The air regime of a building is a combination of factors and phenomena that determine general process air exchange between all its premises and outdoor air, including the movement of air indoors, the movement of air through fences, openings, ducts 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, edge and external.
A general physical and mathematical formulation of the problem of the air regime of a building is possible only in the most generalized form. Individual processes are very complex. Their description is based on the classical equations of mass, energy, and momentum transfer in a turbulent flow.
From the perspective 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, increasing heat loss in the room and reducing 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 air density inside and outside the building are usually not the same, resulting in different gravitational pressure on the sides of the fences. Due to the action of the wind, backwater is created on the windward side of the building, and excess static pressure appears on the surfaces of the fences. On the windward side, a vacuum is formed and the static pressure is reduced. Thus, with wind pressure from outside building differs from the indoor pressure.
Gravity and wind pressure usually act together. Air exchange under the influence of these natural forces is difficult to calculate and predict. It can be reduced by sealing the fences, and also partially regulated by throttling the ventilation ducts, opening windows, frames and ventilation lights.
The air regime is related to the thermal regime of the building. Infiltration of outside air leads to additional heat consumption for its heating. Exfiltration of moist indoor air humidifies and reduces the thermal insulation properties of enclosures.
The position and size of the infiltration and exfiltration zone in a building depend on the geometry, design features, ventilation mode of the building, as well as the construction area, time of year and climate parameters.
Heat exchange occurs between the filtered air and the fence, the intensity of which depends on the location of filtration in the structure of the fence (array, panel joint, windows, air gaps, etc.). Thus, there is a need to calculate 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.
Thermal conditions of the building
General scheme heat exchange in the room
The thermal environment in a 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 air parameters in the plan and height of the room, as well as radiation from surrounding surfaces, depending on their temperature, geometry and radiation properties.
To study the formation of a microclimate, its dynamics and methods of influencing it, you need to know the laws of heat exchange in a room.
Types of heat exchange in a room: convective - occurs between air and the surfaces of fences and heating and cooling system devices, radiant - between individual surfaces. As a result of turbulent mixing of non-isothermal air jets with the air of the main volume of the room, “jet” heat exchange occurs. Internal surfaces external fences mainly transfer heat to the outside air through thermal conductivity through the thickness of the structures.
The heat balance of any surface i in the room can be represented based on the law of conservation of energy by the equation:
where Radiant Li, convective Ki, conductive Ti, components of heat transfer on the surface.
Room air moisture
When calculating moisture transfer through fences, it is necessary to know the humidity state of the air in the room, determined by the release of moisture and air exchange. Sources of moisture in residential premises are household processes (cooking, washing floors, etc.), in public buildings- the people in them, in industrial buildings- technological processes.
The amount of moisture in the air is determined by its moisture content d, g of moisture per 1 kg of dry part humid air. In addition, its moisture state is characterized by elasticity or partial pressure of water vapor e, Pa, or relative humidity of water vapor φ, %,
E is the maximum elasticity at a given temperature.
Air has a certain moisture-holding capacity.
The drier the air, the more strongly it holds water vapor. Water vapor pressure e reflects free energy moisture in the air and increases from 0 (dry air) to maximum elasticity E, corresponding to complete air saturation.
Diffusion of moisture occurs in the air from places with greater elasticity of water vapor to places with less elasticity.
η air = ∆d /∆е.
The elasticity of complete saturation of air E, Pa, depends on the temperature t us and increases with its increase. The value of E is determined:
If you need to know the temperature t us to which a particular value of E corresponds, you can determine:
Air condition of the building
The air regime of a building is a set of factors and phenomena that determine the overall process of air exchange between all its premises and outdoor air, including the movement of air indoors, the movement of air through fences, openings, channels and air ducts and the flow of air around the building.
Air exchange in a building occurs under the influence of natural forces and the work of artificial air movement stimulators. Outside air enters the premises through leaks in fences or through supply ducts ventilation systems. Inside a building, air can flow between rooms through doors and leaks in internal structures. Internal air is removed from the premises outside the building through leaks in external fences and through ventilation ducts exhaust systems.
The natural forces that cause air movement in a building are gravitational and wind pressure.
