|
Layers, materials (item in table SP) |
Thermal resistance R i = i/l i, m 2 ×°С/W |
Thermal inertia D i = R i s i |
Resistance to vapor permeation R vp,i = i/m i, m 2 ×hPa/mg |
|
|
Inner boundary layer | ||||
|
Internal plaster made of cement-sand. solution (227) | ||||
|
Reinforced concrete(255) | ||||
|
Mineral wool slabs (50) | ||||
|
Air gap | ||||
|
External screen – porcelain stoneware | ||||
|
Outer boundary layer | ||||
|
Total () |
* – without taking into account the vapor permeability of screen seams
The thermal resistance of a closed air gap is taken according to Table 7 SP.
We accept the coefficient of thermal technical heterogeneity of the structure r= 0.85, then R req /r= 3.19/0.85 = 3.75 m 2 ×°C/W and the required insulation thickness
0.045(3.75 – 0.11 – 0.02 – 0.10 – 0.14 – 0.04) = 0.150 m.
We take the thickness of the insulation 3 = 0.15 m = 150 mm (multiples of 30 mm), and add it to the table. 4.2.
Conclusions:
In terms of heat transfer resistance, the design complies with the standards, since the reduced heat transfer resistance R 0 r above the required value R req :
R 0 r=3,760,85 = 3,19> R req= 3.19 m 2 ×°C/W.
4.6. Determination of the thermal and humidity conditions of the ventilated air layer
The calculation is carried out for winter conditions.
Determination of movement speed and air temperature in the layer
The longer (higher) the layer, the greater the speed of air movement and its consumption, and, consequently, the efficiency of moisture removal. On the other hand, the longer (higher) the layer, the greater the likelihood of unacceptable moisture accumulation in the insulation and on the screen.
The distance between the inlet and outlet ventilation holes (the height of the interlayer) is taken equal to N= 12 m.
Average air temperature in the layer t 0 is tentatively accepted as
t 0 = 0,8t ext = 0.8(-9.75) = -7.8°C.
The speed of air movement in the interlayer when the supply and exhaust openings are located on one side of the building:
where is the sum of local aerodynamic resistance to air flow at the inlet, at turns and at the exit from the layer; depending on the design solution of the facade system= 3...7; we accept= 6.
Sectional area of the interlayer with nominal width b= 1 m and accepted (in Table 4.1) thickness = 0.05 m: F=b= 0.05 m2.
Equivalent air gap diameter:
The heat transfer coefficient of the surface of the air layer a 0 is preliminarily accepted according to clause 9.1.2 SP: a 0 = 10.8 W/(m 2 ×°C).
(m 2 ×°C)/W,
K int = 1/ R 0.int = 1/3.67 = 0.273 W/(m 2 ×°C).
(m 2 ×°C)/W,
K ext = 1/ R 0, ext = 1/0.14 = 7.470 W/(m 2 ×°C).
Odds
0.35120 + 7.198(-8.9) = -64.72 W/m2,
0.351 + 7.198 = 7.470 W/(m 2 ×°C).
Where With– specific heat air, With= 1000 J/(kg×°C).
The average air temperature in the layer differs from the previously accepted one by more than 5%, so we are clarifying the design parameters.
Speed of air movement in the interlayer:
Air density in the layer
Amount (flow) of air passing through the layer:
We clarify the heat transfer coefficient of the surface of the air layer:
W/(m 2 ×°C).
Heat transfer resistance and heat transfer coefficient of the interior of the wall:
(m 2 ×°C)/W,
K int = 1/ R 0.int = 1/3.86 = 0.259 W/(m 2 ×°C).
Heat transfer resistance and heat transfer coefficient of the outer part of the wall:
(m 2 ×°C)/W,
K ext = 1/ R 0.ext = 1/0.36 = 2.777 W/(m 2 ×°C).
Odds
0.25920 + 2.777(-9.75) = -21.89 W/m2,
0.259 + 2.777 = 3.036 W/(m 2 ×°C).
We clarify the average air temperature in the layer:
We clarify the average air temperature in the layer several more times until the values at neighboring iterations differ by more than 5% (Table 4.6).
.
