Vincent Barraud SOPREMA BASICS OF THERMAL INSULATION

Transcription

1 Vincent Barraud SOPREMA BASICS OF THERMAL INSULATION

2 Summary Part 1 - What is thermal conductivity? Part 2 - How the thermal conductivity is measured? Part 3 How to certify a lambda value?

3 Part 1 - What is thermal conductivity? The common point of all the insulation materials!

4 Part 1 - What is thermal conductivity? Fourier's Law is used to express conductive heat transfer : q =.A.dT / s Where q = heat transfer (W/m²) A = heat transfer area (m 2 ) = thermal conductivity of material (W/(m.K) dt = temperature difference across the material (K) s = material thickness (m)

5 Part 1 - What is thermal conductivity? In International System of Units, thermal conductivity is measured in watts per meter Kelvin (W/(m.K)) and in imperial units thermal conductivity is measured in BTU/(hr.ft. F). Other units which are closely related to the thermal conductivity are in common use in the construction and textile industries. The construction industry makes use of units such as the R-value (resistance) and the U-value (conductivity). Although related to the thermal conductivity of a material used in an insulation product, R and U-values are dependent on the thickness of the product. Likewise the textile industry has several unit including the tog (1 tog = 0.1 m 2.K/W) and the clo which express thermal resistance of a material in a way analogous to the R-values used in the construction industry.

6 Part 1 - What is thermal conductivity? Thermal resistance is the ability of a material to resist to the heat flow. Thermal resistivity is the reciprocal of thermal conductivity and can be expressed as R = s / where R = thermal resistivity (m².k/w) = thermal conductivity (W/(m.K)) s = material thickness (m)

7 Part 1 - What is thermal conductivity? Thermal transmittance from thermal resistance can be expressed as : U = 1 / R = 1 / Σ R i * where R = thermal resistivity (m².k/w) U = overall thermal transmittance (W/m².K) *R i (Rmatrix + Rcell gas + Rconvection + Rradiation)

8 Part 1 - What is thermal conductivity? Typical scales of thermal conductivity Equivalent thickness for U = 0,2 W/m²K

9 Part 1 - What is thermal conductivity? Closed or open Cell Foam Insulation Thermal conductivity is the result of several components : T = M + G + C + R M Thermal conductivity in the matrix G Thermal conductivity through cell gas C Convection in the cell gas R Radiation

10 Part 1 - What is thermal conductivity? Closed or open Cell Foam Insulation The most comprehensive resource for an in-depth description of foam insulation thermal performance, and the source for the discussion in this section, is Glicksman, Total heat transport through foam insulation is typically represented as the superposition of the heat transported via solid conduction, gas conduction, radiation, and convection as shown below. This approximation is valid for lowdensity materials with modest temperature differences. qtotal = ( qsolid + qgas )conduction + q radiation + q convection where: q = heat transfer, W/m².

11 Part 1 - What is thermal conductivity? Closed or open Cell Foam Insulation q solid conduction: The solid conduction performance are determined by the foam morphology, which does not change appreciably over the lifetime of the product. The magnitude of the solid conduction in low density foam is shown below :

12 Part 1 - What is thermal conductivity? Closed or open Cell Foam Insulation q gaz conduction: The remainder of the total conduction is due to gaseous conduction. As gases diffuse in and out of the foam, that mixture changes, and therefore the total thermal conductivity changes. The thermal conductivity of a gas mixture has been characterized by Wassiljewa and Lindsay and Bromley as function of mole fraction, thermal conductivity, viscosity and absolute boiling temperature at one atmosphere pressure:

13 Part 1 - What is thermal conductivity? Closed or open Cell Foam Insulation q gaz conduction: The gas conductivity can be decreased by decreasing the pore size of the material. The collisions between the gas molecules and the solid are elastic which transfer small amounts of energy compared to the collisions between gas molecules. The effect on thermal conductivity, from a lowered pressure is strongly dependent of pore size. Pressure is seen in the denominator for the calculation of the mean free path. A lower pressure gives a longer mean free path which gives a larger Knudsen effect. It is seen that an increase in the mean free path will increase the Knudsen number and thereby decrease the gas conductivity. where : δ [m] is the characteristic system size, which can be interpreted as the distance between two parallel walls, lmean [m] is the mean free path, λg0 [W/(m K)] is the conductivity of the gas when moving freely and β [-] is a constant for the effectiveness of the energy transfer between the gas molecules and the solid pore walls with a value commonly between 1.5 and 2

