Thermal conductivity of the air gap in the wall table. Thermal resistance of a closed air gap

The thickness of the air layer,	Thermal resistance of a closed air gap R vp, m 2 × ° С / W
	horizontal with heat flow from bottom to top and vertical		horizontal with heat flow from top to bottom
	at the air temperature in the interlayer
	positive	negative	Positive	negative

Note. When pasting one or both surfaces of the air gap with aluminum foil, the thermal resistance should be increased by 2 times.

Application 5*

Schemes of heat-conducting inclusions in enclosing structures

Application 6*

(Informative)

Reduced heat transfer resistance of windows, balcony doors and skylights

Filling the light opening	Reduced resistance to heat transfer R o , m 2 * ° C / W
	in wooden or PVC binding	in aluminum binding
1. Double glazing in twin sashes
2. Double glazing in separate sashes
3. Hollow glass blocks (with a joint width of 6 mm) size: 194x194x98	0.31 (without binding) 0.33 (without binding)
4. Profiled box glass	0.31 (without binding)
5. Double plexiglass for skylights
6. Triple plexiglass skylight
7. Triple glazing in separate-paired bindings
8. Single-chamber double-glazed window: From ordinary glass Made of glass with a soft selective coating
9. Double glazing: From ordinary glass (with a glass spacing of 6 mm) From ordinary glass (with a glass spacing of 12 mm) Made of glass with hard selective coating
10. Ordinary glass and single-chamber double-glazed window in separate bindings: From ordinary glass Made of glass with hard selective coating Made of glass with a soft selective coating Made of glass with hard selective coating and argon filling
11. Ordinary glass and double-glazed window in separate bindings: From ordinary glass Made of glass with hard selective coating Made of glass with a soft selective coating Made of glass with hard selective coating and argon filling
12. Two single-chamber double-glazed windows
13. Two single-chamber double-glazed windows in separate bindings
14. Four-layer glazing in two paired bindings

* in steel bindings

Notes:

1. Soft selective glass coatings include coatings with thermal emission less than 0.15, and hard ones - more than 0.15.

2. The values ​​of the reduced resistance to heat transfer of the fillings of the light openings are given for cases where the ratio of the glazing area to the filling area of ​​the light opening is 0.75.

The values ​​of the reduced heat transfer resistances indicated in the table may be used as design values ​​in the absence of such values ​​in the standards or technical specifications for structures or not confirmed by test results.

3. The temperature of the inner surface of the structural elements of the windows of buildings (except for industrial ones) must be at least 3 ° C at the design temperature of the outside air.

Test

on thermal physics No. 11

Thermal resistance of the air gap

1. Prove that the line of temperature decrease in the thickness of the multilayer fence in the coordinates "temperature - thermal resistance" is a straight line

2. What determines the thermal resistance of the air gap and why

3. Causes causing the occurrence of a pressure difference on one and the other side of the fence

temperature resistance air interlayer guard

1. Prove that the line of temperature decrease in the thickness of the multilayer fence in the coordinates "temperature - thermal resistance" is a straight line

Using the equation of heat transfer resistance of the fence, you can determine the thickness of one of its layers (most often insulation - the material with the lowest thermal conductivity), at which the fence will have a given (required) value of heat transfer resistance. Then the required resistance of the insulation can be calculated as, where is the sum of the thermal resistances of layers with known thicknesses, and the minimum thickness of the insulation is as follows: . For further calculations, the thickness of the insulation must be rounded up to a multiple of the unified (factory) values ​​of the thickness of a particular material. For example, the thickness of a brick is a multiple of half its length (60 mm), the thickness of concrete layers is a multiple of 50 mm, and the thickness of layers of other materials is a multiple of 20 or 50 mm, depending on the step with which they are made in factories. When conducting calculations, it is convenient to use resistances due to the fact that the temperature distribution over resistances will be linear, which means that it is convenient to carry out calculations graphically. In this case, the angle of inclination of the isotherm to the horizon in each layer is the same and depends only on the ratio of the difference between the calculated temperatures and the heat transfer resistance of the structure. And the tangent of the angle of inclination is nothing more than the density of the heat flux passing through this fence: .

Under stationary conditions, the heat flux density is constant in time, and hence, where R X- the resistance of a part of the structure, including the resistance to heat transfer of the inner surface and the thermal resistance of the layers of the structure from the inner layer to the plane on which the temperature is sought.

Then. For example, the temperature between the second and third layers of the structure can be found as follows: .