1.3 The building as a single energy system.
2. Heat and moisture transfer through external fences.
2.1 Basics of heat transfer in a building.
2.1.1 Thermal conductivity.
2.1.2 Convection.
2.1.3 Radiation.
2.1.4 Thermal resistance air gap.
2.1.5 Heat transfer coefficients on internal and external surfaces.
2.1.6 Heat transfer through a multilayer wall.
2.1.7 Reduced resistance to heat transfer.
2.1.8 Temperature distribution across the fence section.
2.2 Humidity conditions of enclosing structures.
2.2.1 Reasons for the appearance of moisture in fences.
2.2.2 Negative consequences of wetting external fences.
2.2.3 Relationship between moisture and building materials.
2.2.4 Humid air.
2.2.5 Material moisture content.
2.2.6 Sorption and desorption.
2.2.7 Vapor permeability of fences.
2.3 Air permeability of external fences.
2.3.1 Basic provisions.
2.3.2 Pressure difference on the outer and inner surfaces of the fences.
2.3.3 Air permeability of building materials.
2.1.4 Thermal resistance of the air layer.
To bring uniformity, heat transfer resistance closed air gaps located between the layers of the enclosing structure are called thermal resistance R v.p, m². ºС/W.
The diagram of heat transfer through the air gap is shown in Fig. 5.
Fig.5. Heat exchange in the air layer.
Heat flow passing through the air gap q v.p , W/m² , consists of flows transmitted by thermal conductivity (2) q t, W/m² , convection (1) q к , W/m² , and radiation (3) q l , W/m² .
(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 5ºС. With an increase in the thickness of the layer from 10 mm to 200 mm, the share of heat flux 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%.
Direct calculation of these components is quite cumbersome. Therefore in regulatory documents provides data on the thermal resistance of closed air layers, which were compiled by K.F. in the 50s of the twentieth century. Fokin based on the results of experiments by M.A. Mikheeva. If there is heat-reflecting aluminum foil on one or both surfaces of the air gap, which impedes radiant heat transfer between the surfaces framing the air gap, the thermal resistance should be doubled. To increase the thermal resistance of closed air layers, it is recommended to keep in mind the following conclusions from research:
1) layers of small thickness are effective in terms of heat engineering;
2) it is more rational to make several thin layers in the fence than one large one;
3) it is advisable to place the air gaps closer to the outer surface of the fence, since in this case winter time heat flux by radiation decreases;
4) vertical layers in external walls must be partitioned with horizontal diaphragms at the level of interfloor ceilings;
5) to reduce the heat flux transmitted by radiation, one of the surfaces of the interlayer can be covered with aluminum foil having an emissivity of about ε = 0.05. Covering both surfaces of the air gap with foil practically does not 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 a material?
6. Write the formula for the heat flow transferred by thermal conductivity in a multilayer wall at known temperatures of the internal surfaces tв and external surfaces tн.
7. What is thermal resistance?
8. What is convection?
9. Write the formula for heat flow transferred by convection from air to the surface.
10. Physical meaning convective heat transfer coefficient.
11. What is radiation?
12. Write the formula for heat flux transferred by radiation from one surface to another.
13. Physical meaning of the radiative heat transfer coefficient.
14. What is the heat transfer resistance of a closed air gap in a building envelope called?
15. What type of heat flow does the total heat flow through the air layer consist of?
16. What nature of the heat flow prevails in the heat flow through the air layer?
17. How does the thickness of the air gap affect the distribution of flows in it.
18. How to reduce heat flow through the air gap?
One of the techniques that increases the thermal insulation qualities of fences is the installation of an air gap. It is used in the construction of external walls, ceilings, windows, and stained glass windows. It is also used in walls and ceilings to prevent waterlogging of structures.
The air gap can be sealed or ventilated.
Consider heat transfer hermetically sealed air gap.
The thermal resistance of the air layer R al cannot be defined as the thermal conductivity resistance of the air layer, since heat transfer through the layer with 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 layers, in general case, the air is in motion. In vertical - it moves up along warm surface and down - along the cold one. Convective heat exchange takes place, and its intensity increases with increasing layer thickness, 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 = 0.5 α k + λ a /δ al, (3.23)
where λ a and δ al are the thermal conductivity coefficient of air and the thickness of the air layer, respectively.
This coefficient depends on geometric shape and sizes of air layers, direction of heat flow. By generalization large quantity 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 of the coefficients α" k, calculated by him at average temperature air in a vertical layer t = + 10º C.
Table 3.5
Convective heat transfer coefficients in a vertical air layer
The coefficient of convective heat transfer in horizontal air layers depends on the direction of heat flow. If the top surface is heated more than the bottom, there will be almost no air movement, since warm air concentrated at the top, and cold at the bottom. Therefore, the equality will be satisfied quite accurately
α" k = λ a /δ al.
Consequently, convective heat transfer is significantly reduced, and the thermal resistance of the interlayer increases. Horizontal air layers are effective, for example, when used in insulated basement floors above cold undergrounds, where the heat flow is directed from top to bottom.
If the heat flow is directed from bottom to top, then ascending and descending air flows occur. 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, clause 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 the brickwork. Surface temperatures: t 1 = + 15 ºС, t 2 = + 5 ºС; brick blackness degree: ε 1 = ε 2 = 0.9.