14 Part 1 - What is thermal conductivity? Closed or open Cell Foam Insulation The effect on conductivity by changes in pressure is shown in the figure below (Knudsen effect) : VIP AEROGEL

15 Part 1 - What is thermal conductivity? Closed or open Cell Foam Insulation q radiation: The major factor in the net radiation flux is the emission from the solid portions of the foam. The radiation heat transfer is usually modeled using the Rosseland equation. The extinction coefficient is the reciprocal of the photon mean free path. Reducing the mean free path of the photons, by reducing the cell size, is therefore an effective way to reduce this portion of the total heat transport. The radiation transmission from one side of the foam to the other is very small, so that using reflective layers on the exterior foam surfaces has little impact on the radiation heat transport.

16 Part 1 - What is thermal conductivity? Closed or open Cell Foam Insulation q convection: Convection in porous materials can be separated in two cases, the convection inside the pore cells and the convection through the material on a macro scale. For small pores, the pore cell convection can be neglected, partly because of small temperature differences on the cell walls. That means that the heat transfer through convection often can be neglected for materials with a closed pore system. For materials with open cells, the macro scale convection might have a considerable effect and can thus not be neglected. The macro scale convection is either caused by natural or forced convection. For natural convection the air movement is created by density differences as a consequence of temperature differences, while forced convection is created by a pressure difference due to e.g. wind or fan induced.

17 Part 1 - What is thermal conductivity? Closed or open Cell Foam Insulation q convection: Natural convection is described by the dimensionless quantity Nusselt number which is a function of the modified Rayleigh number. The relation between the Nusselt number and the Rayleigh number is commonly determined empirically for specific geometries and boundary conditions. where ρa [kg/m 3 ] is the density of air, Cpa [J/(kg K)] is the specific heat capacity of air, g [m/s2] is the gravitational acceleration, a [1/K] is the thermal expansion coefficient of air, d [m] is the thickness of the porous material, k [m2] is the permeability, T+ and T- are the temperatures on the warm and the cold side respectively, v [m²/s] is the kinematic viscosity, m [W/(m K)] is the thermal conductivity of the porous material and q [W/m²] is the heat flux through the material.

18 Part 2 - How to measure thermal conductivity? All the products standards for Buildings refer to the EN to determine the thermal conductivity in CE but also its conforms to ASTM C518, JIS A1412, ISO 8301 There is two main tests apparatus : Heat Flow Meters (HFM) Guarded Hot Plate device (GHP)

19 Part 2 - How to measure thermal conductivity? Heat Flow Meter (HFM) Principle of the measurement : A sample is placed between two heated plates, set at different temperatures. The heat flow (q) through the sample is measured by a (calibrated) heat flux transducer. After reaching a thermal equilibrium, the test is done. Only one sample a is used for the analysis. The heat flux transducer output is calibrated with a standard.

20 x T A Q d T T V A N l u hft cal T u T l d A V N 1 Schema of an HFM N calibration factor V voltage of the heat flux transducer Part 2 - How to measure thermal conductivity? Heat Flow Meter (HFM)

21 Part 2 - How to measure thermal conductivity? Heat Flow Meter (HFM) The HFM is an easy-to-operate quality control tool with fast testing times of approx. 15 min. The HFM systems must be calibrated (no absolute test technique) and provide high reproducibility and accuracy (QC). The HFM works between 20 C and 80 C The HFM systems work around room temperature. The HFM systems work under air.

22 Part 2 - How to measure thermal conductivity? Heat Flow Meter (HFM) Illustrations of a Netzsch HFM

23 Part 2 - How to measure thermal conductivity? Guarded Hot Plate (GHP) Principle of the measurement The hot plate and auxiliary heaters (cold plates) are heated such that a well-defined, user-selectable temperature difference is set over the sample thickness. The power input in the hot plate with area is then measures as soon as thermal equilibrium is reached. Using the measured temperatures and power input, the thermal conductivity can be determined from the stationary heat transfer.

