Cement, Energy and Environment

40 consequence of thermal agita on of its composing molecules. Radia on heat transfer can be described by reference to the 'black body' . A black body is a hypothe cal body that completely absorbs all wavelengths of thermal radia on incident on it. Such bodies do not reflect light, and therefore appear black if their temperatures are low enough so as not to be self- luminous. All black bodies heated to a given temperature emit thermal radia on. The radia on energy per unit me from a is black body propor onal to the fourth power of the absolute temperature and can be expressed with Stefan- Boltzmann Law as 4 q = σ T A (1) Where q = heat transfer per unit me (W) -8 2 4 σ = 5.6703 10 (W/m K ) - The Stefan-Boltzmann Constant T = absolute temperature in Kelvin's (K) 2 A = area of the emi ng body (m ) The Stefan-Boltzmann Constant in Imperial Units -8 2 4 σ = 5.6703 10 (W/m K ) -9 2 o 4 = 1.714 10 (Btu/ (h R )) -11 2 o 4 = 1.19 10 (Btu/ (h in R )) Gray Bodies and Emissivity Coefficients For objects other than ideal black bodies ('gray bodies') the Stefan- Boltzmann Law can be expressed as 4 q = ε σ T A (2) where ε = emissivity coefficient of the object (one - 1 - for a black body) For the gray body the incident radia on (also called irradia on) is partly reflected, absorbed or transmi ed. The emissivity coefficient is in the range 0 < ε < 1, depending on the type of material and the temperature of the surface. WHAT ARE RADIATION LOSSES If a hot object is radia ng energy to its cooler surroundings the net radia on heat loss rate can be expressed as 4 4 q = ε σ (T - T ) A (3) h c h Where T = hot body absolute h temperature (K) T = cold surroundings absolute c temperature (K) 2 A = area of the hot object (m ) h CONVECTION Heat transfer through convec on arises when a moving fluid absorbs heat from some surface engineeringtoolbox.com