dust, % where is the degree of air purification from dust j-th faction, % (fractional efficiency - taken according to reference data). Disperse composition of many industrial dusts (at 1< <60 мкм) как и пофракционная степень их очистки и инерционных пылеуловителю подчиняется логарифмически нормальному закону распределения, и общая степень очистки определяется по формуле : wherein where is the diameter of particles captured by 50% in a cyclone with a diameter of Dc at an average air speed in its cross section, – dynamic coefficient of air viscosity (at t=20 °C, =18.09–10–6 Pa–s). Integral (41) is not resolved in quadratures, and its values are determined by numerical methods. In table Figure 9 shows the function values found by these methods and borrowed from the monograph. It is not difficult to establish that this is a probability integral, the tabulated values of which are given in many mathematical reference books (see, for example,). We will consider the calculation procedure using a specific make-up artist. 1. Permissible concentration of dust in the air after its purification in accordance with formula (37) with a maximum permissible concentration in the working area of 10 mg/m 3 () 2. The required degree of air purification from dust according to formula (39) is Such cleaning efficiency for our conditions (µm and kg/m3) can be ensured by a group of 4 cyclones TsN-11 3. Let us determine the required cross-sectional area of one cyclone: 4. Determine the estimated diameter of the cyclone: We select the closest of the normalized series of cyclone diameters (300, 400, 500, 600, 800, 900, 1000 mm), namely m. 5. Determine the air speed in the cyclone: 6. Using formula (43), we determine the diameter of particles captured in this cyclone by 50%: 7. Using formula (42), we determine the parameter X: The obtained result, based on the NIOGAZ method, assumes a logarithmically normal distribution of dust particles by size. In fact, the dispersed composition of dust, in the region of large particles (> 60 microns), in the aspirated air for sheltering conveyor loading areas differs from the normal-logarithmic law. Therefore, it is recommended to compare the calculated degree of purification with calculations using formula (40) or with the methodology of the MOPE department (for cyclones), based on a discrete approach to what is fairly fully covered in the course “Mechanics of Aerosols”. An alternative way to determine the reliable value of the overall degree of air purification in dust collectors is to install special experimental research and comparing them with the calculated ones, which we recommend for in-depth study process of air purification from solid particles. 9. The concentration of dust in the air after cleaning is those. less than acceptable.
, (17)
(18)
(20)
.
particles in the chute
(24)
, g/s (103.536=1260).
). Entry speed into the aspiration funnel – , m/s ( 
).
, (25)
(31)
, =2.931 g/m 3 and we use relation (31) as the calculated ratio for.
(36)
.
(39)
, (41)
, (42)
, (43)
, 
, (44)
, (45)
m 2
m
m/c
µm
.
g/m 3,
