Mathematical model of ventilation systems. Mathematical model of the thermal regime of premises with radiant heating. Supply and exhaust centrifugal fans

The paper considers the processes of modeling ventilation and dispersion of its emissions in the atmosphere. Modeling is based on solving the system of Navier-Stokes equations, the laws of conservation of mass, momentum, and heat. Various aspects of the numerical solution of these equations are considered. A system of equations is proposed that makes it possible to calculate the value of the background turbulence coefficient. For the hyposonic approximation, a solution is proposed, together with the equations of hydrogasdynamics given in the article, for the equation of standing of an ideal real gas and steam. This equation is a modification of the van der Waals equation and more accurately takes into account the size of gas or vapor molecules and their interaction. Based on the condition of thermodynamic stability, a relation is obtained that makes it possible to exclude physically unrealizable roots when solving the equation for volume. The analysis of well-known computational models and computational packages of fluid dynamics is carried out.

modeling

ventilation

turbulence

heat and mass transfer equations

equation of state

real gas

dissipation

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2. Belyaev N. N. Modeling the process of dispersion of toxic gas in the conditions of development // Bulletin of DIIT. - 2009. - No. 26 - S. 83-85.

3. Byzova N. L. Experimental studies Atmospheric diffusion and impurity scattering calculations / N. L. Byzova, E. K. Garger, V. N. Ivanov. - L.: Gidrometeoizdat, 1985. - 351 p.

4. Datsyuk T. A. Modeling of dispersion of ventilation emissions. - St. Petersburg: SPbGASU, 2000. - 210 p.

5. Sauts A. V. Application of cognitive graphics algorithms and methods of mathematical analysis to study the thermodynamic properties of R660A isobutane at the saturation line: Grant No. 2С/10: research report (final) / GOUVPO SPbGASU; hands Gorokhov V.L. 30.- No. GR 01201067977.- Inv. No. 02201158567.

Introduction

When designing industrial complexes and unique facilities, issues related to ensuring the quality of the air environment and normalized microclimate parameters should be comprehensively substantiated. Given the high cost of manufacturing, installation and operation of ventilation and air conditioning systems, increased requirements are imposed on the quality of engineering calculations. To select rational design solutions in the field of ventilation, it is necessary to be able to analyze the situation as a whole, i.e. reveal the spatial relationship of dynamic processes occurring indoors and in the atmosphere. Assess the efficiency of ventilation, which depends not only on the amount of air supplied to the room, but also on the adopted air distribution scheme and the concentration of harmful substances in the outside air at the locations of the air intakes.

Purpose of the article- the use of analytical dependencies, with the help of which calculations of the amount of harmful emissions are performed, to determine the size of channels, air ducts, shafts and the choice of air treatment method, etc. In this case, it is advisable to use the Potok software product with the VSV module. To prepare the initial data, it is necessary to have schemes of the designed ventilation systems with indication of the lengths of sections and air flow rates at the end sections. The input data for the calculation are the description of ventilation systems and the requirements for it. Using mathematical modeling, the following questions are solved:

selection of optimal options for supply and removal of air;
distribution of microclimate parameters by the volume of premises;
assessment of the aerodynamic regime of development;
choice of places for air intake and air removal.

The fields of velocity, pressure, temperature, concentrations in the room and atmosphere are formed under the influence of many factors, the totality of which is rather difficult to take into account in engineering calculation methods without the use of a computer.

The application of mathematical modeling in problems of ventilation and aerodynamics is based on the solution of the system of Navier-Stokes equations.

To simulate turbulent flows, it is necessary to solve the system of mass and Reynolds conservation equations (momentum conservation):

(2)

where t- time, X= X i , j , k- spatial coordinates, u=u i , j , k are the components of the velocity vector, R- piezometric pressure, ρ - density, τ ij are the stress tensor components, s m- mass source, s i are the components of the impulse source.

The stress tensor is expressed as:

(3)

where sij- strain rate tensor; δ ij- tensor of additional stresses arising due to the presence of turbulence.

For information about temperature fields T and concentration With harmful substances, the system is supplemented by the following equations:

heat conservation equation

passive impurity conservation equation With

(5)

where CR- coefficient of heat capacity, λ - coefficient of thermal conductivity, k= k i , j , k- turbulence coefficient.

Basic turbulence factor k bases is determined using the system of equations:

(6)

where k f - background turbulence coefficient, k f \u003d 1-15 m 2 / s; ε = 0.1-04;

The turbulence coefficients are determined using the equations:

(7)

In an open area with low dissipation, the value k z is determined by the equation:

kk = k 0 z /z 0 ; (8)

where k 0 - value kk on high z 0 (k 0 \u003d 0.1 m 2 / s at z 0 = 2 m).

In the open area, the wind speed profile is not deformed;

With unknown atmospheric stratification in an open area, the wind speed profile can be determined:

; (9)

where z 0 - given height (height of the weather vane); u 0 - wind speed at height z 0 ; B = 0,15.

Under condition (10), the local Richardson criterion Ri defined as:

(11)

We differentiate equation (9), equate equations (7) and (8), from there we express k bases

(12)

Let us equate equation (12) with the equations of system (6). We substitute (11) and (9) into the resulting equality, in the final form we obtain the system of equations:

(13)

The pulsating term, following the ideas of Boussinesq, is represented as:

(14)

where μ t- turbulent viscosity, and additional terms in the energy transfer equations and impurity components are modeled as follows:

(15)

(16)

The system of equations is closed using one of the turbulence models described below.

For turbulent flows studied in ventilation practice, it is advisable to use either the Boussinesq hypothesis about the smallness of density changes, or the so-called "hyposonic" approximation. The Reynolds stresses are assumed to be proportional to the time-averaged strain rates. The turbulent viscosity coefficient is introduced, this concept is expressed as:

. (17)

The effective viscosity coefficient is calculated as the sum of the molecular and turbulent coefficients:

(18)

The “hyposonic” approximation involves solving, together with the above equations, the equation of standing for an ideal gas:

ρ = p/(RT) (19)

where p - pressure in environment; R is the gas constant.

For more accurate calculations, the impurity density can be determined using the modified van der Waals equation for real gases and vapors

(20)

where are the constants N and M- take into account the association/dissociation of gas or vapor molecules; a- takes into account other interaction; b" - taking into account the size of gas molecules; υ=1/ρ.

Separating from equation (12) the pressure R and differentiating it by volume (taking into account thermodynamic stability), we get the following relation:

. (21)

This approach makes it possible to significantly reduce the calculation time compared to the case of using the full equations for a compressible gas without reducing the accuracy of the results obtained. There is no analytical solution to the above equations. In this regard, numerical methods are used.

To solve ventilation problems associated with the transfer of scalar substances by a turbulent flow, when solving differential equations use the scheme of splitting by physical processes. According to the principles of splitting, finite-difference integration of the equations of hydrodynamics and convective-diffuse transport of a scalar substance at each time step Δ t is carried out in two stages. At the first stage, hydrodynamic parameters are calculated. At the second stage, diffusion equations are solved on the basis of the calculated hydrodynamic fields.

The influence of heat transfer on the formation of the air velocity field is taken into account using the Boussinesq approximation: an additional term is introduced into the equation of motion for the vertical velocity component, which takes into account the buoyancy forces.

Four approaches are known for solving problems of turbulent fluid motion:

direct modeling "DNS" (solution of non-stationary Navier-Stokes equations);
solution of the averaged Reynolds equations "RANS", the system of which, however, is not closed and needs additional closing relations;
large eddy method "LES » , which is based on the solution of non-stationary Navier-Stokes equations with parametrization of subgrid scale vortices;
DES method , which is a combination of two methods: in the zone of separated flows - "LES", and in the area of "smooth" flow - "RANS".

