Air exchange in rooms (distribution supply air and removal of air from the premises) of industrial and administrative buildings is provided taking into account the mode of their use during the day or year, as well as the available heat, moisture and harmful substances.
Supply air to compensate for exhaust air exhaust system should be served directly to a room with a permanent stay of people. For public and administrative premises, it is allowed to supply up to 50% of the air flow to corridors or adjacent premises.
In production premises, depending on the nature and severity of the factors of the production environment, supply air should be supplied to the working area:
In rooms with significant moisture and heat excesses - in the zones of moisture condensation on the building envelope;
In rooms with dust emission - jets directed from top to bottom from air distributors located in the upper zone;
In rooms for various purposes without dust emission, it is allowed to supply supply air with jets directed from the bottom up from air distributors located in the serviced or working area;
In rooms with slight heat excess, air can be supplied from air diffusers located in the upper zone with jets (vertical, directed from top to bottom; horizontal or inclined - down);
In rooms with sources of emissions of harmful substances that cannot be equipped with local exhausts, supply air is supplied directly to permanent workplaces if they are located at these sources.
Supply air should be directed in such a way that it does not flow through areas with high pollution to areas with less pollution and does not disturb the balance of local suction.
Supply of fresh air by ventilation, as well as air conditioning systems and air heating should be carried out on the basis that the temperature and speed of air movement correspond to the norms of meteorological conditions in the working area, so that there is no fogging and moisture condensation on the surrounding structures.
For industrial premises in which harmful substances or pronounced unpleasant odors, a negative imbalance should be provided for, that is, the excess of the exhaust volume over the inflow volume.
In the cold season in industrial buildings, when justified, a negative imbalance is allowed in the volume of no more than a single air exchange per 1 hour in rooms with a height of 6 m or less and at the rate of 6 m 3 / h per 1 m 2 of floor area in rooms with a height of more than 6 m.
Forced ventilation systems with artificial induction for industrial premises, in which work is performed more than 8 hours a day, must be combined with air heating.
Supply ventilation systems combined with air heating, as well as air heating systems, should be designed with a backup fan or heating unit, or provide for at least two systems connected by an air duct.
The air distribution in the rooms depends on the placement of the supply and exhaust openings. Room ventilation is the process of transferring air volumes from the supply openings, as well as the movement of air due to the suction openings. The air exchange created in the premises by ventilation devices is accompanied by circulation air environment, the volume of which is several times greater than the volume of ventilation air entering the room and removed from it. The circulation of air masses is important for the efficiency of ventilation, since it is the main reason for the spread of harmful emissions throughout the room that enters the air from somewhere.
The nature of the air flow depends on the shape and number of supply openings, their location, as well as the temperature and speed with which the air enters the premises. Options for air movement patterns in industrial premises are shown in fig. 5.8.
Rice. 5.8. Schemes for organizing air exchange in the room:
a- top-up; b
- bottom-down; in
-top down; G
- upwards;
d
- combined; e
- combined
The nature of the distribution of air flows is influenced by work technological equipment and besides - structural elements building. The task of a specialist designing ventilation devices is to take into account the nature of the movement of air masses in the room, so that within working area Satisfactory parameters of the microclimate were provided, namely, temperature and air velocity.
Supply jets. Supply nozzles
At a low speed, the air moves in parallel, non-mixing streams. This type of movement is called laminar and is observed mainly in small channels, thin slots, and also in the absence of directed air movement in various structures. As the speed increases, the jets begin to mix, air particles move more randomly. Vortexes appear in the flow - such a movement is called turbulent. Turbulent motion is characterized by the presence of transverse velocity fluctuations.
The transition from laminar to turbulent motion is observed at certain values of a complex parameter, which is called the Reynolds criterion:
where V– air velocity, m/s; d- the size that determines the movement of air (diameter or hydraulic diameter of the air duct, air outlet), m; ν - kinematic viscosity of air, m 2 / s.
laminar motion in smooth pipes turns into turbulent at Re = 2300. As the roughness increases, this transition occurs at lower values of the Re criterion.
The organization of air exchange largely depends on the nature of the ventilation air jets.
Jet classification
An air jet is a directional flow with finite transverse dimensions. Basically, jets are divided into free and non-free, isothermal and non-isothermal, laminar and turbulent.
Free jets have no obstacles for their free development. A free stream is one that is not limited by walls. Free jets are formed when flowing into a space filled with the same medium, which is in a relatively calm state. Since the air jets move in the same air environment, from the point of view of hydraulics, they are flooded. If the density of the jet and the surrounding air is the same, then the axis of the jet is rectilinear, and at different densities, the axis of the jet is curved. Non-free (constrained) jets - those whose development and aerodynamic structure are influenced by fences; these jets propagate in a space having final dimensions. In isothermal jets, the initial temperature is equal to the ambient air temperature, i.e., in this case, the jet does not participate in heat exchange with the environment. In non-isothermal jets, the initial supply air temperature is higher or lower than the ambient air temperature. A laminar or turbulent jet is characterized by a laminar or turbulent regime, respectively. AT ventilation devices ah, as a rule, turbulent air jets are used.
Energy is expended to move air: thermal, the source of which is heated surfaces, or mechanical, the source of which can be considered, for example, a fan or a combination of thermal and mechanical energy together.
The formation of temperature fields, concentrations of harmful substances (gases) and velocities depends on the patterns of jet propagation and their interaction.
According to the type of energy spent on the formation of a jet, mechanical supply jets are distinguished as isothermal, non-isothermal, and also convective jets.
A free isothermal jet is used to distribute supply air. The jet expands at the exit from the hole, its width grows in proportion to the increase in the distance from the place of expiration. The speed gradually decreases and fades as you move away. Pressure measurements have established that the static pressure in the jet remains constant and equal to the static pressure in the environment.