Using formula (2.13), we find that ε = 0.82. Temperature coefficient θ = 0.91. Then α l = 0.82∙5.7∙0.91 = 4.25 W/(m 2 ·ºС).
The value of α l is much greater than α "k (see Table 3.5), therefore, the main amount of heat through the layer is transferred by radiation. In order to reduce this heat flow and increase the resistance to heat transfer of the air layer, it is recommended to use reflective insulation, that is, covering one or both surfaces, for example, with aluminum foil (the so-called “reinforcement”). This coating is usually placed on a warm surface to avoid moisture condensation, which impairs 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 layer at a constant temperature difference on its surfaces is determined by the formula
Table 3.6
Thermal resistance of closed air layers
| Air layer thickness, m | R al , m 2 ·ºС/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 |
The values of R al for closed flat air layers are given in Table 3.6. These include, for example, layers between layers of dense concrete, which practically does not allow air to pass through. It has been experimentally shown that in brickwork, when the joints between the bricks are insufficiently filled with mortar, a violation of the tightness occurs, that is, the penetration of outside air into the layer 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.
Currently, walls with ventilated air gap (walls with a ventilated facade). A suspended ventilated façade is a structure consisting of cladding materials and a sub-cladding structure, which is attached to the wall in such a way that there is an air gap between the protective and decorative cladding and the wall. For additional insulation of external structures, a thermal insulation layer is installed between the wall and the cladding, so that ventilation gap left between the cladding and thermal insulation.
The design diagram of a 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.
The layers of the structure located between the air gap and the outer surface are not taken into account in the thermal engineering calculations. Therefore, thermal resistance external cladding is not included in the heat transfer resistance of the wall, determined by formula (3.6). As noted in paragraph 2.5, the heat transfer coefficient of the outer surface of the enclosing structure with ventilated air layers 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 during 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. No condensation occurs inside the wall, its heat-shielding properties do not deteriorate, and no additional vapor barrier is required (Chapter 5).
Air flow, which occurs in the interlayer due to a pressure difference, promotes the evaporation of moisture from the surface of the insulation. It should be noted that a significant mistake is the use of a vapor barrier on the outer surface of the heat-insulating layer, since it prevents the free removal of water vapor to the outside.
To bring uniformity, heat transfer resistance closed air gaps located between the layers of the enclosing structure are called thermal resistance Rv.p, m². ºС/W.
The diagram of heat transfer through the air gap is shown in Fig. 5.
Fig.5. Heat exchange in the air layer.
The heat flow passing through the air layer qv.p, W/m², consists 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 resistance to heat transfer. Coefficient of thermotechnical homogeneity of enclosing structures.
25. Standardization of heat transfer resistance based on sanitary and hygienic conditions
, R 0 = *
We normalize Δ t n, then R 0 tr = * , those. in order for Δ t≤ Δ t n It is necessary
R 0 ≥ R 0 tr
SNiP extends this requirement to reduced resistance. heat transfer
R 0 pr ≥ R 0 tr
t in - design temperature of internal air, °C;
accept according to the standards for design. building
t n - - estimated winter outside air temperature, °C, equal to the average temperature of the coldest five-day period with a probability of 0.92
A in (alpha) - heat transfer coefficient of the internal surface of enclosing structures, accepted according to SNiP
Δt n - standard temperature difference between the temperature of the internal air and the temperature of the internal surface of the enclosing structure, adopted according to SNiP
Required heat transfer resistance R tr o doors and gates must be at least 0.6 R tr o walls of buildings and structures, determined by formula (1) at a 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 heat transfer resistance of internal enclosing structures in formula (1), it should be taken instead t n-calculated air temperature of the colder room.
26. Thermal engineering calculation of the required thickness of the fencing material based on the conditions for achieving the required heat transfer resistance.
27. Humidity of the material. Reasons for dampening the structure
Humidity - a physical quantity equal to the amount of water contained in the pores of the material.
Available in mass and volume
1) Construction moisture.(during the construction of a building). Depends on the design and method of construction. Solid brickwork worse than ceramic blocks. The most favorable is wood (prefabricated walls). w/w not always. Should disappear within 2=-3 years of operation. Measures: dry the walls
Ground moisture. (capillary suction). Reaches a level of 2-2.5 m. waterproofing layers, with correct device does not affect.
2) Ground moisture, penetrates into the fence from the ground due to capillary suction
3) Atmospheric moisture. (slanting rain, snow). Particularly important for roofs and eaves... solid brick walls do not require protection if jointing is done correctly. Reinforced concrete, lightweight concrete panels pay attention to joints and window units, a textured layer of waterproof materials. Protection=protective wall on slope
4) Operating moisture. (in workshops of industrial buildings, mainly in floors and lower parts of walls) solution: waterproof floors, drainage system, cladding of the lower part ceramic tiles, waterproof plaster. Protection = protective lining with internal sides
5)Hygroscopic moisture. Due to the increased hygroscopicity of materials (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 influence of humidity on the properties of structures
1) With increasing humidity, the thermal conductivity of the structure increases.
