Issue 37

E. Mihailov et alii, Frattura ed Integrità Strutturale, 37 (2016) 297-304; DOI: 10.3221/IGF-ESIS.37.39 299 In the mathematical model is accepted, that the heat removed by cooling water (Q c.el ) and resultant heat flux are equal: r el c el c Q Q . .  (2) For the heat losses by the roof can be taken very well know equation: el c w w r r el c F R T T Q . . 1     (3) The heat float falling through the flat cooling element can be determined by the following equation [2]:       elc r rad g a sum elc ef elc r r s l F T T Ck P F F k k k Q el c . 4 4 . . 100 100 1 1 1 .                                 (4) Determining of the heat lose according Eq. (3) T r and  w are unknown. The coefficient  w can be defined from the equation [4]: 0,14 s w 0,33 0,8 w p w Pr 0,027Re D Nu             For the purposes of the study, a mathematical model of the external heat exchange, under steady state thermal condition of walls and metal where the electric arc is viewed as energy point-source, was made. For computer implementation of the mathematical model an algorithm (illustrated by a simplified block diagram on Fig. 3) was developed. The object of the study was the construction of real 100t secondary steelmaking electric arc installation. A N EFFECT OF HEAT INSULATION PARAMETERS ON THERMAL LOSSES OF WATER - COOLED ROOFS hermal losses through cooling water have been calculated for various insulation thicknesses and types of materials. The thermophysical characteristics of silica, magnesite, chromitee, shamote, magnesite-chromite and chromite- magnesite refractory of various thicknesses and their own emissivity have been used as input data for the study. For faultless operation of the water-cooled roof panel, water temperature at the outlet shall not exceed 60 О С. It was assumed that water temperature at the panel outlet was 30 О С, and water temperature difference between the panel inlet and outlet was  T=10 °C. As a result of the calculations, the thermal losses, the average heat transfer coefficient to the water, and the required water flow rate ensuring compliance with the restrictions imposed on outlet water temperature were obtained. The resulting heat losses and temperature changes on the insulation surface depending on its thickness for silica, magnesite, chromitee, shamote, magnesite-chromite and chromite-magnesite are presented in Fig. 4 to 9. The thermophysical characteristics of these materials are presented on Tab. 1. From Fig. 4 it can be seen that, in the case of silica insulation depending on the insulation thickness 1mm to 20 mm, the thermal losses decrease with 50% from 1.83MW to 0.92 MW, or 8.3 % from the heat generated from the electric arcs. At the same time surface thermal resistance increases up to 0.01 m 2 K/W and temperature - to 950°C. Similar values can be seen in the case of cromite insulation (Fig. 5) where the surface temperature increases up to 820 O C and temperature resistance- up to 0.013 m 2 K/W. The magnesite insulation (Fig. 6) ensures heat losses in the range of 13% to 12.8% from the heat generated from the electric arcs, i.e. thermal losses decrease with very low values of 0.2%. This is in consequence of the high thermal conductivity with values of thermal resistance 0.003 m 2 K/W. The surface temperature increases up to 300 °C. T