Consequently, since the static pressure along the jet remains constant, the energy losses are compensated in it at the expense of kinetic energy, so the velocity decays. Since the jet ejects (sucks in) particles of ambient air, the flow rate in it increases with distance from the inlet and its cross section increases. In this case, the speed of the particles, due to the deceleration exerted by the surrounding air, constantly decreases.
On fig. 5.9 shows a diagram of a free isothermal jet that flows out of a round hole.
Rice. 5.9. Structure of a free isothermal jet
Two sections are distinguished in the jet - initial and main. In the initial section a-b the flow velocity at all points of the section is the same. Axial speed over length l about the initial section is the same and equal to the velocity in the exit section V o.
In the area of the triangle abs(on distance l o) at all points of the jet is preserved same speed V o.
The structure of the jet is affected by the initial turbulence. The higher the turbulence of the jet before exiting the nozzle, the more intense its mixing with the surrounding air, the greater the jet expansion angle α in the initial section, the shorter the length of the initial section, and vice versa. In the main section, due to turbulent mixing with the surrounding air, the mass of the supply jet increases with distance from the supply hole, and the speed in it continuously decreases both on the axis of the jet and in the peripheral part. The lateral boundaries of the jet correspond approximately to rays emanating from a point called the pole (point 0 ). Since the position of the jet pole and the boundary of the initial section depend on the degree of jet turbulence, the poles of the initial and main sections of the jet may not coincide. The lateral expansion angle of the main section of the jet is 12º25´.
The free jet is practically independent of the Reynolds criterion ( Re) (the jets are self-similar). One of the main properties of a turbulent free jet is the conservation of a constant momentum along its length:
m V = const, (5.42)
where m is the mass of the supply jet in its cross section; V is the air velocity in the same section of the jet.
This allows you to move large masses of air over long distances, which is widely used in ventilation practice.
It is known that a free jet emerging from a rectangular hole is deformed, taking on a cross-sectional shape approaching a circle.
In industrial premises, chambers, etc. due to the presence of enclosing surfaces, the free jet is deformed and its parameters change. The conditions for the jet entering a particular room can be varied, and this determines the speed, temperature, and air distribution.
The air flow in the area of the suction opening behaves differently. Air flows to the suction port from all sides. Suction efficiency is characterized by suction spectra and appears at short distances from the suction openings. The behavior of the air flow near the suction port is discussed in section 5.9.
The specific features of the supply and suction jets must be taken into account and used in ventilation.
On the dynamics of the air environment of the room big influence have convective currents arising from the presence in the room of various kinds of surfaces, the temperature of which is different from the temperature of the surrounding air. Convective currents can be ascending and descending.
When creating specially organized artificial (mechanical) jets, one must take into account convective air currents, i.e., use convective flows as a factor that, under certain conditions, can significantly contribute to the improvement of labor in the working area.
Inlet openings are usually formed with nozzles, which are made in the form of grilles, shades, diffusers, branch pipes with the ability to control the direction of distribution of the supply air. Some design options for inlet openings are shown in Fig. 5.10.
 |  |
 |  |
 |
Rice. 5.10. Jet shapes:
a- plane-parallel laying; b- axisymmetric; in- conical; G- fan (radial); d- spreading; e- annular section; well- flowing through the grate; α - forced scattering angle
Flat supply jets are formed when air flows out of a long slit-like air diffuser.
It should be noted that when the aspect ratio of the holes is less than 1:3, the jet, which takes the shape of a hole at the place of its origin, quickly transforms into an axisymmetric one. With an aspect ratio of more than 1:10, the jet is considered to be flat. But even in this case, the jets can turn into axisymmetric ones, but only at a large distance from the place of their formation.
In addition to axisymmetric and flat, there can be the following types jets, which also differ in the shape of the air outlet:
Fan jets at an angle α = 90°, which are formed when the flow is forced to dissipate at a certain angle. For full fan jets, the angle of air distribution in space is 360 °, with a smaller angle the jet will be incomplete fan;
Annular, if the jet flows out of the annular slot at an angle to the axis of the air supply channel β< 180°, при β около 135° – полой конической, при β = 90° – полной веерной;
Beam, when air enters the room through a large number of equal openings in the form of a stream consisting of parallel streams. However, some distance from supply device individual streams form a common stream.
In addition, depending on the location of the air distributor, the jets may not overlap or may overlap on the plane of the fences.
Restricted jets can also be divided into dead-end, transit, transit-dead. In dead ends, supply air enters and leaves the room through supply and exhaust openings located on the same side of the room. In transit, the jet enters the space limiting it on one side, and leaves on the other; in transit-dead-end air exits from the room both from the side of its entrance, and from the opposite side.
Perforated (perforated) panels are used mainly in low rooms for uniform distribution of supply air. With this method of air supply, a sharp decrease in speed and temperature equalization is ensured, despite high parameters air distributed throughout the room. So, the allowable temperature difference between the supplied air and the room Δ t less than or equal to 15°C, feed rate V less than or equal to 4 m/s (with a speed test in the working area). An example of the organization of air exchange is shown in fig. 5.11.
Rice. 5.11. Air distribution through perforated (perforated)
a - design scheme ceiling; b - placement of holes in the ceiling; c, d - ways to arrange air distribution through perforated grilles
Openings in the ceiling through which air is supplied must have small size to ensure that air is forced out of the distribution duct (chamber) mainly under the influence of static pressure. In this case, in order to best mix the air jets, the mode of air movement into the holes should be turbulent. When air flows out through the holes of a perforated ceiling, according to research, the turbulent regime is provided already at a criterion value of Re = 1500.
Downdraft, can be used to create an appropriate meteorological environment at fixed workplaces (or recreational areas). An air stream is fed into the area where a person is located from top to bottom. large diameter at low speed. This air supply is called downdraft air spraying, fig. 5.12.
Rice. 5.12. Supply ventilation for a fixed workplace
downflow method (dimensions in meters)
Lecture 15 The purpose of the lecture: to study the physical and mathematical description of turbulent jets. To give the basic principles of air supply and removal.
