Description:

Trends modern construction residential buildings, such as increasing the number of storeys, sealing windows, increasing the area of ​​apartments, pose difficult tasks for designers: architects and specialists in the field of heating and ventilation to ensure the required indoor climate. Air mode modern buildings, which determines the process of air exchange between rooms with each other, rooms with outside air, is formed under the influence of many factors.

Air mode of residential buildings

Consideration of the influence of the air mode on the operation of the ventilation system of residential buildings

Technology system mini preparation stations drinking water low productivity

On each floor of the section there are two two-room apartments and one one-room and three-room apartments. One-room and one two-room apartments have one-sided orientation. The windows of the second two-room and three-room apartments face two opposite sides. The total area of ​​a one-room apartment is 37.8 m 2, a one-sided two-room apartment - 51 m 2, a two-sided two-room apartment - 60 m 2, a three-room apartment - 75.8 m 2. The building is equipped with dense windows with air permeability resistance of 1 m 2 h / kg at a pressure difference D P o = 10 Pa. To ensure the flow of air in the walls of the rooms and in the kitchen of a one-room apartment, AERECO supply valves are installed. In fig. 3 shows aerodynamic characteristics valve at full open position and covered by 1/3 state.

The entrance doors to the apartments are also quite dense: with an air permeability resistance of 0.7 m 2 h / kg at a pressure difference D P o = 10 Pa.

The residential building is served by systems natural ventilation with two-way connection of satellites to the shaft and non-adjustable exhaust grilles. In all apartments (regardless of their size), the same ventilation systems are installed, since in the building under consideration, even in three-room apartments, air exchange is determined not by the rate of inflow (3 m 3 / h per m 2 of living space), but by the rate of exhaust from the kitchen, bathroom and toilet (total 110 m 3 / h).

Calculations air mode the buildings were made taking into account the following parameters:

Outside air temperature 5 ° C - design temperature for the ventilation system;

3.1 ° C - average temperature heating season in Moscow;

10.2 ° C - the average temperature of the coldest month in Moscow;

28 ° C - design temperature for the heating system with a wind speed of 0 m / s;

3.8 m / s - average wind speed for the heating period;

4.9 m / s - design wind speed for choosing the density of windows in different directions.

Outside air pressure

The pressure in the outside air is made up of the gravitational pressure (the first term in formula (1)) and the wind pressure (the second term).

The wind pressure is higher on tall buildings, which is taken into account in the calculation by the coefficient k dyn, which depends on the openness of the area (open space, low or high buildings) and the height of the building itself. For houses up to 12 floors, it is customary to consider k dyn as constant in height, and for higher structures, an increase in the value of k dyn along the height of the building takes into account the increase in wind speed with distance from the ground.

The value of the wind pressure of the windward facade is influenced by the aerodynamic coefficients of not only the windward, but also the leeward facades. This situation is explained by the fact that the absolute pressure at the leeward side of the building at the level of the air-permeable element farthest from the earth's surface through which air movement is possible (the mouth of the exhaust shaft on the leeward facade) is taken as the conditional zero pressure, P conv,:

R conv = R atm - r n g N + r n v 2 s s k dyn / 2, (2)

where c z is the aerodynamic coefficient corresponding to the leeward side of the building;

H is the height above the ground of the upper element through which air movement is possible, m.

The total overpressure formed in the outside air at a point at the height h of the building is determined by the difference between the total pressure in the outside air at this point and the total conditional pressure P conv:

R n = (R atm - r n g h + r n v 2 s s k dyn / 2) - (R atm - r n g N +

R n v 2 s s k din / 2) = r n g (N - h) + r n v 2 (s - s s) k din / 2, (3)

where c is the aerodynamic coefficient on the design facade, taken by.

The gravitational part of the pressure increases with an increase in the temperature difference between the indoor and outdoor air, on which the air density depends. For residential buildings with a practically constant temperature of the indoor air during the entire heating period, the gravitational pressure increases with a decrease in the outdoor temperature. The dependence of the gravitational pressure in the outside air on the density of the inside air is explained by the tradition of referring the internal gravitational excess (above atmospheric) pressure to the external pressure with a minus sign. This, as it were, takes out the variable gravitational component of the total pressure in the internal air outside the building, and therefore the total pressure in each room becomes constant at any height of this room. In this regard, P int in is called conditionally constant air pressure in the building. Then the total pressure in the outside air becomes equal to

Р ext = (H - h) (r ext - r int) g + r ext v 2 (c - c h) k dyn / 2. (4)

In fig. 4 shows the change in pressure along the height of the building on different facades under different weather conditions. For simplicity of presentation, we will call one facade of the house northern (upper in the plan), and the other southern (lower in the plan).