Design pressure difference:
The 1st part is gravitational pressure, the 2nd part is wind pressure.
where H is the height of the building from the ground surface to the top of the cornice.
Max from average speeds by point of reference for January.
C n, C p - aerodynamic coefficients from the leeward and windward surfaces of the building fence.
K i -coefficient taking into account changes in wind speed pressure.
The temperature and air density inside and outside the building are usually not the same, resulting in different gravitational pressure on the sides of the fences. Due to the action of the wind, backwater is created on the windward side of the building, and excess static pressure appears on the surfaces of the fences. On the windward side, a vacuum is formed and the static pressure is reduced. Thus, when there is wind, the pressure on the outside of the building is different from the pressure inside the premises. The air regime is related to the thermal regime of the building. Infiltration of outside air leads to additional heat consumption for its heating. Exfiltration of moist indoor air humidifies and reduces the thermal insulation properties of enclosures. The position and size of the infiltration and exfiltration zone in a building depend on the geometry, design features, ventilation mode of the building, as well as on the construction area, time of year and climate parameters.
Heat exchange occurs between the filtered air and the fence, the intensity of which depends on the location of the filtration in the structure (solid mass, panel joint, windows, air gaps). Thus, there is a need to calculate 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.
Infiltration is the penetration of air into a room.
Exfiltration is the removal of air from a room.
Subject of building thermophysics
Building thermophysics is a science that studies the problems of thermal, air and humidity conditions of the internal environment and enclosing structures of buildings for any purpose and deals with the creation of a microclimate in premises, using air conditioning systems (heating, cooling and ventilation) taking into account the influence of the external climate through fences.
To understand the formation of microclimate and determine possible ways impact on it, it is necessary to know the laws of radiant, convective and jet heat transfer in a room, the equations of general heat transfer of room surfaces and the equation of air heat transfer. Based on the patterns of heat exchange between a person and the environment, the conditions for thermal comfort in the room are formed.
The main resistance to heat loss from the room is provided by the heat-shielding properties of the fencing materials, therefore the laws of the heat transfer process through the fencing are the most important when calculating the space heating system. The humidity regime of the fence is one of the main ones when calculating heat transfer, since waterlogging leads to a noticeable decrease in the heat-shielding properties and durability of the structure.
The air regime of the fencing is also closely related to the thermal regime of the building, since the infiltration of external air requires the expenditure of heat to heat it, and the exfiltration of moist internal air moistens the material of the fencing.
Studying the issues discussed above will make it possible to solve the problems of creating a microclimate in buildings in conditions of efficient and economical use of fuel and energy resources.
Thermal conditions of the building
The thermal regime of a building is the totality of all factors and processes that determine the thermal environment in its premises.
The set of all engineering means and devices that provide the specified microclimate conditions in the premises of a building is called a microclimate conditioning system (MCS).
Under the influence of the difference between external and internal temperatures, solar radiation and wind, the room loses heat through the fence in winter and heats up in summer. Gravitational forces, the action of wind and ventilation create pressure differences, leading to the flow of air between communicating rooms and to its filtration through the pores of the material and leakage of the fences.
Precipitation, moisture release in rooms, the difference in humidity between indoor and outdoor air lead to moisture exchange in the room, through fences, under the influence of which it is possible to moisten materials and deteriorate the protective properties and durability of external walls and coatings.
The processes that shape the thermal environment of a room must be considered in an inextricable connection with each other, since their mutual influence can be very significant.
There are basic parameters of the air environment that determine the possibility of human existence on open area and in the home. In particular, this is the concentration of various impurities in the indoor air, depending on the air, thermal and gas conditions of the building. Harmful impurities in the ground layer of the atmosphere can be in the form of aerosols, dust particles, various gaseous substances at the molecular level.
When distributed in the air under the influence of coagulation or various chemical reactions harmful impurities can change quantitatively and in chemical composition. The gas regime of the building consists of three interconnected parts. The external part is the processes of distribution of harmful impurities in the ground layer of the atmosphere with air flows washing the building and moving harmful substances.
The edge part is the process of penetration of harmful impurities into the building through cracks in the external enclosing structures, open windows, doors, other openings and through mechanical ventilation systems, as well as the movement of impurities throughout the building. Interior— the process of distribution of harmful impurities in the premises of a building (gas regimes of premises).
