Thermal resistance of a closed air gap. Determination of speed and air temperature in the interlayer

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

Fig. 5. Heat exchange in the air gap.

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

24. Conditional and reduced heat transfer resistance. The coefficient of thermotechnical uniformity of enclosing structures.

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

, R 0 \u003d *

Normalize Δ t n, then R 0 mp \u003d * , those. so that Δ t≤ Δ t n

R 0 ≥ R 0 tr

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

R 0 pr ≥ R 0 tr

t in - the estimated temperature of the internal air, ° C;

accepted according to the standards for the design. building

t n - - calculated winter outdoor temperature, ° С, equal to the average temperature of the coldest five-day security of 0.92

A in (alpha) is the heat transfer coefficient of the inner surface of the building envelope, taken according to SNiP

Δt n - normative temperature difference between the temperature of the internal air and the temperature of the inner surface of the building, taken according to SNiP

Required Heat Resistance 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 outdoor air equal to the average temperature of the coldest five-day security of 0.92.

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

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

27. The moisture content 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 in mass and volumetric

1) Building moisture. (during the construction of the building). Depends on the design and method of construction. Solid brickwork is worse than ceramic blocks. The most favorable wood (prefabricated walls). reinforced concrete is not always. Should disappear in 2 \u003d -3 years of operation. Measures: wall drying

Ground moisture. (capillary absorption). It reaches a level of 2-2.5 m. Waterproofing layers, with the correct device does not affect.

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

3) Atmospheric moisture. (oblique rain, snow). It is especially important for roofs and curtain rods .. solid brick walls do not require protection with correctly made jointing. Reinforced concrete, lightweight concrete panels, attention to joints and window blocks, textured layer of waterproof materials. Protection \u003d protective wall on the slope

4) Operating moisture. (in the workshops of industrial buildings, mainly in the floors and the lower part of the walls) solution: waterproof floors, drainage device, facing the lower part with ceramic tiles, waterproof plaster. Protection \u003d protective lining with ext. the parties

5) Hygroscopic moisture. It is caused by increased hygroscopicity of mat.-fishing (the ability 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. The effect of humidity on the properties of structures

1) With increasing humidity, the thermal conductivity of the structure increases.

2) Humid deformation. Humidity is much worse than thermal expansion. Peeling the plaster as a result of 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 acquires creep with increasing humidity., Plywood swelling, delamination.

3) Decreased durability-years of uptime design

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

5) Loss of aesthetic appearance

Therefore, when choosing materials, their humidity regime is taken into account and materials with low humidity 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 loss of durability and design and its frost-resistant sv. Some materials with high humidity lose their mechanical strength, change shape. For example, gypsum acquires creep with increasing humidity., Plywood swelling, delamination. Corrosion of metal. deterioration in appearance.

29. Sorption of water vapor builds. Mater. Sorption mechanisms. Sorption hysteresis.

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 molecule of pairs with the surface of the pores and adhesion to this surface (adsorption) 2. Direct dissolution of moisture in the body volume (absorption). Humidity increases with an increase in relative elasticity and a decrease in temperature. "Desorption" if the wet sample is placed in desiccators (sulfuric acid solution), then it gives off moisture.

Sorption mechanisms:

1. Adsorption

2. Capillary condensation

3.Volume micropore filling

4. Filling the interlayer space

Stage 1 Adsorption is a phenomenon in which the surface of the pores is coated 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. Saturated vapor pressure over a concave surface is less than over a flat liquid surface. In capillaries of small radius, moisture forms concave miniski, therefore, the possibility of capillary condensation appears. If D\u003e 2 * 10 -5 cm, then there will be no capillary condensation.

Desorption -the process of naturally drying the material.

Hysteresis (“difference”) sorption is the difference in the sorption isotherm obtained by wetting the material from the desorption isotherm obtained from the dried material. shows the% difference between the weight humidity during sorption and the weight with desorption humidity (desorption 4.3%, sorption 2.1%, hysteresis 2.2%) when the sorption isotherm is moistened. When desorption dries.

30. Moisture transfer mechanisms in building materials. Vapor permeability, capillary absorption of water.

1.In winter, due to temperature differences and at different partial pressures, a stream of water vapor (from the inner surface to the outer) passes through the fence - water vapor diffusion.In the summer, on the contrary.

2. Convective water vapor transfer (with air flow)

3. Capillary water transfer (seepage) through porous mater.

4. Gravity leakage of water through cracks, holes, macropores.

Vapor permeability -the quality of the material or structure made of them, to pass water vapor through it.