12.1 Fundamentals of the theory of turbulent jets
The jet of gas is called free, if it is not limited by solid walls and propagates in a medium of the same physical properties. A jet propagating in a stream is called flooded, and if the temperature of the jet differs from the temperature of the medium, then it is called non-isothermal if not different, then – isothermal.
12.1.1 Propagation of an isothermal turbulent jet
If from a nozzle (Figure 12.1) with a diameter d If a jet flows out at a velocity greater than the critical one into a medium of the same temperature with a uniform velocity field in the exit section of the nozzle, then vortices appear on the interface between the jet and the medium, randomly moving along and across the flow. Between the jet and the medium, there is an exchange of finite masses of gas, which results in a transverse transfer of the momentum. The gas from the adjacent layers of the environment is entrained into the jet, and the jet itself is decelerated; the mass of the jet and its width increase, while the velocity near the boundaries decreases. With distance from the nozzle, this perturbation spreads to an increasing number of layers of the surrounding gas. On the other hand, particles of the surrounding gas penetrate deeper and deeper into the jet until they reach the axis of the jet (point C). Further mixing of the jet with gas from environment occurs over the entire cross section of the jet and is accompanied by an increase in its width and a decrease in velocity on the axis.
Figure 12.1
The region of mixing of the substance of the jet with the gas from the environment is called turbulent boundary layer or jet mixing zone. From the outside, the boundary layer comes into contact with the surrounding gas, forming a jet boundary along the surface, at all points of which the velocity component parallel to the axis of the submerged jet is equal to zero, and at the co- jet boundary, the co-flow velocity. On the inner side, the boundary layer borders on the unperturbed potential core of constant velocities of the ABC jet, in which the velocity is equal to the velocity of the outflow from the nozzle.
The cross section of the jet at point C, where the unperturbed core ends, is called transitional; area before it primary, and after it - main. The point O of the intersection of the outer boundaries of the jet is called pole.
Longitudinal velocity in a potential core Uabout remains constant, due to the constant static pressure, and the transverse component V 1 =0.
The rearrangement of the kinematic structure of the jet occurs in the transition section, the length of which is assumed to be zero.
In a turbulent jet, the transverse velocity components are small compared to the longitudinal ones, and they are neglected in engineering calculations.
In the initial section in the undisturbed core, the velocity is constant and equal to the velocity at the exit from the nozzle, while in the boundary layer the velocity drops from this value to zero at the boundary of the submerged jet or to the velocity of the environment in the cocurrent.
The velocity distribution curves in different sections of the main section have a maximum at the jet axis, and as the distance from it decreases, the velocity decreases and near the boundary becomes equal to the cocurrent velocity or zero when the jet is flooded. As the distance from the nozzle increases, the jet becomes wider and the velocity profile becomes lower.
In dimensionless coordinates, the velocity profiles in various sections in the initial section have a universal character, described by the formula:
(12.1)
where Uo, U and U 2 – respectively, the speed in the undisturbed core of the jet, equal to the speed of the outflow from the nozzle; velocity at an arbitrary point of the boundary layer of the initial section; co-current velocity;
is the dimensionless coordinate;
b= r 1 - r 2 is the width of the boundary layer of the axisymmetric jet;
r 1 and r 2 are the radii of the potential core and the outer boundary of the axisymmetric jet;
at is the current ordinate counted from the X axis running from the nozzle edge parallel to the jet axis.
In the main section of the jet, the universal dimensionless velocity profile is described by the equation:
(12.2)
where U m is the velocity on the jet axis in the considered section (maximum velocity);
= y/r is the dimensionless coordinate for an axisymmetric jet;
r is the cross-sectional radius of the axisymmetric jet in the main section.
To determine the boundaries of the jet, the characteristic of the jet expansion is required, which is determined by the transverse pulsations of the jet. It is established that the increase in the width of the mixing zone of the submerged jet has a linear law:
W=Sz X, (12.3)
where Sz is the angular coefficient of expansion of the mixing zone of the submerged jet;
X is the abscissa measured from the pole of the main section during the outflow of gases with a uniform velocity field in the initial section of the jet and from the nozzle edge - in the initial section.
Thus, the longitudinal section of the submerged jet is limited by straight lines and, when flowing from a round nozzle, have the form of a cone.
2006-11-27Why local exhaust ventilation more efficient than general exchange? As a rule, a certain amount of harmful emissions (heat, moisture, dust, gases) from the operation of the equipment and its maintenance personnel enters the air of the premises of buildings for various purposes.
- GOST 12.1.005–88. General sanitary and hygienic requirements for the air of the working area. - M., 1981.
- GN 2.2.5.1313–03. Hygienic standards. Maximum Permissible Concentrations (MPC) of harmful substances in the air of the working area. - M., 2003.
- GN 2.2.5.1314–03. Hygienic standards. Approximate safe levels of exposure (SHL) of harmful substances in the air of the working area. - M., 2003.
- SNiP 2.04.05–91*. Heating, ventilation and air conditioning. - M., 1999.
- SNiP 41-01-2003. Heating, ventilation and air conditioning. - M., 2004.
- Baturin V.V. Fundamentals of industrial ventilation. Ed. 4th .- M .: "Profizdat", 1990.
- Shepelev I.A. Aerodynamics of air flows in the room. - M .: "Stroyizdat", 1978.
- Taliev V.N. Aerodynamics of ventilation: Proc. allowance for universities. - M.: "Stroyizdat", 1979.
- Elterman V.M. Ventilation of chemical industries. Ed. 3rd - M .: "Chemistry", 1980.
- Posokhin V.N. Calculation of local suctions from heat and gas generating equipment. - M.: "Engineering", 1984.
- Aerodynamic bases of aspiration: Monograph. I.N. Logachev, K.I. Logachev.- St. Petersburg: "Khimizdat", 2005.
- Ventilation and heating of workshops of machine-building enterprises. M.I. Grimitlin, G.M. Pozin, O.N. Timofeeva and others - M .: "Engineering", 1993.