The reduced resistances to heat transfer of inhomogeneous enclosing structures or their sections (fragments) should be determined from the reference book, the reduced resistances of flat enclosing structures with heat-conducting inclusions should also be determined from the reference book.

2. What determines the thermal resistance of the air gap and why

In addition to heat transfer by thermal conduction and convection in the air gap, there is also direct radiation between the surfaces that limit the air gap.

Radiation heat transfer equation: , where b l - heat transfer coefficient by radiation, which depends to a greater extent on the materials of the interlayer surfaces (the lower the radiation coefficients of the materials, the lower and b k) and the average air temperature in the interlayer (with increasing temperature, the coefficient of heat transfer by radiation increases).

So where l eq - equivalent coefficient of thermal conductivity of the air layer. Knowing l eq, it is possible to determine the thermal resistance of the air gap. However, resistance R vp can also be determined from the reference book. They depend on the thickness of the air layer, the air temperature in it (positive or negative) and the type of layer (vertical or horizontal). The amount of heat transferred by thermal conduction, convection and radiation through vertical air layers can be judged from the following table.

Layer thickness, mm	Heat flux density, W / m 2	Amount of heat transferred in %	Equivalent coefficient of thermal conductivity, m o C / W	Thermal resistance of the interlayer, W / m 2o C
		thermal conductivity	convection	radiation
Note: the values ​​given in the table correspond to the air temperature in the interlayer equal to 0 o C, the temperature difference on its surfaces 5 o C and the emissivity of the surfaces C = 4.4.

Thus, when designing external barriers with air gaps, the following must be taken into account:

1) an increase in the thickness of the air gap has little effect on reducing the amount of heat passing through it, and thin layers (3-5 cm) are thermally efficient;

2) it is more rational to make several layers of small thickness in the fence than one layer of large thickness;

3) it is expedient to fill thick layers with low heat-conducting materials to increase the thermal resistance of the fence;

4) the air layer must be closed and not communicate with the outside air, that is, the vertical layers must be blocked by horizontal diaphragms at the level of interfloor ceilings (more frequent blocking of the layers in height is of no practical importance). If there is a need to install layers ventilated with outside air, then they are subject to special calculation;

5) due to the fact that the main part of the heat passing through the air gap is transmitted by radiation, it is desirable to place the layers closer to the outer side of the fence, which increases their thermal resistance;

6) in addition, it is recommended to cover the warmer surface of the interlayer with a material with a low emissivity (for example, aluminum foil), which significantly reduces the radiant flux. Covering both surfaces with such a material practically does not reduce heat transfer.

3. Causes causing the occurrence of a pressure difference on one and the other side of the fence

In winter, the air in heated rooms has a higher temperature than the outside air, and, therefore, the outside air has a higher volumetric weight (density) compared to the inside air. This difference in volumetric weights of air creates a difference in its pressure on both sides of the fence (thermal pressure). Air enters the room through the lower part of its outer walls, and leaves it through the upper part. In the case of air tightness of the upper and lower fences and with closed openings, the air pressure difference reaches its maximum values ​​near the floor and under the ceiling, and is equal to zero at the middle of the height of the room (neutral zone).

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For uniformity, heat transfer resistance closed air gaps located between the layers of the building envelope, called thermal resistance Rv.p, m². ºС/W.
The scheme of heat transfer through the air gap is shown in Fig.5.

Fig.5. Heat transfer in the air gap.

The heat flux passing through the air gap qv.p, W/m², consists of flows transmitted by thermal conductivity (2) qt, W/m², convection (1) qc, W/m², and radiation (3) ql, W/m².

24. Conditional and reduced resistance to heat transfer. Coefficient of thermotechnical homogeneity of enclosing structures.

25. Rationing of resistance to heat transfer based on sanitary and hygienic conditions

, R0 = *

We normalize Δ t n, then R 0 tr = * , those. in order for Δ t≤ Δ t n Necessary

R 0 ≥ R 0 tr

SNiP extends this requirement to the reduced resistance. heat transfer.

R 0 pr ≥ R 0 tr

t in - design temperature of internal air, °С;

accept. according to design standards. building

t n - - calculated winter temperature of the outside air, ° С, equal to the average temperature of the coldest five-day period with a security of 0.92

A in (alpha) - heat transfer coefficient of the inner surface of enclosing structures, taken according to SNiP

Δt n - standard temperature difference between the temperature of the internal air and the temperature of the inner surface of the enclosing structure, taken according to SNiP

Required resistance to heat transfer R tr about doors and gates must be at least 0.6 R tr about walls of buildings and structures, determined by the formula (1) at the calculated winter temperature of the outside air, equal to the average temperature of the coldest five-day period with a probability of 0.92.