Permeability coefficient - Fizich. the value is numerically equal to the number of steam passing 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 .. When decreasing. Temperature, mu decreases, with increased humidity mu increases.

Vapor Resistance: R \u003d thickness / mu

Mu-coefficient of vapor permeability (determined by SNIP 2379 heat engineering)

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

The thinner the capillaries, the greater the strength of capillary absorption, but in general, the transfer rate decreases.

The capillary transfer can be reduced or eliminated by the installation of an appropriate barrier (a small air gap or a 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 by SNIP 2379 heat engineering)

Fizich. the value is numerically equal to the number of steam passing 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 plate sides e 1 Pa. [mg / (m 2 * Pa)]. The smallest mu has a ruberoid of 0.00018, the largest min.vat \u003d 0.065g / m * h * mmHg, window glass and metals are vapor-tight, air is the largest vapor permeation. With a decrease. Temperature, mu decreases, with increased 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 (in a tree, along the fibers \u003d 0.32, across \u003d 0.6).

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

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

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

Heat and moisture transfer through exterior fencing

Building Heat Transfer Basics

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

Thermal conductivity

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

Most building materials are porous bodies. There is air in the pores that can move, that is, transfer heat through convection. It is believed that the convective component of the thermal conductivity of building materials can be neglected due to its smallness. Inside the pore between the surfaces of its walls, radiant heat transfer occurs. The heat transfer by radiation in the pores of materials is determined mainly by the pore size, because the larger the pores, 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 the pores together.

The building envelope is usually flat parallel wallsheat transfer in which is carried out in one direction. In addition, it is usually accepted in thermotechnical calculations of external building envelopes that heat transfer occurs when stationary thermal conditions, that is, with the constant in time of all process characteristics: heat flow, temperature at each point, thermophysical characteristics of building materials. Therefore it is important to consider the process of one-dimensional stationary thermal conductivity in a homogeneous material, which is described by the Fourier equation:

where q T - surface heat flux densitypassing through a plane perpendicular heat flux, 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, o C / m, and is indicated grad t. The temperature gradient is directed towards an increase in temperature, which is associated with the absorption of heat and a decrease in heat flux. The minus sign on the right side of equation (2.1) indicates 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 the material. As follows from equation (2.1), the thermal conductivity of a material is a measure of the heat conductivity of a material, numerically equal to the heat flux passing through 1 m2 of area perpendicular to the direction of the flow, with a temperature gradient along the flow equal to 1 ° C / m (Fig. 1). The larger the value of λ, the more intense the heat conduction process in such a material, the greater the heat flux. Therefore, materials with a thermal conductivity of less than 0.3 W / m are considered to be thermal insulation materials. about S.

Isotherms; - ------ - heat flow lines.

Change in thermal conductivity of building materials with a change in their density 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 of 0.1 W / m o C (in plastic) to 14 W / m o C (in crystalline substances with a heat flow 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 leads to differences in 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 values \u200b\u200bof thermal conductivity:

Table 1.

Thermal conductivity of materials with the same density of 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 skeleton of the material 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, thermal conductivity by radiation becomes significant for heat-insulating materials with low density and significant pore sizes.

The thermal conductivity of the material increases with increasing temperature at which heat transfer occurs. The increase in 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 does not matter much.d. To convert the thermal conductivity of materials obtained at temperatures up to 100 ° C to their values \u200b\u200bat 0 ° C, the empirical formula O.E. Vlasova:

λ o \u003d λ t / (1 + β. t), (2.2)

where λ about - thermal conductivity of the material at 0 about C;

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

β is the temperature coefficient of change in thermal conductivity, 1 / о С, for various materials, equal to about 0.0025 1 / о С;

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

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

where τ 1, τ 2- temperature values \u200b\u200bon the wall surfaces, о С.

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

Fig. 2. Temperature distribution in a flat uniform wall

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

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

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

Convection

Convection - heat transfer 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. In this case, heat transfer and thermal conductivity occur. 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. In the heat transfer of the interior surfaces of the room, convection plays a significant role. At different values \u200b\u200bof the surface temperature and the air adjacent to it, the transition of heat to a lower temperature occurs. The heat flux transmitted by convection depends on the mode of motion of the liquid or gas washing the surface, the temperature, density and viscosity of the moving medium, the surface roughness, and the difference between the temperatures of the surface and the medium washing it.

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

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

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

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

, (2.6)

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

t a - temperature of the air washing the wall surface, о С;

τ - wall surface temperature, о С;

α to - convective heat transfer coefficient on the wall surface, W / m 2. о С.

Fig. 3 Convective heat transfer 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 by convective heat transfer with the difference between the air temperature and the surface temperature of the body equal to 1 about C.