2) Humidity deformations. Humidity is much worse than thermal expansion. Peeling of plaster due to accumulated moisture underneath, then the moisture freezes, expands in volume and tears off the plaster. Non-moisture-resistant materials become deformed when moistened. For example, gypsum begins to creep when humidity increases, plywood begins to swell and delaminate.
3) Reduced durability - number of years of trouble-free operation of the structure
4) Biological damage (fungus, mold) due to dew
5) Loss of aesthetic appearance
Therefore, when choosing materials, they are taken into account humidity conditions and choose materials with the highest moisture content. Also, excessive indoor humidity can cause the spread of diseases and infections.
From a technical point of view, it leads to losses in the durability of the structure and its frost-resistant properties. Some materials high humidity lose mechanical strength, change shape. For example, gypsum begins to creep when humidity increases, plywood begins to swell and delaminate. Corrosion of metal. deterioration in appearance.
29. Water vapor sorption 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 pair molecule with the surface of a pore and adhesion to this surface (adsorption)2. Direct dissolution of moisture in the body volume (absorption). Humidity increases with increasing relative elasticity and decreasing temperature. “desorption”: if a wet sample is placed in desiccators (sulfuric acid solution), it releases moisture.
Sorption mechanisms:
1.Adsorption
2.Capillary condensation
3.Volume filling of micropores
4. Filling the interlayer space
Stage 1. Adsorption is a phenomenon in which the pore surface is covered with one or more layers of water molecules (in mesopores and macropores).
Stage 2. Polymolecular adsorption - a multilayer adsorbed layer is formed.
Stage 3. Capillary condensation.
CAUSE. Pressure saturated steam above a concave surface is less than above a flat surface of a liquid. In capillaries of small radius, moisture forms concave miniskies, so capillary condensation becomes possible. If D>2*10 -5 cm, then there will be no capillary condensation.
Desorption – the process of natural drying of the material.
Hysteresis (“difference”) of sorption lies 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 weight humidity during sorption and the weight humidity of desorption (desorption 4.3%, sorption 2.1%, hysteresis 2.2%) when humidifying the sorption isotherm. When drying desorption.
30. Mechanisms of moisture transfer in building construction materials. Vapor permeability, capillary suction of water.
1. In winter, due to temperature differences and at different partial pressures, a flow of water vapor passes through the fence (from the inner surface to the outer) - water vapor diffusion. In summer it's the other way around.
2. Convective transport of water vapor(with air flow)
3. Capillary water transfer(percolation) through porous materials.
4. Gravity water leaking through cracks, holes, macropores.
Vapor permeability – the ability of a material or structure made from them to allow water vapor to pass through it.
Pore permeability coefficient- Phys. a value numerically equal to the amount of steam passing through the plate with a unit area, with a unit pressure drop, with a unit thickness of the plate, with a unit time with a partial pressure difference on the sides of the plate e 1 Pa.. With a decrease. Temperatures, mu decreases, with increased humidity, mu increases.
Vapor permeation resistance: R=thickness/mu
Mu - vapor permeability coefficient (determined according to SNIP 2379 heat engineering)
Capillary absorption of water by building materials – ensures constant transfer of liquid moisture through porous materials from an area of high concentration to an area of low concentration.
The thinner the capillaries, the greater the force of capillary suction, but overall the transfer rate decreases.
Capillary transfer can be reduced or eliminated by installing an appropriate barrier (small air gap or capillary-inactive layer (non-porous)).
31. Fick's law. Vapor permeability coefficient
P(amount of steam, g) = (ev-en)F*z*(mu/thickness),
Mu– coefficient vapor permeability (determined according to SNIP 2379 heating engineering)
Phys. a value numerically equal to the amount of steam passing through the plate with a unit area, with a unit pressure drop, with a unit thickness of the plate, with a unit time with a partial pressure difference on the sides of the plate e 1 Pa. [mg/(m 2 *Pa)]. The smallest mu has a roofing material of 0.00018, the largest min.cotton wool = 0.065 g/m*h*mm.Hg., window glass and metals are vapor-tight, air has the greatest vapor permeability. When decreasing Temperatures, 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 (for wood along the grain = 0.32, across = 0.6).
Equivalent resistance to vapor permeation of a fence with a sequential arrangement of layers. Fick's law.
Q=(e 1 -e 2)/R n qR n1n =(e n1n-1 -e 2)
32 Calculation of the distribution of partial pressure of water vapor across the thickness of the structure.