The most attractive from the point of view of the accuracy of the results obtained, undoubtedly, is the method of direct numerical simulation. However, at present, the capabilities of computer technology do not yet allow solving problems with real geometry and numbers. Re, and with resolution of vortices of all sizes. Therefore, when solving a wide range of engineering problems, numerical solutions of the Reynolds equations are used.

At present, certified packages such as STAR-CD, FLUENT or ANSYS/FLOTRAN are successfully used to simulate ventilation problems. With a correctly formulated problem and a rational solution algorithm, the resulting amount of information allows you to choose at the design stage best option, but performing calculations using these programs requires appropriate training, and their incorrect use can lead to erroneous results.

As " base case» you can consider the results of generally recognized balance calculation methods that allow you to compare the integral values characteristic of the problem under consideration.

One of important points when using universal software systems for solving ventilation problems is the choice of a turbulence model. By now it is known a large number of various models turbulence, which are used to close the Reynolds equations. Turbulence models are classified according to the number of parameters for turbulence characteristics, respectively one-parameter, two- and three-parameter.

Most of the semi-empirical models of turbulence, one way or another, use the "hypothesis of the locality of the turbulent transport mechanism", according to which the mechanism of turbulent momentum transfer is completely determined by setting local derivatives of the averaged velocities and physical properties liquids. The influence of processes occurring far from the considered point is not taken into account by this hypothesis.

The simplest are one-parameter models that use the concept of turbulent viscosity "n t”, and the turbulence is assumed to be isotropic. A modified version of the "n t-92" is recommended for modeling jet and separated flows. A good agreement with the experimental results is also given by the one-parameter model "S-A" (Spalart - Almaras), which contains the transport equation for the quantity .

The disadvantage of models with a single transport equation is that they lack information about the distribution of the turbulence scale L. By the amount L transfer processes, methods of formation of turbulence, dissipation of turbulent energy influence. Universal dependency to define L does not exist. Turbulence Scale Equation L often turns out to be exactly the equation that determines the accuracy of the model and, accordingly, the area of its applicability. Basically, the scope of these models is limited to relatively simple shear flows.

In two-parameter models, except for the scale of turbulence L, use as the second parameter the rate of dissipation of turbulent energy . Such models are most often used in modern computational practice and contain the equations of turbulence energy transfer and energy dissipation.

A well-known model includes equations for the transfer of turbulence energy k and the rate of dissipation of turbulent energy ε. Models like " k- e" can be used both for near-wall flows and for more complex separated flows.

Two parameter models are used in low and high Reynolds versions. In the first one, the mechanism of interaction between molecular and turbulent transport near a solid surface is taken into account directly. In the high-Reynolds version, the mechanism of turbulent transport near a solid boundary is described by special near-wall functions that relate the flow parameters to the distance to the wall.

At present, the SSG and Gibson-Launder models, which use the nonlinear relationship between the Reynolds turbulent stress tensor and the average strain rate tensor, are among the most promising. They were developed to improve the prediction of separated flows. Since all tensor components are calculated in them, they require large computer resources compared to two-parameter models.

For complex separated flows, some advantages were revealed by the use of one-parameter models "n t-92", "S-A" in terms of the accuracy of predicting the flow parameters and the count rate in comparison with two-parameter models.

For example, the STAR-CD program provides for the use of models of the type " k- e", Spalarta - Almaras, "SSG", "Gibson-Launder", as well as the method of large eddies "LES", and the method "DES". The last two methods are better suited for calculating the movement of air in conditions of complex geometry, where numerous separated vortex regions will occur, but they require large computational resources.

The calculation results depend significantly on the choice of the computational grid. Currently, special programs are used to build grids. Grid cells can have different shape and dimensions best suited to the solution specific task. The simplest type of grid, when the cells are the same and have a cubic or rectangular shape. Universal computing programs currently used in engineering practice make it possible to work on arbitrary unstructured grids.

To perform calculations of numerical simulation of ventilation problems, it is necessary to set the boundary and initial conditions, i.e. values of dependent variables or their normal gradients at the boundaries of the computational domain.

The task with a sufficient degree of accuracy of the geometric features of the object under study. For these purposes, packages such as SolidWorks, Pro / Engeneer, NX Nastran can be recommended for building three-dimensional models. When constructing a computational grid, the number of cells is chosen so as to obtain a reliable solution with a minimum computation time. One of the semi-empirical turbulence models should be chosen, which is the most efficient for the considered flow.

AT conclusion we add that a good understanding of the qualitative side of the ongoing processes is necessary in order to correctly formulate the boundary conditions of the problem and evaluate the reliability of the results. Modeling of ventilation emissions at the design stage of facilities can be considered as one of the aspects of information modeling aimed at ensuring the environmental safety of the facility.

Reviewers:

Volikov Anatoly Nikolaevich, Doctor of Technical Sciences, Professor of the Department of Heat and Gas Supply and Air Protection, FGBOU VPOU "SPbGASU", St. Petersburg.
Polushkin Vitaly Ivanovich, Doctor of Technical Sciences, Professor, Professor of the Department of Heating, Ventilation and Air Conditioning, FGBOU VPO "SPbGASU", St. Petersburg.

Bibliographic link

Datsyuk T.A., Sauts A.V., Yurmanov B.N., Taurit V.R. MODELING OF VENTILATION PROCESSES // Modern problems of science and education. - 2012. - No. 5.;
URL: http://science-education.ru/ru/article/view?id=6744 (date of access: 10/17/2019). We bring to your attention the journals published by the publishing house "Academy of Natural History"

Forecasting thermal regime in serviced areas is a multifactorial task. It is known that the thermal regime is created with the help of heating, ventilation and air conditioning systems. However, when designing heating systems, the impact of air flows created by other systems is not taken into account. This is partly justified by the fact that the effect of air flows on the thermal regime can be insignificant with the normative air mobility in the serviced areas.

The use of radiant heating systems requires new approaches. This includes the need to comply with human exposure standards at workplaces and taking into account the distribution of radiant heat over the internal surfaces of building envelopes. Indeed, with radiant heating, these surfaces are mainly heated, which, in turn, give off heat to the room by convection and radiation. It is due to this that the required temperature of the internal air is maintained.

As a rule, for most types of premises, along with heating systems, ventilation systems are required. So, when using gas radiant heating systems, the room must be equipped with ventilation systems. The minimum air exchange of premises with the release of harmful gases and vapors is stipulated by SP 60.13330.12. Heating ventilation and air conditioning and is at least once, and at a height of more than 6 m - at least 6 m 3 per 1 m 2 of floor area. In addition, the performance of ventilation systems is also determined by the purpose of the premises and is calculated from the conditions of assimilation of heat or gas emissions or compensation of local exhausts. Naturally, the amount of air exchange must also be checked for the condition of assimilation of combustion products. Compensation of volumes of the removed air is carried out by supply ventilation systems. At the same time, a significant role in the formation of the thermal regime in the serviced areas belongs to the supply jets and the heat introduced by them.

Research method and results

Thus, there is a need to develop an approximate mathematical model of complex processes of heat and mass transfer occurring in a room with radiant heating and ventilation. The mathematical model is a system of air-heat balance equations for the characteristic volumes and surfaces of the room.

The solution of the system allows you to determine the parameters of the air in the serviced areas when various options placement of radiant heating devices, taking into account the influence of ventilation systems.

We will consider the construction of a mathematical model using the example of a production facility equipped with a radiant heating system and not having other sources of heat generation. Heat fluxes from radiators are distributed as follows. Convective flows rise to the upper zone under the ceiling and give off heat to the inner surface. The radiant component of the heat flux of the radiator is perceived by the inner surfaces of the outer enclosing structures of the room. In turn, these surfaces give off heat by convection to the internal air and by radiation to other internal surfaces. Part of the heat is transferred through the external enclosing structures to the outside air. Design scheme heat transfer is shown in fig. 1a.