Internal air pressure

Different outdoor air pressures along the height of the building and on different facades will cause air movement, and in each room with number i, its own total excess pressures P in, i will form. After the variable part of these pressures - the gravitational one - is referred to the external pressure, a point characterized by the total excess pressure P in, i, into which air enters and leaves, can serve as a model of any room.

For brevity, in what follows, the total excess external and internal pressure will be called external and internal pressures, respectively.

With the complete formulation of the problem of the air regime of a building, the basis of the mathematical model is the equations of the material balance of air for all rooms, as well as nodes in ventilation systems and the equations of energy conservation (Bernoulli's equation) for each air-permeable element. Air balances take into account the air flow through each air-permeable element in a room or unit of a ventilation system. Bernoulli's equation equates the pressure difference on different sides of the air-permeable element D P i, j to the aerodynamic losses arising when the air flow passes through the air-permeable element Z i, j.

Consequently, the model of the air regime of a multi-storey building can be represented as a set of points connected to each other, characterized by internal P in, i and external P n, j pressures, between which there is air movement.

The total pressure loss Z i, j during air movement is usually expressed in terms of the air permeability resistance characteristic S i, j element between points i and j. All air-permeable elements of the building envelope - windows, doors, open openings - can be conditionally attributed to elements with constant hydraulic parameters. The S i, j values ​​for this group of resistances do not depend on the costs G i, j. Distinctive feature path of the ventilation system is the variability of the resistance characteristics of fittings, depending on the desired air flow rate for individual parts of the system. Therefore, the characteristics of the resistance of the elements of the ventilation duct have to be determined in an iterative process, in which it is necessary to link the available pressures in the network with the aerodynamic resistance of the duct at certain air flow rates.

In this case, the densities of the air moving along the ventilation network in the branches are taken according to the temperatures of the internal air in the corresponding rooms, and along the main sections of the trunk - according to the temperature of the air mixture in the unit.

Thus, the solution of the problem of the air regime of the building is reduced to the solution of the system of equations of air balances, where in each case the sum is taken over all air-permeable elements of the room. The number of equations is equal to the number of rooms in the building and the number of nodes in ventilation systems. Unknown in this system of equations are the pressures in each room and each node of the ventilation systems P in, i. Since the pressure differences and air flow rates through the air-permeable elements are related to each other, the solution is found using an iterative process, in which the flow rates are first set, and as the pressures are refined, they are corrected. The solution of the system of equations gives the desired distribution of pressures and flows throughout the building as a whole, and due to its large dimension and nonlinearity, it is possible only by numerical methods using a computer.

Air-permeable building elements (windows, doors) connect all premises of the building and the outside air into a single system. The location of these elements and their characteristics of resistance to air permeation significantly affect the qualitative and quantitative picture of the distribution of flows in the building. Thus, when solving the system of equations for determining the pressures in each room and node of the ventilation network, the influence of aerodynamic resistances of air-permeable elements is taken into account not only in the building envelope, but also in internal fences. According to the described algorithm, at the Department of Heating and Ventilation of MGSU, a program for calculating the air mode of the building was developed, which was used to calculate the ventilation modes in the investigated residential building.

As follows from the calculations, the internal pressure in the premises is influenced not only by weather conditions, but also by the number of supply valves, as well as the draft of the exhaust ventilation. Since in the house in question in all apartments the ventilation is the same, in a one-room and two-room apartments the pressure is lower than in a three-room apartment. When open interior doors in an apartment, the pressure in rooms oriented to different sides practically does not differ from each other.

In fig. 5 shows the values ​​of the pressure change in the premises of the apartments.

Differences in pressure on breathable elements and air flows passing through them

The flow distribution in apartments is formed under the influence of pressure differences on opposite sides of the air-permeable element. In fig. 6, on the plan of the last floor, arrows and numbers show the directions of movement and air flow rates under various weather conditions.