With this approach, the whole complexity of the physical process of convective heat transfer lies in the heat transfer coefficient, a to. Naturally, the magnitude of this coefficient is a function of many arguments. For practical use, very approximate values \u200b\u200bare accepted a to.

Equation (2.5) is conveniently rewritten in the form:

where R to - convective heat transfer resistance on the surface of the enclosing structure, m 2. о С / W, equal to the difference between the temperature on the surface of the fence and the air temperature during the passage of heat flow 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 surface to 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 having 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 a wavelength. Radiation, which is perceived as thermal and having 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. The radiant heat exchange between the inner surfaces of the room fences and the surface of the heater is important. In all these cases, air is the radiation-transparent medium that transmits thermal waves.

In the practice of calculating the heat flux during 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 \u200b\u200bof the surfaces exchanging radiant heat, о С;

α l - the coefficient of radiant heat transfer on the wall surface, W / m 2. about 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 when the difference between the surface temperatures is 1 ° C.

Introduce the concept resistance to radiant heat transfer R l on the surface of the enclosing structure, m 2. о С / W, equal to the temperature difference on the surfaces of the fences exchanging radiant heat, when passing from the surface to the surface of the heat flux with a surface density of 1 W / m 2.

Then equation (2.8) can be rewritten in the form:

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 spaceslocated between the layers of the building, called thermal resistanceR in. p, m 2. about C / W.

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

Fig. 5. Heat exchange in the air gap

Heat flow through the air gap q in. 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 in. n \u003d q t + q k + q l . (2.12)

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

Direct calculation of these components is rather cumbersome. Therefore, normative documents provide data on the thermal resistances of closed air gaps, which were compiled by K.F. in the 1950s. Fokin according to the results of experiments M.A. Mikheeva. If there is a heat-reflecting aluminum foil on one or both surfaces of the air gap that impedes the radiant heat exchange between the surfaces framing the air gap, the thermal resistance should be doubled. To increase the thermal resistance by closed air gaps, it is recommended to keep in mind the following conclusions from studies:

1) interlayers of small thickness are effective in the heat engineering relation;

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

3) it is desirable to arrange the air spaces closer to the outer surface of the fence, since in this case the heat flux 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, it is possible to cover one of the surfaces of the interlayer with aluminum foil having an emissivity of about ε \u003d 0.05. Foil coating of both surfaces of the air gap practically does not reduce heat transfer compared to coating 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 a material?

6. Write the formula for the heat flux transmitted by the thermal conductivity in the multilayer wall at known temperatures of internal t in and external t n surfaces.

7. What is thermal resistance?

8. What is convection?

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

10. The 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. The physical meaning of the coefficient of radiant heat transfer.

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

15. What kind of heat fluxes is the nature of the total heat flux through the air gap?

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

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

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

One of the methods that increase the thermal insulation quality 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 may be sealed or ventilated.

Consider heat transfer tight air gap.

The thermal resistance of the air gap R al cannot be defined as the thermal conductivity of the air layer, since 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 small (0.026 W / (m · ºС)).

In the interlayers, 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 increasing thickness of the interlayer, since the friction of air jets against the walls decreases. During convection heat transfer, 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 together with convection and heat conduction, the convective heat transfer coefficient α "k, usually 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 size of the air layers, the direction of heat flow. By summarizing a large amount of experimental data based on the theory of similarity, M.A. Mikheev established certain patterns for α "k. Table 3.5 shows, as an example, the values \u200b\u200bof the coefficients α" k calculated by him at an average air temperature in the vertical layer t \u003d + 10º С .

Table 3.5

Coefficients of convective heat transfer in a vertical air gap

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

α "k \u003d λ a / δ al.

Consequently, convective heat transfer decreases significantly, and the thermal resistance of the interlayer increases. Horizontal air spaces are effective, for example, when they are used in insulated basement ceilings over cold undergrounds, where the heat flux is directed from top to bottom.

If the heat flow is directed from the bottom up, then there are ascending and descending air flows. Convection heat transfer 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, clause 2.5).

Using formulas (2.13), (2.17), (2.18) we determine the heat transfer coefficient by radiation α l in the air gap between the structural layers of masonry. Surface temperatures: t 1 \u003d + 15 ºС, t 2 \u003d + 5 ºС; brick blackness: ε 1 \u003d ε 2 \u003d 0.9.