- Lifshits G.D. The study of exhaust torches of local suction by the method of "features" .- Izvestiya VUZov. Series "Construction and Architecture", No. 4/1977.
- Lifshits G.D. On the calculation of suction flows of local suctions. - " Engineering systems» ABOK North-West, No. 4(19)/2005.
- Guidelines on the design of local air inlets built into soldering and tinning equipment. E.M. Elterman, G.M. Pozin.- L.: VNIIOT, 1980.
- Pozin G.M. Calculation of the influence of limiting planes on the suction spectra. Scientific works labor protection institutions. - M.: "Profizdat", 1977.
- Ventilation and Air Conditioning: A Designer's Handbook. Part 3, book. 1, ch. 8. Local suction. - Ed. 4th - M .: "Stroyizdat", 1992.
- Grimitlin M.I., Pozin G.M. Efficiency mark ventilation systems. Technical tests and adjustment of ventilation and air conditioning systems.- L .: LDNTP, 1980.
Air exchange in the premises (distribution of supply air and removal of air from the premises) of industrial and administrative buildings is provided taking into account the mode of their use during the day or year, as well as the available heat, moisture and harmful substances.
Supply air to compensate for that removed by the exhaust system should be supplied directly to a room with a permanent presence of people. For public and administrative premises, it is allowed to supply up to 50% of the air flow to corridors or adjacent premises.
In production premises, depending on the nature and severity of the factors of the production environment, supply air should be supplied to the working area:
In rooms with significant moisture and heat excesses - in the zones of moisture condensation on the building envelope;
In rooms with dust emission - jets directed from top to bottom from air distributors located in the upper zone;
In rooms for various purposes without dust emission, it is allowed to supply supply air with jets directed from the bottom up from air distributors located in the serviced or working area;
In rooms with slight heat excess, air can be supplied from air diffusers located in the upper zone with jets (vertical, directed from top to bottom; horizontal or inclined - down);
In rooms with sources of emissions of harmful substances that cannot be equipped with local exhausts, supply air is supplied directly to permanent workplaces if they are located at these sources.
Supply air should be directed in such a way that it does not flow through areas with high pollution to areas with less pollution and does not disturb the balance of local suction.
The supply of supply air by ventilation, as well as by air conditioning and air heating systems, should be carried out so that the temperature and air velocity correspond to the norms of meteorological conditions in the working area, so that there is no fogging and moisture condensation on the surrounding structures.
For industrial premises in which harmful substances or pronounced unpleasant odors are emitted, a negative imbalance should be provided, that is, the excess of the exhaust volume over the inflow volume.
In the cold season in industrial buildings, when justified, a negative imbalance is allowed in the volume of no more than a single air exchange per 1 hour in rooms with a height of 6 m or less and at the rate of 6 m 3 / h per 1 m 2 of floor area in rooms with a height of more than 6 m.
Forced ventilation systems with artificial induction for industrial premises, in which work is performed more than 8 hours a day, must be combined with air heating.
Supply ventilation systems combined with air heating, as well as air heating systems, should be designed with a backup fan or heating unit, or at least two systems connected by an air duct should be provided.
The air distribution in the rooms depends on the placement of the supply and exhaust openings. Room ventilation is the process of transferring air volumes from the supply openings, as well as the movement of air due to the suction openings. The air exchange created in the premises by ventilation devices is accompanied by the circulating movement of the air, the volume of which is several times greater than the volume of ventilation air entering and leaving the premises. The circulation of air masses is important for the efficiency of ventilation, since it is the main reason for the spread of harmful emissions throughout the room that enters the air from somewhere.
The nature of the air flow depends on the shape and number of supply openings, their location, as well as the temperature and speed with which the air enters the premises. Options for air movement patterns in industrial premises are shown in fig. 5.8.
Rice. 5.8. Schemes for organizing air exchange in the room:
a- top-up; b
- bottom-down; in
-top down; G
- upwards;
d
- combined; e
- combined
The nature of the distribution of air flows is influenced by the operation of technological equipment and, in addition, the structural elements of the building. The task of a specialist designing ventilation devices is to take into account the nature of the movement of air masses in the room, so that within the working area, satisfactory microclimate parameters are provided, namely, temperature and air velocity.
Supply jets. Supply nozzles
At a low speed, the air moves in parallel, non-mixing streams. This type of movement is called laminar and is observed mainly in small channels, thin slots, and also in the absence of directed air movement in various structures. As the speed increases, the jets begin to mix, air particles move more randomly. Vortexes appear in the flow - such a movement is called turbulent. Turbulent motion is characterized by the presence of transverse velocity fluctuations.
The transition from laminar to turbulent motion is observed at certain values of a complex parameter, which is called the Reynolds criterion:
where V– air velocity, m/s; d- the size that determines the movement of air (diameter or hydraulic diameter of the air duct, air outlet), m; ν - kinematic viscosity of air, m 2 / s.
Laminar motion in smooth pipes turns into turbulent at Re = 2300. With an increase in roughness, this transition occurs at lower values of the Re criterion.
The organization of air exchange largely depends on the nature of the ventilation air jets.
Jet classification
An air jet is a directional flow with finite transverse dimensions. Basically, jets are divided into free and non-free, isothermal and non-isothermal, laminar and turbulent.
Free jets have no obstacles for their free development. A free stream is one that is not limited by walls. Free jets are formed when flowing into a space filled with the same medium, which is in a relatively calm state. Since the air jets move in the same air environment, from the point of view of hydraulics, they are flooded. If the density of the jet and the surrounding air is the same, then the axis of the jet is rectilinear, and at different densities, the axis of the jet is curved. Non-free (constrained) jets - those whose development and aerodynamic structure are influenced by fences; these jets propagate in a space that has finite dimensions. In isothermal jets, the initial temperature is equal to the ambient air temperature, i.e., in this case, the jet does not participate in heat exchange with the environment. In non-isothermal jets, the initial supply air temperature is higher or lower than the ambient air temperature. A laminar or turbulent jet is characterized by a laminar or turbulent regime, respectively. In ventilation devices, as a rule, turbulent air jets are used.
