Thermal conductivity of Lagerstroemia indica in Shenyang
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In the cold winter in the north, when the temperature of the inner surface of the outer wall is lower than the dew point temperature of the indoor air, the inner surface of the outer wall condenses and the inner wall becomes moldy. This phenomenon will not only affect people's comfort and health, but also cause damage to indoor appliances and building structures.
1 Calculation of theoretical heat transfer coefficient
In order to solve the problem of dew condensation on the inner surface of the external wall, it is necessary to choose a peripheral protection structure with small heat transfer coefficient and sufficient thickness, so that the temperature of its inner surface will not be too low and condensed water will not be generated on its surface, that is, the heat transfer coefficient K of the external wall is less than the local maximum heat transfer coefficient Kmax in winter. The value of the heat transfer coefficient k of the external wall is directly related to the thickness of the external wall and the materials used in the external wall.
k = 1/〔 1/αβ+∑(δ/λ)+ 1/αH〕
( 1)
Where: α β is the sensible heat coefficient; λ is thermal conductivity; δ is the wall thickness; αH is the heat dissipation coefficient.
kmax =αβ×:tβ-(τ+ 1.5)∞/(tβ-tH)
Taking Shenyang as an example, when the lowest indoor heating temperature (tβ) is 16℃, the lowest outdoor temperature (tH)-33℃, the sensible heat coefficient α β is 7.5, the indoor relative humidity is 50%, the indoor temperature is 16℃, and the condensation temperature τ=6℃, KMAX =/kloc can be obtained.
2. The external envelope adopts clay brick method.
2. Checking calculation of heat transfer coefficient of1370 mm thick clay brick exterior wall
At this stage, most of the external walls in Shenyang are 370mm thick clay brick walls, the internal walls are plastered with 20mm thick mixed mortar, and the external walls are plastered with 20 mm thick cement mortar or water brush stone. The heat transfer coefficient is as follows:
(1) The wall without ring beam and constructional column is calculated according to formula (1), k1=1.51w/(m2.k).
(2) The wall with ring beam and constructional column is calculated according to formula (1), and k2 =1.94w/(m2 k).
It can be seen from the above that the heat transfer coefficient of 370mm thick clay brick wall is equal to the maximum heat transfer coefficient in this area, which just meets the minimum condition of no condensation; If the indoor temperature is lower than 16℃, dew condensation will occur on the inner surface of the external wall, and the heat transfer coefficient at the ring beam and structural column is greater than the maximum heat transfer coefficient in this area, thus laying a hidden danger for the mildew phenomenon caused by dew condensation on the inner surface of the external wall.
In the actual investigation, we also found that when the 370mm thick clay brick wall is used for the external wall, the part of the internal surface of the external wall that causes mold growth starts from the ring beam and constructional column and gradually spreads to other parts of the wall. It can be seen that the 370mm thick clay brick wall can only meet the structural strength requirements, but can not guarantee the dew condensation and heat preservation requirements on the inner surface of the external wall.
2.2 Checking calculation of heat transfer coefficient of 490 mm thick clay brick exterior wall
When the external wall is a 490mm thick clay brick, the heat transfer coefficient is calculated empirically, and there is no ring beam and structural column k1=1.236 w/(m2 k), but there is ring beam and structural column k2 =1.51w/(m2 k). However, increasing the thickness of the external wall will reduce the indoor usable area, at the same time, the weight of the whole building will also increase, the bearing capacity of the foundation must also be improved accordingly, and the whole project cost needs to be improved accordingly.
3. The shell is made of composite material.
3. 1 Sandwich polystyrene board method is adopted for external wall masonry.
Take a 240mm clay brick wall as an example, with a 50mm thick polystyrene board sandwiched in the middle, an inner 120mm clay brick wall, an inner 20mm thick mixed mortar and an outer 20mm thick cement mortar. Check the heat transfer coefficient, and check that there is no ring beam or constructional column K 1=0.55W/(m2. K), meet the conditions. K2= 1.85W/(m2。 K) It does not meet the requirements, and at the same time, the construction operation is troublesome, the integrity of the wall is poor, and the problem of condensation has not been solved.
3.2 Spray polyurethane rigid foam on the inner surface of the external wall.
3.2. 1 external wall structure
Taking the exterior wall as 370mm thick clay brick, the interior wall sprayed with 25mm thick polyurethane rigid foam insulation material, the interior wall plastered with 20mm thick mixed mortar, and the exterior wall plastered with 20mm thick cement mortar (see figure 1) as an example, the calculation of heat transfer coefficient can meet the conditions of non-condensation, with good thermal insulation performance, high strength and strong adhesion. Due to the site construction, the whole wall has good sealing performance and convenient operation.
Figure 1 Schematic Diagram of External Wall Structure
Construction method
(1) Block the hole in the wall and clean the floating ash with a broom.
(2) Dilute and prepare the polyurethane and put it into a spray gun, powered by an air compressor. Construction can be started after the preparatory work is completed, and its thickness can be controlled by hanging lines. After spraying polyurethane foam for 24 hours, the next working procedure can be started.
(3) On the wall sprayed with polyurethane foam, cement mortar (composed of cement, EC glue and water) is sprayed in proportion, and after drying, the interior wall is painted.
(4) Interior wall plastering materials can be mixed mortar, cement mortar and marble mosaic.
3.2.3 Checking calculation of heat transfer coefficient
Wall k1= 0.523 w/(m2 k) without ring beam and constructional column; Wall with ring beam and constructional column: K2 = 0.558W/(m2 k), K 1, and K2 is smaller than the heat transfer coefficient of the above two methods because the actual heat transfer coefficient is smaller than the theoretical maximum. Therefore, spraying 25mm thick polyurethane foam on the inner wall can not only prevent condensation, but also reduce indoor heat loss, which has the effect of heat preservation and energy saving.
4 Benefit analysis and engineering examples
4. 1 Benefit Analysis: Take the story height of 2.7m as an example. In order to ensure that the inner wall does not condense, the above different materials are used for heat preservation. See table 1 for economic analysis.
Table 1 Economic Analysis of Different Thermal Insulation Materials Used in Exterior Wall
Using the method of per linear meter
expense
(Yuan/m2) Advantages and disadvantages of increasing or decreasing the area
Impact (and 1800
Yuan/square meter)
(meta) comprehensive analysis
(yuan)
490mm brick wall 55.73 One-time investment reduces effective area.
The foundation needs to be expanded and its income needs to be reduced.
2 16 reduce income
27 1.73
240mm brick wall+
50 mm polystyrene board
+120mm brick wall 57.2 The indoor area is increased.
Ring beam and structural column with good thermal insulation performance
One exposure does not solve the problem of increasing income.
126 yuan increases income.
68.8
370 mm brick wall+
Increase the indoor area of 25mm thick polyurethane foam insulation material 8 1.
The heat preservation performance is the best
In the later stage, energy is saved, the effect is good, and the one-time investment greatly increases the income.
180 yuan increases income.
99
4.2 Engineering example Shenyang Hong Xin Building has a building area of 27277m2, with frame-shear structure, 2 floors underground and 22 floors above ground. Polyurethane foam thermal insulation material is sprayed on the inner surface of building exterior wall, and cement mortar, mixed mortar and marble are plastered on polyurethane foam. After more than three years of inspection, there are no hollows and cracks on the wall, which not only prevents condensation, but also saves heat and energy.
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