24 x T A Q 2 T x A R I eff Part 2 - How to measure thermal conductivity? Guarded Hot Plate (GHP) Schema of a GHP device

25 Part 2 - How to measure thermal conductivity? Guarded Hot Plate (GHP) The GHP method is an absolute test technique No calibration required The GHP works between 160 C and 700 C (mean sample temperature). The GHP works under well-defined atmospheres: vacuum inert oxidizing

26 Part 2 - How to measure thermal conductivity? Guarded Hot Plate (GHP) Illustrations of a Netzsch GHP device

27 Part 3 - How to certify a lambda value? The declared values R D and D shall be derived from the calculated values R 90/90 and 90/90 The k values, common to all the products, are given in Table A.1 in the next slide.

28 Part 3 - How to certify a lambda value?

29 Part 3 - How to certify a lambda value? Two cases depending of the insulation materials : 1 Thermal insulation products for buildings without influence of the ageing : EN Factory made mineral wool (MW) products EN Factory made expanded polystyrene (EPS) products EN Factory made cellular glass (CG) products EN Factory made wood wool (WW) products EN Factory made cellular glass (CG) products EN Factory made wood fibre (WF) products EN In-situ formed loose fill cellulose (CG) products

30 Part 3 - How to certify a lambda value? Procedure : After conditioning according : Stabilization at 23 C and 50% relative humidity until constant weight with sometimes 6, 48 or 72 hours at a temperature 50 C or 70 C. Or in on oven at a temperature at 70 C until constant weight Thermal resistance and thermal conductivity may also be measured at mean temperatures 10 C then we obtain directly the value declared.

31 Part 3 - How to certify a lambda value? 2 Thermal insulation products for buildings with an ageing procedure or fixement increment : EN13164 Factory made extruded polystyrene (XPS) products EN Factory made polyurethane foam (PU) products EN13166 Factory made phenolic foam (PF) products EN and 2 In-situ formed sprayed rigid polyurethane (PU) and polyisocyanurate (PIR) foam products

32 Part 3 - How to certify a lambda value? EN Factory made extruded polystyrene foam (XPS) products Slicing (open facing): Cutting 10mm slices from the product to accelerate gas diffusion. Storage of 10mm slices at 23 0 C for days depending on product thickness. λ- measuring of assembled stuck of slices for aged value at mean temperatures 10 C. We obtain directly the value declared by adding a correction of the thermal conductivity due to damaged surface by deducting 0,007 or 0,001 W/(m.K). Ageing procedure (tight facing): Thermal resistance and thermal conductivity may also be measured at mean temperatures 10 C after 60 days minimum. We obtain directly the value declared by adding a conventional correction of 0,001 W/(m.K) to take into account the statistical variation of that production batch.

33 Part 3 - How to certify a lambda value? EN Factory made polyurethane (PU) products and EN and 2 In-situ formed sprayed rigid polyurethane (PU) and polyisocyanurate (PIR) foam products The PU board manufacturer has the choice whether to use heat ageing or fixed increments procedures. Heat ageing procedure: Storage the product sample at 70 0 C for 175 days Thermal resistance and thermal conductivity may also be measured at mean temperatures 10 C. We obtain directly the lamdba declared by adding a safety increment following the type of facing and blowing agent. Fixed increments procedure: Only initial λ value is measured before 8 days after the Production at mean temperatures 10 C. A fixed increment is added for the lambda declared following the type of facing and blowing agent.

34 Part 3 - How to certify a lambda value? EN Factory made phenolic foam (PF) products Heat Ageing procedure (expanded with a gas) : Storage at temperatures 70 0 C for 175 days or C for 14 days Thermal resistance and thermal conductivity may also be measured at mean temperatures 10 C Then we obtain directly the lambda declared by adding a safety increment following the type of facing and blowing agent. Slicing procedure (expanded with high molecular weight gas Conform with CPD) : Cutting 10mm slices from the product to accelerate gas diffusion Storage of 10mm slices at 23 0 C for days depending on product thickness - measuring of assembled stuck of slices for aged value

35 Part 3 - How to certify a lambda value? Each country has its own certification label :