We will consider the construction of a mathematical model using the example of a production facility equipped with a radiant heating system and not having other sources of heat release. Convective flows rise to the upper zone under the ceiling and give off heat to the inner surface. The radiant component of the heat flux of the radiator is perceived by the inner surfaces of the outer enclosing structures of the room

Next, consider the construction of the air flow circulation scheme (Fig. 1b). Let's accept the scheme of the organization of air exchange "top-up". Air is supplied in quantity M pr in the direction of the serviced area and is removed from the upper zone with a flow rate M in = M etc. At the level of the top of the serviced area, the air flow in the jet is M page The increase in air flow in the supply jet occurs due to the circulating air, which is detached from the jet.

Let us introduce the conditional boundaries of flows - surfaces on which the velocities have only components normal to them. On fig. 1b, the flow boundaries are shown by a dashed line. Then we select the estimated volumes: serviced area (a space with a permanent stay of people); volumes of the supply jet and near-wall convective flows. The direction of near-wall convective flows depends on the ratio of the temperatures of the inner surface of the outer enclosing structures and the ambient air. On fig. 1b shows a diagram with a falling near-wall convective flow.

So, the air temperature in the serviced area t wz is formed as a result of mixing air from supply jets, near-wall convective flows, and convective heat from the internal surfaces of the floor and walls.

Taking into account the developed schemes of heat transfer and circulation of air flows (Fig. 1), we will compose the equations of heat-air balances for the allocated volumes:

Here With— heat capacity of air, J/(kg °C); Q from is the power of the gas radiant heating system, W; Q with and Q* c - convective heat transfer from the inner surfaces of the wall within the serviced area and the wall above the serviced area, W; t page, t c and t wz are the air temperatures in the supply jet at the entrance to the working area, in the near-wall convective flow and in working area, °C; Q tp - heat loss of the room, W, equal to the sum of heat losses through the external enclosing structures:

The air flow in the supply jet at the entrance to the serviced area is calculated using the dependencies obtained by M. I. Grimitlin.

For example, for air diffusers that create compact jets, the flow rate in the jet is:

where m is the velocity damping factor; F 0 - cross-sectional area of the inlet pipe of the air distributor, m 2; x- distance from the air distributor to the place of entry into the serviced area, m; To n is the coefficient of non-isothermality.

The air flow in the near-wall convective flow is determined by:

where t c is the temperature of the inner surface of the outer walls, °C.

Equations heat balance for boundary surfaces have the form:

Here Q c , Q* c , Q pl and Q pt - convective heat transfer from the inner surfaces of the wall within the serviced area - walls above the serviced area, floor and coating, respectively; Q tp.s, Q* tp.s, Q m.p., Q tp.pt - heat losses through the corresponding structures; W With, W* c , W pl, W nm are the radiant heat fluxes from the emitter arriving at these surfaces. Convective heat transfer is determined by the known dependence:

where m J is a coefficient determined taking into account the position of the surface and the direction of the heat flow; F J is the surface area, m 2 ; Δ t J is the temperature difference between the surface and the ambient air, °C; J— surface type index.

Heat loss Q tJ can be expressed as

where t n is the outside air temperature, °C; t J is the temperature of the internal surfaces of the external enclosing structures, °C; R and R n - thermal and heat transfer resistance of the outer fence, m 2 ° C / W.

A mathematical model of heat and mass transfer processes under the combined action of radiant heating and ventilation has been obtained. The results of the solution make it possible to obtain the main characteristics of the thermal regime when designing radiant heating systems for buildings for various purposes equipped with ventilation systems

Radiant heat fluxes from emitters of radiant heating systems wj are calculated in terms of mutual radiation areas according to the method for arbitrary orientation of emitters and surrounding surfaces:

where With 0 is the emissivity of an absolutely black body, W / (m 2 K 4); ε IJ is the reduced degree of emissivity of the surfaces involved in heat exchange I and J; H IJ is the mutual radiation area of the surfaces I and J, m 2 ; T I is the average temperature of the radiating surface, determined from the heat balance of the radiator, K; T J is the temperature of the heat-receiving surface, K.

By substituting expressions for heat flows and air flow rates in jets, we obtain a system of equations that is an approximate mathematical model of heat and mass transfer processes in radiant heating. Standard computer programs can be used to solve the system.

Dear members of the certification committee, I present to your attention the final qualifying work, the purpose of which is the development of an automatic control system for the supply and exhaust ventilation of production shops.

It is known that automation is one of the most important factors in the growth of labor productivity in industrial production, increasing the quality of products and services. The constant expansion of the scope of automation is one of the main features of the industry at this stage. The developed graduation project is one of the ideas of inheriting the developing concept of constructing "intelligent" buildings, that is, objects in which the conditions of human life are controlled by technical means.

The main tasks to be solved in the design are the modernization of the existing air ventilation system at the implementation site - the production workshops of JSC VOMZ to ensure its efficiency (savings in energy and heat consumption, reducing system maintenance costs, reducing downtime), maintaining a comfortable microclimate and cleanliness of air in working areas, operability and stability, reliability of the system in emergency/critical modes.

The problem considered in the graduation project is due to the obsolescence and technical obsolescence (wear and tear) of the existing control system of the PVV. The distributed principle used in the construction of the IPV excludes the possibility of centralized control (launch and monitoring of the state). The absence of a clear system start/stop algorithm also makes the system unreliable due to human errors, and the absence of emergency operation modes makes it unstable in relation to the tasks being solved.

The relevance of the problem of diploma design is due to the general increase in the incidence of respiratory tract and colds of workers, the general decline in labor productivity and the quality of products in this area. The development of a new ACS PVV is directly related to the factory's quality policy (ISO 9000), as well as programs for the modernization of factory equipment and automation of life support systems for workshops.

The central control element of the system is an automation cabinet with a microcontroller and equipment, selected based on the results of marketing research (poster 1). There are many market offers, but the selected equipment is at least as good as its counterparts. An important criterion was the cost, energy consumption and protective performance of the equipment.

The functional diagram of the automation of the PVV is shown in drawing 1. The centralized approach was chosen as the main one in the design of the ACS, which makes it possible to bring the system mobile, if necessary, to implementation according to a mixed approach, which implies the possibility of dispatching and communications with other industrial networks. The centralized approach is highly scalable, flexible enough - all these quality properties are determined by the selected microcontroller - WAGO I / O System, as well as the implementation of the control program.

During the design, automation elements were selected - actuators, sensors, the selection criterion was functionality, stability of operation in critical modes, the range of measurement / control of the parameter, installation features, the form of signal output, operating modes. The main mathematical models are selected and the operation of the air temperature control system with the control of the position of the damper of the three-way valve is simulated. The simulation was carried out in the VisSim environment.

For regulation, the method of "parameter balancing" in the area of controlled values was chosen. Proportional was chosen as the control law, since there are no high requirements for the accuracy and speed of the system, and the ranges of input/output values are small. The controller functions are performed by one of the controller ports in accordance with the control program. The simulation results of this block are presented in poster 2.

The system operation algorithm is shown in drawing 2. The control program implementing this algorithm consists of functional blocks, a block of constants, standard and specialized functions are used. Flexibility and scalability of the system is ensured both programmatically (use of FBs, constants, labels and transitions, compactness of the program in the controller memory) and technically (economical use of input/output ports, redundant ports).