When determining the required resistance to heat transfer of internal enclosing structures in formula (1), it should be taken instead of t n- the calculated air temperature of the colder room.

26. Thermotechnical calculation of the required thickness of the fence material based on the conditions for achieving the required resistance to heat transfer.

27. Humidity of the material. Reasons for wetting the structure

Humidity - physical quantity equal to the amount of water contained in the pores of the material.

It happens by weight and volume

1) Building moisture.(during the construction of the building). Depends on the design and construction method. Solid brickwork is worse than ceramic blocks. The most favorable wood (prefabricated walls). w / w not always. Should disappear in 2 = -3 years of operation. Measures: drying the walls

ground moisture. (capillary suction). It reaches a level of 2-2.5 m. Water-proof layers, with the right device does not affect.

2) Ground moisture, penetrates into the fence from the ground due to capillary suction

3)Atmospheric moisture. (slanting rain, snow). It is especially important for roofs and cornices .. solid brick walls do not require protection if the jointing is done correctly. reinforced concrete, lightweight concrete panels, attention to joints and window blocks, a textured layer of waterproof materials. Protection = protective wall on the slope

4) Operating moisture. (in the workshops of industrial buildings, mainly in the floors and lower parts of the walls) solution: waterproof floors, drainage system, lining the lower part with ceramic tiles, waterproof plaster. Protection=protective cladding with ext. sides

5)Hygroscopic moisture. Due to the increased hygroscopicity of materials (property to absorb water vapor from humid air)

6) Condensation of moisture from the air: a) on the surface of the fence. b) in the thickness of the fence

28. Influence of humidity on the properties of structures

1) With an increase in humidity, the thermal conductivity of the structure increases.

2) Humidity deformations. Humidity is much worse than thermal expansion. Peeling of the plaster due to the accumulated moisture under it, then the moisture freezes, expands in volume and tears off the plaster. Non-moisture resistant materials deform when wet. For example, gypsum becomes creeping with increasing humidity, plywood swelling, delamination.

3) Decrease in durability - number of years of failure-free operation of the structure

4) Biological damage (fungus, mold) due to dew

5) Loss of aesthetic appearance

Therefore, when choosing materials, their moisture regime is taken into account and materials with the lowest moisture content are selected. Also, excessive humidity in the room can cause the spread of diseases and infections.

From a technical point of view, it leads to a loss of durability and structure and its frost-resistant properties. Some materials at high humidity lose mechanical strength, change shape. For example, gypsum becomes creeping with increasing humidity, plywood swelling, delamination. Corrosion of metal. deterioration in appearance.

29. Sorption of water vapor builds. mater. Sorption mechanisms. Hysteresis of sorption.

Sorption- the process of absorption of water vapor, which leads to an equilibrium moisture state of the material with air. 2 phenomena. 1. Absorption as a result of the collision of a vapor molecule with the surface of the pores and sticking to this surface (adsorption)2. Direct dissolution of moisture in the volume of the body (absorption). Humidity increases with increasing relative elasticity and decreasing temperature. "desorption" if a wet sample is placed in desiccators (solution of sulfuric acid), then it gives off moisture.

Sorption mechanisms:

1.Adsorption

2. Capillary condensation

3. Volumetric filling of micropores

4.Filling the interlayer space

1 stage. Adsorption is a phenomenon in which the surface of the pores is covered with one or more layers of water molecules (in mesopores and macropores).

2 stage. Polymolecular adsorption - a multilayer adsorbed layer is formed.

3 stage. capillary condensation.

CAUSE. The saturation vapor pressure over a concave surface is less than over a flat liquid surface. In small-radius capillaries, moisture forms concave minisks, so capillary condensation is possible. If D>2*10 -5 cm, then there will be no capillary condensation.

Desorption - natural drying process.

Hysteresis ("difference") of sorption consists in the difference between the sorption isotherm obtained when the material is moistened and the desorption isotherm obtained from the dried material. shows the % difference between the sorption weight moisture and the desorption weight moisture (desorption 4.3%, sorption 2.1%, hysteresis 2.2%) when the sorption isotherm is humidified. When dried, desorption.

30. Mechanisms of moisture transfer in materials of building structures. Vapor permeability, capillary absorption of water.

1. In winter, due to the temperature difference and at different partial pressures, a stream of water vapor passes through the fence (from the inner surface to the outer) - diffusion of water vapor. In summer it's the other way around.

