Issue 37

E. Mihailov et alii, Frattura ed Integrità Strutturale, 37 (2016) 297-304; DOI: 10.3221/IGF-ESIS.37.39 302 0 5 10 15 20 8.0x10 5 1.0x10 6 1.2x10 6 1.4x10 6 1.6x10 6 1.8x10 6 2.0x10 6 0 200 400 600 800 1000 1200 1400 0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 Tp, O C R, (m 2 .K)/W Q Q, W/m 2 d, mm T R 0 5 10 15 20 8.0x10 5 1.0x10 6 1.2x10 6 1.4x10 6 1.6x10 6 1.8x10 6 2.0x10 6 0 200 400 600 800 1000 1200 1400 0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 Tp, O C R, (m 2 .K)/W Q Q, W/m 2 d, mm T R Figure 8: Heat losses, surfaces temperature and thermal resistance of insulation as a function of chrome-magnesite insulation thickness Figure 9: Heat losses, surfaces temperature and thermal resistance of insulation as a function of magnesite-chromite insulation thickness 0 5 10 15 20 6,0x10 5 8,0x10 5 1,0x10 6 1,2x10 6 1,4x10 6 1,6x10 6 1,8x10 6 2,0x10 6 0 200 400 600 800 1000 1200 1400 QSH QD QMg QCr Q, W d, mm T, O C TD TSH TMg TCr Figure 10: Heat losses and surfaces temperature of insulations as a function of insulation thickness. As seen from the obtained results and the thermal balance of the water-cooled element (Fig. 2), with the same insulation thickness and lower thermal conductivity coefficient, the surface temperature is higher compared to that in the case of higher thermal conduction, whereupon the own radiation heat flux (Q own ) of the roof cooling element is higher. In the case of lower insulation thickness, the surface temperatures are lower, and under such conditions and lower emissivity (  <0,5), the reflected heat flux from the surface (Q refl ) has a higher value than the absorbed flux (Q abs ) and the own radiation of the surface is lower. For larger thickness and lower values of thermal conductivity, the surface temperature becomes higher than 500°C, and the own radiation of roof cooling element has a higher effect on the surface's thermal balance. Under such conditions, the cooling element participates in the heat transfer with significant quantities of the radiation heat flux from own radiation, depending, to the highest extent, on the surface temperature and surface emissivity and has the behaviour of a refractory-brick roof with all their shortcomings. For the same insulation thickness and higher thermal conductivity coefficient, the surface temperature is lower compared to that in the case of low thermal conductivity, consequently the own radiation (Q own ) of the water-cooling element is lower and under such conditions and lower emissivity (  <0.5), the reflected heat flux from the surface (Q refl ) has a higher values than the absorbed flux (Q abs ). Obviously, the use of such heat insulating materials for heat losses reduction is restricted by insulation thickness, levels of thermal conductivity coefficient and emissivity. The high level of the surface temperature has to be avoided, because the higher surface temperatures are sources of thermal gradients and thermal stresses and can lead to destruction of insulations.