Programmatically provides system actions in emergency modes (overheating, fan failure, hypothermia, filter clogging, fire). Algorithm of the system operation in the mode fire protection is shown in drawing 3. This algorithm takes into account the requirements of the standards for evacuation time and the actions of fire safety equipment in case of fire. In general, the application of this algorithm is effective and proven by tests. The problem of modernization of exhaust hoods in terms of fire safety was also solved. The solutions found were considered and accepted as recommendations.

The reliability of the designed system depends entirely on the reliability of the software and on the controller as a whole. The developed control program was subjected to the process of debugging, manual, structural and functional testing. Only recommended and certified units have been selected to ensure reliability and compliance with the automation equipment warranty. The manufacturer's warranty for the selected automation cabinet, subject to compliance with warranty obligations, is 5 years.

Also, a generalized structure of the system was developed, a clock cyclogram of the system operation was built, a table of connections and cable markings, an ACS installation diagram were formed.

The economic indicators of the project, calculated by me in the organizational and economic part, are shown on poster No. 3. The same poster shows a strip chart of the design process. Criteria according to GOST RISO/IEC 926-93 were used to evaluate the quality of the control program. Grade economic efficiency development was carried out using SWOT analysis. Obviously, the designed system has a low cost (cost structure - poster 3) and fairly fast payback periods (calculated using minimal savings). Thus, we can conclude about the high economic efficiency of the development.

In addition, issues of labor protection, electrical safety and environmental friendliness of the system were resolved. The choice of conductive cables, air duct filters is substantiated.

Thus, as a result of the thesis, a modernization project was developed that is optimal in relation to all the requirements set. This project is recommended for implementation in accordance with the terms of modernization of the plant equipment.

If the cost-effectiveness and quality of the project are confirmed probationary period, it is planned to implement the dispatcher level using local network enterprises, as well as modernization of ventilation of other industrial premises in order to combine them into a single industrial network. Accordingly, these stages include the development of dispatcher software, logging of system status, errors, accidents (DB), organization of an automated workplace or a control post (CCP). It is also possible to work out the weak points of the existing system, such as the modernization of treatment units, as well as the completion of air intake valves with a freezing mechanism.

annotation

The diploma project includes an introduction, 8 chapters, a conclusion, a list of references, applications and is 141 pages of typewritten text with illustrations.

The first section provides an overview and analysis of the need for designing an automatic control system for supply and exhaust ventilation (ACS PVV) of production workshops, a marketing study of automation cabinets. Are being considered typical schemes ventilation and alternative approaches to solving problems of diploma design.

The second section provides a description of the existing PV system at the implementation site - OJSC VOMZ, as technological process. A generalized block diagram of automation for the technological process of air preparation is being formed.

In the third section, an extended technical proposal is formulated for solving the problems of graduation design.

The fourth section is devoted to the development of self-propelled guns. The elements of automation and control are selected, their technical and mathematical descriptions. An algorithm for controlling the supply air temperature is described. A model has been formed and simulation of the operation of the ACS for maintaining the air temperature in the room has been carried out. Selected and justified electrical wiring. A clock cyclogram of the system operation has been constructed.

The fifth section contains specifications programmable logic controller (PLC) WAGO I/O System. The tables of connections of sensors and actuators with PLC ports are given, incl. and virtual.

The sixth section is devoted to the development of functioning algorithms and writing a PLC control program. The choice of the programming environment is substantiated. Block-algorithms for working out by the system are given emergencies, block-algorithms of functional blocks that solve the problems of starting, controlling and regulating. The section includes the results of testing and debugging the PLC control program.

The seventh section deals with the safety and environmental friendliness of the project. Analysis of hazardous and harmful factors during the operation of ACS PVV, a decision is made on labor protection and ensuring the environmental friendliness of the project. Protection of the system from emergency situations is being developed, incl. reinforcement of the system in terms of fire protection and ensuring the stability of operation during emergency situations. The developed principal functional diagram specification automation.

The eighth section is devoted to the organizational and economic justification of the development. The calculation of the cost, efficiency and payback period of design development, incl. considering the stage of implementation. The stages of project development are reflected, the labor intensity of the work is estimated. An assessment of the economic efficiency of the project using a SWOT analysis of the development is given.

In conclusion, conclusions on the graduation project are given.

Introduction

Automation is one of the most important factors in the growth of labor productivity in industrial production. A continuous condition for accelerating the growth rate of automation is the development technical means automation. The technical means of automation include all devices included in the control system and designed to receive information, transmit, store and convert it, as well as to implement control and regulatory actions on the technological control object.

The development of technological means of automation is a complex process, which is based on the interests of automated consumer production, on the one hand, and the economic capabilities of manufacturing enterprises, on the other. The primary incentive for development is to increase the efficiency of production - consumers, through the introduction of new technology can be appropriate only if quick payback costs. Therefore, the criterion for all decisions on the development and implementation of new tools should be the total economic effect, taking into account all the costs of development, production and implementation. Accordingly, for the development, manufacture should be taken, first of all, those options for technical means that provide the maximum total effect.

The constant expansion of the scope of automation is one of the main features of the industry at this stage.

Particular attention is paid to the issues of industrial ecology and labor safety in production. When designing modern technology, equipment and structures, it is necessary to scientifically substantiate the development of safety and harmlessness of work.

On the present stage development of the national economy of the country, one of the main tasks is to increase the efficiency of social production on the basis of the scientific and technological process and to make fuller use of all reserves. This task is inextricably linked with the problem of optimizing design solutions, the purpose of which is to create the necessary prerequisites for increasing the efficiency of capital investments, reducing their payback periods and ensuring the greatest increase in production for each ruble spent. The increase in labor productivity, the production of quality products, the improvement of working and rest conditions for workers are ensured by air ventilation systems that create the necessary microclimate and air quality in the premises.

The purpose of the diploma project is the development of an automatic control system for supply and exhaust ventilation (ACS PVV) of production shops.

The problem considered in the graduation project is due to the wear and tear of the automatic equipment system of the PVV at JSC "Vologda Optical and Mechanical Plant". In addition, the system is designed distributed, which eliminates the possibility of centralized management and monitoring. The site of injection molding (B-category for fire safety), as well as the premises adjacent to it - the site of CNC machines, planning and dispatching office, warehouses, was chosen as the object of implementation.

The tasks of the graduation project are formulated as a result of a study of the current state of the ACS PVV and on the basis of an analytical review, are given in section 3 "Technical proposal".

The use of controlled ventilation opens up new possibilities for solving the above problems. The developed automatic control system should be optimal in terms of performing the designated functions.

As noted above, the relevance of the development is due to both the obsolescence of the existing self-propelled guns, an increase in the number repair work on ventilation "routes", and a general increase in the incidence of respiratory tract and colds of workers, a tendency to feel worse during long work, and, as a result, a general drop in labor productivity and product quality. It is important to note the fact that the existing self-propelled guns are not connected with fire automatics, which is unacceptable for this kind of production. The development of a new ACS PVV is directly related to the factory's quality policy (ISO 9000), as well as programs for the modernization of factory equipment and automation of life support systems for workshops.

The diploma project uses Internet resources (forums, electronic libraries, articles and publications, electronic portals), as well as technical literature of the required subject area and texts of standards (GOST, SNIP, SanPiN). Also, the development of ACS PVV is carried out taking into account the proposals and recommendations of specialists, on the basis of existing installation plans, cable routes, air duct systems.

It is worth noting that the problem raised in the graduation project takes place in almost all old factories of the military-industrial complex, the re-equipment of workshops is one of the most important tasks in terms of ensuring the quality of products for the end consumer. Thus, the diploma design will reflect the accumulated experience in solving similar problems at enterprises with a similar type of production.

1. Analytical review

1.1 General analysis of the need for designing ACS PVV

The most important source of saving fuel and energy resources spent on heat supply of large industrial buildings with significant consumption of heat and electrical energy, is to increase the efficiency of the system of supply and exhaust ventilation (PVV) based on the use of modern achievements in computer and control technology.