By formula (2.13), we find that ε \u003d 0.82. Temperature coefficient θ \u003d 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 larger than α "k (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 heat transfer resistance of the air interlayer, 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. surface reduces 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 enclosed air spaces

The thickness of the air gap, m	R al, m 2 · ºС / W
for horizontal interlayers with heat flow from the bottom up and for vertical interlayers	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 \u200b\u200bfor closed planar air spaces 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 the mortar, a leakage occurs, that is, the penetration of external air into the interlayer and a sharp decrease in its resistance to heat transfer.

When one or both surfaces of a layer are coated with aluminum foil, its thermal resistance should be doubled.

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

The design diagram of the ventilated facade is shown in Fig. 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. Therefore, the thermal resistance of the outer cladding is not included in the heat transfer resistance of the wall, determined by the formula (3.6). As noted in Section 2.5, the heat transfer coefficient of the outer surface of the building with ventilated air gaps α ext for the cold period is 10.8 W / (m 2 · ºС).

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

“Warm”, since the main temperature difference occurs in the insulating layer. Condensation does not form inside the wall, its heat-shielding properties do not deteriorate, and additional vapor barrier is not required (chapter 5).

The air flow arising in the interlayer due to the pressure drop promotes 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 insulating layer, as it prevents the free removal of water vapor outside.

The table shows the thermal conductivity of air λ depending on temperature at normal atmospheric pressure.

The value of the coefficient of thermal conductivity of air is necessary in the calculation of heat transfer and is part of the similarity numbers, for example, such as the Prandtl, Nusselt, and Bio numbers.

Thermal conductivity is expressed in dimension and is given for gaseous air in the temperature range from -183 to 1200 ° C. For example, at a temperature of 20 ° C and normal atmospheric pressure, the thermal conductivity of air is 0.0259 W / (m · deg).

At low negative temperatures, the cooled air has low thermal conductivity, for example, at a temperature of minus 183 ° C, it is only 0.0084 W / (m · deg).

According to the table shows that with increasing temperature, the thermal conductivity of air increases. So, with an increase in temperature from 20 to 1200 ° C, the thermal conductivity of air increases from 0.0259 to 0.0915 W / (m · deg), that is, more than 3.5 times.

Thermal conductivity of air depending on temperature - table

t, ° С	λ, W / (m	t, ° С	λ, W / (m	t, ° С	λ, W / (m	t, ° С	λ, W / (m
-183	0,0084	-30	0,022	110	0,0328	450	0,0548
-173	0,0093	-20	0,0228	120	0,0334	500	0,0574
-163	0,0102	-10	0,0236	130	0,0342	550	0,0598
-153	0,0111	0	0,0244	140	0,0349	600	0,0622
-143	0,012	10	0,0251	150	0,0357	650	0,0647
-133	0,0129	20	0,0259	160	0,0364	700	0,0671
-123	0,0138	30	0,0267	170	0,0371	750	0,0695
-113	0,0147	40	0,0276	180	0,0378	800	0,0718
-103	0,0155	50	0,0283	190	0,0386	850	0,0741
-93	0,0164	60	0,029	200	0,0393	900	0,0763
-83	0,0172	70	0,0296	250	0,0427	950	0,0785
-73	0,018	80	0,0305	300	0,046	1000	0,0807
-50	0,0204	90	0,0313	350	0,0491	1100	0,085
-40	0,0212	100	0,0321	400	0,0521	1200	0,0915

Thermal conductivity of air in liquid and gaseous states at low temperatures and pressures up to 1000 bar

The table shows the thermal conductivity of air at low temperatures and pressures up to 1000 bar.
Thermal conductivity is expressed in W / (m · deg), the temperature range is from 75 to 300K (from -198 to 27 ° C).

The thermal conductivity of air in a gaseous state increases with increasing pressure and temperature.
Air in liquid state tends to decrease thermal conductivity with increasing temperature.

The bar under the values \u200b\u200bin the table means the transition of liquid air into gas - the numbers under the bar refer to gas, and above it - to the liquid.
The change in the state of aggregation of air significantly affects the value of the coefficient of thermal conductivity - thermal conductivity of liquid air is much higher.

The thermal conductivity in the table is indicated to the degree of 10 3. Remember to divide by 1000!

Thermal conductivity of gaseous air at a temperature of 300 to 800K and various pressures

The table shows the values \u200b\u200bof thermal conductivity of air at various temperatures depending on pressure from 1 to 1000 bar.
Thermal conductivity is expressed in W / (m · deg), the temperature range is from 300 to 800K (from 27 to 527 ° C).

According to the table, it is seen that with increasing temperature and pressure, the thermal conductivity of air increases.
Be careful! The thermal conductivity in the table is indicated to the degree of 10 3. Remember to divide by 1000!