2. Convective transport of water vapor(with airflow)

3. Capillary water transfer(leakage) through porous materials.

4. Gravitational water leakage through cracks, holes, macropores.

Vapor permeability - the property of a material or structure made of them to pass water vapor through itself.

Permeability coefficient- Physical. the value is numerically equal to the number of steam that has passed through the plate at a unit area, at a unit pressure drop, at a unit thickness of the plate, at a unit time at a partial pressure drop on the sides of the plate e 1 Pa. Temperatures, mu decreases, with increasing humidity, mu increases.

Vapor resistance: R=thickness/mu

Mu - vapor permeability coefficient (determined according to SNIP 2379 heat engineering)

Capillary absorption of water by building materials - provides a constant transfer of liquid moisture through porous materials from a region of high concentration to a region of low concentration.

The thinner the capillaries, the greater the force of capillary suction, but in general the transfer rate decreases.

Capillary transport can be reduced or eliminated by providing an appropriate barrier (small air gap or capillary inactive layer (non-porous)).

31. Fick's law. Vapor permeability coefficient

P(amount of steam, g) \u003d (ev-en) F * z * (mu / thickness),

Mu- coefficient. vapor permeability (determined according to SNIP 2379 heat engineering)

Physical the value is numerically equal to the amount of steam that has passed through the plate at a unit area, at a unit pressure drop, at a unit plate thickness, at a unit time at a partial pressure drop on the sides of the plate e 1 Pa. [mg / (m 2 * Pa)]. The smallest mu has roofing material 0.00018, the largest mineral wool = 0.065g / m * h * mm Hg, window glass and metals are vapor-tight, air has the highest vapor permeability. When decreasing Temperatures, mu decreases, with increasing humidity, mu increases. It depends on the physical properties of the material and reflects its ability to conduct water vapor diffusing through it. Anisotropic materials have different mu (for wood, along the fibers = 0.32, across = 0.6).

Equivalent resistance to vapor permeability of the fence with a sequential arrangement of layers. Fick's law.

Q \u003d (e 1 -e 2) / R n qR n1n =(e n1n-1 -e 2)

32 Calculation of the distribution of partial pressure of water vapor over the thickness of the structure.

Heat and moisture transfer through external fences

Fundamentals of heat transfer in a building

The movement of heat always occurs from a warmer environment to a colder one. The process of transferring heat from one point in space to another due to temperature difference is called heat transfer and is collective, as it includes three elementary types of heat transfer: thermal conduction (conduction), convection and radiation. Thus, potential heat transfer is temperature difference.

Thermal conductivity

Thermal conductivity- a type of heat transfer between fixed particles of a solid, liquid or gaseous substance. Thus, thermal conductivity is the heat exchange between particles or elements of the structure of the material environment that are in direct contact with each other. When studying thermal conductivity, a substance is considered as a continuous mass, its molecular structure is ignored. In its pure form, thermal conductivity occurs only in solids, since in liquid and gaseous media it is practically impossible to ensure the immobility of a substance.

Most building materials are porous bodies. The pores contain air that has the ability to move, that is, to transfer heat by convection. It is believed that the convective component of the thermal conductivity of building materials can be neglected due to its smallness. Radiant heat exchange occurs inside the pore between the surfaces of its walls. The transfer of heat by radiation in the pores of materials is determined mainly by the size of the pores, because the larger the pore, the greater the temperature difference on its walls. When considering thermal conductivity, the characteristics of this process are related to the total mass of the substance: the skeleton and pores together.

The building envelope is usually plane-parallel walls, heat transfer in which is carried out in one direction. In addition, it is usually assumed in thermal engineering calculations of external enclosing structures that heat transfer occurs when stationary thermal conditions, that is, with the constancy in time of all the characteristics of the process: heat flow, temperature at each point, thermophysical characteristics of building materials. Therefore, it is important to consider the process of one-dimensional stationary heat conduction in a homogeneous material, which is described by the Fourier equation:

where q T - surface heat flux density passing through a plane perpendicular to heat flow, W / m 2;

λ - thermal conductivity of the material, W/m. about C;

t- temperature changing along the x axis, °C;

Attitude, is called temperature gradient, about S/m, and is denoted grad t. The temperature gradient is directed towards an increase in temperature, which is associated with the absorption of heat and a decrease in the heat flux. The minus sign on the right side of equation (2.1) shows that the increase in heat flux does not coincide with the increase in temperature.