When installing valves in living rooms air movement is directed from rooms to ventilation grilles in kitchens, bathrooms and toilets. This direction of movement remains in one-room apartment where the valve is installed in the kitchen.

Interestingly, the direction of air movement did not change when the temperature dropped from 5 to -28 ° C and when the north wind appeared at a speed of v = 4.9 m / s. No exfiltration was observed throughout heating season and in any wind, which testifies to the sufficiency of the height of the shaft 4.5 m. Dense entrance doors to the apartments prevent horizontal overflow of air from the apartments of the windward facade to the apartments of the leeward facade. A small, up to 2 kg / h, vertical overflow is observed: air leaves the apartments on the lower floors through the entrance doors, and enters the apartments on the upper floors. Since the air flow through the doors is less than allowed by the standards (no more than 1.5 kg / h m 2), it is possible to consider the air permeation resistance of 0.7 m 2 h / kg for a 17-storey building even excessive.

Ventilation system operation

The capabilities of the ventilation system were tested in the design mode: at 5 ° C in the outside air, calmness and open vents. Calculations have shown that, starting from the 14th floor, the exhaust flow rates are insufficient, therefore, the section of the main channel of the ventilation unit should be considered for this building underestimated. By replacing the vents with valves, the costs are reduced by about 15%. It is interesting to note that at 5 ° C, regardless of the wind speed, from 88 to 92% of the air removed by the ventilation system on the ground floor enters through the valves and from 84 to 91% on top floor... At a temperature of -28 ° C, the air supply through the valves compensates for the exhaust air by 80–85% on the lower floors and by 81–86% on the upper ones. The rest of the air enters the apartments through the windows (even with an air permeability resistance of 1 m 2 h / kg at a pressure difference D P o = 10 Pa). At an outdoor air temperature of -3.1 ° C and below, the flow rates of the air removed by the ventilation system and the air supplied through the valves exceed the design air exchange of the apartment. Therefore, it is necessary to regulate the flow both on the valves and on the ventilation grilles.

In cases of fully open valves with negative temperature outside air ventilation costs the air air of the apartments on the first floors is several times higher than the calculated ones. At the same time, the ventilation air consumption of the upper floors drops sharply. Therefore, only at an outside temperature of 5 ° C, the calculations were carried out for fully open valves throughout the building, and for more low temperatures the valves of the lower 12 floors were covered by 1/3. This took into account the fact that the valve has automatic control by the humidity of the room. In case of large air changes in the apartment, the air will be dry and the valve will close.

Calculations have shown that at an outdoor air temperature of -10.2 ° C and below, the entire building is provided with excess exhaust through the ventilation system. At an outside air temperature of -3.1 ° C, the design inflow and exhaust are fully maintained only on the lower ten floors, and the apartments on the upper floors - when close to the design exhaust - are provided with air flow through the valves by 65–90%, depending on the wind speed.

conclusions

1. In multi-storey residential buildings with one riser of the natural exhaust ventilation system per apartment, made of concrete blocks, as a rule, the cross-sections of the trunks are underestimated to allow ventilation air to pass at an outside air temperature of 5 ° C.

2. The designed ventilation system at correct installation operates stably on the hood throughout the entire heating period without "overturning" the ventilation system on all floors.

3. Supply valves must necessarily have the ability to regulate to reduce the air consumption in the cold season of the heating season.

4. To reduce the consumption of extract air, it is advisable to install automatically adjustable grilles in the natural ventilation system.

5. Through tight windows in multi-storey buildings there is infiltration, which reaches 20% of the exhaust flow rate in the building in question, and which must be taken into account in the heat loss of the building.

6. Norm of density entrance doors in apartments for 17-storey buildings is performed with a resistance to air permeability of doors of 0.65 m 2 h / kg at D P = 10 Pa.

Literature

1. SNiP 2.04.05-91 *. Heating, ventilation, air conditioning. M .: Stroyizdat, 2000.

2. SNiP 2.01.07-85 *. Loads and impacts / Gosstroy RF. M .: GUP TsPP, 1993.

3. SNiP II-3-79 *. Construction heat engineering / Gosstroy RF. M .: GUP TsPP, 1998.

4. Biryukov SV, Dianov SN The program for calculating the air regime of a building. articles of MGSU: Modern technologies heat and gas supply and ventilation. M .: MGSU, 2001.