Usually, local automation tools are used to control the ventilation system. The main disadvantage of such regulation is that it does not take into account the actual air and heat balance of the building and real weather conditions: outdoor air temperature, wind speed and direction, atmospheric pressure.

Therefore, under the influence of local automation, the air ventilation system, as a rule, does not work in the optimal mode.

The efficiency of the supply and exhaust ventilation system can be significantly increased if the systems are optimally controlled based on the use of a set of appropriate hardware and software tools.

The formation of the thermal regime can be represented as the interaction of disturbing and regulating factors. To determine the control action, information is needed about the properties and number of input and output parameters and the conditions for the heat transfer process to proceed. Since the purpose of controlling ventilation equipment is to ensure the required air conditions in the working area of buildings at minimal energy and material costs, then with the help of a computer it will be possible to find the best option and develop appropriate control actions on this system. As a result, a computer with an appropriate set of hardware and software forms automated system management of the thermal regime of premises of buildings (ACS TRP). At the same time, it should also be noted that under the computer one can understand both the control panel of the EEW, and the control panel for monitoring the state of the EEW, as well as the simplest computer with the simulation program for the ACS of the EEW, processing the results and operational control based on them.

An automatic control system is a combination of a control object (a controlled technological process) and control devices, the interaction of which ensures the automatic flow of the process in accordance with a given program. In this case, the technological process is understood as a sequence of operations that must be performed in order to obtain a finished product from the feedstock. In the case of PVV, the finished product is the air in the serviced room with the specified parameters (temperature, gas composition, etc.), and the raw material is the outdoor and extract air, heat carriers, electricity, etc.

The basis for the functioning of ACS PVV, as well as any control system, should be based on the principle of feedback (OS): the development of control actions based on information about the object obtained using sensors installed or distributed at the object.

Each specific ACS is developed on the basis of a given technology for processing the inlet air flow. Often, the supply and exhaust ventilation system is associated with an air conditioning (preparation) system, which is also reflected in the design of control automation.

When using stand-alone devices or complete process air handling units, ACS are supplied already built into the equipment and already incorporated with certain control functions, which are usually described in detail in the technical documentation. In this case, the adjustment, maintenance and operation of such control systems must be carried out in strict accordance with the specified documentation.

Analysis of technical solutions of modern PVV of leading manufacturers ventilation equipment showed that control functions can be roughly divided into two categories:

Control functions determined by air handling technology and equipment;

Additional functions, which are mostly service functions, are presented as know-how of the companies and are not considered here.

AT general view the main technological functions of the control of the air-handling equipment can be divided into the following groups (Fig. 1.1)

Rice. 1.1 - The main technological functions of the control of the PVV

Let us describe what is meant by the PWV functions shown in Fig. 1.1.

1.1.1 "Monitoring and recording parameters" function

In accordance with SNiP 2.04.05-91 mandatory parameters controls are:

Temperature and pressure in the common supply and return pipelines and at the outlet of each heat exchanger;

The temperature of the air outside, supply air after the heat exchanger, as well as the temperature in the room;

MPC standards for harmful substances in the air exhausted from the room (presence of gases, combustion products, non-toxic dust).

Other parameters in supply and exhaust ventilation systems are controlled as required by the technical specifications for the equipment or according to the operating conditions.

Remote control is provided for measuring the main parameters of the technological process or parameters involved in the implementation of other control functions. Such control is carried out using sensors and measuring transducers with the output (if necessary) of the measured parameters on the indicator or screen of the control device (control panel, computer monitor).

To measure other parameters, local (portable or stationary) instruments are usually used - indicating thermometers, pressure gauges, devices for spectral analysis of air composition, etc.

The use of local control devices does not violate the basic principle of control systems - the principle of feedback. In this case, it is implemented either with the help of a person (operator or maintenance personnel), or with the help of a control program “hardwired” into the microprocessor memory.

1.1.2 Function "operational and program control"

It is also important to implement such an option as "start sequence". To ensure the normal start-up of the PVV system, the following should be taken into account:

Preliminary opening of the air dampers before starting the fans. This is due to the fact that not all dampers in the closed state can withstand the pressure difference created by the fan, and the time for the full opening of the damper by the electric drive reaches two minutes.

Separation of the moments of starting electric motors. Asynchronous motors can often have large starting currents. If the fans, air damper drives and other drives are started at the same time, then due to heavy load on the electrical network building, the voltage will drop sharply, and the electric motors may not start. Therefore, the start of electric motors, especially of high power, must be spread over time.

Preheating the heater. If the water heater is not preheated, the frost protection may be activated at low outdoor temperatures. Therefore, when starting the system, it is necessary to open the supply air dampers, open the three-way valve of the water heater and warm up the heater. As a rule, this function is activated when the outdoor temperature is below 12 °C.

The reverse option is the “shutdown sequence” When shutting down the system, consider:

Stop delay of the supply air fan in units with an electric heater. After removing the voltage from the electric heater, it should be cooled for some time without turning off the supply air fan. Otherwise heating element heater (thermal electric heater- TEN) may fail. For the existing tasks of diploma design, this option is not important due to the use of a water heater, but it is also important to note it.

Thus, on the basis of the selected options for operational and program control, it is possible to present a typical schedule for turning on and off the devices of the air-handling devices.

Rice. 1.2 - Typical cyclogram of ACS PVV operation with a water heater

This whole cycle (Fig. 1.2) the system should work out automatically, and, in addition, an individual start-up of the equipment, which is necessary during adjustment and preventive maintenance, should be provided.

Equally important are the functions of program control, such as changing the winter-summer mode. The implementation of these functions is especially relevant in today's conditions of shortage of energy resources. In regulatory documents, the performance of this function is advisory in nature - "for public, administrative, domestic and industrial buildings, as a rule, programmatic regulation of parameters should be provided for, ensuring a reduction in heat consumption."

In the simplest case, these functions provide for either turning off the air-conditioning system at a certain point in time, or reducing (increasing) the set value of a controlled parameter (for example, temperature) depending on changes in heat loads in the serviced room.

More efficient, but also more difficult to implement, is software control, which provides for automatic change in the structure of the air-conditioning system and the algorithm for its operation not only in the traditional "winter-summer" mode, but also in transitional modes. The analysis and synthesis of the structure of the EWP and the algorithm of its operation is usually carried out on the basis of their thermodynamic model.

In this case, the main motivation and optimization criterion, as a rule, is the desire to ensure, possibly, the minimum energy consumption with restrictions on capital costs, dimensions, etc.

1.1.3 Function "protective functions and interlocks"

Protective functions and interlocks common to automation systems and electrical equipment (protection against short circuits, overheating, movement restrictions, etc.) are stipulated by interdepartmental regulatory documents. Such functions are usually implemented by separate devices (fuses, residual current devices, limit switches, etc.). Their use is regulated by the electrical installation rules (PUE), fire safety rules (PPB).

Frost protection. Function automatic protection from freezing should be provided in areas with an estimated outdoor temperature for the cold period of minus 5 ° C and below. The heat exchangers of the first heating (water heater) and recuperators (if any) are subject to protection.

Usually, the antifreeze protection of heat exchangers is performed on the basis of sensors or sensors-relays of the air temperature downstream of the apparatus and the temperature of the heat carrier in the return pipeline.

The danger of freezing is predicted by the air temperature in front of the apparatus (tн<5 °С). При достижении указанных значений полностью открывают клапаны и останавливают приточный вентилятор.

During non-working hours, for systems with frost protection, the valve must remain slightly open (5-25%) with the outside air damper closed. For greater reliability of protection when the system is turned off, the function of automatic regulation (stabilization) of the water temperature in the return pipeline is sometimes implemented.