Thermal conductivity λ is one of the main thermal characteristics of a material. As follows from equation (2.1), the thermal conductivity of a material is a measure of the conduction of heat by a material, numerically equal to the heat flux passing through 1 m 2 of an area perpendicular to the flow direction, with a temperature gradient along the flow equal to 1 o C / m (Fig. 1). The greater the value of λ, the more intense the process of thermal conductivity in such a material, the greater the heat flux. Therefore, heat-insulating materials are considered to be materials with a thermal conductivity of less than 0.3 W/m. about S.

Isotherms; - ------ - heat current lines.

Change in the thermal conductivity of building materials with a change in their density is due to the fact that almost any building material consists of skeleton- the main building material and air. K.F. For example, Fokin cites the following data: the thermal conductivity of an absolutely dense substance (without pores), depending on the nature, has a thermal conductivity from 0.1 W / m o C (for plastic) to 14 W / m o C (for crystalline substances with a heat flux along the crystalline surface), while air has a thermal conductivity of about 0.026 W / m o C. The higher the density of the material (less porosity), the greater the value of its thermal conductivity. It is clear that light heat-insulating materials have a relatively low density.

Differences in porosity and thermal conductivity of the skeleton lead to differences in the thermal conductivity of materials, even at the same density. For example, the following materials (Table 1) at the same density, ρ 0 \u003d 1800 kg / m 3, have different thermal conductivity values:

Table 1.

The thermal conductivity of materials with the same density is 1800 kg/m 3 .

With a decrease in the density of the material, its thermal conductivity l decreases, since the influence of the conductive component of the thermal conductivity of the material skeleton decreases, but, however, the influence of the radiation component increases. Therefore, a decrease in density below a certain value leads to an increase in thermal conductivity. That is, there is a certain density value at which the thermal conductivity has a minimum value. There are estimates that at 20 ° C in pores with a diameter of 1 mm, the thermal conductivity by radiation is 0.0007 W / (m ° C), with a diameter of 2 mm - 0.0014 W / (m ° C), etc. Thus, the thermal conductivity by radiation becomes significant for heat-insulating materials with low density and significant pore sizes.

The thermal conductivity of a material increases with an increase in the temperature at which heat transfer occurs. An increase in the thermal conductivity of materials is explained by an increase in the kinetic energy of the molecules of the skeleton of a substance. The thermal conductivity of air in the pores of the material also increases, and the intensity of heat transfer in them by radiation. In construction practice, the dependence of thermal conductivity on temperature is of little importance. Vlasov:

λ o = λ t / (1+β . t), (2.2)

where λ o is the thermal conductivity of the material at 0 o C;

λ t - thermal conductivity of the material at t about C;

β - temperature coefficient of change in thermal conductivity, 1/ o C, for various materials, equal to about 0.0025 1/ o C;

t is the temperature of the material at which its thermal conductivity is equal to λ t .

For a flat homogeneous wall of thickness δ (Fig. 2), the heat flux transferred by thermal conductivity through a homogeneous wall can be expressed by the equation:

where τ 1 ,τ 2- temperature values ​​on the wall surfaces, o C.

It follows from expression (2.3) that the temperature distribution over the wall thickness is linear. The value δ/λ is named thermal resistance of the material layer and marked R T, m 2. about C / W:

Fig.2. Temperature distribution in a flat homogeneous wall

Therefore, the heat flux q T, W / m 2, through a homogeneous plane-parallel wall with a thickness δ , m, from a material with thermal conductivity λ, W/m. about C, can be written in the form

The thermal resistance of the layer is the thermal conductivity resistance, equal to the temperature difference on opposite surfaces of the layer when a heat flux passes through it with a surface density of 1 W/m 2 .

Heat transfer by thermal conductivity takes place in the material layers of the building envelope.

Convection

Convection- transfer of heat by moving particles of matter. Convection takes place only in liquid and gaseous substances, as well as between a liquid or gaseous medium and the surface of a solid body. In this case, there is a transfer of heat and thermal conductivity. The combined effect of convection and heat conduction in the boundary region near the surface is called convective heat transfer.

Convection takes place on the outer and inner surfaces of the building fences. Convection plays a significant role in the heat exchange of the internal surfaces of the room. At different temperatures of the surface and the air adjacent to it, the heat transfers to a lower temperature. The heat flux transmitted by convection depends on the mode of motion of the liquid or gas washing the surface, on the temperature, density and viscosity of the moving medium, on the surface roughness, on the difference between the temperatures of the surface and the surrounding medium.