For this purpose, a multi-zone model of a ventilated room is used, on the basis of which the room is considered as a set of elementary volumes, the relationship and interaction between which occurs across the boundaries of elementary volumes. Within the framework of the gas regime of the building, the convective and diffusion transport of harmful impurities is studied. The amount of air ions in the air is characterized by their concentration per cubic meter of air, and the air ion regime is part of the gas regime of the building.
Aeroions are tiny complexes of atoms or molecules that carry a positive or negative charge. Depending on their size and mobility, there are three groups of air ions: light, medium and heavy. The reasons for air ionization are different: the presence of radioactive substances in the Earth’s crust, the presence of radioactive elements in building and facing materials, natural radioactivity of both air and soil (radon and thoron), and rocks (isotopes K40, U238, Th232).
The main ionizer of air is cosmic radiation, as well as water spraying, atmospheric electricity, friction of sand particles, snow, etc. Air ionization occurs as follows: under the influence of an external factor, a gas molecule or atom is given the energy necessary to remove one electron from the nucleus. The neutral atom becomes positively charged, and the resulting free electron joins one of the neutral atoms, giving it a negative charge, forming a negative air ion.
In a fraction of a second, such positively and negatively charged air ions are joined by a certain number of molecules and gases that make up the air. As a result, complexes of molecules called light air ions are formed. Light air ions, colliding in the atmosphere with other air ions and condensation nuclei, form large air ions - medium air ions, heavy air ions, ultra-heavy air ions.
The mobility of air ions depends on the gas composition of the air, temperature and atmospheric pressure. The sizes and mobility of positive and negative air ions depend on the relative humidity of the air - with increasing humidity, the mobility of air ions decreases. The charge of an air ion is its main characteristic. If a light air ion loses its charge, then it disappears, but if a heavy or medium air ion loses its charge, the decay of such an air ion does not occur, and in the future it can acquire a charge of any sign.
The concentration of air ions is measured in the number of elementary charges per cubic meter of air: e = +1.6 × 10-19 C/m3 (e/m3). Under the influence of ionization in the air, physical and chemical processes of excitation of the main components of air - oxygen and nitrogen - occur. The most stable negative air ions can form the following elements chemical substances and their compounds: carbon atoms, oxygen molecules, ozone, carbon dioxide, nitrogen dioxide, sulfur dioxide, water molecules, chlorine and others.
The chemical composition of light air ions depends on chemical composition air environment. This both affects the gas regime of the building and room and leads to an increase in the concentration of stable molecular air ions in the air. Maximum permissible concentration (MAC) standards have been established for harmful impurities, as for neutral, uncharged molecules. Harmful effects charged molecules of impurities on the human body increases. The “contribution” of each type of molecular ion to discomfort or comfort surrounding a person air environment is different.
How cleaner air, those longer time life of light air ions, and vice versa - when the air is polluted, the lifetime of light air ions is short. Positive air ions are less mobile and live longer in comparison with negative air ions. Another factor characterizing the air-ionic regime of a building is the unipolarity coefficient, which shows the quantitative predominance of negative air ions over positive ones for any group of air ions.
For the surface layer of the atmosphere, the unipolarity coefficient is 1.1-1.2, indicating the excess of the number of negative air ions over the number of positive ones. The unipolarity coefficient depends on the following factors: time of year, terrain, geographical location and the electrode effect from the influence of the negative charge of the Earth's surface, in which the positive direction electric field near the Earth's surface creates predominantly positive air ions.
In the case of the opposite direction of the electric field, negative air ions are predominantly formed. For the hygienic assessment of the air ion regime of a room, an indicator of air pollution has been adopted, which is determined by the ratio of the sum of heavy air ions of positive and negative polarity to the sum of positive and negative light air ions. The lower the air pollution index, the more favorable the air ion regime.
The concentration of light air ions of both polarities depends significantly on the degree of urbanization of the area and on the ecological state of the human habitat. Light air ions have a therapeutic and preventive effect on the human body in a concentration of 5 × 108-1.5 × 109 e/m3. In rural areas, the concentration of light air ions is within the healthy norm for humans.
