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 air mode their buildings are united in 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 wind, the pressure from the outside of the building is different from the pressure inside the premises.
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, panel joint, windows, air gaps, 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.
The air inside the premises can change its composition, temperature and humidity under the influence of a variety of factors: changes in the parameters of the outside (atmospheric) air, the release of heat, moisture, dust, etc. As a result of exposure to these factors, indoor air can take on unfavorable conditions for people. To avoid an excessive deterioration in the quality of indoor air, it is required to carry out air exchange, that is, to change the air in the room. Thus, the main task of ventilation is to provide air exchange in the room to maintain the design parameters of the internal air.
Ventilation is a set of measures and devices that ensure the calculated air exchange in the premises. Ventilation (BE) of the premises is usually provided using one or more special engineering systems- ventilation systems (CBE), which consist of various technical devices... These devices are designed to perform individual tasks:
In addition to the use of technical devices for the normal functioning of ventilation, the implementation of some technical and organizational measures is required. For example, to reduce the noise level, it is required to comply with the normalized air velocities in the air ducts. BE should provide not just air exchange (BO), but calculated air exchange(RVO). Thus, the BE device requires a mandatory preliminary design, in the process of which the WBO, the design of the system and the modes of operation of all its devices are determined. Therefore, BE should not be confused with ventilation, which is unorganized air exchange. When a resident opens a window in a living room, this is not yet ventilation, since it is not known how much air is required, and how much of it actually enters the room. If special calculations have been made, and it has been determined how much air must be supplied to a given room and at what angle the window must be opened so that exactly this amount of it enters the room, then we can talk about a ventilation device with a natural induction of air movement.
Question 46. (+ Question 80). What issues does the internal task of the air regime solve?
The processes of air movement inside the premises, its movement through fences and openings in fences, along channels and air ducts, air flow around the building and the interaction of the building with the environment air unite general concept air mode of the building. When considering the air regime, buildings are distinguished three tasks: internal, regional 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);
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 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.
Question 47. What questions does the boundary problem of the air regime solve?
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, mines and other elements of ventilation systems;
d) the choice of the method of air treatment - giving it certain "conditions": for the inflow - this 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 cold outside air bursting through open openings (external doors, gates, technological openings). For protection, air and air-thermal curtains are usually used.
Question 48. What issues does the external task of the air regime solve?
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 the ventilation of the space near the building and between individual buildings on the 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; verification of the adequacy of the degree of purification of the discharged polluted air.
Due to the temperature difference under the action of gravitational pressure, outside air enters the rooms of the lower floors through the fence; on the upwind side, the effect of the wind increases the infiltration; with the windy one - it reduces it.
Internal air from the first floors tends to penetrate into the upper room (it flows through internal doors and corridors that are connected to the staircase).
From the premises of the upper floors, air leaves through the non-density of the outer fences outside the building.
The premises of the middle floors can be in a mixed mode. The natural air exchange in the building is superimposed by the action of supply and exhaust ventilation.
1. In the absence of wind, gravitational pressure of different magnitudes will act on the surfaces of the outer walls. According to the law of conservation of energy, the average pressure along the height inside and outside the building will be the same. Relative to the average level in the lower part of the building, the pressure of the column of warm indoor air will be less than the pressure of the column of cold outside air from the outer surface of the wall.
The density of zero overpressure is called the neutral plane of the building.
Figure 9.1 - Construction of overpressure diagrams
The magnitude of the overpressure of the gravitational at an arbitrary level h relative to the neutral plane:
(9.1)
2. If the building is blown by the wind, and the temperatures inside and outside the building are equal, then an increase in static pressure or vacuum will be created on the outer surfaces of the fences.
According to the law of conservation of energy, the pressure inside the building with the same permeability will be equal to the average value between the increased windward and lower windward side.
Absolute value of excess wind pressure:
, (9.2)
where k 1, k 2 - aerodynamic coefficients, respectively, from the windward and windward sides of the building;
Dynamic pressure running onto the building with a stream of air.
To calculate air infiltration through an external fence, the difference in air pressures outside and inside the room, Pa, is:
where H w is the height of the mouth of the ventilation shaft from the ground level (the mark of the location of the point of the conditional zero pressure);
H e - the height of the center of the building element under consideration (window, wall, door, etc.) from the ground level;
The coefficient introduced for the speed pressure and taking into account the change in wind speed from the height of the building, the change in wind speed from the outside temperature depends on the area;
The air pressure in the room, determined from the condition of maintaining the air balance;
Excessive relative pressure in the room due to ventilation.
For example, for administrative buildings buildings of scientific research institutes and the like are characterized by balanced supply and exhaust ventilation in operating mode or complete shutdown of ventilation during non-working hours P in = 0. For such buildings, the approximate value:
3. To assess the influence of the building's air regime on the thermal regime, simplified calculation methods are used.
Case A. In a multi-storey building, in all rooms, the ventilation hood is fully compensated by the ventilation flow, therefore = 0.
This case includes buildings without ventilation or with mechanical supply and exhaust ventilation all rooms with equal inflow and exhaust flow rates. The pressure is equal to the pressure in the staircase and the corridors directly connected to it.
The magnitude of the pressure inside individual rooms is between the pressure and the pressure on the outer surface of that room. We assume that due to the difference, the air sequentially passes through the windows and internal doors facing staircase, and corridors, the initial air flow and pressure inside the room can be calculated using the formula:
where are the characteristics of the permeability of the area of the window, the door from the room going into the corridor or staircase.
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. The air regime of modern buildings, which determines the process of air exchange between rooms and rooms with outside air, is formed under the influence of many factors.
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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 the aerodynamic characteristics of the valve when fully open and 1/3 closed.
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).
The calculations of the air regime of the building were carried out 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.
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).
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 - gravitational - is related to the external pressure, a point characterized by the total overpressureР в, i, into which air enters and from which leaves.
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 from the passage of the air flow 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, only 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 aerodynamic resistance breathable elements 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, but also 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 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.
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 is directed from the rooms to the 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. Exfiltration was not observed during the entire heating season and in any wind, which indicates that the shaft height is 4.5 m high. The tight entrance doors to the apartments prevent horizontal air flow from the apartments on the windward facade to the apartments on 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.
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 underestimated for this building. 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 first floor and from 84 to 91% on the top floor comes through the valves. 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 rate of the removed ventilation system air and supply air 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 completely open valves at negative temperature outdoor air ventilation air flow rates of apartments on the first floors are 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 is automatically controlled according to the room humidity. 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 outdoor 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.
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 thick windows v 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. The density norm of entrance doors to apartments for 17-storey buildings is carried out with a resistance to air permeability of the doors of 0.65 m 2 h / kg at D P = 10 Pa.
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 construction thermal physics / RAASN RNTOS NIISF. M., 2002.