Energy is expended on moving air: thermal, the source of which is heated surfaces, or mechanical, the source of which can be considered, for example, a fan or a combination of thermal and mechanical energies together.
The formation of temperature fields, concentrations of harmful substances (gases) and velocities depends on the patterns of jet propagation and their interaction.
According to the type of energy spent on the formation of a jet, mechanical supply jets are distinguished as isothermal, non-isothermal, and also convective jets.
A free isothermal jet is used to distribute supply air. The jet expands at the exit from the hole, its width grows in proportion to the increase in the distance from the place of expiration. The speed gradually decreases and fades as you move away. Pressure measurements have established that the static pressure in the jet remains constant and equal to the static pressure in the environment.
Consequently, since the static pressure along the jet remains constant, the energy losses are compensated in it at the expense of kinetic energy, so the velocity decays. Since the jet ejects (sucks in) particles of ambient air, the flow rate in it increases with distance from the inlet and its cross section increases. In this case, the speed of the particles, due to the deceleration exerted by the surrounding air, constantly decreases.
On fig. 5.9 shows a diagram of a free isothermal jet that flows out of a round hole.
Rice. 5.9. Structure of a free isothermal jet
Two sections are distinguished in the jet - initial and main. In the initial section a-b the flow velocity at all points of the section is the same. Axial speed over length l about the initial section is the same and equal to the velocity in the exit section V o.
In the area of the triangle abs(on distance l o) the same speed is maintained at all points of the jet V o.
The structure of the jet is affected by the initial turbulence. The higher the turbulence of the jet before exiting the nozzle, the more intense its mixing with the surrounding air, the greater the jet expansion angle α in the initial section, the shorter the length of the initial section, and vice versa. In the main section, due to turbulent mixing with the surrounding air, the mass of the supply jet increases with distance from the supply hole, and the speed in it continuously decreases both on the axis of the jet and in the peripheral part. The lateral boundaries of the jet correspond approximately to rays emanating from a point called the pole (point 0 ). Since the position of the jet pole and the boundary of the initial section depend on the degree of jet turbulence, the poles of the initial and main sections of the jet may not coincide. The lateral expansion angle of the main section of the jet is 12º25´.
The free jet is practically independent of the Reynolds criterion ( Re) (the jets are self-similar). One of the main properties of a turbulent free jet is the conservation of a constant momentum along its length:
m V = const, (5.42)
where m is the mass of the supply jet in its cross section; V is the air velocity in the same section of the jet.
This allows you to move large masses of air over long distances, which is widely used in ventilation practice.
It is known that a free jet emerging from a rectangular hole is deformed, taking on a cross-sectional shape approaching a circle.
In industrial premises, chambers, etc. due to the presence of enclosing surfaces, the free jet is deformed and its parameters change. The conditions for the jet entering a particular room can be varied, and this determines the speed, temperature, and air distribution.
The air flow in the area of the suction opening behaves differently. Air flows to the suction port from all sides. Suction efficiency is characterized by suction spectra and appears at short distances from the suction openings. The behavior of the air flow near the suction port is discussed in section 5.9.
The specific features of the supply and suction jets must be taken into account and used in ventilation.
The dynamics of the air environment of the room is greatly influenced by convective currents that arise due to the presence of various kinds of surfaces in the room, the temperature of which is different from the temperature of the surrounding air. Convective currents can be ascending and descending.
When creating specially organized artificial (mechanical) jets, one must take into account convective air currents, i.e., use convective flows as a factor that, under certain conditions, can significantly contribute to the improvement of labor in the working area.
Inlet openings are usually formed with nozzles, which are made in the form of grilles, shades, diffusers, branch pipes with the ability to control the direction of distribution of the supply air. Some design options for inlet openings are shown in Fig. 5.10.
 |  |
 |  |
 |
Rice. 5.10. Jet shapes:
a- plane-parallel laying; b- axisymmetric; in- conical; G- fan (radial); d- spreading; e- annular section; well- flowing through the grate; α - forced scattering angle
Flat supply jets are formed when air flows out of a long slit-like air diffuser.
It should be noted that when the aspect ratio of the holes is less than 1:3, the jet, which takes the shape of a hole at the place of its origin, quickly transforms into an axisymmetric one. With an aspect ratio of more than 1:10, the jet is considered to be flat. But even in this case, the jets can turn into axisymmetric ones, but only at a large distance from the place of their formation.
In addition to axisymmetric and flat, there can be the following types of jets, which also differ in the shape of the air outlet:
Fan jets at an angle α = 90°, which are formed when the flow is forced to dissipate at a certain angle. For full fan jets, the angle of air distribution in space is 360 °, with a smaller angle the jet will be incomplete fan;
Annular, if the jet flows out of the annular slot at an angle to the axis of the air supply channel β< 180°, при β около 135° – полой конической, при β = 90° – полной веерной;
Beam, when air enters the room through a large number of equal openings in the form of a stream consisting of parallel streams. However, at some distance from the supply device, a common jet is formed from individual streams.
In addition, depending on the location of the air distributor, the jets may not overlap or may overlap on the plane of the fences.
Restricted jets can also be divided into dead-end, transit, transit-dead. In dead ends, supply air enters and leaves the room through supply and exhaust openings located on the same side of the room. In transit, the jet enters the space limiting it on one side, and leaves on the other; in transit-dead-end air exits from the room both from the side of its entrance, and from the opposite side.
Perforated (perforated) panels are used mainly in low rooms for uniform distribution of supply air. With this method of air supply, a sharp decrease in speed and equalization of temperatures is ensured, despite the high parameters of the air distributed throughout the room. So, the allowable temperature difference between the supplied air and the room Δ t less than or equal to 15°C, feed rate V less than or equal to 4 m/s (with a speed test in the working area). An example of the organization of air exchange is shown in fig. 5.11.