The process of heat exchange between the surface and the gas (or liquid) proceeds differently depending on the nature of the occurrence of gas motion. Distinguish natural and forced convection. In the first case, the movement of gas occurs due to the temperature difference between the surface and the gas, in the second - due to forces external to this process (fan operation, wind).

Forced convection in the general case can be accompanied by the process of natural convection, but since the intensity of forced convection noticeably exceeds the intensity of natural convection, when considering forced convection, natural convection is often neglected.

In the future, only stationary processes of convective heat transfer will be considered, assuming that the speed and temperature are constant in time at any point in the air. But since the temperature of the elements of the room changes rather slowly, the dependences obtained for stationary conditions can be extended to the process non-stationary thermal conditions of the room, at which at each considered moment the process of convective heat transfer on the inner surfaces of the fences is considered to be stationary. The dependences obtained for stationary conditions can also be extended to the case of a sudden change in the nature of convection from natural to forced, for example, when a recirculation device for heating a room (fan coil or split system in heat pump mode) is turned on in a room. Firstly, the new air movement regime is established quickly and, secondly, the required accuracy of the engineering assessment of the heat transfer process is lower than possible inaccuracies from the lack of heat flux correction during the transition state.

For engineering practice of calculations for heating and ventilation, convective heat transfer between the surface of the building envelope or pipe and air (or liquid) is important. In practical calculations, to estimate the convective heat flux (Fig. 3), Newton's equations are used:

, (2.6)

where q to- heat flux, W, transferred by convection from the moving medium to the surface or vice versa;

ta- temperature of the air washing the surface of the wall, o C;

τ - temperature of the wall surface, o C;

α to- coefficient of convective heat transfer on the wall surface, W / m 2. o C.

Fig.3 Convective heat exchange of the wall with air

Convection heat transfer coefficient, a to- a physical quantity numerically equal to the amount of heat transferred from air to the surface of a solid body by convective heat transfer at a difference between air temperature and body surface temperature equal to 1 o C.

With this approach, the entire complexity of the physical process of convective heat transfer lies in the heat transfer coefficient, a to. Naturally, the value of this coefficient is a function of many arguments. For practical use, very approximate values ​​are accepted a to.

Equation (2.5) can be conveniently rewritten as:

where R to - resistance to convective heat transfer on the surface of the enclosing structure, m 2. o C / W, equal to the temperature difference on the surface of the fence and the air temperature during the passage of a heat flux with a surface density of 1 W / m 2 from the surface to the air or vice versa. Resistance R to is the reciprocal of the convective heat transfer coefficient a to:

Radiation

Radiation (radiant heat transfer) is the transfer of heat from the surface to the surface through a radiant medium by electromagnetic waves that transform into heat (Fig. 4).

Fig.4. Radiant heat transfer between two surfaces

Any physical body that has a temperature other than absolute zero radiates energy into the surrounding space in the form of electromagnetic waves. The properties of electromagnetic radiation are characterized by the wavelength. Radiation that is perceived as thermal and has wavelengths in the range of 0.76 - 50 microns is called infrared.

For example, radiant heat exchange occurs between surfaces facing the room, between the outer surfaces of various buildings, the surfaces of the earth and sky. Radiant heat exchange between the inner surfaces of the room enclosures and the surface of the heater is important. In all these cases, the radiant medium that transmits thermal waves is air.

In the practice of calculating the heat flux in radiant heat transfer, a simplified formula is used. The intensity of heat transfer by radiation q l, W / m 2, is determined by the temperature difference of the surfaces involved in radiant heat transfer:

, (2.9)

where τ 1 and τ 2 are the temperature values ​​of the surfaces exchanging radiant heat, o C;

α l - coefficient of radiant heat transfer on the wall surface, W / m 2. o C.

Heat transfer coefficient by radiation, a l- a physical quantity numerically equal to the amount of heat transferred from one surface to another by radiation at a difference between the surface temperatures equal to 1 o C.

We introduce the concept resistance to radiant heat transfer R l on the surface of the building envelope, m 2. o C / W, equal to the temperature difference on the surfaces of the fences exchanging radiant heat, when passing from the surface to the surface of a heat flux with a surface density of 1 W / m 2.

Then equation (2.8) can be rewritten as:

Resistance R l is the reciprocal of the radiant heat transfer coefficient a l:

Thermal resistance of the air gap

For uniformity, heat transfer resistance closed air gaps located between the layers of the building envelope, called thermal resistance R in. p, m 2. about C / W.

The scheme of heat transfer through the air gap is shown in Fig.5.

