На основании результатов лабораторного определения условий теплового самовозгорания отложений образцов паров горючих жидкостей в воздуховодах вентиляционных систем рассчитаны значения критической толщины этих отложений при штатном и аварийном режимах работы вентиляции с учетом несимметричного теплообмена образующегося слоя. Показано влияние температуры, поперечного сечения воздуховода и скорости потока среды в воздуховодах на условия теплового самовозгорания отложений внутри воздуховодов. Установлено, что влияние поперечного сечения воздуховода и скорости потока среды в воздуховодах на условия самовозгорания отложений является несущественным. Показано, что основным лимитирующим фактором самовозгорания отложений в воздуховодах следует считать процесс накопления осаждающихся слоев до критической толщины (с учетом штатных и аварийных условий эксплуатации технологических участков).
This article describes the results of laboratory tests on determination of conditions of the thermal self-ignition of sediments of vapors of various combustible liquids inside air pipes of ventilating systems. On the basis of tests there were calculated the values of critical thickness of sediments at regular and emergency operating modes of ventilating system taking into account the unsymmetrical heat exchange of the formed layer. In such conditions the temperature of opposite surfaces of sediments are different, and heat exchange of the accumulated layer with environment is unsymmetrical. To determinate the critical parameters of self-ignition of sediments on the surface of various equipment, in conditions of unsymmetrical heat exchange of the material layer, it was used the calculation method with the Frank-Kamenetsky parameter d in oxidation reaction of the combustible substance, considering convective heat exchange at both opposite surfaces of the sediment. For calculation it was accepted that the maximum average daily temperature in regions of Russia during the warm season doesn’t exceed 40 °C. Influence of the temperature inside air pipe on the critical thickness of sediment was determined at temperature values of external air of 40 and 20 °C. Presence of the temperature difference on opposite surfaces of a sediment layer leads to growth of its critical thickness by several times in comparison with requirements of the Russian State Standard 12.1.044-89. Our calculations show that if the temperature of air-gas environment ( Tg ) inside air pipes is equal to 70-80 °С, then the self-ignition of sediments become impossible because of significant growth of their critical thickness. Apparently, ignition of sediments, which thickness is less than 0.05 m, can occur as a result of penetration of the smoldering particles into air pipe or uncontrolled growth of the temperature of air-gas environment higher than 210 °C. Noticeable reduction of the critical thickness of sediments become possible also in air pipes with thermal insulation, if the temperature of external surface of the sediment (contacting with the pipe surface) approximate values of Tg . Influence of the cross section diameter of air pipe, as well as the rate of motion of air flow inside the pipe, on conditions of self-ignition is unobserved in comparison with influence of increase of the sediment layer thickness. The results of estimation of the induction period of self-ignition of the sediment with critical thickness (when Tg = 127.5 °C) inside air pipe with section diameter of 0.5 m and rate of motion of 1.5 m/s, in conditions when the temperature in production room is about 40 °C, indicate that self-ignition of sediments can occur within 1 hour, i. e. during one working shift. Despite the fact that features of heat exchange of sediments can significantly increase the induction period, the process of accumulation of the sediment layers up to critical thickness (taking into account regular and emergency conditions of operation) should be considered as the major limiting factor.
The authors declare that there are no conflicts of interest present.