5. Biryukov SV Calculation of natural ventilation systems on a computer. reports of the 7th scientific-practical conference on April 18–20, 2002: Actual problems of building thermal physics / RAASN RNTOS NIISF. M., 2002.

The processes of air movement inside the premises, its movement through fences and openings in fences, along channels and air dams, air flow around the building and the interaction of the building with the surrounding air are combined general concept air regime of the building. In heating, the thermal regime of the building is considered. These two modes, as well as the humidity mode, are closely related to each other. Likewise thermal conditions When considering the air regime of a building, three tasks are distinguished: internal, edge and external.

The internal task of the air mode includes the following questions:

a) calculation of the required air exchange in the room (determination of the amount of harmful emissions entering the premises, selection of the performance of local and general ventilation systems);

b) determination of the parameters of the internal air (temperature, humidity, speed of movement and content harmful substances) and their distribution over the volume of the premises at different options air supply and removal. Choice optimal options air supply and removal;

c) determination of air parameters (temperature and speed of movement) in jet streams created by forced ventilation;

d) calculation of the amount of harmful emissions escaping from under the shelters of local suction (diffusion of harmful secretions in the air stream and indoors);

e) creating normal conditions at workplaces (shower) or in certain parts of the premises (oases) by selecting the parameters of the supplied supply air.

The boundary value problem of the air regime combines the following questions:

a) determination of the amount of air passing through external (infiltration and exfiltration) and internal (overflow) fences. Infiltration leads to an increase in heat loss in the premises. The greatest infiltration is observed in the lower floors of multi-storey buildings and in high industrial premises... Unorganized air flow between rooms leads to pollution clean rooms and distribution throughout the building unpleasant odors;

b) calculation of the areas of openings for aeration;

c) calculation of the dimensions of channels, air ducts, shafts and other elements of ventilation systems;

d) the choice of the method of air treatment - giving it certain "conditions": for the inflow, it is heating (cooling), humidification (drying), dust removal, ozonation; for the hood - it is cleaning from dust and harmful gases;

e) development of measures to protect premises from the burst of cold outside air through open openings (external two ri, gates, technological openings). For protection, air and air-thermal curtains are usually used.

The external task of the air regime includes the following questions:

a) determination of the pressure created by the wind on the building and its individual elements (for example, deflector, lantern, facades, etc.);

b) calculation of the maximum possible amount of emissions that does not lead to pollution of the territory industrial enterprises; determination of ventilation of the space near the building and between separate buildings at an industrial site;

c) selection of locations for air intakes and exhaust shafts of ventilation systems;

d) calculation and forecasting of atmospheric pollution with harmful emissions; checking the adequacy of the degree of purification of the discharged contaminated air.

Basic ventilation solutions for industrial building.

42. Sound and noise, their nature, physical characteristics... Sources of noise in ventilation systems.

Noise is random vibrations of various physical nature, characterized by the complexity of the temporal and spectral structure.

Initially, the word noise referred exclusively to sound vibrations, but in modern science it has been extended to other types of vibrations (radio, electricity).

Noise is a collection of aperiodic sounds of varying intensity and frequency. From a physiological point of view, noise is any perceived unfavorable sound.

Noise classification. Noises consisting of a random combination of sounds are called statistical noises. Noises with a predominance of any tone that can be detected by ear are called tonal noises.

Depending on the environment in which the sound propagates, structural or body and airborne noises are conventionally distinguished. Structural noises occur when an oscillating body is in direct contact with machine parts, pipelines, building structures etc. and propagate along them in the form of waves (longitudinal, transverse, or both at the same time). Oscillating surfaces impart vibrations to adjacent air particles, forming sound waves. In cases where the noise source is not associated with any structures, the noise it radiates into the air is called airborne noise.

By the nature of its occurrence, noise is conventionally divided into mechanical, aerodynamic and magnetic.

According to the nature of the change in the total intensity over time, noises are subdivided into impulsive and stable noises. Impulse noise has a rapid rise in sound energy and a rapid decay, followed by a long break. Stable noise has little energy change over time.

According to the duration of action, noises are divided into long-term (total duration continuously or with pauses of at least 4 hours per shift) and short-term (duration less than 4 hours per shift).