In resorts and in mountainous areas, the concentration of light air ions is slightly higher than normal, but the beneficial effect remains, and in large cities on streets with heavy traffic, the concentration of light air ions is below normal and can approach zero. This clearly indicates contamination atmospheric air. Negative air ions are more sensitive to impurities compared to positive air ions.
Vegetation has a great influence on the aeroion regime. Volatile plant emissions, called phytoncides, make it possible to qualitatively and quantitatively improve the air ion regime environment. IN pine forest the concentration of light air ions increases and the concentration of heavy air ions decreases. Among the plants that can favorably influence the aeroion regime, the following can be distinguished: snowdrop, lilac, white acacia, geranium, oleander, Siberian spruce, fir.
Phytoncides influence the air ion regime through processes of recharging air ions, due to which the transformation of medium and heavy air ions into light ones is possible. The ionization of air is important for human health and well-being. Staying people in a ventilated room with high humidity and dustiness of the air with insufficient air exchange significantly reduces the number of light air ions. At the same time, the concentration of heavy air ions increases, and dust charged with ions is retained in the human respiratory tract by 40% more.
People often complain about the lack fresh air, fatigue, headaches, decreased attention and irritability. This is due to the fact that the parameters of thermal comfort are well studied, but the parameters of air comfort are not sufficiently studied. Air being processed in the air conditioner, in the supply chamber, in the system air heating, almost completely loses air ions, and the air ion conditions in the room worsen tenfold.
Light air ions have a therapeutic and preventive effect on the human body at a concentration of 5 × 108-1.5 × 109 e/m3. During artificial ionization of air, the resulting light air ions have the same beneficial properties, the same as air ions formed naturally. In accordance with the standards, increased and decreased concentrations of light air ions in the air are classified as physically harmful factors.
There are several types of devices for artificial ionization of indoor air, among which the following types of ionizers can be distinguished: coronary, radioisotope, thermionic, hydrodynamic and photoelectric. Ionizers can be local and general, stationary and portable, regulated and unregulated, generating unipolar and bipolar light air ions.
It is beneficial to combine air ionizers with systems supply ventilation and air conditioning, it is necessary that air ionizers be located as close as possible to the serviced area of the room in order to reduce the loss of air ions during their transportation. Heating the air leads to an increase in the number of light air ions, but the interaction of air ions with the metal parts of heaters and air heaters reduces their concentration, cooling the air leads to a noticeable decrease in the concentration of light air ions, drying and humidification leads to the destruction of all light mobile air ions and the formation of heavy air ions due to water spraying .
Application plastic parts ventilation and air conditioning systems can reduce the adsorption of light air ions and increase their concentration in the room. Heating has a beneficial effect on increasing the concentration of light air ions in comparison with the concentration of light air ions in the outside air. The increase in light air ions during operation of the heating system in winter is compensated by the decrease in these air ions as a result of human activity.
After the irrigation chamber, the decrease in light negative air ions based on the molecules of ozone, oxygen and nitrogen oxide occurs tens of times, and instead of these air ions, air ions of water vapor appear. In underground rooms with limited ventilation, the reduction in the amount of light negative air ions based on the ozone and oxygen molecules occurs hundreds of times, and based on the nitrogen oxide molecule - up to 20 times.
From air conditioning systems, the concentration of heavy air ions increases slightly, but in the presence of people, the concentration of heavy air ions increases significantly. The balance of formation and destruction of light air ions can be characterized by the following significant circumstances: the entry of light air ions with the influx of outside air into the serviced premises (in the presence of light air ions outside), the change in the concentration of light air ions during the passage of air into the serviced premises ( mechanical ventilation and air conditioning reduce the concentration of air ions), a decrease in the concentration of light air ions with a large number of people in the room, high dust levels, gas combustion, etc.
An increase in the concentration of light air ions occurs with good ventilation, the presence of phytoncide-forming plants, artificial air ionizers, good home ecology and successful measures to protect and improve the state of the environment in populated areas. The nature of the change in the concentration of light positive and negative air ions in the surface layer of the atmosphere in the annual regime coincides with fluctuations in the temperature of the outside air, visibility in the atmosphere, and the duration of insolation of the territory in the annual regime.