Rice. 5.11. Air distribution through perforated (perforated)
a - design scheme of the ceiling; b - placement of holes in the ceiling; c, d - ways to arrange air distribution through perforated grilles
Openings in the ceiling through which air is supplied must be small in order to ensure that air is forced out of the distribution duct (chamber) mainly under the influence of static pressure. In this case, in order to best mix the air jets, the mode of air movement into the holes should be turbulent. When air flows out through the holes of a perforated ceiling, according to research, the turbulent regime is provided already at a criterion value of Re = 1500.
Downdraft, can be used to create an appropriate meteorological environment at fixed workplaces (or recreational areas). An air jet of large diameter is fed from top to bottom into the area where a person is located at a low speed. This air supply is called downdraft air spraying, fig. 5.12.
Rice. 5.12. Supply ventilation for a fixed workplace
downflow method (dimensions in meters)
5.8. Supply systems mechanical ventilation. cleaning
supply air. Heaters. Fans
Supply systems are used to supply clean air to serviced premises, the scheme of the system is shown in fig. 5.13.
Rice. 5.13. Scheme of the supply system
1 - louvered grille of the air intake device; 2 - insulated valve;
3 - filter; 4 - intermediate section; 5 - heater section; 6 - transitional section;
7 - fan; 8 - a network of air ducts; 9 - air distributors
The bottom of the air inlet opening in the air intake unit is placed at a height of more than 1 m from the level of stable snow cover, but not lower than 2 m from the ground level. Louvre grille of the air inlet device prevents entry into the air intake unit precipitation. The warmed valve protects system from penetration of cold air. Instead of an insulated valve in individual cases install an insulated damper with an electric actuator.
Pos. 1-7 form the supply chamber. Supply chambers usually use standard ones, developed for various air capacities by Gosstroy organizations and manufactured by enterprises.
To calculate the supply system, you first need to determine the volume L air that must be supplied to the serviced premises, type (water, steam, electricity) and parameters of the heat carrier (heat carrier temperature in the supply t d and vice versa t about pipelines), design outdoor temperature t n, required temperature supply air t pr, as well as speed V r.z air in the working area.
Purification of supply outside and recirculation air in the supply chamber filter is carried out for the following purposes:
a) to reduce the dust content of the air supplied to ventilated buildings, if the concentration of dust in the area of the building or near the place of air intake systematically exceeds the MPC established by hygienic standards;
b) to protect heat exchangers, irrigation devices, automation devices and other equipment of ventilation chambers and air conditioners from dust;
c) to protect valuable interior decoration and equipment of ventilated buildings from pollution by deposits fine dust;
d) to maintain the premises specified in accordance with technological requirements air purity.
MPC in atmospheric air settlements when submitting it to the premises public buildings;
30% MPC in the air of the working area when it is supplied to the premises of industrial and administrative buildings;
30% MPC in the air of the working area with dust particles no larger than 10 microns when it is supplied to the cabins of crane operators, control panels, the breathing zone of workers, as well as during air showering.
To clean the supply air from dust, mainly porous air filters and wash-type electric air filters. In table. 5.10. air filters used in our country are listed.
Table 5.10
Air filter range for supply systems
| Type of | View | Filter efficiency class | Quality criterion | Rated air load on the inlet section, m 3 / (h m 2) | Resistance at nominal air load, Pa | Dustiness-bone upon reaching the specified final resistance, g / m 3 | Average initial dust content of the cleaned air, mg/m 3 | Filter regeneration method | ||
| at the beginning | final at the specified dust capacity | admissible | ultimate | |||||||
| Dry porous | ||||||||||
| Fibrous dry | Cell FyaL-12, FyaL-2 | I | 0,05 | 0,15 | ||||||
| Cellular LAIK | I | According to the catalogs of the association "Soyuzmedinstrument" | 0,01 | 0,05 | Filter change | |||||
| Pocket FyaKP | II | Cleaning and changing the filter material | ||||||||
| Panel FR (FR3, FR4, FR5) | III | 10 000 | 0,10 | 0,50 | Changing the filter material | |||||
| Mesh | Roll FRS* (FRPM) | III | - | 10 000 | - | Cleaning of dusty material (pneumatic) | ||||
| Cell FyaVB | III | Cleaning dusty material by rinsing in water | ||||||||
| sponge dry | Cell FyaPB | III | 0,3 | 0,5 | Same, or pneumatic | |||||
| Wetted porous | ||||||||||
| Fibrous oily | Cell FyaUB | III | 0,3 | 0,5 | Changing the filter material | |||||
| Cell FyaUB | III | 0,3 | 0,5 | Insert change | ||||||
| Oily | Self-cleaning Kd (KdM, Kt, KtTs, FS) | III | 7 – 15** | 0,3 | 0,5 | Continuous flushing of filter elements in oil | ||||
| Cell FyaRB | III | Washing the filter in a soda solution, followed by oiling | ||||||||
| Cell FyaV | III | Same | ||||||||
| Electrical | ||||||||||
| Two-zone flushing | Aggregate FEK and FE-2M | II | 10 000 | Washing with water | ||||||
| * - used to clean the air from fibrous dust ** - in % of the mass of oil poured into the bath |
Porous filters are subdivided into wetted and dry ones: wetted ones include filters covered with thin films of viscous non-volatile lubricants filled with metal plates, wire or polymer nets and nonwoven fibrous layers; to dry - filters filled with non-woven fibrous layers, corrugated nets and sponge, not moistened with lubricant.
Filters are selected taking into account the initial dust content of the air and the permissible residual concentration dust in the air after it has been cleaned, i.e. by their effectiveness. At the same time, the initial resistance of the filter, the change in resistance when the filter is dusted, design and operational features are taken into account.
The filter quality criterion takes into account the efficiency of air purification, initial resistance and dust capacity; the lower this value, the higher the quality of the filter. For filters whose resistance does not change during operation (for example, self-cleaning ones), the quality criterion is the smallest, equal to zero.
