Building Science Thermal Conductivity From last week Conduction
Building Science Thermal Conductivity
From last week • Conduction • Convection • Radiation • i. Learn – please make use of this • Heat transfer / quiz
k values / r value • Thermal conductivity – a measure of the rate in which heat in conducted through a material • Measured in W/m. K • Resistivity – this is the reciprocal of thermal conductivity – a measure of the opposition to heat transfer • Measured in m 2 K/W
k values • Every material has a value. • In construction materials have variations such as water content, density, and location. Insulation also has variations with deterioration.
r values • Quantify how quick heat energy is transferred through a section of construction. • Insulation – minimum conductivity and minimum thickness • Or poor conductivity and maximum thickness • HIGHER R VALUE BETTER THERMAL INSULATION
Fourier’s Law of Conduction • That the rate of heat flow through a single uniform solid is directly proportional to the area and to the change in temperature with respect to the length of the path of the heat flow. • This is an empirical law based on observation.
Fourier’s Law of Conduction • Conduction of heat occurs most readily in pure metals, less so in alloys, and much less readily in non-metals. The very low values of k in some insulators (e. g. cork) is due largely to their porosity, the air trapped within the material acting as an insulator. Gases and liquids are good insulators, but unless a completely stagnant layer of fluid is obtained, heat is transferred by convection currents.
Fourier’s Formula • A slab of material of thickness dx and surface area A has one face at a temperature of t, and the other at a lower temperature (t – dt). Heat flows from the higher temperature face to the lower temperature face and the temperature change in the direction of heat flow is –dt
Fourier’s Formula OR Q= k = A = dt = dx= Rate of Heat Flow Thermal Conductivity Area change of temp change of distance
Example • The inner surface of a plane brick wall is at 40°C, and the outer surface is at 20°C. Calculate the rate of heat transfer per sq metre of surface area of the wall, which is 250 mm thick. The k value for the brick is 0. 52 W/mk.
Example Answer • Thermal Resistance = Material Thickness in m Thermal Conductivity • Thermal Resistance = 0. 25 0. 52 • Thermal Resistance = 0. 48 m 2 K/W
Example Answer Continued • U Value = 1 Thermal Resistance • U Value = 1 0. 48 • U Value = 2. 08 W/m 2 K = 1 RT
Example Answer Continued • Heat Loss = Area x U Value x Temp Change • Heat Loss = 1 x 2. 08 x 20 • Heat Loss = 41. 6 W/m 2
Tutorial • Calculate the heat transfer rate through an aluminium slab 2 m x 1. 6 m, if the temperature on one side is 24°C and 16°C on the other side. The slab is 150 mm thick. • Aluminium Thermal Conductivity = 205 W/mk
Total thermal resistance • So far we have worked out a single material • In construction this is seldom the case • Also we forget about radiation at the surface – where a stationary layer is formed opposing heat flow
Construction • In a typical construction we have many materials to take into account • Along with the surface resistances • We take each heat loss, total these together (Total Thermal Resistance or RT) • Use a formula to calculate the “U” value of the construction • This can be compared to building regulations to look at the effectiveness of the construction
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