From November to March, the concentration of heavy air ions increases and the concentration of light air ions decreases; in spring and summer, the number of all groups of heavy air ions decreases and the number of light air ions increases. In daily mode, the concentration of light air ions is maximum in the evening and night hours, when the air is clean - from eight in the evening to four in the morning, the concentration of light air ions is minimal from six in the morning to three in the afternoon.
Before a thunderstorm, the concentration of positive air ions increases; during a thunderstorm and after a thunderstorm, the number of negative air ions increases. Near waterfalls, near the sea during the surf, near fountains and in other cases of spraying and splashing of water, the number of light and heavy positive and negative air ions increases. Tobacco smoke worsens the air ion conditions in a room, reducing the amount of light air ions.
In a room of about 40 m2 with poor ventilation, depending on the number of cigarettes smoked, the concentration of light air ions decreases. The respiratory tract and human skin are areas that perceive air ions. A larger or smaller part of the light and heavy air ions, when passing through the respiratory tract, give off their charges to the walls of the air-passing tract.
An increased level of light air ions leads to a reduction in morbidity and mortality; ionized air increases the body's resistance to diseases. In the presence of clean air ionized by light air ions, performance increases, the process of restoration of performance after prolonged exercise is accelerated, and the body's resistance to toxic environmental influences increases.
Today it is known that air ionization to a value of 2 × 109-3 × 109 e/m3 has a beneficial, normalizing effect on the human body. Higher concentrations - more than 50 × 109 e/cm3 of ionization - are unfavorable, the desired level is 5 × 108-3 × 109 e/m3. The effectiveness of the air ion regime is directly related to compliance with air exchange standards. Ionized air must be dust-free and free of chemical contaminants of various origins.
Due to the temperature difference under the influence of gravitational pressure, outside air penetrates into the rooms of the lower floors through the fence; on the windward side, the action of the wind increases infiltration; with the windward one it decreases.
Internal air from the first floors tends to penetrate into the upper room (it flows through interior doors and corridors that are connected to the staircase).
From the premises upper floors air escapes through non-density external fences outside the building.
The premises on the middle floors may be in mixed mode conditions. The natural air exchange in the building is affected by the action of supply and exhaust ventilation.
1. In the absence of wind, the surfaces of external walls will act different sizes gravitational pressure. According to the law of conservation of energy, the average height pressure 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 warm internal air column will be less than the pressure of the external cold air column from the outer surface of the wall.
Zero density overpressure called the neutral plane of the building.
Figure 9.1 – Construction of excess pressure diagrams
The magnitude of excess gravitational pressure 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 will be created on the external surfaces of the fences static pressure or discharge.
According to the law of conservation of energy, the pressure inside a building with the same permeability will be equal to the average value between the increased value on the windward side and the decreased value on the windward side.
Absolute value of excess wind pressure:
, (9.2)
where k 1 ,k 2 are the aerodynamic coefficients on the windward and downwind sides of the building, respectively;
Dynamic pressure applied to a building by a stream of air.
To calculate air infiltration through the external enclosure, the difference in air pressure outside and inside the room, Pa, is:
where N w is the height of the mouth of the ventilation shaft from ground level (mark of the location of the conditional pressure zero point);
H e – the height of the center of the building element in question (window, wall, door, etc.) from ground level;
A 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;
Air pressure in the room, determined from the condition of maintaining air balance;
Excessive relative pressure in the room due to ventilation.
For example, administrative buildings of research institutes and similar buildings are characterized by balanced supply and exhaust ventilation during operating mode or complete shutdown of ventilation during non-working hours P in = 0. For such buildings, the approximate value is:
3. To assess the influence of the building’s air regime on the thermal regime, simplified calculation methods are used.
Case A. IN multi-storey building in all rooms ventilation hood is completely compensated by the ventilation inflow, 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.
Pressure inside separate rooms is between the pressure and the pressure on the outer surface of this room. We assume that due to the difference, the air sequentially passes through the windows and internal doors facing the staircase, and corridors, the initial air flow and pressure inside the room can be calculated using the formula:
where are the permeability characteristics of the window area, door from the room opening onto the corridor or staircase.