Fig.5. Heat transfer in the air gap

Heat flux passing through the air gap q c. P, W / m 2, consists of flows transmitted by thermal conductivity (2) q t, W/m 2 , convection (1) q to, W/m 2 , and radiation (3) q l, W/m 2 .

q c. p \u003d q t + q k + q l . (2.12)

In this case, the share of the flux transmitted by radiation is the largest. Let us consider a closed vertical air layer, on the surfaces of which the temperature difference is 5 ° C. With an increase in the thickness of the layer from 10 mm to 200 mm, the proportion of heat flow due to radiation increases from 60% to 80%. In this case, the share of heat transferred by thermal conductivity drops from 38% to 2%, and the share of convective heat flow increases from 2% to 20%.

The direct calculation of these components is rather cumbersome. Therefore, the regulatory documents provide data on the thermal resistance of closed air spaces, which were compiled by K.F. Fokin based on the results of experiments by M.A. Mikheev. If there is a heat-reflecting aluminum foil on one or both surfaces of the air gap, which hinders radiant heat transfer between the surfaces framing the air gap, the thermal resistance should be doubled. To increase the thermal resistance by closed air spaces, it is recommended to keep in mind the following conclusions from the studies:

1) thermally efficient are interlayers of small thickness;

2) it is more rational to make several layers of small thickness in the fence than one large one;

3) it is desirable to place air gaps closer to the outer surface of the fence, since in this case the heat flux by radiation decreases in winter;

4) vertical layers in the outer walls must be blocked by horizontal diaphragms at the level of interfloor ceilings;

5) to reduce the heat flux transmitted by radiation, one of the interlayer surfaces can be covered with aluminum foil having an emissivity of about ε=0.05. Covering both surfaces of the air gap with foil does not significantly reduce heat transfer compared to covering one surface.

Questions for self-control

1. What is the heat transfer potential?

2. List the elementary types of heat transfer.

3. What is heat transfer?

4. What is thermal conductivity?

5. What is the thermal conductivity of the material?

6. Write the formula for the heat flux transferred by thermal conductivity in a multilayer wall at known temperatures of the inner t in and outer t n surfaces.

7. What is thermal resistance?

8. What is convection?

9. Write the formula for the heat flux transferred by convection from air to the surface.

10. Physical meaning of the coefficient of convective heat transfer.

11. What is radiation?

12. Write the formula for the heat flux transmitted by radiation from one surface to another.

13. Physical meaning of the radiant heat transfer coefficient.

14. What is the name of the resistance to heat transfer of a closed air gap in the building envelope?

15. Of what nature does the total heat flow through the air gap consist of heat flows?

16. What nature of the heat flow prevails in the heat flow through the air gap?

17. How does the thickness of the air gap affect the distribution of flows in it.

18. How to reduce the heat flow through the air gap?

One of the techniques that increase the thermal insulation qualities of fences is the installation of an air gap. It is used in the construction of external walls, ceilings, windows, stained-glass windows. In walls and ceilings, it is also used to prevent waterlogging of structures.

The air gap can be sealed or ventilated.

Consider heat transfer sealed air layer.

The thermal resistance of the air layer R al cannot be defined as the thermal conductivity resistance of the air layer, since the heat transfer through the layer at a temperature difference on the surfaces occurs mainly by convection and radiation (Fig. 3.14). The amount of heat,

transmitted by thermal conductivity is small, since the coefficient of thermal conductivity of air is low (0.026 W / (m ºС)).

In the layers, in general, the air is in motion. In vertical - it moves up along the warm surface and down - along the cold. Convective heat transfer takes place, and its intensity increases with an increase in the thickness of the interlayer, since the friction of air jets against the walls decreases. When heat is transferred by convection, the resistance of the boundary layers of air at two surfaces is overcome, therefore, to calculate this amount of heat, the heat transfer coefficient α k should be halved.

To describe heat transfer jointly by convection and thermal conductivity, the convective heat transfer coefficient α "k is usually introduced, equal to

α" k \u003d 0.5 α k + λ a / δ al, (3.23)

where λ a and δ al are the thermal conductivity of air and the thickness of the air gap, respectively.

This coefficient depends on the geometric shape and dimensions of the air spaces, the direction of the heat flow. By summarizing a large amount of experimental data based on the theory of similarity, M.A. Mikheev established certain patterns for α "to. In Table 3.5, as an example, the values ​​\u200b\u200bof the coefficients α" to, calculated by him at an average air temperature in a vertical layer t \u003d + 10º C .