By efficiency, air filters are divided into three classes (Table 5.11).
Table 5.11
Characteristics of the main classes of air filters
With a high initial concentration of dust or when particularly thorough air purification is required, multi-stage purification is used.
Bimetallic or plate heaters installed in supply chambers are used to heat the air supplied to industrial premises. The heat carrier can be water, steam, electricity.
Bimetallic heaters with spirally-rolled fins can be single-pass with a vertical arrangement of tubes and multi-pass with a horizontal arrangement of tubes. Lamellar heaters are made only multi-pass with a horizontal arrangement of tubes.
When the coolant is water, multi-pass heaters and their serial connection along the coolant should be used. Parallel connection along the heat carrier of rows of heaters located in series along the air flow is allowed.
The calculation of the heating surface area of heaters for ventilation and air conditioning systems combined with air heating and designed to supply outdoor air in the quantities necessary for ventilation during the cold season should be made, taking the design parameters B (for agricultural buildings - according to parameters A ).
The actual consumption of heat supplied to the heater is determined by the sum of the heat consumption for heating and ventilation, corresponding to the consumption at the calculated outdoor temperature in the cold season according to the calculated parameters B.
Air heaters for the first heating of air conditioning systems and supply ventilation systems with humidification of the supply air with a water coolant must be checked for operating modes corresponding to outdoor temperature and temperatures at the breakpoints of the water temperature graph in heating networks, and on the water temperature at the outlet of the heater.
Calorifiers are calculated in the following order.
1. Given the mass air velocity Vρ 1, kg / (m 2 s), determine required area front section of air heaters:
f 1 = G/ (Vρ) 1 , m 2 , (5.43)
where G– heated air consumption, kg/s.
2. Using the technical data on heaters and based on the required area of the frontal section, select the number and number of heaters installed in parallel and find the actual area of their frontal section f. The number of heaters should be kept to a minimum.
3. Determine the actual mass air velocity in the heaters
Vρ = G/ f, kg / (m 2 s). (5.44)
When the coolant is water, the volumetric flow rate of water passing through each heater is calculated by the formula
G water = 
, m 3 / s, (5.45)
where Q– heat consumption for air heating, W; t mountains and t arr - water temperature at the inlet to the heater and at the outlet of it, °С; n- the number of heaters connected in parallel along the coolant; 4.2 - specific heat water, kJ/(kg K).
Find the speed of water in the tubes of heaters
W = G water / f tr, m/s, (5.46)
where f tr - living section of tubes of heaters for the passage of water, m 2.
By Mass Velocity V×ρ and water velocity (with steam only by mass velocity) according to reference literature or catalogs for heaters, find the heat transfer coefficient of the heater To, W / (m 2 ·°С).
4. Calculate the required area F at the heating surface of the calorific unit
, m 2 , (5.47)
where t cp is the average coolant temperature, °С; t n is the initial temperature of the heated air, °C; t k is the final temperature of the heated air, °C.
Average coolant temperature
With water coolant
t cf = ( t mountains + t arr)/2, °С; (5.48)
With saturated steam pressure up to 0.03 MPa t cp = 100ºС;
With saturated steam pressure over 0.03 MPa t cf = t pair,
where t steam - temperature, °C, of saturated steam, corresponding to its pressure.
5. Determine total number installed heaters:
where F k - heating surface area of one heater of the selected model, m 2.
Rounding the number of heaters to a multiple of their number in the first row n, find the actual heating surface area, setting:
M 2 . (5.50)
The heat flow of the selected heater should not exceed the calculated one by more than 10%. The excess heat flow of the heater will be:
, (5.51)
With an excess heat flow of more than 10%, another model or number of the heater should be used and re-calculated.
According to tables from reference literature or catalogs of air heaters, the aerodynamic resistance of the air heater installation, as well as the resistance of the air heater installation to the passage of the coolant, are determined by the mass air velocity.
For air resistance, a margin of 10% should be given, for water resistance - 20%.
Fans in mechanical ventilation systems are used radial (centrifugal) and axial.
Radial (centrifugal) fans are divided into the following groups: low pressure(up to 1 kPa), medium pressure (from 1 to 3 kPa) and high pressure (from 3 to 12 kPa). Low and medium pressure fans are usually used in supply and exhaust ventilation installations, air conditioning installations and for air curtains, and high-pressure fans in process units.
Axial fans are usually used with relatively low resistance of the ventilation network (up to about 200 Pa) or without an air duct network.
Depending on the conditions of their operation, fans are manufactured in the usual design - to move clean or low-dust air with a temperature of up to 80°C; in anti-corrosion design (from vinyl plastic and other material) - to move air with impurities that destroy ordinary steel; in spark-proof design - for moving combustible and explosive mixtures. In the latter case, the wheels and inlet pipes are made of a material softer than steel, such as aluminum, to avoid sparking. To move air with a dust content of more than 100 mg / m 3, dust fans are used, which have increased wear resistance.
Fans are usually driven by electric motors to which they are connected in one of the following ways:
Directly on the shaft or through an elastic coupling (version 1);
V-belt transmission with a constant gear ratio (version 5 or 6);
Adjustable stepless transmission through hydraulic and inductive slip clutches.
Fans can be of right rotation, when their wheel rotates clockwise (when viewed from the suction side), and left rotation, when their wheel rotates counterclockwise. The sizes of fans, both radial and axial, are characterized by the numbers assigned to them, which numerically express the value of the impeller diameter in dm (for example, fan No. 5 has a wheel with a diameter of 500 mm). The larger the fan number, the more air is supplied by the fan.
On fig. 5.14 shows a general view of a radial (centrifugal) fan.