Sound, in broad sense- elastic waves propagating longitudinally in the medium and creating mechanical vibrations in it; in a narrow sense - the subjective perception of these vibrations by the special senses of animals or humans.

Like any wave, sound is characterized by amplitude and frequency spectrum. Usually, a person hears sounds transmitted through the air in the frequency range from 16-20 Hz to 15-20 kHz. Sound below the human hearing range is called infrasound; higher: up to 1 GHz - ultrasound, from 1 GHz - hypersonic. Among the audible sounds, phonetic, speech sounds and phonemes (of which oral speech) and musical sounds (of which music is composed).

The source of noise and vibration in ventilation systems is a fan, in which non-stationary processes of air flow through the impeller and in the casing itself take place. These include speed pulsations, the formation and separation of vortices from the fan elements. These factors are the cause of aerodynamic noise.

E. Ya. Yudin, who studied the noise of ventilation units, points to three main components of aerodynamic noise generated by a fan:

1) vortex noise - a consequence of the formation of vortices and their periodic breakdown when the air flow around the fan elements;

2) noise from local flow irregularities generated at the inlet and outlet of the wheel and leading to unsteady flow around the blades and fixed elements of the fan located near the wheel;

3) rotation noise - each moving blade of the fan wheel is a source of disturbance in the air environment and the formation of vortices. The proportion of rotational noise to the overall fan noise is usually negligible.

Vibrations of structural elements ventilation unit, often due to inadequate wheel balancing, are the cause of mechanical noise. Mechanical noise of the fan usually has an impact character, an example of this is knocking in the clearances of worn bearings.

Dependence of noise on the circumferential speed of the impeller at different characteristics The network for a centrifugal fan with forward-curved blades is shown in the figure. It follows from the figure that at a peripheral speed of more than 13 m / s the mechanical noise of the ball bearings is "masked" by the aerodynamic noise; at lower speeds, bearing noise prevails. At a peripheral speed of more than 13 m / s, the level of aerodynamic noise increases faster than the level of mechanical noise. Have centrifugal fans with backward-curved blades, the level of aerodynamic noise is somewhat less than that of fans with forward-curved blades.

In ventilation systems, in addition to the fan, noise sources can be vortices formed in the elements of air ducts and in ventilation grilles, as well as vibrations of insufficiently rigid walls of air ducts. In addition, penetration through duct walls and ventilation grilles is possible. extraneous noise from adjacent rooms through which the duct runs.

Basic parameters of physical and climatic factors

Climate is a set of weather conditions that repeat from year to year. The climate is influenced by: altitude, geographical position, proximity of large bodies of water, current, prevailing winds. Air (temperature, humidity, wind), temperature and soil moisture, precipitation, solar radiation.

Factors that determine the indoor climate

The thermal environment in the room is determined by the combined action of a number of factors: temperature, mobility and humidity of the room air, the presence of jet currents, the distribution of the air condition parameters in the plan and along the height of the room (all of the above characterizes the air mode of the room), as well as the radiation radiation of the surrounding surfaces, depending on their temperature, geometry and radiation properties (characterizing the radiation regime of the room). A comfortable combination of these indicators corresponds to conditions under which there is no tension in the process of human thermoregulation.

Air and radiation conditions of the premises

The processes of air movement inside the premises, its movement through fences and openings in fences, through channels and air ducts, air flow around the building and the interaction of the building with the surrounding air environment are united by the general concept of the air mode of the building. In heating, the thermal regime of the building is considered. These two modes, as well as the humidity mode, are closely related. Similarly to the thermal regime, when considering the air regime of a building, three tasks are distinguished: internal, edge and external.

The internal task of the air mode includes the following questions:

a) calculation of the required air exchange in the room (determination of the amount of harmful emissions entering the premises, selection of the performance of local and general ventilation systems);

b) determination of the parameters of the internal air (temperature, humidity, speed of movement and the content of harmful substances) and their distribution over the volume of the premises with various options for supplying and removing air. Selection of the best options for air supply and removal;

c) determination of air parameters (temperature and speed of movement) in jet streams created by forced ventilation;

d) calculation of the amount of harmful secretions escaping from under the shelters of local suction (diffusion of harmful emissions in the air stream and indoors);

e) creating normal conditions at workplaces (shower) or in certain parts of the premises (oases) by selecting the parameters of the supplied supply air.