1.1.4 Function "protection of technological equipment and electrical equipment"

1. Filter contamination control

Filter clogging control is assessed by the pressure drop across the filter, which is measured by a differential pressure sensor. The sensor measures the difference in air pressure before and after the filter. The permissible pressure drop across the filter is indicated in its passport (for pressure gauges presented on factory air routes, according to the data sheet - 150-300 Pa). This difference is set during system commissioning on the differential sensor (sensor setting). When the setpoint is reached, the sensor sends a signal about the maximum dustiness of the filter and the need for its maintenance or replacement. If the filter is not cleaned or replaced within a certain time (usually 24 hours) after the dust limit signal is issued, it is recommended to provide an emergency shutdown of the system.

Similar sensors are recommended to be installed on fans. If the fan or fan drive belt fails, the system must be shut down in emergency mode. However, such sensors are often neglected for reasons of economy, which greatly complicates system diagnostics and troubleshooting in the future.

2. Other automatic locks

In addition, automatic locks should be provided for:

Opening and closing of outdoor air valves when fans are turned on and off (damper);

Opening and closing valves of ventilation systems connected by air ducts for full or partial interchangeability in case of failure of one of the systems;

Closing valves of ventilation systems for rooms protected by gas fire extinguishing installations when the fans of the ventilation systems of these rooms are turned off;

Ensuring the minimum flow of outdoor air in systems with variable flow, etc.

1.1.5 Control functions

Regulatory functions - automatic maintenance of the set parameters are the main ones by definition for supply and exhaust ventilation systems operating with variable flow, air recirculation, air heating.

These functions are performed using closed control loops, in which the feedback principle is present in an explicit form: information about the object coming from sensors is converted by control devices into control actions. On fig. 1.3 shows an example of a supply air temperature control loop in a ducted air conditioner. The air temperature is maintained by a water heater through which the coolant is passed. The air passing through the heater heats up. The air temperature after the water heater is measured by a sensor (T), then its value is fed to the comparison device (US) of the measured temperature value and the setpoint temperature. Depending on the difference between the setpoint temperature (Tset) and the measured temperature value (Tmeas), the control device (P) generates a signal that acts on the actuator (M - three-way valve electric drive). The actuator opens or closes the three-way valve to a position where the error is:

e \u003d Tust - Tism

will be minimal.

Rice. 1.3 - Supply air temperature control circuit in the air duct with a water heat exchanger: T - sensor; US - comparison device; P - control device; M - executive device

Thus, the construction of an automatic control system (ACS) based on the requirements for accuracy and other parameters of its operation (stability, oscillation, etc.) is reduced to the choice of its structure and elements, as well as to the determination of the controller parameters. Usually, this is done by automation specialists using classical control theory. I will only note that the controller settings are determined by the dynamic properties of the control object and the chosen control law. The regulation law is the relationship between the input (?) and output (Ur) signals of the regulator.

The simplest is the proportional law of regulation, in which? and Ur are interconnected by a constant coefficient Kp. This coefficient is the setting parameter of such a controller, which is called the P-regulator. Its implementation requires the use of an adjustable amplifying element (mechanical, pneumatic, electrical, etc.), which can function both with and without an additional energy source.

One of the varieties of P-controllers are positional controllers that implement a proportional control law at Kp and form an output signal Ur having a certain number of constant values, for example, two or three, corresponding to two- or three-position controllers. Such controllers are sometimes called relay controllers due to the similarity of their graphical characteristics with those of a relay. The setting parameter of such regulators is the value of the dead zone De.

In the technology of automation of ventilation systems, on-off controllers, due to their simplicity and reliability, have found wide application in controlling temperature (thermostats), pressure (pressure switches) and other parameters of the process state.

Two-position regulators are also used in systems of automatic protection, blocking and switching of equipment operation modes. In this case, their functions are performed by sensors-relays.

Despite these advantages of P-regulators, they have a large static error (for small values of Kp) and a tendency to self-oscillate (for large values of Kp). Therefore, with higher requirements for the regulatory functions of automation systems in terms of accuracy and stability, more complex control laws are also used, for example, PI and PID laws.

Also, the regulation of the air heating temperature can be performed by a P-regulator, which works according to the principle of balancing: increase the temperature when its value is less than the set value, and vice versa. This interpretation of the law has also found application in systems that do not require high accuracy.

1.2 Analysis of existing typical schemes for automatic ventilation of production shops

There are a number of standard implementations of the automation of the supply and exhaust ventilation system, each of which has a number of advantages and disadvantages. I note that despite the presence of many standard schemes and developments, it is very difficult to create such an ACS that would be flexible in terms of settings relative to the production where it is being implemented. Thus, for the design of ACS for air and gas supply, a thorough analysis of the existing ventilation structure, an analysis of the technological processes of the production cycle, as well as an analysis of the requirements for labor protection, ecology, electrical and fire safety are required. Moreover, often designed ACS PVV is specialized in relation to its field of application.

In any case, the following groups are usually considered as typical initial data at the initial design stage:

1. General data: territorial location of the object (city, district); type and purpose of the object.

2. Information about the building and premises: plans and sections indicating all dimensions and elevations relative to ground level; indication of the categories of premises (on architectural plans) in accordance with fire safety standards; availability of technical areas with indication of their sizes; location and characteristics of existing ventilation systems; characteristics of energy carriers;

3. Information about the technological process: drawings of the technological project (plans) indicating the placement of technological equipment; specification of equipment with indication of installed capacities; characteristics of the technological regime -- the number of work shifts, the average number of workers per shift; equipment operation mode (simultaneity of operation, load factors, etc.); the amount of harmful emissions into the air (MAC of harmful substances).

As the initial data for calculating the automation of the PVV system, they take out:

The performance of the existing system (power, air exchange);

List of air parameters to be regulated;

Limits of regulation;

The operation of automation when receiving signals from other systems.

Thus, the execution of the automation system is designed based on the tasks assigned to it, taking into account the norms and rules, as well as general initial data and schemes. Drawing up the scheme and selection of equipment for the ventilation automation system is carried out individually.

Let us present the existing standard schemes of supply and exhaust ventilation control systems, we will characterize some of them regarding the possibility of using them to solve the problems of the graduation project (Fig. 1.4 - 1.5, 1.9).

Rice. 1.4 - ACS direct-flow ventilation

These automation systems have found active use in factories, factories, office buildings. The object of control here is the automation cabinet (control panel), the fixing devices are channel sensors, the control action is on the motors of the fan motors, damper motors. There is also a heating/cooling ATS. Looking ahead, it can be noted that the system shown in Fig. 1.4a is a prototype of the system that must be used in the injection molding section of OAO Vologda Optical and Mechanical Plant. Air cooling in industrial premises is ineffective due to the volume of these premises, and heating is a prerequisite for the correct functioning of the automatic control system of the air-handling equipment.

Rice. 1.5- ACS ventilation with heat exchangers

The construction of an automatic control system for PVV using heat recovery units (recuperators) allows solving the problems of excessive consumption of electricity (for electric heaters), the problems of emissions into the environment. The meaning of recuperation is that the air that is irrevocably removed from the room, having a temperature set in the room, exchanges energy with the incoming outside air, the parameters of which, as a rule, differ significantly from those set. Those. in winter, the warm extract air that is removed partially heats the outdoor supply air, while in summer the cooler extract air partially cools the supply air. In the best case, recuperation can reduce the energy consumption for supply air treatment by 80%.

Technically, recovery in supply and exhaust ventilation is carried out using rotating heat exchangers and systems with an intermediate heat carrier. Thus, we get a gain both in heating the air and in reducing the opening of the dampers (more idle time of the motors controlling the dampers is allowed) - all this gives an overall gain in terms of saving electricity.