Table 3.5

Coefficients of convective heat transfer in a vertical air gap

The coefficient of convective heat transfer in horizontal air layers depends on the direction of the heat flow. If the upper surface is heated more than the lower surface, there will be almost no air movement, since warm air is concentrated at the top and cold air at the bottom. Therefore, the equality

α" to \u003d λ a / δ al.

Consequently, the convective heat transfer decreases significantly, and the thermal resistance of the interlayer increases. Horizontal air gaps are effective, for example, when used in insulated basement ceilings above cold underground floors, where the heat flow is directed from top to bottom.

If the heat flow is directed from the bottom up, then there are ascending and descending air flows. Heat transfer by convection plays a significant role, and the value of α" k increases.

To take into account the effect of thermal radiation, the coefficient of radiant heat transfer α l is introduced (Chapter 2, p. 2.5).

Using formulas (2.13), (2.17), (2.18), we determine the coefficient of heat transfer by radiation α l in the air gap between the structural layers of brickwork. Surface temperatures: t 1 = + 15 ºС, t 2 = + 5 ºС; the degree of blackness of the brick: ε 1 = ε 2 = 0.9.

By formula (2.13) we find that ε = 0.82. Temperature coefficient θ = 0.91. Then α l \u003d 0.82 ∙ 5.7 ∙ 0.91 \u003d 4.25 W / (m 2 ºС).

The value of α l is much greater than α "to (see Table 3.5), therefore, the main amount of heat through the interlayer is transferred by radiation. In order to reduce this heat flux and increase the resistance to heat transfer of the air layer, it is recommended to use reflective insulation, that is, a coating of one or both surfaces, for example, with aluminum foil (the so-called "reinforcement"). Such a coating is usually arranged on a warm surface to avoid moisture condensation, which worsens the reflective properties of the foil. "Reinforcement" of the surface reduces the radiant flux by about 10 times.

The thermal resistance of a sealed air gap at a constant temperature difference on its surfaces is determined by the formula

Table 3.6

Thermal resistance of closed air spaces

Air layer thickness, m	R al, m 2 °C / W
for horizontal layers with heat flow from bottom to top and for vertical layers	for horizontal layers with heat flow from top to bottom
summer	winter	summer	winter
0,01	0,13	0,15	0,14	0,15
0,02	0,14	0,15	0,15	0,19
0,03	0,14	0,16	0,16	0,21
0,05	0,14	0,17	0,17	0,22
0,1	0,15	0,18	0,18	0,23
0,15	0,15	0,18	0,19	0,24
0,2-0.3	0,15	0,19	0,19	0,24

R al values ​​for closed flat air gaps are given in Table 3.6. These include, for example, interlayers between layers of dense concrete, which practically does not allow air to pass through. It has been experimentally shown that in brickwork with insufficient filling of the joints between the bricks with mortar, there is a violation of tightness, that is, the penetration of outside air into the interlayer and a sharp decrease in its resistance to heat transfer.

When covering one or both surfaces of the interlayer with aluminum foil, its thermal resistance should be doubled.

At present, walls with ventilated air layer (walls with a ventilated facade). A hinged ventilated facade is a structure consisting of cladding materials and a substructure, which is attached to the wall in such a way that an air gap remains between the protective and decorative cladding and the wall. For additional insulation of external structures, a heat-insulating layer is installed between the wall and the cladding, so that a ventilation gap is left between the cladding and thermal insulation.

The design scheme of the ventilated facade is shown in Figure 3.15. According to SP 23-101, the thickness of the air gap should be in the range from 60 to 150 mm.

Structural layers located between the air gap and the outer surface are not taken into account in the heat engineering calculation. Consequently, the thermal resistance of the outer cladding is not included in the heat transfer resistance of the wall, determined by formula (3.6). As noted in clause 2.5, the heat transfer coefficient of the outer surface of the building envelope with ventilated air spaces α ext for the cold period is 10.8 W / (m 2 ºС).

The design of a ventilated facade has a number of significant advantages. In paragraph 3.2, the temperature distributions in the cold period in two-layer walls with internal and external insulation were compared (Fig. 3.4). A wall with external insulation is more

“warm”, since the main temperature difference occurs in the heat-insulating layer. There is no condensation inside the wall, its heat-shielding properties do not deteriorate, additional vapor barrier is not required (chapter 5).

The air flow that occurs in the layer due to the pressure drop contributes to the evaporation of moisture from the surface of the insulation. It should be noted that a significant mistake is the use of vapor barrier on the outer surface of the heat-insulating layer, as it prevents the free removal of water vapor to the outside.