Thermal conductivity of air at high temperatures and pressures from 0.001 to 100 bar

The table shows the thermal conductivity of air at high temperatures and pressures from 0.001 to 1000 bar.
Thermal conductivity is expressed in W / (m · deg), temperature range from 1500 to 6000K (from 1227 to 5727 ° C).

With increasing temperature, the air molecules dissociate and the maximum value of its thermal conductivity is achieved at a pressure (discharge) of 0.001 atm. and temperature 5000K.
Note: Be careful! The thermal conductivity in the table is indicated to the degree of 10 3. Remember to divide by 1000!

Test

thermophysics No. 11

Thermal resistance of the air gap

1. To prove that the line of temperature reduction 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. The causes of the occurrence of the pressure difference on one or the other side of the fence

temperature resistance air layer fencing

1. To prove that the line of temperature reduction in the thickness of the multilayer fence in the coordinates "temperature - thermal resistance" is a straight line

Using the equation of resistance to heat transfer of the fence, you can determine the thickness of one of its layers (most often insulation - the material with the lowest coefficient of thermal conductivity), at which the fence will have a given (required) value of resistance to heat transfer. Then, the required insulation resistance can be calculated as, where is the sum of the thermal resistances of layers with known thicknesses, and the minimum insulation thickness is:. For further calculations, the thickness of the insulation must be rounded up to a multiple of unified (factory) values \u200b\u200bof the thickness of a 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 of the 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 but the density of the heat flux passing through this fence:.

Under stationary conditions, the heat flux density is constant in time, and therefore, where R x - resistance of a part of the structure, including resistance to heat transfer of the inner surface and 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 layer of the structure can be found as follows:.

The reduced heat transfer resistances of inhomogeneous building envelopes or their sections (fragments) should be determined by reference, the reduced resistance of flat building envelopes with heat-conducting inclusions should also be determined by reference.

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

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

The heat transfer equation by radiation:, where b l - the heat transfer coefficient by radiation, which is more dependent on the materials of the interlayer surfaces (the lower the emissivity of the materials, the less b l) and the average air temperature in the interlayer (with increasing temperature, the heat transfer coefficient of radiation increases).

So where l eq - the equivalent coefficient of thermal conductivity of the air gap. Knowing l eq, you can determine the thermal resistance of the air gap. However, the resistance R VP can be determined by reference manual. They depend on the thickness of the air gap, the air temperature in it (positive or negative) and the type of air gap (vertical or horizontal). The amount of heat transmitted by heat conduction, convection and radiation through vertical air spaces can be judged by the following table.

The thickness of the layer, mm	Heat flux density, W / m 2	The amount of heat in% transmitted	Equivalent coefficient of thermal conductivity, m o C / W	Thermal resistance of the interlayer, W / m 2 ° C
		thermal conductivity	convection	radiation




Note: the values \u200b\u200bgiven in the table correspond to the air temperature in the interlayer equal to 0 ° C, the temperature difference on its surfaces 5 ° C and the emissivity of surfaces C \u003d 4.4.

Thus, when designing exterior fencing with air gaps, the following should be considered:

1) an increase in the thickness of the air gap has little effect on reducing the amount of heat passing through it, and interlayers of small thickness (3-5 cm) are effective in the heat engineering relation;

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

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

4) the air gap must be closed and not connected with the outside air, that is, vertical layers must be partitioned with horizontal diaphragms at the level of interfloor ceilings (more frequent partitioning of the layers by height is not practical). If there is a need for interlayers ventilated with outside air, then they are subject to special calculation;

5) due to the fact that the bulk of the heat passing through the air gap is transmitted by radiation, it is desirable to place the interlayers 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 material with a low emissivity (for example, aluminum foil), which significantly reduces the radiant flux. Coating both surfaces with such a material practically does not reduce heat transfer.

3. The causes of the occurrence of the pressure difference on one or the other side of the fence

In winter, the air in heated rooms has a temperature higher than the outside air, and, therefore, the outside air has a greater bulk density (density) compared to the inside air. This difference in volumetric air weights creates differences in its pressures on both sides of the enclosure (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 airtightness of the upper and lower fences and with closed openings, the difference in air pressure reaches maximum values \u200b\u200bat the floor and under the ceiling, and is zero in the middle of the room height (neutral zone).

Thermal resistance of a closed air gap. Determination of speed and air temperature in the interlayer

Thermal conductivity of air in liquid and gaseous states at low temperatures and pressures up to 1000 bar

Thermal conductivity of gaseous air at a temperature of 300 to 800K and various pressures

Thermal conductivity of air at high temperatures and pressures from 0.001 to 100 bar

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