Heat recovery systems are promising and active and are being introduced to replace older ventilation systems. However, it is worth noting that such systems cost additional capital investments, however, their payback period is relatively short, while the profitability is very high. Also, the absence of a constant release into the environment increases the environmental performance of such an organization of automatic equipment. Simplified operation of the system with heat recovery from air (air recirculation) is shown in Fig. 1.6.

Rice. 1.6 - Operation of the air exchange system with recirculation (recuperation)

Cross-flow or plate heat exchangers (Fig. 1.5 c, d) consist of plates (aluminum), representing a system of channels for the flow of two air streams. The duct walls are common for supply and exhaust air and are easy to transfer. Due to the large exchange surface area and the turbulent air flow in the channels, a high degree of heat recovery (heat transfer) is achieved with a relatively low hydraulic resistance. The efficiency of plate heat exchangers reaches 70%.

Rice. 1.7 - Organization of air exchange of ACS PVV based on plate heat exchangers

Only the sensible heat of the extract air is utilized, since The supply and exhaust air do not mix in any way, and the condensate formed during the cooling of the exhaust air is retained by the separator and removed by the drainage system from the drain pan. To prevent freezing of condensate at low temperatures (up to -15°C), the corresponding requirements for automation are formed: it must ensure periodic shutdown of the supply fan or the removal of part of the outside air into the bypass channel bypassing the heat exchanger channels. The only limitation in the application of this method is the mandatory crossing of the supply and exhaust branches in one place, which in the case of a simple modernization of the ACS imposes a number of difficulties.

Recuperation systems with an intermediate coolant (Fig. 1.5 a, b) are a pair of heat exchangers connected by a closed pipeline. One heat exchanger is located in the exhaust duct, and the other in the supply duct. A non-freezing glycol mixture circulates in a closed circuit, transferring heat from one heat exchanger to another, and in this case, the distance from the air handling unit to the exhaust unit can be very significant.

The efficiency of heat recovery with this method does not exceed 60%. The cost is relatively high, but in some cases this may be the only option for heat recovery.

Rice. 1.8 - The principle of heat recovery using an intermediate heat carrier

Rotary heat exchanger (rotating heat exchanger, recuperator) - is a rotor with channels for horizontal air passage. Part of the rotor is located in the exhaust duct, and part is in the supply duct. Rotating, the rotor receives heat from the exhaust air and transfers it to the supply air, and both sensible and latent heat, as well as humidity, are transferred. The efficiency of heat recovery is maximum and reaches 80%.

Rice. 1.9 - ACS PVV with a rotary heat exchanger

The limitation on the use of this method is imposed primarily by the fact that up to 10% of the exhaust air is mixed with the supply air, and in some cases this is unacceptable or undesirable (if the air has a significant level of pollution). The design requirements are similar to the previous version - the exhaust and supply machines are located in the same place. This method is more expensive than the first and is rarely used.

In general, systems with recovery are 40-60% more expensive than similar systems without recovery, however, operating costs will differ significantly. Even at today's energy prices, the payback period for a recovery system does not exceed two heating seasons.

I would like to note that energy saving is also affected by control algorithms. However, it should always be taken into account that all ventilation systems are designed for some average conditions. For example, the outdoor air flow rate was determined for one number of people, but in reality the room may be less than 20% of the accepted value, of course, in this case, the calculated outdoor air flow rate will be clearly excessive, the operation of ventilation in excess mode will lead to an unreasonable loss of energy resources. It is logical in this case to consider several operating modes, for example, winter / summer. If automation is able to set such modes, the savings are obvious. Another approach is related to the regulation of the outdoor air flow depending on the quality of the gas environment inside the room, i.e. the automation system includes gas analyzers for harmful gases and selects the value of the outdoor air flow so that the content of harmful gases does not exceed the maximum permissible values.

1.3 Marketing research

Currently, all the world's leading manufacturers of ventilation equipment are widely represented on the market of automation for supply and exhaust ventilation, and each of them specializes in the production of equipment in a particular segment. The entire market of ventilation equipment can be divided into the following areas of application:

Household and semi-industrial purposes;

Industrial purpose;

Ventilation equipment for "special" purposes.

Since the graduation project considers the design of automation for the supply and exhaust systems of industrial premises, in order to compare the proposed development with those available on the market, it is necessary to select similar existing automation packages from well-known manufacturers.

The results of the marketing research of the existing ACS PVV packages are presented in Appendix A.

Thus, as a result of the marketing research, several of the most commonly used self-propelled guns from different manufacturers were considered, by studying their technical documentation, the following information was obtained:

The composition of the corresponding ACS PVV package;

The brand of the programmable logic controller and its equipment (software, command system, programming principles);

Availability of connections with other systems (is communication with fire automatics provided, is there support for local area network protocols);

Protective design (electrical safety, fire safety, dust protection, noise immunity, moisture protection).

2. Description of the ventilation network of the production workshop as an object of automatic control

In general, based on the results of the analysis of existing approaches to the automation of ventilation and air preparation systems, as well as the result of analytical reviews of typical schemes, it can be concluded that the tasks considered in the graduation project are relevant at the present time, actively considered and studied by specialized design bureaus (SKB).

I note that there are three main approaches to the implementation of automation for the ventilation system:

Distributed approach: the implementation of the automation of the PVV on the basis of local switching equipment, each fan is controlled by the corresponding device.

This approach is used to design the automation of relatively small ventilation systems in which no further expansion is foreseen. He is the oldest. The advantages of the approach include, for example, the fact that in the event of an accident on one of the controlled ventilation branches, the system makes an emergency stop of only this link/section. In addition, this approach is relatively simple to implement, does not require complex control algorithms, and simplifies the maintenance of ventilation system devices.

Centralized approach: the implementation of the automatic ventilation system based on a group of logic controllers or a programmable logic controller (PLC), the entire ventilation system is controlled centrally in accordance with the programmed data and the program.

The centralized approach is more reliable than the distributed approach. All management of VVV is rigid, carried out on the basis of the program. This circumstance imposes additional requirements both on writing the program code (it is necessary to take into account many conditions, including actions in emergency situations), and on the special protection of the control PLC. This approach has found application for small administrative and industrial complexes. It is distinguished by the flexibility of settings, the ability to scale the system to reasonable limits, as well as the possibility of mobile integration of the system according to a mixed organization principle;

Mixed approach: used in the design of large systems (a large number of controlled equipment with huge performance), is a combination of a distributed and centralized approach. In the general case, this approach assumes a level hierarchy headed by a control computer and slave "microcomputers", thus forming a global control production network in relation to the enterprise. In other words, this approach is a distributed-centralized approach with system dispatch.

In terms of the task to be solved in graduation design, the most preferable is a centralized approach to the implementation of the automation of the PVV. Since the system is being developed for small industrial premises, it is possible to use this approach for other objects with the aim of their subsequent integration into a single ACS of the IPV.

Often, ventilation control cabinets are provided with an interface that allows monitoring the state of the ventilation system with information displayed on a computer monitor. However, it is worth noting that this implementation requires additional complications of the control program, training of a specialist who monitors the state and makes operational decisions based on visually obtained data from the sensor survey. In addition, there is always a factor of human error in emergency situations. Therefore, the implementation of this condition is rather an additional option to the design of the PVV automation package.

2.1 Description of the existing automatic control system for supply and exhaust ventilation of production shops

To ensure the basic principle of ventilation of production shops, which consists in maintaining the parameters and composition of air within acceptable limits, it is necessary to supply clean air to the places where workers are located, followed by distribution of air throughout the room.

Below in fig. 2.1 shows an illustration of a typical supply and exhaust ventilation system, similar to which is available at the implementation site.