Radiation regime. Radiant heat transfer.

An important component of a complex physical process that determines the thermal regime of a room is heat exchange on its surfaces.

Radiant heat exchange in a room has a peculiarity: it occurs in a closed volume under conditions of limited temperatures, certain radiation properties of surfaces and the geometry of their location. Thermal radiation of surfaces in a room can be considered as monochromatic, diffuse, obeying the laws of Stefan-Boltzmann, Lambert and Kirchhoff, infrared radiation of gray bodies.

As one of the types of surfaces in a room, window glass has peculiar radiation properties. It is partially transparent to radiation. Window glass, which transmits short-wave radiation well, is practically opaque for radiation with a wavelength of more than 3-5 microns, which is typical for heat transfer in a room.

When calculating radiant heat transfer between surfaces, room air is usually considered a radiant medium. It consists mainly of diatomic gases (nitrogen and oxygen), which are practically transparent to heat rays and do not emit thermal energy themselves. The insignificant content of polyatomic gases (water vapor and carbon dioxide) with a small thickness of the air layer in the room practically does not change this property.

The air regime of a building is a combination of factors and phenomena that determine the general process of air exchange between all its rooms and outside air, including the movement of air inside the premises, the movement of air through fences, openings, channels and air ducts and the flow of air around the building. Traditionally, when considering individual issues of the air regime of a building, they are combined into three tasks: internal, regional and external.

The general physical and mathematical formulation of the problem of the air regime of a building is possible only in the most generalized form. The individual processes are quite complex. Their description is based on the classical equations of transfer of mass, energy, momentum in a turbulent flow.

From the standpoint of the specialty "Heat supply and ventilation" the following phenomena are most relevant: infiltration and exfiltration of air through external fences and openings (unorganized natural air exchange, which increases the heat loss of the room and reduces the heat-shielding properties of external fences); aeration (organized natural air exchange for ventilation of heat-stressed rooms); air flow between adjacent rooms (unorganized and organized).

The natural forces that cause air movement in a building are gravity and wind pressure. The temperature and density of the air inside and outside the building are usually not the same, as a result of which the gravitational pressure on the sides of the fence is different. Due to the action of the wind, a backwater is created on the windward side of the building, and excessive static pressure arises on the surfaces of the fences. On the leeward side, a vacuum is formed and the static pressure is reduced. Thus, with the wind, the pressure c outside building differs from indoor pressure.

Gravitational and wind pressures usually work together. Air exchange under the influence of these natural forces is difficult to calculate and predict. It can be reduced by sealing barriers, and also partially regulated by throttling ventilation ducts, opening windows, transom and ventilation lanterns.

The air regime is related to the thermal regime of the building. Infiltration of outside air leads to additional heat consumption for heating it. Exfiltration of humid indoor air humidifies and reduces the heat-shielding properties of fences.

The position and size of the infiltration and exfiltration zone in the building depends on the geometry, design features, ventilation mode of the building, as well as on the construction area, season and climate parameters.

Heat exchange takes place between the filtered air and the fence, the intensity of which depends on the place of filtration in the fence structure (array, joint of panels, windows, air spaces etc.). Thus, there is a need for calculating the air regime of a building: determining the intensity of infiltration and exfiltration of air and solving the problem of heat transfer of individual parts of the fence in the presence of air permeability.

Similarly to the thermal one, there are 3 problems when considering the HRZ.

Internal

Regional

External.

TO internal task refers to:

1.calculation of the required air exchange (determination of the amount of harmful emissions, productivity of local and general ventilation)

2.determination of parameters of indoor air, content of harmful substances

and their distribution over the volume of the premises at different schemes ventilation;

choice optimal schemes air supply and removal.

3. determination of the temperature and air speed in the jets created by the inflow.

4.calculation of the number of hazards escaping from technological shelters

equipments

5. creating normal working conditions, spraying and creating oases by choosing the parameters of the supply air.

The boundary value problem is:

1.determination of overflows through external fences (infiltration), which leads to an increase in heat loss and the spread of unpleasant odors.

2.calculation of openings for aeration

3.calculation of the dimensions of channels, air ducts, mines and other elements

4. selection of the method of processing the transfer air (heating, cooling, cleaning) for the exhaust air - cleaning.