Rice. 5.14. Radial fan:
1 - fan casing; 2 - electric motor; 3 - frame; 4 - vibration isolators
The fan and electric motor are placed on a frame, under which vibration isolators are installed to reduce the impact of vibration on supporting structures. A wheel with blades is placed inside the casing (the axis of the wheel is horizontal). When the impeller rotates in the direction of rotation of the volute casing, air is sucked in through the inlet and, under the action of centrifugal force, is ejected through the outlet. The wheel blades can have different shapes (forward-curved, radial or backward-curved). The greatest pressure is generated with forward curved blades, but fans with backward curved blades are more efficient and, in addition, they create less noise.
Radial fans are also made with a vertical arrangement of the wheel axle. This arrangement of the wheel axle is typical for roof fans, fig. 5.15. They are used in the device of general ventilation, placing on the roof industrial buildings without air duct system, as well as for smoke exhaust systems. The air is taken by the fan directly from under the roof of the building and released into the atmosphere.
Rice. 5.15. Radial roof fan
Axial fans are used in ventilation systems, air heating systems and for other industrial and technological purposes, in smoke protection systems for buildings to supply air to escape routes in case of fires. On fig. 5.16 shows the design of an axial fan, which is a blade wheel located in a cylindrical casing.
Rice. 5.16. Axial fan:
1 - vane wheel; 2 - casing; 3 - electric motor
As the wheel rotates, air flows through the fan along its axis. Hence the name of the fan - axial. The pressure created by the axial fan is not more than 200 Pa. Dimensions axial fans, as well as radial ones, are characterized by their numbers.
The selection of fans is carried out according to air performance L and pressure P that the fan must provide.
Ventilation types are presented great variety systems various kinds and appointments. Systems are divided into several types based on common features. The main ones are the methods of air circulation in the building, the service area of the unit, and the design features of the facility.
Natural way of air exchange
Considering the types of ventilation devices, you should start with this type. In this case, air movement occurs for three reasons. The first factor is aeration, that is, the temperature difference between the indoor and outdoor air. In the second case, air exchange is carried out as a result of exposure to wind pressure. And in the third case, the pressure difference between the room used and exhaust device also leads to air exchange.
The aeration method is used in places with high heat generation, but only when the incoming air contains no more than 30% harmful impurities and gases.
This method is also not used in cases where treatment of the incoming air is necessary or the inflow of outside air leads to the formation of condensate.
In ventilation systems, where the basis for air movement is the pressure difference between the room and the exhaust device, the minimum height difference must be at least 3 m.
In this case, the length of the sections located horizontally should not exceed 3 m, while the air speed is 1 m/s.
These systems do not require expensive equipment; in this case, exhaust hoods are used, located in bathrooms and kitchen areas. The ventilation system is durable, for its use it is not required to purchase additional devices. Natural ventilation is simple and cheap to operate, but only if it is set up correctly.
However, such a system is vulnerable, since it is necessary to create additional terms for air intake. For this purpose, cut interior doors so that they do not interfere with air circulation. In addition, there is a dependence on the air flow that blows the building. It depends on him natural system ventilation.
An example of this type is open window. But with this action or the tie-in of the hoods, another problem arises - a large amount of noise coming from the street. Therefore, despite its simplicity and economy, the system is vulnerable to a number of factors.
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Means for artificial air exchange
The artificial system, which is also mechanical, uses additional devices for ventilation that help air enter and leave the building, thereby organizing a constant exchange. For this purpose, various devices are used: fans, electric motors, air heaters.
A big disadvantage in the operation of such systems is the cost of energy, which can reach rather big values. But this type has more advantages, they fully pay off the costs of using the funds.
To positive moments it is necessary to attribute the movement of air masses to the desired distance. In addition, such ventilation systems can be regulated, on the basis of which the air can enter or be removed from the rooms in the right amount.
Artificial air exchange does not depend on environmental factors, as is observed with natural ventilation. The system is autonomous, and in the process of work can be used additional functions e.g. heating or humidifying the incoming air. With the natural type, this is impossible.
However, in this moment it is popular to use both air supply systems at once. This allows you to create the necessary conditions indoors, reduce costs, improve the efficiency of ventilation in general.
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Supply air supply
This type of ventilation system is used to ensure a constant supply fresh air. The system can carry out the preparation of air masses before they enter the apartment. For this purpose, air purification, heating or cooling is carried out. Thus, the air acquires the desired qualities, after which it enters the room.
The system includes air handling units and air outlets, and the installation that provides air supply, in turn, includes a filter, heaters, a fan, automatic systems and soundproofing.
When choosing such devices, you should pay attention to a number of factors. Great importance is the volume of air entering the building. This indicator can be equal to several tens or several tens of thousands of cubic meters of air entering the room.
An important role is played by indicators such as the power of the heater, air pressure and the noise level of the device. In addition, these types of ventilation devices have automatic regulation, which allows you to adjust the power consumption and set the level of air consumed. Devices with timers allow you to set the unit to work on a schedule.
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Combination of two methods: supply and exhaust type
This system is a combination of two types of ventilation - supply and exhaust, which allows you to use positive traits both systems at the same time and leads to improved air exchange.
As in the previous version, there is a means of filtering and regulating the incoming air masses. This type can create the necessary conditions in the room, adjust the humidity level of the incoming masses, create the desired temperature by heating or cooling the air. To filter the air masses received from the outside is also included in the functionality of the unit.
The supply and exhaust system will help to reduce costs, which is achieved by removing the heat that is used to heat the incoming air. This process takes place in a heat exchanger - a special-purpose heat exchanger.
Exhaust air masses at room temperature enter the device, after which they transfer their temperature to the heat exchanger, which heats the air entering from outside.
In addition to the above advantages, supply and exhaust ventilation has another quality that is well suited for people suffering from changes in blood pressure. We are talking about the ability to create high and low pressure compared to the environment.
The device is self-contained, independent of environmental conditions, so it can be used all year round. However, the system is not without negative qualities. Among them is the need fine adjustment. If both methods - exhaust and supply - are not balanced with each other, then a person using this type of ventilation risks getting drafts in the house.