The ventilation system of the industrial premises consists of fans, air ducts, outside air intakes, devices for cleaning the air entering and emitted into the atmosphere, and an air heating device (water heater).

The design of the existing supply and exhaust ventilation systems was carried out in accordance with the requirements of SNiP II 33-75 “Heating, ventilation and air conditioning”, as well as GOST 12.4.021-75 “SSBT. Ventilation systems. General requirements”, which specifies the requirements for installation, commissioning and operation.

Purification of polluted air emitted into the atmosphere is carried out by special devices - dust separators (used at the injection molding production site), air duct filters, etc. It should be taken into account that dust separators do not require additional control and are triggered when exhaust ventilation is turned on.

Also, cleaning of the air extracted from the working area can be carried out in dust settling chambers (only for coarse dust) and electrostatic precipitators (for fine dust). Air purification from harmful gases is carried out using special absorbent and decontaminating substances, including those applied to filters (in filter cells).

Rice. 2.1 - Supply and exhaust ventilation system of the production workshop 1 - air intake device; 2 - heaters for heating; 3- supply fan; 4 - main air duct; 5 - branches of the duct; 6 - supply nozzles; 7 - local suction; 8 and 9 - master. exhaust air duct; 10 - dust separator; 11 - exhaust fan; 12 - shaft for ejection of purified air into the atmosphere

The automation of the existing system is relatively simple. The technological process of ventilation is as follows:

1. the beginning of the work shift - the system of supply and exhaust ventilation is started. The fans are driven by a centralized starter. In other words, the control panel consists of two starters - for start and emergency stop / shutdown. The shift lasts 8 hours - with an hour break, that is, the system is idle on average 1 hour during working hours. In addition, such a “blocking” of control is economically inefficient, as it leads to an over-expenditure of electricity.

It should be noted that there is no production need for exhaust ventilation to work constantly, it is advisable to turn it on when the air is polluted, or, for example, it is required to remove excess heat energy from the working area.

2. The opening of the dampers of the air intake devices is also controlled by the local starting equipment, the air with the parameters of the external environment (temperature, purity) is drawn into the air ducts by the supply fan due to the difference in pressure.

3. The air taken from the external environment passes through the water heater, heats up to acceptable temperature values, and is blown into the room through the air ducts through the supply nozzles. The water heater provides significant heating of the air, the control of the heater is manual, the electrician opens the damper damper. For the summer period, the heater is turned off. Hot water supplied from the internal boiler house is used as a heat carrier. There is no automatic air temperature control system, as a result of which there is a large overrun of the resource.

Supply and exhaust centrifugal fans

A conventional centrifugal fan is a wheel with working blades located in a spiral casing, during the rotation of which the air entering through the inlet enters the channels between the blades and moves through these channels under the action of centrifugal force, is collected by the spiral casing and directed to its outlet. The casing also serves to convert dynamic head to static head. To increase the pressure, a diffuser is placed behind the casing. On fig. 4.1 shows a general view of a centrifugal fan.

A conventional centrifugal wheel consists of blades, a rear disc, a hub, and a front disc. A cast or turned hub, designed to fit the wheel on the shaft, is riveted, screwed or welded to the rear disc. The blades are riveted to the disk. The leading edges of the blades are usually attached to the front ring.

Spiral casings are made of sheet steel and installed on independent supports; for low-power fans, they are attached to the beds.

When the wheel rotates, part of the energy supplied to the engine is transferred to the air. The pressure developed by the wheel depends on the density of the air, the geometric shape of the blades and the circumferential speed at the ends of the blades.

The exit edges of the blades of centrifugal fans can be bent forward, radial and bent back. Until recently, the edges of the blades were mainly bent forward, as this made it possible to reduce the overall dimensions of the fans. Nowadays, impellers with backward curved blades are often found, because this allows to increase the efficiency. fan.

Rice. 4.1

When inspecting fans, it should be borne in mind that the outlet (in the direction of the air) blade edges should always be bent in the direction opposite to the direction of rotation of the impeller to ensure impact-free entry.

The same fans, when changing the rotational speed, can have a different supply and develop different pressures, depending not only on the properties of the fan and the rotational speed, but also on the air ducts connected to them.

Fan characteristics express the relationship between the main parameters of its operation. The complete characteristic of the fan at a constant shaft speed (n = const) is expressed by the dependencies between supply Q and pressure P, power N and efficiency. Dependencies P (Q), N (Q) and T (Q) are usually built on one chart. They select a fan. The characteristic is built on the basis of tests. On fig. 4.2 shows the aerodynamic characteristics of the centrifugal fan VTS-4-76-16, which is used as a supply fan at the implementation site

Rice. 4.2

The fan capacity is 70,000 m3/h or 19.4 m3/s. Fan shaft speed - 720 rpm. or 75.36 rad/sec., the power of the drive asynchronous fan motor is 35 kW.

The fan blows outside atmospheric air into the heater. As a result of heat exchange of air with hot water passed through the tubes of the heat exchanger, the passing air is heated.

Consider the scheme for regulating the operation mode of the fan VTS-4-76 No. 16. On fig. 4.3 shows a functional diagram of the fan unit with speed control.

Rice. 4.3

The transfer function of the fan can be represented as a gain, which is determined based on the aerodynamic characteristics of the fan (Fig. 4.2). The fan amplification factor at the operating point is 1.819 m3/s (minimum possible, experimentally established).

Rice. 4.4

experimental It has been established that in order to implement the required fan operation modes, it is necessary to supply the following voltage values to the control frequency converter (Table 4.1):

Table 4.1 Supply ventilation operating modes

At the same time, in order to increase the reliability of the electric motor of the fans of both the supply and exhaust sections, there is no need to set their operating modes with maximum performance. The task of the experimental study was to find such control voltages at which the norms of the air exchange rate calculated below would be observed.

Exhaust ventilation is represented by three centrifugal fans VC-4-76-12 (capacity 28,000 m3/h at n=350 rpm, asynchronous drive power N=19.5 kW) and VC-4-76-10 (capacity 20,000 m3 /h at n=270 rpm, asynchronous drive power N=12.5 kW). Similarly to the supply for the exhaust branch of ventilation, the values of the control voltages were experimentally obtained (Table 4.2).

To prevent the state of "oxygen starvation" in the working shops, we calculate the air exchange rates for the selected fan operation modes. It must satisfy the condition:

Table 4.2 Operating modes of exhaust ventilation

In the calculation, we neglect the supply air coming from outside, as well as the architecture of the building (walls, ceilings).

The dimensions of the rooms for ventilation: 150x40x10 m, the total volume of the room is Vroom? 60,000 m3. The required volume of supply air is 66,000 m3 / h (for a coefficient of 1.1, it was chosen as the minimum, since the air inflow from the outside is not taken into account). It is obvious that the selected operating modes of the supply fan satisfy the set condition.

The total volume of exhaust air is calculated using the following formula

To calculate the exhaust branch, the modes of "emergency extraction" are selected. Taking into account the correction factor of 1.1 (since the emergency operation mode is taken as the least possible), the volume of exhaust air will be equal to 67.76 m3 / h. This value satisfies condition (4.2) within the limits of permissible errors and previously accepted reservations, which means that the selected fan operation modes will cope with the task of ensuring the air exchange rate.

Also in the electric motors of the fans there is a built-in protection against overheating (thermostat). When the motor temperature rises, the thermostat relay contact will stop the motor. The differential pressure sensor will record the stop of the electric motor and give a signal to the control panel. It is necessary to provide for the response of the ACS of the PVV to an emergency stop of the fan motors.

Mathematical model of ventilation systems. Mathematical model of the thermal regime of premises with radiant heating. Supply and exhaust centrifugal fans

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