5.calculation of protection against air burst through open openings ( air curtains)

The external task includes:

1.determination of the pressure created by the wind on the building

2. calculation and determination of the ventilation of the prom. playgrounds

3.Selection of locations for air intakes and exhaust shafts

4.calculation of MPE and verification of the adequacy of the degree of purification

1. Local exhaust ventilation. Local suction, their classification. Exhaust hoods, requirements and calculation.

Benefits of Local Exhaust Ventilation (MVV)

Removal of harmful secretions directly from their places of excretion

Relatively low air consumption.

In this regard, MVV is the most effective and economical way.

The main elements of MVV systems are

2 - air duct network

3 - fans

4 – cleaning devices

Basic requirements for local suction:

1) localization of harmful secretions in the place of their formation

2) removal of polluted air outside the premises with high concentrations, much more than with general ventilation.

The requirements that are presented to the Ministry of Defense are divided into sanitary and hygienic and technological ones.

Sanitary and hygienic requirements:

1) maximum localization of harmful secretions

2) the removed air must not pass through the respiratory organs of the workers.

Technological requirements:

1) the place of formation of harmful secretions should be as covered as possible as far as it allows technological process, and open working openings should be of minimum dimensions.

2) MO should not interfere with normal work and reduce labor productivity.

3) Harmful discharge, as a rule, should be removed from the place of their formation in the direction of their intensive movement. For example, hot gases go up, cold gases go down.

4) The design of the MO should be simple, have small aerodynamic drag, easy to mount and dismantle.

MO classification

Structurally, MO is designed in the form of various shelters for these sources of harmful emissions. According to the degree of source isolation from the surrounding space, the MO can be divided into three groups:

1) open

2) half-open

3) closed

To MO open type include air ducts located outside the source of harmful emissions above it or on the side or below, examples of such MO are exhaust panels.

Semi-open shelters include sources of harm. The shelter has an open working opening. Examples of such shelters are:

Fume cupboards

Ventilation chambers or cabinets

Shaped shelters from rotating or cutting tools.

Fully closed suction units are a casing or a part of the device, which has small leaks (in the places where the casing touches the moving parts of the equipment). Currently, some types of equipment are performed with built-in MO (these are painting and drying chambers, wood processing machines).

Open MO. They resort to open MO when it is impossible to apply half-open or completely closed MO, which is due to the peculiarities of the technological process. The most common open type MOs are umbrellas.

Exhaust umbrellas.

Exhaust hoods are called air inlets made in the form of truncated peramides located above the sources of harmful emissions. Exhaust hoods are generally only used for upwardly capturing streams of hazardous substances. This occurs when the harmful emissions are heated and a stable temperature flux is formed (temperature> 70). Exhaust hoods are widely used, much more than they deserve. For umbrellas, it is characteristic that there is a gap between the source and the air inlet, the space is unprotected from air environment... As a result, the ambient air flows freely to the source and deflects the flow of harmful emissions. As a result, umbrellas require significant volumes, which is the disadvantage of an umbrella.

Umbrellas are:

1) simple

2) in the form of visors

3) active (with slots around the perimeter)

4) with air blowing (activated)

5) group.

Umbrellas are arranged both with local and mechanical exhaust ventilation, but the main condition for the use of the latter is the presence of powerful gravitational forces in the stream.

For the operation of the umbrellas, the following must be observed

1) the amount of air sucked off by the umbrella must be no less than that which is emitted from the source and joins on the way from the source to the umbrella, taking into account the influence of lateral air currents.

2) The air flowing to the umbrella must have a supply of energy (mainly thermal enough to overcome gravitational forces)

3) The dimensions of the umbrella must be larger than the dimensions of the leaking medium /

4) An organized flow is required to avoid overturning of the draft (for natural ventilation)

5) Effective work umbrella is largely determined by the uniformity of the section. It depends on the umbrella opening angle α. α = 60 then Vts / Vc = 1.03 for a circular or square section, 1.09 for a rectangular α = 90 1.65. The recommended opening angle α = 65, at which the greatest uniformity of the velocity field is achieved.

6) Dimensions of a rectangular umbrella in plan A = a + 0.8h, B = b + 0.8h, where h is the distance from the equipment to the bottom of the umbrella h<08dэ, где dэ эквивалентный по площади диаметр источника

7) The volume of sucked air, determined depending on the thermal power of the source and the air mobility in the room, Vn at low thermal power, is carried out according to the formulas L = 3600 * F3 * V3 m3 / h where f3 is the suction area, V3 is the suction speed. For non-toxic emissions, V3 = 0.15-0.25 m / s. For toxic ones, V3 = 1.05-1.25, 0.9-1.05, 0.75-0.9, 0.5-0.75 m / s should be taken.

With significant heat release, the volume of air sucked out by the umbrella is determined by the formula L 3 = L k F 3 / F n Lk - the volume of air rising to the umbrella with a convective jet Qk - the amount of convective heat released from the surface of the source Q k = α k Fn (t n -t in).

If the calculation of the umbrella is carried out for the maximum release of harmfulness, then you can not arrange an active umbrella, but manages with an ordinary umbrella.

1. Suction panels and onboard suction, features and calculation.

In those cases when, for design reasons, the coaxial suction cannot be located close enough above the source, and therefore the suction performance is excessively high. When it is necessary to deflect the jet rising above the heat source so that harmful emissions do not fall into the zone of movement of the worker, suction panels are used for this.

Structurally, these local suctions are divided into

1 - rectangular

2 - panels of uniform suction

rectangular suction panels are of three types:

a) one-sided

b) with a screen (to reduce volumetric suction)

c) combined (with suction sideways and downwards)

the volume of air removed by any panel is determined by the formula where c is the coefficient. depending on the design of the panel and its location relative to the heat source, Qk is the amount of convective heat emitted by the source, H is the distance from the upper plane of the source to the center of the panel's suction openings, B is the length of the source.

The combined panel is used to remove heat flux containing not only gases, but also surrounding dust 60% is removed to the side, and 40% down.

Uniform suction panels are used in welding shops, inclined panels that provide a deviation of the torch of harmful substances from the face of the welder have become widespread. One of the most common is the Chernoberezhsky panel. The suction opening is made in the form of a lattice, the free section of the cracks is 25% of the area of ​​the panel. The recommended air speed in the open section of the slots is taken equal to 3-4 m / s. The total air consumption is calculated at a specific flow rate equal to 3300 m3 / h per 1 m2 of the suction panel. This is a device for removing air together with harmful emissions in the bathroom where heat treatment takes place. Suction takes place along the sides.

Distinguish:

Single-sided suction when the suction slot is located along one of the long sides of the bath.

Double-sided, when the slots are located on both sides.

On-board suction is simple when the slots are in a vertical plane.

Overturned when the slot is horizontal.

There are solid, sectional with blowing.

The more toxic the discharge from the bath mirror, the closer they need to be pressed against the mirror so that the harmful discharge does not get into the breathing zone of the workers. To do this, all other things being equal, it is necessary to increase the volume of suctioned air.

When choosing the type of onboard suction, consider the following:

1) simple suction should be used when the solution level in the bath is high, when the distance to the suction slot is less than 80-150 mm; at a lower standing, inverted suction is used, which requires much less air flow.

2) Single-sided ones are used if the bathtub width is much less than 600mm, if more then double-sided ones.

3) If, in the course of blowing, large things are lowered into the bathtub that can disrupt the operation of a single-sided suction, then I use double-bottomed ones.

4) Solid structures are used for lengths up to 1200mm, and sensory ones for lengths not exceeding 1200mm.

5) Use blower suction with a bath width of more than 1500mm. When the grout surface is completely smooth, there are no protruding parts, there is no dipping operation.

The efficiency of trapping harmful substances depends on the uniformity of suction along the length of the slot. The task of calculating on-board suction is reduced to:

1) the choice of design

2) determination of the volumes of suctioned air

several types of calculation of on-board suction have been developed:

method M.M. Baranov's volumetric air flow for onboard suction is determined by the formula:

where a is the tabular value of the specific air consumption depending on the length of the bath, x is the correction factor for the depth of the liquid level in the bath, S is the correction factor for the air mobility in the room, l is the length of the bath.

On-board suction with blowing is a simple one-board suction activated by air with the help of a jet directed to the suction along the bath mirror so that it overlaps it, while the jet becomes more long-range and the flow rate in it decreases, the volume of air for blowing is L = 300kB 2 l